# Re: starship-design: relativity

Kyle R. Mcallister writes:
> Maybe what really happens is this: the object moving at .5c towards you
> really doesn't send out a beam that goes 1.00c. Maybe the beam really goes
> 1.00c with respect to you, but only .5c with respect to him. .5c+.5c=1.00c.
> The thing is, he sees the beam to be coming from his laser (or whatever
> emitter) at 1.00c because of the effects of length contraction and time
> dilation. But if you believe that, then you have to ask, which observer is
> really at rest? Are you or is he? Then you have to go further and say that
> one frame of rest is special: it is _really_ at rest, and everything is in
> motion with respect to it. Why don't we teach this view? Because so far, no
> one knows how to: A. Measure this supposed absolute reference frame, and B.
> It would have to cover the predictions of general relativity, not just the
> simpler special relativity. If faster than light signals are determined to
> exist, then this will probably change the views we currently hold on how
> constant the speed of light really is. But at this point, it is just
> speculation, and not needed. The preceding was for thought provocation
> only...

The deal is that the object moving at 0.5c relative to you emits a beam
of light that appears to move at 1c relative to you and at 1c relative
to him too.  Other than that, however, you and he have rather different
ideas of the spacetime coordinates of pretty much everything.

Something to remember is that the speed of light is itself a result of
the electrical and magnetic properties of the vacuum.  If you could see
different speeds for different light beams, then you would also have to
be seeing different electromagnetic properties in the regions those
light beams propagate through.  And if those properties were different,
then a lot of the fundamental behavior of atoms would also be different,
since electrons are bound to nuclei by electrical forces and so on.  In
other words, if c wasn't a constant, then the properties of matter would
also be non-constant.

This line of thinking is more or less why Einstein proposed that
Maxwell's equations of electromagnetism are more fundamental than
Newton's laws of motion and notions of absolute time.  Other physicists
of the day were already groping towards that idea; Einstein was just the
one who sat down and worked out the important consequences.