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The Theory of Relativity for Dummies

Most people think it was Einstein who, in the first decade of the twentieth century, came up with the theory of relativity – as if Albert was quietly working away in his patent office in Switzerland and, entirely on his own, managed to come up with a completely new theory of space and time. Actually, it wasn’t quite like that, but because the history of science is a dreadfully tedious subject, we will skip Albert’s many predecessors and get straight to the best bits of the theory of relativity.

Question: Why is it called a theory of RELATIVITY?
Because time and length are no longer absolutes. You’ve got your digital watch on your wrist and a metre ruler on your desk. These seem like absolutes: a second and a centrimetre for you must be the same as they are for me, and the same as they are on Alpha Centauri. But they’re not.

If I stay on my balcony while you start a career as an astronaut flying round the galaxy at an incredible speed (and it would have to be pretty close to the speed of light: 300,000km/sec), and if you could later whiz past my balcony so that we could somehow compare watches and rulers, your metre ruler would be smaller and your watch would be going slower than mine. (Actually that wouldn't be possible because the human eye can't spot things moving at that kind of speed, and spaceship rockets do nasty things to balconies that are only a few metres away. But if it were practically possible, it would be fun.)

While you’re out in space travelling at some unbelievable speed nothing seems to you to have changed. It’s only if you have a chance to compare measurements of time and length with those back home that you see that something odd has happened.

Q: All the introductions to Einstein talk about the twin paradox. What's that?
One 25 year old twin stays on earth while the other, fresh out of astronaut school, sets off on a space voyage travelling at 90% of the speed of light. After 10 years in space, with her mission accomplished, she turns round and heads back to earth. By the time she lands she knows from her on-board clock that 20 years have passed. She is now 45 years old. Fortunately, her study of relativity has prepared her for the shock when she sees her twin sister, who is now 71 years old.

Conclusion: Space travel, when it is really, really fast, is also time travel: you travel into the future without getting that much older yourself.

So is everything relative?
Not exactly. Actually, the idea of time and length being relative to our speed was proposed first as a way of explaining an observation that puzzled everyone.

Some people in the nineteenth century devised a very sensitive piece of apparatus to measure the speed of light as we on earth rotate in space. The idea behind the experiment is easier to grasp if we think of spacecraft and the tiny particles of light called photons. If you were accelerating away from the sun wearing special goggles that enabled you to see individual photons, as you approached 300,000km/sec you would expect to see photons moving ever more slowly past the side window of the spacecraft. And common sense would say if you put your foot on the gas a bit more, you should overtake the photons and leave them crawling along behind as your spacecraft exceeds the speed of light.

What the scientists discovered, to everyone’s surprise, was that if you move faster, light doesn’t whiz past your window more slowly. It always whizzes past at the same speed. (In other words, the photons always win – nothing travels faster than light.)

To explain this bizarre finding, scientists (even before Einstein) suggested the following: the result only makes sense if, the faster you travel relative to the speed of light, the shorter your unit of length becomes and the slower your measurement of time becomes.

To an outside observer looking at your superfast starship, the photons might be moving past your side windows really slowly as your speed approaches 300,000km/sec, but if your on-board clock has slowed down by the same amount and your measurement of length has been compressed those same photons seen from inside the starship will seem to be whizzing past at the same speed as they had when you were still in first gear.

Is there any proof for all this?
Yes. Although spacecraft are still way too slow for astronauts to notice the effects of relativity, research into the behaviour of subatomic particles gives clear support to the theory. There is a laboratory deep within a Swiss mountain where they watch what happens to subatomic particles as they whiz through a circular tunnel attaining speeds close to that of light. Weird things happen, such as unstable particles staying alive for a lot longer than they normally would, and these weird things can only be expained by the theory of relativity.

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