## November 08, 2006

### Shed no light

Mr Black Hole, we used to call my college buddy the inveterate borrower. Magazines, books, sugar, money ... he borrowed them all. But he never was much of a returner. Always seemed like anything that got anywhere close to him simply vanished. As if into a black hole. For that's what happens to things that get near black holes, even those of a more cosmic breed than my friend.

We cannot see black holes directly, for reasons that will soon be apparent. But we know they exist; Einstein even predicted as much in his theory of relativity. Put simply, a black hole is a region of space (or, more correctly, spacetime, but we'll let that pass here) where gravity is so strong that nothing can escape it. Nothing.

Not even light.

At first glance, this is an amazing idea. Gravity holds back light? But look a little more closely. There's gravity on Earth, which is why we don't float off into space. Then some of us do, in space shuttles and the like. How do space shuttles escape Earth's gravity when we can't, no matter how high we jump? That's because shuttles hitch a ride on the backs of rockets that get up to a very high speed. A speed that's actually called the escape velocity. Rockets, or anything at all, can escape the Earth's gravity only by exceeding that escape velocity, about 11 km/s. Compare to the speed a Boeing 747 reaches: about 1 km every 3 seconds. Compare to how long the fastest human would take to cover 11 km: just over half an hour.

If Earth's gravity was stronger, this escape velocity would be higher. For example, the same rocket would have to touch about 62 km/s to escape giant Jupiter's gravity. To get away from the Sun's fiery clutches (well, that's unlikely by itself), it would have to be ten times faster still -- 617 km/s.

Einstein thought about all this, all the way to the logical conclusion. At the end of a star's life, the matter in it collapses on itself, getting packed more and more densely into a smaller and smaller volume. This means that its gravity increases accordingly, as does its escape velocity.

Now stars close to death can collapse dramatically indeed. Imagine that the Sun, for example, has shrunk to a ball only 3 km across. At that size, its gravity is so strong that the escape velocity will exceed the speed of light, 300,000 km/s.

Nothing, but nothing, travels at the speed of light ... except light. Yet not even travelling at that speed is enough to escape the gravity of some collapsed stars. Here is the core, the soul, of this profound thought experiment: not even light can escape a body like this.

Not even light.

Think of this. What do we have here? There are objects out there in our Universe, said Einstein, that we have no way of seeing. Because no light can escape from them.

Black holes, you see.

Imagine the uproar this created in the scientific world of the early 20th Century. Used to looking through telescopes at celestial objects, scientists now had to comprehend that there were very real objects that they simply could not see. Astronomers immediately began searching for ways to actually observe such objects. Finding a black hole would be compelling proof of the validity of Einstein's theories, though they had already been proved in other ways.

But how would they find one? Light emitted inside a black hole cannot get out. Light emitted close to a black hole gets, in a sense, captured by the black hole and cannot be detected. And black holes are small: an average star like the Sun would produce a black hole only a few kilometres across. Even if it did emit light, something that small is not easily seen over cosmic distances. So yes, how do you observe a black hole?

Easy, actually: by how it relates to other matter. Overwhelmed by a black hole's powerful gravity, nearby celestial matter falls towards it. In doing so, the stuff gains energy and heats up rapidly. If heated enough, it gives off X-rays. If that happens before the matter gets entirely trapped by the black hole, the rays can escape and we can observe them.

For these reasons, sources of X-rays are good candidates for black holes. By now, astronomers have identified hundreds of powerful X-ray sources. Some of these are probably black holes. "Probably", because the evidence has different interpretations. Still, it is strong enough and relativity is a sound enough theory that we can be pretty much sure of the existence of black holes. For example, two well-known "globular clusters" are suspected to contain black holes; besides, it is now thought that most galaxies have a black hole at their centre. Including our own Milky Way.

Then again, long before I heard of any of these, I was sure of the existence of at least one black hole. It still owes me Rs 250 and Caravan to Vaccares by Alistair Maclean.

Anirudh said...

Good one. You should write about science more often.

k.r.a.k.t.i.k said...

Very absorbing.

No pun :-)

Red Watch said...

Dear Mr.DDS,

Excellent column.

Red Watch

Ramesh said...

Small correction - escape velocity is the speed required to escape gravity 'without propulsion'. Rockets carry fuel and burn it up to move higher - so they dont need to reach speeds of 11kmps, and they dont. Another way to understand this is to think of throwing a rock out into space - when the rock leaves your hand, it would need a 11 kmps speed to escape earth's gravity and fly into space, else it will fall back. If you had a tall enough ladder, you could just walk out of earth's gravity into space - you dont need to climb the ladder at 11 kmps!!

annie said...

this post reminds me of why I cannot stop thinking of you as teacher... you should teach. this is so lucid!

Corporate Serf said...

You should read up science history a little bit more carefully. Einstein did not predict the existence of black holes. He simply wrote down the equations of general relativity and (I believe) some simple solutions of it. Later Schartschild (sp?) gave a black hole solution to einstein equations and demonstrated the existence of black holes in this theory. (All these were non-quantal theories) Chandrasekhar provided a dynamical theory which set limits on how big a star could get without collapsing into a black hole (as well as othe exotic astronomical objects)

I vaguely remember reading on wiki that there was a suggestion of black holes (in the sense of a body from which light could not escape) in a newtonian setting, but I can't believe anyone took it seriously: the idea of c being the ultimate speed is einstein's as well as the idea that gravity can affect the propagation of light.