Introduction to relativity: part 2
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.
Eh?
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.
Eh?
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 1920�s:
�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.
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