Scientists from the LIGO (Laser Interferometer Gravitational-wave Observatory) research centre based in America announced during a press conference last Thursday the first experimental evidence of gravity waves, 100 years after Albert Einstein predicted their existence in his theory of General Relativity.

The phenomena that LIGO observed was the coalescing of two black holes 1.3 billion light years away. These black holes, of 36 and 29 solar masses, each only a hundred km across, with a density roughly equivalent to putting Mount Everest inside an exercise ball, caused a monumental shockwave to echo through the cosmos. Circling ever closer to each other until eventually, in the split second when their boundaries (event horizons in science speak) touched, they fused and unimaginable quintillions of tonnes of mass was redistributed.

However the final product weighed 3 solar masses less than the sum of the two initial black holes, and this colossal mass was released as energy to permeate through space. The orbiting black holes had been losing energy as gravitational waves for millions of years slowly circling closer together, however these effects were too small to measure. At the last moment the crashing together of the two black holes released such a vast amount of energy as gravitational waves that LIGO was able to measure the effect on earth.

The gravitational waves arise due to disturbances in space-time, the fabric of the universe that Einstein described. Space-time is deformed by objects of great mass; imagine placing a heavy ball on a taut sheet, the dipping in the sheet represents the gravity that the object exerts. In the case of a black hole the dip in the sheet goes on forever and life gets very confusing. Suffice to say that when two black holes join together the space time fabric is greatly disturbed and gravitational waves are released like ripples through the universe.

LIGO works using two 4km long tunnels at right angles to each other. A light beam containing light in phase with itself is split so that some light travels up each of the tubes, is reflected and returns to the detector. There the light is combined and the intensity is measured. If one beam of light has had to travel any further than the other then they will be slightly out of phase causing the intensity to drop a little. The tubes are exactly the same length so that the light is in phase originally and any change can be measured.

When the gravitational waves hit the detector they will affect the space-time the light is travelling through, causing it to travel a bigger or smaller distance and arrive slightly early or late in its return to the detector. The gravitational waves will cause a larger effect on one of the legs compared to the other, so when the beam recombines the light will be slightly out of phase and the intensity will drop, exactly what LIGO detected.

This effect can be detected on earth only with great difficulty. The size of the waves decreases by four every time you double the distance you are measuring from. We are measuring from 1.3 billion light years away so the effect is absolutely tiny. As a consequence, LIGO is the most accurate measuring device ever built – it measured the extra distance the light had to travel correct to within 1/1000th of the width of a proton. If you scaled up the accuracy of this measurement, it is equivalent to measuring the distance to Alpha Centauri, the star closest to the sun, correct to within the width of a human hair.

The consequences of this could be huge. The team behind it are almost certainly going to win the Nobel Prize and it is another piece of evidence supporting Einstein’s theory. However most excitingly this could open up another window onto the universe. Supermassive objects much further away could now be measured. For example if you go far enough back in time towards the Big Bang then light and all the other types of radiation we currently use to ‘look’ at the cosmos were not able to form. Since light takes time to reach us this is equivalent to saying if we look far enough away our current techniques stop working. Gravitational waves on the other hand have been able to permeate from before that time, so this detection technique could help us see further back in time and improve our knowledge of the Big Bang.

The LIGO project actually includes two detectors, in Louisiana and Washington, which allowed the signal to be measured twice, effectively making sure it wasn’t fluke. A network of such detectors are either under construction or planned, in India, China and Europe. Together these could be a very useful astronomical tool in the future. Much like the realisation that we could use radiation other than light, such as radio waves, to observe astronomical objects extending our senses, this method could be a gateway to things previously only imagined.