Regarding the "aether", I understand that the concept was
dropped from physics due to the acceptance of the theory of relativity
and experiment by Michaelson and Morley showing that the speed of light
is a constant despite linear motion. My question is how to interpret the
following thought experiment: imagine that we are in an "Einstein
elevator" i.e. we are enclosed so we have no external referents.
Relativity implies that we cannot distinguish gravitational force acting
on our elevator from acceleration. OK, but consider this: suppose I
the elevator and I have 2 small weights connected by a spring scale.
Assume for simplicity that there is no gravity or linear acceleration so
I am functionally weightless. Now at the start of the thought experiment
I hold the scale by its center of gravity, it will measure 0. Now I give
the weights and scale a spin about their center of gravity. The scale
indicates a force. But the scale will continue to indicate a force even
if the elevator is spun in synchrony with the scale. This indicates that
rotational motion is not relative, it is absolute and it points to an
absolute frame of reference...no?
No. Different viewers will still disagree on the linear speed of the
scale system as a whole, so it doesn't define an absolute frame of
reference as the aether was thought to do. It's true that everyone
will agree the scale system is rotating, and it's true this is because
rotational motion involves acceleration, which is not relative.
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Update: June 2012