Einsteiniana's standard lie: The speed of light has been measured to be constant countless times, both before (Michelson-Morley experiment) and after Einstein devised his special relativity. Now a different song:

http://www.huffingtonpost.com/victor...b_5672842.html
Victor Stenger: "One of the axioms of Einstein's special theory of relatively is that the speed of light in a vacuum is equal to an absolute constant c that is independent of the motion of the source or observer of the light.. The first common myth I wish to address is that c is a measurable quantity and that it is possible to disprove the special theory by measuring a different speed in different frames of reference. (...) You can go ahead and try to measure the speed of light in a vacuum if you like. If your measuring instruments are properly calibrated by an atomic clock, then you are going to get 299,792,458 meters per second every time. If you don't, then all this will prove is that your light beam did not travel at c, not that Einstein was wrong. Einstein could someday turn out to be wrong, because that's how science works. But that fact will not be discovered by measuring the speed of light."

Pentcho Valev