5 ms·

In case someone wants to read a really good explanation about this; http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmat

In case someone wants to read a really good explanation about this; http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti... Generally speaking, all three of FTL (travel/communication), causality and relativity cannot all be true at the same time. If you can FTL-travel/communicate (they are the same really), then you can come up with a scenario in which an observer can see an event happening, and then see the cause of that event after it happened. Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox.

codethief 4y ago
> Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox This is a guess, i.e. one possible outcome physicists are considering. People have proposed alternative outcomes of FTL like the (in my opinion much more sensible) Novikov consistency principle, which roughly proposes that spacetime and the entities it contains (e.g. an observer's wordline) should be looked upon as a whole, in the sense that they need to be self-consistent. Spacetime is not time-dependent and does not evolve, so it does not make much sense to say "something something leads to a [spacetime] paradox".
- codethief 4y ago
  Addendum: Self-consistency basically amounts to periodic boundary conditions – not really surprising when you have (almost) closed time-like curves.
  - zachf 4y ago
    Periodicity in time is unusual to think about in physics, partly because you start to get wacky results. If you could establish a CTC in nature, it would allow for computers that can efficiently (i.e. in polynomial time) solve not just NP problems but actually even all of PSPACE [0]. You can interpret this in two ways. There’s the hopeful way: “we should spend a boatload of money trying to find CTCs in our universe since they’ll let us create super-ultra computers”. And then there’s the pessimistic way, that nature probably isn’t going to give us a free lunch like that. Sadly, I’m in the pessimist camp on this one! [0] https://www.scottaaronson.com/papers/ctc.pdf https://www.scottaaronson.com/papers/ctc.pdf
yellowapple 4y ago
This explanation, like previous explanations for "FTL implies time travel" I've read, presupposes that the signal is actually moving at superluminal speeds, i.e. actually covering a distance greater than 299,792,458 meters in a single second from the signal's perspective. This would not be relevant for Alcubierre drives or wormholes or anything else that warps spacetime, since the whole point of such things is to stretch/contract space such that the thing is traveling a much shorter distance per second. In short, if I were to instantaneously poof out of existence here on Earth, poof into existence on Mars a second later, grab a rock, poof out of existence again, and poof back into existence back on Earth again another second later and hand you that rock, it doesn't seem reasonable to assert that I traveled backward in time when I clearly traveled forward by two seconds. The whole lecture on that external observer is irrelevant, since there's nothing to observe unless the observer happens to be in my bubble/wormhole/whatever - and even then, one'd only be observing subluminal actions/signals within that region of spacetime.
- xenadu02 4y ago
  The problem is that if FTL travel exists then our existing theories about spacetime are wrong in some way (despite making lots of great predictions) or time paradoxes are possible in which case we have no idea what the consequences could be. Your example assumes there is some underlying rate at which time advances for the universe (or at least Earth and Mars) and that spacetime as we know it (including relativity and time dilation) are just some kind of modifier on top. But theory and experiment so far point to that not being the case. There is no "pop out of existence here and pop in over there" without time travel (as best as we can tell). The whole light cone / worldline explanations are more formal explanations of that. Now you can magic this problem away by proposing any number of schemes... like saying the entire universe's worldline is exists within a metaworldline and time travel actually resets the state of the universe as it was in the past then re-runs the universe... but all of that always proceeds forward in the metaworldline. In other words all past histories existed in a causal order, changing the past just adds "new commits" to the universe but history is never really rewritten. Bam! Our magic theory solves all paradox problems without requiring billions of parallel universes and allows time travel! But it's not a theory we can test or make predictions with so it isn't a useful scientific theory. It might as well be literal magic. Similarly you could propose that GR is wrong... but your new theory is gonna need to match GR's predictions that have proven true while making some new ones we can test, while also avoiding or explaining causality and paradoxes.

codethief 4y ago

> Extending that logic, if the observer can also move at superluminal speeds, he could prevent the cause of the event after seeing the event happen, leading to a paradox This is a guess, i.e. one possible outcome physicists are considering. People have proposed alternative outcomes of FTL like the (in my opinion much more sensible) Novikov consistency principle, which roughly proposes that spacetime and the entities it contains (e.g. an observer's wordline) should be looked upon as a whole, in the sense that they need to be self-consistent. Spacetime is not time-dependent and does not evolve, so it does not make much sense to say "something something leads to a [spacetime] paradox".

codethief 4y ago
Addendum: Self-consistency basically amounts to periodic boundary conditions – not really surprising when you have (almost) closed time-like curves.
- zachf 4y ago
  Periodicity in time is unusual to think about in physics, partly because you start to get wacky results. If you could establish a CTC in nature, it would allow for computers that can efficiently (i.e. in polynomial time) solve not just NP problems but actually even all of PSPACE [0]. You can interpret this in two ways. There’s the hopeful way: “we should spend a boatload of money trying to find CTCs in our universe since they’ll let us create super-ultra computers”. And then there’s the pessimistic way, that nature probably isn’t going to give us a free lunch like that. Sadly, I’m in the pessimist camp on this one! [0] https://www.scottaaronson.com/papers/ctc.pdf https://www.scottaaronson.com/papers/ctc.pdf

yellowapple 4y ago

This explanation, like previous explanations for "FTL implies time travel" I've read, presupposes that the signal is actually moving at superluminal speeds, i.e. actually covering a distance greater than 299,792,458 meters in a single second from the signal's perspective. This would not be relevant for Alcubierre drives or wormholes or anything else that warps spacetime, since the whole point of such things is to stretch/contract space such that the thing is traveling a much shorter distance per second. In short, if I were to instantaneously poof out of existence here on Earth, poof into existence on Mars a second later, grab a rock, poof out of existence again, and poof back into existence back on Earth again another second later and hand you that rock, it doesn't seem reasonable to assert that I traveled backward in time when I clearly traveled forward by two seconds. The whole lecture on that external observer is irrelevant, since there's nothing to observe unless the observer happens to be in my bubble/wormhole/whatever - and even then, one'd only be observing subluminal actions/signals within that region of spacetime.

xenadu02 4y ago
The problem is that if FTL travel exists then our existing theories about spacetime are wrong in some way (despite making lots of great predictions) or time paradoxes are possible in which case we have no idea what the consequences could be. Your example assumes there is some underlying rate at which time advances for the universe (or at least Earth and Mars) and that spacetime as we know it (including relativity and time dilation) are just some kind of modifier on top. But theory and experiment so far point to that not being the case. There is no "pop out of existence here and pop in over there" without time travel (as best as we can tell). The whole light cone / worldline explanations are more formal explanations of that. Now you can magic this problem away by proposing any number of schemes... like saying the entire universe's worldline is exists within a metaworldline and time travel actually resets the state of the universe as it was in the past then re-runs the universe... but all of that always proceeds forward in the metaworldline. In other words all past histories existed in a causal order, changing the past just adds "new commits" to the universe but history is never really rewritten. Bam! Our magic theory solves all paradox problems without requiring billions of parallel universes and allows time travel! But it's not a theory we can test or make predictions with so it isn't a useful scientific theory. It might as well be literal magic. Similarly you could propose that GR is wrong... but your new theory is gonna need to match GR's predictions that have proven true while making some new ones we can test, while also avoiding or explaining causality and paradoxes.