That time slows down and distance shortens as matter moves faster, to keep the speed of light constant, has interesting implications. In a classic thought experiment, Einstein imagined a twin leaving on a rocket who returns after five years of high-speed space travel to find his twin brother on earth is an old man of eighty! This could happen because a muon traveling at 99.5% of the speed of light should travel 300 meters in its millionth of a second life, but actually travels 3,000m, 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 it would return to an earth that is four million years older (Harrison, 1986, p157). The facts agree that time slows down as matter moves faster, but what does this mean?
Newton believed that God painted matter objects on a universal canvas but Einstein replaced his view with equations that change the space-time canvas. If we keep the analogy, then what is painted isn’t just the object but also its time, as if movement and time use the same paintbox, painting the first more leaves less paint for the second. The next section explores the concept that a finite source produces both movement and time in more detail.
Einstein’s equations make time stop at the speed of light, so a matter clock sitting on a photon wouldn’t tick at all. Light from the Andromeda galaxy takes 2.5 million years to arrive on earth, but according to relativity, no time at all passes for the light 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 alternative is that matter time doesn’t apply to light.