Time travel implications with various philosophies

noAxioms

I request some discussion of the implications of various philosophical positions on the what might be potential time travel. There was another thread on the topic, but hardly comprehensive.

First of all, the time travel has to at least be a tiny bit plausible under physics. The alternative is magic, and if you posit magic, you can also conclude anything you want.

SEP envisions time travel as some sort of vehicle (Doctor, Leap, Putnam) or other device that takes the occupant to a destination time selected by the occupant. Other versions are closed time curves (CTC) as envisioned by Godel and Einstein, and also differential aging (Einstein).

I will pretty much dismiss most of these as magic. Differential aging is dismissed as not time travel, else pacing would qualify. The CTC thing works given exotic matter, creating a sort of portal that needs to be set up at both ends, not just at one end. I would also accept any form of retrocausality that preserves mass, energy and momentum.

SEP denies that sleep, coma, cryonics, VR, and crystal balls are time travel. I agree except for the crystal option. If you have a crystal ball that actually gets information from a future time, then that's blatant retrocausality, and time travel can be constructed from that.

So I see two main visions of how it works: Given exotic matter, one has wormholes at fixed locations/times and one can supposedly make one big enough to get a living thing through (just an information stream is more likely). This is sort of like the movie Kate & Leopold.

Another is teleportation, where a person is scanned to sufficient detail to reproduce, and then reassembled at another time. If there's any retrocausality, that time can be a past one.

Finally, there is the usual branching model, a little like MWI except alternate worlds (in Hilbert space) are created by said device and not by quantum measurement events.

All that said, the list of topics I would like to discuss is meant to be somewhat comprehensive, so it's not short:

Presentism
If there is not a past or future, then there is no destination to which travel can target. It's off the table. Growing block at least leaves a past to target, but it being a past, there's no way to 'change' it, so the branching model is the only option. Moving spotlight seems to similarly only be compatible with the branching model unless one can specify how any form of presentism deals with closed time loops. Most travel models presume eternalism: The lack of a premise of a a preferred frame and time.

Identity
Traveling to a minute ago lets you meet another version of yourself. It is inherently a cloning machine, in which case new conventions are needed to designate who is the original and who is the copy. Identity is a convention usually, with nothing physical to back it.
One of my travel scenarios is very similar to teleportation, and the same questions about identity arise from being cloned by teleportation as opposed to time travel.
Also part of the identity issue is what is the boundary between you and not-you. So Captain Jack has this wrist time-travel thing. You strap it on and push the button and it takes you to the time you want. How does the device know to take your clothes but not the hand-railing your grasping? Where the boundary of what it takes?

Free Will
For purooses of discussion, free will is defined by choices not being a function of physical processes. This is not to be confused by fate, which is that minor variations in initial state have insignificant long term consequences. I deny fate, and any assertion of it needs to be justified and include a working knowledge of chaos theory which denies that assertion.

Quantum interpretations
All the interpretations have potential impact for time travel, but MWI is interesting because what if several worlds (1% of them say) all have somebody who decides to travel back to some similar time in their mutually shared prior state? What is the consequence of locality, or of counterfactuals?

ReEntry
How does the traveler target a location free of obstacles? What does it do with the material that is there? Replace it? It can't add to it, since physics really falls apart when the conservation laws are violated. Perhaps this is only an issue with the concepts deemed 'magic' above, but the branching idea seems to require a resolution to it.
How does the machine know what location in space to go to? It's not like 'Paris' is in the same place it way 2 hours ago. Why do all the time travel fictions ignore this issue? I've seen only a few comics like xkcd make fun of this obvious oversight.

Dualism, Philosophy of Mind
If there's clones, what is it like to be one of them and not the original?
How about epiphenomenalism, and simply doing something to move your personal spotlight to a different physical body in another time? That is very plausible as time travel, but what would be the experience of doing that?

Subjective Experience
What is the experience especially of the non-traveler. The movies always show the point of view of the protagonist who travels. In BTTF, consider Marty's father, who is a loser in 1985, and then transforms into the confident father. What is it like to be him? What's it like to be the Marty that was born to and grew up with the confident Dad? What becomes of that Marty?
BTTF depicts the 'changing' of history. What does that wording even mean? I consider BTTF to be one of the cases dismissed as magic.

Idealism
Is time travel meaningful under idealism? You just suddenly start having experiences of a presumably different time/world, sort of like going to Disneyland. It It's still linear, and actual 'travel' isn't really a meaningful term under idealism, so perhaps it can be dismissed.

Last Tuesdayism
This topic is well aided by people who know how to debate both sides of the Last-Tuesdayism stance. It is especially relevant to the branching view of time travel.

Multiple machines
What if time travel vehicles are common and everyone is doing it? What if time-jumps by others alters either the source or destination of your jump, 'while' (if that has any meaning) you are en-route?

Did I miss anything? I meant the list to be comprehensive.

Vera Mont

Did I miss anything? — noAxioms

Just the fact that time has no physical locations.

jgill

a little like MWI except alternate worlds (in Hilbert space) — noAxioms

Hilbert space? Don't end up there. You'll be a sentient vector upon which linear operators prey, trying to leave the premises but caught there by completeness, finally manhandled by an ominous inner product.

I don't think there is any hope for time travel.

Tom Storm

↪noAxioms

Do we know if time exists outside of human cognition?

BC

↪noAxioms

The important question here is cost and the likelihood of prompt and complete refunds if time travel doesn't deliver on the promised delights of the Roman Baths, the thrill of watching T Rexes mate, or the satisfaction of shooting J. W. Booth on his way to the Ford Theater on the evening of April 14, 1865. Or is time travel strictly caveat emptor?

Philosophim

Time doesn't exist somewhere on a hard drive. The universe is in a state at one moment, then another state in the next. The reason why the universe is in one state is because of the forces and matter in the previous state. Meaning that if we could reshape the forces and matter to what it was 1000 years ago, we would be in the state of the universe 1000 years ago. But we can't go back. There is only now, and what was before.

noAxioms

Thank you all for your initial responses.

time has no physical locations — Vera Mont

Well I didn't say 'travel to where they keep time'.

Time travel seems to be the presence of a person with memories of another time (a future time, or a discontinuously past time). A coma accomplishes this, but is trivially dismissed as mere 'waiting'.
In block terms, time travel is either a discontinuous worldline, or a worldline that isn't everywhere time-like.

I don't think there is any hope for time travel. — jgill

No argument, but also not the point of the topic.

Do we know if time exists outside of human cognition? — Tom Storm

Well clocks measure a kind of time (proper time), so that type is real, at least if you consider the clock to exist outside Human cognition. Coordinate time is another type, and that one is purely an abstraction (or it is under relativity, but not so much if you don't accept its premises).
Then there's the 3rd type of time, which is the one usually meant when asking the question of if time is real. It is sort of the progression of the present moment. Like the teapot orbiting past Jupiter, there is zero empirical evidence for it,

So it seems that the latter two qualify as a product of cognition, but certainly not limited to human cognition.

The important question here is cost — BC

Engineering issues are not a concern to this discussion, only the implications on current philosophies if it could be pulled off.
I am dismissing the Disney-version of time travel, where the entire universe is destroyed and rebuilt as a replica of what the machine thinks the selected time ought to look like. That's giving it omnipotent powers far beyond mere cost.

the thrill of watching T Rexes mate — BC

It was brought up elsewhere. If you go to watch the T Rexes mate, you either were always there, or you changed something. If a change was made, there's no going back to the world from which you departed. Humanity beyond what you brought with you is gone.

What that is like to the people you left behind is one of the items of discussion. A similar answer to the caveat emptor query.

↪Philosophim

Your post seems to presume a sort of growing block model, which is absolutist, and classically causal in nature. You can go with that assumption, but the conclusions would then only follow if that premise was true.

The universe is in a state at one moment, then another state in the next. — Philosophim

This part is specifically absolutist, since the assertion isn't true without a preferred foliation of all spacetime events.

The reason why the universe is in one state is because of the forces and matter in the previous state.

This implies the classical causality. In a classic sense, this works, but given closed time curves (CTC), there is not an objective ordering of causal events, and CTCs are valid solution in relativity theory. I'm not sure how growing block handles a CTC.

Meaning that if we could reshape the forces and matter to what it was 1000 years ago, we would be in the state of the universe 1000 years ago. But we can't go back.

'Reshape' is the omnipotent power thing I'm disallowing, so we either utilize one of those closed curves, or we actually 'travel' to 1000 years ago and make some local difference (similar to wave function collapse) that spawns a different causal progression from the mildly altered state (similar to MWI).

There is only now, and what was before.

That finally seems like a direct assertion of growing block view. You can 'go to' the past since it exists, but you can't go to the nonexistent future. But growing block says the past can't return to the state of 'is happening', so at best your presence there would be epiphenomenal, or again, the creation of a new branch, which is more sideways travel than backwards.

BC

↪noAxioms

I was joking. I do not believe time travel is possible -- though it is a fertile topic of science fiction.

On a more theoretical level, the past can not be altered. Remembered? Yes. Misremembered? Yes -- that's the usual experience. Reinterpreted? Yes. Denied? Some try. Longed for? Yes, usually that's a mistake--the good old days were terrible. Visited? No.

I look at the past as a crystal -- the atoms in the crystal stay organized the way they are and the universe remains intact. Time's arrow does not turn around and go wherever we might want it to go. We may not like the present, but we've already screwed things up in the past, and we would screw it up again--because we are not merely fallible, we are dead ringers for making big mistakes.

God, who some conceive as omnipresent -- present in the past, present, and future and everywhere all at once -- has not, according to our founding fictions. seen fit to do over any part of the past.

Vera Mont

Well I didn't say 'travel to where they keep time'. — noAxioms

But you did say:

SEP envisions time travel as some sort of vehicle (Doctor, Leap, Putnam) or other device that takes the occupant to a destination time selected by the occupant. — noAxioms

This means a physical body in a physical container, being transported from a point of departure to a destination, which would have to be an actual place where an actual body can land. That would be the only version that could be called time travel. Comas and aging and messed-up, unverifiable memories don't count.

In block terms, time travel is either a discontinuous worldline, or a worldline that isn't everywhere time-like. — noAxioms

The what now?

The most you can hope for is that someone in the past made a faithful virtual recording of some aspect of their world, and you can access that recording through some device. Like old movies.

noAxioms

I do not believe time travel is possible. — BC

I'm not saying it is, but that simply isn't the point of the topic. In a stretch, it could be, especially with exotic matter. Thing is, exotic matter, while mathematically consistent with theory, is not something that can be manufactured or manipulated.

So within reasonable confines, what if it were possible? What would be the implications of all the points I brought up?

I look at the past as a crystal

I personally don't put a whole lot of stock into the concept of 'the past', and most (but not all) of my discussion kind of assumes the concept is meaningless.

God ... has not, according to our founding fictions. seen fit to do over any part of the past.

OK. And what if God has seen fit to do exactly that, but not write about it? What would that be like? It's a valid point, and one that I neglected to include in my list.

But you did say:
":SEP envisions time travel ... to a destination time selected"
Yes I did say that. Travel to a time is like travel to a spatial location, and not to a place where 'space' is stored. Time travel is no different in that respect.
— Vera Mont

This means a physical body in a physical container, being transported from a point of departure to a destination, which would have to be an actual place where an actual body can land.

Pretty much, yea.

messed-up, unverifiable memories don't count.

No memory is completely verifiable, so I disagree with this statement. If I went back a year, I could make some (but not all) predictions about things I remember, so that very much does sound like a verifiable memory. I say 'some' because I'm a firm denier of fate, and my presence a year ago would change many things. The BTTF sequel with the sports almanac wouldn't work, but predicting comets and close meteor encounters would.

In block terms, time travel is either a discontinuous worldline, or a worldline that isn't everywhere time-like.
— noAxioms
The what now?

It's spacetime terminology from relativity theory. Hard to discuss that if you're unfamiliar.
I do realize that I am posting in a forum with members often unfamiliar with the physics involved, but it is after all a philosophical topic that doesn't belong in a physics forum. But I also don't see how proper philosophy on most of the above mentioned topics can be done by anybody not familiar with recent (at least up to 1960) physics.

The most you can hope for is that someone in the past made a faithful virtual recording of some aspect of their world, and you can access that recording through some device. Like old movies.

Anyone can look at the past, which isn't any sort of retrocausality. I mean, that's exactly what hte archaologists do. It's looking forward or causing some effect backwards that's the trick. Most of the plausible scenarios I have in mind require cooperation at both ends. No travel to a time that isn't expecting you, but rather a portal deliberately held open at both ends to let information or more through. So in that scenario, there's no 'changing' of the earlier time since the travel back to that point was always there. That's the nature of a CTC. SEP had some examples of this, but I find them implausible.

So to pick one of my points, I start with identity since so many other points rely on it. I have a cloning booth with three doors. A person walks in the middle door and that person walks out of both of the other two doors. The question is, which is the original? This contrived scenario is deliberately symmetrical, but with time travel or mere teleport machines, the symmetry usually isn't there.
This well might happen naturally all the time such as with MWI, but nobody sees both of the clones, so the convention is easy. The one you see is the original, and the one you don't isn't. Each of them considers himself to be the original, and that's not a contradiction because there's no evidence of one.

With a teleport/cloning machine where the clones can interact, that convention falls apart, and a new one is needed. It's a convention, meaning a deliberate choice. It's not something that can be physically verified because physics is mute on the topic.

Vera Mont

Pretty much, yea. — noAxioms

And time is not physical.

No memory is completely verifiable, so I disagree with this statement. — noAxioms

No amount of circumlocution will turn any form of memory into any form of time travel.

If I went back a year, — noAxioms

There is no there there.

jorndoe

Just the fact that time has no physical locations. — Vera Mont

Isn't it more that events have temporal locations?
Anyway, duration and simultaneity are meaningful enough, and suggest some temporal structure taken together.
Processes/events can be reasonably clear temporally, and less clear spatially.
Objects can be reasonably clear spatially, and less clear temporally; volume and place are meaningful enough, and suggest some spatial structure taken together.

Vera Mont

Isn't it more that events have temporal locations? — jorndoe

No. What you've got there is a metaphor. You can mess with language, but you can't mess with physics. Time has no substance, no length and width, no spatial orientation, no compass points, no loci, no tangible co-ordinates at which you can land a 3-dimensional object of any kind.
(Not even if you were unconcerned about displacing an object or collection of objects of the same mass and/or volume that occupied the same space at that time. If you time-zapped yourself into the 17th century, might you not worry that Rene Descartes would be zapped into the present? Or a skinny med student and a half-killed dog? Worse yet, you could not return him to his proper time, because the processes and changes he was supposed to undergo through the duration of the absence did not take place; the Descartes of a day or a week later no longer exists. Nor, of course does the you of tomorrow or next week.But we're not concerned about that, right?)

Anyway, duration and simultaneity are meaningful enough, and suggest some temporal structure taken together. — jorndoe

In the human mind.

Processes/events can be reasonably clear temporally, and ~~less clear~~ [meaningless] spatially. — jorndoe

Objects can be reasonably clear spatially, and less clear temporally; — jorndoe

Objects with a longer life than a muon's exist in space-time. You can ask : "When was this ball red?" and the correct answer might be "Before Rex chewed the paint off it." But there is no place in time where you can go and see that ball as red; in order to remember it, you had to have seen it while the paint lasted.

volume and place are meaningful enough, and suggest some spatial structure taken together. — jorndoe

Volume and place, sure; duration and process, not.

sime

Anyone can look at the past, which isn't any sort of retrocausality. I mean, that's exactly what hte archaologists do. It's looking forward or causing some effect backwards that's the trick. Most of the plausible scenarios I have in mind require cooperation at both ends. No travel to a time that isn't expecting you, but rather a portal deliberately held open at both ends to let information or more through. So in that scenario, there's no 'changing' of the earlier time since the travel back to that point was always there. That's the nature of a CTC. SEP had some examples of this, but I find them implausible. — noAxioms

I think that the concept of non-local causal cooperation that you allude to is interesting and useful, but i think CTCs are empirically inconsistent and theoretically unnecessary. For any proposed loop, if you could experience going around it more than once, then the proposed loop would be falsified (since the second iteration would be distinguishable from the first). But if you cannot experience going around the loop, then how you do know the loop exists to begin with? A theory containing a CTC cannot have empirically observable consequences on pain of contradiction. So a CTC can at most be an uninterpretable expression of mathematical convenience rather than a representation of a physically verifiable entity.

As for archeology, how do you know that the practice isn't retrocausal? Consider that the effect of digging into the ground can be expected to produce both predictable consequences that we might call "forwards directed" e.g the dig producing a hole next to a mound of earth, as well as unpredictable consequences that we might call "backwards directed", e.g the dig revealing of a Roman hoard of treasure. For why should the hoard of treasure be assumed to exist before it was discovered in the hole? Why shouldn't the archeologist take some credit for the hoard's physical existence?

Sure, before the dig was commissioned archeologists might have discovered other archeological evidence the day before, that implied that the hoard would be found where the hole would later be dug. But that merely moves the goal posts; why should the hoard of treasure that was unearthed be assumed to exist prior to the establishment of the archeological evidence that they determined day before?

So in short, i think the concept of non-local causal cooperation (Synchronicity?) is a causally permissible concept that aligns with experience, but I cannot say the same about closed time-loops.

jorndoe

No. What you've got there is a metaphor. You can mess with language, but you can't mess with physics. — Vera Mont

What am I to do over at the bus stop, when I find that the bus is scheduled to arrive in a few minutes...?
Do I deny that "events have temporal locations", and ehh head over to the pub instead? :)

Vera Mont

Anyone can look at the past, which isn't any sort of retrocausality. I mean, that's exactly what hte archaologists do. — noAxioms

No, it isn't. What archeologists look at is bits of pottery and metal and the remnant of walls that they dig up in old habitation sites. From those physical bits and pieces, they construct a story of what the people who lived there may have been doing and how long ago. What paleontologists look at is layers of rock and fossils, from which they construct a story of what may have happened in the past.
You can mess with language, not with physics.

Vera Mont

What am I to do over at the bus stop, when I find that the bus is scheduled to arrive in a few minutes...?
Do I deny that "events have temporal locations", and ehh head over to the pub instead? — jorndoe

The bus is expected (barring unforeseen events) to arrive at a bus stop: a specified physical location. If its arrival were tied to a location in time, it wouldn't matter whether you waited for it at the bus stop, in the pub or in the Tokyo stock exchange, it would still get to you at the designated time.

BC

I personally don't put a whole lot of stock into the concept of 'the past', and most (but not all) of my discussion kind of assumes the concept is meaningless. — noAxioms

“The past is a foreign country: they do things differently there,” wrote L.P. Hartley in his 1953 novel “The Go-Between.” How we understand the past and how we come to terms with our own memories, is an unpaid debt that all humans share.

“The past is never dead. It's not even past.” - William Faulkner

I do not know with any certainty how long "the present" is. A second? A nanosecond? A minute? A vanishing moment between the untouchable past and the expected--but not guaranteed--future? The past seems more certain -- at least 13+ billion years worth. It sometimes seems like the present is an ever-vanishing moment; at other times the present seems stabile.

I do not understand how the past can be meaningless. You have a past; if you did not, you would not be speculating about time travel, or anything else. You have been a member of TPF for 8 years worth of the past. Your past isn't meaningless. You -- we -- are connected to the past by links on a chain stretching back to the beginning of the universe. Our basic body plan (vertebrate) appeared about 518 million years ago. The book Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body by Neil Shubin explains our physical connection to the past.

But carry on.

noAxioms

For any proposed loop, if you could experience going around it more than once, then the proposed loop would be falsified (since the second iteration would be distinguishable from the first). — sime

That was sort of the main simple example in the SEP article. Fred sneaks into a museum, steals the time machine, goes back a bunch of years, and donates the machine to the museum (explaining what they're doing with one). He doesn't use the machine again, so nobody goes around twice. But the machine violates entropy. It cannot have an odometer or any other evidence of age without invoking the contradiction you point out. So the machine (never created) is self perpetuating, and will do so forever according to its circular worldline.

But if you cannot experience going around the loop, then how you do know the loop exists to begin with? — sime

A time curve doesn't require 'the same thing' going around more than once, but if one end is in the light cone of the other, it's technically closed, and thus a loop. It has little to do with 'experience', but one of the presumptions of the definition of 'time travel' is that a person does it, not just his dog or his class ring.

A theory containing a CTC cannot have empirically observable consequences on pain of contradiction. — sime

I don't think this is true, but the dangers are definitely there. A CTC would be by definition consistent with itself, but that means that any information going back cannot be leveraged for the purpose of preventing the future state of the other end of the thing. So a closed loop can be (must be) contradiction free, but it seems to allow the contradictions, which is unacceptable.

So a CTC can at most be an uninterpretable expression of mathematical convenience rather than a representation of a physically verifiable entity. — sime

I don't think I in any way presented a sound rebuttal of that conclusion.

As for archeology, how do you know that the practice isn't retrocausal? Consider that the effect of digging into the ground can be expected to produce both predictable consequences that we might call "forwards directed" e.g the dig producing a hole next to a mound of earth, as well as unpredictable consequences that we might call "backwards directed", e.g the dig revealing of a Roman hoard of treasure. — sime

I'm sorry, but the Roman hoard isn't backwards directed. The Romans were first and caused the hoard. The dig did not. Archaeology was my example of forward causation that didn't involve light the whole way, unlike say 'looking at' light from a star 100 million light years away.

For why should the hoard of treasure be assumed to exist before it was discovered in the hole? — sime

Under what view would it not? Idealism, sure, but they don't ever think it exists, only the experience of it. While I don't presume counterfactuals, the hoard doesn't count as one. I'm not considering any epistemological definition of things unless specifically discussing a view where such things are fundamental. And the archaeologist is very much getting credit for the epistemological existence of the hoard.
I think even idealism has only forward classical causality. First dig, that causes knowledge of hoard, which causes ideal of Roman story, all in forward progression. The Romans are not in the past. They're the last step, the story crafted.

why should the hoard of treasure that was unearthed be assumed to exist prior to the establishment of the archeological evidence that they determined day before? — sime

Depends when that assumption of prior existence is being made. The answer is quite different if you've already dug it up.

So in short, i think the concept of non-local causal cooperation (Synchronicity?) is a causally permissible concept that aligns with experience, but I cannot say the same about closed time-loops. — sime

I'm unclear on how your example illustrated non-locat causal cooperation. Sometimes I'm a bit slow.

I do not know with any certainty how long "the present" is. — BC

In language, the duration is context dependent. Mathematically, only zero duration avoids contradictions.
I've seen arguments about quantum time, where the duration between successive moments isn't infinitesimal, and has no 'nows' between them, at least not at a given point in space.

I do not understand how the past can be meaningless. — BC

'The past' can be meaningless since it means 'times prior to the present', which, in the absence of a view that includes a premise of the existence of 'the present', renders the the phrase about as meaningful as 'one KM northward of the teapot that orbits beyond Jupiter'. That phrase simply doesn't define an actual location in space without an additional premise of the existence of the teapot out there. If the premise is made, then the phrase has meaning, even if the location of the teapot is unknown.

You have a past — BC

If I went back a year,
— noAxioms
There is no there there. — Vera Mont

Your opinions. They're fine, but only opinions, and as stated above, much of the discussion revolves around a different view where there is a there there.That view isn't falsified by assertions and laying and kicking legs in the air.

Physics has moved on since this 19th century view. I suspect you don't deny things like the big bang or black holes, but both of those come only from a theory that denies your assertions.

Isn't it more that events have temporal locations?
Anyway, duration and simultaneity are meaningful enough, and suggest some temporal structure taken together. — jorndoe

Simultaneity seems only meaningful in coordinate time (if there is no teapot time), or teapot time if there is.
Your input is appreciated. An open mind sees more than one valid interpretation of things.
Why do these topics always devolve into presentist foaming about an alternate view of which they've no understanding?

What archeologists look at is bits of pottery and and metal and walls that they dig up in old habitation sites. — Vera Mont

In what way is that not forward causality. I mean, I think that sime above attempted to explain something on those lines, but not sure if I got it.

Vera Mont

f I went back a year,
— noAxioms
There is no there there. — Vera Mont

Your opinions. They're fine, but only opinions, and as stated above, much of the discussion revolves around a different view where there is a there there. — noAxioms

Okay. So time is a physical entity, with form, spatial co-ordinates and dimensions, which co-exists with the world in which we experience time only as processes, events and changes. In theory, a person can step from the 3-dimensional world into the stream of time and back again.
The body of evidence for this is found in which scientific discipline?

jorndoe

↪Vera Mont

, so a location in time and space, both?

↪noAxioms

:up: I'm thinking that duration and simultaneity are reliable/meaningful enough that we depend on them, both in everyday life and science. The ontological examination (philosophy) is another matter perhaps.

Vera Mont

so a location in time and space, both? — jorndoe

How do you figure that?
You mean this?:
" If its arrival were tied to a location in time, it wouldn't matter whether you waited for it at the bus stop, in the pub or in the Tokyo stock exchange, it would still get to you at the designated time." But this is not the case.
The bus is located only in space. It moves; it changes location. You measure an abstract concept incidentally named 'time' by the processes, events and changes you experience, including the movement of a bus and the changing of numbers on a digital timepiece. Nothing and no-one is located 'in time', since time has no inside or outside. The bus moves along a street surface and, no matter when it's scheduled to arrive at any specific location along that street surface, it can't go into another time zone, or time-line or parallel reality. Location is of space; duration is of time; the two concepts are not interchangeable.

*You know what? This stopped being funny about a page ago.

jorndoe

↪Vera Mont

, I'm not quite seeing the difficulty.
When the bus arrives (temporal location) + where the bus arrives (spatial location) — the spacetime location — means I can then get on the bus, unless I went to the pub instead (spatial), or missed the bus (temporal).
FYI, I've done it a few times myself, i.e. have justification to find it meaningful. ;)
Do you not find those notions meaningful/reliable?
Also, it takes some time (duration) to get to my destination, and there's usually room on the bus (volume).
Roughly, I suppose we might say that when+duration are to events/processes (temporal) what where+volume are to objects (spatial), or something like that anyway.
On another note, I don't know if space or time are considered physical as such; in some contexts, physical tends to mean objects (or something that's conserved or persists).
Apologies

↪noAxioms

, didn't mean to distract the time travel discussion.

Vera Mont

I'm not quite seeing the difficulty. — jorndoe

That's very sad.

noAxioms

I think that the concept of non-local causal cooperation that you allude to is interesting and useful, but i think CTCs are empirically inconsistent and theoretically unnecessary. For any proposed loop, if you could experience going around it more than once, then the proposed loop would be falsified (since the second iteration would be distinguishable from the first). But if you cannot experience going around the loop, then how you do know the loop exists to begin with? A theory containing a CTC cannot have empirically observable consequences on pain of contradiction. — sime

I am going to disagree, but draw a similar conclusion for different reasons.

First of all, a CTC doesn't come in iterations, so if there's a loop, it's like a portal that's open for a while. One can go through (back a day say), and do it again in a day, but not a third time. That's not a contradiction since there's no iteration, only one loop with two different people going through, possibly holding hands. Secondly, no person needs to experience the trip. The loop is likely not something a living being can survive, but getting information through is enough. If at the past end of the loop, data is received concerning news of tomorrow (such as a sports score), that is evidence that it worked, without anybody having to experience it first hand. The sports score constitutes an empirically observable consequence.

OK, so why it might not work seems to come from the requirement for a unification of relativity and quantum mechanics. Relativity is strictly a classic deterministic theory. Any describption of spacetime curvature is a deterministic one, but if action needs to take place at both ends of the CTC, that action is conditionally going to take place at some computable probability. The future unified theory would have to be able to describe that, a sort of probabilistic shape of spacetime, which would involve completely different ways to describe it than what GR says.
Point is, while the current deterministic GR theory allows for a CTC, the new one may have no way to describe it. This is similar to Newton's theory allowing the generation of a uniform gravitational field by various methods, including an infinite sheet of material with some known mass density per area. Under the replacement theory (GR), there is actually no way to describe that sheet, and thus a uniform gravitational field is not possible under GR.

So to illustrate my concerns, suppose the sports game score is a function of some quantum measurement. There's a probability of it being this, and also of that, and maybe the game might be cancelled. So the information going back through the CTC (which only probably exists in the first place) might be all of those, unreadable. If a person goes through, different people from different futures might all come through, despite only one going in at any given future event. This was mentioned above with the MWI implications. If it's possible that any score might come through, it might be possible that all of them do.

Time travel opens the door to the concept of "can do otherwise" since suddenly the state of things allows a person to empirically witness different outcomes. Imagine the free will implications.

Apologies ↪noAxioms, didn't mean to distract the time travel discussion. — jorndoe

No worries, but I'll leave it mostly to you. There's not much traffic that you're interrupting, but you're banging against a wall with your efforts. Opinions held deliberately in ignorance are not usually changed. Evidence of that:

Okay. So time is a physical entity, with form, spatial co-ordinates and dimensions, which co-exists with the world in which we experience time only as processes, events and changes. In theory, a person can step from the 3-dimensional world into the stream of time and back again.
The body of evidence for this is found in which scientific discipline? — Vera Mont

There is no body of evidence supporting the picture you just described.

You still haven't identified which kind of time you're making all your assertions about.
That's perhaps understandable given your deliberate efforts to remain ignorant of, as I put it, the last century and a half of physics.

Vera Mont

You still haven't identified which kind of time you're making all your assertions about. — noAxioms

There are different species of time? I haven't seen, touched or stepped in any of them.
"All my assertions" are only two: 1. Time is a concept, with no physical dimensions and 2. You can mess with language; not with physics.

jorndoe

Hmm. Well. Cannon ball trajectories (roughly) form a parabola over time. The position (spatial) can be expressed as a function of time. Physics. (High school if memory serves.) Meaningful, reliable. But, yes, our expressions are descriptive, not pre/proscriptive. Anyway, carry on. :)

noAxioms

Not to let the main track die, let me poke at the dualism implications.
If there is a dualistic mind/body relationship, the identity convention is usually tied to the mind, at least in a view where the mind has an identity.
So perhaps MWI would be incompatible with such a view since the mind can only 'be' one body, and the ones in other worlds are either p-zombies or not real. Not real is simply denial of MWI.
But if the mind is a different sort of substance, or a different property of matter, then it still has location and travels with the time traveler, and the magic mind gets cloned along with the body. Such a view would be compatible with MWI and with time travel. For that matter, eternalism isn't entirely compatible with a lot of notions of dualism. I could start a whole different topic about this, but I fear that my naive understanding of dualism would just fill the OP with straw man arguments.

The only way I can see dualism (true separate supernatural entity version) being compatible with eternalism is via some kind of epiphenomenal relationship. It's no wonder they're all presentists.

Well. Cannon ball trajectories (roughly) form a parabola over time. The position (spatial) can be expressed as a function of time. Physics. (High school if memory serves.) Meaningful, reliable. — jorndoe

Agree with all, but it is Newtonian physics, the stuff of 19th century and before. Reliable indeed, since that's what was used to put people on the moon.
Interestingly, Einstein did away with the Newtonian concept of gravity being a force. The cannon ball actually goes straight (follows a geodesic to be more precise), and it is the ground that approximately follows a parabola since it is the ground experiencing the force and is thus accelerating per F=ma. Sure, the ball follows a parabolic trajectory in the accelerating reference frame of the ground, but that's a coordinate effect, not an example proper acceleration.

There are different species of time? — Vera Mont

Solid evidence that you don't even read the posts, and confirmation of my earlier assessment.
The three kinds have been enumerated in multiple places above.

I suspect the time to which you refer is a fourth kind: one that has a location in space, is tangible, and that, if you traveled to where it is, you can step in it. I can add that one to the list and call it straw time.

Vera Mont

I suspect the time to which you refer is a fourth kind: one that has a location in space, is tangible, and that, if you traveled to where it is, you can step in. — noAxioms

Since no other kind can be relevant to time travel, I refer to that version as impossible, yes.
You and others have proffered fanciful alternative realities, curly time, elastic time, ragtime, Miller time or whatever, which I admit to not reading with close attention.
Bottom line: No, you can't travel in any of them.
But good luck on your attempts!

Corvus

Bottom line: No, you can't travel in any of them.
But good luck on your attempts! — Vera Mont

:up: :fire:
Logical possibility doesn't mean factual possibility.

sime

First of all, a CTC doesn't come in iterations, so if there's a loop, it's like a portal that's open for a while. One can go through (back a day say), and do it again in a day, but not a third time. That's not a contradiction since there's no iteration, only one loop with two different people going through, possibly holding hands. Secondly, no person needs to experience the trip. The loop is likely not something a living being can survive, but getting information through is enough. If at the past end of the loop, data is received concerning news of tomorrow (such as a sports score), that is evidence that it worked, without anybody having to experience it first hand. The sports score constitutes an empirically observable consequence. — noAxioms

But if iteration isn't permitted, then is sending information backwards proof of a loop? Isn't the hypothetical possibility of a temporal contradiction the very motive for the loop interpretation? For if contradictions are ruled out a priori, then what justifies the use of a loop topology?

E.g suppose that it is possible to send sports results backwards in time. If this action "changed" history, then many people (including myself) would interpret this as merely referring to the action producing significantly non-local effects in our present, so that we can preserve the meaning of the word "history" as referring to immutability. On the other hand, if the action cannot "change" history, then what is the proof that anything has actually been sent backwards?

To return to the presentist reasoning I sketched earlier, It is logically consistent to believe that the past of our world is generated 'on the fly', as in a roguelike video game that generates the content of the game world as an effect of the adventurers present actions. In such worlds it might appear that information is sent backwards. E.g the adventurer is in an unknown dungeon with a closed door. Only after he opens the door does the game decide what lies beyond the door. Every adventurer's action has a predictable "forwards" effect e.g pushing a closed door causes it to open, and an unpredictable "backwards" effect, referring to what the action reveals about the world. But no information is actually sent backward, unless the adventurer is allowed to choose the revealed information, say as a consequence of using a magic spell . So the adventurer's ordinary present actions enable the course of history but without controlling the course of history (which in general is decided by a Dungeon Master of which one of his responsibilities is the logical consistency of the game world).

Notably, players don't typically interpret "history change" as time travel, e.g when an adventurer uses a magic spell to re-roll the state of the dungeon around him, but merely as magic affecting the global state of the present. Amusingly, a philosophical dispute once arose between players of the single-player roguelike game Nethack. In that dungeon crawler there exists the "Potion of Amnesia", which if drunk by an adventurer causes the game to delete it's record of the adventurer's knowledge of the game world,whilst leaving the actual game world in tact, meaning that the player must rely on their personal memories when their adventurer navigates and relearns the content of old locations. But isn't that cheating? Shouldn't a true potion of amnesia change the world itself? Players are divided.

Time travel implications with various philosophies

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