Einstein was wrong. A century ago he first predicted the existence of gravitational waves (ripples in spacetime generated by accelerating objects), correctly calculating that the amplitude of gravitational waves would be small, but he thought that they were too small to have any imaginable practical application. As we mark World Space Week 2017, scientists are using gravitational-waves to learn about our Universe, transforming our understanding of astrophysical objects like black holes.
The coalescence of two black holes is the perfect source of gravitational waves—two massive objects, (many times the mass of our Sun), collide at high speeds (significant fractions of the speed of light). As the two black holes merge together to become one, vast amounts of energy are radiated away as gravitational waves: if the gravitational waves emit were translated to light, a single merger would briefly outshine all the stars in the visible Universe. While these events are cataclysmically violent, by the time the gravitational waves have travelled to reach Earth they are tiny. Gravitational waves are a stretching and squashing of spacetime, and a loud gravitational wave may stretch and squash by one part in 10^21. Einstein was correct that they are small, but with the imagination and perseverance of hundreds of scientists, they are detectable.
The first detection of gravitational waves came in 2015. This was the culmination of decades of work designing and building gravitational-wave detectors by a global collaboration of research groups (including the University of Birmingham). The discovery, which won this year’s Nobel Prize for physics, verified one of the key predictions of Einstein's general relativity, namely the existence of gravitational waves, but this was only the beginning of its scientific impact. Gravitational waves give us a new way to do astronomy - from the measured form of the signal, we can infer the properties of its source. The first detection was not only the first time we measured gravitational waves, but the first time we have discovered a black hole binary (two black holes orbiting each other), the first time we had observed the merger of two black holes into one, and the first time we had evidence for black holes with masses about 30 times of our Sun's.
Gravitational-wave astronomy gives us a new way to study the cosmos, one that complements traditional electromagnetic astronomy. With gravitational waves we can study systems like binary black holes which are invisible using light, and potentially gain a new insight into objects like neutron stars, which would otherwise be inaccessible. Two years ago, we had no knowledge of binary black holes, now we have a growing catalogue of detections which may help reveal how these systems form.
The most recent detection was announced last week. Its source was two black holes, about 31 and 25 times the mass of Sun. This new system is entirely consistent with our growing understanding of the binary black hole population, but is remarkable because it is the first to be detected by three gravitational-wave observatories. In addition to the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO), located in Louisiana and Washington, this event was observed by the European Virgo detector. The addition of the third detector dramatically improves how well we can localise the source - a huge benefit in the search for an electromagnetic counterpart to a gravitational-wave signal. In this case the uncertainty in the sky position was reduced by more than a factor of 10.
Gravitational waves also give us a way to test the theory of general relativity. With them we can probe the strongest gravitational fields, and check their behaviour when they are rapidly varying. These extreme conditions are exactly where we would expect that any deviations from the predictions of general relativity would manifest. Having a network of three detectors also lets us test, for the first time, the polarization of gravitational waves or how they stretch and squash. So far, no deviations from general relativity have been observe. Einstein is right.
The University of Birmingham's Institute for Gravitational Wave Astronomy is involved in all areas of gravitational-wave science, from detector design to unravelling the astrophysics of black hole formation.