Today the LIGO Scientific Collaboration, including many physicists from the University of Birmingham, announced the first direct detection of a gravitational wave. The long-awaited announcement sparked strong public and media interest, but why is this so exciting?
Firstly, the simple fact of the detection is phenomenal: about a billion years ago in a galaxy far away, two black holes were orbiting around each other, slowly getting closer and closer, until they eventually merged in a truly cosmic explosion that took just a fraction of a second to convert three times the mass of our sun into pure energy. This energy spread through the universe as a ripple in space and time, until on September 14 2015 at 10:50:45 (UK time) a part of this wave passed through the two LIGO detectors in the USA.
The LIGO detectors have been built to search for gravitational waves. Each detector consists of a 4km-long interferometer that uses an ultra-stable laser to monitor the distances between very special, and large, glass blocks. These blocks act as mirrors for the laser light and at the same time serve as test masses for any changes in space and time. Gravitational waves interact only very slightly with matter, but they stretch and squash space a little. When the gravitational wave passed through the LIGO detectors it wiggled the length of the 4km-long interferometers by about 0.002 femtometers, which is about one thousandth of the diameter of an atom’s core.
The LIGO instruments registered this change and recorded a strong and clean signal. This direct measurement of a gravitational wave was the result of a 50-years-long quest in experimental physics to achieve the precision and sensitivity necessary for this discovery. Einstein never believed that it would be possible to measure a gravitational wave directly. Now, a large international collaboration using modern technology, after decades of preparation, is excited to prove this particular prediction wrong.
In 1915 Einstein presented his new theory of general relativity that associated gravity with a bending of spacetime (the combination of space and time into one set of coordinates). One year later he described some of the mathematical consequences of his theory and predicted the existence of gravitational waves, ripples in spacetime that travel through the universe at the speed of light.
In the same year Karl Schwarzschild showed that Einstein’s theory also permitted the existence of strange objects with such immense gravity that not even light could escape their gravitational field: these became known as black holes.
In the hundred years since, Einstein’s theory of gravity has passed many tests with flying colours. We now have evidence from astronomical observations that black holes and ripples in spacetime exist. However, the signal measured by LIGO is the first probe of this type into a system of black holes. For the first time we have a direct measurement that indicates that the black holes in Nature behave like the theoretical constructs described by Einstein’s theory. We can measure their properties and understand more about their behaviour. The signal, now called GW150914, will be the first entry in a brand new catalogue of astronomical events. The era of gravitational wave astronomy has begun.
The gravitational wave community is truly international and the team at the University of Birmingham supports several international projects. As part of the GEO collaboration we have been involved with the Advanced LIGO project that constructed the current LIGO detectors from the beginning. For over ten years students and staff in our group have built sensors and actuators that are now part of LIGO, supported the operation of the machines, and now provide key input to the analysis of the data and the understanding of the signal. During the last months our offices were buzzing with activity, as the writing of several scientific papers published by the collaboration today were coordinated by members of our team. Now we experience the rare moment when a new field of science is emerging and history is being made. Our success was borne out of the open and collaborative spirit that the founders of our field have shown and which can still be felt in the large collaborations we work in today.
The discovery will surely bring changes to our field, with many opportunities and new challenges. First, the LIGO detectors look forward to periods of commissioning and installation of upgrades over the next few years to increase their sensitivity even further. Other detectors around the world will come online soon (Virgo, and KAGRA), and a new generation of detectors is already being planned for operation on the Earth as well as in space (eLISA and LISA Pathfinder). Pulsar Timing Arrays (International Pulsar Timing Array and The European Pulsar Timing Array) are taking data to observe the giant black holes at the center of galaxies. Soon, we will see a continuous stream of gravitational wave signals from black holes, neutron stars and other dark cosmic objects we have never seen before. There are exciting times ahead in gravitational wave astronomy.
Professor Andreas Freise
Professor of Experimental Physics, School of Physics and Astronomy, University of Birmingham