Experimental proof (published in Science) that the speed of light is not constant - light is slowed down at the source and then travels in vacuum at a speed lower than c:

http://rt.com/news/225879-light-speed-slow-photons/
"Physicists manage to slow down light inside vacuum (...) ...even now the light is no longer in the mask, it's just the propagating in free space - the speed is still slow. (...) "This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum," co-author Romero explains in the University of Glasgow press release."

http://www.upi.com/Science_News/2015.../1191422035480
"The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A group of them just proved that light can be slowed down, permanently."

https://www.youtube.com/watch?v=rM-fGNxxmSE
Glasgow researchers slow the speed of light

Pentcho Valev