Experiments have unequivocally shown that the speed of light is not a constant:

http://phys.org/news/2016-03-optical-slower.html
"Researchers at the University of Ottawa observed that twisted light in a vacuum travels slower than the universal physical constant established as the speed of light by Einstein's theory of relativity. [...] In The Optical Society's journal for high impact research, Optica, the researchers report that twisted light pulses in a vacuum travel up to 0.1 percent slower than the speed of light, which is 299,792,458 meters per second. [...] If it's possible to slow the speed of light by altering its structure, it may also be possible to speed up light. The researchers are now planning to use FROG to measure other types of structured light that their calculations have predicted may travel around 1 femtosecond faster than the speed of light in a vacuum."

http://science.sciencemag.org/content/347/6224/857
"Spatially structured photons that travel in free space slower than the speed of light" Science 20 Feb 2015: Vol. 347, Issue 6224, pp. 857-860

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."

http://www.gmanetwork.com/news/story...ut-touching-it
"Although the maximum speed of light is a cosmological constant - made famous by Einstein's Special Theory of Relativity and E=mc^2 - it can, in fact, be slowed down: that's what optics do."

https://www.youtube.com/watch?v=NxJ7_tbbIsg
"Glasgow researchers slow the speed of light"

http://www.cbsnews.com/news/faster-t...peed-of-light/
"For generations, physicists believed there is nothing faster than light moving through a vacuum -- a speed of 186,000 miles per second. But in an experiment in Princeton, N.J., physicists sent a pulse of laser light through cesium vapor so quickly that it left the chamber before it had even finished entering. The pulse traveled 310 times the distance it would have covered if the chamber had contained a vacuum. Researchers say it is the most convincing demonstration yet that the speed of light -- supposedly an ironclad rule of nature -- can be pushed beyond known boundaries, at least under certain laboratory circumstances. [...] The results of the work by Wang, Alexander Kuzmich and Arthur Dogariu were published in Thursday's issue of the journal Nature."

http://www.nature.com/nature/journal.../406277a0.html
Nature 406, 277-279 (20 July 2000): "...a light pulse propagating through the atomic vapour cell appears at the exit side so much earlier than if it had propagated the same distance in a vacuum that the peak of the pulse appears to leave the cell before entering it."

Pentcho Valev