Hi, The theory of special relativity is based on assumption that the speed of light in vacuum does not depend on the source or observer. I have two questions regarding this. 1) Which is constant, speed or velocity? Is the direction also constant? 2) Are there experimental verifications for the the constancy of speed of light in vacuum that it does not depend on the source or observer? If yes, what are these experiments? I appreciate any replies. Thank you very much.