cfgauss

Jesse

Answerer, I believe the people in the articles you cite are hoping for some kind of new physics which will allow faster-than-light communication using entanglement (for example, if quantum randomness was actually based on some pseudorandom deterministic law it might be possible)...I think it's been proven pretty definitively that quantum entanglement cannot be used to transmit information faster-than-light given the current known laws of physics.

cfgauss

It can't at all. If you could, ugly things could happen!

Jesse

cfgauss:
It can't at all. If you could, ugly things could happen!

I assume you are referring to sending information into the past and causality violations. However, faster-than-light signalling only implies these things if you assume there are no preferred reference frames (Lorentz invariance); if you break this symmetry and allow things to travel faster-than-light (but not back in time) only relative to a single preferred reference frame, then you can avoid the possibility of transmitting information backwards in time. Most physicists don't believe Lorentz invariance will be violated in this way, but it can't be ruled out.

Anyway, the fact that a theory allows backwards-in-time signalling is not a reason to automatically dismiss it. Classical general relativity allows this "ugly" possibility in certain circumstances (see Kip Thorne's work on wormholes and time travel in GR), although most physicists suspect that when we combine GR with quantum mechanics this sort of thing will be ruled out.

Wyz_sub10

You know what else makes ears bleed? Yelling "NOOOO!!!" and so forth.

cfgauss

Jesse:
It's misleading to say it "allows" it. It's more correct to say that it doesn't forbid it. And it's not just a "violation" of causality! The whole causal structure breaks down! Since that *doesn't* happen, you can't go faster than light.

There's also no reason what-so-ever (and in fact, are several reasons against!) having a preferred reference frame!

Wyz_sub10:
Yes, but it's so much more dramatic to say it that way!

Jesse

cfgauss:
It's misleading to say it "allows" it. It's more correct to say that it doesn't forbid it.

I am not sure how you are using these words, but I would use "doesn't forbid" to mean just that no one has proven something is impossible within a theory, while reserving "allows" to mean someone has come up with a positive demonstration that something is possible within that theory. In that sense, Kip Thorne has shown that classical general relativity does indeed allow "closed timelike curves", ie travelling into one's own past (Kurt Godel showed it even earlier, but only in the case of a rotating universe; in contrast, wormholes do not require any assumptions about the universe as a whole).

As I said, though, most physicists suspect that when you replace classical general relativity with a theory of quantum gravity, closed timelike curves will be forbidden. But the fact remains that general relativity does allow it, in its current form.

cfgauss:
And it's not just a "violation" of causality! The whole causal structure breaks down! Since that *doesn't* happen, you can't go faster than light.

If by "breaks down" you mean that backwards time travel automatically leads to paradoxes, then many physicists would disagree with you, including Kip Thorne. He argued (using some examples of billiard balls travelling through wormholes) that it will always be possible to find a consistent history, so you just need to assume that the existence of global constraints on histories involving time travel which only allow consistent events. This would contradict free will--if you tried to go back and assasinate your grandfather, somehow events would always conspire to stop you--but then, so do all theories of classical mechanics.

See the Time Travel and Modern Physics article, which talks a lot about why time travel need not lead to paradoxes, and the idea of global constraints on histories involving time travel (including Thorne's billiard-ball thought-experiments, in the section 'Slightly More Realistic Models of Time Travel')

cfgauss:
There's also no reason what-so-ever (and in fact, are several reasons against!) having a preferred reference frame!

All the evidence so far has suggested there are no preferred reference frames, which is probably a good reason to suspect there aren't any. But I don't know of any theoretical reasons why it would be disastrous if there was a preferred reference frame (though my knowledge could easily be incomplete here), it seems like it would just involve a minor modification to SR (albeit one that would cause it to lose a lot of its 'elegance'). Here's the homepage of a physicist who is doing experiments to look for evidence of tachyons, and in his FAQ he suggests that one might avoid the problem of causality violations using a preferred reference frame:

http://www.physics.gmu.edu/~e-physics/bob/t.htm

cfgauss

Jesse:

I say we shouldn't say "allows" because it doesn't say it's cool, it just doesn't say no. We can manipulate it to do funky things, but we need funkier things like exotic matter and imaginary time to exist to do it! It would be like saying Newtonian mechanics "allows" imaginary distance to exist because we can still do math with it. Yeah, we can, but talking about traveling 5i meters at 3 meters/sec doesn't mean anything!

And yeah, I do mean it necessarily leads to paradoxes. The only ways to avoid them involve logically nonsensical structures with no empirical backing. We've never seen anyone or anything from the future, nor do any of our theories imply that we can (i.e., like particle physics implies the existence of quarks). *However*, I do believe that it may be possible for small particles to briefly do funky things like travel through time, but I don't believe that this is any more controllable than other quantum effects, and is no more likely to happen to us than it is likely that I will quantum tunnel myself to a Hawaiian vacation!

We did have a theory of a preferred frame, it was called the aether. It was postulated as the medium for the propagation of light. This was proven to not exist. It was later investigated (after SR) if there could be a preferred frame of some kind, and it was discovered that the only way one could exist and be consistent with SR is if it was completely undetectable and did not effect us in any way (which is equivalent to not existing at all). If there is a preferred frame, somewhere SR and GR will fail due to it.

Jesse

cfgauss:
I say we shouldn't say "allows" because it doesn't say it's cool, it just doesn't say no. We can manipulate it to do funky things, but we need funkier things like exotic matter and imaginary time to exist to do it!

I think that all you need to have backwards time travel in GR is negative energy, which we already have reason to think is possible due to the Casimir effect. It's true that this is a quantum-mechanical effect and thus we have to leave classical GR to talk about it, but it's a lot less exotic than your example of bringing imaginary quantities into classical Newtonian physics.

cfgauss:
And yeah, I do mean it necessarily leads to paradoxes. The only ways to avoid them involve logically nonsensical structures with no empirical backing.

What is "logically nonsensical" about global constraints? Perhaps you could show explicitly how this idea leads to a self-contradiction, unless you didn't actually mean "logically" in the literal sense there.

It's true that there is no empirical backing for these sorts of global constraints, but then the only circumstances where they'd lead to empirical predictions would be in the neighborhood of wormholes or other strange forms of twisted spacetime. Do you wish to rule out wormholes entirely simply because we have not empirically observed them? Even physicists who don't believe in time travel generally don't go that far--the more common idea is that wormholes might be possible, but if you try to move the mouths around in such a way as to create a time machine, you'll get a buildup of feedback from radiation which will destroy the wormhole (see the bottom section of this article). And if you dismiss strange consequences of GR just because they have no "empirical backing", you'd have dismissed black holes when they were first postulated as well.

cfgauss:
We've never seen anyone or anything from the future, nor do any of our theories imply that we can (i.e., like particle physics implies the existence of quarks).

That's a pretty weak argument--even if it's possible that doesn't mean anyone will ever do it. And even if someone does, they would not be able to travel back farther than the origin of the wormhole anyway.

cfgauss:
We did have a theory of a preferred frame, it was called the aether. It was postulated as the medium for the propagation of light. This was proven to not exist. It was later investigated (after SR) if there could be a preferred frame of some kind, and it was discovered that the only way one could exist and be consistent with SR is if it was completely undetectable and did not effect us in any way (which is equivalent to not existing at all).

But I already said that a preferred frame would not be consistent with SR--it would necessarily involve a modification to it. It's conceivable that a preferred frame could exist, but that most particles would behave the same regardless of which frame they were in, while only a few--tachyons, for example--would behave differently (thus the modified theory would 'reduce to' SR in most situations, just as SR reduces to Newtonian mechanics in the low-velocity limit). As far as I can tell this would not contradict any existing observations, nor would it force us to throw out any existing theories completely, just modify them slightly. Personally I don't think it's at all likely that we'll need to make this sort of modification (just like I don't think backwards time travel using wormholes is very likely), but I also don't think your apparent absolute certainty that there is no way this could be true is warranted, so I'm playing devil's advocate here. You haven't really provided any substantive arguments to justify your confidence, as far as I can tell, and there seem to be a number of respected physicists who would disagree with you on these points.

Friar Bellows

cfgauss, can you explain to me why the aether is a preferred frame and the "Hubble flow" isn't a preferred frame? I thought I understood it before, but now I'm confused again.