I see it that indirect realism demands the literal exact opposite. An indirect realist would say your visual experience of your house is NOT just your house as it is. That's okay, that's not required for "seeing", it's just a fact — flannel jesus

I said that indirect realists demand that you see your house as it is in itself. I was referring to the thing-in-itself in the Kantian sense. See here, for example. Or, as I said earlier, a God's-eye view.

I've made a point previously in the thread that indirect realists can (and in real life, not in this thread, usually do) use the word "see" in a completely intuitive, conventional way. — flannel jesus

I had a quick look but couldn't find this reply. Could you direct me to it?

I don't need anybody to jump through hoops to know what I'm saying when I say "I can see my house from here". — flannel jesus

I'm sure you don't, but do you mean that you can see your house as it is in itself, as (I believe) the indirect realist demands, or just that you can see your house, as an average person might say it?

At least on the forum, productive discussions of direct vs indirect realism tend to require pinning down where the disagreement is between disputants. — fdrake

As I understand it (which is not very much), direct realists use the words "see" or "perceive" in a conventional manner, taking into account the filter that is the human perceptory apparatus in the act of "seeing" an object. On the other hand, indirect realists, who are unsatisfied with our human all too human perceptory filter, use the word "see" or "perceive" in an unconventional manner that eschews our human filter, demanding a God's-eye-view or view from nowhere in their use of the word "see" or "perceive", all the while pointing out that we have a human filter that colours the real objects of our perceptions.

Not to prejudge the issue, but indirect realists are misusing the language.

I don't consider it a 'part', no. I don't see perdurantist language in the field, so I don't use it. A part of a 4D object would be a smaller 4D object. — noAxioms

Do you at least acknowledge that Floyd is a 4D object according to eternalism? A 4D object requires higher dimensions within which to move, but that’s not the sort of motion you describe in your Floyd example.

The 3D cross section you describe corresponds to a state of Floyd in presentism. Floyd is in one state at noon, and a different state at 1. None of those states move since each is at but one location ever. But Floyd is still said to move in presentism. — noAxioms

What exists in a presentist universe is continuously changing. 3D objects can change their location over time in a presentist universe even if presentists cannot measure that change or do not acknowledge the existence of any other times.

Your argument seems to be equivalant to Floyd not moving because none of his states do. — noAxioms

It depends what sort of object Floyd is. Are you talking about the "states" of 3D Floyd or of 4D Floyd? Your previous descriptions indicate that it is 3D Floyd who supposedly moves. For example, you are presumably not talking about changes in 4D Floyd's spatial location(s), but about changes in 3D Floyd's spatial locations at different times. If 3D Floyd is what you call a "state" of Floyd, then yes, no 3D part/state of 4D Floyd changes its location or moves. 3D Floyd is not earlier at time t₀ and then later at time t₁, because two different 3D Floyd parts co-exist at each of those times. 3D Floyd doesn't change his location between those times because there exists more than one 3D Floyd at, and between, those times.

I will also say that, given a frame of reference to define the hyperplane of simultaneity referred to as 'at noon', then 'Floyd at noon' defines a set of events that comprise a 3D spatially extended region, and that those events are a subset of all the events that are considered to be Floyd.
That's pretty close to the perdurantist wording, but without all the ambiguity and terms with loading meaning. Funny thing is, the statement works under presentism as well, except the specification of the frame wouldn't be necessary. — noAxioms

Okay, but there is still the assumption that the same 3D Floyd object changes its location over time. In order for there to be motion, an eternalist must ignore that more than one 3D Floyd object exists over that time.

The noon-part of Floyd doesn't change its temporal or spatial location, like you assume.

There you go again, putting straw man assumptions in my mouth. — noAxioms

If you don't assume that Floyd-at-noon is the same 3D Floyd object as Floyd-at-1pm, then how could you coherently refer to it (i.e. Floyd the 3D object) as having changed its spatial location (i.e. as having moved) or not? You must assume that the noon-part of Floyd changes its spatial and temporal location in order for it to move.

The definition of motion is a change in a 3D object's position over time.

And reiterating discriminatory definitions as well. I showed that definition to be false even in presentism (the shadow), and you didn't counter it, but rather came up with irrelevant comments about its causes. — noAxioms

Okay then, an object with less than four dimensions. It's still either the same shadow that can change its spatial location over time (per presentism) or else it’s many different shadows that cannot change their spatial locations over time (per eternalism).

I will continue to refer to 3D objects instead of "objects with less than four dimensions" though, only because I assume 3D objects are much more common.

So you've proven what nobody seems to be able to do, which is to falsify eternalism. Kindly detail some empirical falsification test, Love to hear it. — noAxioms

No empirical test is required. It's what eternalism logically entails.

'Floyd at noon' indeed describes a 3D object, yes. — noAxioms

You refuse to acknowledge that Floyd at noon is but a 3D part of a 4D object.

Floyd at 1 is a different 3D object, — noAxioms

Technically, Floyd-at-noon and Floyd-at-1pm are two different 3D parts (of Floyd the 4D object).

but it is all still Floyd, — noAxioms

It is all still Floyd the 4D object. But it is not Floyd the 3D object, which departs grandma's house at noon and arrives home at 1pm. That's because both (noon and 1pm) parts of Floyd co-exist. The noon-part of Floyd doesn't change its temporal or spatial location, like you assume. You appear to suppose that a 3D part of Floyd departs from its temporal and spatial position at noon and then arrives at (and replaces?) the 3D part of Floyd that exists at 1pm. (Not to mention all the 3D parts that exist between noon and 1pm.)

and the difference in Floyd's location over time is, by definition, motion. — noAxioms

Floyd is a 4D object. The differences over time that you refer to here are between different 3D parts of Floyd. Those 3D parts all co-exist. It is not - as you suppose - Floyd the 3D object at one time and then Floyd the same 3D object at another time. It is only a 3D part of Floyd existing at one time and then another, completely different 3D part of Floyd existing at another time. Those 3D parts are not the same 3D object; they are two different 3D parts co-existing at different times. The first 3D part does not move from its temporal location at noon to (replace?) the second 3D part at its temporal location at 1pm. Instead, each 3D part exists at its own time and never moves.

The definition of motion is a change in a 3D object's position over time. This definition does not apply to two diffferent 3D objects or to two different 3D parts of a 4D object; it applies to a single, enduring (same) 3D object over time.

Is velocity also forbidden then? — noAxioms

Generally speaking, no. But in an eternalist universe, yes. Nothing moves in an eternalist universe.

The constant c apparently has no meaning in physics. Hmm... Somebody ought to tell them that they're all talking bunk. — noAxioms

I never claimed anyone was talking bunk. I'm only saying there's technically no motion in an eternalist universe. This needn't imply that there's no motion in our universe, only that if there is motion in our universe, then our universe is not (purely) eternalist. You are borrowing presentist concepts when you treat 4D Floyd as an enduring 3D object over time, which can change its location between noon and 1pm.

It is the 4D thing said to move (change locations over time), not the parts. — noAxioms

As I just explained:

Otherwise, you could argue that what moves is Floyd as a 4D object (or some 4D part of Floyd, or the block universe as a whole). However, that would require higher (e.g. 5th, 6th, etc.) dimensions in which the 4D part/object/universe can move. This is not comparable to the motion of 3D objects. — Luke

You are treating Floyd as a 3D object, not as a 4D object. That is not consistent with eternalism.

Since the 4D object as a whole does not move (or since such higher dimensional motion is irrelevant to our dispute), and since no 3D part of a 4D object can ever move, then there is no motion in an eternalist universe.

But motion isn't defined as an event having more than one set of coordinates. It is a difference of location at different times, and Floyd meets that definition. — noAxioms

Yes, motion of a 3D object. No 3D part of a 4D object can ever be at a different location or time other than the location and time at which it eternally exists.

To meet your discriminatory definition maybe. Floyd is home at noon and at grandma's at 1. That is motion by the definition. That's how the language is used by an eternalist. — noAxioms

No, that’s how the language is used by a physicist who ignores the internal logic of eternalism.

Presentism is a theory of existence, whereas endurantism is a theory of persistence.
— Luke

According to the article you linked, both are alternate interpretations of persistence. Despite what various articles might call them, neither is a theory since they both lack any empirical falsification test. — noAxioms

I think you've misread. I said presentism, not perdurantism.

Surely you mean that a 3D part of the 4D object has one location at a given time and a different 3D part of the 4D object has another location at a different time.
— Luke

That wasn't so hard, was it? — noAxioms

Huh? No, it wasn't hard to correct you.

You need to explain how two different 3D parts of a 4D object can produce the change required for your definition of motion, when neither of those 3D parts ever changes its temporal or spatial location in the block universe.
— Luke

It produces motion by exactly fitting the (not my) definition: Floyd is at one location at one time, and a different location at another. Floyd moves even if what a perdurantist calls his temporal parts do not. — noAxioms

You are again assuming that Floyd is a 3D object. However, under eternalism, Floyd is a 4D object. One 3D part of Floyd is at one location at one time, and a different 3D part of Floyd is at a different location at another time. Both 3D parts of Floyd co-exist at two different times. In fact, each 3D part of Floyd exists at a different time and all 3D parts of Floyd co-exist. Therefore, no 3D part of Floyd changes its temporal (or spatial) location. However, a 3D part of Floyd must change its temporal (or spatial) location in order to meet the definition of motion.

Otherwise, you could argue that what moves is Floyd as a 4D object (or some 4D part of Floyd, or the block universe as a whole). However, that would require higher (e.g. 5th, 6th, etc.) dimensions in which the 4D part/object/universe can move. This is not comparable to the motion of 3D objects.

Okay, but which preferred method of slicing allows for a 3D part of a 4D object to change its temporal or spatial location?
— Luke

Any slicing does this. — noAxioms

How does any method of slicing allow for a (post-sliced) 3D part to change its temporal or spatial position?

Alice2 can only clone herself.
— Luke

All the Alices are herself, and Alice1 made it across the tracks without crashing. Alice4 dies immediately upon coming into existence, and is the shortest-lived Alice. — noAxioms

How does Alice4 (Alice1's clone) come into existence? You say it's a result of Alice2's time travel but you haven't explained how.

presentism holds that only present objects exist.
— Luke

Then time travel isn't possible under that definition of presentism since it would constitute travel to some destination that doesn't exist. — noAxioms

True, but time travel is also not possible under eternalism since nothing moves in a 4D universe.

Objects lack temporal extension under both presentism and endurantism. Both theories face the same problem if there are two or more frames of reference (or "present moments") involved.
— Luke

Presentism doesn't face this problem, because only at most one of those frames can be correct, and probably neither are.
Eternalism doesn't face the problem since the phrase 'present moments' is meaningless. — noAxioms

Fair point. Presentism is a theory of existence, whereas endurantism is a theory of persistence. Therefore, I suppose you're right that endurantism is not positing the existence of any present moment (or of objects at any present moment), but instead says only how objects persist. The way that objects persist according to endurantism is entirely consistent with there being a present moment, since those objects are said to have no temporal extension and are wholly present whenever they exist.

The definition of motion is confined to a presentist view, I agree

No, your definition is thus confined, worded specifically to exclude a view you find undesirable. 'The definition' : 'to change position' isn't so confined. — noAxioms

No, there is no motion in an eternalist universe, as I have argued. Therefore, the definition of motion is only consistent with a presentist view.

An eternalist universe contains 4D objects
— Luke

No, a purdurantist universe contains this. Don't confuse the two. — noAxioms

An eternalist (block) universe is 4-dimensional by definition, and so are its objects.

4D objects are divisible into different/discrete 3D parts
Each 3D part of a 4D object exists at a different time
No 3D part of a 4D object exists at more than one time
A 3D part must exist at more than one time in order to be able to change over time
No 3D part of a 4D object can change over time
No 3D part of a 4D object can change its location over time
Therefore, no 3D part of a 4D object can move, according to the given definition of motion
— Luke

It does not follow that the lack of motion of a 3D 'part' implies the 4D object does not meaningfully exhibit motion. — noAxioms

A 4D object could only possibly move in a higher (e.g. 5th) dimension. A 4D object does not "meaningfully exhibit motion" because no 3D part of it can ever change its temporal or spatial location.

At no point in any of that do you mention that the 4D object has one location at a given time, and a different location at a different time (which is how an eternalist would word it), which is, by definition (not by your definition), motion. — noAxioms

Surely you mean that a 3D part of the 4D object has one location at a given time and a different 3D part of the 4D object has another location at a different time. You need to explain how two different 3D parts of a 4D object can produce the change required for your definition of motion, when neither of those 3D parts ever changes its temporal or spatial location in the block universe.

One can easily just assume that one or more of those 3D parts can move, but it's not consistent with the static nature of the 4D block universe posited by eternalism. To simply assume there can be motion in an eternalist universe, while telling me I'm wrong without actually addressing my argument, is begging the question.

You cannot have two temporal slices at the same time.
— Luke

Nonsense. That's what a frame change is, slicing through the same point (a given event, which has a specific time) at a different angle, which makes for two very different temporal slices. I take it by this that you're entirely unfamiliar with Minkowskian geometry. — noAxioms

Okay, but which preferred method of slicing allows for a 3D part of a 4D object to change its temporal or spatial location?

The only one being "cloned", or the only one who has two versions of themself in existence at the same time, is the time traveller.
— Luke

That was the convention I had initially chosen. We switched to yours. — noAxioms

What you say "was the convention" is my convention; that's what I'm telling you. Whatever you switched to isn't mine.

Where did Alice4 come from?
— Luke

Clone of Alice1, made by the travel of Alice2. — noAxioms

Alice2 can only clone herself. What makes you think she clones Alice1?

Your scenario, as I now understand it, is that Alice1 time travels backwards and "clones" Alice2, such that Alice1 and (Alice1's younger self) Alice2 now co-exist at the same time. If Alice2 now time travels backwards, then she will clone Alice3 (Alice2's younger self) and Alice2 and Alice3 will co-exist at the same time. Alice1 will no longer exist, just as all the people on the timeline when old Bob departs and time travels backwards no longer exist. That's what it means to overwrite the timeline; the timeline reverts back to its earlier state at the traveller's arrival time, except that that time now also includes the time traveller and their time machine.

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. — noAxioms

I didn't realise there were two different definitions of 'happens'. What is the eternalist definition of 'happens'? Can you clarify how it is distinct from the definition of 'exists'?

There are several forms of presentism, so perhaps endurantism is but one of them, — noAxioms

Endurantism is not a form of presentism, it is a theory of how objects persist over time. However, it is entirely consistent with presentism because it holds that objects are wholly present whenever they exist, and presentism holds that only present objects exist.

perhaps 3D presentism, as opposed to growing block, spotlight, and other 4D versions of it. — noAxioms

Presentism is also a theory in its own right, which you could call "3D presentism", I suppose, or simply "presentism". That's the theory I am referring to whenever I use the term 'presentism'. I don't use the term "presentism" to refer to any "4D versions of it".

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. — noAxioms

Agreed.

Actually, there is no Andromeda paradox under presentism, in any of its forms. Presentism denies both premises of special relativity — noAxioms

Objects lack temporal extension under both presentism and endurantism. Both theories face the same problem if there are two or more frames of reference (or "present moments") involved.

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

You seem to be mixing views in that query, rendering the question meaningless. — noAxioms

The question was basically asking if you are a presentist (endurantist) or an eternalist (perdurantist). That's not "mixing views", nor is it a meaningless question.

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

OK, then your definition is confined to a presentist view. — noAxioms

The definition of motion is confined to a presentist view, I agree, and that's because it is inconsistent with the 4D objects/parts of eternalism, as my argument shows.

The shadow of a pole moves, and it isn't a 3D thing. — noAxioms

The shadow of a pole is caused by the movements of the sun (probably) and the pole, which are 3D things. But none of them moves in an eternalist universe.

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.
— Luke

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. — noAxioms

The motion of 3D objects is what we typically mean by "motion" (or change in location over time). I have been considering the (im)possible motion of 3D parts of a 4D object only because it is most comparable to the motion of 3D objects.

You seem to be forgetting that, according to your own eternalist view, "you" are a 4D object. In order for a 4D object to move, additional dimensions would be required, including at least another temporal dimension. If you want to argue that 4D objects move, then that's a whole other discussion.

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. — noAxioms

I'm not using any private definition. I'm using the same definition of motion you gave earlier.

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. — noAxioms

According to presentism, Floyd is a 3D object that changes location over time.
According to eternalism, Floyd is a 4D object that has different (3D) parts existing at different times.

It is simply inconsistent with your own definition of motion to maintain that some 3D part of Floyd can change its location over time, given that all the different 3D parts of Floyd exist at different times and none of them ever changes its location. Which part of the following argument do you disagree with?

An eternalist universe contains 4D objects
4D objects are divisible into different/discrete 3D parts
Each 3D part of a 4D object exists at a different time
No 3D part of a 4D object exists at more than one time
A 3D part must exist at more than one time in order to be able to change over time
No 3D part of a 4D object can change over time
No 3D part of a 4D object can change its location over time
Therefore, no 3D part of a 4D object can move, according to the given definition of motion

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

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. — noAxioms

You cannot have two temporal slices at the same time.

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

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

Could you explain further?

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. — noAxioms

I was drawing a comparison between a 3D (lack of) change in the 4th dimension (time) in the case of 3D parts, and a 2D (lack of) change in the 3rd dimension (length) in the case of the steel bar. So, I understood that the steel bar example wasn't a change over time. That's why it was an analogy.

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. — noAxioms

I'm not interested in the physical possibility of time travel. I'm not trying to build a time machine. I'm interested in the philosophy of time, and the implications on the different theories of time. Let's just assume that it can actually transport people from one time to another.

So Alice2 and Alice4 collide at the tracks, and what happens thereafter depends on your collision resolution description that you're reluctant to describe. — noAxioms

I've said several times that they both die. Why won't you accept it?

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. — noAxioms

That's one way of looking at it, I guess. But it also overwrites the timeline and deletes the timeline that the traveller departs from. I wouldn't call that cloning. The only one being "cloned", or the only one who has two versions of themself in existence at the same time, is the time traveller.

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 — noAxioms

Where did Alice4 come from? Alice1 is still Alice1 after she time travels. She is the original. Time travelling does not create a clone of the time traveller (as a time traveller). The "clone" is the younger version of the time traveller who already exists in the past. If it helps, you can think of young Bob as the "clone" and old Bob as the time traveller.

So it is Alice1 who lands on Alice2 and they die as a result, and then the timeline continues without any Alices.

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. — noAxioms

The observation that "those words can be applied to a block view" doesn't make it logically consistent (with eternalism) to do so. We can observe many uses of words that are nonsensical or logically inconsistent with one theory or another. I don't believe that it is logically consistent, within the theory of eternalism, to apply terms like "happen", "move" or "flow" to objects or events in a block universe.

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. — noAxioms

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

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’).
— SEP Temporal Parts

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. — noAxioms

Then I believe you've misunderstood the article. The introduction states (my emphasis):

Some philosophers believe that you take up time by having different temporal parts at different times. Your spatial parts are things like your head, your feet and your nose; your temporal parts are things like you-yesterday, you-today and you-tomorrow. If you have different temporal parts, this would explain how you can exist at different times, and it would also explain how you can have different properties at different times (you-yesterday hasn’t heard of temporal parts, you-tomorrow is an expert). According to these philosophers, then, persisting through time is pretty much like extending through space: it’s all a matter of parts. — SEP Temporal Parts

This is the perdurantist view. It is consistent with eternalism due to the temporal extension of its objects which are, therefore, divisible into temporal parts. The introduction continues (my emphasis):

Other philosophers reject this picture. They argue that you persist through time as a whole: it’s not just a part of you sitting in front of the computer right now, it’s you, the whole you! — SEP Temporal Parts

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.

To repeat, from the article: "Endurantists believe that ordinary things do not have temporal parts; instead, things are wholly present whenever they exist (things persist by ‘enduring’)." Therefore, the phrase "wholly present" is not, as you say, "a reference to all events in the object's worldline". That is, unless it refers to a presentist object's worldline, which extends no further than the present moment.

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. — noAxioms

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?

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. — noAxioms

Are your tonsils part of "you" in 2000 or not? If not, then the whole presence of "you" in 2000 does not include your tonsils. There is no contradiction.

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. — noAxioms

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

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

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

What I meant was that the given definition of motion is only logically consistent with 3D objects, and that it is logically inconsistent with 4D objects (and their parts). Since each 3D part (of a 4D object) exists at a different time, then no 3D part can move or change its location over time.

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

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. — noAxioms

No, it's entailed by the logic of eternalism.

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.

If it's not the same 3D part/object at one time as it is at another time, then you can't say that the 3D part/object moves or changes its location over time. That's purely a result of the eternalist view of objects as 4D and temporally extended.

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, — noAxioms

You still end up with different temporal parts no matter how you slice it, so my argument against motion still holds.

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'. — noAxioms

If you don't slice the 4D object, then you just have a whole, unsliced 4D object with no parts.

The slicing of spatial parts is irrelevant and therefore so are "0D events". What is relevant to the temporal parts discussion is only (3D) objects with or without temporal (4D) extension. Those are the temporal parts we've been discussing.

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

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

It is analogous because no 3D part of a 4D object can change its location over time, just as no part of a rigid steel bar can change its location along its own length (disregarding the possibility of a steel bar with moveable parts, etc.). If they could do so, then they would conform to the definition of motion given. I've provided an argument for why 3D parts can't move. The only reason 3D temporal parts don't conform to the definition of motion given is because they can't move. Eternalists want to treat time as a spatial dimension, so this should be no surprise.

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? — noAxioms

Does air die/explode? It would be no different to moving the time machine to a particular location in normal time. 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. I don't see understand why you are pressing this point. What difference does it make?

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

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? — noAxioms

It lands on it/collides with it. What's so important about this? Surely we can imagine that the time machine can arrive safely. I'm not interested in defining "safely". I've already said that the machine and its contents can be destroyed, but let's assume it has the technology to avoid it.

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. — noAxioms

Perhaps, but the person who travels backwards the furthest still survives, and time travel is still logically possible. 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.

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?
— Luke

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? — noAxioms

I would refer to the one that time travelled as the original, since there cannot be two versions of a person existing at the same time without a time travel event.

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. — noAxioms

In your previous post, the first Alice lands exactly on the second Alice, so let's stick with that.

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

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. — noAxioms

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.

If Alice2 is the 30-seconds younger version of Alice1 and if Alice2 time travels 5 seconds before crossing the tracks, then Alice2 will not proceeed to cross the tracks, as Alice1 does, and will not time travel after crossing the tracks, as Alice1 does.

However, Alice2 must do exactly what Alice1 does up until (30 seconds before) Alice1 time travels, because Alice2 is just a 30-seconds younger version of Alice1. There are not two version of Alice existing at the same time until Alice1 has time travelled.

If Alice1 lands on and kills Alice2 following Alice1's time travel event, then Alice2 cannot time travel. Alice2 cannot time travel before Alice1 lands on her because Alice1 is the original time traveller; the first one to time travel. And, if Alice2 dies without time travelling, then there is no Alice3.

From last week: — noAxioms

I'll try and get back to this at a later time.

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. — noAxioms

Are you referring to my attempts at such descriptions? Or your attempts? Or just attempts in general?

If you mean the former, then no, I do not see eternalism as containing any flow or motion. 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. Otherwise, you still have not made clear how the existence of an event differs from the fact that it happens. I don't believe there is any difference under eternalism, but you claim there is.

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. — noAxioms

Glad I could help. This is The Philosophy Forum, not The Physics Forum.

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 — noAxioms

The article describes the two main views of temporal parts (or persistence): perdurantism and endurantism. Perdurantism is more aligned with eternalism and endurantism is more aligned with presentism. The way I remember it is they have the opposite starting letter; eternalism = perdurantism, presentism = endurantism. Perdurantism has temporal extension; endurantism does not.

The two most popular accounts of persistence are perdurance theory (perdurantism) and endurance theory (endurantism). 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’). This looks like a straightforward ontological disagreement, a dispute about what exists. Perdurantists think that objects have both spatial and temporal parts, while endurantists think that they only have spatial parts. — SEP Temporal Parts

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. Eternalism has 4D objects (with 3D parts). Presentism has 3D objects.

What was you argument against that again? — noAxioms

My argument is that the definition of motion as 'a difference of location over time' is something that can only apply to 3D objects. This is due to my argument against motion in an eternalist universe or argument against motion of 3D parts (of a 4D object).

My argument against 3D parts (of a 4D object) moving or changing location over time is that all the 3D parts of a 4D object are different and exist at different times. Since each 3D part (of the 4D object) exists at a different time, then no 3D part moves or changes its location over time. It would be analogous to part of a steel bar "moving" along its own length; it doesn't happen.

Do you deny this definition, or deny that it applies to either view? — noAxioms

I don't deny the definition. It applies to both views, but there is no change of location of any 3D part in eternalism.

Also, it was you that took the break, never replying to anything from my post a week ago. — noAxioms

What are you talking about? Before you took a break, I posted this reply to you. You never responded and were absent from the discussion for a week afterwards. Then, two days ago, you posted:

Been away a while. — noAxioms

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

Then time travel is mostly impossible the way you envision it since there is always something (air, dust, bugs, trees, whatever) at the destination, — noAxioms

How would air, dust or bugs at the destination prevent time travel? If the machine can time travel, then it can probably find a safe place to arrive.

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. — noAxioms

The quote was from over a week ago. I didn't "suddenly suggest" it.

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,. — noAxioms

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

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? Apparently not, since those two Alices die after one lands on the other. So, where did "Alice behind" come from? However, now a third Alice approaches the tracks to find the wreck of the collision that killed the other two Alices. Where did third Alice come from? Was it only the first Alice who time travelled? If these are different people then why did you call them all Alice? This is very confusing.

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. — noAxioms

You've made this unsupported accusation several times. What makes you think I'm ignorant of the theory of eternalism?

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'. — noAxioms

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.

All this is a side topic. — noAxioms

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

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? — noAxioms

I'm not being "evasive". I've already answered this:

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. — Luke

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. — noAxioms

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

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.
— Luke

3D parts of what?? — noAxioms

3D parts of the 4D object. I thought I made that clear. I said the 4D object consists of 3D parts.

Any object (a car part say) occupies a 4D volume of spacetime. I can't think of a 'part' that is 3D. — noAxioms

Whether or not objects have extension through time is the subject of our temporal debate, not a given. Otherwise, you are begging the question.

If all objects are 4D objects and have temporal extension, then at any given time only a (3D, temporal) part of that 4D object exists, and the different parts of that object exist at different times. A presentist might say instead that the object does not have temporal extension and that the same 3D object exists at different times. For example, you are the same object/person you were as a child. Whereas an eternalist might say that you-as-an-adult and you-as-a-child are different (3D) parts of the same 4D object.

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. — noAxioms

Yes, I'm referring to a whole 3D object at any given time (of its existence) or, alternatively, to a 3D part of a 4D object at any given time (of its existence).

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. — noAxioms

No, it's not true under both views. Under presentism, the same 3D object exists at different times. Under eternalism, different 3D parts (of the same 4D object) exist at different times. That's why 3D parts cannot be said to move; it's not the same 3D part at two different times.

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. — noAxioms

Right, that was my point. I don't understand why you are directing these comments at me instead of at @Michael.


Also, please learn how to quote text on this forum. The formatting of your posts makes them difficult to follow (and to reply to). After you highlight some text, a little box should pop up somewhere on the screen with the word "quote" in it. Just click on the little box.

I'm simply presenting an alternative view, I'm not trying to argue against your view. — Michael

But I am arguing against your view. My argument against motion in an eternalist universe does not allow for the motion of consciousness over time if that consciousness has a physical basis; where the motion of that consciousness is dependent on the motion(s) of the physical host.

The traditional view is a presentist one, where 3D objects move over time.
— Luke

Yes, and this is apparently in conflict with general relativity (and time reversibility?). — Michael

How is it in conflict?

So I'm offering a hypothetical solution that might resolve the conflict between this and our everyday experience of the (one-way) passage of time. — Michael

Your solution seems to be simply that consciousness moves when nothing else does. This does not explain why we experience a one way passage of time, it only allows for us to do so; it presupposes the experience of time that we have.

If consciousness is underpinned by physical stuff, then how can it move when none of the physical stuff does?
— Luke

I don't know. Much like I don't know how, according to physical presentism, physical objects can move through the unmoving space(time) that underpins them. Again, I'm spit balling. I don't have some consistent and complete mathematical model at hand. — Michael

It’s not really analogous. We can see the effects on consciousness from damage/changes to the brain, for example. There is no similar study we can do to see how damage/changes to “unmoving space(time)” affects physical objects.

If dualism is correct then the physical and the mental need not necessarily behave according to the same laws. — Michael

That’s not something I believe, and I doubt it takes into account the facts as we best understand them.

The traditional view is a presentist one, where 3D objects move over time. You are simply ignoring my argument. If consciousness is underpinned by physical stuff, then how can it move when none of the physical stuff does? Why should you be able to treat consciousness as a presentist object in an otherwise eternalist universe?

I understand the concept. As per my quote of Herman Weyl in the other discussion I referred to recently:

The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the life line of my body, does a section of this world come to life as a fleeting image in space which continuously changes in time. — Hermann Weyl, Philosophy of Mathematics and Natural Science (1949)

But if the host’s consciousness supervenes on the physical body of the host, then you need to counter my argument to explain how that consciousness can move, given that the physical host does not.

My random idea is that the physical host is something like a tunnel and consciousness the occupant. The tunnel is fixed in time and space with consciousness travelling through it. — Michael

I understand. However, I’m assuming that the consciousness has a physical basis on the host. I’ve given arguments for why neither the 4D host nor a 3D part of that 4D host moves. Therefore, I can’t see how the consciousness of either the 4D host or a 3D part could move.

I suppose you could posit some sort of disembodied consciousness, but I think any reasonable philosophy needs to take into account the facts as we best understand them.

Just spit balling but how about:

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

The consciousness of what? If you mean the consciousness of the 4D host, then that is extended across time and doesn’t move. If you mean the consciousness of a 3D part of that 4D host, then all the 3D parts are different and none of them moves.

I have presented my views on time previously on the forum, e.g. in this discussion. The argument I present below can also be found in the same discussion at this post, although I have tried to strengthen it a little more here.

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 that an object is 3D and traverses time. 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.

If we consider that the motion of an object is basically a change in its position over time, 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.

If I travel from London to Paris, then I am considered as a 3D object that departs from London and arrives in Paris. Before I have departed from London, I do not yet exist in Paris and after I arrive in Paris, then I no longer exist in London. However, this is not the case in eternalism, where I exist as a 4D object, not as a 3D object. In eternalism, I exist across time with a part of me existing in London at one time and another part of me existing in Paris at another time. But the 3D part of me that exists in London is not the same as the 3D part of me that exists in Paris. No part of me departed from London because that (London) part always exists there, and no part of me arrived in Paris because that (Paris) part of me always exists there. And all the parts of me in between London and Paris always exist there. In a 4D object, there is no change in position or time of a 3D part, because all the 3D parts are different.

It is only in presentism, where the same 3D object can change its position over time, that an object can move.

Eternalists may treat a 3D part as though it were a 3D object, but in doing so they should recognise that they are adopting a presentist view.

I'm aware of the different theories. I'm not keen on committing to the objective present moment of the Moving Spotlight Theory,

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.

As I explained previously in the discussion:

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). — Luke

I believe that a combination of both views of presentism and eternalism are required to coherently account for time. — Luke

So, I don't consider myself to be a presentist, either, but I do think that presentism should not be entirely rejected. Its progression - the natural progression of a present moment - is very important to our concept of time, and this belongs entirely to presentism imo. Even if the concept of an objective present moment is rejected, I don't believe that progression - the march of time - associated with it should be also. This temporal progression is too easily assumed as equally belonging to eternalism or as logically coherent with eternalism, but I don't believe it is. This progression is also a very difficult concept to quantify or to describe in language. I believe this is partly why it is so commonly rejected by eternalists.

Well, I think any reasonable philosophy needs to take into account the facts as we best understand them. According to General Relativity time is the fourth dimension of spacetime. Talking about "overwriting" the "timeline" is like talking about "overwriting" the "heightline" or the "widthline" or the "lengthline". It seems pretty nonsense.

What does it mean to "overwrite" a direction in space(time)? — Michael

We were discussing time travel. The question posed by the OP is: is time travel to the past hypothetically possible? I'm sure you've heard of time travel before, where a time machine is used to transport the user to a different time. I never suggested that time travel was genuinely physically possible according to our known physics and/or technology, but if it were at all hypothetically possible, then the time traveller would supposedly travel to a time that they had never visited before (at least, in their role and at their age as a time traveller). If we assume backwards time travel, where the arrival time is earlier than the departure time, then, prior to the time traveller's arrival at their destination time, there exists a history which is has not yet been visited by the time traveller. Once the time traveller arrives at their destination, then that unvisited history changes, or is overwritten, by the time traveller's inclusion in that history. That's what it means to overwrite a timeline, or history, or an earlier time, as supposed in many fictional time travel scenarios.

I've never made any claims about overwriting a direction in space(time). That would assume an eternalist view of time, in which time is treated much like a length, or as another spatial dimension. Whereas - prior to the untimely demise of this discussion - I was seeking to explore the limitations of eternalism, such as its logical omission of progress, happening or motion; characteristics that I consider to be absent from eternalism but logically aligned with the opposing view of presentism. However, many eternalists disagree.

I find it worthy of an interesting philosophical discussion, but I suppose you've made up your mind already. 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". You could always just mind your own business.

Well, I think any reasonable philosophy needs to take account of the facts. According to General Relativity, time is the fourth dimension of spacetime. — Michael

I'm reasonably sure General Relativity is a theory, not a fact.

Talk of "overwriting" the "timeline" is like talking about "overwriting" the "depthline" or the "widthline" or the "heightline". It seems pretty nonsense. — Michael

It might seem nonsense if you adopt an eternalist view of time, but that's not a fact either.

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. — noAxioms

If Alice0 is the original, then how is she able to see Alice1 driving off on the other side of the tracks as you claim? I thought that the car on the other side of the tracks was the original Alice after she had time travelled. Since she hasn't yet time travelled, then she cannot possibly see a copy of herself driving off on the other side of the tracks. The car on the other side of the tracks must be the original Alice for the scenario to make any sense.

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. — noAxioms

What is Alice1's origin story? How do two Alices exist before any time travel occurs?

You seem only capable of imagining the traveler, just like Hollywood only follow the protagonist. — noAxioms

I think I'm quite capable of imagining both perspectives.

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. — noAxioms

If Alice0 has never time travelled, then where did Alice1 come from? As I said,

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? — noAxioms

The time travel in my model does overwrite the timeline, yes. I was referring to your Alice and the train tracks scenario, which does not. Hence, your scenario is an ineffective argument against my model.

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. — noAxioms

Your argument is supposedly that my presentist model entails a blank future universe. However, as I said, I am neither a presentist nor an eternalist, but a mixture of both. I could say that the future has a definite physical existence prior to the time travel, but it gets overwritten by the forward time travel event anyway.

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. — noAxioms

You can also go forward a few hours without having to experience waiting simply by sleeping (or by being in a coma). However, the SEP article designates sleep, coma, cryogenics and waiting as not actual time travel.

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. — noAxioms

Really? 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).

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. — noAxioms

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? I imagine that the time machine can immediately transport the passenger from one time to another.

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. — noAxioms

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?

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. — noAxioms

Right, tables don't happen but events do. I should have qualified this to say "happens" is no different to "exists" in relation to events. But I thought this was already implied by the context.

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. — noAxioms

This isn't making any distinction between "exists" and "happens". You could exchange these words in your above statement without changing the statement's meaning.

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. — noAxioms

I didn't drag them in; you did with your Titanic example. The time at which an event happens, or is happening, is the preferred moment ("preferred" in the sense that it has not already happened and is not yet to happen but is, as you say, "somewhere in between").

In your Titanic example, you referred to a time when the event had not yet happened and to a time when the event had happened, but you also want to distance yourself from saying that the event is ever happening. This is illogical. If it is going to happen (relative to some time earlier than the event) or if it did happen (relative to some time later than the event), then there must also be a time when the event is happening (relative to some time simultaneous with the event).

You noted that your Titanic description of what it means for events to "happen" in an eternalist universe is "not much different than presentism". I agree. I think "happens" is a presentist term (i.e. which only makes sense in a presentist universe) which makes little sense in an eternalist universe. It is my view that nothing ever happens in an eternalist universe. If nothing is ever happening, then nothing will ever happen or did ever happen. The word loses its meaning, except as a synonym for "exist".

You clearly reject any sense of the word "happen" which is associated with a "preferred moment", yet you say your Titanic description involving the word "happen" is exactly like presentism, only without the preferred moment. I don't think anything remains of presentism if you subtract the preferred moment. Therefore, if "happen" has the same meaning in eternalism as it does in presentism only without the preferred moment, then I think the word "happen" (in relation to events) under eternalism loses its conventional meaning and can only be used synonymously with "exist".

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. — noAxioms

Right, "happen" should not refer to any transpiration or progression of events in eternalism. I agree. For the sake of consistency, it should be used to mean nothing other than "exist".

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? — noAxioms

You referred to these different "ontological states" in your Titanic example. Here it is again:

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'. — noAxioms

I thought your example was intended to clarify the meaning of "happen" in eternalist terms.

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. — noAxioms

The reason I asked was because you said:

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. — noAxioms

You seemed to be arguing that there are no events before the big bang even though there are times before the big bang, therefore falsifying my statement that "there exist events at each (and every) time. Do you believe that there is time (or that there are times) before the big bang? If not, then I don't follow your argument.

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. — noAxioms

Under eternalism?

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. — noAxioms

I said that it is not required that every event is accounted for, so I don't have any "bit about a requirement for all events being accounted for".

The phrase "nothing is happening" is not a meaningful one in an eternalist universe, — noAxioms

Right, even the use of the word "happening" isn't meaningful. That's because nothing happens (in the conventional sense) in an eternalist universe.

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. — noAxioms

Your examples of the scenario were:

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. — noAxioms

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. — noAxioms

The problem is that you provided very little detail so it is difficult to follow what is happening in this scenario. Presumably, the DeLorean is a time machine, like in Back to the Future. So, Alice gets to the train tracks and has to stop because the gate comes down. 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. What I don't understand is, after she does this, why is there another DeLorean behind her getting stuck at the gates? The time travel event in your scenario does not overwrite the timeline.

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. — noAxioms

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. 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? 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? In that case, I don't understand why the time machine is needed.

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). — noAxioms

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

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. — noAxioms

I made a mistake in my last post. Sorry, I've had COVID recently and wasn't thinking straight. I started out writing the post thinking that you don't meet yourself going forwards, but then I somehow reasoned myself out of it. So 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.

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. — noAxioms

I never said that. Once again, what I said was:

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. — Luke

The timeline that would have existed if there were no time travel events gets overwritten by a timeline with a time travel event. It gets overwritten in the same sense as backwards time travel: it inserts a time traveller into the timeline that originally had no time travel events. And, as per my correction, since Bob departs an earlier time for a later time, then there isn't a copy of him left behind who ages normally.

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? — noAxioms

It is. I've said that what would have existed gets overwritten. We can imagine how the timeline would have existed if there had never been any forwards time travel, just as we witnessed how the timeline did exist without any backwards time travel just prior to the backwards time travel event.

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. — noAxioms

Right, thanks for the correction.

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. — noAxioms

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

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. — noAxioms

Why do the observers need to prove it?

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. — noAxioms

I don't know why you keep wanting to treat processes like points/events. I don't see the relevance.

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. — noAxioms

It follows from this that "happens" is no different to "exists". It happens when it exists and exists when it happens - there is no distinction.

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? — noAxioms

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

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

There are no such ontological differences. — noAxioms

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'." This implies that there is a time when the sinking has not yet happened, and a later time when the sinking has happened, and a time between these when the sinking happens.

I can see how the view might be difficult to learn from that source. — noAxioms

I didn't find it difficult.

Only the latter statement is true under eternalism, and the paragraph above seems not to clarify which meaning is meant. — noAxioms

The article distinguishes between "x exists now" in the temporal location sense and in the ontological sense. The distinction is clearly made.

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. — noAxioms

Is "before the big bang" part of spacetime? 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.

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' — noAxioms

.

It is only meaningful under eternalism to say that the Titanic has not yet sunk or that it has sunk and that "Somewhere between those two events does the sinking 'happen'? It seems like the past or future tense of 'happen' is acceptable, but that nothing happens (except sinking?) in an eternalist universe.

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. — noAxioms

She simply decides not to hit the button again. You didn't provide any information in your scenario about why she time travels. Presumably she does it to avoid being hit by an oncoming train. Maybe she realises she can't keep looping back every 30 seconds forever and tries something different instead. Maybe she tries jumping from the car. Maybe she resigns to her fate.

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. — noAxioms

What fictions involve time travelling to a future time where nothing exists; a "blank universe"? What fictions involve waiting for the future to happen first, before time travelling to it?

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.

It takes 11 years to write that future state (assuming an 11 year jump. — noAxioms

Time traveller Bob departs the current time to arrive 11 years in the future. He does not stay behind.

It also clones Bob. — noAxioms

The only cloning that happens is if Bob travels to some time within his own lifetime and, even then, you would probably consider it cloning only if his time travel departure and arrival times were very close to each other, e.g. if he time travelled to 5 minutes ago or a day forward. I wouldn't really consider an 11-year younger or older version of Bob to be a clone or a copy of Bob. And, although my concept of time travel may have these sorts of strange consequences, you are yet to have proven it illogical.

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. — noAxioms

If Bob succeeds in time travelling, then F-Bob does not exist. F-Bob only exists if Bob fails to time travel

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). F-Bob only exists in timeline A and S-Bob (the time traveller) only exists in timeline B.

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. — noAxioms

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

Anyhow, your example doesn't leave a copy behind. When Bob time travels to a later time, he departs from the earlier time; he doesn't stay behind. However, he will meet an older version of himself in the later time (assuming that he travels to a time within his lifetime).

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.

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'. — noAxioms

That's right. If there is no time travel event then the machine doesn't disappear. The machine only disappears if there is a time travel event. I didn't say that there both is and isn't a time travel event, such that S-Bob and F-Bob both exist on the same timeline. What I said was:

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

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. — noAxioms

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. You spoke of the time before the Titanic event when "it has not yet sunk" and the time after the event when "it is at the bottom of the ocean" (i.e. when it has sunk). You also said that "Somewhere between those two events does the sinking 'happen'." You noted "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'."

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.

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."

Does eternalism allow only for the different ontological states of 'not yet happened' and 'happened', but not for the ontological state of happening? Does eternalism have anything to say about the process that changes an event's ontological state from 'not yet happened' to 'happened', or about any ontological state(s) between those two?

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. — noAxioms

It seemed to follow from your Titanic example, where you spoke of times when the event had not yet happened and when it had happened, and you also referred to the "ontological state" of 'happening'. You will need to clarify whether any such events can happen or do happen or are happening in an eternalist universe.

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. — noAxioms

To be clear, I was using the word "exist" in the latter, "ontological sense" given here:

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

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. — noAxioms

You should learn that there are two ways of interpreting my sentence. I wasn't saying eternalists hold that each event exists at all times. 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.

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

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

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. — noAxioms

What I said is that I fail to see how it ends the timeline. You seem to refuse to comment on that. Your examples show only that my concept of time travel could have terrible side effects, such as the end of humanity or the possibility of cloning an army, which I don't see as being relevant to whether or not my concept of time travel is logically consistent. Even if your examples were to show that the timeline does end, this would obviously be disastrous, but it would not prove my concept of time travel to be impossible.

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. — noAxioms

It paces back and forth only for as long as Alice continues to hit the button to go back every 30 seconds. As far as I can tell, nothing forces her to keep hitting the button.

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. — noAxioms

1990 does not have multiple states, because it is either overwritten or it is not.

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

Neither did I. — noAxioms

I'm quite sure you did, because it's exactly what you proceeded to say:

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. — noAxioms

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.

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

It makes sense to leave a copy of Bob behind? No time-travel fiction portrays it that way. — noAxioms

Why would it leave a copy of Bob behind?

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. — noAxioms

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

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

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. — noAxioms

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 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. — noAxioms

I think I understand the scenario. What I don't get is how/why the timeline ends as a result.

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

Not sure how to word it differently. The Titanic sinks on some 1912 night. That is a statement of something that happens. 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'. — noAxioms

To 'sink' typically denotes a process wherein an object descends from the top to the bottom of a body of water or a liquid. 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. This "preferred" moment is the state that is happening, as opposed to those that have happened or have not yet happened. As you note, this sounds a lot like presentism. You are apparently saying that in order for an event to happen, it must be present. However, I note that this is inconsistent with your earlier agreement to my statement that, "There is no past tense of events having existed or having happened for the Eternalist."

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

You said events happen repeatedly. — noAxioms

I said an event would happen repeatedly at the [its] same temporal location.

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

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. — noAxioms

I don't think there's any difference. I was only aiming for more emphasis and clarity with 'en masse'.

I was trying to say that, if an event exists then it is happening. Since eternalists hold that events exist at all times, then they must also happen at all times, without the pesky requirement that they only happen when they are present. Therefore, all events are always happening - en masse, at once? - at each of their respective times, rather than them happening only when they are present. That's why they must also be repeatedly happening at each their respective times. Otherwise, the Titanic would have sunk before 1912 became present. The sinking of the Titanic event/process, with a short duration of a day or two, would have stopped happening way before 1912 if it were not repeatedly happening.

BTW, this is my argument ad absurdum against events happening in an eternalist universe, not what I think actually...happens.

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

So an event needs to be present in order to happen? Or can past and future events be happening (now) even if they are not present, given that those non-present events presently exist at their respective temporal locations?

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

Nothing of the sort is suggested. — noAxioms

You suggested it with your Titanic example:

The Titanic sinks on some 1912 night. That is a statement of something that happens. 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'. — noAxioms

The "preferred" moment is the state that is happening, as opposed to those that have happened or have not yet happened.

The whole point of the train track exercise is to figure out how to get Alice out of the loop. — noAxioms

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

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. — noAxioms

I fail to see how your examples of multiple time travel events end the timeline. I only see that they end the existence of humanity, which is not the same. With truncation or overwrite, time still marches on.

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. — noAxioms

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?

No. You need the 2nd line to order all the different times that a given year appears. — noAxioms

No. A given year appears on the graph only once but that same year can be overwritten. Since events are overwritten, the same graph line returns to 1990 and starts (writing) again. This is virtually the same as if the timeline were truncated back to 1990. There is no 2nd new line. The time machine's creation event and departure from 2024 get deleted/overwritten in the history of events. Your graph shows a timeline that is not overwritten, but I'm saying that it is.

Forwards time travel is just like backwards time travel.
— Luke

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. — noAxioms

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

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.
— Luke

Why would you want that? There seems to be no point. — noAxioms

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

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. — noAxioms

I don't believe I have ignored it. You said the result would be a bunch of cars all arriving in the same location causing a black hole. This does not explain how the timeline ends. Otherwise, I do not understand how the timeline is supposed to end in your Alice example. As I said earlier, your examples all indicate the end of humanity's progression but I don't follow how they indicate the end of the timeline's progression.

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
— Luke

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. — noAxioms

You seem to have lost track of the discussion. I never suggested that eternalism involves overwriting or writing. We were talking about my time travel example, which does involve overwriting the timeline. I said - in relation to my time travel example - that I understand that an eternalist would prefer for the overwritten section to remain in existence, precisely because eternalism does not involve change or things going out of existence.

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. — noAxioms

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

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

I think not. — noAxioms

Okay, then the terms "happen" and "occur" are synonymous with the word "exist" (at least, in relation to events).

I mean, by calling it an event, an implication is made that the event exists at a point in spacetime, — noAxioms

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

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'. — noAxioms

You've just told me that the terms "happen" and "occur" do not mean anything other than that the event exists. Now you're saying those terms do mean something other than that the event exists? "Happen" and "occur" are not synonymous with the word "exist"?

Also, since eternalists treat time as a spatial dimension, then 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?

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. — noAxioms

As I said, I had in mind events with a duration of more than a point in time (i.e. processes). Those events (i.e. processes) have a beginning and end point. An event (i.e. process) is conventionally considered to end at the end point of the duration of the event. However, if eternalist events do not cease to exist (as eternalism claims), and if "exist" and "happen" are synonymous under eternalism (at least, in relation to events), then eternalist events do not cease to happen; they never stop happening.

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.
— Luke

Ouch. No! There is no repeat. They happen once. An event cannot have multiple temporal locations. — noAxioms

I never said an event "has multiple temporal locations". I said "all events are happening en masse at their respective times."

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. — noAxioms

Sure, eternalism can allow for a sequence of events, but what does it mean for those events 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. — noAxioms

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. Incidentally, I would have thought that the sinking of the Titanic was a process rather than an event.

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
— Luke

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. — noAxioms

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. All events exist and happen at each of their respective times. Since they all exist and happen at all times, I don't follow why they should happen in a sequence from earlier to later, so that they apparently happen one after another. This suggests that there is a "special event which is 'current', which moves along [the] worldline," which sounds very much like 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.

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."
— Luke

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. — noAxioms

I don't see two different usages of the same word ("progression"). Please define the two different usages/meanings.

Not sure what black holes have to do with our timelines. I don't anticipate either of our lines being in a black hole. — noAxioms

This was part of your Alice example, which is what I replied to, in which a black hole resulted from the Deloreans all arriving at the same location.

Someone could see it disappear, I suppose
— Luke

This seems contradictory. — noAxioms

Okay, maybe you're right. Nobody can witness the departure of the time machine.

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".
— Luke

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. — noAxioms

I don't see that there's much difference between 'overwrite' and 'truncation'. Post-time travel, on the truncate model, the history of events from 1990 to 2024 is deleted and the resulting blank period from 1990-2024 gets overwritten by a different history of events. On the overwrite model, the history of events from 1990-2024 is retained but gets overwritten by a different history of events. It makes little difference post-time travel.

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

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. — noAxioms

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.

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. — noAxioms

If I understand you correctly, the two dimensions of time are date order/calendar time and "the time containing the first kind of time". It's not clear to me what you're referring to by "the time containing the first kind of time".

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. — noAxioms

It would be reset to a prior state if not for the additional insertion and subsequent effects of the time traveller and their time machine into that earlier time.

And how does the graph look if the (1990-2024) history is deleted/overwritten by the new timeline? Because that's what I'm saying.

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. — noAxioms

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

One cannot fully understand your view unless forward travel is described. Sticking to this one-backward-jump case leaves several open questions. — noAxioms

Forwards time travel is just like backwards time travel. 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.

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

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

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; because its going out of existence is at odds with eternalism.

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

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. — noAxioms

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, especially since, as you say, "there is nothing that 'progresses' at all." Do the terms "happen" and "occur" mean anything other than that the event exists?

If Eternalists take "exist" to be synonymous with "happen" then, since Eternalist events do not cease to exist, they must also not cease to happen. There is no past tense of events having existed or having happened for the Eternalist. 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. Just as Eternalist events do not cease to exist, they also do not begin to exist, and therefore they also do not begin to happen; they have no beginning or end. 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, and they are happening over and over again (at their respective times). Or, it may not be the period of each event (of a different duration) that repeats; it could be more regular periods such as minutes or seconds that repeat. Otherwise, nothing happens on an Eternalist timeline and everything simply exists.

Therefore, there is no such thing as travel?
— Luke

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

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."

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. — noAxioms

Since the pre-time travel timeline gets overwritten by the post-time travel timeline, then the time machine's creation would be overwritten. My view supports that.

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. — noAxioms

Even with the black hole option - which I'm not ruling out - I don't see that time travel is impossible or logically inconsistent. So I don't see that it really matters whether I say that the time travelling cars can all occupy the same space or not. Black holes exist in our universe and they haven't ended the timeline.

This is just a repeat of what was said before, without answering any of the questions. — noAxioms

Your responses indicated that you were unsure of my concept of time and time travel. I was attempting to clarify it for you.

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? — noAxioms

Roberta won't have any experience if she is overwritten, because she will cease to exist. As I said in my last post, some people "may simply disappear from the timeline" as a result of time traveller Bob's actions post-time travel.

What is the experience of Sue when she still exists, but has her marriage and all her history overwritten? — noAxioms

It will be whatever her experience is in the post-time travel timeline. I'm not sure what sort of answer you are seeking. Since older Bob (the time traveller) has killed his younger self, then Sue won't meet young Bob or have children with him.

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

Someone could see it disappear, I suppose, but yes, basically.

You've given no clue how it can go forward to some piece of history that has yet to be written. — noAxioms

Because I asked early on in the discussion for us to get clear on one backwards time travel event first.

I have no issue with the word 'truncated'.
— Luke

You balked at that before. — noAxioms

I wasn't sure what you meant by it before. I still don't believe that the timeline gets truncated or shortened permanently.

So no overwrite, but just truncation, and a new building onto 1990, not overwriting some alternate future that no longer will happen. — noAxioms

If this is your interpretation of my view, then it's incorrect. 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".

You are correct that the pre-time travel period of 1990-2024 "no longer will happen", but only because it already did happen. That pre-time travel 1990-2024 period which did happen gets overwritten by the post-time travel history from 1990 onwards that happens subsequent to it.

Robert is immediately gone, and never was, and never will be, in the world timeline which is presently at 1990. — noAxioms

I assume you mean Roberta, and yes, this is all true once the time traveller kills young Bob.

The time machine now exists without having been created since its creation has been truncated off. It doesn't exist and never will. — noAxioms

The time machine was created pre-time travel, circa 2024. I think you mean that it won't be created again on the post-time travel timeline (exactly as it was created originally by young Bob on the pre-time travel timeline). This is true. However, it definitely does exist in 1990 post-time travel. Why wouldn't it? It transported the time traveller to 1990 from 2024. It will continue to exist (post-time travel) until its destruction.

Sure, its creation exists on Bob's line, but most of Bob's line is not part of the universe, but just a memory. — noAxioms

Okay then, it's just a memory. 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 don't really need to maintain the existence of the pre-time travel timeline for that purpose. Although I understand why an Eternalist would prefer for that section of the timeline to remain in existence.

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

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. — noAxioms

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

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. — noAxioms

Doesn't this imply that nothing ever happens in an Eternalist universe? Therefore, there is no such thing as travel? That would be a different type of grandfather "paradox".

...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.

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

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

So you seem to envision two dimensions of time. — noAxioms

No.

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. — noAxioms

I don't know what you mean by horizontal and perpendicular dimensions.

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. — noAxioms

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.

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. — noAxioms

I have no issue with the word 'truncated'. You claimed that the timeline could be permanently truncated. I still don't follow how or why that could be.

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. — noAxioms

Since the timeline gets overwritten, whatever effects Bob has on the timeline from 1990 onwards (post-time travel) will ripple through to 2024. The people and history of events in 2024 pre-time travel are affected by Bob's actions from 1990 onwards (post-time travel). Some people who are at 2024 after Bob departs in his time machine may simply disappear from the timeline thereafter, like family members in Marty McFly's photograph. Whatever effects time traveller Bob has on 2024 will only be witnessed when 2024 comes around again. However, we can consider the post-time travel effects from the 2024 perspective without waiting for 2024 to be present again. That is, I'm not suggesting that 1990 and 2024 are both progressing simultaneously. After all, I'm not an Eternalist.

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. — noAxioms

The history of the time machine is that it was built circa 2024 pre-time travel, it then time travelled to 1990 and continues to exist thereafter. It is a similar history for the time traveller. The important fact is that 1990 post-time travel is necessarily continuous with, and subsequent to (can occur only after), the time travel event in 2024.

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. — noAxioms

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

I think you used the word 'rewind'. — noAxioms

You used the word "rewind". I followed your usage to point out that time travel does involve a sort of rewinding of time. However, as I have repeatedly said, in addition to rewinding time, time travel also inserts something into the earlier destination time that wasn't there previously: a time traveller (and, perhaps, their time machine).

If that's how it works, then the tape will never reach year 3000 because somebody (not always the same person) keeps rewinding it. — noAxioms

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

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. — noAxioms

If we can stick to only one time travel event, then the model works like this: 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. The second time around with the inclusion of a time traveller, 1985 will likely proceed very similarly to how it did the first time around, except for whatever effects the time traveller has to change things from how they were the first time around. It is probable that most of the changes will be localised around the time traveller's location. Time will progress in its usual fashion, just as it did before the occurrence of the time travel event in 2024.

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

I'm happy to discuss the inconsistencies if you'd care to name them.

The VCR tape resumes recording at 1985 and progresses no problem. — noAxioms

Exactly, except that 1985 now contains a time traveller, whereas it didn't before the "rewind".

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. — noAxioms

I don't believe that it would be very chaotic, or that many of the people born would be different ones, unless the time traveller was very powerful and/or dangerous and was willing and able to make a lot of global change/damage. Anyhow, so what if it is very chaotic? The time travel event has occurred and makes sense.

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. — noAxioms

I wouldn't expect events to turn out identically.

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

The time travel event which occurs in 2024 and sends the time traveller back to 1985 is all a continuation of the same timeline from its beginning, albeit with a changed/changing history of events. But it should be expected that a time travel event changes the history of events, given that it involves inserting a time traveller into a time (the second time around) which didn't previously contain one (the first time around). An Eternalist might argue that there can be only one history of events, but I would argue that that singular history should contain all events that ever occur, including those of the first time around, without a time traveller, and those of the second time around, with one.

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. — noAxioms

I've said that it gets overwritten.

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. — noAxioms

I have read about chaos theory, thanks. Is there something in particular you could cite which would explain why it is necessarily the case that humanity could not evolve if a time traveller were to appear in the Cretaceous period?

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. — noAxioms

I've never said that the timeline is "truncated". By "truncated", do you mean "shortened"? In what sense is the timeline shortened? What I'm suggesting is not significantly different to Marty McFly's time travel in Back to the Future. I don't see how he "truncates" the timeline after his time travel to 1985 or why the history of the timeline would be unable "to grow beyond any backwards travel departure" after one or more time travel events.

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. Unless the time traveller does something catastrophic, then I would imagine that many of the same people will be born and many of the same things will happen as they did prior to the time travel event's occurrence in 2024.

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? The time machine is not used, or perhaps even created, until 2024.

That's enough to end the universe, according to the 'rewind/truncate' thing you've been pushing lately. — noAxioms

I haven't been "pushing" this "truncate thing"; you have. And the "rewind" thing is just time travel.

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? — noAxioms

Where did I say that "everything from there to 1990 ceases to be part of the universe"? I've said only that the time travel event would change the history of the timeline, starting with the time traveller's insertion into the earlier time (as a time traveller).

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

There is but the one timeline, but the history of that timeline must be able to be altered by a time travel event. That is, if 2024 is the departure time and 1985 is the destination time, then there was originally a 1985 without a time traveller, until 2024 when the time traveller departed and returned to 1985. This is what time travel involves. It can only be following this time travel event in 2024 that a time traveller arrives in 1985.

Otherwise, you are just stipulating that time travel is impossible or that there was never a period without a time traveller from 1985 to 2024, which both amount to the same thing. If there was never a period without a time traveller from 1985 to 2024, then there was never a time travel event which transported the time traveller back from 2024 to 1985. The time traveller was just always there in 1985 and didn't - or didn't need to - time travel. No time machine required.

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. — noAxioms

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

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. — noAxioms

How does he necessarily alter history to one in which he is never born?

That's popularized by the term 'butterfly effect'. Chaos theory is very clear on points like this. — noAxioms

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

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. — noAxioms

Yes, I have.

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. — noAxioms

Could you explain further why the universe cannot go on? I don't follow. This is a causal loop, I take it? You said that a causal loop only appears to occur once for any outside observer. How is the rest of the universe destroyed or affected?

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. — noAxioms

No, I think most people understand that time travel involves departing from the present time to arrive at a past time without them being confused about which is the present time. I offered to speak in B-series terms because you were being difficult about it and because it makes no difference to my point.

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. — noAxioms

Does that mean we can't think about it, then? Is it impossible that there was a period of time before time travel first occurred; before there were any effects of time travel?

There's no obvious correct way to compare moments between timelines. — noAxioms

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

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? — noAxioms

The time machine has time travelled from 2024 to 1990, so why would it be un-built in 1990? It seems that you now wish to simply reject the possibility of time travel out of hand.

Also, I don't know what you mean by the "truncate model". You still have not yet fully explained how the universe truncates or ceases to exist or how the population dwindles following a time travel event.

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

I noticed. — noAxioms

Well, how do you account for their adult existence at a time which is earlier than the time of their birth? Was a time travel event the cause, or was it some other cause?

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. — noAxioms

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

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

Are you saying that humans did only evolve on a timeline without a time machine, or that humans can only evolve on a timeline without a time machine. If the latter, then why can't humans evolve on a timeline that has a time machine in the Cretaceous period?

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. — noAxioms

As I explained in my previous post, my use of "before" is in relation to (before and after) the time travel event, not in relation to the linear order of the timeline. But it seems this has made things unclear. Instead of having referred to timelines before and after a time travel event, perhaps I should have referred to timelines that are with and without a time travel event. It is the comparison between timelines with and without a time travel event that I wish to make. The first and most obvious difference is that a timeline with a time travel event contains a time traveller, whereas a timeline without a time travel event does not contain a time traveller.

However, the existence of the timeline without time travel must precede the existence of the timeline with time travel. The time traveller departs from the timeline without time travel and creates a timeline with time travel by doing so. The time traveller's destination is on the timeline without time travel, so the time travel event alters the timeline without time travel to become a new timeline (with time travel) from that point on, after the time traveller inserts himself into it. We can then consider the effects of the time travel event by comparing the timelines with and without time travel (from the destination time onwards).

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

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. — noAxioms

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.

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

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

Yes, however, I also said:

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

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

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

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. — noAxioms

The timeline is not "currently being rewritten from there" (i.e. from 1990), because I have been referring to a time travel event which has not yet occurred in 2024, but that will occur tomorrow (or at some future time relative to your reading this post in 2024). Since 1990 is in the past of (or earlier than) the present time of 2024, there is no inconsistency in my referring to 1990 as "the past".

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. — noAxioms

Why would the time machine be un-built in 1990? Did the time traveller dismantle it after he arrived?

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. — noAxioms

I still don't understand why the population must be dwindling. It's one timeline. The only way the population could be dwindling as an effect of the time travel is if the time traveller's actions somehow prevent people in the future from being born. That is not obviously the case. The time traveller could visit the dinosaurs and then be killed 5 seconds later, having been crushed by a dinosaur. I can imagine scenarios in which the time traveller somehow prevented the evolution of humanity, but it's not necessarily so.

Why was he never born?
— Luke

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. — noAxioms

His birth event does exist. That's my point: that the original timeline must still exist (in some sense) because it contains the causal history of the time travel event and the new timeline. You could think of it as eternalism for the time traveller's timeline.

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

Although the time traveller can obliterate their birth event from the original timeline, this isn't necessarily the case. But I'm not entirely sure what you mean by "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.
— Luke

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. — noAxioms

What makes this a logical sequence on the linear timeline? How is it logical that Bob was never born and that he first appears with the dinos?

Also, an explanation is required for why Bob's birth event did not occur. His actions may have removed the possibility of his own birth, but I don't see why it's a given.

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

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)... — noAxioms

How is this necessarily what I'm describing? You may find it perfectly logical for a person to exist before they are born, but I do not. At least, not on the time traveller's timeline.

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

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

I was referring to the sequence of events of a person's life. On the the time traveller's timeline, their birth precedes all the other events of their life.

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

Ah, you actually identify a line. — noAxioms

Yes, and earlier I actually mentioned another line, the linear timeline.

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. — noAxioms

Why not? Did he somehow prevent it from happening?

Being alive is pretty good evidence of having been born.
— Luke

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

Yes, I still maintain the strange rules that one must be born before they can travel to an earlier time, and that people cannot exist as adults at a time before their birth event without having first been born and then having time travelled. Why should those rules not apply?

If you're talking about the departure event, the appearance of the machine in 1990 does not follow that event. 1990 is before 2024
— noAxioms

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?
— Luke

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. — noAxioms

Then you acknowledge that the appearance of the time machine in 1990 does follow its departure event in 2024?

you seem incapable of understanding why the jump counter in a loop would be a contradiction. — noAxioms

I do understand why it would be a contradiction. As I said, I acknowledge that it must read 'x' jumps in the causal loop. I'm just saying that it must have read 'x-1' jumps prior to the time travel (which happens outside the causal loop).

Although we would never know it. — jgill

True, but this doesn't make time travel any less possible. Also, if we were on a timeline which had had time travellers in its past then we can imagine that there might be reports or records of it.

They didn't in the [spawn new timeline] scenario, so nothing to explain. — noAxioms

I'm not sure what you mean by "spawn new timeline scenario". Two posts ago, I rejected the view that there is more than one timeline. However, 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.

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

...I suppose it depends on which moment on the new timeline is consdiered to be 'the present'. — noAxioms

In this context, the present is simply the departure time of the time travel event. The time traveller departs from the present and arrives in the past.

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 — noAxioms

The time traveller does not depart from the present of the spawned timeline, but from the present of the original timeline. They travel to the past. Assuming this is the first time they have ever used the time machine, then they were never in the past (as a time traveller) prior to this departure.

Did he travel from 'the future'? No. He came from a different line is all. — noAxioms

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.

The 2024 of this timeline does not have him going back. — noAxioms

Right, nobody has yet time travelled (that we know of). If they were to time travel (at some time after this present time), then they would alter the past and present as we know it.

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 — noAxioms

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? I don't believe you think it's only because Earth's population was smaller then.

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. — noAxioms

There's only one timeline.

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

Why was he never born?

That state doesn't exist in the one timeline. — noAxioms

Why not? Aren't we considered to be on one timeline? Are we never born?

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. — noAxioms

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?

The only logical sequence of events is that the time traveller is first born and then time travels to visit the dinosaurs. This implies that there must exist a linear time without any time traveller up until the time traveller's birth and subsequent time travel. THEN, when the time traveller visits the dinosaurs, they change the history of the timeline from what it was, without a time traveller, to what it is, with a time traveller.

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. — noAxioms

I don't know why you keep bringing it up, but your religious references are wasted on me.

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

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. — noAxioms

In this context, I'm measuring it on the traveller's timeline; on the linear sequence of events of a person's life. Marty must first be born in order for him to time travel. This is true given a time machine. He cannot travel back to 1955 until after he is born.

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. — noAxioms

Being alive is pretty good evidence of having been born.

Contradicting the fact that you just said it reads x+1, — noAxioms

I didn't say that it reads x+1. I asked you to clarify the contradiction of it reading x+1. I acknowledge that it would always read x in the causal loop, but logically, since you say that the guy in the box has time travelled, then it must have previously read x-1 prior to its time travel (assuming the odometer works).

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. — noAxioms

The time machine's appearance in the Cretaceous period is not uncaused. It's appearance is caused by the functioning of the time machine, which enables it to transport a traveller from some future time to the Cretaceous time.

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. — noAxioms

I agree that the past is only altered from the time travel destination time onwards (or from the earliest destination time onwards if we are considering more than one time travel event). I thought I had said this previously.

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

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, — noAxioms

The time travel "action" hasn't "just been done". As I said, the time travel "action" will happen tomorrow. Now is 2024.

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

You mean 1984? — noAxioms

No, I mean the 1985 without any time travellers that currently exists prior to any time travel event. Since I'm time travelling tomorrow, no time travel has yet occurred, which means there are/were no time travellers in 1985 according to history as it currently stands. Tomorrow's time travel event will change all of that history, because there will henceforth be a time traveller in 1985.

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

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? — noAxioms

No, I'm not talking about a different jump. This is no different to saying that you cannot have arrived in Paris (as a space traveller) before you have travelled to Paris. Likewise, you cannot have arrived in 1990 (as a time traveller) before you have time travelled to 1990.

Before that is 1989. 1991 is after that. — noAxioms

I am talking about before the time travel event, not before the destination time of 1990. As I said, you cannot have arrived in 1990 (as a time traveller) before you have time travelled.

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. — noAxioms

It seems like it's confusing enough to discuss one time travel event. Why compound the confusion by discussing more than one? Let's agree to the consequences of one time travel event before we discuss more than one.

Again, it doesn't follow a time travel event, it is the event. — noAxioms

The time travel event is both the departure of the time machine from 2024 and its arrival in 1990. I can refer to it as a time travel "journey" or "trip" or "jump" instead of an "event" if you'd prefer. It is a bit of a quibble to say that the arrival of the time machine is no different to the appearance of the time machine such that I cannot refer to the appearance of the time machine as being an effect of the time travel 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. — noAxioms

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?

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. — noAxioms

If you can suspend disbelief, I suppose.

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. — noAxioms

Okay, then where is the inconsistency?

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).
— Luke

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. — noAxioms

He was necessarily born before he time travelled to the Cretaceous period. Again, by "before", I'm talking about what happens before the time travel event; or better still, before the time machine departs. What I'm saying is that the time traveller must be born before the time machine departs (with the time traveller); I'm not saying that the time traveller was born before the Cretaceous period in linear time.

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. — noAxioms

The first "me" was not taught how to time travel by my older, time-travelling self (pre-time travel). It is only the young "me" in the causal loop who learned how to time travel by being taught by my older, time-travelling self (post-time travel).

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? — noAxioms

Let's just assume that it works flawlessly and does no serious damage or harm. It's time travel technology and it works.

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. — noAxioms

It looks like we're saying the same thing.

However, this does not explain how a time traveller can have travelled to the past before their first ever time travel event. Bob will one day grow up to build a time machine, but before he has ever grown up or built his time machine, his older self appears from the future in a time machine. If this is a causal loop, then where are the causes or the determinism that brought the time machine into existence? There must have been an original version of the past before Bob ever created his time machine in which he was not visited by his older self.

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. — noAxioms

In the case of this dinosaur visitor, surely this person had to have been born before they could ever travel to the past? Therefore, there must have been an original version of the past that existed before the dinosaur visitor ever visited. How could the time traveller be in that past time before they had ever time travelled or before they were born in the first place? Surely, their birth must precede all the other events of their life, including their time travel event. If their birth must precede all the events of their life (given determinism and causality), then there must be a (different) version of history without a dinosaur visitor which exists until this person was born and subsequently time travelled to visit the dinosaurs. This original version of history without a time traveller gets overwritten following the time travel event, but must have existed.

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. — noAxioms

If you consider history from the end of time, at the end of history, then obviously nobody has the free will to change any of that. Nothing more can be done.

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

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. — noAxioms

If it's a causal loop, then it will repeat the same time travel event over and over again. 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). This may not be part of the causal loop, but it logically (and causally) must be the case.

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. — noAxioms

Sure, but I'm considering the possibility of time travel, not trying to save lives or the stability of the timeline.

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
— Luke

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? — noAxioms

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

We might say or believe that up until now there have been no time travel events. If I were to time travel tomorrow, back to 1985, then I would be altering history as we now know it. After that, history will contain my time travel event, but it must also contain the "unaltered" history that preceded my time travel event (the history as we presently know it, before any time travel events). My time travel event causes the alteration of the original history, so this is consistent with causality and determinism.

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

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. — noAxioms

Not sure that I understand what you mean here, but in order to make sense of your "travel to" 1990, you cannot already have arrived at 1990 (as a time traveller) before you have time travelled to 1990.

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. — noAxioms

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.

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

But all your scenarios describe exactly that, including pretty much every fictional story that I can think of. — noAxioms

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

Nobody can demonstrate the typical definition of free will. — noAxioms

Can somebody demonstrate the typical definition of determinism?

It's a hazard of living in such a world is that your life expectancy outside the machine is moments at a time — noAxioms

Ain't that the truth.

and no better inside the machine since no time passes in there (unless you assert otherwise I guess). — noAxioms

I imagine the time travel duration to be instantaneous, but I don't think it's important.

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.
— Luke

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

But we can imagine how a causal loop can come to exist. 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. 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).

It has a first appearance?
— Luke

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?

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

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. — noAxioms

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

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? — noAxioms

Yes, we are talking about the original timeline. 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

I think you've acknowledged this is not possible. It is why I was arguing that more than one timeline is required.

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. — noAxioms

Sorry, but I don't see the contradiction. The time machine starts with an odometer reading of x (representing the number of time travel "trips" made by the time machine), then it makes a (another) time travel "trip" and records a reading of x+1. Why does it need to revert to a reading of x again 8 seconds later? Isn't it correct that the number of "trips" made by the time machine has increased from x to x+1?

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. — noAxioms

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.

In fact, causal loops can be avoided in the examples we've discussed because the time traveller can just do something differently at any time after any of their time travel events. For example, there is nothing forcing the 8-second guy to time travel again after 8 seconds.

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. — noAxioms

Perhaps they're logically consistent, but I find the magical, uncaused existence of the causal loop universes (and their contents) to be too far-fetched and unsatisfactory. Someone who gets their time travel technology knowledge from their older time travelling self does not explain how the older time travelling self got this knowledge in the first place, before they first time travelled. And I find the idea that there is no first time travel event unrealistic.

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

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.

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. — noAxioms

I'm not using 'first' as an objective ordering of things but as a human ordering of things. I don't assume the determination of a 'first' event is independent of some group of language users (particularly, English speakers).

Nope. It came into existence when it first appears... — noAxioms

It has a first appearance?

But single-timeline travel isn't necessarily contradictory so long as one does not make choices known to be different than those made before. — noAxioms

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

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

If you time travel to the past, by definition you end up somewhere 'before' the event where you initiated the travel. — noAxioms

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.

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. — noAxioms

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

That would be a contradiction. — noAxioms

What's the contradiction?

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

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. — noAxioms

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.

Things that are impossible in this universe are not impossible in this alternate universe where time travel makes for different causal rules. — noAxioms

What are these different causal rules? There are still causes and effects, it seems. 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.

So some causal rules are okay, but not others? We may never ask/explain how a time machine came into existence in the universe, but it's okay to ask/explain how a time machine came into existence in a museum?

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. — noAxioms

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. But how is it logically self-consistent that there was never a first time travel event? Does time or causality work differently in these scenarios such that it would be impossible to trace back to the first time travel event? If so, then how do time or causality work differently? Why is it impossible to trace back to the first time travel event? Or, why can't there be a first time travel event?

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.

So maybe this time traveling creature never evolved, but just is. Again, there are movies depicting pretty much this. — noAxioms

We're dispensing with evolution, too? I'm unfamiliar with those movies/scenarios.

The scenarios in the SEP article and those I've been considering all involve human time travel with a time machine. I suppose I could alter P2 to say that time machines involved in human time travel need to be created at some point. 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).

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

You are effectively telling me to ignore how the time machine came into existence originally. That doesn't "falsify" P2.

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

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. — noAxioms

There is no problem in saying that time machines or waffle irons have always existed in a causal loop, which is what you suggest when you say they are uncaused. This wording need not imply that these things exist at all times. My point, however, was that time machines, like waffle irons, are man-made for a particular purpose, and so it seems unlikely that they would naturally exist without any human (or other sentient beings') intervention. Also, I note that we were talking about human time travel using time machines until recently, but 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.

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.
— Luke

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. — noAxioms

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

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

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. — noAxioms

Right, that's why I've been arguing that time travel only makes sense on multiple timelines, and why I've been arguing that a causal loop (or that time travel on a single timeline) does contradict its existence. I don't see why I should ignore there being a first time travel event just for the sake of maintaining the consistency of a causal loop. I'm arguing that they do contradict their existence.

Not following. There a possibility of a loop that doesn't involve time travel? Example please. — noAxioms

No, 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.