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.
Q: I want to live as long as possible. Can relativity help me?
Time ticks by more slowly if you travel really fast, but this won't help you to enjoy living longer. On the spaceship nothing seems to have changed. If you make it to 80, despite all the health risks of space travel (osteoporosis, exposure to some really nasty radiation, etc) you will still look old and wrinkled. You would, however, be able to come back to earth and find that you had lived longer than your old mates (now at peace in the cemetery) but that doesn't sound like fun, so those who want to live longer would be better off sticking to a healthy diet and regular exercise, coupled with marriage and a sincere belief in God (on average, married believers live longer than unmaried atheists).
Is there just one theory of relativity?
Unfortunately, there are two. The earlier one about space and time and the speed of light is known as the special theory of relativity. Later, Einstein realised he had made a few important omissions: gravity and acceleration (which turned out to have some striking similarities). So he developed the general theory of relativity to add to and complete the earlier theory. Again, Einstein wasn?t the first to say some pretty weird things about light and gravity and space, but we?re not going to bother with the boring historical details. Let's concentrate on the weird stuff.
What's weird about reality according to the general theory of relativity?
Well, for one, space is curved.
If space wasn't curved, whenever we shone a beam of light (like a laser) it would travel in a line that would seem perfectly straight from wherever you were in the universe, and it would go on for ever and ever in the same direction. This is exactly as Euclid would have predicted (Euclid being the ancient Greek guy who was the founding father of high school geometry, and who assumed that space just had to be flat). This is not what happens, though. Light is bent by gravity, so a beam of light passing through galaxies curves when it comes close to a strong gravitational field.
Some people even think that gravity bends the space of the entire universe into a huge sphere. In practise, this would mean that if you tried to shine a laser beam out beyond the edge of the universe, gravity would bend it and send it in a huge circle running round the perimeter of the universe. (There would be no way of looking beyond or travelling beyond the edge of a universe like this.)
Is that about as weird as it gets?
Not exactly. The theory predicted (not for the first time) the existence of black holes. If gravity bends light then it is possible that if a star became dense enough, its gravitational field could be so great that the light it previously emitted could no longer escape.
Let's begin like this: To launch a spaceship from the surface of the earth, it has to reach a velocity of about 40,000km/hour (11km/sec) otherwise gravity will either pull it into an orbit or back to the surface of the earth. This escape velocity increases relative to the size of the planet or the star (or even the galaxy) and its density. From the surface of the sun (much bigger and slightly more dense) the escape velocity would be 624km/sec. That would cause problems for terrestrial spacecraft but it causes no problems for light (travelling at 300,000km/sec).
When stars reach the end of their life strange things start to happen and they start to collapse. Eventually the atoms are squeezed together so tightly that their nucleii start to touch one another. That makes collapsed stars incredibly dense, the consequence of which is an incredibly strong gravitational field. If this were to happen to our sun, and if it were to become so compact that its diameter were a mere 1.47 km, gravity at the surface would be so high that the light of the dying star would no longer be able to escape.
As one physicist put it in the 1920s:
There could come a time when the sun is shrouded in darkness, not because it has no light to emit but because its gravitational field will be impermeable to light.
The sun would have become a black hole.
Hang on. If a collapsed star can become a black hole, black holes can't really be holes, can they?
True. Actually, they weren't originally called black holes, and the word "hole" is a bit confusing because it makes you think that there is really nothing there, which isn't true because there is only a black hole when there is something which is either very very big or very very dense.
Another thing. Didn't you say light always travels at 300,000km/sec? Now you tell us that gravity makes light travel more slowly and could even bring it to a standstill.
If you were somewhere near a black hole and you measured the speed of light coming from your on-board laser, you would be disappointed to find it was still travelling at the usual speed. This is because gravity also does weird things to the clocks and rulers you would use to measure the speed of light. Close to very strong sources of gravity clocks tick away more slowly and rulers shrink (not that you would notice this inside the spaceship). These distortions of time and space are what they call a warp in spacetime.
If black holes do weird things to clocks, could they help me live longer?
If you could find a nearby black hole that was spinning, you could fly your spaceship into the whirling ring of material around it, and then with a quick burst from your booster rockets you could pull the ship out of the orbit before it got sucked into the blackness. Your on-board atomic clock might indicate that the hair-raising trip just lasted a couple of hours. But back on the mother ship hundreds of years might have elapsed. Again, all your old mates would be dead, which isn't much fun.
However, there is a happier lesson to be learnt for those of us back on earth. You should bear in mind that clocks tick slower in stronger gravitational fields when you next look for somewhere new to live. Physicists have put atomic clocks (that can measure a billionth of a second) in the basements and on the top floors of skyscrapers, and they have proved that clocks in basements run more slowly. So you should stop looking for a room with a view, you should get all your mates together and share one big flat underground.