[lmkg]

Anything which is going faster than C in one inertial frame of reference, is going backwards in time in another inertial frame of reference, which allows for a violation of causality. I'm sure the experimental set-up would be complicated as balls, but a qualitative property like causality is probably more robust to measurement.

[CJefferson]

One small disagreement with you. While general relativity suggests that going faster than C causes a violation of causality, it might be the case that things can go faster than C without causing violation of causality, and we need a new model for such cases.

Part of the point of experimentation is to find holes in our existing models, so it is reasonable for scientists to look for faster than C without violation of causality.

[btilly]

It does not necessarily allow for a violation of causality.

As an example, suppose that we were able to go faster than C in the reference frame of the fixed distant stars, but not in other reference frames. There are reference frames where you can go backwards in time, but none in which you can violate causality.

(That said, General Relativity allows for causality violations. However setting them up is well beyond any engineering ability our species is likely to develop...)

[Symmetry]

Right, but the establishment of privileged reference frames like that doesn't just overturn Einstein, it overturns Galileo.

[btilly]

That is true, however that particular reference frame already seems somewhat privileged thanks to Mach's principle.

Of course I state this as a theoretical point only - there is absolutely no reason to suspect that any such thing is possible.