Relativity says the speed of light is constant because time slows and distance shortens as matter moves faster. In a classic thought experiment, Einstein imagined a twin leaving on a rocket who 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 when it should travel only 300 meters in its millionth of a second life, so speed extends its life tenfold. Relativity lets in a rocket accelerating at one g get to our nearest galaxy and back in 60 years but a traveler would return to find the earth four million years older (Harrison, 1986, p157). The evidence suggests that matter time slows down as matter moves faster but what does this mean?
For Newton, God “painted” matter objects on a universal canvas of space and time. Einstein replaced this view with equations that we know are true but can’t explain. If we keep the analogy, then what is painted isn’t just the object but also its time. This is hard to imagine, but if the object’s time and its movement come from the same finite “painter”, increasing one demand by moving an object faster will decrease the other, so time passes slower for faster moving objects because the same finite source gives rise to both. This is what the theory of relativity implies.
According to Einstein’s equations, time stops at the speed of light so a matter clock on a photon wouldn’t 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 starts and ends its journey at the same location by length contraction! Needless to say, this makes no sense, as how can light move at all if its time stops? The obvious conclusion is that matter time doesn’t apply to light.