What were Einstein's predictions?

Gravitational waves: this is something that Einstein predicted 100 years ago with his theory of general relativity and scientists have since been scouring the universe for them. This month, the team at the Laser Interferometry Gravitational Observatory, LIGO, announced they'd finally done it... But what are these gravitational waves, how were they found, and why does the discovery matter?

To explain, first we need to go back several hundred years to the birth of the concept of gravity itself and the Cambridge scientist Isaac Newton. He was at Trinity College and - appropriately enough - so is the Astronomer Royal Martin Rees. He spoke to Chris Smith about the history of gravity, beginning with Newton's groundbreaking insights...

Martin - It was the first great unification; he realised that the force that holds us to the ground and makes the apple fall is the same force that holds the moon in it's orbit around the earth and hold the planets in their orbits around the sun.  He learnt that this force affects all substances equally and that it obeys the so-called inverse square law, which means that if two objects get twice as far away from each other, the force gets four times weaker and this law explained very well all that was known at that time about the orbits of the planets.  But it breaks down in two ways; one way is if things go at nearly the speed of light and the other way is if gravity was very strong.  Gravity in the earth and in the sun is not that strong but we imagine that there are objects in the universe where gravity is much stronger...

Chris - Like black holes for example?

Martin - An extreme phenomenon is a black hole, which was one of the great consequences of Einstein's theory, and Newton's theory breaks down under those extreme conditions.

Chris - So along comes Einstein, quite a bit later, but how did Einstein change that?

Martin - Well Einstein didn't really overthrow Newton, he extended and transcended Newton.  And his theory allows us to correctly describe what happens under extremes of strong gravity and high speed, but also it gave us a deeper understanding into what gravity was.  It wasn't really clear to Newton why it should be inverse square law, why all objects should fall at the same speed whatever they were made of, but that became natural when Einstein saw that this was really a consequence of space itself.  Space interacts with mass and the mantra is: matter tells space how to curve; space tells matter how to move; and as an interaction between the behaviour of space and the matter in it.

Chris - Einstein puts forward the idea of this concept of space time, where the fabric of the universe is the notional entity space time and big things that are very gravitationally active will exert an effect or an influence on that space time...

Martin - Yes, space itself becomes a sort of active arena where things happen and the strongest gravity is around black holes.  And if gravity changes, if for instance two black holes fall together, then there was an issue in that we thought that nothing could travel faster than light.  So, if two black holes crash together, for instance, something must go at the speed of light in order to cause a change in the gravitational pull felt by distant objects, and so there must be some sort of wave that transmits information.  And so, Einstein generally predicted that if things change, then they must emit gravitational waves and the trouble is that these waves are extremely weak and they're only emitted by very violent events indeed.  That's why they've been so hard to find.

Chris - And how do we know that Einstein got it right?

Martin - Well, there've been lots of tests of Einstein's theory.  Classically, soon after he proposed the theory there were tests of how light was bent when it passed close to the sun during an eclipse and astronomers have found evidence for black holes, but we'd really like to have detailed models for what black holes are like.  Theorists can calculate what a black hole ought to be like, what shape it would be.  But it's been this discovery of gravitational waves which has really helped to clinch that because what's been discovered is that we get this chirp of gravitational radiation that we've just heard earlier on and that's thought to be due to two black holes spiraling together.  They orbit around each other, they omit gravitational waves that takes away energy and eventually they coalesce and merge and then form a single black hole, and this effect is predicted by Einstein's theory.  We can calculate what ought to happen and what's marvelous is that what's been observed to happen is exactly what you would expect.