Comment by jeroenvlek 4 days ago
Am I interpreting this correctly to say that if you travel through the universe (at relativistic speeds?) and you arive at your destination, then you are reset to be the same person as when you started the journey?
Well in the model of General Relativity. Laws of physics are human descriptions of how we think nature operates based on current observations. It's not like we have a wormhole available to test time travel, assuming wormholes actually exist in nature. We don't really know if nature "conspires" to keep things consistent like that. Physicists do have a desire to come up with consistent theories though.
> It's kind of a cool result. The laws of physics conspire to keep the universe consistent even in the presence of time travel.
Indeed. I find this very cool and this paper gives some interesting examples of how this might unfold including Einstein clocks and the grandfather paradoxon.
It might make more sense of you think of spacetime as literally one thing, with one constant value. That value being c (or some meta value that boils down to the same thing).
Energy in all its forms (including velocity), mass, etc. or the lack thereof being ‘space’, and time being what you have ‘left over’ when you subtract ‘space’.
The more mass, or velocity, etc. you have, the less ‘time’ you get left over. That is time dilation, both in the presence of masses and when you’ve got a lot of velocity (because having a lot of velocity means you have a lot of energy).
That is an alternative formulation of e=mc^2. [https://en.m.wikipedia.org/wiki/Mass%E2%80%93energy_equivale...].
At the point your velocity hits c (somehow), you have no ‘time’ left over from your perspective, so wherever you go, you go there instantly from your perspective. No time has passed for you. Same if you are ‘inside’ a singularity like a black hole.
Space time curvature (aka gravity) may arise from that effect not just being a point one, but a subtle cumulative area effect.
In that model, time travel, FTL, and any other lack of causality (aka effect after cause) make no sense, because there is no ‘lever’ for such a thing to ever happen.
Maybe if someone could invent negative mass/energy (we currently have no evidence/idea such a thing could exist!), or a way to manipulate the fundamental factors that make spacetime spacetime. We have no concrete idea how to even conceive of trying such a thing idea right now though.
That result is terrifyingly boring in its implications though, which is why we try to avoid it.
> That result is terrifyingly boring in its implications though, which is why we try to avoid it.
What result? A result that the entire Universe is deterministic and already determined, like a movie already recorded to tape that we are somehow watching play from inside?
That could be one.
But I was referring to the possibility that it’s energetically impractical to ever get out of the solar system (or even travel within our solar system) in a way that any human is likely to ever want to do it. (Aka no FTL, or even near light speed)
And this likely applies to anything we’d call ‘life’ too.
And that time is actually one way, so no do overs, and no time travel. (Either because of the 2nd law of thermodynamics, or the 2nd law is an effect of this!)
If we accepted that, it would kill what - 95% of all Sci-fi ever?
A closed timelike curve is the name in General Relativity for a time machine: you go forward in time and wind up in your past, and you go around and around the loop forever.
The point is that when you get to the same point in the loop your state must be what it was the last time you were at that point in the loop.
If you have a relativistic trajectory that doesn't form a loop in time there's no reset effect.
Wouldn't the fact that entropy must increase mean that you can never get to the exact same state as you were before?
Consider that Heisenberg's uncertainty theorem states that we cannot know precisely the position and velocity of a particle at the same time, not even in theory. Thus, they don't have precise values even in theory. Then how could you ever return back to the precisely same state which never had a precise value to begin with?
> Wouldn't the fact that entropy must increase mean that you can never get to the exact same state as you were before?
What you're running into is the observation that near closed timelike curves quantum mechanics inevitably becomes important.
> Consider that Heisenberg's uncertainty theorem states that we cannot know precisely the position and velocity of a particle at the same time, not even in theory. Thus, they don't have precise values even in theory.
This is a popular-level understanding of QM. The first sentence is true because it includes the all important 'at the same time'. The second is... trickier.
Position and momentum are observables, not state variables (unlike classical mechanics). They are incompatible so they suffer from Heisenberg uncertainty. But that doesn't mean a state can't have a definite position or momentum.
You can construct a wavefunction with a definite position. By Heinsenberg uncertainty, if you try to measure its momentum you can get any value. Likewise you can construct a wf with a definite momentum but if you measure its position you can get any value.
Nevertheless the system has a definite wf. For these two different wfs it is perfectly fair to say the first has a definite position and the second a definite momentum.
> Then how could you ever return back to the precisely same state which never had a precise value to begin with?
If the system goes around a CTC and returns to the same wavefunction it's the same. Quantum mechanical systems have definite states. But weirdly in QM position + momentum are not state variables.
"A" universe, but not "the" (i.e. our) universe.
Specifically: https://en.wikipedia.org/wiki/Gödel_metric
It's specifically a universe where time travel definitely happens.
From the paper:
> Finally, we stress again that our main results are valid in an arbitrary background spacetime (including charged Kerr black holes [51, section 12.3]), provided that the CTC of interest is the orbit of a periodic one-parameter family of symmetries of the metric. This happens in all axisymmetric models whose rotation Killing field becomes timelike somewhere.
I think it's more like: "Quantum mechanics is consistent with what we expect to happen with matter that exists in a closed timelike curve: everything is reset upon return to the starting spacetime point."
> Am I interpreting this correctly to say that if you travel through the universe (at relativistic speeds?) and you arive at your destination, then you are reset to be the same person as when you started the journey?
If you manage to arrive at the same place and time that you started from (i.e. because you time-travelled, e.g. by going through a wormhole), then you are necessarily the same person when you arrive as you were when you departed.
It's kind of a cool result. The laws of physics conspire to keep the universe consistent even in the presence of time travel.