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Old 02-20-2020, 02:06 AM
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How would time travel actually work in nature (under known laws of physics)?


According to Wikipedia, "time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light."

Fine. So based on the current known laws of physics or quantum mechanics, there are some mathematical formulas that solve to "go back and stop Hitler".

And there has been endless debates about the philosophical implications of pop culture science fiction from the likes of H.G. Wells, Kurt Vonnegut, James Cameron and Robert Zemeckis. Warnings about paradoxes and sleeping with your grandmother and whatnot.

But how would time travel actually WORK in what we know of how the universe actually works?

What I mean to say is this. Let's say I'm sitting here in the year 2029 with the rest of you. That is happening right now from my perspective. I go back in time to 1979. Does that mean that 1979 "exists" somewhere in the universe (along with every other moment in history, perhaps as some offshoot of the "Many Worlds" theory)? And if so, why is my "now" in 2029 any more "now" than that of seven year old me in 1979?

Or do I effectively have to "rewind" the entire universe (or at least from my perspective) so that basically nothing after 1970 exists yet?

Theoretically, IF one could create a wormhole and IF one could accelerate one entrance to some relativistic speed for a hundred years, one could travel backwards in time to the "younger" end. Still seems weird to me in that it still seems to require the simultaneous existence of two "universes". One where I go into some Stargate that's been orbiting the sun at some fraction of the speed of light while the other has been sitting there 100 years, and another universe where the second Stargate is not that old.



Basically, I'm having trouble wrapping my brain how one could go through a door and 1979 exists on the other side, even if some weird math says it might be possible.
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Old 02-20-2020, 05:52 AM
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Swap space and time in all of your questions. If I'm sitting here, but I can go through a door and be someplace else instead, does "here" really exist?
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Old 02-20-2020, 06:20 AM
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Originally Posted by msmith537 View Post
According to Wikipedia, "time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light."

Fine. So based on the current known laws of physics or quantum mechanics, there are some mathematical formulas that solve to "go back and stop Hitler".

And there has been endless debates about the philosophical implications of pop culture science fiction from the likes of H.G. Wells, Kurt Vonnegut, James Cameron and Robert Zemeckis. Warnings about paradoxes and sleeping with your grandmother and whatnot.
Sure, but there’s also pop-culture science fiction in which time travel means you can’t “go back and stop Hitler”: in which it’s as if a slightly-older you is already there and then, failing to stop Hitler; and in which you can, here and now, fire up a time machine and go back to take your shot, and fail, and paradox lost.

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Old 02-20-2020, 06:55 AM
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Well the answer of course is we don't know, and can only speculate, including speculating that there is no way for macroscopic objects to do anything like travel backwards in time.

From what we can gather so far though, it looks like the universe is fine with going backwards as long as causality is preserved -- you can ask kinds of weird shit, like the quantum eraser experiment, but you can't use any of these phenomena to send the winning lottery numbers to yourself. Heck, you can send the winning numbers back as long as you're also sending noise such that it's impossible for your past self to read the message.

So it appears a human going back would likely be of the kind where you're powerless to change anything, and whatever you do is what already happened.
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Old 02-20-2020, 11:39 AM
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I would suspect that the extent to which time travel exists as reality, the physical laws governing it would be such that it would be effectively useless. I could imagine that individual quantum mechanical particles could go back in time but that doing so would be uncontrollable, such that no information could be passed backwards in time, similar to the way that quantum entanglement allows fuzzy action at a distance but can't be used to send messages faster than light. At best you could detect a closed loop in time after the fact, so no going back in time to kill Hitler.
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Old 02-20-2020, 12:12 PM
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According to Wikipedia, "time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light."
As far as I know, faster-than-light travel with respect to the local spacetime region is itself a violation of the known laws of physics. You could have apparent time travel by which you move from point A to point B so fast that a neutral observer sees you exit from B before you walk into A. But that is just an illusion due to the relative distortion of spacetime between the observer and points A & B. It would be like the observer is watching two cameras fixed at the ends of a tunnel, where one camera has a ten second delay.

I could, of course, be very wrong about all of this.

~Max
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Old 02-20-2020, 12:47 PM
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My theory is, just as passing from point A to point B in space requires going through all points in a path between A and B, moving through time from time A to time B requires going through all points in time between A and B - and if you try to go backwards in time, you will run into something - namely, yourself - that blocks you just as a wall between space points A and B would.
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Old 02-20-2020, 12:51 PM
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My theory is, just as you can't travel in space from point A to point B without passing through all points in a path between A and B, you can't travel in time from time X to time Y without passing through all points in time between X and Y - and if you try to go backward, you will be stopped by something that was already there - namely, yourself - just as you would be stopped from going from A to B by, say, a wall somewhere on the path.
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Old 02-20-2020, 12:58 PM
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I think you just hit a wrinkle in time, there.
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Old 02-20-2020, 01:06 PM
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We'd need to spend all day talking about time travel, making diagrams with straws.
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Old 02-20-2020, 02:02 PM
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My theory is, just as passing from point A to point B in space requires going through all points in a path between A and B, moving through time from time A to time B requires going through all points in time between A and B - and if you try to go backwards in time, you will run into something - namely, yourself - that blocks you just as a wall between space points A and B would.
Whereas my theory is that in order to move from point A to point B in space, you must also move from A to B in time, and vice versa. I think it is consistent with the laws of physics to go from B to A (backwards) in time, but that would also entail everything - even your mental state - going exactly from B to A in space. Coupled with hard determinism, backwards time travel would be useless. Just like rewinding a movie - the characters won't remember the "future" and when it is played back, nothing changes.

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Old 02-20-2020, 04:10 PM
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My personal theory, which preserves "free will", is that any time travel immediately changes "the past", which is now the new present. Nothing that happened between the 1979 of your arrival and now exists any longer. Lost, forevah lost! All those events, all those things, not only gone but never was.

And it can never be remade.

Even if your changes to the past were so small that on a macroscopic level no one can tell, you're still not going back to the world you cane from.

Not only can you change the past in time travel, you MUST change the past. Your mere presence has changed things. For one, since the atoms in your body are already there in 1979, the total mass of the universe has increased. Congratulations, you've violated thermodynamics. But the universe doesn't care. If it did, you couldn't time travel. ipso facto.

Go ahead, kill Hitler. Kill your grandfather. Won't change you - you're still there. But now you have no connection to the current reality.

Last edited by Just Asking Questions; 02-20-2020 at 04:11 PM.
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Old 02-21-2020, 12:21 AM
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As far as I know, faster-than-light travel with respect to the local spacetime region is itself a violation of the known laws of physics. You could have apparent time travel by which you move from point A to point B so fast that a neutral observer sees you exit from B before you walk into A. But that is just an illusion due to the relative distortion of spacetime between the observer and points A & B. It would be like the observer is watching two cameras fixed at the ends of a tunnel, where one camera has a ten second delay.

I could, of course, be very wrong about all of this.

~Max
But is the traveler actually moving faster than light, or is he simply taking advantage of a convenient shortcut known as a wormhole?
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Old 02-21-2020, 02:26 AM
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Almost all the laws of physics are symmetric in time, but some things (memory, volition) depend on time's direction. Tommy Gold, Ph.D., F.R.S. proposed a model in which creatures from the very distant future would be traversing time in the opposite direction from us! (Agents from the far distant universe might have no way to arrive in ours — they'd first need to pass through an entropy maximum — but let's ignore that detail.) This might provide a scientific basis for creatures like White's Merlyn! (Physicists including Stephen Hawking once accepted the 'Gold Universe' as a possibility, but it is now believed to be contradicted by cosmological measurements.)

It is difficult to image how time-reversed agents could interact with the normal world. (Where would they even get food? The wheat in our fields would have the wrong causality direction to be used by Merlyn!) But if they could survive at all, they could communicate with us, though with difficulty. So we could send Merlyn a message: "When you find Hitler, kill him!" (Or "!mih llik ,reltiH dnif uoy nehW" if you will.) But how would Merlyn act on the request?

I fully expect to be ridiculed by the Board's professional physicists. But I still have my asbestos suit on from last time.
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Old 02-21-2020, 07:59 AM
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(Please note IANA physicist, so a real physicist would probably say that I'm wrong, but I'd be interested to see exactly what mistakes I've made in the following essay).

Lets's imagine some bizarre kind of reflecting device that could convert an entity like us from something going forward in time into something going backwards in time. Maybe it looks like a twist in space-time, possibly resembling a four-dimensional Moebius strip.

Under the rules of Charge, Parity, Time symmetry, an entity reflected backwards in time would be made from antimatter. So it would explode as soon as it touched anything made of matter. Ouch.

After passing through such a reflecting device, the entity concerned would be converted into antimatter, and would explode as soon as it met something travelling in the normal direction. Probably the first thing it would meet would be itself, before the conversion, travelling in the opposite direction and attempting to occupy the same space - and the result would be gamma rays.

Alternately, imagine that we get one of Tommy Gold's backwards-travelling entities, and reflect it so that it was travelling through time in the same direction as us (so that we can communicate with it more conveniently); the first thing it would meet would be itself, going the other way, and it would annihilate. Bang!

Unfortunately, to my untrained eye, this method of time travel seems to result in the efficient creation of an antimatter bomb. Good for ordnance, not so good for temporal tourism.
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Old 02-21-2020, 08:05 AM
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Congratulations, you've violated thermodynamics. But the universe doesn't care. If it did, you couldn't time travel. ipso facto.

Go ahead, kill Hitler. Kill your grandfather. Won't change you - you're still there. But now you have no connection to the current reality.
Does it violate thermodynamics? If time (t) is treated as just another dimension (x,y,z), then would moving a mass from 2029 to 1979 be any different from moving the same mass across the room? You haven't added or subtracted any matter or energy from the entire "universe"

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Old 02-21-2020, 08:18 AM
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No, but imagine a fleet of Tardises bringing refugees back from the Heat Death of the Universe to our time. They will increase the mass of our universe significantly.

Imagine that these refugees manage to live long enough to reach the Heat Death again - back they come in their time machines and repeat the process. Eventually the Current Era consists of 90% by mass of temporal refugees; this causes the mass of the universe in the Current Era to increase until it collapses in a Big Crunch.

You don't want that to happen.

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Old 02-22-2020, 09:52 AM
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No, but imagine a fleet of Tardises bringing refugees back from the Heat Death of the Universe to our time. They will increase the mass of our universe significantly.

Imagine that these refugees manage to live long enough to reach the Heat Death again - back they come in their time machines and repeat the process. Eventually the Current Era consists of 90% by mass of temporal refugees; this causes the mass of the universe in the Current Era to increase until it collapses in a Big Crunch.

You don't want that to happen.
Why stop at refugees? What if you brought the entire mass of the history of the universe to 2019? Although I suppose it would all collapse into a giant black hole where time and space are meaningless anyway.
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Old 02-22-2020, 05:10 PM
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There was no enormous crowd on the grassy knoll. Armies of faithful did not interrupt the crucifixion. There weren't six million visitors to Max Yasgur's farm. No welcoming committee came to Serenity Base.

Generalizing from mathematically describable quantum event states to multi-trillion molecule four dimensional transport events in two different temporal/spatial sets of loci are unrelated and entirely dissimilar levels of mathematical abstractions. Much of what can be mathematically described cannot exist.

If there could ever be time travelers, where the heck are they now?
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Old 02-22-2020, 08:08 PM
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Here's an introduction to the (fictional) concept of time travel. Enjoy! https://qntm.org/models
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Old 02-23-2020, 06:19 AM
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Okay, for the sake of completeness, here are a few models of how time might work, some of which allow time travel, others don't.

1/Presentism. In this model only the present exists, the past has gone, and the future does not exist yet (and when it does exist, it will be the present). I suspect that this is the model that msmith537 subscribes to in the OP, either consciously or otherwise.
More details here
https://en.wikipedia.org/wiki/Philosophical_presentism
In Presentism there would be no past or future to travel to.
But there are other models.

2/Eternalism. In this model the past and future both exist forever, outside of time as we know it, and we pass through time in a similar way to the way we move though space. So we could in theory travel back to the past, which stiull exists elsewhere in space time, and experience the same events again. In some models of time, the entire universe, past present and future, exists as a solid four-dimensional block, through which we move like a three-dimensional slice, always moving in one direction. This is the block universe concept, and if both [past and future exist as a single, unchanging block, then there is perfect predestination and events are fixed. Some might say that in a block universe there is no free will, but this is a completely different question.
More details here
https://en.wikipedia.org/wiki/Eterna...osophy_of_time)
In Eternalism there is a past to go to, but if the universe is a solid four-dimensional block we can't change it withouit completely destroying the existing pattern.

Another variant of the block universe concept is the 'growing block universe' concept, where only the past exists as a solid block, but the future does not exist yet. The growing block universe looks like it might allow time travel at first glance, since the past exists and can be travelled to; but if the future does not exist, then there would be no future to travel from.

Another different view of time is the multiple timelines possibility, which means you could travel to the past and the future, but they need not be the past or future you are familiar with. I think that, if expressed in 'block universe' terms, this implies that the universe has (at least) another dimension, one in which events can vary significantly. Multiple timelines are popular with science fiction authors, but I think they are less popular with physicists and cosmologists.

Last edited by eburacum45; 02-23-2020 at 06:22 AM.
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Old 02-24-2020, 07:22 AM
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Does it violate thermodynamics? If time (t) is treated as just another dimension (x,y,z), then would moving a mass from 2029 to 1979 be any different from moving the same mass across the room? You haven't added or subtracted any matter or energy from the entire "universe"
In some theories of TT, no it might not. A deterministic universe would have the mass of the time machine as twice during, say, 1970-2019, but after 2019 the "original" machine time travels and subtracts it self out of the universe, and everything balances.

But in my theory, which is sort of like Terminator or the first movie of BTTF, there are inescapably two time machines in the universe, forever.

A way out of this I consider is that matter cannot be created or destroyed, but the "books" only have to balance at the end of the universe. And since there might not be one, you're fine.
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Old 02-24-2020, 08:24 AM
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In some theories of TT, no it might not. A deterministic universe would have the mass of the time machine as twice during, say, 1970-2019, but after 2019 the "original" machine time travels and subtracts it self out of the universe, and everything balances.
In this model the total mass of the universe fluctuates over time. If there are locations in history that are particularly popular with time travellers then the universe weighs more at those times. Conversely, locations in space-time that are unpopular weigh less.

As I suggested in my thought experiment, mass movement of time travellers from unpopular or dangerous parts of the universe (such as the heat death) will make those regions less massive, and other popular locations in space time will become more massive, with potentially dangerous consequences.
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Old 02-24-2020, 05:04 PM
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Basically, I'm having trouble wrapping my brain how one could go through a door and 1979 exists on the other side, even if some weird math says it might be possible.
I'd be happy to be proven wrong, but you can only skip to the future but you cant go backwards.
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Old 02-25-2020, 11:45 AM
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One wrinkle suggested by Larry Niven -- if one postulates that it is possible to travel back in time and alter the timeline, the "eventual" result is that somebody alters the timeline in such a way that nobody discovers how to travel back in time, thus creating a "final" stable timeline.
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Old 02-25-2020, 12:41 PM
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In this model the total mass of the universe fluctuates over time. If there are locations in history that are particularly popular with time travellers then the universe weighs more at those times. Conversely, locations in space-time that are unpopular weigh less.

As I suggested in my thought experiment, mass movement of time travellers from unpopular or dangerous parts of the universe (such as the heat death) will make those regions less massive, and other popular locations in space time will become more massive, with potentially dangerous consequences.
That's not what Just Asking Questions was saying. What you are describing violates the theory of time translation symmetry (and thus mass-energy conservation). What Just Asking Questions was saying is that the time machine itself would manage the mass-energy on both ends of travel. So when departing from 2019, the time machine would gain exactly the same mass as whatever is being sent back in time; when arriving in 1963, the time machine would lose exactly the same mass.

Usually this weird time travel machine accomplishes its function by already existing at both ends of the trip, and by using certain hypothetical "exotic matter" with negative mass.

~Max
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Old 02-25-2020, 01:06 PM
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Armies of faithful did not interrupt the crucifixion.
Let's see: per standard Christian theology, if they had, then we would not have been saved by Jesus' sacrifice because it didn't happen, so after a couple of decades had passed, nobody would have heard of this Jesus person anyway, and nobody would have gone back in time to stop his crucifixion, so it would have happened after all, Christianity as we know it would exist, so people would have gone back in time and interrupted the crucifixion, and...

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Old 02-25-2020, 01:11 PM
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I'd be happy to be proven wrong, but you can only skip to the future but you cant go backwards.
Well to each his own, but I'm not skipping to the future, I'm slowly slipping into the future while sitting here at my desk.

Why, it's almost ten minutes further into the future than it was I opened this thread!

And what do you mean by 'the' future, anyway?
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Old 02-25-2020, 01:28 PM
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That's not what Just Asking Questions was saying. What you are describing violates the theory of time translation symmetry (and thus mass-energy conservation). What Just Asking Questions was saying is that the time machine itself would manage the mass-energy on both ends of travel. So when departing from 2019, the time machine would gain exactly the same mass as whatever is being sent back in time; when arriving in 1963, the time machine would lose exactly the same mass.

Usually this weird time travel machine accomplishes its function by already existing at both ends of the trip, and by using certain hypothetical "exotic matter" with negative mass.

~Max
As generally spacetime doesn't have global time translational symmetry I would say that conservation of energy is a moot point anyway. That's a conventional, but perhaps not entirely universal view in general relativty, but when you have time travel you can't even globally define space in order to do the bookkeeping on, so it is a definiitely moot when time travel is involved.
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Old 02-25-2020, 03:56 PM
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Ah, yes. I've heard of this 'cosmic bookeeping' idea before. It suggests that wormholes might become charged with an excess of negative energy to balance the books, so much so that they might be detectable as divergent lenses in deep space.

But that is only one theory, and one model of time travel. There are others. And global conservation laws may not apply anyway, as Asympotically Fat mentioned.
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Old 02-25-2020, 04:54 PM
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And what do you mean by 'the' future, anyway?
Its cheating, but you can go to the future, through the magic of time dilation
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Old 02-25-2020, 05:39 PM
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As generally spacetime doesn't have global time translational symmetry I would say that conservation of energy is a moot point anyway. That's a conventional, but perhaps not entirely universal view in general relativty[...]
That's news to me. I found some mentions of a newly discovered state of matter called "time crystals" but, it is beyond my comprehension.

~Max
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Old 02-25-2020, 05:51 PM
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To explain why global bookkeeping of energy fails when time travel is involved:

When a spacetime describes a physical situation you can often slice it, much like a joint of ham, so that each slice represents space at given moment in time. This is a natural way for us to view sapce (and time), though it must be noted there is always more than one way to perform the slicing. Time-translational symmetry leads to global conservation of energy and if a spacetime has global time transational symmetry then you must be able to slice it so that each slice is identical, but as gravity is the curvature of spacetime, you cannot do this for gravitationally-dynamic systems.

This casual dumping of the conservation of energy in general relativity has never been seen as a big problem because we all know in order to get conservation of energy we have to include graviational energy (GE) too, which is not modelled as energy in general relativity. The obvious way to recover global conservation of energy is to account for GE, the underlying problem though is that gravity is the curvature of spacetime and the curvature can always be made to vanish at any point unlike energy. For classes of spacetimes you can use certain tricks to account for GE, but there's no univerasl method to account for GE. It is reasonable to suppose that such a method may exist for large classes of spacetime, but at this point, if the conservation of energy is such an obscure concept, you have to ask if it is useful general concept in the theory at all.

Going back to time travel and the slicing of spacetime, each slice has two requirements: firstly it is spacelike, and secondly each possible trajectory of a particle crosses it exactly once. As requirements these are pretty much self explanatory. However if you have time travel then it is not possible to find slices where each possible trajectory crosses it exactly once, so you can't even create a global spatial background in order which to have global conservation of energy on.
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Old 02-25-2020, 06:03 PM
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That's news to me. I found some mentions of a newly discovered state of matter called "time crystals" but, it is beyond my comprehension.

~Max
Time crystals would be substances that display discrete symmetries in time, but conservation laws are related to continuous symmetries.
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