## Time travel to the past hypothetically possible?

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It can be a fun exercise to consider how one might change the rules of the board game Monopoly, in order to turn the spatial loop into a time loop. What should the state of the board be for the first player to pass Go? If he lands on a square and decides to buy a house, then which players does this effect and how should the state of the game be backtracked and updated?
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However, this does not explain how a time traveller can have travelled to the past before their first ever time travel event.Luke
They didn't in the [spawn new timeline] scenario, so nothing to explain. I suppose it depends on which moment on the new timeline is consdiered to be 'the present'. If, say, the present is designated to be 'the present' in this spawned timeline, then the traveler (if there is one) must be present at 'the past', 5 years prior. Did he travel there? I suppose he did. Did he travel from 'the future'? No. He came from a different line is all. The 2024 of this timeline does not have him going back. He dies before then, presuming he doesn't exist the line by a subsequent usage of the machine.

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

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

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

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

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

If it's a causal loop, then it will repeat the same time travel event over and over again.
Only from the PoV of the machine and its contents. Per the outside observer, there is but the one jump. Yet again, you need to specify which timeline is being referenced when making statements like that.

However, the odometer reading of "x" (jumps) is after the time travel event. Therefore, if the odometer actually works, then its reading before the time travel event must have been "x-1" (jumps).
Contradicting the fact that you just said it reads x+1, a number to which it was set 7 seconds ago and not altered since. That would be a contradiction, and thus cannot be the case.

Concerning the 'rewrite the one timeline' interpretation:

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

Before anyone has ever time travelled, history will be a particular way, and this particular way (or version of history) will be altered by the time travel event to create a different version of history.
The time travel event (the appearance of the box) only has a causal effect on subsequent events, not on the prior ones that are the 'history'. The machine doesn't alter history, but it truncates it to a point and starts a new rewrite.

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

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

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

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

(the history as we presently know it, before any time travel events).
It is now presently 1985 and there is no 'we' there, so no, that statement makes no sense.

you cannot already have arrived at 1990 (as a time traveller) before you have time travelled to 1990.
Traveling to 1990 and arriving there is the same thing. That arrival event IS the time travel event. Are you talking about a different jump? Before that is 1989. 1991 is after that. The traveler has a memory of a nonexistent 1991, it being nonexistent because it's a future time, yet to actually be.

To simplify matters, we might only consider one time travel event rather than several. Also, in this discussion I'm interested in the possibiilty and consequences of time travel, not in preserving the stability of the population or the timeline.
If you're interested in consequences, you need to address the case of multiple machines crossing each other. I thought we were deliberately ignoring the lack of possibility. If you're actually interested in it, then exploring consequences is moot until you find a way that it's actually viable. SEP seems to suggest that pacing counts, but that's hardly something with interesting consequences.

Obviously, a time machine will appear in the past to come "out of nowhere" following the time travel event.
Again, it doesn't follow a time travel event, it is the event. If you're talking about the departure event, the appearance of the machine in 1990 does not follow that event. 1990 is before 2024.

That is not what I am talking about. I am talking about the history of a time machine's construction being erased in a causal loop, such as in the museum donation scenario.
The loop does not erase its construction. It isn't something that is constructed at all. It's a solid example that 'things' in that universe don't necessarily need a construction phase.

This is the sort of appearance from "out of nowhere" that I am referring to; that a time machine or its technology comes to exist without any causal history.
It has a causal history. It's just a retro-causal history is all. As I said, you're going about finding the inconsistency all wrong. Stop trying to find the end of a loop that doesn't have one. That's not where the inconsistency is.

The same applies to the dinosaur visitor who can exist in the past (which is necessarily post-time travel) prior to ever having been born (which is necessarily pre-time travel).
He was necessarily born pre-Cretaceous? That makes no sense to me. It can make sense in the branching case, depending on how one chooses to order events that are not on the same timeline.

Can somebody demonstrate the typical definition of determinism?
That is QM (or time) interpretation dependent, and no,. there is no way to falsify the interpretations that are not deterministic in one way or another.

For example, I spend my life working out time travel technology and build a working time machine. I then time travel back to 1990 and teach my younger self how to time travel. My younger self grows up, uses the knowledge to time travel back to 1990, and teaches my/their younger self how to time travel.
Why? He's already got the first 'you' teaching him. How many of you does it take? You're not making a loop by doing this. You're making a crowd control situation.

A causal loop follows the initial time travel event, but it has a different history prior to the first time travel event (an original history in which I figured out time travel without having been taught it by my time travelling self).
Well, you just had two different people (both you) time travel to the same spot. What if the coordinates are exact and second one obliterates whatever was at the spot at which it appear? I mean, you've never really specified what happens when the machine pops into existence somewhere. What happens to the bugs and other contents of that location? If there's a person there, or half of one, or the middle of a jet engine in flight? What if you manifest a mile underground? Never mind you being somewhat stuck, but what happens to the rock that was there a moment ago?

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

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

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

I thought your post was too large, but mine is even larger.
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Again, on which timeline are you measuring this?

The word "timeline" is, of course, vital in the study of history. Over an era there is a timeline of wars, a timeline of governance, illnesses, etc. But the word used in this thread is a many-worlds fabrication. Its twin brother is "alternate history".

Is there any evidence of the existence of timelines in the physical world beyond time dilation?
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They didn't in the [spawn new timeline] scenario, so nothing to explain.

I'm not sure what you mean by "spawn new timeline scenario". Two posts ago, I rejected the view that there is more than one timeline. However, I retain the idea that there must have been one version of history before any time travel events and a different version of history after the first time travel event (a history which henceforth includes a time traveller), at least different starting from the destination time of the time travel.

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

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

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

If, say, the present is designated to be 'the present' in this spawned timeline, then the traveler (if there is one) must be present at 'the past', 5 years prior. Did he travel there? I suppose he did

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

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

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

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

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

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

If I were to make the first-ever time travel journey tomorrow to arrive at the destination time of 1985, why would the population suddenly decrease from my POV as a result of the time travel? I don't believe you think it's only because Earth's population was smaller then.

That's interpretation dependent. Empirically, the guy will remember being born, sure. Given the copy/past interpretation, yes, he was actually born in some timeline somewhere, one of many, but not this one.

There's only one timeline.

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

Why was he never born?

That state doesn't exist in the one timeline.

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

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

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

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

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

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

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

Again, on which timeline are you measuring this? Given a time machine, this would obviously not be true or a calendar timeline. Marty is in 1955, well before say 1968 when he is born, contradicting your statement.

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

On Marty's timeline, he is in what appears to be 1955, and has 17 years of memories, which include stories of his birth. If the memories were perfect, yea, he'd remember that birth. Whether that birth event actually exists is a matter of interpretation, just as is my birth event. Per last-Tuesdayism, there is no way I can prove that I was ever born. We all just assume it by convention.

Being alive is pretty good evidence of having been born.

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

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

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

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

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

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

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

The word 'now' in that sentence is ambiguous. Presumably you are still planning to go back to 1985, and thus it is still 'now' 2024, and there is still a 'we' to know such things.
If the action has just been done, then 'now' is 1985,

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

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

You mean 1984?

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

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

Traveling to 1990 and arriving there is the same thing. That arrival event IS the time travel event. Are you talking about a different jump?

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

Before that is 1989. 1991 is after that.

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

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

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

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

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

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

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

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

If you can suspend disbelief, I suppose.

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

Okay, then where is the inconsistency?

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

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

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

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

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

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

Let's just assume that it works flawlessly and does no serious damage or harm. It's time travel technology and it works.
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If they were to time travel (at some time after this present time), then they would alter the past and present as we know it.Luke

Although we would never know it.
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Although we would never know it.

True, but this doesn't make time travel any less possible. Also, if we were on a timeline which had had time travellers in its past then we can imagine that there might be reports or records of it.
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If they were to time travel (at some time after this present time), then they would alter the past and present as we know it.Luke
The "If" part needs backing proofs with evidence before the whole sentence could be accepted as a meaningful statement.
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I retain the idea that there must have been one version of history before any time travel events and a different version of history after the first time travel event (a history which henceforth includes a time traveller), at least different starting from the destination time of the time travel.Luke
If the 2024 that doesn't yet include the time traveler is before the 1990 that includes the time traveler, then if would seem a stretch to call what he has done 'travel to the past'. It seems to be just a re-setting of the present state (the part outside of the machine) to what things looked like back then, but no actual travel anywhere.

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

The time traveller does not depart from the present of the spawned timeline, but from the present of the original timeline.
You said you were rejecting the 'spawned timeline' idea that occupied so many of our posts.

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

He did travel to the past from a time which is in the relative future of that past time. He did not travel to the past from a different timeline; his time travel will change the history of the same timeline. The changes will begin from the time traveller's date of arrival, starting with the addition of the time traveller in that time.
This is the truncation I mentioned, the overwrite scenario instead of spawn new line scenario. The inconsistency is calling 1990 'the past'. If the universe is currently being rewriten from there, then 1990 is the present, and there is no original history of making the machine. Those dates have yet to be written since they are in 'the future'. So now you have a machine sitting there un-built, but not un-caused. It was caused by a nonexistent retro-causal occurrence.

If I were to make the first-ever time travel journey tomorrow to arrive at the destination time of 1985, why would the population suddenly decrease from my POV as a result of the time travel?
The people there now have access to time machine technology, so that timeline is likely to be overwritten at any point. Eventually somebody will erase all of human existence and that will be that. It takes just one traveler going back a million years or so.

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

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

Why was he never born?
This isn't hard. His birth event doesn't exist (assuming he/somebody/something truncates the present to a date prior to the birth date. If he isn't the guy in the machine, then he doesn't exist either (at all). So not even a memory of being born.

Okay, in the linear time there are dinosaurs, and a time traveller and their time machine have appeared uncaused. Nobody was born, yet the time traveller exists. How is this consistent with causality and determinism?
We're in a universe with retro-causality here, one that a cause obliterates its own existence from the one history.

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

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

Surely, their birth must precede all the other events of their life
So 2024 precedes year -100,000,000, a funny interpretation of the word 'precedes'.

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

Being alive is pretty good evidence of having been born.
Not if your earliest appearance was from a time machine. You keep thinking the rules of this universe apply to this retro-causal one.

The arrival of the time machine in 1990 does not follow its departure from 2024? But isn't that exactly what a time machine does?
From the PoV of the machine, sure, That's the same as memory. 2024 feels like 'the recent past' to the machine and its contents. If we're talking about the stacking model, it actually still is the past, and sure, the machine was in fact built at some point. That model is empirically different than the other ones we've been discussing.

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

The "If" part needs backing proofs with evidence before the whole sentence could be accepted as a meaningful statement.
We were deliberately ignoring all that, since the possibility of this as described isn't there at all.

The word "timeline" is, of course, vital in the study of history. Over an era there is a timeline of wars, a timeline of governance, illnesses, etc.
Here you seem to be using the word 'timeline' to mean something like 'period of time'. That's not how it is being used in our posts. One timeline with Hitler losing WWII. One with him winning. Others with no Hitler. Other timelines with no humans at all, ever.

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

The timeline we speak of here is a specific history of everything, not just the path traced through spacetime of a single object.
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The "If" part needs backing proofs with evidence before the whole sentence could be accepted as a meaningful statement.
— Corvus
We were deliberately ignoring all that, since the possibility of this as described isn't there at all.
Sure. The point is not a criticism or condemnation by any means. It is just to clarify the statement is unsupported in any meaningful manner without proofs and evidences, hence all the following arguments would be just speculative conjectures.

Because of the fact the premise "IF" describes the possible physical and empirical events, and also the conclusion part is soley dependent on the premise, it should have given even brief explanations how the IF part could be possible, for it to be accepted as a valid assumption for the further arguments.
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The "If" part needs backing proofs with evidence before the whole sentence could be accepted as a meaningful statement.
— Corvus
We were deliberately ignoring all that, since the possibility of this as described isn't there at all.
Come to think of it, what prevents you from trying to prove the assumption? Wouldn't it be actually an interesting attempt, and all the emanating arguments from the proofs (if it were possible to come to some sort of proofs with evidences) would be more exciting? :D
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The word "timeline" is, of course, vital in the study of history. Over an era there is a timeline of wars, a timeline of governance, illnesses, etc. But the word used in this thread is a many-worlds fabrication— jgill

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

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

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

I admit, I am stretching a point. I'm looking for any sort of evidence of change of movement through time. And time dilation shows one individual moving at a different rate, but through the same "timeline". Actually, I think of each instant as triggering an infinity of timelines, an unimaginable bifurcation that we wander through without second thought. As I punch this key I choose or create a timeline. A study of the structure of this web of timelines would seem appropriate as a prelude to this thread.
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If the 2024 that doesn't yet include the time traveler is before the 1990 that includes the time traveler, then if would seem a stretch to call what he has done 'travel to the past'. It seems to be just a re-setting of the present state (the part outside of the machine) to what things looked like back then, but no actual travel anywhere.

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

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

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

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

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

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

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

Yes, however, I also said:

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

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

Keep in mind that I'm not a presentist, either. As I said earlier in the discussion and as I have explained previously on these forums, I believe that a combination of both views of presentism and eternalism are required to coherently account for time.

The inconsistency is calling 1990 'the past'. If the universe is currently being rewriten from there, then 1990 is the present, and there is no original history of making the machine.

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

then 1990 is the present, and there is no original history of making the machine. Those dates have yet to be written since they are in 'the future'. So now you have a machine sitting there un-built, but not un-caused. It was caused by a nonexistent retro-causal occurrence.

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

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

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

Why was he never born?
— Luke

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

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

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

Although the time traveller can obliterate their birth event from the original timeline, this isn't necessarily the case. But I'm not entirely sure what you mean by "a cause obliterates its own existence from the one history".

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

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

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

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

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

Two kinds of time mixed there, unless the history line is never truncated, and the machine simply writes the current universe a new way without traveling at all. That model (I'll call it the stacking model)...

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

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

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

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

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

Ah, you actually identify a line.

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

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

Why not? Did he somehow prevent it from happening?

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

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

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

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

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

From the PoV of the machine, sure, That's the same as memory. 2024 feels like 'the recent past' to the machine and its contents.

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

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

I do understand why it would be a contradiction. As I said, I acknowledge that it must read 'x' jumps in the causal loop. I'm just saying that it must have read 'x-1' jumps prior to the time travel (which happens outside the causal loop).
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It is just to clarify the statement is unsupported in any meaningful manner without proofs and evidences.
There is evidence one way or the other. There is rarely 'proof' of anything. In this case, there are valid non-local interpretations of physics with superluminal cause/effect. That opens the door for retrocausality. But none of the interpretations allow superluminal information transfer. That pretty much closes the door.
Physics mathematically allows for tachyons, which can 'go backwards' in time, but nobody has ever found a tachyon or other necessary exotic matter such as things with negative mass and such.

Because of the fact the premise "IF" describes the possible physical and empirical events, and also the conclusion part is soley dependent on the premise, it should have given even brief explanations how the IF part could be possible, for it to be accepted as a valid assumption for the further arguments.
I suppose in the end it would matter how it works, before we go about presuming the properties and possible interpretations of the thing.

The time traveller departs from the timeline without time travel and creates a timeline with time travel by doing so.Luke
Again, I thought you were abandoning the interpretation with creation of timelines in favor of modifying the one and only line.

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

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

If it will help make things clearer, I can try to dispense with (McTaggart's) A-series terms. The time traveller departs from the year 2024 and arrives in the year 1990.
Seemingly an admission that time travel with presentism don't particularly mix. I mean it does. SEP discusses it, but says very much that the arrival event occurs decades before the departure event, back when the arrival event was the present, which only happens once. That model doesn't have a history between those times where time travel hasn't yet happened.

Keep in mind that I'm not a presentist, either.
You use a lot of A-series terms, which make no sense without presentism. Yes, learn to dispense with the concept. It helps. There's no evidence for it other than intuition, a pragmatic lie that makes us fit.

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

As I said earlier in the discussion and as I have explained previously on these forums, I believe that a combination of both views of presentism and eternalism are required to coherently account for time.
Maybe. I mean, it;s not possible, so you'd probably get a hard contradiction with eternalism as well. Doing so given an impossible premise wouldn't falsify either view.

Why would the time machine be un-built in 1990?
I don't mean disassembled. I mean something exists which never came into being. But this is in the truncate-model, which I'm rejecting because we could never have existed in such a universe.
I know you consider the machine to have been built, despite that process not existing, and 'was built' (a past tense reference) 30 years from now. As Dr Who said in his Xmas party: Didn't you get me this next year?

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

I was referring to the sequence of events of a person's life.
But that's just a memory. It is a memory of nonexistent events.
His birth event (say in 1975) is nonexistent. He can't for instance take the machine back to it and witness it.
Why not? Did he somehow prevent it from happening?
Yes. A machine appeared in the Cretaceous and humans evolve only on the timeline without the machine.

I admit, I am stretching a point. I'm looking for any sort of evidence of change of movement through time.
'Change of movement through time'. What an interesting way of putting it. You'd like the SEP definition of time travel then, which is whenever clocks don't agree for reasons other than a faulty clock.

Funny thing about time dilation is that there's no way to tell which individual is the one 'moving through time' at the faster or slower rate.
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'Change of movement through time'. What an interesting way of putting it.

"Time travel" has been beaten to a pulp through pulp science fiction. I like "timelines", but only those I initiate. Time is like a murky, viscous liquid that covers your feet - the older you get the harder it is to make progress.
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The time traveller departs from the timeline without time travel and creates a timeline with time travel by doing so. — Luke

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

Yes, I have.

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

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

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

Seemingly an admission that time travel with presentism don't particularly mix.

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

SEP discusses it, but says very much that the arrival event occurs decades before the departure event, back when the arrival event was the present, which only happens once. That model doesn't have a history between those times where time travel hasn't yet happened.

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

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

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

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

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

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

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

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

I noticed.

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

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

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

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

Are you saying that humans did only evolve on a timeline without a time machine, or that humans can only evolve on a timeline without a time machine. If the latter, then why can't humans evolve on a timeline that has a time machine in the Cretaceous period?
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I'm disappointed that nobody has invented time-travel Monopoly. As in the ordinary case, a game consists of players traversing a spatial loop by rolling dice to land on squares , then making decisions and interacting with other players, and repeating this process until they reach or pass Go. Call this an iteration of the loop.

For a trivial implementation of the time-travel variant, say that whenever a player passes Go, he is forced to land on Go and must wait for the other players to finish the loop. After all players finish the loop, their individual wealth is reset to that of the beginning of a new game (£200?). During the next iteration of the board, the players must perform exactly the same actions as they did in the previous iteration and must re-use the dice-values they previously rolled.

Obviously that example is completely useless and boring, but it expresses the desirable property of an iteration of a Monopoly time-loop, namely that the iteration of the loop is stable, in the sense that the next iteration must proceed in exactly the same way as the last.

To put this condition more generally, a stable iteration must be a fixed-point of some functional F of the players combined observations and decisions. In the case where we always restart the players from Go and reset their wealth to the beginning of game and force them to play as they did in the last loop , then every possible iteration of the board is a fixed-point, since there is no new information leaking from one iteration to the next. This corresponds to setting the functional F to the identity functional.

So the challenge of Board-game game design here, is to design a functional F that allows partial leakage of information from one iteration to the next, such that players have freedom to make decisions that spans several iterations of the loop, albeit with that freedom decreasing from one iteration to the next as the game converges towards a fixed-point, such that the result of the game expresses time-travel, or rather a time-loop, although the iterated gameplay beforehand does not.
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Physics mathematically allows for tachyons, which can 'go backwards' in time, but nobody has ever found a tachyon or other necessary exotic matter such as things with negative mass and such.
Tachyon is a hypothetical object which is in the domain of a fiction.

I suppose in the end it would matter how it works, before we go about presuming the properties and possible interpretations of the thing.
In Modal Logic, when you say X is possible, it implies that X was possible in real sense. For example,
X = I can order wine instead of beer.
X =I can have pizza instead of sandwich or McDonald.
X = I can read Hume instead of Kant. These are all possible options that can happen in real life. It is just a matter of one's choice, hence legitimate ground for being premises for further discussions.

But if X = I can walk on the planet Jupiter, or I can fly faster than light. then it would be rejected by most people unless there were some explanations on how that would be possible, because there is no logical ground or scientific possibilities for that statement to make sense on their own out of blue. Therefore it is not fit for being a premise for any intelligible discussions.
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There is a difference between the concept of changing the past versus the concept of affecting the past. Changing the past is that which only a time-lord could do on the basis of his transcendental privilege, as illustrated by an unrestricted variant of the time-travel monopoly game, whose rules permit the players actions to diverge from any fixed-point of the game, which is essentially Classical Monopoly, that incidentally isn't identified as expressing time travel for that very reason (for fictional time-travel is a bunch of inconsistent narrative requirements).

But the ability to merely affect, or rather to construct the past is a weaker condition that only stipulates that the past is a creation made out of available information, including information that is the consequence of players present and future actions, but it doesn't assume that the revealed information is mutable.

Of course, a mere mortal cannot know as to whether the information at his disposal "comes from the future or the past", in order to rule out the possibility of him using that information to cause causal contradictions, and this epistemic restriction isn't modeled in the suggested time-lords game I previously suggested, which grants players transcendental knowledge of the future/past distinction and merely forbids them from acting upon it.

The only means of eliminating temporal omniscience from the game, is to restrict the game to a single iteration of the board loop. But to distinguish the resulting game from a case of single-iteration classic monopoly requires a different approach to the rules and constraints. For example, by granting a random event such as "Bank error in your favor, collect £200" the capacity to impose constraints onto the future actions of the players. Say, by that event triggering the potential introduction of a "cause" card into a deck of cards that some player is guaranteed to draw in the future (but without his prior knowledge).
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Tachyon is a hypothetical object which is in the domain of a fiction.
It is a hypothetical object in the domain of science. Can't help it if the fiction folks are the ones that latched onto it.
The particle is lumped onto various headings of 'exotic matter' (including various virtual particles), and exotic matter is seemingly a hard requirement for time travel.

But if X = I can walk on the planet Jupiter, or I can fly faster than light. then it would be rejected by most people unless there were some explanations on how that would be possible, because there is no logical ground or scientific possibilities for that statement to make sense on their own out of blue. Therefore it is not fit for being a premise for any intelligible discussions.
Closed time loops are valid solutions to Einstein's field equations. They would probably involve exotic matter, and would already be there, forming small close time loops. Classically (unrealistic), this is equivalent to a 'rift in space & time' (definitely a fiction term), sort of like in the Kate & Leopold movie. There's no machine, no punching in a desired destination. You just compute where and when they are and leverage them.

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

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

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

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

This is a causal loop, I take it? You said that a causal loop only appears to occur once for any outside observer.
This is a different kind of loop since it doesn't involve the same machine traveling over and over. It only makes but the one trip. That's enough to end the universe, according to the 'rewind/truncate' thing you've been pushing lately.

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

Does that mean we can't think about it, then?
You can, but it would be really nice if the discussion was free of more contradictions than just the impossibility of time travel (besides the pacing).

Is it so difficult to distinguish a timeline which contains a time traveller from one which does not?
There is but the one timeline, unless we're changing stories again.

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

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

Are you saying that humans did only evolve on a timeline without a time machine
They only evolve from a Cretaceous state that doesn't include a time machine, yes. More precisely, humans don't evolve from a Cretaceous state that is in any way different than the Cretaceous state from which we evolved. That's popularized by the term 'butterfly effect'. Chaos theory is very clear on points like this.

I like "timelines", but only those I initiate.
And what definition are you using this time? What is this sort of timeline, and how does one go about initiating one?
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Your new suggestion says that the original (and only) timeline is truncated back to the destination event upon somebody time traveling backwards. If it subsequently (30 seconds later) is truncated again, by 30 seconds, then there is no way for the history of the timeline to grow beyond any backwards travel departure. The only way for it to go forward significantly is if there is never again a backwards time travel event. I don't know about forward time travel You've given seemingly no thought as to how that might work.

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

Let's say the time traveller travels from 2024 to 1985. The time travel event will change the history of the timeline from 1985 onwards, compared to the history of the timeline as it was before the time travel event took place in 2024. But I don't see why any time after 1985 should not exist, post-time travel. Unless the time traveller does something catastrophic, then I would imagine that many of the same people will be born and many of the same things will happen as they did prior to the time travel event's occurrence in 2024.

On that note, do you agree that the time travel event does not occur until 2024, given that the time traveller departs from 2024 to arrive in 1985? The time machine is not used, or perhaps even created, until 2024.

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

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

I didn't say destroyed. I say it ends. Your idea posits that: If I go back to 1990, everything from there to 1990 ceases to be part of the universe. Is not the entire universe affected by this, or do we just rewind some limited region like Disneyland?

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

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

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

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

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

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

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

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

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

How does the butterfly effect of the time travel event necessarily prevent the evolution of humanity?
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They have quantum teleporters, which means they actually have teleported a small object from here to there

Wow :gasp:

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

What is this sort of timeline, and how does one go about initiating one?

My own. Replying to your comment does the job.
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Tachyon is a hypothetical object which is in the domain of a fiction.
— Corvus
It is a hypothetical object in the domain of science. Can't help it if the fiction folks are the ones that latched onto it.
The particle is lumped onto various headings of 'exotic matter' (including various virtual particles), and exotic matter is seemingly a hard requirement for time travel.
What is exactly the 'exact matter' including various virtual particles?

None of this rewinds reality, but actual retro-causal (or FTL) information transfer opens things up to paradoxes.
So does it not prove that the whole story is just a fiction itself?

Yet another thread about time. I was thinking yesterday of a very vague idea that I'd like opinions on. So then if we took a "snapshot" of this very moment with it's totality, that being: The position of every single object, cell and et cetera. And had the ability to manipulate matter in such a way that we can reposition new "environmental" circumstances into the ones that we have snapshotted, would that not be considered time travel? If anybody ever has watched "Watchmen" and know of Dr.manhattan I ask this question as regarding his fictional abilities. Those being the ability to manipulate a surrounding environment to a total extent, whatever that may be.

The OP seems to have already taken into account of time travel is a fiction, but asked how it could be possible even hypothetically. Shouldn't how one could change the past events follow after fictitious successful time travel has been achieved, rather than before the travel? Have you achieved fictitious time travel into the past or future in actuality?
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They have quantum teleporters, which means they actually have teleported a small object from here to there.

Isn't quantum teleportation essentially just the transfer of information though?
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What is exactly the 'exact matter' including various virtual particles?
I didn't mention 'exact matter'. Perhaps you misread 'exotic'. One can simply google 'exotic matter' for a more specific list.

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

You quote the OP asking this same thing:

And had the ability to manipulate matter in such a way that we can reposition new "environmental" circumstances into the ones that we have snapshotted, would that not be considered time travel?
One can scan a person down to the biochemical level: the location of every cell and connection, the chemical makeup of all fluids everywhere. That's still a classical measurement. It's trying to scan down to the atomic level where things get impossible.

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

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

Shouldn't how one could change the past events follow after fictitious successful time travel has been achieved, rather than before the travel? Have you achieved fictitious time travel into the past or future in actuality?
I cannot parse this. How does something follow something that is fictitious?

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

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

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

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

Let's say the time traveller travels from 2024 to 1985. The time travel event will change the history of the timeline from 1985 onwards, compared to the history of the timeline as it was before the time travel event took place in 2024. But I don't see why any time after 1985 should not exist, post-time travel.
I didn't say otherwise. The VCR tape resumes recording at 1985 and progresses no problem.

Unless the time traveller does something catastrophic, then I would imagine that many of the same people will be born
Well, from about 1986 on, the people born will be different ones. That's a very chaotic function.

On that note, do you agree that the time travel event does not occur until 2024, given that the time traveller departs from 2024 to arrive in 1985?
If this new timeline also has a time travel event in 2024, then the rewind happens again. If there is no time travel event there, then no rewind takes place then. That's why I came up with the 30 second train-track example, where the subsequent time travel decision is very likely. Over 40 years, it is very unlikely that events will turn out identically, especially if Bob goes back to 1985 explicitly to prevent the creation of the time machine.

Where did I say that "everything from there to 1990 ceases to be part of the universe"?
What does rewind do to the 40 years over which we backtrack? It either erases as it goes or that part of history gets overwritten as the recording resumes. Either way it is not part of the universe. That's the problem of using the same tape to record something new: you lose what was on there before.

Why does the first time travel event allow history to "all get rewritten again" but the second time travel event does not?
I didn't say that.

How does the butterfly effect of the time travel event necessarily prevent the evolution of humanity?
Who gets born is very much a function of exactly when people have sex, and which sperm wins. Which species come about is very much a function of random mutations and environmental chance. All these things are altered by chaotic things in the environment.

Read up on chaos theory. I can't possibly explain it to you in this context. There is no strange attractor for a specific person being born, or for a specific species to evolve. There would probably be mammals around since those existed in the Cretaceous, but probably no mammal that you'd recognize.
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I didn't mention 'exact matter'. Perhaps you misread 'exotic'. One can simply google 'exotic matter' for a more specific list.
Yes, it must have been my mistyping. I try not to google too much if I can help it. The underlying implication for asking the question was not just the meaning of the concept, but also your explanation on how it works with time travel.

So does it not prove that the whole story is just a fiction itself?
Not really. CTCs are allowed, and might actually exist at quantum scales. Their existence is not inherrently contradictory. To open one at a classical scale probably leads to necessary contradictions, and since all the time travel stories are classical, I'd have to actually answer that such stories are necessarily fiction.
Not saying they are not allowed, but trying to focus more on the possibility of the travel before what one can do in the past or future when arrived there.

One can scan a person down to the biochemical level: the location of every cell and connection, the chemical makeup of all fluids everywhere. That's still a classical measurement. It's trying to scan down to the atomic level where things get impossible.
But there are loads of the other aspects that you must think of such as the mental contents = memories, thoughts and the consciousness of the past, such as if you travelled to 1761, would you still contain the present mind, or would the content of your mind be wiped out, and replaced by the 1761 mind, or would it become total blank due to the travel?

Shouldn't how one could change the past events follow after fictitious successful time travel has been achieved, rather than before the travel? Have you achieved fictitious time travel into the past or future in actuality?
— Corvus
I cannot parse this. How does something follow something that is fictitious?
Fictitiously.

Not saying time travel is total baloney, but I am interested in how it might be possible, as well as what you could do in the past or future when you arrived there.
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It is entirely quantum, not a classical copy.

:up:
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The particle is lumped onto various headings of 'exotic matter' (including various virtual particles), and exotic matter is seemingly a hard requirement for time travel.

I didn't mention 'exact matter'. Perhaps you misread 'exotic'. One can simply google 'exotic matter' for a more specific list.
Yep, got the description for "exotic matter", but you still need to explain why and how exotic matter is required for time travel. How does it supposed to work?
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Read up on chaos theory. I can't possibly explain it to you in this context.

Sensitive to initial conditions is easily understood as the butterfly effect, but the other two conditions (both of which may imply SIC) are not as easily digested when one attempts to apply chaos theory to the real world. The theory assumes a dynamical system, which means a simple iteration of a single complex function. And the iteration of points must follow stringent patterns. Bringing up CT is like bringing up QT.
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But there are loads of the other aspects that you must think of such as the mental contents = memories, thoughts and the consciousness of the past, such as if you travelled to 1761, would you still contain the present mind, or would the content of your mind be wiped out, and replaced by the 1761 mind, or would it become total blank due to the travel?
Why would any of that occur? I mean, sure, if one was to travel to 1990, they'd find me there, but without 2024 memories, but why would the teleporter leave you in a different state when it by definition doesn't?

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

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

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

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

Not saying time travel is total baloney, but I am interested in how it might be possible, as well as what you could do in the past or future when you arrived there.
Well, the usual physical explanations disallow the concept of 'change the past'. That means much of our discussion is moot. The machine (presuming unrealistically that the requirement is a vehicle of sorts) comes first, then the development of it. More realistic is the idea that the connection is established at both ends and there's no surprise when something appears uncaused 'from nowhere' so to speak.

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

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

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

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

and all that we've discussed (who gets conceived/born, which creatures evolve) is very much a function of the weather, among countless other factors, most notably wave function collapse.

Huh. How did wave functions sneak in? But I'm being picky . . . please continue time travel speculation.
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