a little like MWI except alternate worlds (in Hilbert space) — noAxioms
Well I didn't say 'travel to where they keep time'.time has no physical locations — Vera Mont
No argument, but also not the point of the topic.I don't think there is any hope for time travel. — jgill
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).Do we know if time exists outside of human cognition? — Tom Storm
Engineering issues are not a concern to this discussion, only the implications on current philosophies if it could be pulled off.The important question here is cost — 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.the thrill of watching T Rexes mate — BC
This part is specifically absolutist, since the assertion isn't true without a preferred foliation of all spacetime events.The universe is in a state at one moment, then another state in the next. — Philosophim
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.The reason why the universe is in one state is because of the forces and matter in the previous state.
'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).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.
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.There is only now, and what was before.
But you did say:Well I didn't say 'travel to where they keep time'. — 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.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
The what now?In block terms, time travel is either a discontinuous worldline, or a worldline that isn't everywhere time-like. — noAxioms
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.I do not believe time travel is possible. — 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.I look at the past as a crystal
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.God ... has not, according to our founding fictions. seen fit to do over any part of the past.
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
Pretty much, yea.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.
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.messed-up, unverifiable memories don't count.
It's spacetime terminology from relativity theory. Hard to discuss that if you're unfamiliar.In block terms, time travel is either a discontinuous worldline, or a worldline that isn't everywhere time-like.
— noAxioms
The what now?
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.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.
Just the fact that time has no physical locations. — Vera Mont
Isn't it more that events have temporal locations? — jorndoe
In the human mind.Anyway, duration and simultaneity are meaningful enough, and suggest some temporal structure taken together. — jorndoe
Processes/events can be reasonably clear temporally, andless clear[meaningless] spatially. — 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.Objects can be reasonably clear spatially, and less clear temporally; — jorndoe
Volume and place, sure; duration and process, not.volume and place are meaningful enough, and suggest some spatial structure taken together. — jorndoe
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
No. What you've got there is a metaphor. You can mess with language, but you can't mess with physics. — Vera Mont
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.Anyone can look at the past, which isn't any sort of retrocausality. I mean, that's exactly what hte archaologists do. — noAxioms
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
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
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.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
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.But if you cannot experience going around the loop, then how you do know the loop exists to begin with? — 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.A theory containing a CTC cannot have empirically observable consequences on pain of contradiction. — sime
I don't think I in any way presented a sound rebuttal of that conclusion.So a CTC can at most be an uninterpretable expression of mathematical convenience rather than a representation of a physically verifiable entity. — 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.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
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.For why should the hoard of treasure be assumed to exist before it was discovered in the hole? — sime
Depends when that assumption of prior existence is being made. The answer is quite different if you've already dug it up.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
I'm unclear on how your example illustrated non-locat causal cooperation. Sometimes I'm a bit slow.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
In language, the duration is context dependent. Mathematically, only zero duration avoids contradictions.I do not know with any certainty how long "the present" is. — 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.I do not understand how the past can be meaningless. — BC
You have a past — BC
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.If I went back a year,
— noAxioms
There is no there there. — Vera Mont
Simultaneity seems only meaningful in coordinate time (if there is no teapot time), or teapot time if there is.Isn't it more that events have temporal locations?
Anyway, duration and simultaneity are meaningful enough, and suggest some temporal structure taken together. — jorndoe
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.What archeologists look at is bits of pottery and and metal and walls that they dig up in old habitation sites. — 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
How do you figure that?so a location in time and space, both? — jorndoe
I am going to disagree, but draw a similar conclusion for different reasons.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
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:Apologies ↪noAxioms, didn't mean to distract the time travel discussion. — jorndoe
There is no body of evidence supporting the picture you just described.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
You still haven't identified which kind of time you're making all your assertions about. — noAxioms
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.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
Solid evidence that you don't even read the posts, and confirmation of my earlier assessment.There are different species of time? — Vera Mont
Since no other kind can be relevant to time travel, I refer to that version as impossible, yes.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
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
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