Why FTL implies time travel (2016) | Hacker News Reader

Why FTL implies time travel (2016)

physicsmatt.com

12 pointsladberg5 comments

The flaw in this argument comes here:

    ...if we could just say that there was only one frame of reference where 
    we needed to set up cause and effect, then we could have FTL without 
    worrying about causality. However, there is no special frame of reference,
    there cannot be one if relativity is to be true. And relativity is true...

This just assumes the conclusion. Obviously, FTL is not possible using physical forces as they are currently understood. If it's to be useful, the argument must be trying to show that FTL cannot possibly be achieved using some new force, that has not yet been discovered. But there is no reason to think that such a not-yet-discovered force obeys the principle of relativity. It could very well (if it exists!) allow communication that is faster than light in one particular reference frame - maybe that of the cosmic background radiation? - in which case the argument that causality would also be violated does not go through.

As you approach the speed of light, in any direction, due to time dilation, once you get above 0.866 c, as you add thrust, it still seems like you're accelerating normally, but you are instead accelerating yourself more in the time dimension than in a spacial dimension. Time continues to slow for the rest of the universe from your POV, until you reach 0.999... c, at which point time you are moving so fast, the rest of the universe appears to not experience time anymore. The same as how galaxies at the edge of visible universe are receding away from us at the speed of light, and are deeply redshifted, but if we could correct that redshift, we would see galaxies that don't experience time, because they are moving away so quickly, that we are receiving their photons at a heavily reduced rate. Photons do not experience any time, from their POV, they are created and destroyed at the same time. Thus going FTL implies travelling backwards in time.

You're just explaining how physical forces as currently understood don't allow faster-than-light travel. You don't need an argument like the one in the article that involves causality violations to show that - it's just a consequence of the currently-understood laws of physics.

None of this tells you whether faster-than-light travel or communication might be possible using some as-yet-undiscovered force obeying yet-to-be-discovered laws of physics.

In physics it's implied that an argument assumes known physics is true.

Ah! Sort of like how Kelvin determined that the Earth was only 20 to 100 millions years old? (See https://blogs.scientificamerican.com/roots-of-unity/lord-kel...)

For a sensible argument, one has to assume that current physics is true in the domain in which it has been tested - eg, that physicists haven't been faking all the results from the LHC. But assuming that physicists have already discovered everything is just hubris.

The linked article says "...faster-than-light communication (which includes travel) breaks something very fundamental about physics, something that is often ignored by sci-fi..." So they're trying to prove that all those science fiction stories in which galactic civilizations communicate instantly by "ansibles" that operate using hyper-local disturbances in the quasi-immanent K-field are nonsense. Well, they are nonsense of course, being just made up. And I do think that FTL communication is very unlikely to be possible. But that's not the same thing as proving it is impossible.

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