There are more things in heaven and earth, Horatio,
Hamlet, Act 1, Scene 5
Than are dreamt of in our philosophy.
Consider the following two sentences.
1. I saw two cars crashing in a head on collision.
2. Scientists detected collision of two black holes.
Which sentence has a significant effect on you? Most of us would be able to visualize the first event, some may well remember something similar they have seen or experienced. What about the second statement? If you watched the movie Interstellar, you may have some vague idea but I doubt if any of us can really imagine the event described in statement 2. Unless, of course, you have been gifted with an imagination like Richard Feynman, who could visualize as complex a phenomenon as electrons travelling in a metal. For us mere mortals, words like black hole mean practically nothing. Our brains are not equipped with the mechanisms needed to imagine the phenomenon that the words indicate. Their circuitry still deals with distances and times that are suitable for hunter-gatherers.
One of the most important scientific developments of last century was Einstein’s theory of relativity. We live in a world of three dimensions but we do not consider time as a dimension. Special relativity tells us that space and time are intimately related and cannot be separated from each other. Thus, we refer to the four dimensional space-time, with time as a fourth dimension. Now as far as our day-to-day lives are concerned, this does not affect us. Treating space-time together will not help you in any way to get on time to your meeting. In fact, this is the reason why time has been considered as a separate entity all along. We do not see any evidence of time being connected to space in out daily lives. It is only when we go to the extreme conditions that things begin to get interesting. And by interesting, I mean things so esoteric that you would think that they belong in science fiction.
What do we mean by extreme conditions? Very high speeds close to the speed of light which is 3,00,000 kms/sec or very high gravitational fields. Such high speeds can be obtained here on Earth for very small particles like protons – this what the experiments at particle accelerators like CERN do. High gravitational fields require very high masses. We think of the Earth as very big, right? But it’s nowhere near as big as we require for those interesting things to happen. What about the Sun? Yes, the Sun is very big. You can fit about 1.3 million Earths inside the Sun. Even then the Sun is not the biggest star around.
Stars are fascinating. A star is formed when a massive gas nebula – consisting mainly of hydrogen and helium gases – collapses due to its own gravity. The core of the star gets denser and denser and a nuclear reaction starts, converting that hydrogen into helium. The star starts shining, radiating light and energy. This internal reaction is what stops the star from collapsing further. It’s a tug of war – the gravity trying to pull all of the star inwards and the reactions inside resisting the pressure. How long can this go on? As long as the reaction goes on. When the star runs out of fuel, its core stops creating energy and the star dies. If the star is small enough, the collapse eventually settles.
If the star is very massive – more then 2.5 times the Sun – then interesting things happen when it dies. In this case, nothing can prevent the collapse of the star and it goes on shrinking till the gravity becomes so high that nothing can escape from its clutches. It becomes a black hole. No light can escape a black hole so you cannot see it. Black holes have very large mass but they are very small. So a black hole which is 20 times heavier than the sun can have a diameter of 10 miles. There is an upper limit on how big the black holes can be. Are you ready? They can be as big as 50 billion Suns. There is a black hole in the center of our galaxy. It’s about as heavy as 4 million suns. Such immense gravity can have some fascinating effects. If a person can manage to get close to a black hole, time actually slows down for him, just like in Interstellar.
Black holes are hungry creatures. They eat everything they come into contact with. What happens when two black holes meet? Well, first they circle around each other for a few million years, like two lions sizing each other up. As they come closer, their speed of rotation keeps increasing until finally, they merge to form one single black hole. This is one of the most violent events in the universe. When these black holes merge, they send out an energy burst in form of waves. These are the gravitational waves. If you drop a pebble in a pond, it produces ripples. Similarly, these gravitational waves produce ripples in space-time.
Scientists at the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US set up an L shaped detector with each of its arms 4 kms long. They passed laser beam through each arm and a mirror at the end which reflected it back. When a gravitational wave passes, it will change the length of the arm and we will be able to detect it. Just to be sure, they set up two such detectors, one in Washington State and the other 3000 km away, in Louisiana. The idea is, when gravitational waves strike, both detectors should record it. Otherwise, it is some other random event.
And then you wait.
You wait for two massive black holes to collide and send waves. On 14 September 2015, they hit the jackpot. Two black holes merging into each other were “seen”. And on 26 December 2015, they detected one more event, proving that the first one was not a fluke. This is one of the most important discoveries of this century and a very hot contender for this year’s Nobel Prize in Physics.
Here’s another interesting tit-bit. The last event that was recorded happened 1.3 billion years ago. I have no idea how long that is. I am unable to process that information. To keep things in perspective, the age of the universe is 13.82 billion years.
Astronomy is fascinating for many reasons. One of them is a sense of sheer awe that it instills.
