“Einstein proposed two things, really quite innocuous when you hear them. One is that, just like on a train, if you’re sitting on a train at the station you momentarily kind of nod off, you wake up and the only thing you can see is another train next to you and it’s moving with respect to you, and in that split second, you can’t tell whether you’re moving or whether it’s the train next to you, and it’s the idea that, well actually, all you can say is “I’m moving relative to something else”. So that was the first idea, that saying that actually, all the laws of physics are the same no matter how you’re moving, it’s the same, there’s no particular special speed or reference in the universe.
In other words, I mean that’s how, you know an air hostess can serve coffee on a plane, she doesn’t have to correct for the speed of the plane, she just pours the coffee, because the laws of physics are exactly the same. So that’s the first idea. That’s not so hard to swallow.
The second idea is slightly harder, and that is the idea that – which again we go by experiment – is the idea that unlike any… anything else, speed of light is constant for whoever observes it, and it’s the same value. So okay let me just explain what that means. I mean, normally if you’re… you know if somebody throws a cricket ball at you, and you’re stationary, you feel a certain hit in your hands as you’re…. as it hits you. If you’re running away from it, the cricket ball reaches you at an actually slower relative speed, so it hurts less when you catch it. If you run away faster than the cricket ball – I personally can’t do that – but if you did, the cricket ball would never… it wouldn’t actually catch up with you. So in that sense, the cricket ball is….. changes it’s speed relative to what you’re actually doing.
Now light doesn’t do that, it doesn’t matter how fast you run, you will always measure the speed of light to be exactly the same. Now how on Earth did Einstein arrive at this conclusion? What he did, he – so the story goes – he imagined himself looking into a mirror, as he approached, and going faster and faster trying to catch up with light. What would the image in the mirror look like? Because if light is like the cricket ball, of course, if he’s going at the speed of light, the light will never actually leave his face, hit the mirror and get back to him, so his image will actually disappear.
But we’ve already said that well, there’s no special… there’s no way of telling whether you are moving or somebody else is moving, so if his image disappeared, he would be able to tell that he was moving and that somebody else wouldn’t. He’d just make a telephone call back and say “Well my image has disappeared”, and that breaks then, the first postulate, it goes against the first one. So combine those two things together, and suddenly all…. you know the heavens open, the notions of time change.”
– Neil Johnson, In Our Time (Programme 54) “Time”