
Why FTL implies time travel (2016) - ladberg
http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel
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radford-neal
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.

~~~
hnuser123456
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.

~~~
radford-neal
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.

