The Principles of Special Relativity
Einstein was lead to his special theory of relativity by his
believe that there is no way to detect absolute motion. This dictated
that the measured speed of light must be the same in all
inertial frames of reference. If this was not so, the laws of physics
would have been different in inertial frames that move relative to each
other.
Inertial frames are uniformly moving coordinate systems, far
away from gravitational effects or any other form of influence, where
inertia is isotropic, meaning a given force will cause the same
acceleration on identical masses in whatever direction the force is
applied.
The Aether Abolished
Einstein also argued that if there was a 'rest-frame' for light
(the luminiferous aether), we could in principle set up an inertial
frame in which light would not propagate in the forward direction at
all, e.g., if the frame moves at the speed of light relative to the
aether. Einstein was still very young, when he reportedly contemplated
if he would still be able to see his own face in a mirror if they were
both at rest in such a frame - moving at the speed of light through the
aether.
Einstein realized that it is paradoxical to assume the same light ray can actually move with the same speed c
(in an absolute Newtonian sense) relative to all inertial frames. This
would require that light adapt its "absolute speed" to the frame that
measures it. He decided that either time intervals or distance
intervals (or both) must change if measured by observers in different
inertial frames in relative motion.
This led to the famous and sometimes controversial notion that "moving clocks tick slower than stationary clocks". The problem is that one cannot really decide which clock is moving and which clock is stationary - at least not if both clocks are strictly inertial...
I'm inviting questions and/or discussion on this problem.
A more complete description can be found on my website: http://www.relativity-4-engineers.com under the tag: What is Relativity?
(I was unsuccessful in generating a hotlink using this editor - my apologies for any inconvenience caused.)
Burt Jordaan