Einstein’s Two Postulates
“The phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest. They suggest rather that .. the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good.”
- That is, the results of experiments are independent of the translational motion of the frame of reference in which they are performed.
“Light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body”
- That is, the speed of light is the same ( = c) for all observers regardless of motion of the observer or the source.