On my break, I had a chance to read, or at least get half way through, a book called “The Elegant Universe”, by… er… someone.
Its about superstring theory, which is the current grand unified theory, which attempts to unify Einsteins relativity with quantum mechanics. Its all quite interesting, and complicated.
But… Ive always struggled to get a feel for the theory of relativity. This book had a nice explanation which I will attempt to describe in a kinda simplified way… I will try and get it right, let me know if I make any mistakes!
1. You have to believe the speed of light is constant. This is hard to do, because it means if someone throws a ball at you at 10kmh and you run towards the ball and 10kmh, then the ball will hit you at 20kmh. This doesnt happen with light. Light will always be heading towards you at the speed of light, or away from you at the speed of light, regardless of how fast you are heading towards or away from the source. This has evidently been experimentally proven since ages ago. But this is the key point.
2. Imagine a atomic clock, where a photon bounces back and forth between two plates. Bounce, bounce, bounce. At the speed of light. The number of bounces are like the ticks of a clock.
3. Now, if the clock is not moving, it just goes, bounce up, bounce down, bounce up, bounce down at the speed of light. Isnt bounce a funny word…?
4. But, imagine if someone puts it in a bus. And the bus is moving, and you are just watching from the footpath. What do you see? Bounce up, bounce down, sure… but because the bus is moving, the photon appears to move in a sine wave kinda thing. So far so good right?
5. So, from your point of view, the photon is travelling a longer distance, over the sine wave thing. Now, we know that Distance=Speed*Time right? Distance has just increased, so either speed or time must increase too. But… from point 1, speed is constant. So…. time increases. ie, time slows down for the clock. Similar things can be shown for length and mass.
and this has been experimentally established. What a weird world we live in!