Many experiments have been made in the laboratory as well as light-year distances. None have indicated any differences in the speed of light. One example of an astronomical observation is to look at blue light and red light arriving from a supernova many light-years distant. Both colors arrive at the same time. This is a very fundamental point. If, for example, photons had a very small mass then there should be a difference.
LeeH
(published on 04/21/2008)
