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

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

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

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

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

If we can stick to only one time travel event, then the model works like this: 1985 progresses without the appearance of any time traveller, until 2024 when someone first time travels and they arrive back in 1985. Everything about 1985 (the second time around) is almost the same as it was without the time traveller (the first time around), except that now it has a time traveller in it. In this way, it is very much like Back to the Future. The second time around with the inclusion of a time traveller, 1985 will likely proceed very similarly to how it did the first time around, except for whatever effects the time traveller has to change things from how they were the first time around. It is probable that most of the changes will be localised around the time traveller's location. Time will progress in its usual fashion, just as it did before the occurrence of the time travel event in 2024.

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

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

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

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

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

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

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

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

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

I wouldn't expect events to turn out identically.

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

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

What does rewind do to the 40 years over which we backtrack? It either erases as it goes or that part of history gets overwritten as the recording resumes. — noAxioms

I've said that it gets overwritten.

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

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

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

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

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

If time is some physical entity running itself somewhere in the universe, and if there were different timelines running in different physical spaces, then perhaps you could get into the space via the teleport or whatever bending spacetime and what have you, maybe then you could say your  mind and body of 2024 can travel to whatever year you choose without losing the memory, thoughts or consciousness.

But if time is just a mental concept for measuring the intervals between the start and end of changes of the objects in the physical world, then the whole topic would be just a fiction.  Even if you accept whatever premises for time travel hypothetically and keep speculating what you would do in the past or future in time travel, you still have to accept the most foundational universal law, that all minds and bodies are subject to change through time. 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

and if there were different timelines running in different physical spaces

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

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

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

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

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

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

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

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

How did wave functions sneak in?

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

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

It is just one of the different scenarios of what the nature of time might be.

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

That wasn't anything to do with a supernatural tangent. It was just an expression to emphasise that you cannot reverse time, and no one in the whole universe can. No one said that was your problem.

And BTW, 'object' is very much just an ideal. There seem to be anything physical about what constitutes an object.
I'm getting pretty far off topic here. — noAxioms

You have been for sure. OK, please carry on. I am bowing out here.

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

No.

One is Earth coordinate time, as measured with numbers like 1990, moving horizontal to larger numbers, and the other is perpendicular, and moves 'up' with each travel event. — noAxioms

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

You seem to assert a single physical space that is 'overwritten', which is a lot like a VCR tape, except there are perhaps no spools to rewind since you seem to balk at that word. — noAxioms

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

So there is the tape which holds the entire history of the universe up to a 'present' where the write-head is writing. It writes up to 2024 say and then Bob goes back to 1990. A write head goes back to 1990 (without erasing, which would be truncation, another word you don't like) and starts overwriting there. — noAxioms

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

It is unclear if this is a second write head (leaving the 2024 one to continue writing a universe without Bob, or if the history stops there and waits for the write head 34 years prior to catch up. — noAxioms

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

With this model, Bob goes back, and the history of the creation of the time machine in 2023 still exists, but the writing is going on in the 90's and when it finally gets to 2023, it overwrites the creation of the time machine, leaving a time machine without a creation event in any of history. — noAxioms

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

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

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

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

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

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

I have no issue with the word 'truncated'.

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

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

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

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

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

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

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

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

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

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

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

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

Bob is born 1985, meets Sue in 2002, married in 2007, and has a daughter Roberta, born in 2010. Bob kills young-Bob in 1990, so what is the experience of Roberta when she gets overwritten? — noAxioms

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

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

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

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

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

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

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

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

You balked at that before. — noAxioms

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

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

If this is your interpretation of my view, then it's incorrect. I said that the timeline gets overwritten, but you've somehow interpreted that (to be the opposite of what I said) as "no overwrite, but just truncation".

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

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

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

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

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

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

Okay then, it's just a memory. My argument is that time travel and the act of time travelling to kill one's own grandfather (or their younger self) is hypothetically possible and logically consistent. I don't really need to maintain the existence of the pre-time travel timeline for that purpose. Although I understand why an Eternalist would prefer for that section of the timeline to remain in existence.

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

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

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

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

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

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

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

It sounds like this truncates Alice's existence, but I don't see how it permanently truncates the timeline.

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

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

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

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

Someone could see it disappear, I suppose — Luke

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Therefore, there is no such thing as travel?

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

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

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

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

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

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

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

Someone could see it disappear, I suppose
— Luke

This seems contradictory. — noAxioms

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

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

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

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

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

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

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

This is what I mean by you referencing two dimensions of time. One is the time I'm talking about, where 1990 comes before 2024, and the other is the time containing the first kind of time. — noAxioms

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

Along the x axis, the present is at 1990 more than once, and the 2nd 1990 happens after the first 2024, but all of it 'happens' at some point. That corresponds more to no time travel at all, and history isn't deleted at all, but rather the state of the universe is simply reset to a prior state the exception of the contents of the machine which are protected from the overwrite everywhere else. — noAxioms

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

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

I think, but am not sure, than when you get in you machine and set the coordinates for some destination, that you select a value on the y axis and not on the x axis, but it isn't really clear. — noAxioms

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

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

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

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

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

I know that things don't remain or go in or out of existence under eternalism. That's why I said that an Eternalist would prefer for the overwritten section to remain in existence; because its going out of existence is at odds with eternalism.

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

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

I'm aware that the words "happen" and "occur" are usually synonymous, but it's unclear what it means for an event to "happen" or to "occur" on an Eternalist timeline, especially since, as you say, "there is nothing that 'progresses' at all." Do the terms "happen" and "occur" mean anything other than that the event exists?

If Eternalists take "exist" to be synonymous with "happen" then, since Eternalist events do not cease to exist, they must also not cease to happen. There is no past tense of events having existed or having happened for the Eternalist. This implies that, instead of the usual sequential progression of events wherein later events occur after earlier events, on an Eternalist timeline all events are happening en masse at their respective times and each event happens repeatedly. Just as Eternalist events do not cease to exist, they also do not begin to exist, and therefore they also do not begin to happen; they have no beginning or end. Like all events on the Eternalist timeline, the sinking of the Titanic in 1912 and humanity's extinction event in 2316 are both always happening, and they are happening over and over again (at their respective times). Or, it may not be the period of each event (of a different duration) that repeats; it could be more regular periods such as minutes or seconds that repeat. Otherwise, nothing happens on an Eternalist timeline and everything simply exists.

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

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

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

That opens the door to the paradoxes, but it also allows a time machine to exist uncreated. Your view I think doesn't support that. — noAxioms

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

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

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

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

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

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

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

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

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

Forwards time travel is just like backwards time travel.

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

Agree

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

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

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

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

therefore [events]also do not begin to happen

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

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

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

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

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

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

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

It makes little difference post-time travel.
— Luke

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

An event 'happens' at the location of the event. Not sure how else to say it. — noAxioms

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

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

I think not. — noAxioms

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

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

I find it odd that you refer to an event as occurring at a single point in time. I suppose the word could be used in this way, but I typically think of events as having a duration; lasting for a period of time.

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

No, that's not true. The length of my table might exist, but it's not something that 'happens'. — noAxioms

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

Also, since eternalists treat time as a spatial dimension, then why wouldn't they say that the length of your table happens, just like the length of an event (i.e. a process) happens? What's the difference?

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

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

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

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

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

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

Sure, eternalism can allow for a sequence of events, but what does it mean for those events to happen?

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

Ah okay, I see now that I've been using the term "event" to refer to what you call a "process". I will adopt your terminology henceforth. I note that a process requires progress. Incidentally, I would have thought that the sinking of the Titanic was a process rather than an event.

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

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

Right. I'm suggesting that, in order to say that all events exist/happen under eternalism, then all times must be, in a sense, present. All events exist and happen at each of their respective times. Since they all exist and happen at all times, I don't follow why they should happen in a sequence from earlier to later, so that they apparently happen one after another. This suggests that there is a "special event which is 'current', which moves along [the] worldline," which sounds very much like a progression. Instead, eternalism entails that events all happen en masse at their respective times, rather than in a sequence, one after another. But in that case, each event must happen repeatedly, without beginning or end.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

I think not.
— noAxioms

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

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

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

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

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

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

I did. I misread the question.

Also, since eternalists treat time as a spatial dimension

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

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

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

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

You said events happen repeatedly.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[Black holes] was part of your Alice example

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

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

I did it with one time travel event, a scenario you seem to refuse to comment on directly except to say you apparently don't get it. — noAxioms

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

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

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

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

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

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

Neither did I. — noAxioms

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

Bob is traveling to it, but it must happen first before he can arrive, else he ends up in a blank universe not yet written. It would presumably be subjectively instant to Bob, just like it is backwards. — noAxioms

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

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

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

Why would it leave a copy of Bob behind?

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

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

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

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

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

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

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

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

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

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

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

You said events happen repeatedly. — noAxioms

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

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

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

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

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

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

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

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

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

Nothing of the sort is suggested. — noAxioms

You suggested it with your Titanic example:

The Titanic sinks on some 1912 night. That is a statement of something that happens. The night before it has not yet sunk, and the night after it is at the bottom of the ocean. Somewhere between those two events does the sinking 'happen'. — noAxioms

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

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

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

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

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

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

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

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

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

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

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

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

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

Why would it leave a copy of Bob behind?

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

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

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

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

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

As you note, this sounds a lot like presentism.

Because the comment IS presentist.

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

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

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

True (and meaningful only) under presentism.

Since eternalists hold that events exist at all times

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

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

The "preferred" moment is the state that is happening

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

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

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

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

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

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

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

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

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

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

It also clones Bob. — noAxioms

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

Sure, from the traveler's viewpoint (the only one you ever consider), it looks like he just appears there, in 2035 with F-Bob sitting there much in the same way that none of the fictions depict. — noAxioms

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

There is either a timeline without a time travel event or there is a timeline with a time travel event. Call the timeline without a (any) time travel event timeline A and call the timeline with a time travel event timeline B. If there is no time travel event then timeline A results. If there is a time travel event then timeline B results (and timeline A gets overwritten by timeline B). F-Bob only exists in timeline A and S-Bob (the time traveller) only exists in timeline B.

Why would it leave a copy of Bob behind?

You said that it goes to a "future timeline that would have existed if there had been no time travel events". If there had been no travel events, Bob would still be in the timeline instead of the machine, so aging F-Bob (the one that is not in the machine) is a copy of the not-aging S-Bob in the machine. — noAxioms

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

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

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

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

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

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

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

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

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

I'm just trying to get a better understanding of the distinction between the meanings of "happen(s)" and "exist(s)" in relation to an event/process under eternalism. You spoke of the time before the Titanic event when "it has not yet sunk" and the time after the event when "it is at the bottom of the ocean" (i.e. when it has sunk). You also said that "Somewhere between those two events does the sinking 'happen'." You noted "It's not much different than presentism except there is no preferred moment that has to somehow glide across that event in order for the event to cycle through the different ontological states of 'happening' and then 'happened'."

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

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

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

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

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

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

Since eternalists hold that events exist at all times

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

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

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

Really, learn the view before you start asserting what it must say. It hurts, the way you're murdering a view with which you obviously don't hold. — noAxioms

You should learn that there are two ways of interpreting my sentence. I wasn't saying eternalists hold that each event exists at all times. I was saying eternalists hold that there exist events at each (and every) time, i.e. that there also exist events that are not present.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

you are yet to have proven it illogical.

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

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

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

F-Bob only exists if Bob fails to time travel

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Besides the explicit reference to a preferred moment?

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

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

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

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

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

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

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

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

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

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

Your examples of the scenario were:

Back to the train tracks, Alice gets there just as the gates go down, but watches a very similar car ahead of here make it across. So she hits the button and goes back 30 seconds. That destroys the 30 seconds. She ends up at the tracks, and in time to scoot across. The world ends 30 seconds later when the car behind here truncates it there. There is no future after that. The universe cannot go on. — noAxioms

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

The problem is that you provided very little detail so it is difficult to follow what is happening in this scenario. Presumably, the DeLorean is a time machine, like in Back to the Future. So, Alice gets to the train tracks and has to stop because the gate comes down. She decides to use her DeLorean time machine to go back 30 seconds so that she can floor it and cross the tracks before the gate comes down (the second time around). All well and good. What I don't understand is, after she does this, why is there another DeLorean behind her getting stuck at the gates? The time travel event in your scenario does not overwrite the timeline.

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

I asked you which works of fictions involve time travel to a blank universe which has not been "written" yet. You tell me that there are no such works of fiction. Okay then, which works of fiction wait for the future destination to be written before time travel to that future destination occurs? By "wait", I assume you mean in the usual fashion, like you might wait for a bus? So you are saying that, in all works of fiction, there is no time travel to a future time which occurs before people have waited for that future time to happen? In that case, I don't understand why the time machine is needed.

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

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

As I said, the machine has always been a cloning device. Bob goes back to 1990 where he meets another Bob. Two Bobs means one is a clone. Using this technique, you can make as many Bobs as you want, and you can do it quickly, in minutes instead of decades. So getting a clone by going forward is admittedly consistent with your going-back description, even if none of the fictions seem to depict that consistency. Hollywood has developed a rule that you can meet yourself if you go backwards, but not if you go forwards. — noAxioms

I made a mistake in my last post. Sorry, I've had COVID recently and wasn't thinking straight. I started out writing the post thinking that you don't meet yourself going forwards, but then I somehow reasoned myself out of it. So I agree with the Hollywood version; you don't meet yourself or clone yourself going forwards. You depart from an earlier time to a later time, so there's no other version of you left behind who continues aging normally once you depart from the earlier time for the later time. You can only "clone" yourself (in a sense) going backwards.

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

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

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

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

Why can't the B line just be written from the start since F-Bob and the rest of the A line is doomed before the first moment is written? — noAxioms

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

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

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

Right, thanks for the correction.

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

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

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

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

Why do the observers need to prove it?

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

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

Since time is one of the dimensions of spacetime, the word 'happens' is meaningful. The event happens at the location in spacetime of that event, which I realize is circular, but that's the nature of a tautology. — noAxioms

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

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

Besides the explicit reference to a preferred moment? — noAxioms

I defined the preferred moment as "the state that is happening, as opposed to those that have happened or have not yet happened". What's wrong with that? Does eternalism allow for events to have happened, and for events to have not yet happened, but not for events to happen? Why?

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

There are no such ontological differences. — noAxioms

You described them as such in your Titanic example. You described a time when the Titanic had not yet sunk, and a later time when it had sunk, and then you said "Somewhere between those two events does the sinking 'happen'." This implies that there is a time when the sinking has not yet happened, and a later time when the sinking has happened, and a time between these when the sinking happens.

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

I didn't find it difficult.

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

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

Also not sure about the first part, that there exist events at each (and every) time. For instance, do there exist events before the big bang? I think not. Do all events have a time coordinate? I can't think of a single coordinate system that assigns coordinate values to every event that is part of spacetime, so even that isn't true. — noAxioms

Is "before the big bang" part of spacetime? The statement "there exist events at each (and every) time" does not require every event to be accounted for, as long as there exists at least one event at each and every time.

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

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

.

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

The timeline that would have existed if there were no time travel events gets overwritten by a timeline with a time travel event. — Luke

Does physics describe what the above even means?

Does physics describe what the above even means? — Michael

:lol:

Does physics describe what the above even means? — Michael

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Is "before the big bang" part of spacetime?

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

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

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

Nothing is happening in an eternalist universe?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

I don't think many works of fiction explicitly rely on this growing-block model that you have going on here, so concepts like a new history growing simply don't apply. — noAxioms

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

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

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

Presumably backwards time travel works differently. Why should the machine have to wait in forwards time travel if it is not required to wait in backwards time travel? I imagine that the time machine can immediately transport the passenger from one time to another.

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

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

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

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

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

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

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

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

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

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

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

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

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

Meaningless due to the implicit references to the present. — noAxioms

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

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

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

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

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

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

Is "before the big bang" part of spacetime?

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

The reason I asked was because you said:

Also not sure about the first part, that there exist events at each (and every) time. For instance, do there exist events before the big bang? I think not. — noAxioms

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

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

First of all, the statement is false since I can think of a time that has no events. — noAxioms

Under eternalism?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

I find it worthy of an interesting philosophical discussion, but I suppose you've made up your mind already. What I don't understand is why you felt compelled to interrupt a discussion that's been developing for over five pages only to denounce it as "pretty nonsense". You could always just mind your own business.

The only possible way is if the multiverse is true, if all probabilities has their own branch, but then there's no point in going back in time to do anything as you cannot change the future you came from. It would be closer to traveling to other universes rather than specifically traveling back in time. And any change would only just fraction into new branches ... — Christoffer

:up: :up:


https://thephilosophyforum.com/discussion/comment/690858 :nerd:

That would assume an eternalist view of time, in which time is treated much like a length, or as another spatial dimension. Whereas - prior to the untimely demise of this discussion - I was seeking to explore the limitations of eternalism, such as its logical omission of progress, happening or motion; characteristics that I consider to be absent from eternalism but logically aligned with the opposing view of presentism. However, many eternalists disagree. — Luke

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

If you're arguing for presentism then this might be interesting:

Presentists Should Not Believe in Time Travel

The general gist being that the very concept of traveling to the past depends on the past existing in some sense as a location to travel to, and so requires either the growing block universe or eternalism.

If presentism is correct then any supposed time machine would work by rebuilding the universe into a facsimile of one of its past states, which isn't really time travel.

there exists a history — Luke

If there exists a history then presentism is false.

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

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

As I explained previously in the discussion:

As you may recall from previous discussions on time, my ontology of time involves a blend of presentism and eternalism (in short, that without presentism there is no 'progression of events', and without eternalism there is no timeline(s) of events). — Luke

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

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

The Moving Spotlight Theory? Seems to be a hybrid view that allows for both eternalism and a dynamic time.

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

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

Argument against motion in eternalism

A major difference between presentism and eternalism is their differing concepts of an object. Presentism takes the commonly held view that an object is 3D and traverses time. Eternalism takes the uncommonly held view that an object is 4D, that the 4D object exists across time, and that it consists of 3D parts.

If we consider that the motion of an object is basically a change in its position over time, then it can be shown that this can only apply to (the presentist view of) a 3D object. This is because, according to presentism, the same 3D object exists at different times. However, according to eternalism, the same 3D part (of a 4D object) does not exist at different times.

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

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

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