A Simple Way of Understanding Black Holes

What is a black hole?

A black hole is a region where gravity is so strong that any light that tries to escape gets dragged back. Since nothing can travel faster than light, everything else gets dragged back too. So it is believed that you can fall into it and never get out again. It has always been thought of as the ultimate prison from which there is no escape.

The edge of the black hole is called “Horizon”. It is like the edge of a waterfall. If you are above the edge, you can get away if you paddle fast enough. But once you pass the edge, you are doomed. As more and more things fall into it, it gets bigger and bigger and the horizon moves farther out. It is like feeding a pig: the more you feed it, the larger it gets.

How is a black hole made?

Think of squashing a very large amount of matter into a very small space. Then the pull of gravity will be so strong that everything including the light will be dragged back, without being able to escape.

One way that these are formed is when a star that has burned up its energy explodes. This effect is called a supernova. The explosion will drive off the outer layers of the star. In a giant exploding shell of gas, gravity will pull the central regions inwards. If the star is more than a few times the size of our sun, a black hole will form.

Our own milky way has a black hole at its centre, several trillion times (a million times million) the mass of our sun!!

Stretching and squeezing

A more fascinating thing is to imagine oneself falling into a black hole. You can fall into a black hole just as you can fall into a waterfall. If you fall in feet first, your feet will be nearer to it than your head, and will be pulled harder by its gravity. So you will be stretched out lengthwise and squeezed in sideways. This effect is weaker, the bigger the black holes are.

How can you see black holes?

You can’t, because no light can get out of a black hole! It is like looking for a black cat in a dark room! A black hole can be detected by the way its gravity pulls on other things. We see stars that are orbiting something invisible which we know can only be a black hole. We also see discs of gas and dust revolving around a central object that is invisible. This is nothing other than a black hole. This way we can detect or predict the existence and position of a black hole.

Law of conservation of energy

People used to think that nothing would ever get out of a black hole. After all, that is why they were called “black holes”. Anything that fell into it was lost and gone forever. But this contradicted the law of nature which says that very information or energy in this universe is conserved. But black holes were thought to disobey this very law of nature.

Hawking radiation

Then it was discovered that this picture of black holes was not quite right. The fluctuations in time and space meant that these were not the perfect traps they were once thought to be. Instead they would slowly leak particles in the form of “Hawking Radiation.” The ratio of leakage is slower, the bigger the black hole is. These radiations would cause the black hole to gradually evaporate. The rate of evaporation will be slower at first, but it will speed up as the black hole gets smaller and smaller. Eventually after billions of years, the black hole will disappear. So they are not the eternal prisons, after all! But what about their prisoners? – The things that made the black hole or fell in later. They will be recycled into energy and particles. If you examine carefully what comes out of the black hole, you can reconstruct what was inside. This means that if you fall into a black hole by any means, then we can reconstruct you by collecting the energy and radiation that the black hole emits by recycling you!

So, the memory of what falls into a black hole is not lost forever, but just lost for a very long time!