I’d somehow managed to live for thirty-two years without hearing about Einstein’s light clock, only to be enlightened by Professor Brian Cox at the Uncaged Monkeys show.
Some super-clever Australians explain it very well here, complete with neat animations. I gather that the premise is that, because the speed of light is constant whatever your own position, time is (to deploy a technical term) stretchy.
Very clever man, that Einstein.
I can’t mention Uncaged Monkeys and not link to one of the many many Pale Blue Dot videos on the internet (attending Uncaged Monkeys leaves you with the not unpleasant feeling of having been inducted into a fundamentalist Saganite cult). So here’s one, and here’s another:
There’s also a religious take on the p.b.d. to be found on GodTube (no, really), and it’s an excellent exercise in advanced point-missing, but I won’t link to it, as just watching it made me do a bit of a sick in my mouth.