According to Einstein, the speed of light stays the same because time slows and distance shortens as matter moves faster. In the equations, time stops at the speed of light so a matter clock carried by a photon would not tick at all. A photon from the Andromeda galaxy takes 2.5 million years to get here but according to relativity, no time at all passes for the photon itself! It also says that light starts and ends its journey at the same location by length contraction. Needless to say, this makes no sense, as if time stops for light, how does it move at all? Clearly something is not right here, unless matter time doesn’t apply to light.
In his classic thought experiment, Einstein imagined a twin leaving on a rocket that returns after five years of high-speed travel in space to find his twin brother is an old man of eighty! This could happen, because a muon traveling at 99.5% of the speed of light travels 3000 meters in its millionth of a second life when it should travel only 300 meters, i.e. speed extends its life tenfold. Relativity lets a traveler in a rocket accelerating at one g to get to our nearest galaxy and back in their 60 year life but they would return to find the earth four million years older (Harrison, 1986, p157). The evidence is clear that matter time really does slow down as matter moves faster.