[ExI] Faster than light??

Anders Sandberg anders at aleph.se
Tue Sep 27 11:36:26 UTC 2011

Tomaz Kristan wrote:
> I don't think that enough Relativity remains, that the "light cone" 
> for example, is a meaningful concept anymore.

The light cone can still be defined as the events that are hit by a 
spherically expanding wavefront of light. Even if you throw out the 
theoretical apparatus of SR that gives it a somewhat privileged position 
it remains as a set of points.

And we have fairly good empirical evidence that for timelike 
trajectories inside observed lightcones the symmetries of space behave 
like SR says, so any generalisation that turns into SR for "low" 
velocities must also have nearly the same symmetries inside the 
lightcone. I am no expert, but it looks like one needs a rather 
nonlinear symmetry group to patch that together with a very non-SR 
behavior (whatever is needed to save causality) outside the light cone.

One of the key conceptual selling points of SR is that it is the 
simplest theory you can get by insisting on identical physics in 
inertial frames and constancy of observed lightspeed; you get the 
Poincare group from that. Of course, a trans-SR theory might drop 
symmetries like boost invariance, but then it better either provide 
amazingly good empirical evidence or some profoundly satisfying 
conceptual explanation. Ever since Emma Noether we have a pretty solid 
reason to regard symmetries as important.

Anders Sandberg,
Future of Humanity Institute 
James Martin 21st Century School 
Philosophy Faculty 
Oxford University 

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