Dr. Barry Wardell is a postdoctoral researcher working in the Complex and Adaptive Systems Laboratory at University College Dublin, and a Research Associate in the Astronomy Department at Cornell University.
One hundred years ago, almost to the day, Einstein published his famous theory of General Relativity. The culmination of over ten years of work, Einstein’s theory was the biggest breakthrough in our understanding of gravity since Newton proposed his law of universal gravitation more than two hundred years earlier. Although General Relativity and Newtonian gravity agree well when gravity is weak – as is typically the case in our everyday life – Einstein’s theory makes bold predictions which disagree with Newtonian gravity in a strong gravitational field. Many of these, such as the precession of Mercury’s orbit around the sun and the bending of light by a gravitational field, have already been conclusively confirmed by experiments. Others, like the possibility of wormholes and time travel, are still firmly in the realm of science fiction. In between, there is a wealth of exotic phenomena predicted by General Relativity which has yet to be confirmed by experiments.
Solidly on the side of science-fact is the prediction that our universe is filled with black holes and neutron stars, extremely dense objects which are formed after a star has burned all of its fuel and collapsed under the weight of its own gravity. Traditional electromagnetic telescopes routinely observe signals from neutron stars, but black holes are much more elusive. They get their name from the fact that they are so dense that even light cannot escape their strong gravitational field. Without light coming from them, it is impossible to observe black holes directly. But there is an indirect way to infer their existence: observations of the centre of the Milky Way have shown conclusively that there is an extremely dense, but invisible, object at the centre, and the only viable explanation is that this is a supermassive black hole.
Exciting as these indirect glimpses are, we know we can do better. It turns out that there is a way to observe black holes, and again it is Einstein’s theory which comes to the rescue. If two black holes happen to be nearby each other (an unlikely occurrence on average, but still highly likely given the enormous number of black holes in the universe), then their strong gravitational pull attracts them together and causes them to gradually spiral inwards towards each other. As part of this inspiral process, they radiate gravitational waves which permeate throughout the universe. It is exactly these gravitational waves that provide the direct signal we are looking for.
Now, on the eve of the centenary of General Relativity, we are on the verge of the next great breakthrough in our understanding of gravity. For over twenty years, hundreds of physicists across the globe have been tirelessly working towards a landmark achievement: the direct detection of gravitational waves from inspiralling black holes. An irrefutable consequence of Einstein’s theory, gravitational waves were first predicted by Einstein just months after the publication of his initial paper. Despite this prediction, for decades even Einstein himself did not believe that it would ever be possible to actually detect gravitational waves. It was not until the latter part of the last century that it was realised that the observation of gravitational waves was a realistic goal. Now, thanks to the efforts of countless physicists we finally have not one, but multiple experiments capable of detecting gravitational waves. Two of these – called the LIGO detectors, one of which is pictured above – are based in the US: one in Livingston, Louisiana and another in Hanford, Washington. After a twenty year build-up, this October they finally started taking data with enough sensitivity to actually measure signals from black holes. A third detector – the VIRGO interferometer based in Italy – is close behind, and is expected to be operational within the next year.
It cannot be overstated what an immense achievement these detectors are. As a gravitational wave passes by the Earth, it causes space and time to distort ever so slightly. Once the numbers have been crunched, the prediction that comes out is that the distance between two points separated by 4km (as is the case for LIGO) will oscillate by less than one-thousandth the diameter of a proton (that’s one part in 1,000,000,000,000,000,000,000)! It may seem impossible to measure such a small change in distance, but nevertheless the LIGO experiment has managed it. This has been no small task. LIGO has had to deal with all sorts of problems from the highly technical (the momentum of the photons in the lasers it uses can cause the detector to move more than a gravitational wave would) to the practical (trains and logging nearby cause otherwise unnoticed vibrations which can completely wash out any potential signal). Even an earthquake thousands of miles away in Chile has knocked the LIGO detectors out of action. Despite these difficulties, the LIGO team have finally gotten the machine tuned to the point where the first direct gravitational wave detections are almost certainly just around the corner. So, stay tuned for exiting results in the near future!
A free online course has been created by the team working on the eLISA mission, to explain in simple terms the many fascinating aspects of gravitational waves and black holes. The course is absolutely free of charge and does not require a scientific background.