The observation that "those words can be applied to a block view" doesn't make it logically consistent (with eternalism) to do so. — Luke

Totally agree. My usages of 'happens' for instance, in eternalist context, are logically consistent, and many of yours are not. Perhaps you are trying to use the presentist definition of the word in a non-presentist context.

They all imply motion which, I believe, is the more fundamental difference between the two views.

Two of the three imply motion. Motion is not the fundamental difference since both have it. I've said repeatedly: the fundamental different is that presentism posits a preferred moment in time, and eternalism doesn't. That, and only that, is the fundamental difference. All the rest just follows.

it’s all a matter of parts.

The perdurantist position seems to very much be about parts, yes. That's for the perdurantists to defend. I've posted some inconsistencies I've found with that.

This is the endurantist view. It is consistent with presentism due to the lack of temporal extension of its objects which are, therefore, not divisible into temporal parts.

OK, I think I did misread that. The question comes down to then: Is there a difference between somebody claiming to be endurantist and claiming to be presentist? There are several forms of presentism, so perhaps endurantism is but one of them, perhaps 3D presentism, as opposed to growing block, spotlight, and other 4D versions of it.

Therefore, the phrase "wholly present" is not, as you say, "a reference to all events in the object's worldline".

Yes, I withdraw that. The concept of a worldline implies 4D spacetime, and 3D presentism does not have meaningful worldlines, but 4D versions of it do still have worldlines.

I agree that the answer depends on which reference frame is present and so may be considered as ambiguous.
However, why do you say that presentists don't have this problem?

Actually, there is no Andromeda paradox under presentism, in any of its forms. Presentism denies both premises of special relativity: 1) Physics is the same in any frame. Well, it isn't. The whole point of presentism is a preferred frame, and all the others are wrong. 2) Speed of light is the same in any frame. Under presentism, that's false. The speed varies depending on which direction it is going, relative to any frame which is one of the 'wrong' ones.
So with the Andromeda thing, there is only one current moment in Andromeda, and one's choice of frame has nothing to do with it. The motion of any object is irrelevant to which events are simultaneous. The paradox is a non-starter since presentism is an absolutist view. There is no 'relativity' at all.

Are "you" a 3D object that is wholly present at each time or are "you" a 4D object temporally extended over time?

You seem to be mixing views in that query, rendering the question meaningless. If you're asking about eternalism, then keep it to those terms. I've never heard an eternalist talk about something being 'wholly present at some time', which seems not even wrong.

If you're a 4D object then a temporal part of you is home at noon and a different temporal part of you is at grandma's house at 1.

That is a decent description of movement in perdurantist terms, which I find needlessly complicated. The science community never uses such cumbersome terminology to say something so simple, which is why the 'temporal parts' page was largely educational for me.

My argument is that the definition of motion as 'a difference of location over time' applies only to 3D objects.

OK, then your definition is confined to a presentist view. That doesn't mean that a non-presentist must use that definition. The definition I gave works for both, and I've never seen a dictionary restrict the definition to 3D things. In short, my google query says 'move' means to change position. The shadow of a pole moves, and it isn't a 3D thing.

The 4D object is all "you", but it's not the same temporal part (3D part/object) of you at one time as it is at another time.

So per the perdurantists that use that sort of language, 'you' change position over time, but the parts don't. It's still you doing the moving. You're just trying to leverage your private definition onto a view that defines the word differently, which of course makes it contradictory. But that's a straw man fallacy.

Remember that the two views are fundamentally identical except for that one extra premise of an additional entity. So the two views can use all the same language so long as no reference to that additional entity is made (B-series language). If such a reference is made (A-series), then it is a presentist statement only. So saying 'Floyd moves from home to grandma's house over that hour' works just fine in both views because no reference to that additional entity is made.
I don't know what purpose you think is being served by trying to argue otherwise.

You still end up with different temporal parts no matter how you slice it

Yes, but one slice can be at gradmas house and another (at the same time) is not, so I find it to be a problem. The 3D things posited to 'exist at a time' are ambiguous without also positing a preferred frame.

It is analogous because no 3D part of a 4D object can change its location over time

Not true actually. You just need to slice it the right way.

just as no part of a rigid steel bar can change its location along its own length

This implies that all the points of a steel bar are at the same location at a given time. The bar changes its location over length instead of a change in location over time. This fits the definition of change, if not motion. Other examples of change not over time: The air pressure changes with altitude.

Just saying...

Does air die/explode?

No, but I do if I'm suddenly in the same place as air that wasn't there just before. If the machine is nothing but an air-filled balloon, then suddenly twice the air would be in there, and it might very well explode from the extra pressure.

It would be no different to moving the time machine to a particular location in normal time.

No, that is coming from one side, pushing aside what was there. OK, so maybe it pushes stuff aside. In what direction? Does it do it instantly? That would be a nuke explosion. So it takes time, perhaps expanding outward from a point, which will certainly destroy a Delorean inside of which this growing object suddenly appears. But in such a case, the new machine is alive, and any object already there is shoved aside, possibly crushing or exploding it. The tree would not take it well, and the remainder would probably fall and crush the machine that just teleported under it.

Let's say that whatever happens to the material already present at the target destination if we moved the time machine there in normal time is the same/similar to what would happen if we moved the time machine there via time travel.

Doesn't work since the form physics is normal motion, say from one side. Where does that start? From how far away does it effectively come? If it comes from a side, then somewhere it has to initiially appear, and not come from even further to the side. So far, the answer is that it teleports in somewhat off-center of target (destroying whatever is there), and then forcibly moving over to the actual target spatial location, possibly pushing/crushing the additional objects that are there, and of course crashing your own machine, since a vehicle collision is what happens when two things move into the same location in normal motion.

Sorry if I'm ragging on the answer, but I need to know how it actually appears. If the machine pops fully into existence somewhere (off to the side or not), it needs to deal with the material already there. If it starts at a point and expands gradually outward, then that solves the whole expel at infinite-speed problem, but it also destroys anything inside of which the expansion takes place. And if it takes time, how long? Does it slowly grow into existence over a minute? A second? 9 months?

I don't see understand why you are pressing this point. What difference does it make?

The Alice story cannot proceed without knowing this. Also the extreme example of setting your machine to go back half a second.

Surely we can imagine that the time machine can arrive safely

No we can't. My examples are specifically designed to reduce the odds of safety to zero. I'm finding flaws in the view envisioned, which I thought was the purpose of all these posts. The half-second just is obviously going to lang on the machine that is there. Destroying it isn't such a bad thing in that case, but I need to know if that's what happens. If the jump finds somewhere more (but not completely) 'empty' nearby, would it teleport there instead? That's a different solution than the bang-and-push thing you described before. It results in different problems.

but let's assume it has the technology to avoid it.

What does it do to avoid it? Go to the moon instead? NASA would love it if your machine did exactly that. So much effort saved. Who cares that it's a time machine. It's also a space teleporter.

You seem more concerned about the ramifications of time travel - the end of humanity or the destruction caused by the time machine - than you are with the possibility of time travel.

But the possibility of time travel, as you describe it, has exactly those ramifications. If you don't want that, then a different model should be assumed.

According to my convention, Alice1 is the original; the time traveller. Alice2 is the 30-seconds younger version of Alice1 who exists in the past (just as young Bob exists in the past of time traveller old Bob). I cannot see how both:
(i) Alice1 will time travel back 30 seconds after crossing the tracks; and
(ii) Alice2 will time travel back 30 seconds, 5 seconds before crossing the tracks.

Alice 1 has already traveled and will not do so again. Alice2 will travel back when she gets to the track, cloning everybody on that timeline, so I guess Alice1 vanishes as does everybody not in a machine that goes back in time.
Alice3 is 30 seconds away from the tracks, and has never traveled. Alice4 is at the crossing, a clone of Alice1 that did the first travel. (I neglected to name here Alice4 in my prior description, but by your convention, two new Alices get created when Alice2 goes back. So Alice2 and Alice4 collide at the tracks, and what happens thereafter depends on your collision resolution description that you're reluctant to describe. Alice3 will get to the scene in 25 seconds, and based on what she finds there, she may or may not decide to just wait for the train, or go back more than 30 seconds to avoid the accident scene, or some other choice.

I don't like your identity convention since it clones everybody in the universe except the occupants of the machine, but I am using your convention above.

and if Alice2 time travels 5 seconds before crossing the tracks

Everybody time travels at noon+30 seconds, back to exactly noon. At noon+25 seconds each virgin Alice gets to the tracks and has 5 seconds to assess the situation and decide to go back 30 seconds or not.

then Alice2 will not proceeed to cross the tracks

Maybe. She makes it to the crossing too late, hits the button, goes back 30 seconds, and if her collision with Alice4 isn't noticed, she probably considers it mission accomplished and proceeds to cross the tracks just before the gates start coming down. But I don't think the collision will go unnoticed, which likely will effect whether she proceeds across the tracks or not.

If Alice1 lands on and kills Alice2

Alice1 is the first to jump, and lands on nobody. She proceeds across and is truncated out of existence when Alice2 pushes her button. Alice1 is the only happy Alice, so it's a shame her life ends so abruptly.

It is your assumption that events happen (which you differentiate from mere existence) in an eternalist universe which suggests some sort of flow or motion in an eternalist universe. — Luke

It is not an assumption, but rather an observation that those words can be applied to a block view, and that they don't mean that it is an assumption that time itself is what flows or moves.
Water flows. The wheels on the bus move. The sinking of the Titanic happens in 1912. None of those statements imply a presumption of a preferred moment in time, and that one presumption is the only fundamental difference between the views.

Perdurantism has temporal extension; endurantism does not.

Perdurantists believe that ordinary things like animals, boats and planets have temporal parts (things persist by ‘perduring’). Endurantists believe that ordinary things do not have temporal parts; instead, things are wholly present whenever they exist (things persist by ‘enduring’).

I didn't read it that way. The endurantists statements you make seem to consider objects to have temporal extension (since a reference to 'wholly present is a reference to all events in the object's worldline, and that is, in the absence of a preferred moment presumption, an eternalist stance.
The endurantist stance, as stated, needs clarification since it seems contradictory. First of all, there is the statement about being present (not absent) when it exists, but 'when it exists' is ambiguous. Consider the Andromeda 'paradox'. Is the en-route invasion of Earth fleet wholly present in 2024 or does it absent, according to endurantists? The answer is ambiguous due to relativity of simultaneity. The presentists don't have this problem with the Andromeda scenario.
The other contradiction I see:is that I wholly am present in the year 2000, which includes my tonsils, but my tonsils in particular are absent in 2000, so they are both present (as part of something present) and absent in 2000 (as just the tonsils), a contradiction. So as I said, clarification is needed to clean up such examples.

The science community cares not at all about such distinctions, and the time travel question becomes a scientific one once we have empirical descriptions of how it all works.

Motion in a block universe is a difference of location over time, just as it is in presentism.
— noAxioms

Motion and/or location of what, though?

Objects of course. I'm at home at noon, and at grandma's house at 1, a different location (relative to the frame of the surface of Earth) over an hour's time.
If you get anal and take my attempt at the the endurantists wording of the situation, then "Relative to the coordinate system of the surface of Earth in timezone X, the events in my worldline that have the temporal coordinate 'noon' have the same spatial coordinates as 'home', and the events in my worldline that have the temporal coordinate 'at 1' have the same spatial coordinates as 'grandma's house'.
But that's a mouthful much more easily expressed with "Between noon and 1, I move from home to grandma's house".

My argument is that the definition of motion as 'a difference of location over time' applies only to 3D objects.

Well I just applied that definition to a 4D object just above.

Since each 3D part (of the 4D object) exists at a different time, then no 3D part moves or changes its location over time.

OK, this is just a refusal to use the typical identity convention, that me at one moment is not the same me a second later, but rather two separate entities. Regardless of a presentist or eternalist stance, if that identity convention is used, then indeed, nothing can move, by definition. There are valid attacks on the usual identity convention, so this can be a reasonable alternate convention. I think I can disassemble any identify convention by choosing the right example, so I don't suggest any one convention is necessarily correct.

It sort of has all sorts of moral implications, that one cannot be held responsible for something a different entity did yesterday. It's an interesting exercise to argue why that statement is not so much true, but rather meaningless given the assumptions made.

Another counterargument to the whole 'separate 3D parts' interpretation is that a 3D part is coordinate system dependent. There are different was to slice a 4D worldline into 3D cross sections, and absent a preferred angle of slicing, there are not actually any 3D parts, but rather only utterly separate 0D events that are the 'parts'. The perdurantist stance doesn't seem to get into this, perhaps because the adherents are not really up on the physics from which all these eternalist views sprung in the first place.
The SEP article on temporal parts seems to mention some of these problems in section 7, but without resolving any of them.

It would be analogous to part of a steel bar "moving" along its own length; it doesn't happen.

That usage of 'move' does not conform to the definition given, so no, it isn't analogous.

I have no idea why you think I never replied to your post from a week ago.

My bad. Some of the notifications are not coming through. Will try to reply to parts not covered since.

How would air, dust or bugs at the destination prevent time travel?

I asked for how you envision interaction with material already present at the target destination. Your answer was simply 'die/explode'. So perhaps the answer needs to be changed. Maybe it handles air better, by what, pushing it aside first? Absorbing it (which probably covers 'die' pretty well)? The answer you gave does not imply that it simply replaces what was there with a new state (terminator style, except with electrical effects preceding).
So if explode/die is the wrong answer, then what is the actual answer? If air is treated differently than other material, where is the line drawn, and how about the bugs, which are definitely not air? How about the tree I mentioned?

If the machine can time travel, then it can probably find a safe place to arrive.

That's a different answer. So it assesses the target, and selects somewhere close? Does it have a limit as to how far (both spatially and temporally) it is willing to look for a satisfactory point in which to insert itself? What does it do with the stuff that is already at the selected point?
Alice hits the button to go back 30 seconds and finds herself on the tracks with the gates already down (just like in BTTF) and with a train 3 meters away. Hey, it was the nearest available spot...

I need to know the rules so I can illustrate the contradictions that result from those rules. We've not even attempted everybody having such a machine yet. I can't imagine how many questions it's going to take to get a clear model of that, but it probably won't happen because the machine you envision erases history, so in very short order, all those other machines will be erased from history by the person who travels backwards the furthest.

You've lost me here. There are three Alices?

Two travel events (both by younger Alice, traveling for the first time ever), each one making a clone, so yes, three of them. Did you forget the machine makes clones?

Alice goes back 30 seconds. Okay. Then there is also an "Alice behind". Is she the same Alice as the one who just went back 30 seconds?

Depends on your identity convention. Which do you consider to be the original in the just-truncated history, the one that traveled, or the younger one that has not, but is about to? When she does, at noon there are two or three Alices, depending on the microsecond timing. If the 2nd destination event happens ever so slightly sooner than noon, it erases the noon event of the appearance of the Alice that makes it across the tracks, and there still remain two Alices, the one that just appears, and the one 30 seconds back that is approaching the crossing and is going to hit the button in 30 seconds.
If the timing is the other way (which it must be eventually), the 2nd travel event lands exactly on the first one, and the whole explode/die thing occurs, leaving only the younger Alice who will get to the explosion scene 25 seconds after noon.

Apparently not, since those two Alices die after one lands on the other. So, where did "Alice behind" come from?

She is always there. Nobody traveled back far enough to erase her from history. She's the one that has never traveled before, and is late for her appointment.

However, now a third Alice approaches the tracks to find the wreck of the collision that killed the other two Alices.

It seems you convention is to consider the traveler to the original, and the other in the timeline to be the clones.
So in the Bob thing, the original Bob goes back and kills his younger clone, who is not Bob, but rather clone-Bob.
I had been using a different convention, but which one used doesn't matter except when we assign names.

Where did third Alice come from? Was it only the first Alice who time travelled?

Using your convention, the original goes back (Alice1), who crosses the tracks,. Alice2 is 30 a clone, 30 seconds younger, and will get to the track in 25 seconds and will decide to go back 30 seconds to make it across. Alice2 goes back to noon, explodes and dies in a collision with identically aged Alice1 who also appears just there, and Alice3 (30 seconds younger than 1 and 2) will get there in 25 seconds.

If these are different people then why did you call them all Alice? This is very confusing.

They are all Alice, but I put numbers on them to keep track of the clones. I used your convention.


From last week:

Much of the confusion early in that post is my using a different identity convention, where I consider, in a timeline resulting from a travel destination event, that the traveler is the original and those pre-existing in the timeline are the orginals. The story as reworded above utilizes the opposite convention where the traveler is designated as the original.

Your argument is supposedly that my presentist model entails a blank future universe.

If it doesn't wait for the destination to be written, then yes, it is blank. If it just makes up a state to write into that blank space, then fine, it puts something there, all very BTTF. Nobody can tell anything is weird except those who witness (or better, catch on video) the appearance of the time machine out of nowhere.

I could say that the future has a definite physical existence prior to the time travel

Your model had truncation. This statement seems in contradiction with that term, which sort of implies that when the present is moved back to 1990, the written state of things between 1990 and 2024 is reverted back to a blank state. Now you suggest otherwise. All very self contradictory. Perhaps more clarification is needed as to what exactly happens to the 34 years between when the present is moved back to 1990.

You say you're not necessarily a presentist, but you've been describing something that matches only growing-block theory, and matches nothing else. This more recent statement is more like moving-spotlight, where 'the future' is sort of written (exists), but is not yet at the preferred moment.

I think many works of fiction depict time travel as I depict it in my model, where the time traveller travels to, and inserts themselves into, a time they have never visited before (as a time traveller). For example, Marty McFly was never in 1955 prior to his first time travel event, and his time travel results in changes to the 1985 he departed (i.e. he overwrites the timeline).

And encounters a slow version of the grandfather paradox where he is threatened with nonexistence by changing the circumstances leading to his birth, a different story than the one you tell. Anyway, that story is full of contradictions, and it doesn't explicitly call out the interpretation of time it is using. The movie probably contradicts any valid interpretation of time.

Presumably backwards time travel works differently. Why should the machine have to wait in forwards time travel if it is not required to wait in backwards time travel?

In a growing block model, the past exists but the future does not, but will eventually. Hence the wait. In a moving spotlight model, both exist, and it is merely a matter of 1, moving the spotlight, and 2, creating a destination state that is compatible with the identity convention of choice. In raw presentism, backwards time travel is impossible because the destination doesn't exist, and never will again. Under eternalism, a branching model in Hilbert space is probably the best, but world creation is not really time travel without a simultaneity convention between separate worlds.

Why do the events happen in a sequence when they don't exist in a sequence? That is, events do not flow into and out of existence sequentially in an eternalist universe, like they do in a presentist universe. So, why do they happen sequentially in an eternalist universe?

Putting them in a sequence is a choice, a natural choice, as I've illustrated. I can create a series of pictures that a child can order in apparent causal order, not necessarily in the order in which the pictures were drawn.

You seemed to be arguing that there are no events before the big bang even though there are times before the big bang,

I don't argue for meaningful time 'before the big bang', given a realist definition of the universe as 'all there is', there would probably be more than what is accounted for by just the spacetime that we know. The ability to temporally order the other parts is likely meaningless, so different language is needed to discuss such things.

What makes you think I'm ignorant of the theory of eternalism? — Luke

It's just that every attempt at describing things in eternalist terms still adds references to flow or other implications of a special moment in time.

You do seem to be more familiar with the glossary as used in the philosophy sites. I come from more of a physics background where such terms and distinctions are not important. I've never heard a physicist refer to a 3D part of a 4D object, but apparently SEP is full of that sort of thing, and you linking to those sites has helped me see what the language is all about.

For someone who regularly accuses me of ignorance of concepts in the philosophy of time, I find it amusing that you are obviously unfamiliar with the concept of temporal parts.

It seems I am.
The SEP site describes spatial parts that are extended (hand, feet and such), but when it comes to temporal parts, it seems not to allow any extension to them, which seems an inconsistent use of the term 'parts' to me.
The article is supposed to be describing a form of eternalism, but it still makes plenty of references to 'the present, past, and future', which begs a different view.

Physics doesn't seem to care about the distinction between perdurantism and endurantism, and the difference seems merely one of language. The views don't seem actually different in any physical way, so I couldn't really say which of the two I'd side with if I had to choose one.
It seems one finds meaning to the question of 'does a 1947 event exist in 2047?', and the other view does not find the question meaningful as worded.

Before your break, we were discussing whether events can happen (or be happening) in eternalism, so I don't consider a further discussion of the implications of eternalism to be a side topic.

This is also mostly a choice of how to use the language, but the tense 'can be happening' in the absence of an explicit time, constitutes an implicit reference to the present, and such references should be avoided. I've said this repeatedly.

You appeared to be arguing that eternalism is the only theory that can make sense of time travel.

Not at all, but it treats it differently. Different interpretations work in one interpretation or the other, but most not in both.

Besides, you completely ignored my argument against motion in an eternalist universe, just as prior to your break, you never replied to my argument that Alice0 cannot be the original Alice.

Motion in a block universe is a difference of location over time, just as it is in presentism. What was you argument against that again? Do you deny this definition, or deny that it applies to either view?

As for which Alice is the original, I simply chose a convention. I never argued that a different convention was necessarily wrong. The Alice story can be told using either convention.
Also, it was you that took the break, never replying to anything from my post a week ago.

Yes, if the car/person jumps to the same location as another car/person then they would all die/explode/cause a black hole/etc.

Then time travel is mostly impossible the way you envision it since there is always something (air, dust, bugs, trees, whatever) at the destination, unless one chooses to materialize in deep space, and none of your scenarios do that. But here you suddenly suggest that materialization at a location that already has something results in the destruction of the machine and whatever was there before.

If it materialized in deep space, the machine would be wonderfully useful for budget space travel. Other worlds could be populated effortlessly, a task currently not feasible.

So Alice goes back 30 seconds, crosses the track, and the Alice behind travels back 30 seconds later and lands on the first traveling Alice, and both traveling Alices die, leaving just the younger Alice approaching the tracks, who finds the wreckage of the collision there, and thinks twice about adding herself to the heap. Problem solved, but Alice misses her interview appointment and doesn't land the desperately needed job,.

Yes, because most people are not physicists that understand relativity theory. Hence, "commonly held". — Luke

Alright, but when in a discussion where the implications of a specific theory (or its alternatives) are very relevant, coming into the discussion in ignorance of that theory doesn't put you in a position where your view can be coherently argued.
For instance, you seem to be able to discuss black holes probably because you've heard the term on pop-science sites or the news or whatever, but only Einstein's theory predicts them. They cannot exist under presentism of any kind. So the commonly held view is also self-contradictory, which is simply not a concern of the average guy on the street. Probably 99.9% of everybody holds views somewhere that are mutually in contradiction. But most of those people don't argue on forums for the consistency of the specific points that are in contradiction with each other.

3D parts of the 4D object.

I see what you're saying. It's a funny way of putting it, but I suppose so. I would have called them cross sections instead of 'parts'.


All this is a side topic. We need to make progress since almost none is being made in a 200+ post topic.
None of the post was about time travel, and your rules continue to be evasive.
Suppose I take my (stationary) machine and go back half a second. There's obviously a machine sitting at the targeted destination, so where do we materialize? Does the machine of 1/2 second ago get trod upon and destroyed, both machines destroyed (car crash style), or does it find somewhere/somewhen else to materialize? What's the rule here?
I really couldn't make progress on the Alice example without knowing how you envision this.

Been away a while. Much of the below is yet more repetitious corrections.

What I don't understand is why you felt compelled to interrupt a discussion that's been developing for over five pages only to denounce it as "pretty nonsense" — Luke

An I don't understand why you let the interruption halt the discussion.

Talking about "overwriting" the "timeline" is like talking about "overwriting" the "heightline" or the "widthline" or the "lengthline". It seems pretty nonsense. — Michael

You're confusing the timeline with the time axis/dimension. The latter is nothing more than a sort of state of what happens in a particular world.

I assume you're also against the growing block theory of time? — Michael

I'm not against the growing block theory, per se, but I don't necessarily consider it to be the view that I hold. — Luke

All of your responses (since branching had been abandoned) seem to describe pretty much a growing-block view, with travel to the past truncating the block and resetting the present to the new destination time. You shows little understanding of a view that isn't some kind of presentism (as evidenced below). I can't think of a label that better describes what you've been describing.

Presentists Should Not Believe in Time Travel — Michael

I was trying to work with it. It is actually 'travel' under presentism, as opposed to a sort of discontinuous (or at least not time-like) worldline you get under eternalism. But yes, traveling to a time that isn't the present creates all sorts of problems, solved by the apparent god-like ability of the machine to rewrite the present state of the entire universe.

Growing block is a form of presentism, and under that, there is at least a past to which one can travel, but getting that state to be the new 'present' is the big trick.
Moving spotlight is another form of presentism, but once again, requires a god-like power to control the spotlight, hard to do if everybody has such a device.

If there exists a history then presentism is false.

Growing block is a form of presentism, and has a history.

Argument against motion in eternalism

A major difference between presentism and eternalism is their differing concepts of an object. Presentism takes the commonly held view — Luke

Yes, commonly held, but not by physicists that understand relativity theory.

Presentism takes the ... view that an object is 3D and traverses time.

The present is 3D. Growing block and moving spotlight are also presentism (positing a preferred moment that traverses time), but still have 4D spacetime.

Eternalism takes the uncommonly held view that an object is 4D, that the 4D object exists across time, and that it consists of 3D parts.

3D parts of what?? Any object (a car part say) occupies a 4D volume of spacetime. I can't think of a 'part' that is 3D. One can take a 3D cross section (in any direction, not just space-like), resulting in a 3D subset. I think that's what you're referencing.

If we consider that the motion of an object is basically a change in its position over time. ...
according to presentism, the same 3D object exists at different times.

Yes, true under both views.

, then it can be shown that this can only apply to (the presentist view of) a 3D object. This is because, according to presentism, the same 3D object exists at different times. However, according to eternalism, the same 3D part (of a 4D object) does not exist at different times.

Physical objects are 4D objects extended in space and time as per eternalism.
Consciousness is a non-physical 0D "object" bound to some physical object.

Time doesn't flow but consciousness travels through (its physical host's) time. — Michael

That sound a lot like moving spotlight, but in the absence of presentism (with a yet unwritten future), it boils down to epiphenomenalism, sort of like watching a movie where the experiencer is in no way capable of influencing the character being experienced.

Time travel is possible though this, but instead of old-Bob physically appearing in the past, the mind experiencing Bob just 'switches channels' and moves his personal spotlight to something (presumably a different person) at some other time.

Sorry, it was special relativity, not general relativity — Michael

Both imply (but don't explicitly require) a lack of an objective present. SR is nice, but is a local theory, only describing one's immediate environment and not the universe in which we actually live.
GR does not posit (or imply) a present, but there is a sort of preferred frame in which the mathematics is easier. It isn't an inertial frame. That part is kind of in conflict with the first SR postulate that the physics is the same in all frames. Well it isn't. It's more complicated in the other ones. Einstein was not pleased with this outcome.

Well, yes. I think it self-evident that I experience the passage of time. I want a theory of time that can account for that.

The experience of time is the same under both views. Relativity theory is not in any way a theory about how biological experience works.

There is no similar study we can do to see how damage/changes to “unmoving space(time)” affects physical objects. — Luke

Spacetime does not change. It isn't embedded in time, so it cannot evolve over time. Objects ARE contained by time, and thus change over time. Treating spacetime as an object is a category error.

You state a disbelief in spacetime and relativity theory. That's fine, but a lack of understanding doesn't put you in a position to criticize the consensus* view.

*among those with understanding.


All this aside, I'm trying to put together a comprehensive analysis (probably naive) of all the different interpretations of relevant ideas, so show how some views are blatantly in contradiction with time travel, and others might not be. The branching seems to avoid most of the contradictions, but as @Christoffer points out, it isn't really travel then, is it?

I would post the think in this topic, but it's so far down the rabbit hole and making no progress that I think it better to be it's own topic, one where I'm driving instead of just replying to ideas of others.

Does physics describe what the above even means? — Michael

No, it doesn't. It is kind of like asking what physics has to say about if the sun suddenly wasn't there. Would Earth continue to orbit for 8 minutes or would it immediately commence a straight trajectory?
Another question: Does an infinite sheet of material (a meter-thick slab of concrete say) result in a uniform gravitational field?

Physics has nothing to say about either case since there is no way to describe what any of the above even means.

Luke is exploring a philosophical question about the implications of various philosophical models on the concept of time travel. The current model seems to be a sort of growing block model, which is full of contradictions, most of which have been left unexplored due to the slow pace of working through even the trivial bits.

I made a mistake in my last post. ... I agree with the Hollywood version; you don't meet yourself or clone yourself going forwards. You depart from an earlier time to a later time, so there's no other version of you left behind who continues aging normally once you depart from the earlier time for the later time. You can only "clone" yourself (in a sense) going backwards. — Luke

Good. This is more in line with the typical pop vision of the time-traveling vehicle. Given our growing block model, the machine still has to wait for 2035 to come around before it can materialize in it. There's problems with that, but not obvious when there's but a single time travel event in consideration.

Also good that you recognize that any backward time travel machine is a cloning device. The traveler is the clone. The think I descried with the army-creator makes thousands of clone soldiers, plus a group of originals that have never time traveled. Keep all that in mind when considering the Alice example. The Alice that appears at the tracks and makes it across is the clone. The original Alice is the one that doesn't make it to the crossing on time. We can number the clones if there's more than one.

So, Alice gets to the train tracks and has to stop because the gate comes down.

That's Alice0, yes. She's the original. She's never time traveled, not backwards at least.

She decides to use her DeLorean time machine to go back 30 seconds so that she can floor it and cross the tracks before the gate comes down (the second time around). All well and good.

By 'second time around' you mean the 2nd writing of those 30 seconds, yes. Alice1 makes it across the tracks. Alice0 is a half km back from the crossing and will get there in 30 seconds, 5 seconds after the gate goes down.

What I don't understand is, after she does this, why is there another DeLorean behind her getting stuck at the gates?

You seem only capable of imagining the traveler, just like Hollywood only follow the protagonist. Think about the others in the world at noon. Remember that Alice0 is in that world, half a km up the road, who is fretting about how tight her time is to make her appointment. She thinks about little else at the moment. Alice1 makes it across but Alice0 is about to erase Alice1's victory by hitting the button for the very first time in her life, truncating the history where Alice1 made it across. It sort of turns into a Groundhog-Day situation, except in Groundhog Day, the protagonist has memory of all the times through the loop. Alice doesn't. Alice0 has no memory of ever having time traveled.

The time travel event in your scenario does not overwrite the timeline.

It doesn't? You say it does. You said Bob going back to 1990 truncates history back to 1990 so it can be overwritten with older-Bob in it now, which is exactly what Alice0 is doing, except this time younger Alice0 is working the controls, not older Alice1. Are we changing the story again?

I asked you which works of fictions involve time travel to a blank universe which has not been "written" yet. You tell me that there are no such works of fiction.

None that I know of anyway. Langoliers comes closest. The travelers arrive at a sort of blank future, but stay put at the moment of arrival until the 'present' catches up with them and suddenly everybody appears. It's one of the few stories that really leans on presentism, where the author is very aware of his model and tries to be consistent with it.

Okay then, which works of fiction wait for the future destination to be written before time travel to that future destination occurs? By "wait", I assume you mean in the usual fashion, like you might wait for a bus?

No, waiting for a bus takes subjective time, experienced by the waiter. The experience of the traveler is no waiting. The world is simply there when they arrive, sort of like super-fast spaceship and time dilation. I can go forward 11 years in a moment without having to experince waiting, if my ship is fast enough. And SEP apparently designates that as actual time travel, despite my protests.

I don't think many works of fiction explicitly rely on this growing-block model that you have going on here, so concepts like a new history growing simply don't apply. But also, many (most?) time travel stories never depict the viewpoint of somebody other than the traveler. Dr Who has gotten a lot better about that since it was resurrected. The writing has gotten better and many stories are told from different viewpoints, including episodes mostly without the doctor or machine in it at all.

So you are saying that, in all works of fiction, there is no time travel to a future time which occurs before people have waited for that future time to happen?

The machine has to wait. The people never do, since the experience is instantaneous to them.

Cryonics is not a time machine; not the sort we have been discussing, so not relevant to the discussion.

I didn't say Cryonics was time travel. I said the experience is essentially the same to the traveler: (Step in, step out into some future year). The experience of the outside observer is not the same because they can see the machine with Cryonics, and it 'disappears' presumably if it's a time machine. Both machines have to wait for 2035 to happen, but the time machine apparently waits in some inaccessible dimension or some such. No explanation is yet given as to where it is en route.

It doesn't have to wait. It just travels there and overwrites what would have been.

OK, this is new. It just makes up a plausible state for 2035? None of the intervening years actually happen, the state is just put there? How very last-Tuesdayism. BTW, I am a total fan of last-Tuesdayism, not that I assert it, but it is something everybody needs to attempt (and fail) to falsify.
I'm fine with that. It's consistent with the God-like powers the machine needs anyway to go backwards, so if it can set the present to 1990, why not make up a 2035? You don't even need growing block then. It can just be raw presentism, where the 1990 it creates isn't actually what the real 1990 looked like back then, but it's consistent with what is known about 1990 in 2024.

This is also very consistent with the 2nd Back to the Future movie, going to a totally made-up 2015 that looks nothing like what the actual 2015 would have looked like had the machine waited for the real timeline to grow to that point.

It follows from this that "happens" is no different to "exists".

I gave examples of the difference between the words, where substituting one for the other in a sentence would result in a wrong statement. So no, they're not synonymous.

It happens when it exists and exists when it happens - there is no distinction.

I never said either. It happens at the time of the event. It exists in spacetime. All events exists in spacetime, but they don't all happen at any given time since the time of one event may be different than the time of another.

I defined the preferred moment as "the state that is happening, as opposed to those that have happened or have not yet happened". What's wrong with that?

Yes, that's how a presentist might define the preferred moment. But that moment is not postulated in eternalism. If you want to understand eternalism, don't drag in definitions and premises from an incompatible view.

Does eternalism allow for events to have happened, and for events to have not yet happened, but not for events to happen? Why?

Meaningless due to the implicit references to the present. One can say that relative to 2080, 2070 has already happened. That's an explicit relation reference. Tensed verb work as long as the reference moment is explicitly stated.

Does eternalism allow only for the different ontological states of 'not yet happened' and 'happened'

Both are meaningless. They are both references to the present. How can you not see this?

You described them as such in your Titanic example. You described a time when the Titanic had not yet sunk, and a later time when it had sunk, and then you said "Somewhere between those two events does the sinking 'happen'."

Yes, all references to explicit times, not implicit references to the present.

Is "before the big bang" part of spacetime?

Not our spacetime. The geometry outside our spacetime is not really known, It isn't know if 'geometry' is the right word for it even.[/quote]

The statement "there exist events at each (and every) time" does not require every event to be accounted for, as long as there exists at least one event at each and every time.

First of all, the statement is false since I can think of a time that has no events. Secondly, I know of no coordinate system that accounts for every event (assigns a value to its coordinates), so the bit about a requirement of all events being accounted for is not there for a coordinate system, but it kind of is there for spacetime. Spacetime is physical. Coordinate systems are abstractions.

Nothing is happening in an eternalist universe?

Not even wrong.
The phrase "nothing is happening" is not a meaningful one in an eternalist universe, so the truth of the phrase cannot be assessed.

She simply decides not to hit the button again. You didn't provide any information in your scenario about why she time travels. — Luke

There is no 'again'. She's hitting the button for the first and only time, because she's late for a very important appointment (a job interview say) and cannot afford to wait for the slow train. She hits the button the one and only time to go back 30 seconds to before the train gate coming down, and thus proceeds across the crossing to make her appointment. There's was never a repeated hitting of the button. Somebody else (the younger Alice back there) will hit the button for the first and only time, for reasons already explained.

Presumably she does it to avoid being hit by an oncoming train.

I never gave any indication that she's stuck on the tracks. She's at the crossing, having to wait for it, a wait she cannot afford.

Maybe she realises she can't keep looping back every 30 seconds forever and tries something different instead.

It's her first time. There's no loop of which she can be aware, except she knows that any use of travel to the past makes the past happen again, a loops of sorts. Look at Bob who goes and makes 1990 happen a 2nd time, but differently. That's a loop of sorts, but one that only 'happens' twice since his actions there prevent young-Bob from doing his 2024 thing.

What fictions involve time travelling to a future time where nothing exists; a "blank universe"?

None, which is why you model, if the machine doing a forward jump doesn't wait for the destination to be written, would match any of the typical fictions.
So either the machine must wait for the destination to be written, or if it doesn't, the machine appears in an unwritten future, which is blank.

What fictions involve waiting for the future to happen first, before time travelling to it?

All of them. It's not a wait from the traveler perspective of course. He arrives having aged but a moment with no memory of any waiting.

It would be a pointless time machine if the user had to wait for the future to happen before one could time travel to it. You don't need a time machine in order to wait for the future to happen.

Sure you do. Jumping to Y3000 with a machine gets you to Y3000 just like Phillip Fry (who does it via Cryonics, an identical experience). Jumping to Y3000 via waiting gets you very very dead.

The only cloning that happens is if Bob travels to some time within his own lifetime

2035 is withing his own lifetime, so F-Bob (who I'm designating as the clone) is not yet dead, but he's 50. S-Bob (the time-traveling original) is 39 and meets his clone fact to face.
As I said, the machine has always been a cloning device. Bob goes back to 1990 where he meets another Bob. Two Bobs means one is a clone. Using this technique, you can make as many Bobs as you want, and you can do it quickly, in minutes instead of decades. So getting a clone by going forward is admittedly consistent with your going-back description, even if none of the fictions seem to depict that consistency. Hollywood has developed a rule that you can meet yourself if you go backwards, but not if you go forwards. There's no reason time travel has to conform to what Hollywood finds desirable.

you are yet to have proven it illogical.

Never claimed it was. Just an unusual choice of rules, since Hollywood does have an influence on most people's vision of what time travel would be like.

If Bob succeeds in time travelling, then F-Bob does not exist.

That's not what you said. You said the line is written as if the travel had not taken place (so it has F-Bob in it), but with S-Bob appearing in 2035, the destination event, which thus has both of them in it.

F-Bob only exists if Bob fails to time travel

No, you said the line is written as if the travel had failed, so F-Bob very much exists in the line to which S-Bob travels.

You can of course abandon that assertion and say the line proceed as if the travel succeeded. Then the experience of Roberta is to see the machine disappear, and she's without her dad for 11 years. That's the typical hollywood depiction, but then the cloning property only works in reverse travel, not forward travel. You can still build the unlimited army, but the algorithm is slightly different.

There is either a timeline without a time travel event or there is a timeline with a time travel event. Call the timeline without a (any) time travel event timeline A and call the timeline with a time travel event timeline B. If there is no time travel event then timeline A results. If there is a time travel event then timeline B results (and timeline A gets overwritten by timeline B).

OK, so A exists, the machine waits 11 years for line A to get to 2035, and then when it does, the history (with F-Bob) gets truncated back to 2024 and the machine has to wait an additional 11 years for the B line (no Bob at all) to get to 2035? Why can't the B line just be written from the start since F-Bob and the rest of the A line is doomed before the first moment is written?

If there had been no time travel events then F-Bob wouldn't exist.

No, if there had been no travel event, then S-Bob (the traveling one) doesn't exist.

I need to make a correction here. I said earlier that forward time travel would change the timeline from the arrival time onwards. I should have said that forward time travel would change the timeline (from timeline A to timeline B) from the departure time onwards.

Yes, as described just above. The machine has to wait 22 years now for two different histories to play out over 11 years each. Weird, but not contradictory.

If there is no time travel event then the machine doesn't disappear.

Fine. The Robert in line A sees the machine stay put (fail), and a dejected F-Bob gets out The Roberta in line B sees it disappear and eventually meets S-Bob 11 years later.

The machine only disappears if there is a time travel event.

Well, there was a time travel event in line A, but the observers in it have no way to tell. They would have been able to tell in 2035, but their line ends there, so they have no experience that would constitute a falsification test.

I'm just trying to get a better understanding of the distinction between the meanings of "happen(s)" and "exist(s)" in relation to an event/process under eternalism.

Processes are comprised of multiple events, and just like Earth (with spatial extension) can be treated as a point in some calculations, so can a process (a concert say) be treated as a point event so long as our precision is low enough that it doesn't matter.

Events are members of spacetime, thus exist in spacetime, just like locations exist in a 2D plane and thus exist within it. Since time is one of the dimensions of spacetime, the word 'happens' is meaningful. The event happens at the location in spacetime of that event, which I realize is circular, but that's the nature of a tautology.

You spoke of the time before the Titanic event when "it has not yet sunk"

Yes. The event of the Titanic Sunday Apr 14 has the Titanic in a state of 'not yet sunk'. It means that the sinking event (Monday, around 2AM) is a subsequent event in the ordering of all the events along the Titanic worldline. One can say that event A is prior to B, or A is in the past of B. Such relations are valid, It is the implicit reference to a preferred moment that is meaningless.
The statement about the Sunday event being a state of 'not yet sunk' simply says that the sinking event lies in the future of that Sunday.

It seems very much as though there was a time before the event when the sinking had not yet happened, and a time after the event when the sinking had happened, and then somewhere in between those two times when the sinking was happening.

There are a couple (bold) implicit references to the present in all that. To reword:
There is a time before the [sinking] event when the sinking had not yet happened, and a time after the [sinking] event when the sinking had happened, and somewhere in between those two times when the sinking happens.
The bit in brackets is not a correction, but just there for clarity so we know which event is being referenced.

Eternalism is unintuitive because A-series statements are just part of everyday language and is very hard-coded into our instincts. People mistake language for truth instead of the pragmatic utility that it and the instincts are. It is hard to remove an assumption that is so integral with one's everyday life. The assumption is put there very long ago by evolution because anything making such an assumption is more fit than something that doesn't. So to embrace eternalism, one has to set aside that intuition that protests at every step.

Several people were working on relativity theory, some getting a good head start on Einstein. But Einstein had the ability to ignore intuition when the intuition contradicted his findings. Others (notably Lorentz, Poincare) had a harder time with the implications of frame invariance and frame independent of light speed.

What was wrong with my depiction that "while the event is happening, it is cycling through successive ontological states with each successive state being the "preferred" moment," where "the "preferred" moment is the state that is happening, as opposed to those that have happened or have not yet happened."

Besides the explicit reference to a preferred moment?

Does eternalism allow only for the different ontological states of 'not yet happened' and 'happened'

There are no such ontological differences. There is no division between such ontological differences.

[/quote]To be clear, I was using the word "exist" in the latter, "ontological sense" given here:[/quote]There are a lot of ways to use that word, and interpreting it one way doesn't mean that all references to the word mean that interpretations.

It might be objected that there is something odd about attributing to a non-presentist the claim that Socrates exists now, since there is a sense in which that claim is clearly false. In order to forestall this objection, let us distinguish between two senses of “x exists now”. In one sense, which we can call the temporal location sense, this expression is synonymous with “x is present”. The non-presentist will admit that, in the temporal location sense of “x exists now”, it is true that no non-present objects exist now. But in the other sense of “x exists now”, which we can call the ontological sense, to say that “x exists now” is just to say that x is now in the domain of our most unrestricted quantifiers. Using the ontological sense of “exists”, we can talk about something existing in a perfectly general sense, without presupposing anything about its temporal location. When we attribute to non-presentists the claim that non-present objects like Socrates exist right now, we commit non-presentists only to the claim that these non-present objects exist now in the ontological sense (the one involving the most unrestricted quantifiers). — SEP article on time

Wow, what a mix of multiple meanings and preferred moment references in a paragraph trying to clarify a view that denies the referent. I can see how the view might be difficult to learn from that source. Apparently there are using 'is present' to mean 'currently exists', which suggests that eternalism asserts that Socrates exists in 2024, which, itself can be interpreted as either 'Some of the events of the worldline of Socrates have a time coordinate of 2024', or as "All events exist, and a reference time of 2024 doesn't change that'. Only the latter statement is true under eternalism, and the paragraph above seems not to clarify which meaning is meant.

I was saying eternalists hold that there exist events at each (and every) time, i.e. that there also exist events that are not present.

By 'not present', I am guessing that you mean 'not at the present moment' (as opposed to 'absent', which of course is not an eternalist statement.
Also not sure about the first part, that there exist events at each (and every) time. For instance, do there exist events before the big bang? I think not. Do all events have a time coordinate? I can't think of a single coordinate system that assigns coordinate values to every event that is part of spacetime, so even that isn't true.
BTW, by 'exists', I usually mean 'is a member of' relation. So an event existing means it is a part of the implied spacetime, the thing of which it is a member.

Nothing is happening in an eternalist universe? The sinking of the Titanic happened but was never happening?

The statements as worded are both meaningless under eternalism, so instead of being true or false, both are more 'not even wrong'.

Through this process of accelerating, is there any difference in the length of the train *in the reference frame of the train itself* — flannel jesus

Train has no wheels since relativistic wheels are a whole new problem. So it's sort of a mag-lev situation where the track is there but doesn't touch.
There are three trains actually, called F, R, and U.
Train F is like a normal train with the engine in front, pulling all the stuff behind it.
Train R is like a rocket, with the thrust coming from the rear
Train U has every car in the train self-powered, thus achieving uniform acceleration.

F will stretch under tension. The front begins to accelerate at say 1g but the rear stays put for say a minute until the acceleration wave travels at the speed of sound to the rear when it finally begins to accelerate, and at more than 1g at first. The sound waves bounce back and forth and an accelerometer at the rear wavers up and down until dampers put a stop to that nonsense. While the bouncing is going on, the different parts of the train are not stationary in the frame of the engine, and the train has no proper length. Once the waves stop (a few hours maybe?), the train will have a longer proper length under tension than it did parked at the station unstressed. The accelerometer at the rear will read a slightly higher value that 1g. There's another set of bouncing waves when the acceleration ceases and the rest of the train takes time to notice. The people in the rear get kind of seasick at the start and end of the acceleration.

R is similar. We accelerate the rear at the slightly more than 1g level. The train compresses, but eventually stops bouncing with a shortened (under strain) proper length. The train is accelerating at 1g as measured at the front.

U is powered everywhere, and independently. Each thruster knows when to start and when to cut off. Hence there is no stress, and thus no strain ever on this train. It exhibits rigid motion unlike the other two trains. We don't even need couplers between the cars. The proper length of U never wavers and is constant always. The front accelerates at 1g and the rear a bit more.

At 1 light second and 1g, the difference in acceleration between front and rear isn't much. If the train was a light year long, the difference grows to the point of impossibility.

What I said is that I fail to see how it ends the timeline. — Luke

We set the universe to a state where time is truncated by 30 seconds, in 30 seconds. The same state (sort of) is set each time, so a way out of the loop needs to be identified. For that, I need to know more rules than those you've given me.

As far as I can tell, nothing forces her to keep hitting the button.

She's in a state where she's going to hit the button in 30 seconds. She's enough in that state that she does it. The question is, what's different about the nth time around that she doesn't, given the same initial state? There's not time for chaos theory to do its thing. Events 30 seconds from now are essentially determined, except for this machine appearing not quite in the sight of Alice who's going to hit the button in 30 seconds.

She simply doesn't press the time travel button again.

What changes, that she makes a different decision than the one we know she makes, for reasons specified?

I never said that future events must actually happen before there can be time travel to a future time.

OK, I presume they must. If they've not happened, wouldn't Bob appear in a blank universe, at a time where nothing had yet been written? The machine moves the present to a universe state that is nonexistent, leaving a universe with only Bob and his machine in it. It would make sense (and match all the fictions) if the machine waited for the writing of the target destination before appearing there.

I never said that the future timeline is "a blank universe not yet written.
" I referred to the future timeline that would have existed if there had been no time travel events.

It takes 11 years to write that future state (assuming an 11 year jump. It also clones Bob. Sure, from the traveler's viewpoint (the only one you ever consider), it looks like he just appears there, in 2035 with F-Bob sitting there much in the same way that none of the fictions depict.

Why would it leave a copy of Bob behind?

You said that it goes to a "future timeline that would have existed if there had been no time travel events". If there had been no travel events, Bob would still be in the timeline instead of the machine, so aging F-Bob (the one that is not in the machine) is a copy of the not-aging S-Bob in the machine.
So if you have a machine that holds a thousand passengers, and set it to go 1 second into the future, you now have 1000 cloned people. Hence the soldier factory.
Of course you could always have done the same trick with travel 1 second to the past, with precautions, so the clone-making property of the machine was always there.

The machine disappears. You did not explain why it shouldn't.

It disappearing would not be consistent with a timeline where 'there had been no time travel event'.

It could be argued that, while the event is happening, it is cycling through successive ontological states with each successive state being the "preferred" moment.

No, not under eternalism. There is no preferred moment in it. You know that, yet you persist with comments like that.

As you note, this sounds a lot like presentism.

Because the comment IS presentist.

You are apparently saying that in order for an event to happen, it must be present.

Your words, not mine. I would never have used the word 'present' (as in not-absent) in that way, in that context.

I was trying to say that, if an event exists then it is happening.

True (and meaningful only) under presentism.

Since eternalists hold that events exist at all times

No, they don't say that. Each event exists at a specific time, and not at the others. The comment is analogous to saying Paris and London exist in all places, and not distinct ones.

Really, learn the view before you start asserting what it must say. It hurts, the way you're murdering a view with which you obviously don't hold. It's not absurd at all when one accepts only its premises and not premises borrowed from an incompatible view.

The "preferred" moment is the state that is happening

My eternalism titanic example comments never say anything 'is happening'. That is a reference to a present that the view denies.

In Newtonian physics... I don't know how to do that version of transformation between S and S', but it would seem to me that once you as an observer account for the time it took the light to reach you for each of your observations, wouldn't your results look exactly like they would look from the perspective of a stationary observer? — flannel jesus

The part I said was true under Newtonian physics was the bit about which runner was first observed to finish, which is a function of where the observer is at the time of that observation and not at all a function of how fast or what direction the observer is moving. There's a set of events that the light from both runners finishing reaches simultaneously. That set of events forms a 3D hyperplane in spacetime. If the observation of the observer is made on one side of that plane, the one runner is first observed finishing, else the other runner is first observed finishing.

The part asked at the end (which runner actually finishes first in S') is not the same in Newtonian physics, the latter which says that if the race is a tie, it's a tie in all frames since time is absolute. Frame rotations don't involve changing any time coordinate in Newtonian physics.

And if someone wasn't moving in S, wouldn't they just see the same thing as the original description?

The original description doesn't say where the observer is ("in the stands" but where in S are 'the stands'?). Anyone in S would compute that the race was a tie, but the nearer runner would first be observed finishing. For instance, each guy with the stopwatch would see the other runner finish 2 seconds later.

Things not specified:
The starting point is presumed relatively stationary to both finish lines, in a frame we'll call S.

OK, the guys with the stop watches at the finish lines measure the time from the light signal to the runners getting there.

Both stop watches are stopped at the exact same time. — flannel jesus

Both are stopped at the same time in S, but not simultaneous relative to some other frame.

If there's an observer in some relativistic frame of reference, travelling at some significant fraction of the speed of light to the left or the right (you choose, it doesn't matter), how do they perceive the race? — flannel jesus

They'd observe the closer runner finishing first. They'd also see that the stoped watches read identical values. All this is true even under Newtonian physics, and the observer in question doesn't have to be moving in S to observe any of this.
The moving observer would probably not consider the race to be a tie in that moving frame S' because the stop watches were not started simultaneously in that frame. They'd consider the slower (in S') runner to have reached his finish line first (a bit unintuitive, I know)

Do they think the runners ran the same speed? — flannel jesus

Not relative to S', of course not.

both runners are running at 25% of the speed of light relative to the people in the audience, they reach their finish lines after 4 seconds of running (so they ran 1 full light-second), and the observer watching the race in a relativistic frame of reference is travelling to the right at 50% the speed of light. — flannel jesus

The guy running to the right (slow, at about -.286c relative to S') wins the race. The guy running directly left moves at 2/3 c relative to S' and gets to his destination some time after (in S') the first guy does.

I fail to see how your examples of multiple time travel events end the timeline. — Luke

I did it with one time travel event, a scenario you seem to refuse to comment on directly except to say you apparently don't get it.
We've not considered multiple travel events hardly at all, so I'm not sure if the consistency of a particular interpretation will ever be explored.

I only see that they end the existence of humanity, which is not the same. With truncation or overwrite, time still marches on.

March it does, but in the example I gave, it just paces back and forth. That needs to be resolved I think before we consider multiple machines.

Where on your graph does it show that the timeline is overwritten from 1990 onwards and that the pre-time travel 1990-2024 period ceases to exist?

One can shade all the regions below the line. Those are events that exist (history that is written) at a given time on the x axis. One cannot ask what the state of 1990 is (a time on the y axis) because it has multiple states, being written more than once.

I never said that future events must actually happen before there can be time travel to a future time.

Neither did I. Bob is traveling to it, but it must happen first before he can arrive, else he ends up in a blank universe not yet written. It would presumably be subjectively instant to Bob, just like it is backwards.

I don't really want that; it's just how forward time travel makes sense to me.

It makes sense to leave a copy of Bob behind? No time-travel fiction portrays it that way. Doesn't make it wrong, but it makes it into a cloning machine. The army would love it. Millions of somewhat disposable trained soldiers at the push of some buttons.

We can imagine a timeline of future events; of how things could/would have been if there were no time travel events. This imaginary future timeline is what gets overwritten by a time travel event. This is similar to backwards time travel, except that backwards time travel has an actual history of events (without time travel) that gets overwritten by a time travel event. It's not really any different because in either case the time travel event overwrites what the timeline was/would have been.

The typical depiction is that the machine disappears, which results in the writing of history as if the travel had actually happened. If it doesn't happen, the car/machine doesn't disappear.

You said the result would be a bunch of cars all arriving in the same location causing a black hole.

That's sort of one outcome depending on the answers to questions I've asked: What happens when multiple travel events target the exact same space and time? In my example, they're all the same travel event, but happening repeatedly in a different sort of loop that causes collisions. There can be an odometer this time, but outside the machine, not inside.

This does not explain how the timeline ends. Otherwise, I do not understand how the timeline is supposed to end in your Alice example.

Then comment on the example. Where does my description of it go wrong? All I have is 'I don't get it'. I need to know what part you don't get.
I spelled it out in considerable detail a couple posts ago. No comments on that.

That's not very helpful. I'm asking you what it means.

Not sure how to word it differently. The Titanic sinks on some 1912 night. That is a statement of something that happens. Relative to the night before, it has not yet sunk, and the night after, it is at the bottom of the ocean. Somewhere between those two events does the sinking 'happen'. It's not much different than presentism except there is no preferred moment that has to somehow glide across that event in order for the event to cycle through the different ontological states of 'happening' and then 'happened'. All events have equal ontology. Besides that, there is very little difference with the standard definition of 'happens'.

Do the terms "happen" and "occur" mean anything other than that the event exists?
— Luke

I think not.
— noAxioms

That was a bad answer. I think the two words mean essentially the same thing as each other, but you didn't ask that. You asked if the words mean 'exist'. No, the words do not mean 'exist'. The laws of physics might be said to exist, but they're not something that 'happen' or 'occur'. There's not a place at which the laws of physics specifically occur.
I also gave the example of the table length as something that exists, but doesn't 'happen'.

I find it odd that you refer to an event as occurring at a single point in time.

No, at a point in spacetime. Time is 1 dimensinal, but spacetime is 4D. An event is a point in 4D spacetime, just like a location is a point in 3D space. The latter, plus a moment in time, are all frame dependent things. Events are invariant: They're not dependent on a frame choice.

I suppose the word could be used in this way, but I typically think of events as having a duration; lasting for a period of time.

That's the colloquial definition. I'm talking about the physics definition. Yes, an event can be bigger than a point. The sinking of the Titanic took place over a kilometers and a few hours, but from a distance, that's a point, just like Earth is treated as a massless spatial point in something like the twins paradox.

You've just told me that the terms "happen" and "occur" do not mean anything other than that the event exists.

I did. I misread the question.

Also, since eternalists treat time as a spatial dimension

Dimension yes, but it is a temporal one. One can still translate seconds to meters if you want. The units are interchangeable under the constant c.

why wouldn't they say that the length of your table happens, just like the length of an event (i.e. a process) happens? What's the difference?

I suppose you can say the table 'happens'. Mine is of size 40 years and its current length started 'happening' perhaps 34 years ago, and counting, all depending on how one chooses to measure its length of course. But when I speak of an event, I'm usually talking about something that is best treated as a point.

I never said an event "has multiple temporal locations".

You said events happen repeatedly.

I said "all events are happening en masse at their respective times."

OK. I'm unclear on the distinction between all the events happening at their respective times, and them all happening en masse at their respective times. The latter wording would seem to be opposed to some of the events happening at their respective times, but other not.

Ah okay, I see now that I've been using the term "event" to refer to what you call a "process". I will adopt your terminology henceforth. I note that a process requires progress.

OK. A fire begins to happen, and goes out at a later time, both ends being different events, with the fire being the process between. And yes, if you use 'event' to describe something with duration, like a concert, then it obviously begins to happen and later ceases to happen.

Right. I'm suggesting that, in order to say that all events exist/happen under eternalism, then all times must be, in a sense, present.

Horrible word choice, but I suppose so. That is not to say that they all exist at a present time, but 'present' in the sense of 'present and accounted for'.

All events exist and happen at each of their respective times. Since they all exist and happen at all times,

They don't happen at all times. Each event has a time coordinate and only happens at that time.

I don't follow why they should happen in a sequence from earlier to later, so that they apparently happen one after another.

That's just causality doing its thing. Classically, a later state is a function of prior states,. That works in both directions, but there is the arrow of time which indicates which way is forward.
So I can throw you a pile of pictures of the Titanic, and you could very likely put then in order, despite none of the pictures being the cause of any other.

This suggests that there is a "special event which is 'current', which moves along [the] worldline,"

Nothing of the sort is suggested. That is an additional premise, for which zero evidence exists. There's no empirical test for it (or, similar to the teapot orbiting past Jupiter, for its absence). Both sides have proposed all sorts of attempts at arguments for their side, but most arguments don't revolve around anything empirical.

The integers are ordered, but there is similarly no obvious integer which is the preferred one, despite each integer perhaps thinking it is the preferred one. The integers are ordered, but do not constitute a progression.

Instead, eternalism entails that events all happen en masse at their respective times, rather than in a sequence, one after another. But in that case, each event must happen repeatedly, without beginning or end.

Again, you drag repetition into a view that implies no such thing.

[Black holes] was part of your Alice example

Oh right... It was one of the solutions to the problem of the universe being unable to progress. Time travel (without the wormhole) violates mass conservation, but we're ignoring physics violations, so there is no limit to how many machines we can put in one place. Too much mass results not so much a black hole, but rather enough gravity to kill Alice and put a stop to what she's doing. The whole point of the train track exercise is to figure out how to get Alice out of the loop.

I don't see that there's much difference between 'overwrite' and 'truncation'.
...
It makes little difference post-time travel. — Luke

It makes no difference in the single-travel-event scenario, and 60 posts into this, multiple events remain out of consideration.

The time machine's departure from 2024 did happen before its arrival in 1990. Otherwise, you are simply prohibiting the possibility of time travel by stipulating that all events - and all use of tensed language - must obey date order.

As I said, you seem to order events per the x axis, and I tend to order events along the y axis. I presume you saw my picture. You will note the absence of numbers along the x axis since it was unclear what to put there. One could put Bob's age there, but that would only work if Bob takes part in every time jump ever.

If you accept that history gets overwritten, then I think there would be only one axis/timeline.

No. You need the 2nd line to order all the different times that a given year appears. My graph shows that, and all you posts reference this x-axis kind of time. Machine gets created. After that, machine gets used. After that, creation of machine gets overwritten. All nice and causal.

Forwards time travel is just like backwards time travel.

It can't be. There is no future, since it needs writing first. The machine would, at minimum, be forced to wait for the destination to come around, holding its occupant in stasis all the while similar to cryonics but without the cold.

The timeline that would have existed if there had been no time travel events gets overwritten (from the time machine's arrival date onwards) by a new timeline which includes a time travel event and its effects.

Why would you want that? There seems to be no point.
If the line continues as if no travel event occurred, the Roberta (age 14) watches Bob push the button for destination 2035. You say the timeline continues as if the travel event had not occurred, so Roberta keeps her dad, who appears to be a failure. We call him F-Bob. Meanwhile Bob is actually successful, and is traveling to 2035. We call him S-Bob (success). 2035 goes by for some reason and S-Bob doesn't show up. A long time goes by (you don't say how far it goes) and suddenly S-Bob appears in 2035, truncating history back to that point, and F-Bob's 50th birthday party, who says "Who the f*** is that? I thought it didn't work!". 39-year old S-Bob replies "It sure as s*** worked!". Yes, F-Bob drops the F-bomb and S-Bob drops the S-bomb. Sorry, couldn't resist that one.

Anyway, the usual description says that 'history' proceeds as if Bob had actually traveled, and Bob is not in 2025 at all, and Roberta doesn't see him again until 2035 when he shows up out of thin air. History is not in need of truncation at all since it just then got to that point.

All that said, if you agree to the latter, we can demonstrate issues that result. The way you word it makes it into a cloning machine. You can make an army of soldiers in minutes using such a machine, just by setting it for one second from now.


Are we going to discuss the contradictions that might arise by having Bob (or others) make more than just the one jump? It all works great and intuitive for a single jump, but the differences in the interpretations really comes out when everybody has one.
I also notice that you've dropped the discussion of Alice at the tracks, ending the universe. That was one consequence of the truncate interpretation: a universe that cannot progress.

I know that things don't remain or go in or out of existence under eternalism. That's why I said that an Eternalist would prefer for the overwritten section to remain in existence

No, there can be no overwriting or anything. There is no writing at all. There is but the one timeline (or more if you want), but they don't change. Change is something applicable to something contained by time.

I'm aware that the words "happen" and "occur" are usually synonymous, but it's unclear what it means for an event to "happen" or to "occur" on an Eternalist timeline

An event 'happens' at the location of the event. Not sure how else to say it. The time coordinate assigned to the event might be frame dependent, but the event itself is objective.

Do the terms "happen" and "occur" mean anything other than that the event exists?

I think not. I mean, by calling it an event, an implication is made that the event exists at a point in spacetime, and all points in spacetime have a location on the time dimension, just like they have a location in the spatial dimensions.

If Eternalists take "exist" to be synonymous with "happen"

No, that's not true. The length of my table might exist, but it's not something that 'happens'. It was the word 'event' that carries an implication of being part of spacetime, and that, coupled with a premise that spacetime exists, implies that an event exists.

then, since Eternalist events do not cease to exist, they must also not cease to happen.

Not sure what 'cease to happen' means, but events, by definition, 'happen' somewhere. They would perhaps be said to exist in the spacetime of which they are part.

There is no past tense of events having existed or having happened for the Eternalist.

Agree

This implies that, instead of the usual sequential progression of events wherein later events occur after earlier events, on an Eternalist timeline all events are happening en masse at their respective times and each event happens repeatedly.

Ouch. No! There is no repeat. They happen once. An event cannot have multiple temporal locations. An except to this is the usage of a coordinate system that does not exhibit a 1-1 correspondence of events to coordinates. Under such coordinate systems (such as a variable acceleration one), events can have multiple valid sets of coordinate values, and thus 'happen' more than once, and in more than one location. One of the best illustrations of this is the Andromeda paradox, which leverages such a coordinate system.

Events can be ordered. One can say that event A happens before event B. If the two events are not spacelike separated, then that ordering is objective. If the events are spacelike separated, then their ordering is frame dependent,. per relativity of simultaneity.

Point is, there is still a sequence for the sort of events you're imagining: Titanic sinks before WWII.
What eternalism lacks is the premise of a 'present' moment, objectively separating all events to three ontological states of 'past, present, and future'.. Any reference to the thing not posited is meaningless under eternalism. Hence the lack of tensed verbs, since such verbs carry a reference to the thing not posited.

therefore [events]also do not begin to happen

I am not sure how you distinguish the terms 'happen' from 'begin to happen', but events do happen. A process that has duration (a house fire say) is something that begins to happen, but an event, being a point in spacetime, has no duration.

Like all events on the Eternalist timeline, the sinking of the Titanic in 1912 and humanity's extinction event in 2316 are both always happening

No. 'Is happening' is a reference to the present. Please don't make up your own ideas for eternalism. There is no repeat to it.

Travel is something which happens or occurs, and the word "travel" usually means there is something which progresses (in this case, Ted) from one place to another. Yet, you say "there is nothing which 'progresses' at all."

Different usage of the same word. Yes, Ted's life is a progression from his early times (conception) to his death. All those events exist. They all happen. They are ordered, so in that sense, there is a progression. There is no special event which is 'current', which moves along his worldline. In that sense of the word, there is no progression.

Black holes exist in our universe and haven't ended our timeline.

Not sure what black holes have to do with our timelines. I don't anticipate either of our lines being in a black hole.
And no, there are unfalsified theories that don't allow black holes, so their existence is not fact, but the consensus is that the one theory that predicts them has been dang successful, so their existence is presumed.

Roberta won't have any experience if she is overwritten, because she will cease to exist. — Luke

noAxioms]Apparently nobody can witness the departure event of the time machine, at least not if it goes backwards —

Someone could see it disappear, I suppose — Luke

This seems contradictory.
Robert and Sue are watching Bob get into the time machine. He reaches for the button and Roberta ceases to exist and Sue is currently a child with zero memory of 2024. That makes the departure pretty much impossible to witness.

I said that the timeline gets overwritten, but you've somehow interpreted that (to be the opposite of what I said) as "no overwrite, but just truncation".

Overwrite means the time between 1990 and 2024 still existrs, but gets changed as time makes its way across that period. Truncation means it is gone, and the new write is added to the end of existing history, which is at 1990. The two are the same after 2024 is reached again, or until there is another travel event.

You are correct that the pre-time travel period of 1990-2024 "no longer will happen", but only because it already did happen.

You have a funny definition of 'did happen'. Those are future events, and if it's 1990, they're not in the past and thus the use of past tense is misleading.

This is what I mean by you referencing two dimensions of time. One is the time I'm talking about, where 1990 comes before 2024, and the other is the time containing the first kind of time. So you can lay out a graph with two time axes, and graph where the present is (y axis, calendar time) for a given 2nd kind of time (x axis, Luke time), which would show a steady line up to 2024 where it jumps to 1990 and continues upward again. Two dimensions of time, and it being a simple exercise to plot out all the jumps this way.

https://postimg.cc/jDtnftnt
I tried linking an image I drew, but the site apparently doesn't support images. Click the link.

Along the x axis, the present is at 1990 more than once, and the 2nd 1990 happens after the first 2024, but all of it 'happens' at some point. That corresponds more to no time travel at all, and history isn't deleted at all, but rather the state of the universe is simply reset to a prior state the exception of the contents of the machine which are protected from the overwrite everywhere else. If it does that, then yes, the pre-time travel period of 1990-2024 did very much happen since that 2024 is in the past of the second 1990. That is sort of an 'append' view where nothing gets deleted. Roberta still exists (the thick maroon part), but her worldline ends abruptly when the universe is rewritten to a state that doesn't include her. The time travel machine thus would have access to any of those states (such as pre-travel 1996) and could reset the universe to that state if those coordinates are chosen. They still exist, so you could 'go there'.

Interesting side effect. You're at a time travel convention, and 20 of you with similar machines all decide to eat at Joe's in 1936 where the food is cheap. Only one machine (the first to leave) makes it, the rest are erased from history before they can follow.

I think, but am not sure, than when you get in you machine and set the coordinates for some destination, that you select a value on the y axis and not on the x axis, but it isn't really clear. One cannot fully understand your view unless forward travel is described. Sticking to this one-backward-jump case leaves several open questions.

My argument is that time travel and the act of time travelling to kill one's own grandfather (or their younger self) is hypothetically possible and logically consistent.

I grant that. It has universe-ending consequences, but the grandfather thing isn't itself paradoxical in this view. Presentism does buy you that. The paradox has more teeth when you take presentism away.

Although I understand why an Eternalist would prefer for that section of the timeline to remain in existence

Things don't 'remain' or 'go in or out of' existence under eternalism. You seem to not understand the view.

That isn't truncating the timeline; it's truncating human existence. Time continues to "move forward" with or without us.

The train example may or may not permanently end time for the entire universe, depending on answers to questions concerning how subsequent jumps are handled.

Doesn't this imply that nothing ever happens in an Eternalist universe?

No. The Titanic sinks in 1912. Humanity goes extinct in 2316. Those are eternalist statements since they contain no references to the present. Events still occur at specifiable times, which is what 'happens' means.

Therefore, there is no such thing as travel?

Ted is home at 7AM, Ted is at school at noon. Ted must travel to be at different places at different times.

Time travel under eternalism would be illustrated by a picture showing the state of things at each time. There would hopefully be but the one dimension, so 1990 is before 2024 unconditionally. There is no 'first 1990 and second 1990'. That opens the door to the paradoxes, but it also allows a time machine to exist uncreated. Your view I think doesn't support that.

...so either there is an explosion and the Alices all die (yay for the universe), or one wins and truncates the other out of existence, and the cycle continues.
— noAxioms

It sounds like this truncates Alice's existence, but I don't see how it permanently truncates the timeline.
Whether it permanetly truncates the history of the universe depends on what Alice does as she approaches the tracks. If there is a massive wreck of DeLoreans at the crossing, she might be reluctant to hit the end-universe button, and will simply miss her appointment. That's the way out of the pickle. She (the Alice who has never time traveled yet) needs to make a different decision based on what the future Alices have chosen to do. I've given her only 30 seconds to realize that, but I think it's enough.

The question is unresolved until you clarify how subsequent time travels work,. In particular, what happens to the object at the location where the machine 'appears'? Does it murder the person there? Does it look for a relatively harmles place to appear? What if a million machines all try to go to the same spot? Eventually space will run out for them all, and Earth collapses into a black hole from too many DeLoreans.

Did you mention the solution already or are you keeping it to yourself?

Work through the Alice example. I didn't keep it to myself.

The time traveller originally passes through 1990 without any time travel events (as a child, say). They subsequently grow up and build a time machine. Subsequent to this, in 2024, they travel back to 1990 (as an adult time traveller). There is no time traveller (who has time travelled) in 1990 until after the 2024 time travel event. 1990 is only "overwritten" (post-time travel) in the sense that it now contains a time traveller, whereas it did not contain one before the 2024 time travel event. It is also "overwritten" in the sense of whatever effects the time traveller has on the timeline from 1990 onwards post-time travel that they did not have on the timeline from 1990 to 2024 pre-time travel. — Luke

This is just a repeat of what was said before, without answering any of the questions. It's always described only from the PoV of Bob.

Bob is born 1985, meets Sue in 2002, married in 2007, and has a daughter Roberta, born in 2010. Bob kills young-Bob in 1990, so what is the experience of Roberta when she gets overwritten? What is the experience of Sue when she still exists, but has her marriage and all her history overwritten?
Apparently nobody can witness the departure event of the time machine, at least not if it goes backwards. You've given no clue how it can go forward to some piece of history that has yet to be written.

I have no issue with the word 'truncated'.

You balked at that before. So no overwrite, but just truncation, and a new building onto 1990, not overwriting some alternate future that no longer will happen. Robert is immediately gone, and never was, and never will be, in the world timeline which is presently at 1990. The time machine now exists without having been created since its creation has been truncated off. It doesn't exist and never will. You seem to not like that, but that part doesn't bother me. Sure, its creation exists on Bob's line, but most of Bob's line is not part of the universe, but just a memory.

You claimed that the timeline could be permanently truncated. I still don't follow how or why that could be.

The train track scenario illustrated that, but it depends on your answers. The truncation interpretation does result in that, yes. Time cannot move forward. The machine has God-like powers and can actually take control of where the present is and put it somewhere else. Any alien with this technology can effortlessly wipe out human existence simply by truncating us off of history.

This is why I don't like the truncating interpretation. Too much power for a simple machine. The rewind/overwrite interpretation has the same problem. Not all interpretations do.

I'm not suggesting that 1990 and 2024 are both progressing simultaneously. After all, I'm not an Eternalist.

Eternalism suggests no such thing. There is nothing that 'progresses' at all.

I'm asking that we get clear about a single time travel event first.

OK, the train track thing is a single event (sort of), and I don't see how the universe can ever get to tomorrow with it.

At time 12:00:25, the train gates go down. At 12:00:30, Alice gets to the crossing, who's in a hurry and she's driving the DeLorean. She hits the button to go back 30 seconds.
At noon, a DeLorean appears at the tracks and proceeds across. 400 meters back, a DeLorean approaches the crossing.
At time 12:00:25, the train gates go down. At 12:00:30, Alice gets to the crossing. She hits the button to go back 30 seconds. The Alice on the other side of the crossing is truncated out of existence.
At noon a DeLorean appears at the tracks, almost exactly in the same place as the other one that appears there. OK, so there is some sort of resolution of a car appearing at the location of a car already there, so either there is an explosion and the Alices all die (yay for the universe), or one wins and truncates the other out of existence, and the cycle continues.

I have to admit that there is a solution to the problem that I didn't see before.

You used the word "rewind". I followed your usage to point out that time travel does involve a sort of rewinding of time. — Luke

The word you used was 'overwrite'. I've been trying to explore the implications of various models, but perhaps I have it wrong. To quote:

On reflection, I want to reject my suggestion that there is more than one timeline. You've helped me to see that this is not really what I had in mind. What I have in mind is that there is only a single timeline but that the effects of the first time travel event overwrite the past of the original timeline (starting from the destination time of the time travel event, e.g. 1990). This might create a causal loop or it might not. However, the main idea I've been trying to convey all along is that there must be an original version of "the past" prior to the first ever time travel event, which gets overwritten and is necessarily different to the version of "the past" that exists post-time travel. This helps to retain "normal" causality, thus removing the need for the magical appearance of time machines or technology "out of nowhere", existing uncaused (as in a causal loop) and thus removes the impossibility of killing one's own grandfather (as in the grandfather paradox). It also removes the unpopular idea that time travel creates a "copy" of the original timeline. And it retains free will. — Luke

So you seem to envision two dimensions of time. One is Earth coordinate time, as measured with numbers like 1990, moving horizontal to larger numbers, and the other is perpendicular, and moves 'up' with each travel event.
You seem to assert a single physical space that is 'overwritten', which is a lot like a VCR tape, except there are perhaps no spools to rewind since you seem to balk at that word.
So there is the tape which holds the entire history of the universe up to a 'present' where the write-head is writing. It writes up to 2024 say and then Bob goes back to 1990. A write head goes back to 1990 (without erasing, which would be truncation, another word you don't like) and starts overwriting there. It is unclear if this is a second write head (leaving the 2024 one to continue writing a universe without Bob, or if the history stops there and waits for the write head 34 years prior to catch up.
With this model, Bob goes back, and the history of the creation of the time machine in 2023 still exists, but the writing is going on in the 90's and when it finally gets to 2023, it overwrites the creation of the time machine, leaving a time machine without a creation event in any of history.
It of course exists in the timeline left behind (in the 'down' direction of the 2nd kind of time) described by the part you bolded above, but that line isn't the one actual timeline, it has been overwritten.

Did I get anything right this time, or is the model completely different than that?
There are implications, but if I got the model wrong, there's no point in discussing them.

Not if we only discuss one time travel event, like I keep asking.

So you want to limit the discussion by imposing a single travel event restriction. This would prevent us from exploring the plausibility of the model. Apparently avoiding that exploration is something you want.;

Let's discuss Alice and the train tracks then. That's one 30-second travel event, sort of. I don't know how to analyze that since I don't know if I got your model right.

If we can stick to only one time travel event, then the model works like this

A description that works only in one case isn't a model.

1985 progresses without the appearance of any time traveller, until 2024 when someone first time travels and they arrive back in 1985. Everything about 1985 (the second time around) is almost the same as it was without the time traveller (the first time around), except that now it has a time traveller in it. In this way, it is very much like Back to the Future.

You seem only to describe the traveler, not what it's like to be left behind, to be 'overwritten'. Back to the Future (BttF) never shows what it's like for his loser parents to be overwritten by the confident parents. These are the parts missing from your model.

It is probable that most of the changes will be localised around the time traveller's location.

For a brief time, maybe. BttF seems to adopt an unrealistic fatalistic approach without chaos theory. It's entertainment and isn't supposed to be consistent with physics.

I don't believe that it would be very chaotic, or that many of the people born would be different ones

You can hold this belief all you want, but the mathematics says otherwise. Things turning out the same way assumes a very hard variant of determinism, even without the appearance of something that can't be there.
Why is it important to hold this belief? I don't see the problem with history unfolding a completely different way after a while.

On that note, do you agree that the time travel event does not occur until 2024, given that the time traveller departs from 2024 to arrive in 1985?

On the vertical time axis, yes, as described above. But that sort of runs into problems when there is more than one travel event, an avenue you seem reluctant to face.

Besides, I thought your example was supposed to end the timeline somehow, but I still don't follow how it does.

I don't know your model clearly. I can't discuss this.

How does the butterfly effect of the time travel event necessarily prevent the evolution of humanity?

Evolution of specifically humans was less likely that a 1 in a gazillion chance. Countless uncaused random events needed to happen just so. So the odds of rolling the same gazillion sided die and getting the same number is effectively nil.

Which species come about is very much a function of random mutations and environmental chance.

Yep, and we're changing the environment, and also letting all the random events have a 2nd try, and they'd all have to come out the same..

If time is some physical entity running itself somewhere in the universe — Corvus

Not really clear what might be meant by that...

and if there were different timelines running in different physical spaces

That sounds like a multiverse of sorts, levels I-III if that means anything to you. But the whole point of them being a multiverse is that the states in the various physical spaces don't interact. If they do, it's one universe, not multiple.

maybe then you could say your  mind and body of 2024 can travel to whatever year you choose without losing the memory, thoughts or consciousness.

OK, you seem to separate mind from the physical state, so it's on you to figure out how the two might keep track of each other.

But if time is just a mental concept for measuring the intervals between the start and end of changes of the objects in the physical world, then the whole topic would be just a fiction.[/quote]This sounds like the idealism hand-wave. We interact with anything (an object say) via mental concepts. There is no other interface. If you want to draw the line there and say that the physical state corresponding to that ideal supervenes on the ideal, then the story stops there. And BTW, 'object' is very much just an ideal. There seem to be anything physical about what constitutes an object.
I'm getting pretty far off topic here.

Under the law that even God cannot intervene, your mind will be that of the people who lived in the world of whatever year you travel to, and you body as well.  Perhaps your body will need a few deaths, resurrections and new births to reach the time you are supposed to travel to if it is a few hundred years away from the present moment.

We seem to have gone off on a supernatural tangent. Not my problem.

I assume you are not talking about "real functions" as compared with "complex functions", but what we find in nature. — jgill

Um, yes. I'm talking about the complicated functions of reality as opposed to the simple functions often used to demonstrate chaotic behavior in textbooks.

How did wave functions sneak in?

Quantum randomness is a critical part of especially mutations. Given a different starting state (or even the same starting 'state' but without hard determinism), a completely different outcome will collapse out of the wave function of all possible futures of that initial state.

But there are loads of the other aspects that you must think of such as the mental contents = memories, thoughts and the consciousness of the past, such as if you travelled to 1761, would you still contain the present mind, or would the content of your mind be wiped out, and replaced by the 1761 mind, or would it become total blank due to the travel? — Corvus

Why would any of that occur? I mean, sure, if one was to travel to 1990, they'd find me there, but without 2024 memories, but why would the teleporter leave you in a different state when it by definition doesn't?

Given a physical monist philosophy of mind, one would presume the person to arrive with all memories, experiencing nowt but a sort of change of environment, very much like getting on and off an elevator.

Given a supernatural philosophy of mind, I suppose one has to address whether that part goes with you or not, and what happens to it if not, and if something replaces it if not. All that is similarly discussed in scenarios like the Star Trek transporter.

Given a closed time loop, there's absolutely no reason to worry about it since there is no 'moment of actual travel', no reason for one's consciousness to not follow along like it always has. Not my problem anyway, it's the problem of the dualist.

Take time dilation, which SEP says is time travel: Suppose I get in a fast ship that dilates me to 1000th of the usual rate. Physics says I would not notice, but the dualists with a model of a mind experiencing the objective flow of time, the experience would be that it would take an hour to draw a breath, something you'd likely not remember to do for an entire hour. The boredom and inability to function would kill you. The falsification test is safely behind a wall of technological capability. Nothing we have can test a human accelerated enough to empirically tell the difference, and the machines that have don't count since humans are special in this regard.

Not saying time travel is total baloney, but I am interested in how it might be possible, as well as what you could do in the past or future when you arrived there. — Corvus

Well, the usual physical explanations disallow the concept of 'change the past'. That means much of our discussion is moot. The machine (presuming unrealistically that the requirement is a vehicle of sorts) comes first, then the development of it. More realistic is the idea that the connection is established at both ends and there's no surprise when something appears uncaused 'from nowhere' so to speak.

but you still need to explain why and how exotic matter is required for time travel. How does it supposed to work? — Corvus

Ask those who have worked out valid solutions to Einstein's field equations. Apparently it cannot be done without utilizing negative energy and such. The Alcubierre drive (NASA reportedly working on it) requires it as well, at it very much would constitute time travel if it worked. All these require bending spacetime in a manner that isn't possible with ordinary positive energy. Neither of us knows the mathematics of it well enough to understand their explanations.

Tachyons for example need more (negative) energy to go slow than they do to go fast. They can approach c (from the >c side), but not reach

The theory assumes a dynamical system, which means a simple iteration of a single complex function. — jgill

The simplest models exhibiting chaotic behavior may be simple, but real functions are anything but. The weather for instance is not a simple iteration of a single complex function, and yet it is very chaotic, and all that we've discussed (who gets conceived/born, which creatures evolve) is very much a function of the weather, among countless other factors, most notably wave function collapse.

What is exactly the 'exact matter' including various virtual particles? — Corvus

I didn't mention 'exact matter'. Perhaps you misread 'exotic'. One can simply google 'exotic matter' for a more specific list.

So does it not prove that the whole story is just a fiction itself?

Not really. CTCs are allowed, and might actually exist at quantum scales. Their existence is not inherrently contradictory. To open one at a classical scale probably leads to necessary contradictions, and since all the time travel stories are classical, I'd have to actually answer that such stories are necessarily fiction.

You quote the OP asking this same thing:

And had the ability to manipulate matter in such a way that we can reposition new "environmental" circumstances into the ones that we have snapshotted, would that not be considered time travel? — unintelligiblekai

One can scan a person down to the biochemical level: the location of every cell and connection, the chemical makeup of all fluids everywhere. That's still a classical measurement. It's trying to scan down to the atomic level where things get impossible.

I had also asked this question when I brought up the Studebaker back around post 55 or so. (I wish this site numbered its posts). SEP is strangely mute on this particular case, and it has quite a list. In the end, it 1) only works forward (but so do some of the other cases designated as time travel in SEP), and 2) it is arguably a copy, especially since the environmental circumstances can be positioned in multiple locations at once, something not possible with the teleportation I mentioned, which is otherwise arguably a similar thing, even down to the quantum scale.

I think to qualify as time travel, it would require some sort of getting to what appears to be a past state. So for instance, in my Disney example, we take a scan of a town in 1955. Then Disney, in 2055, makes a big box in which the town fits. They put a copy of the 1955 town in there, and let you in. The people inside don't know. They're not actors. Have I time-traveled? Have the townsfolk?

Shouldn't how one could change the past events follow after fictitious successful time travel has been achieved, rather than before the travel? Have you achieved fictitious time travel into the past or future in actuality? — Corvus

I cannot parse this. How does something follow something that is fictitious?

Isn't quantum teleportation essentially just the transfer of information though? — Pantagruel

Good question. Yes and no. Yes, the state of the source side was somehow reduced to what might be construed as information (something one might shove through a wormhole??), but not information that could be monitored or saved in any way. The ability to do that would violate Heisenberg's uncertainty. But whatever was 'transmitted' to the destination 'booth' (I don't know the actual words they use), it reproduced the state of the source exactly, which necessarily does not leave the source behind. It is entirely quantum, not a classical copy. If the particle was entangled with some other particle, it still is after the teleport. That would not be true of a copy.

I've never said that the timeline is "truncated". By "truncated", do you mean "shortened"? — Luke

I think you used the word 'rewind'. It seemed to work like a VCR tape recording all of history everywere. Anytime somebody travels back, you rewind the tape to 1985, and start recording from there. If that's how it works, then the tape will never reach year 3000 because somebody (not always the same person) keeps rewinding it.
The original idea you pushed was the branching one. Whenever somebody goes back (and maybe forward, don't know), a new tape starts recording from the arrival event and the original tape keeps recording, which includes the time machine just vanishing permanently. You abandoned that model.

Anyway, if I got things wrong, you need to correct me on how the model actually works because I don't see how the tape can make forward progress if anybody anywhere has the power to rewind it arbitrarily far at any moment.

As for Back to the Future, that movie has holes. It isn't self consistent.

Let's say the time traveller travels from 2024 to 1985. The time travel event will change the history of the timeline from 1985 onwards, compared to the history of the timeline as it was before the time travel event took place in 2024. But I don't see why any time after 1985 should not exist, post-time travel.

I didn't say otherwise. The VCR tape resumes recording at 1985 and progresses no problem.

Unless the time traveller does something catastrophic, then I would imagine that many of the same people will be born

Well, from about 1986 on, the people born will be different ones. That's a very chaotic function.

On that note, do you agree that the time travel event does not occur until 2024, given that the time traveller departs from 2024 to arrive in 1985?

If this new timeline also has a time travel event in 2024, then the rewind happens again. If there is no time travel event there, then no rewind takes place then. That's why I came up with the 30 second train-track example, where the subsequent time travel decision is very likely. Over 40 years, it is very unlikely that events will turn out identically, especially if Bob goes back to 1985 explicitly to prevent the creation of the time machine.

Where did I say that "everything from there to 1990 ceases to be part of the universe"?

What does rewind do to the 40 years over which we backtrack? It either erases as it goes or that part of history gets overwritten as the recording resumes. Either way it is not part of the universe. That's the problem of using the same tape to record something new: you lose what was on there before.

Why does the first time travel event allow history to "all get rewritten again" but the second time travel event does not?

I didn't say that.

How does the butterfly effect of the time travel event necessarily prevent the evolution of humanity?

Who gets born is very much a function of exactly when people have sex, and which sperm wins. Which species come about is very much a function of random mutations and environmental chance. All these things are altered by chaotic things in the environment.

Read up on chaos theory. I can't possibly explain it to you in this context. There is no strange attractor for a specific person being born, or for a specific species to evolve. There would probably be mammals around since those existed in the Cretaceous, but probably no mammal that you'd recognize.

Tachyon is a hypothetical object which is in the domain of a fiction. — Corvus

It is a hypothetical object in the domain of science. Can't help it if the fiction folks are the ones that latched onto it.
The particle is lumped onto various headings of 'exotic matter' (including various virtual particles), and exotic matter is seemingly a hard requirement for time travel.

But if X = I can walk on the planet Jupiter, or I can fly faster than light. then it would be rejected by most people unless there were some explanations on how that would be possible, because there is no logical ground or scientific possibilities for that statement to make sense on their own out of blue. Therefore it is not fit for being a premise for any intelligible discussions. — Corvus

Closed time loops are valid solutions to Einstein's field equations. They would probably involve exotic matter, and would already be there, forming small close time loops. Classically (unrealistic), this is equivalent to a 'rift in space & time' (definitely a fiction term), sort of like in the Kate & Leopold movie. There's no machine, no punching in a desired destination. You just compute where and when they are and leverage them.

They have quantum teleporters, which means they actually have teleported a small object from here to there. Do that with a worm hole and you have retro-causal information transfer. If you can teleport a small thing, theoretically you can do it to a big one. Something is sent from one teleport booth to the other, so send that 'something' through the wormhole and reassemble your person, Amazon package, or whatever you're sending.

None of this rewinds reality, but actual retro-causal (or FTL) information transfer opens things up to paradoxes.

Nobody has ever detected what would be considered a wormhole. A lot of this stuff can be verified only by privileged verification, where only privileged people can possibly test certain things, and there is no way to convey the results of the test to non-privileged people. For example, I posit the existence of an afterlife. So you die, and if there's an afterlife, you know it but can't tell those still living. If there is no afterlife, you can't be in a state of knowing that.
Presentism is another example. There's a test: Jump into a large black hole. If you can be in there, presentism is falsified to you, but there is no way to inform those outside (the non-privileged folk) of this finding. If presentism is true, then like the lack of afterlife, you can't be in a state of knowing that.

Could you explain further why the universe cannot go on? I don't follow. — Luke

Your new suggestion says that the original (and only) timeline is truncated back to the destination event upon somebody time traveling backwards. If it subsequently (30 seconds later) is truncated again, by 30 seconds, then there is no way for the history of the timeline to grow beyond any backwards travel departure. The only way for it to go forward significantly is if there is never again a backwards time travel event. I don't know about forward time travel You've given seemingly no thought as to how that might work.

This is a causal loop, I take it? You said that a causal loop only appears to occur once for any outside observer.

This is a different kind of loop since it doesn't involve the same machine traveling over and over. It only makes but the one trip. That's enough to end the universe, according to the 'rewind/truncate' thing you've been pushing lately.

How is the rest of the universe destroyed or affected?

I didn't say destroyed. I say it ends. Your idea posits that: If I go back to 1990, everything from there to 1990 ceases to be part of the universe. Is not the entire universe affected by this, or do we just rewind some limited region like Disneyland? So now everyone in Disney thinks it's 1990 (they're pretty good at that sort of thing), but people outside the park think it's still 2024. That's not time travel, it's just fooling the guy in the machine by putting him in a live action role playing game.

Does that mean we can't think about it, then?

You can, but it would be really nice if the discussion was free of more contradictions than just the impossibility of time travel (besides the pacing).

Is it so difficult to distinguish a timeline which contains a time traveller from one which does not?

There is but the one timeline, unless we're changing stories again.

Also, I don't know what you mean by the "truncate model".

It's your model, the one you are not pushing instead of the branching model. You didn't really give it a name, so I did. In it, travel to 1990 deletes 34 years of history and lets it all get rewritten again, but with a different 1990 state this go around. That 34 year scenario might well not end the universe, if the second go around can not only destroy that machine, but preventing anything anywhere (including other galaxies) from ever making one. This cannot occur in the 30-second story with the train tracks. No way to stop that one, so the universe ends there.

Why couldn't he use the time machine to witness his own birth?

In the context you didn't include, it was because he travels to a time before his birth, thus altering 'history' to one in which he (or any other human for that matter) is never born.

Are you saying that humans did only evolve on a timeline without a time machine

They only evolve from a Cretaceous state that doesn't include a time machine, yes. More precisely, humans don't evolve from a Cretaceous state that is in any way different than the Cretaceous state from which we evolved. That's popularized by the term 'butterfly effect'. Chaos theory is very clear on points like this.

I like "timelines", but only those I initiate. — jgill

And what definition are you using this time? What is this sort of timeline, and how does one go about initiating one?

It is just to clarify the statement is unsupported in any meaningful manner without proofs and evidences. — Corvus

There is evidence one way or the other. There is rarely 'proof' of anything. In this case, there are valid non-local interpretations of physics with superluminal cause/effect. That opens the door for retrocausality. But none of the interpretations allow superluminal information transfer. That pretty much closes the door.
Physics mathematically allows for tachyons, which can 'go backwards' in time, but nobody has ever found a tachyon or other necessary exotic matter such as things with negative mass and such.

Because of the fact the premise "IF" describes the possible physical and empirical events, and also the conclusion part is soley dependent on the premise, it should have given even brief explanations how the IF part could be possible, for it to be accepted as a valid assumption for the further arguments.

I suppose in the end it would matter how it works, before we go about presuming the properties and possible interpretations of the thing.

The time traveller departs from the timeline without time travel and creates a timeline with time travel by doing so. — Luke

Again, I thought you were abandoning the interpretation with creation of timelines in favor of modifying the one and only line.

I thought of a simple killer for the modify-original timeline: The universe would end pretty much abruptly any time the machine was used to go back, but in a way that doesn't prevent the departure event.
Back to the train tracks, Alice gets there just as the gates go down, but watches a very similar car ahead of here make it across. So she hits the button and goes back 30 seconds. That destroys the 30 seconds. She ends up at the tracks, and in time to scoot across. The world ends 30 seconds later when the car behind here truncates it there. There is no future after that. The universe cannot go on.

So if that's how it works, using it is a doom to any future event unless you end up in a world where no further use of the machine will ever take place.
Maybe we should go back to the spawn-new-timeline model, which has infinite series of Alice crossing those tracks, but at least each of them gets to her appointment on time.

If it will help make things clearer, I can try to dispense with (McTaggart's) A-series terms. The time traveller departs from the year 2024 and arrives in the year 1990.

Seemingly an admission that time travel with presentism don't particularly mix. I mean it does. SEP discusses it, but says very much that the arrival event occurs decades before the departure event, back when the arrival event was the present, which only happens once. That model doesn't have a history between those times where time travel hasn't yet happened.

Keep in mind that I'm not a presentist, either.

You use a lot of A-series terms, which make no sense without presentism. Yes, learn to dispense with the concept. It helps. There's no evidence for it other than intuition, a pragmatic lie that makes us fit.

The branching model works reasonably well in a block model.. There's no obvious correct way to compare moments between timelines.

As I said earlier in the discussion and as I have explained previously on these forums, I believe that a combination of both views of presentism and eternalism are required to coherently account for time.

Maybe. I mean, it;s not possible, so you'd probably get a hard contradiction with eternalism as well. Doing so given an impossible premise wouldn't falsify either view.

Why would the time machine be un-built in 1990?

I don't mean disassembled. I mean something exists which never came into being. But this is in the truncate-model, which I'm rejecting because we could never have existed in such a universe.
I know you consider the machine to have been built, despite that process not existing, and 'was built' (a past tense reference) 30 years from now. As Dr Who said in his Xmas party: Didn't you get me this next year?

You may find it perfectly logical for a person to exist before they are born, but I do not.

I noticed.

I was referring to the sequence of events of a person's life.

But that's just a memory. It is a memory of nonexistent events.
His birth event (say in 1975) is nonexistent. He can't for instance take the machine back to it and witness it.

Why not? Did he somehow prevent it from happening?

Yes. A machine appeared in the Cretaceous and humans evolve only on the timeline without the machine.

I admit, I am stretching a point. I'm looking for any sort of evidence of change of movement through time. — jgill

'Change of movement through time'. What an interesting way of putting it. You'd like the SEP definition of time travel then, which is whenever clocks don't agree for reasons other than a faulty clock.

Funny thing about time dilation is that there's no way to tell which individual is the one 'moving through time' at the faster or slower rate.

I retain the idea that there must have been one version of history before any time travel events and a different version of history after the first time travel event (a history which henceforth includes a time traveller), at least different starting from the destination time of the time travel. — Luke

If the 2024 that doesn't yet include the time traveler is before the 1990 that includes the time traveler, then if would seem a stretch to call what he has done 'travel to the past'. It seems to be just a re-setting of the present state (the part outside of the machine) to what things looked like back then, but no actual travel anywhere.

The time traveller departs from the present and arrives in the past.

So he's in 1990 despite it presently being 2024? What's it like to be in a place that isn't the present? I think the Steven King book/movie Langoliers had a plot like that.

The time traveller does not depart from the present of the spawned timeline, but from the present of the original timeline.

You said you were rejecting the 'spawned timeline' idea that occupied so many of our posts.

Keep in mind that I'm not a presentist, and am sort of having fun seeing how a presentist can phrase time travel coherently.

He did travel to the past from a time which is in the relative future of that past time. He did not travel to the past from a different timeline; his time travel will change the history of the same timeline. The changes will begin from the time traveller's date of arrival, starting with the addition of the time traveller in that time.

This is the truncation I mentioned, the overwrite scenario instead of spawn new line scenario. The inconsistency is calling 1990 'the past'. If the universe is currently being rewriten from there, then 1990 is the present, and there is no original history of making the machine. Those dates have yet to be written since they are in 'the future'. So now you have a machine sitting there un-built, but not un-caused. It was caused by a nonexistent retro-causal occurrence.

If I were to make the first-ever time travel journey tomorrow to arrive at the destination time of 1985, why would the population suddenly decrease from my POV as a result of the time travel?

The people there now have access to time machine technology, so that timeline is likely to be overwritten at any point. Eventually somebody will erase all of human existence and that will be that. It takes just one traveler going back a million years or so.

Point is, every use of the machine(s) in the backwards direction truncates history a little further. The population would empirically slowly dwindle in the branch timelines, but here you have no branches, only the original, and in that line, the present keeps moving backwards at frequent intervals.

In the alter-history interpretation, no he was never born.
— noAxioms

Why was he never born?

This isn't hard. His birth event doesn't exist (assuming he/somebody/something truncates the present to a date prior to the birth date. If he isn't the guy in the machine, then he doesn't exist either (at all). So not even a memory of being born.

Okay, in the linear time there are dinosaurs, and a time traveller and their time machine have appeared uncaused. Nobody was born, yet the time traveller exists. How is this consistent with causality and determinism?

We're in a universe with retro-causality here, one that a cause obliterates its own existence from the one history.

The only logical sequence of events is that the time traveller is first born and then time travels to visit the dinosaurs.

That is not a logical sequence on the linear timeline. First he appears with the dinos. Then, much later, the time eventually comes that matches the year he remembers being born. There is no birth event of Bob at that time. The memory was false.

This implies that there must exist a linear time without any time traveller up until the time traveller's birth and subsequent time travel.

Two kinds of time mixed there, unless the history line is never truncated, and the machine simply writes the current universe a new way without traveling at all. That model (I'll call it the stacking model) doesn't easily support forward time travel, but not sure if any of them do. You ought to think about how forward works. Funny, but the stacking model does allow one to witness one's own birth. Not the actual one since it doesn't involve actual travel to the past, but a copy of it. One can restore all the people eliminated by the dinosaur stint. There are no loops in the stacking model.

Surely, their birth must precede all the other events of their life

So 2024 precedes year -100,000,000, a funny interpretation of the word 'precedes'.

In this context, I'm measuring it on the traveller's timeline

Ah, you actually identify a line. Sure, on that line, 2024 precedes -100M. But it's just a memory. His birth event (say in 1975) is nonexistent. He can't for instance take the machine back to it and witness it.

Being alive is pretty good evidence of having been born.

Not if your earliest appearance was from a time machine. You keep thinking the rules of this universe apply to this retro-causal one.

The arrival of the time machine in 1990 does not follow its departure from 2024? But isn't that exactly what a time machine does?

From the PoV of the machine, sure, That's the same as memory. 2024 feels like 'the recent past' to the machine and its contents. If we're talking about the stacking model, it actually still is the past, and sure, the machine was in fact built at some point. That model is empirically different than the other ones we've been discussing.

Okay, then where is the inconsistency?

Take 8 second-man, but make it 50 years. A young guy steps out the machine, and the same guy 50 years older travels back to the arrival event, and not looking like some old guy. That's an odometer, and I cannot explain it better when you seem incapable of understanding why the jump counter in a loop would be a contradiction.
Hence me saying that 8-second guy can't be human. A human ages. He can't.

The "If" part needs backing proofs with evidence before the whole sentence could be accepted as a meaningful statement. — Corvus

We were deliberately ignoring all that, since the possibility of this as described isn't there at all.

The word "timeline" is, of course, vital in the study of history. Over an era there is a timeline of wars, a timeline of governance, illnesses, etc. — jgill

Here you seem to be using the word 'timeline' to mean something like 'period of time'. That's not how it is being used in our posts. One timeline with Hitler losing WWII. One with him winning. Others with no Hitler. Other timelines with no humans at all, ever.

Is there any evidence of the existence of timelines in the physical world beyond time dilation?

Here I think perhaps you're confusing the word with 'worldline', a term for a physical path of an object through spacetime, that sometimes comes up in discussion of relativity and block universes, although the term is not directly related to time dilation, which is just an abstract coordinate effect.

The timeline we speak of here is a specific history of everything, not just the path traced through spacetime of a single object.

However, this does not explain how a time traveller can have travelled to the past before their first ever time travel event. — Luke

They didn't in the [spawn new timeline] scenario, so nothing to explain. I suppose it depends on which moment on the new timeline is consdiered to be 'the present'. If, say, the present is designated to be 'the present' in this spawned timeline, then the traveler (if there is one) must be present at 'the past', 5 years prior. Did he travel there? I suppose he did. Did he travel from 'the future'? No. He came from a different line is all. The 2024 of this timeline does not have him going back. He dies before then, presuming he doesn't exist the line by a subsequent usage of the machine.

And as I said, the empirical experience of everybody is the same between the copy/paste interpretation and the 'alter the original' interpretation. Either way results in a general de-population of Earth from the travelers PoV, or if the use of the machines becomes commonplace for everybody.

In the case of this dinosaur visitor, surely this person had to have been born before they could ever travel to the past?

That's interpretation dependent. Empirically, the guy will remember being born, sure. Given the copy/past interpretation, yes, he was actually born in some timeline somewhere, one of many, but not this one. In the alter-history interpretation, no he was never born. That state doesn't exist in the one timeline. No earlier time had his birth in it, and only an earlier time qualifies for that verb tense.

Therefore, there must have been an original version of the past that existed before the dinosaur visitor ever visited.

You are using past-tense in a mixed way. Be specific. In the linear timeline, there are dinosaurs and a time machine that has appeared uncaused, all in the present. There are no other people on that timeline unless the guy brings a breeding population with him. Nobody was born. There is no 'must have been' about it since earlier times do not contain his birth.

On the traveler's timeline, there is a memory of a birth, a memory of a time that doesn't exist. Memories are thought of as 'past; things, so one could meaningfully said that he must have been born, but it's more like Adam and Eve and insisting that they must have been born which reportedly they actually had not. One wonders what their very first memories were. Did they have to learn to eat and not poop in your bed and had invent language? Our time traveler seems to have all that experience already, so he's better off.

Surely, their birth must precede all the other events of their life

Again, on which timeline are you measuring this? Given a time machine, this would obviously not be true or a calendar timeline. Marty is in 1955, well before say 1968 when he is born, contradicting your statement.
On Marty's timeline, he is in what appears to be 1955, and has 17 years of memories, which include stories of his birth. If the memories were perfect, yea, he'd remember that birth. Whether that birth event actually exists is a matter of interpretation, just as is my birth event. Per last-Tuesdayism, there is no way I can prove that I was ever born. We all just assume it by convention.

If it's a causal loop, then it will repeat the same time travel event over and over again.

Only from the PoV of the machine and its contents. Per the outside observer, there is but the one jump. Yet again, you need to specify which timeline is being referenced when making statements like that.

However, the odometer reading of "x" (jumps) is after the time travel event. Therefore, if the odometer actually works, then its reading before the time travel event must have been "x-1" (jumps).

Contradicting the fact that you just said it reads x+1, a number to which it was set 7 seconds ago and not altered since. That would be a contradiction, and thus cannot be the case.


Concerning the 'rewrite the one timeline' interpretation:

I am referring to pre-time travel; before the time travel event has ever occurred.

OK, on hte Earth timeline, we're talking about dinosaurs then, just before the machine appears somewhere uncaused with an odometer reading 207. Before that Cretaceous time, no time travel event has ever occurred. History is a particular way then, but the Cretaceous is the present, so it goes only that far, and the rest is yet to be written.

Before anyone has ever time travelled, history will be a particular way, and this particular way (or version of history) will be altered by the time travel event to create a different version of history.

The time travel event (the appearance of the box) only has a causal effect on subsequent events, not on the prior ones that are the 'history'. The machine doesn't alter history, but it truncates it to a point and starts a new rewrite.

We might say or believe that up until now there have been no time travel events.

None before the Cretaceous, no. We don't know that, but we have strong reasons to believe it. Any prior time travel event would arguably have to have been made by something not human or human-created, and probably wouldn't be on Earth.

That, btw, is another problem rarely addressed: How does the machine know where to go in space? Almost all the stories have them setting only 1 coordinate, not 4. Earth is moving. If I just back a week, what reason do I have that it will also transport me sideways to where Earth was (the surface of it no less) a week ago?

If I were to time travel tomorrow, back to 1985, then I would be altering history as we now know it.

The word 'now' in that sentence is ambiguous. Presumably you are still planning to go back to 1985, and thus it is still 'now' 2024, and there is still a 'we' to know such things.
If the action has just been done, then 'now' is 1985, there is no we, and there is no history to be known, although you do know of it as a sort of fiction.

After that, history will contain my time travel event, but it must also contain the "unaltered" history that preceded my time travel event

You mean 1984? Yes, it contains that. If you mean 2023, then now, since it is now 1985 and 2023 has yet to be, and least per this 'rewrite' interpretation.
You seem to be trying to refer to what was the original timeline in the branching interpretation. If you've switched back to that, you need to indicate so, but I think not since you're explicitly referencing the alteration (truncation) of the original line.

(the history as we presently know it, before any time travel events).

It is now presently 1985 and there is no 'we' there, so no, that statement makes no sense.

you cannot already have arrived at 1990 (as a time traveller) before you have time travelled to 1990.

Traveling to 1990 and arriving there is the same thing. That arrival event IS the time travel event. Are you talking about a different jump? Before that is 1989. 1991 is after that. The traveler has a memory of a nonexistent 1991, it being nonexistent because it's a future time, yet to actually be.

To simplify matters, we might only consider one time travel event rather than several. Also, in this discussion I'm interested in the possibiilty and consequences of time travel, not in preserving the stability of the population or the timeline.

If you're interested in consequences, you need to address the case of multiple machines crossing each other. I thought we were deliberately ignoring the lack of possibility. If you're actually interested in it, then exploring consequences is moot until you find a way that it's actually viable. SEP seems to suggest that pacing counts, but that's hardly something with interesting consequences.

Obviously, a time machine will appear in the past to come "out of nowhere" following the time travel event.

Again, it doesn't follow a time travel event, it is the event. If you're talking about the departure event, the appearance of the machine in 1990 does not follow that event. 1990 is before 2024.

That is not what I am talking about. I am talking about the history of a time machine's construction being erased in a causal loop, such as in the museum donation scenario.

The loop does not erase its construction. It isn't something that is constructed at all. It's a solid example that 'things' in that universe don't necessarily need a construction phase.

This is the sort of appearance from "out of nowhere" that I am referring to; that a time machine or its technology comes to exist without any causal history.

It has a causal history. It's just a retro-causal history is all. As I said, you're going about finding the inconsistency all wrong. Stop trying to find the end of a loop that doesn't have one. That's not where the inconsistency is.

The same applies to the dinosaur visitor who can exist in the past (which is necessarily post-time travel) prior to ever having been born (which is necessarily pre-time travel).

He was necessarily born pre-Cretaceous? That makes no sense to me. It can make sense in the branching case, depending on how one chooses to order events that are not on the same timeline.

Can somebody demonstrate the typical definition of determinism?

That is QM (or time) interpretation dependent, and no,. there is no way to falsify the interpretations that are not deterministic in one way or another.

For example, I spend my life working out time travel technology and build a working time machine. I then time travel back to 1990 and teach my younger self how to time travel. My younger self grows up, uses the knowledge to time travel back to 1990, and teaches my/their younger self how to time travel.

Why? He's already got the first 'you' teaching him. How many of you does it take? You're not making a loop by doing this. You're making a crowd control situation.

A causal loop follows the initial time travel event, but it has a different history prior to the first time travel event (an original history in which I figured out time travel without having been taught it by my time travelling self).

Well, you just had two different people (both you) time travel to the same spot. What if the coordinates are exact and second one obliterates whatever was at the spot at which it appear? I mean, you've never really specified what happens when the machine pops into existence somewhere. What happens to the bugs and other contents of that location? If there's a person there, or half of one, or the middle of a jet engine in flight? What if you manifest a mile underground? Never mind you being somewhat stuck, but what happens to the rock that was there a moment ago?

8 second guy has a first and only appearance, yes. From his looping timeline, there is no first anything. It's a circular timeline.
— noAxioms

Is that a "yes" or a "no" on the first?

From the world timeline, it's a yes: first and only. I said that. From the circular timeline, there is no first.

But logically (and causally), those non-existent times did exist, prior to the time travel event.

I don't see how they can both be nonexistent and also 'did exist' when the time of their existence hasn't yet happened. Nothing at those times exists yet. That's the nature of 'the future'. It's what makes using the same machine to travel to future times somewhat contradictory. It would have to just go into a stasis state (Per Larry Niven's universe), wait for the prescribed time, or in the case of Niven, waiting for conditions outside to be non-fatal. The thing is, where is the machine while it's doing this? Can others see the box waiting there, or does it vanish into another realm while it waits for its destination to come into being? And of course, what happens if the departure in history suddenly ceases to be a part of history?

I thought your post was too large, but mine is even larger.

As I said earlier:
...it would be logically impossible to travel to 'the past' (i.e. on a single timeline) unless I was somehow already there before I time travelled. But how could I already be there before I time travel?
— Luke

Yes, we are talking about the original timeline. — Luke

Given physics where there is a timeline that is the original one, that line cannot have a time traveler in it at all. All the copy lines have but the traveler(s) that created that line (assuming the machine had one or more passengers). So in those lines, any traveler was already there at its start.

Given the physics of a single timeline, various machines might travel here and there, but there would be presumably some earliest one (to see dinosaurs say). In such a situation, there is no traveler before that earliest Cretaceous period. I don't think you're asking that, but who knows...

There are valid scenarios with such a single timeline, but the traveler (if it is human) is part of 'the past' then and lacks the free will to do otherwise. I agree this runs into serious problems if he knows what he does (say a minute ago) and goes back explicitly to do a different thing. No amount of determinism is going to prevent that. Determinism is not a thing with a will different than yours. Nobody seems to realize that.

Why does it need to revert to a reading of x again 8 seconds later?

Because it reads x when it appears 8 seconds before that. You know that. It's on the outside and you read it. You can't read it being x, x+1, and all the other numbers. The number has to match at both ends, or it didn't come from that 8-seconds hence jump. It wouldn't be a loop, just a stopover, and a different party popper than the one he took from you.

On reflection, I want to reject my suggestion that there is more than one timeline. You've helped me to see that this is not really what I had in mind. What I have in mind is that there is only a single timeline but that the effects of the first time travel event overwrite the past of the original timeline (starting from the destination time of the time travel event, e.g. 1990).

OK. That's not something we discussed yet. How does it empirically differ from the branch thing? The old 'history' goes away, so there's nobody to witness the population of the world going down. There are a whole mess of uncaused events going on, but besides the classical impossibility of that, no other contradictions. You have people who don't have valid identification. Maybe no other people at all. So the empirical experience of those you don't take with you is irrelevant, and the empirical experience of the traveler is identical to the experience of the guy in the branching model. So this sounds like a different interpretation of the exact same experience.

Those are just my thoughts before going on to read what you said about it:

This might create a causal loop or it might not. However, the main idea I've been trying to convey all along is that there must be an original version of "the past" prior to the first ever time travel event

OK, back before the earliest time, before the destination of any retro-time traveler.
Or do you mean 'first' on the timeline of some traveler instead of on the one world timeline? You don't ever specify. I think you mean the latter, in which case, what do you mean by 'must be'? How can something 'be' if it doesn't exist at any time? What definition of 'be' are you using?

and is necessarily different to the version of "the past" that exists post-time travel.

Suppose I travel to 1990. How is what you call the original line (the one I remember with cellphones and all) is 'the past'? It's not before 1990, and for that matter, it's not after either. It just isn't at all.

It all sounds like a re-growing-block model, except that disallows forward time travel since the destination specifed doesn't yet exist. I set sights for the year 3000 (like in Futurama), but while my machine is waiting for Y3000 to come around, somebody else uses a time machine to go back to 1985, thus obliterating me and the destination I targeted. same fate awaiting all those people paying for cryonic preservation. It requires a stability that just isn't there.

This helps to retain "normal" causality, thus removing the need for the magical appearance of time machines or technology "out of nowhere", existing uncaused

But all your scenarios describe exactly that, including pretty much every fictional story that I can think of. Time travel, as envisioned, necessitates technology or at least some object/person appearing uncaused from nothing, or worse, replacing what was otherwise at that spot. Remember terminator? This sphere of space replaces what was there with some air and a naked person. Nobody says what happens to the stuff that unfortunately happened to be where that ball appears, which by chance might possibly be half of another naked person.

And it retains free will.

Well, the paradoxes are gone at least. Nobody can demonstrate the typical definition of free will.

In fact, causal loops can be avoided

They'd not be loops at all then. The 8-second guy would simply die in moments the same way the half-of-naked person did. It's a hazard of living in such a world is that your life expectancy outside the machine is moments at a time, and no better inside the machine since no time passes in there (unless you assert otherwise I guess).

OK. The 8 second machine is created in front of me at some point, and un-created 8 seconds later when it vanishes. Works for me.
— noAxioms

That doesn't explain how the time machine was created for the time traveller.

It's not created for the time traveler any more than the time traveler is created or has an age.

It has a first appearance?

8 second guy has a first and only appearance, yes. From his looping timeline, there is no first anything. It's a circular timeline.

But is single-timeline travel necessarily contradictory, even if one does make different choices post-time travel?

What you describe above is a single preferred timeline scenario, with all the non-preferred timelines being nonexistent. I am not sure if there are 'different choices' involved since there is but the one timeline, and thus one choice being made at any point in time. Sure, you remember making different choices, but those are memories of nonexistent times.

I was talking about the time before the first time travel event; before you've ever time travelled. You're talking about what happens if (or after) you time travel, so you're not talking about the time before you've ever time travelled. — Luke

OK, Bob makes the machine and uses it to go from 2024 to a new timeline starting at 1990. Any point on the original timeline before Bob vanishes from it is the time before the first travel event. There is no time on the new timeline before the first travel since it starts there, kind of per last-Tuesdayism.
I lost track of the question about this 'time before'. Are we talking about say 2023 on the original timeline or am I still getting it wrong?

Why can there be no odometer on the time machine counting jumps?

There can't be one on the machine that jumps in the loop. Bob's machine can have an odometer, no problem.
The contradiction: Suppose, just before the jump, the odometer reads x. It arrives at its destination (8 seconds in the past in my popper example) and immediately increments the thing to x+1. This contradicts it leaving 8 seconds later with a reading of x.

Sorry to be unclear again. What I meant was: how could I be in the past as a time traveller prior to the first use of the time machine.

Just repeating the same question doesn't make it clear. Are we talking about Bob and the copy-timeline scenario? If so, you need to specify which timeline you're referencing when talking about one thing being prior to another.
In general, if one considers that Bob builds the machine and first uses it in 2024 and uses it to go to 'the past' (no timeline specified), then since 'the past' is typically considered to be prior to 'the present', Bob is in the past as a time traveler (in 1990 or whatever) prior to 2024 since 1990 is often considered to be prior to 2024. So that's how he's in the past prior to first using the thing. It's the whole point of the machine to be able to do this.

What are these different causal rules?

That's what you are apparently trying to figure out. I don't know either, so I'm also exploring. What I don't do is presume the usual rules, such as that a place that almost looks like the state of things in 1990 is prior to the state of things in 2024. I also don't presume that the cause of a thing is necessarily prior to the thing. That's a pretty obvious one to throw out.

There are still causes and effects, it seems.

Agree. We're trying to keep that. The loop is causally closed, so I don't see it as a contradiction. The cause of the 8-second guy is his own travel event 8 seconds later.

The older self can teach the younger self about time travel technology and the younger self can then use that knowledge in order to time travel from the future to the past. Or, the younger self can steal a time machine from the museum and then later use that time machine in order to donate the time machine back to the museum. The only different causal rule appears to be that there can be no first time travel event or that we are not allowed to talk about the first time travel event, for some unspecified reason.

There is no first time for the loop, or if there is, it's the only time. There is after all but the one jump, per the external timeline, presuming its a simple loop. Only the machine's timeline has multiple jumps, plus its contents if those contents go from arrival all the way back into the machine at departure.

So some causal rules are okay, but not others?

The ones not OK lead to contradictions. The looping machine having its own 'first time' leads to a contradiction. It would effectively be an odometer going from 0 to 1, and we showed how that is a contradiction.

We may never ask/explain how a time machine came into existence in the universe

It came into existence by traveling from 'the future'. You can ask and that's the answer. That universe allows that sort of causality.

but it's okay to ask/explain how a time machine came into existence in a museum?

By being donated of course.

I think the rule you find hard to discard is that all seemingly artificial things must somehow be invented and assembled at some point, and the examples we show are consistent without all those steps. Sure, the machine is built in the teaching loop, but the technology knowledge (the inventing) is the loop, information that is never gleaned, but is merely passed on.

It's logically self-consistent as long as we never consider a loop as having a first time travel event or what preceded it, it seems.

You're trying to find a logical inconsistency, and I don't see one. Before the loop, the machine simply doesn't exist, nor does it after. The 8-second machine exists but for 8 seconds. Not time to study and figure out how its done, something the museum guys might decide to attempt.

But how is it logically self-consistent that there was never a first time travel event?

The same way that the lack of the most eastern point isn't a logical inconsistency? It's only inconsistent if you presume there must be a first time (on the machine's timeline), so that's apparently a wrong thing to presume. There's a first time on the world's timeline. Isn't that enough? This presumes that the external world is itself not a loop. There are hypotheses that suggest otherwise, a sort of cyclic model of the universe.

Does time or causality work differently in these scenarios such that it would be impossible to trace back to the first time travel event?

The infinite-age universe hypothesis similarly suggests the impossibility of tracing back to a first event. A loop without a beginning is not in contradiction with anything.

Not if it is part of a loop. The whole 'must be a first time for everything' is only a rule in a universe like ours, intuitive to us, but not true in the sort of scenario we're discussing.
— noAxioms

How is it "not true"? It doesn't seem to me that it's not true; it seems that you just want me to ignore it.

It's true in our universe because I cannot think of a scenario where at some earlier time there is not a mug, and at a later time there is a mug, and that there it a beginning to the mug's timeline. The timeline of the mug and that of the rest of the universe is completely parallel, so there must be a 'first moment' for it. In this alternate universe, the mug timeline might not be parallel. It still has a first (and only) time in the universe timeline, but not on its own timeline, which isn't parallel to the one 'outside'.

We're dispensing with evolution, too?

If a machine that loops and is never created can exist in some consistent way, then so can a creature than has no evolutionary ancestory. It just appears from some retrocausal event, and its existence somehow eventually plays a role in that eventual retrocausal event.
So it's like humans have no evolutionary ancestors (despite the biology folks suggesting otherwise). But some time far in the future, when humanity is near its end but they develop time travel, the put a couple back to year -4000 and name them Adam & Eve.
Geez, the religious folks would jump on this if it wasn't supposed to be blamed on God instead of retrocausality.

I suppose I could alter P2 to say that time machines involved in human time travel need to be created at some point.

OK. The 8 second machine is created in front of me at some point, and un-created 8 seconds later when it vanishes. Works for me.

Or, better still, P2 could say that there must be a first human time travel event associated with the human use of a time machine or time travel device/technology (assuming that any such events occur).

Dangerous to use the word 'first' when the temporal ordering of things is not objective. I think that's where a lot of the trouble comes from.

You are effectively telling me to ignore how the time machine came into existence originally.

Nope. It came into existence when it first appears, not 'uncaused'. It doesn't exist at any time before that, so that is it coming into existence. It gets donated to the museum some time later and yet later is stolen and vanishes from existence forever after as it causes the earlier event.

now you're invoking fanciful beings that can time travel without any time machines and other magical shenanigans in order to try and save the "self-consistent" logic of causal loops.

I just made them up as another example which isn't directly self contradictory.

I should probably withdraw my language that these things are not created. They are, but the causes of their creation are events that are future events as measured by the world-outside timeline, a timeline which we are presuming to be reasonably linear and therefore orderable.

Am I supposed to be the guy in the box/time machine, because this doesn't sound like a causal loop

No, you are the spectator who has somebody use your popper and then take it from you. The person in the box is, well ... something else. It is along those lines that you should tear this apart. A human makes a great odometer, and you can't have an odometer, so the guy is perhaps not human?

it's just a guy using a time machine to go back in time every 8 seconds to do the same thing repeatedly. It's unlike the other causal loop scenarios because it's not clear that I ever become the guy in the box/time machine. Or was that part left unsaid?

Yes to the first. No, it's never you. You're left behind being befuddled, remember? You never see him again. It very much is a loop, and a very tight one.

I thought by "un-created" you meant that the time machine was not created or did not exist. Did you mean "uncaused"?

Un-create means to cease existing. From the perspective of the linear timeline, Any traveler uncreates his machine and himself. It's just gone leaving not even disassembled parts. Of course on the machine's timeline, it just has an external environment change and isn't an act of creation or uncreation at all.

Somebody could catch a video of uncreation on their phone. You're were taking a video of a train crossing because you heard a train in the distance. The video records a blue car suddenly appearing (an act of spontaneous creation) at the crossing which then crosses and continues on. The lights go on and the gates come down, just as an identical blue car pulls up and vanishes (uncreates) at the exact spot where the prior car appeared. You have a video of time travel in action, and it even worked in the universe of Bob where a new timeline is created each time. The video is subsequently sold to the time machine sales people who use it to pitch their product.

Right, that's why I've been arguing that time travel only makes sense on multiple timelines

For the most part I agree. But single-timeline travel isn't necessarily contradictory so long as one does not make choices known to be different than those made before. It does require a sort of lack of free will as it is often defined.

I was just trying to restrict it only to causal loops that do involve time travel, in case you were about to bring up any causal loops that don't.

Any loop in time is contrary to the sort of linear ordering of all events that we find intuitive. No, it doesn't have to be labeled 'time travel'. A cyclic universe is a nice loop that isn't considered time travel because there is no linear timeline laid alongside the loop.

so it does not follow that every timeline would quickly have a population of zero. — Luke

Agree that if the people (especially those on the original timeline) fear the readily available devices, they wouldn't get used and the population remains.

The people on the alternate timelines would have solid evidence of it working. A car gets caught by a train crossing, so it goes back 1 minute in time and sneaks across the tracks before the gates come down. The traveler can see her own car doing it as she approaches, and the bystanders can witness it as well. The person doesn't always reappear. Sometimes the traveler is gone forever. Kind of pot luck, a game of Russian roulette.

What I meant was: how could I already be in the past before I have ever time travelled?

If you time travel to the past, by definition you end up somewhere 'before' the event where you initiated the travel. I kind of lost track of the context. Are we talking about the loop here?

I could already be in the past (on a single timeline) if I had time travelled before but, given causality, there must have been a first time that I ever used the time machine to time travel.

I don't see how that follows with the loop scenario. There would be no 'first time' to a loop. As I said, there can be no odometer on the machine counting jumps. That would be a contradiction.
If it's just teaching the younger-self how to do it, then every jump is the only jump, so I guess that would count as the first (and only) time, or at least the one jump that defines the simple loop.

How could I already be in the past prior to that?

I don't understand this. If the jump is from 2024 to 1990, then 1990 is 'the past' destination, and you are not in a past that is prior to that except perhaps as a young person, the one that you teach.

That sort of logic only holds water because there are no time machines possible.
— noAxioms

Why are no time machines possible? That's not something I've said.

We're presuming they're possible, hence the logic you give being fallacious. Things that are impossible in this universe are not impossible in this alternate universe where time travel makes for different causal rules. A loop is valid under the new rules. It doesn't violate anything except the rules of this universe.

We can just discard causality and assume that time machines don't need to have had a first ever use, and we can conveniently disregard whatever history led up to that first ever use?

You need to discard the causality rules of this universe, yes. The rules are different in the universe we're discussing. With the loop scenario, there is no 'first ever' to it. You can't count them. The loop is just there, and is self-consistent.

but if a creature evolved a way to do it, then there must have been a first time that they ever time travelled.

Not if it is part of a loop. The whole 'must be a first time for everything' is only a rule in a universe like ours, intuitive to us, but not true in the sort of scenario we're discussing. Yet again, a simple counterexample falsifies your assertion. So maybe this time traveling creature never evolved, but just is. Again, there are movies depicting pretty much this.

P2 is unacceptable. It's like trying to prove God by asserting that the universe needs creating at some point (which is itself a self-refuting argument). An un-created time machine does not violate any rules in a universe where time travel is possible.
— noAxioms

If you accept that one cannot travel without a time machine (P1) - at least, for the sake of argument - then it follows that a time machine (or the means for time travel) must be created or have evolved or somehow brought into existence in some manner.

I will not. We're discussing the possibility of closed loops, and loops falsify P2.

it's merely assuming the universe must have been brought into existence (which is quite self-evident).

I don't find that evident at all. It violates Einstein's theories for starters, which suggests that time is part of the universe, and not something in which the universe is contained and in need of being created.

Hopefuly we can agree to the standard scientific view that the universe's existence began with the big bang,

Pop science view maybe. OK, if one confines one's definition of 'the universe' to just what evolved from the big bang, then a good deal of them would suggest a larger structure from which that bang was initiated. But there is no before/after without the sort of time that boiled out of the bang, so calling it 'before' is misleading.
If you consider the universe to be the entire quantum structure, which includes all the stuff that 'springs' from it, our bang being one of them, then that structure is not something 'created' or 'caused'. It cannot be, both terms implying a larger container for something we're defining to be the largest container.

but even if we might assume that the universe has always existed

Sorry to be so buggy, but I don't buy that either. The phrase once again implies a universe contained by time, and not the other way around. Yes, there are those that suggest something like that, in denial of Einstein's postulates.

To say that time machines have always existed is more like saying that waffle irons have always existed.

Your wording suggests that the machine exists at all times, which isn't the case. It exists in the loop in the museum case. It doesn't exist at other times.
In the teaching case, it is built by young Bob in say 2022 with knowledge from his older self. Bob can then use to travel all over the place, here and there, to say the restaurant at the end of the universe. Eventually he goes to 1990 and teaches his younger self the secrets. Then he's off again to see even more wonders. Point is, there no point in time where that machine cannot be unless it has a limited range or something.

An un-created time machine does not violate any rules in a universe where time travel is possible.
— noAxioms

Sure, but it would imply no time travel.

Same counterexample falsifies this.

Imagine you're holding one of those party poppers that you pull and it explodes a bit of confetti around. You're about to do it and a box appears in front of you from which a some guy jumps out and explodes his own party popper as he says "three!". Then he grabs yours (unpopped), and apologizes, says the box is a time machine that goes back 8 seconds, the says "One, two, ..." and the box disappears, leaving you simply befuddled. That's what an 8 second loop looks like.
It's not too hard to take that one apart, but not by the logic you've been attempting.

Why can't there be a first time in a loop? Loops are immune to causality?

The kind of causality rules you're thinking of don't exist in a universe with time travel. A first time for a loop would contradict its existence, which is travel from the other end of the loop and not somewhere else.

P4 [the initial time travel event cannot cause itself (e.g. by a prior time travel event using the newly- invented time machine).] is not irrelevant. I'm saying that if a loop involves time travel (as the examples in the SEP article do), then we can consider the first ever time travel event in that loop and what preceded it. Unless you are arguing that there is no causality in a loop or that time travel loops and time machines in loops have always existed? Why should causal loops be immune from causality; from having been caused? It seems like a bit of magic.

Not following. There a possibility of a loop that doesn't involve time travel? Example please.
You're then referencing the 'first ever' go-around for a loop that cannot have such a thing. So that's what you mean by 'initial travel event'. There is no such thing for a loop, so I must withdrawm my 'is OK' assessment of it.

Unless more than one person used the same time machine to time travel together. — Luke

Right. Neglected that bit.

I don't understand why the planet would quickly have a population of zero in all timelines though.

well, if everybody had one and knew it worked, I suppose they'd all use it and exit any particular timeline. It's sort of like heaven: The sales pitch is great, but if it's such a better place, why does nobody voluntarily hit the button and go there? It's because from the perspective of the original timeline, it just looks like you vanish, never to be seen again. There is zero evidence that it is safe, let alone works.

Bob would continue to exist on any timeline he travelled to (at least, until he dies).

And Bob is missing from every timeline except one. Of course on the other timelines, there may be many people that attest to having traveled, and the evidence is there that it works. Those timelines would empty out faster than the original, if only from people going back to times when there were still people to meet.
Nobody on these worlds knows who the actual time traveler is (the one that created this world), not even Bob.

But how could I already be there before I time travel?

With a time machine of course. That sort of logic only holds water because there are no time machines possible.

If I go back 250 million years to see the early evolution of mammals, I'm sorry, but humans will never evolve from that timeline.

How?

Evolution is a chaotic function. The popular term for it is 'butterfly effect'. The killer asteroid is not chaotic, so you don't alter that, but evolution is a random process, and you've totally altered that. People are not an inevitable result of the state of 250 million years ago. It's an inexpressibly low chance even without the traveler mucking things up. OK, that last statement presumes a lack of hard determinism. Our discussion has a lot of quantum interpretation implications as well as implications for interpretation of time. The SEP article didn't mention the former.

Maybe it all boils down to this. I'm arguing that causal loops require a start; that there must be an initial time travel event which causes the loop in the first place

No. The whole point of them is that they are uncaused. They'd not be a loop if they were caused. That it doesn't fit in with your notion of singular causality is irrelevant since all those rules must be discarded with reverse causality.

My premises would be that:
P1 - one cannot time travel without a time machine
P2 - time machines need creating at some point
P3 - there must be an initial time travel event following the creation of the time machine, when the time machine is first used to time travel
P4 - the initial time travel event cannot cause itself (e.g. by a prior time travel event using the newly- invented time machine).

I added bold labels. Let me know if I did it wrong.
P1: I said I would accept this for this purpose, but there is no such requirement. If time travel was possible, somebody might be able to do it just by willing it. If a machine can do it, why can't a creature evolve a way to do it. The premise is something like saying you cannot get to grandma's house without a car. Well, that's false since evolution has given us a means of machineless locomotion.
P2 is unacceptable. It's like trying to prove God by asserting that the universe needs creating at some point (which is itself a self-refuting argument). An un-created time machine does not violate any rules in a universe where time travel is possible.

The argument falls apart there: an unacceptable premise, which again, has a counterexample which falsifies it.

P3 seems false. I might make a time machine but never use it. We presume you mean the machine in the loop, so yes, it just happens to get used (the 'first time' say) in the story you are creating. I put 'first time' in scare quotes because there can't be a first time in a loop: there cannot be an odometer on the machine that records how many jumps it has taken.

P4 is OK, but seemingly irrelevant since your story involves only a single time travel event, no loop at all.
I listed them all as postulates (and no conclusions) since none are worded as conclusions. I don't see any conclusion of the impossibility of a closed loop. The whole things doesn't seem to discuss loops at all. It discusses only a created time machine, not a looping one.

By your rules, a person can only be in the presence of but one actual time traveler, even if other people on the timeline also remember time traveling.
— noAxioms

I'm considering Bob to be the first ever time traveller.. — Luke

You're not reading my comment. I said that by your rules, a person can be in the presence of at most one actual time traveler. We could have a factory that made them like bags of cheetos, and everybody used them to get to appointments and catch the traffic light that just went yellow. If they were used like that, the planet would quickly have a population of zero in not just the original, but all the timelines. Despite that prediction, no person would ever be in the presence of more than one actual time traveler, which is the one and only person that created the specific timeline the person finds himself in (if he's still in it and hasn't left already).
Actually, nobody would use the machines, due to the overwhelming evidence of it being nothing more than a self-annihilation machine. So good thing Bob is the only person that has one, and only Bob fails to exist in pretty much any of the timelines.

if they were to travel to the past then that would be altering something about the past.

But the way you describe it, it isn't really the past, just a different timeline which maybe looks like 'the' past, but is actually just another line, 'a' past at best, one of many. There is only one 'the' past, and you didn't go there.

Well, I wouldn't call [a read-only verision] "travelling to the past", That is just somehow viewing the past at the present time of the viewers.

Except he can't leave (turn off the TV so to speak). OK, I agree that it stretches the definition too much. But if he's there at all, history is gone. If I go back 250 million years to see the early evolution of mammals, I'm sorry, but humans will never evolve from that timeline. Your very presence destroys that, although it doesn't prevent the asteroid that wipes out whatever is there instead of the dinosaurs.

the time machine must have been built by someone else in order for it to have been stolen and then donated to the museum. — Luke

Does not follow. That sort of reasoning is only valid if time travel is not possible. The whole point is that it was never built. — noAxioms

You will need to explain why my objection does not follow.

Try to state the logic of your statement formally. What are the premises? How does your conclusion (that the closed-loop machine must have been built) follow? One of your premises is perhaps that all things need creating at some point, but that premise begs a universe with no closed time curves.

If you don't have a time machine then you can't time travel

I accept that premise, at least for purposes of this issue.

so you can't then obtain that time machine (or its technology) via time travel.

Why not? It works, does it not? This is worded as a conclusion, not an additional premise. I don't accept it since 1) it doesn't follow from the premise, and 2) it is easily falsified by counterexample.

Again, try to word it more formally, and the errors will stand out better. I think the example of learning the technology from the future works better than actually being handed the machine.

Which hole are you referring to? Entropy?

The one we discussed: the machine needing to exist for infinite time without showing any wear. Hence better to be handed the plans than to be handed the machine. The movie predestination works that way. It depicts a closed loop, without the infinite-age issue.

The loop could have started only 10 years ago.

It wouldn't be a loop if it had. Loops don't have a start.

he should not be surprised by his sudden appearance at an earlier time, unlike everyone else on the new timeline (who we would assume have never encountered a time traveller before). — Luke

By your rules, a person can only be in the presence of but one actual time traveler, even if other people on the timeline also remember time traveling.
Any witnesses to Bob's first moment's on that timeline will be surprised by the apparent sudden appearance of Bob, despite the fact that he has been there all along and by definition doesn't appear somewhere where he wasn't just before. There simply isn't a 'just before' on that timeline.

Also, instead of considering the new timeline as a copy, you could consider it as a re-writing of history, but one which does not eliminate the original timeline.

That would be a different convention. The new timeline is a rewrote-history according to traveled-Bob, and the old timeline becomes the copy from which he originated.

if they were to travel to the past then that would be altering something about the past

There are stories/scenarios in which nothing is altered. It's more like watching the past on TV since nothing there can detect you.

Incidentally, based on my very amateur understanding, I had thought that once the Einsteinian "time traveller" had returned to Earth, the same amount of time must have elapsed on Earth as it has for the traveller, given the time dilation effects of turning their ship around in order to return. When I read about the twin paradox long ago, I figured that although one twin can be in the future of the other, there is no way to transmit information to the Earthbound twin which could give them advanced knowledge about the future and that they must both return to the same proper time when they meet again. However, I admit that I don't fully understand these things and I'm probably way off. Besides, those sorts of time travel scenarios involving that type of "time travel" are not what I had in mind here.

The twin goes out and comes back, and the two twins are no longer the same age. Not sure what you've been reading, but the proper time going out and back is less than the proper time of a direct path between the two events where the depart and meet up again. None of this requires anything communicating or having knowledge of what the other is doing.
It works better with pregnant women, who make great clocks. Betty and Veronica both get knocked up the same day and Betty takes off for the stars. Betty comes back in 9 months her time and has her baby. She meets who she presumes to be Veronica also giving birth in the same hospital, but it turns out the woman is actually Veronica's daughter giving birth to Veronica's grandchild. 20 some years have passed on Earth since Betty left and the other woman is merely the spitting image of her mother.

Anyway, SEP considers that to be time travel.

The scenario wants us to imagine that this is a logically-sealed causal loop. However, the time machine must have been built by someone else in order for it to have been stolen and then donated to the museum.

Does not follow. That sort of reasoning is only valid if time travel is not possible. The whole point is that it was never built.

It wasn't the time traveller that built it, so it cannot be the donation by the time traveller that causes the existence of the time machine.

The existence is caused by its own time travel to the past. Such is the nature of closed loops. Still, in my prior post I pointed out a hole in that story.

...imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self.

That version works better since it mostly solves the problem I identified.

I don't buy the fact that the time traveller could not have done something else.

The closed-loop scenarios illustrate free will (or more precisely, the lack of it) better than any discussion about reality where there's no pragmatism to it.

There are several fiction stories with closed loops. Predestination is one of them.

[The machine] stays perfectly new at all times, which isn't plausible for something that is thousands of years old.
— noAxioms

I don't follow why it must be thousands of years old.

It takes say 10 years from donation to museum to getting stolen. It ages 10 years during that time after which it goes back 10 years and does it again, and again... Infinite age since it's stuck in a loop. Somebody has to do one excellent refurbishment effort somewhere during each 10 years.

I imagine the causal loop in these scenarios to be a much shorter period than thousands of years.

It's a loop. It has no finite length, just a period, just like there's no end to walking east.

It's not a copy of Old Bob, since he time travels from the original timeline to the new timeline. — Luke

That part is just you saying it. It could be just as easily said that everybody (including old Bob) in the copy timeline is a copy. The machine could split Bob just like it splits everything else. The story doesn't go like that, but the story could go like that. It would still be time travel of a sort, especially from the PoV of the Bob on the created timeline.

What's the point (or possibility) of time travelling to the past if it is to leave the past completely unchanged?

I take it you're not a historian. Those guys would love a machine that lets them go back, even in a way that cannot alter anything, just watch.

This is the only way to avoid contradictions, paradox and violations of causality.

I think we're doing considerable damage to causality if any of this were plausible. OK, the Einstein time travel doesn't violate causality, but I personally don't think that one counts even if it meets the SEP definition.

Right, but I'm attempting to point out why I think single-timeline examples of time travel are senseless, and why I believe that a second timeline is necessary to avoid contradiction or paradox.

The SEP article gives several examples of a single timeline without paradox, Some of the best are the loop ones, including a case where you don't even need to invent/build the machine. You just give it to your younger self when you're done with it.
There is a sort of paradox with that scenario which is how the machine experiences no entropy: It stays perfectly new at all times, which isn't plausible for something that is thousands of years old.

Most designations of identity have pragmatic reasoning and are thus not arbitrary. This doesn't, so the question needs asking, and the answer needs justification.
— noAxioms

I am arguing that Old Bob cannot have been in the past originally, because Young Bob had not yet grown up to build a time machine or to time travel. — Luke

You don't seem to understand my point, which is that there is not obvious convention as to if the old-Bob in the copy timeline is the same old-Bob from the original timeline. The usual conventions for saying this person is the same person that looked like him yesterday. "I bought a can of beans yesterday": True? By convention, yes, the person who bought the can of beans is the same person that submitted this post. We know that because we know the convention. There is no convention for crossing timelines. To me it looks like old-Bob commits suicide, but builds a copy of himself (and the machine) in a timeline with a copy of everything else. The convention could just as easily say that.

This could be achieved by something like 'waiting', which the SEP article categorises as not time travel. — Luke

The SEP does consider travel to a parallel timeline (Meiland, 1974 or Deutsch and Lockwood, 1994) to be time travel despite the lack of rigorous simultaneity convention between separate timelines. Your idea seems to illustrate the same issue. What was this alternate timeline doing before Bob traveled to it? Was it different in any way? Did it have a 'present' 2024 that was altered by Bob's appearance in what was considered to be 1990 at the time?
I don't think your timelines are parallel like the ones discussed in SEP. The question seems more approriate for the above mentioned authors.

However, you have suggested that the inhabitants of the second timeline are merely "copies". — Luke

You seem to regard them as copies yourself, as evidenced by several comments (my bold):

There is no point or possibility of travelling to a destination if you are already there.

So by this wording, the young Bob that gets killed is not Bob. He is not already there, but is rather killing a copy, somebody else, having left the young Bob that is actually himself back in the original timeline unkilled.

The younger self does not time travel; the older self does.

If the two of them were the same person, this would be a direct contradiction. But you seem to regard them as not the same person. So if (actual) Bob goes to some parallel world in 1990, and waits several years for the perfect opportunity to take out the young-Bob copy1 that is there. The moment comes, and he fires his gun only to find it wasn't loaded. Opportunity lost, and there won't be another one. But he has a time machine, so he goes back a day and loads the gun that yesterday-Bob (also a copy) can use to complete his task (of killing young-Bob copy2, leaving young-Bob copy1 un-shot back in the first alternate timeline).

My point of all that is that your comment is true only if you assume yesterday-Bob is a copy. Yesterday-Bob (the one with the empty gun) has time traveled (he's the original Bob). Yesterday-Bob copy has not, him being a different person with a false memory of having time traveled. If he was the same person as Bob, then he very much as time traveled, explaining his presence in 1995 and his memory of 2024.

I'm saying time travel is senseless on a single timeline — Luke

All the examples of 'is time travel' at the top of the SEP article are single-timeline examples. I'm not saying that traveling 'sideways' to a different line is or is not time travel, but I'm saying that those examples cannot all be senseless. Yes, they all have potential paradoxical consequences, all discussed in the article.

My answer was that it is Old Bob from the original timeline who time travels and kills his younger self (on the new timeline). — Luke

My reason for asking was to figure out justification of that assertion. I'm not saying it's wrong, just an arbitrary designation. Most designations of identity have pragmatic reasoning and are thus not arbitrary. This doesn't, so the question needs asking, and the answer needs justification.
You wanted to explore the implications. I'm trying to do that.

Fiction is just that: Non-evidence, so it doesn't in any way constitute an argument one way or another.
Apparently Kirk was also split by the transporter, but not identical. So the story changes as the plot requires.

But it should, in principle, be possible to make a complete copy (à la Thomas Riker), who feels, thinks.. exactly the same as Will Riker. — Walter

This seems to be an assertion against body-soul dualism, not for it. The trek writers have always sort of somewhat presumed monism, but the subject has come up before with Dr. McCoy disliking the transporter since he considered it a copy/suicide machine. He said it always made a copy and destroyed the original. He simply chose a different convention.

Presuming that one isn't lacking something supernaturally critical (such as memories stored in the mind instead of physically), neither would know which is the original, and the designation seems to fall to convention. I might say that the transporter successfully beamed him up and mistakenly left a copy behind. The one on the planet is the copy. That's just a different convention. Identity is just an abstract convention, and the human identity convention stops working in sufficiently alien situations like say mitosis. Say I have 3 fuse strings tied at a common point. One end is lit, creating a flame. The flame gets to the junction and goes both ways down the other two fuses. Which is the original flame? Our convention doesn't answer that, and neither does physics.

You use the word 'soul', which has connotations of an entity with identity that gets held responsible in the afterlife for its choices made in life. If it doesn't hold memory, then it cannot know it is being judged, let alone why. If it does, then it will be quite obvious which Riker was the copy since the copy would have no memories. Maybe, rather than be a zombie (which you seem to assert otherwise), it would be assigned a new 'soul', which seems to absolve it of all the sins of the original, such as that time he took a leak on the captain's chair, earning him the informal title of 'number one'.


All this said, MWI interpretation says you are copied all the time, with no distinct 'original'. The convention is that each copy (and everybody that interacts with it) assumes it is the original. Is dualism then totally incompatible with MWI? I think not. If it were somehow proven, the dualists would find a way to bend their story, but the 'soul' thing would really take a hit. Which one is responsible for some random bad choice made at some point?

I wouldn't call that time travel in the relevant sense. The SEP article attempts to draw the relevant distinction: https://plato.stanford.edu/entries/time-travel/#WhaTimTra — Luke

I read almost all of the SEP article. Thanks for the link. Didn't know it had a page on the subject.
Apparently what I am doing right now does count as time travel, so long as I move. Motion-related time dilation apparently counts according to the article. I would not have said that.

The definition they referenced (but were not totally satisfied with) came from Lewis (1976) who says that time travel is when two different kinds of time (coordinate time and proper time say) don't match. The clock on the wall says one thing, but my watch says another. But physics allows that sort of stuff all the time. One is always proper time, but the other is something else. The SEP article seems not to know physics enough to actually use the correct term of 'proper time', the length (interval) of any worldline, which is what a clock measures if it follows that worldline.

The article doesn't ever reference 'branching', but it talks about traveling to other parallel timelines, like those lines already existed, and apparently already have the time machine appearing from nothing in them, much to the surprise of any witnesses.

The article does seem to accept travel to a 'nonexistent' time in the past under presentism. All of section 4 discusses the various interpretations of time and their implications.

My point was that it is senseless for Bob to travel to the past if he is already there — Luke

Him already being there was the point: To alter what he (younger self) would have otherwise done. I see no reason why the younger self cannot have already time travelled before. Another mistake could be made, 'necessitating' a second correction. I put it in quotes because the mistake cannot be corrected on the more original (more real?) timeline.

I don't think so. I'm assuming that Bob returns to the same past that he lived through when he was younger — Luke

Poor assumption. If I'm to 'kill grandfather', I'd have to go back at least a century. Maybe I want to witness the asteroid taking out the dinosaurs. You can't put in a rule that says you can only travel a short ways to some past with you in it somewhere.

You also contradict yourself. You say on one hand that it is senseless to go to a time when you exist, and on the other hand you're presuming Bob does this 'senseless' thing.

I don't; that's the scenario of the Grandfather paradox. — Luke

Any travel to the distant past will destroy the history you know. Everyone talks about critical events that make a change, but just appearing and stepping on a bug is enough. That said, killing grandpa isn't necessarily paradoxical. Maybe you're not actually related to him, but rather the mailman. I know my grandfather was a cheater. Why can't grandma be?

Call it a copy if you will but this is the only way that time travel is possible. — Luke

Well, besides the fact that it isn't possible in the first place, there are valid scenarios discussed in SEP that allow travel to the original timeline. CTCs are one example.

I assume that it is the Bob (or Luke) from the original timeline — Luke

I don't make that assumption. I try to work it out.

As I replied earlier, I wouldn't call this recreation of another time in the present time to be time travel. — Luke

I'm not recreating a time. I'm just moving a Studebaker forward in time by a century. That's pretty much exactly what you're describing, except in the forward direction. So tell me why that's not what I did. How do you back the assertion that the car didn't travel through time, but Bob (also going forward say) did. Was it the lack of a fancy time machine looking device with blinking lights and stuff? There's plenty of fictions where the machine isn't necessary.

They've done quantum teleportation, which counts as time travel according to SEP. They put something in a booth and it teleported it to another a couple hundred km away. Arguably not time travel, but my question is: Is the thing at the far booth (and no longer at the near one) the same object, or a perfect copy? They were asked this question, and replied: "What possible difference does it make?".

The time traveller was either never at the destination time and cannot return there without contradiction (having two conflicting histories on a single timeline), or else they were always there and therefore cannot "return" there. — Luke

How is the 2nd clause different from the first? Both just seem to say that you can't travel to your own timeline, which is partly silly because I am doing it now. IOW, does forward time travel necessitate a branch in timelines?

One can pretty easily do it just by having a space ship that travels faster than light, and fiction is full of that as well.

Bob must travel to, and insert himself into, a past time at which he didn't always already exist as a time traveller.

Why this restriction? I go back to 1955 (standard destination). Hang around until 1970, and go back to 1960 this time, where "I" already am as a time traveler. What's wrong with that? Can he also make a 3rd branch off the original timeline? Can I, having just made the machine, branch a new line off some other timeline where I never existed in the first place, say some version of 1980 where my parents didn't survive WWII?

Meanwhile, why do you want to kill anybody? The young-Luke you find back there is not you since 'you' is presumably on the original timeline. You've no reason to kill this other person or for that matter, anybody. If you kill yourself, have you killed Luke, or did a copy kill himself?
I think you need to consider the question I asked about the Studebaker in my first post in this thread (about post 57). Is that time travel? If not, why not? What is your machine doing that my example with the Studebaker did not?

I often bring up the famous rhetorical question that Albert Einstein asked his friend on an afternoon walk (I think it was Abraham Pais): 'Does the moon continue to exist when we're not looking at it?'

I think the answer is obviously 'yes' but the question I would like to ask is, why did he feel compelled to ask it in the first place? Why did it bother him? — Wayfarer

The quip was said in the early days of quantum theory when what is now known as the Copenhagen interpretation was 1) pretty much all they had, and 2) was strictly an epistemological interpretation, concerning what was known about a system and not what was. Ontologically, only the Wigner interpretation (leading to solipsism) suggests that human observation has anything to do with what is.

So Einstein perhaps was vocalizing some of the apparent implications of quantum experiments, that one cannot know the state of a system (moon in this case) between measurements. And indeed this is true. The moon is not in a specific state relative to anybody on Earth since it is over one second away and any measurement of it is quite old. That statement is wrong if one presumes counterfactuals.

They give philosophical questions very short shrift. — Wayfarer

That they do. Wrong forum to ask that sort of stuff. But most of the forums that do allow it don't have the sort of expertise found there. I mean, I'm a mod on one of them, and apparently 'top dog' on things like relativity and maybe QM, which is pathetic since I would utterly fail a college level exam on either subject. I learned enough to glean informed implications of both theories on philosophical topics, but not enough to actually do the higher mathematics.

I dabbled in mathematical Platonic realism (especially as it applies to Tegmark's MUH) and found it lacking in explanatory capability. The kind of ontology that makes sense to me simply does not apply to most mathematical structures functionally different than our own. I'm of course not 'most other contexts'.

So many of the actual philosophy forums suffer from a lack of posting standards, resulting in a negligible signal to noise ratio.

A: Yes, but only up to the point where the rate is so high that the interaction between different electrons can no longer be neglected. — Wayfarer

OK, should have thought of that. I was kind of thinking photons, which don't interact with their neighbors nearly as significantly as something like a charged particle. So I pictured a laser weapon aimed at the slits...

It's the nature of that existence which is the philosophical conundrum. It's not as if it's precise position and momentum is unknown, but that it's indeterminable. It will be found whenever it is observed, but the sense in which it exists when not being observed is what is at issue.

The moon was measured. It's still there despite it not being measured at the moment (like it's possible to ever not measure the moon from anywhere as close as Earth). The proton is like that, but with not quite as many 9's to express the probability of it still 'existing'.
The conundrum of which you speak seems to be that the proton in fact hasn't an objective location/momentum at all between measurements. The moon, being classical, isn't like that. But quantum theory doesn't say that the particle doesn't 'exist'. A few interpretations say it does in fact have these properties at all times, but they're just interpretations. The others might still say it 'exists', in the manner of say energy, charge, baryon & lepton number conservation. It can't just not-exist. It just lacks objective properties that put it in a specific state.

Note that if I say something different from the physics-forum guys, they trump me. There are some really solid experts over there, and I don't often respond to questions for fear of putting my foot in my mouth.

Another note: I've not been reading this thread, so not sure how the non-classical nature of QM has to do with 'best argument for (or against) physicalism'.

The Schrödinger equation's solution is called a wave function. If one simplifies the equation considerably it has the form dQ/dt=kQ, which has solutions involving e^it=cost+isint, giving it repetitive or wave-like characteristics. — jgill

Thanks for the clarification, which was mostly about the terminology. Yes, it definitely has wave-like characteristics.

But what is the probability wave, other than a distribution of probabilities? The answer to the question ‘where is the particle’ just IS the equation, right up until the time it is registered or measured. So the answer to the question ‘does the particle exist’ is not yes or no. The answer is given by the equation. So you can’t unequivocally say ‘it exists’ - you can only calculate the possibility that it might. (This torpedoes Democritus ‘atoms and the void’ by the way.)

So - does that mean ‘yes it is?’ - let’s ask noAxioms. — Wayfarer

The probability of measuring some part of a system can be computed from the wave function. I've not heard the result of that computation being referred to as a 'wave', but I'm sure it is somewhere.
Does the particle exist? That's a counterfactual, so there is only a yes/no answer given an interpretation that posits counterfactuals. Quantum theory would simply give a probability of measuring it here or there, or not at all. You can confidently say about some proton that it 'exists' mostly because outside of the sun, protons are pretty stable * and don't just cease existing, so it exists but you don't know exactly where it will be next measured.

I was asked if 'yes it is' is correct, in reference to:

But the wave form of the particle is not the probability wave of the particle is it? — jgill

No it is not. The wave function of the particle describes its quantum state. The probability of where it might be computed from that wave function, but the wave function itself is not a 'probability wave'.

it is well-known that if only one particle at a time is fired in the double-slit experiment, a wave interference pattern still occurs.

Right. This shows that the interference pattern (from a continuous beam say) is not due to the photons interacting with each other.

But the intriguing thing is that even if you increase the rate, you still get the same pattern (up to a point).

Up to a point? What happens if you go beyond that point, other than the slits melting or something? Got a citation?


* ¹⁵O (with a half-life of a couple minutes) decaying into ¹⁵N is an example of an everyday non-violent end of a proton that might be observed in a lab here on Earth. A PET scanner apparently uses exactly this reaction to study oxygen / blood flow.

You said you're creating a new world
— noAxioms
I have never said this. — Luke

Not exactly in those words:

there must be two (or more) parallel timelines in order for time travel to make sense. The timelines branch off into two or more timelines following the first time travel event. — Luke

That says a parallel timeline [world] is needed, created since it doesn't otherwise exist. The 2nd sentence implies the 1990 new timeline branches off the 2024 'travel' event, which means no actual travel, just a universe creation event at 2024.
You say this, but perhaps worded it poorly and meant actual travel to an actually existing 1990, so it isn't something (a whole universe) that you need to manufacture.

my ontology of time involves a blend of presentism and eternalism (in short, that without presentism there is no 'progression of events', — Luke

OK, you acknowledge that the concept of a timeline implies the lack of presentism. There is no need for a 'progression of events'. Time travel under eternalism simply involves a worldline that is discontinuous, or doesn't follow a timelike path. So we ditch the presentism altogether, and that gives us a 1990 destination which we select as our target.

Now we do some retro-causality magic and branch a new timeline off of that point, which doesn't alter the original line at all except for Bob's abrupt disappearance from 2024, a violation of a bunch of conservation laws that we are ignoring. How is this Bob in the new timeline the same Bob as the old timeline? I mean, with the usual parallel timelines (MWI here), the Luke in some other world is not you, but somebody else.

There's no contradictions with it because killing the copy young-Bob isn't killing old-Bob's actual ancestor.
— noAxioms

You can call it a "copy" if you like. There are two parallel timelines, after all: one timeline in which Young Bob grows up to build a time machine and travel to 2024 and another in which Young Bob gets killed by Old Bob. — Luke

Sounds like a copy to me. Old Bob is a continuation of the not-murdered original young Bob, not the Bob that gets murdered.

However, what supposedly happens to Old Bob in your single timeline scenario after he murders Young Bob?

I don't have a single-timeline scenario. Heck, I don't have a scenario at all. Just trying to figure yours out. I've changed my guess significantly based on what you've said and based on some past comments that I read again. Is it better now?
What was our point here? If we can do this impossible thing, no contradiction need exist (except for the magic in doing it). I've long since expressed that the branching solution resolves the grandfather paradox.

This would be the only realistic result — AmadeusD

Sort of like Marty (or his picture of his older siblings) beginning to fade as he slowly destroys any possibility of his parents hooking up. Hollywood loves this idea despite the paradox it creates.

Could you explain why it must be a "new thing, a copy" of 1990 recreated in 2024 and why Old Bob cannot actually travel back to 1990? — Luke

No I can't. You won't let me discuss interpretations at all. You said you're creating a new world, not altering the original, in effort to avoid the paradox. That means an act of creation of a new world.
You said that 2024 is the antecedent state, so that means the alternate (copy) 1990 state was created at that time. It's all I have to work with. I see why the video says it needs a lot of energy.

The original 1990 doesn't exist anymore. You can't travel to somewhere that doesn't exist. You have to create it, a copy of it. I'm running with that assumption when trying to understand what you're suggesting.

There's no contradictions with it because killing the copy young-Bob isn't killing old-Bob's actual ancestor.

What precedes old Bob's appearance in 1990 is the use of the time machine in 2024 — Luke

It sounds like your machine doesn't travel at all then. It manufactures a new world in 2024 that looks like how things were in 1990. It's a new thing, a copy. The time is still 2024, but the calendar hung on the wall is set to 1990. Rather than going through the bother of putting a copy of old-Luke (and the machine) in this newly created world, it would save effort by just creating the world like it was but without young-Luke.

The original 2024 timeline marches on, without you and the machine if the universe-creation process involves the destruction of the machine and its occupant, and still with you if it doesn't involve that and only places a copy of you and it in the new world created.

Anyway, if you hand-wave away all the physical reasons why this cannot be done, I have no problem envisioning time-travel scenarios that are free of paradoxes.

Not sure if there was anyone witnessing the Big Bang — Corvus

We did. It's not like it happened a finite distance away and the view of the bang has already passed us by. Of course the really early events are obscured by the opaque conditions back then. The window through which we look took a third of a million years or so to turn transparent. By that measure, nothing could 'see' the big bang since it was all obscured behind a blanket until then.

Sure, I am not saying it is not allowed to have conjectures and hypothesis on time travel. My point was the claim that "If X, Y, Z, then time travel is possible." remains as a hypothesis until X, Y, Z had been proved as truths which complies to the objective facts in the actual world. — Corvus

Nothing ever gets proved. I can go to grandma's house if I have a car, and the weather is acceptable, and if I draw breath. But technically I cannot prove any of those.
Point is, requiring 'proof' is going to far. Evidence of X,Y Z is probably enough for plausible time travel. Right now, that evidence is very negative.

I am not sure what the physical clock measures. — Corvus

It measures proper time, which is very defined in both interpretations of time. It doesn't measure the advancement of the present, or the rate of the flow of time. That sort of time is more abstract, and there is no empirical way to detect it, let alone measure it. So maybe we're talking past each other when I reference the sort of time that clocks measure, vs you referencing the latter.

But one might still demand to prove the existence of the parallel time lines, before progressing further. — Corvus

Only if it is claimed that they necessarily must be. We're assuming them here to see if it makes time travel possible. It doesn't, but it does remove some of the issues and paradoxes.

From my own perspective, time doesn't exist. It is a mental concept. — Corvus

A physical clock measures something. Hard to deny the existence of something that can be measured.
You seem to get around this by defining time differently than, well than how physics defines it, which boils down to 'what a clock measures'. I agree that the coordinates we assign to time is pure abstraction.

I just transcribed most of the very short video. — Luke

Saved me from typing it. Most of the thanks was for that.

— Luke

As you may recall from previous discussions on time, my ontology of time involves a blend of presentism and eternalism (in short, that without presentism there is no 'progression of events', and without eternalism there is no timeline(s) of events). If eternalism solves a problem for time travel, that's great. — Luke

How about a growing block model then? The past exists. You can go to it, but since it is 'the past', you cannot change it. So a new branch is created (MWI style, but with physics violations), very much like your Bob story. I think that would satisfy both of us. The video presumes (I think) one would have to recreate the entire past state of the universe, hence the excessive energy required.

Oh, then we are in agreement and I've wasted my keystrokes. I thought the grandfather paradox indicated that time travel itself is paradoxical? — Luke

Wasn't wasted. Your Bob example showed how that paradox can be easily avoided.
Another way is to scratch the parallel world and let Bob simply destroy his younger self, and the time machine appears in 1990 uncaused. It's going to do that anyway (in violation of physics), but we're supposed to be ignoring known physics for this exercise.

OK, I said it wasn't paradoxical, but it's still a violation of the physics that we're ignoring. If sending information outside of the cause's future light cone constitutes a paradox, then its still a paradox.

And apparently Bob fails in his effort to destroy the bad thing resulting from his technology.
— noAxioms
Not with the spawning of a new, second timeline (once old Bob time travels back from 2024). — Luke

The old timeline still has the bad technology. It just doesn't have Bob anymore. If it's just Bob that's the problem, he could fix that quick without bothering to build the machine.

The antecedent state would be old Bob's time machine transporting him from 2024 to 1990 — Luke

No, the antecedent state would be 1990 minus 1 second. That cannot produce an old-Bob.
Either that or he didn't actually go to 1990, but simply rearranged the entire state of the 2024 universe to correspond to what it looked like in 1990, which seemingly is what the video envisioned.

Physics doesn't allow a vehicle to just materialize from nothing. But I'm told to ignore this inconvenient problem. Hollywood depicts it frequently, and they can't be wrong, right?