How would time travel actually work in nature (under known laws of physics)?

Great Debates
21

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.
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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.
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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.

22

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.

23

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.

24

I’d be happy to be proven wrong, but you can only skip to the future but you cant go backwards.

25

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.

26

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

27

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…:smiley:

28

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! :wink:

And what do you mean by ‘the’ future, anyway? :slight_smile:

29

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.

30

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.

31

Its cheating, but you can go to the future, through the magic of time dilation

32

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

33

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.

34

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