Yesterday's announcement by the international LIGO collaboration of the direct detection of gravitational radiation on September 14, 2015, initiates a new phase in the exploration of the universe and our search for the physical laws that govern it. Of course the details of the measurement must and will get the critical scrutiny they deserve—this is the way of science—but the data shown seem very convincing.
This report represents a triumph on many fronts. It is a reward for a long line of theorists. starting with Albert Einstein a century ago this year, who gradually figured out how to represent, calculate and seek the observable properties of these waves of gravitation. It is a tribute to the battalion of experimental and computational physicists and engineers, experts on optics, lasers, signal processing and much more, who worked on this quest for more than three decades, gradually honing their techniques to be able to measure distances smaller than a thousandth of a proton diameter and detect true signals in an unforgiving background of seismic, thermal and laser noise. It is a vindication of government agencies around the world, notably the financially strapped US National Science Foundation, who kept faith with an unproven technique, in the face of skepticism and intense competition from safer science.
The LIGO collaboration attributed the signal their instruments detected to the merger of two pre-existing black holes more than a billion light years from Earth. Each had a mass of roughly thirty times that of the Sun, and before the merger they orbited around each other faster and faster, coming closer and closer. This diabolical dance ended with the formation of a violently shaking and spinning patch of spacetime that quickly collapsed, exhausted, to form a single stationary black hole— three solar masses lighter than the combined mass before merger. The rest was converted into energy that was carried off by the gravitational waves. This is E=mc2 on a grand scale!
Before today, most physicists were talking about detecting a pair of merging neutron stars. These binary objects can be observed as radio pulsars long before they coalesce, and their orbits have been measured to shrink at the rate predicted by the theory of general relativity with an accuracy of better than one in a thousand. We have a relatively good understanding of how often this is likely to happen, and it was expected that LIGO might take a year of operation to see them, as its sensitivity gradually improved. By contrast, there were widely varying views on the rate of black hole mergers. We now suspect that the optimists were right! It is too soon to know the provenance of binaries like these. They might be made in the nuclei of galaxies or in globular star clusters. They might even be the last remains of the oldest stars in the universe, formed when galaxies were ten times closer together than they are today.
A bonus from detecting binary black holes is that the frequencies of gravitational waves they emit as they merge are well-matched to LIGO’s capabilities. They are also pure, gravitational unions, unaffected by the messy behavior of the nuclear matter inside neutron stars. We can compute accurately what should be observed from black hole mergers and use observations to test this most vigorous application imaginable of classical general relativity. Those who desire to put Einstein’s theory of relativity to the sword should be especially pleased by this observation.
What comes next? Advanced LIGO still has to reach its full design sensitivity, which will allow it to find new sources at a rate thirty times greater than it can today. Five years of operation with a fully sensitive LIGO could make measurements with a hundred times more detail. Conversely, we should also be able to see sources out to cosmological distance and, perhaps, even determine whether Einstein’s cosmological constant is all that we need to account for the accelerating universe.
Neutron star binaries are thought to be associated with gamma-ray bursts and this hypothesis is likely to be tested soon. More generally, searches for electromagnetic signals associated with gravitational wave events are now possible, from radio wavelengths to gamma-ray energies, as are searches for associated bursts of neutrinos. As happened after the first exoplanet was announced in 1995, we could be in for a string of surprises. Perhaps black hole binaries, contrary to expectation, will be detected electromagnetically!
LIGO-like interferometers have already begun operating in Europe and Japan and, we hope, will also start up in India and the southern hemisphere. Combining these facilities improves the sensitivity and should enable locating sources accurately. Completion and upgrade of these facilities will surely now get high priority.
Three other ways to detect gravitational radiation are being pursued. Waves with periods in the range of minutes should be seen by a proposed space-borne interferometer called, generically, LISA. An important feasibility test – the LISA Pathfinder – is currently demonstrating critical technologies in space while simpler, though less sensitive implementations than the original proposed mission, which had been deferred to the 2030s, are being studied.
Waves with periods on the order of years, expected from massive black hole binaries in galactic nuclei, are being sought, using the International Pulsar Timing Array and important limits on the combined signal from these sources have already been set. Thirdly, evidence for waves with ten billion year periods, predicted to come from the time of inflation, will be sought in the famous microwave background “B-modes” using a new generation of radio telescopes. Each of these approaches is likely to be boosted by today’s announcement.
Congratulations, then, to the LIGO team—now a thousand strong—and good fortune as they build on a wonderful discovery which exemplifies some valuable truths: that the universe can still surprise and delight us with unscripted discovery, that the future of big science is international and that scientific audacity, when combined with skill and patience, can pay off.