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.

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.

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. — Luke

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.

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.

The idea is that consciousness is something that supervenes on the physical, much like any traditional dualism, and so is not separable from it. The 4D object doesn’t move as you mean it but this 0D consciousness travels through the time dimension creating the illusion of physical movement.

So time flows for consciousness but not for the physical host. At the very least it could tentatively explain the current conflict between general relativity seeming to imply physical eternalism and our everyday experience seeming to imply presentism.

Of course this would seem to entail determinism, unless something like the many worlds hypothesis is true and consciousness can choose which “world branch” of the 4D host it travels down.

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.

Well, I would say that the traditional view is that physical objects supervene on and move through static space(time). I’m just pushing this up a level and suggesting that consciousness supervenes on and moves through the time dimension of static 4D physical objects.

But again, I’m just spit balling.

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?

You are simply ignoring my argument. — Luke

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

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?). 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.

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.

Why should you be able to treat consciousness as a presentist object in an otherwise universe? — Luke

Because, as above, it may resolve the conflict between general relativity and our everyday experience of the passage of time. If dualism is correct then the physical and the mental need not necessarily behave according to the same laws.

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.

How is it in conflict? — Luke

Sorry, it was special relativity, not general relativity:

B-theorists typically emphasize how special relativity eliminates the past/present/future distinction from physical models of space and time. Thus what seems like an awkward way to express facts about time in ordinary English is actually much closer to the way we express facts about time in physics.

For example, see the conventionality of simultaneity.

it presupposes the experience of time that we have. — Luke

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

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

There's perhaps gravity and any curvature of space(time) in general.

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

It's something I believe. I'm unconvinced that physics alone can explain the hard problem of consciousness.

But I am arguing against your view. — Luke

Again, I'm spit balling, not trying to argue for it. It was really just an off-hand remark, not something I intended to lead to a rigorous discussion.

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

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

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

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

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

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

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

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

Presentists Should Not Believe in Time Travel — Michael

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

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

If there exists a history then presentism is false.

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

Argument against motion in eternalism

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

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

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

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

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

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

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

Yes, true under both views.

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

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

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

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

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

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

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

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

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

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

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

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

*among those with understanding.


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

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

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.

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

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

3D parts of the 4D object.

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

Perdurantism has temporal extension; endurantism does not.

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

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

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

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

Motion and/or location of what, though?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


From last week:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

it’s all a matter of parts.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Just saying...

Does air die/explode?

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

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

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

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

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

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

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

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

Surely we can imagine that the time machine can arrive safely

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

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

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

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

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

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

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

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

and if Alice2 time travels 5 seconds before crossing the tracks

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

then Alice2 will not proceeed to cross the tracks

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

If Alice1 lands on and kills Alice2

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

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.

I didn't realise there were two different definitions of 'happens'. — Luke

"To take place, occur" is what I get from a google query. That works fine, since the definition isn't specifically crafted to exclude the undesirables. To exist means 'to have being', to be real. I can be an eternalist (or presentist for that matter) without being a realist, so an event need not exist in order to happen.
'Exist; has somewhat different meaning in mathematics, e.g. a positive integer is not prime if there exists a positive integer other than itself or 1 that divide the number evenly.

What is the eternalist definition of 'happens'?

I could probably craft one that excludes the undesirable presentist view, but doing so wouldn't in any way constitute evidence that a view excluded is wrong.

presentism holds that only present objects exist.

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

I don't use the term "presentism" to refer to any "4D versions of it".

I do, because all of the alternate versions still posit a preferred moment in time, which is the fundamental different between any of them and eternalism.

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.

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.

The question was basically asking if you are a presentist (endurantist) or an eternalist (perdurantist).

I try not to hold hard beliefs. I know both, and can discuss either. The purdurantist wording seems silly to me. I've never seen its terminology used in any practical discussion, such as in the science community. And science definitely uses both eternalism (especially in a discussion of cosmology, relativity, physics, chemistry), and presentism (astronomy, climate science, biology, anthropology). I never hear anybody use 'temporal parts' or 'wholly present'. One context uses B-series terminology, and other contexts use A-series.

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.

An eternalist universe contains 4D objects

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

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

It does not follow that the lack of motion of a 3D 'part' implies the 4D object does not meaningfully exhibit motion.. 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. The 3D references are perdurantist phrasing, and the argument above is still doesn't demonstrate that the object doesn't move, only that a specific temporal part doesn't, which of course it cannot since it would need time to do the moving.

You cannot have two temporal slices at the same time.

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.

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?

If I slice a 4D object across a spatial axis instead of across the time axis, I end up with a 3D object that has one temporal dimension and two spatial dimensions. The location in 2D space changes over time.
My example would be a car in a drivers-ed parking lot course, sliced through z= half a meter above the lot. That reduces the 4D car to a 3D object that moves over time. It gets weirder if the driving course has hills in it.

I'm interested in the philosophy of time, and the implications on the different theories of time.

How it handles collision is critical to identifying the implications. If I don't know how the machine handles targetting an event where there's already something else, then we cannot explore the implications of a trivial situation where that necessarily occurs.

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

You change the story several times, so I wasn't sure which you had settled on. OK, so they both die, Alice3 comes upon the death scene and perhaps doesn't decide to add herself to the wreckage, and chooses to miss her important appointment instead. The universe doesn't end (this time).

Your machine is very dangerous then, since it seems to require one to place a bet that the destination selected is free of anything larger than dust. It might succeed the first few time, but travel to millions of years ago? Due to erosion and plate techtonics, the destination is almost certainly in the middle of bedrock somewhere, or is way in the air, under water, or even in space. Unless the vehicle can deal with those situations, the traveler dies.

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.

There's four Alices,. Sounds like cloning to me.

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.

That was the convention I had initially chosen. We switched to yours. My convention had only three Alices (not four), and everybody else (Alice or otherwise) was an original. In a way your convention is better, because each person (traveler or not) has a unique history. My convention has a given person (the guy mowing his lawn nearby say) multiple histories that play out in different ways, which violates identity rules.

Where did Alice4 come from?

Clone of Alice1, made by the travel of Alice2. Alice4 lives but a moment and is gone in the collision with Alice2. Alice2 lives 30 seconds, and dies in collision with Alice4. I did say that Alice1 is the only happy Alice. It sucks to be any of the others.

Alice1 is still Alice1 after she time travels. She is the original.

Right. Her travel creates Alice2. Alice1 never time travels again. She lives but 30 more seconds and is truncated into oblivion.

So it is Alice1 who lands on Alice2 and they die as a result

No, Alice2 lands on Alice4. Alice1 doesn't land on anybody, which contributes heavily to her being the happy one.

and then the timeline continues without any Alices

Well, 1 is gone, 2 and 4 die in a crash, so only Alice3 survives (if she chooses to lay off the button). If she still hits the button (but in a different place than where the wreck is, and for maybe a different jump than 30 seconds, then she can make a whole bunch more dead Alices, herself included, since no actual traveler survives the experience.

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.

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

Time travel under eternalism is simply any non-timelike worldline, and, if you take the SEP definition, any non-straight worldline. The sort of travel you've been envisioning would be a discontinuous worldline. A continuous but not timelike worldline would have an undefined proper time, meaning it's not clear what the subjective duration of the travel should be, but the external experience of the machine would be much like the description of Putnam in SEP. Funny that his machine sort of has to accelerate to some speed (88 mph just like in BTTF) to make the jump.

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

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.

there is no motion in an eternalist universe, as I have argued.

And I've shown otherwise, so you're simply wrong. The eternalists use all the same language as do the presentists, but formally, only references to the nonexistent extra thing is what makes a statement meaningless. Motion has meaning under eternalism since a statement such as 'Floyd takes an hour to move from A to B' has meaning.
Really, why the brutal discrimination here? What purpose is served by your refusal to accept normal usage of language? You seem to seek only to prevent people that hold an alternate view from being able to discuss anything, when clearly the statements have meaning.

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.

That wasn't so hard, was it?

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.

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.

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

Any slicing does this. The positing of a preferred way is known as 'absolutism'. The first premise of relativity is that there isn't a preferred way, but it's a premise, meaning relativity isn't proof against a theory that doesn't accept that premise. The slice can be odd shaped. It need not be flat, but it does need to be space-like, else you end up with events that occur out of causal order.

Alice2 can only clone herself.

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.
If your convention is that only Alice2 gets cloned, then I suppose Alice4 and Alice1 are just Alice1, who experiences two different fates, a contradiction of identity. X crosses the tracks. X does not cross the tracks. That violates the law of non-contradiction, and the problem is solved by differentiating Alice1 and Alice4.
It's all a nitpick what names we give them. The scenario was solved. Time travel is super dangerous. Can we move on? There's so many more problems to discover.

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.

Mostly right. You didn't mention the Alice that collides and dies with Alice2 in that description (so 3 Alices coexisting at once, but two of them dead). The time machines were cloned as well, so there were 4 of those, one truncated away, two crashed into each other, and the only one remaining is the one never used.

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 is a theory of existence, whereas endurantism is a theory of persistence.

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

I did misread it, so thanks.
I guess I'm not clear on the difference between the two. Both are essentially ontological stances, which is in the end, existence. 'Persisting through time' and 'existing in time' seem to be just different ways of saying the same thing, so perhaps I'm missing an important distinction.

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

But I never disagreed with the 'corrected' statement.

You are again assuming that Floyd is a 3D object.

I never said any such thing, in the context of eternalism. The 3D things are (per the perdurantists) separate 'parts' of the 4D thing. It is the 4D thing said to move (change locations over time), not the parts.

No 3D part of Floyd changes its temporal or spatial location

I gave an example where this wasn't true, but I know what you mean. To summarize, by definition, no event that is part of Floyd can be at different coordinates in an inertial coordinate system. It's true of a 0d event, even if not necessarily true of 'parts' consisting of 1-3 dimensions. 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.

which is what a 3D part must do in order to meet the definition of motion.

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. The language is serving its purpose, which is to have meaning, and it does so without needing to change the definition from 'change locations over time'.
The 'over time' part is necessary, because my one hand is at a different location than the other, at one given time. That isn't motion of Floyd. It's a difference in location of parts, sure, but not over time. Extension alone is not usually considered to be motion.

Any slicing does this.
— noAxioms
How?

Do you understand a 3D cross section of a 4D object? All the events on the arbitrary slice can be assigned the same time coordinate so long as the slice is space-like. Angle the slice a different way and a different set of events (except those events at the intersection of the different slices) are now assigned the same time coordinate. This is essentially a change of reference frame, coupled with relativity of simultaneity, with which I suspect you are not familiar else you'd not be asking that question. A loaf of bread is often the analogy (slicing a 3D object, with time being the long dimension say) along 2D spatial planes, arbitrarily oriented. A slice through a given event (the center of the loaf say) can be angled in many ways and still include that one event, so all the other events are only part of some slices and not part of the others. That's relativity of simultaneity in bakery terms.

I bring all this up because perdurantists seem to slice Floyd up into 3D disjoint parts, but that can only be done if there is a preferred frame. If arbitrary frames are allowed, 3D cross sections can intersect (be in different locations at 1 given time), and I don't think perdurantists intended that. So without resolution of this issue, they seem to require a preferred frame (absolutism, without the presentism), which violates the relativity theory of which they're presumably in favor. A change in location over frame rotations (instead of over time) is also not considered to be motion by most, but the change in location can be quite large.

You can probably tell that I'm not impressed with the perdurantist shtick.

How does Alice4 (Alice1's clone) come into existence?

Alice1, at the tracks at t=12:00:30 travels back 30 seconds to being there at exactly noon. So Alice1 is at the tracks at noon. Alice2, at t=12:00:30 also selects that same noon event as her destination, so she clones the Alice1 there and the first-noon version of Alice2 (not at the tracks), to create two new clones Alice4 and Alice3 respectively. Alice 2 and 4 are occupying the same space at the tracks simultaneously, and one doesn't survive that.

Do I have to explain it yet again? I don't know if I can get any more detailed.

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.

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

'Floyd at noon' indeed describes a 3D object, yes. Floyd at 1 is a different 3D object, but it is all still Floyd, and the difference in Floyd's location over time is, by definition, motion. It is entirely consitent with B-series language which any eternalist uses without contradiction.

Is velocity also forbidden then? I mean, velocity in block view is either a rate of change of position over time (generic definition), or it is the slope of the object's worldline (an alternate definition that is not compatible with 3D presentism).

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


The conversation has ceased being about time travel. I apparently cannot discuss an eternalist view with all the restrictions placed on language, all under the guise of 'logic'.

'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.