Gravitational-wave astronomy starts in earnest


THE timing was impeccable, to the point where one might wonder if it had been stage-managed. Less than two weeks after Sweden’s Royal Academy of Science announced that it was awarding this year’s Nobel physics prize “for decisive contributions to the LIGO detector and the observation of gravitational waves”, that detector has come up with its most interesting finding yet.

LIGO is the Laser Interferometer Gravitational-Wave Observatory. Actually, it is two observatories, 3,002km apart in the American states of Louisiana and Washington—a degree of separation ensuring that only disturbances registered by both are considered as coming from outer space. Its purpose, as its name suggests, is to detect gravitational waves. These are ripples in space, propagated at the speed of light, that are created by tumultuous astronomical events involving gargantuan bodies. Their existence was predicted, just over a century ago, by the mathematics of Albert Einstein’s general theory of relativity, which is actually a theory of gravity.

LIGO first detected such a wave in September 2015, though the discovery was not made public until February 2016. Since then, until this week’s announcement, it had seen three others. A fifth might thus be thought unexceptional news. But it is not. For this detection marks the beginning of what LIGO’s supporters always claimed the project would lead to, the use of gravitational waves as an additional window on the universe, through which events observable in other ways can also be seen. That is because, for the first time, the event that created the waves was also noticed by telescopes that look at parts of the electromagnetic spectrum. This means optical, radio-frequency, X-ray and gamma-ray observations can all be correlated with the gravitational data.

X-rays mark the spot

The difference between the fifth gravitational wave and the other four is its origin. The others were the results of two black holes merging. This one was caused by two neutron stars colliding.

Neutron stars are the remnants of supernova explosions. They are, as the name suggests, made almost entirely of neutrons—particles that can pack closely together. Neutron stars are thus small and dense. They are just a few kilometres across even though, typically, they weigh more than the sun.

Because normal stars (some of which will end up as neutron stars) are frequently found orbiting each other as pairs in binary systems, astronomers think that binary neutron stars should also be common. Rotating binary neutron stars emit energy and gradually spiral inward towards each other, eventually merging. When they collide, a burst of gravitational waves is produced. Such a merger will also emit energy all across the electromagnetic spectrum, from radio waves to gamma rays. That is not true of a merger between black holes, the strong gravity of which prevents any electromagnetic radiation escaping.

Though announced on October 16th, the latest gravitational wave was actually observed on August 17th. LIGO, and also Virgo, a detector in Italy, both saw a wave consistent with the merger of neutron stars with masses 1.1 and 1.6 times that of the sun. Those in charge soon found out that another telescope—the Gamma Ray Burst Monitor, aboard a satellite called the Fermi Gamma-ray Space Telescope—had picked up a short-lived burst of gamma rays from the same part of the sky two seconds later. Astronomers around the world then got to work, training other telescopes on the part of the sky the gamma-ray burst had come from and poring through data that had already been collected. Sure enough, optical telescopes pinned the event down within an hour to a galaxy called NGC 4993 (the fuzzy blob in the picture above), which is about 130m light-years away. A campaign over the next two weeks produced more observations in visible, ultraviolet and infrared light through a network of ground-based telescopes. X-ray and radio telescopes also saw the merger.

The events in NGC 4993 were not, however, visible through every astronomical window—and that, in itself, is significant. Astronomers looked for the merger signal in two neutrino telescopes, IceCube (at the South Pole) and ANTARES (in the Mediterranean Sea), and in the Pierre Auger Observatory (in Argentina), which records the arrival of cosmic rays. None of these observatories saw anything. That, according to Szabolcs and Zsuzsanna Marka of Columbia University, in New York, two of the physicists involved in LIGO, could be because the merger just observed is at the weak end of gravitational-wave-producing events. Neutrinos (a type of extremely light, electrically neutral subatomic particle) would be produced in bulk only by more energetic collisions, as might cosmic rays, most of which are high-energy protons. The Drs Marka hope, though, that in the future LIGO will indeed detect such events, permitting them to be observed in four different ways, namely electromagnetically, gravitationally, and by their emissions of neutrinos and protons.

For those behind LIGO, Virgo and other detectors now under construction, these are exciting times. In particular, neutron-star mergers are thought to be an important source of the heaviest chemical elements, such as gold, platinum and uranium. These are formed by the addition of neutrons (some of which then decay into protons) to lighter atomic nuclei. An explosive collision between neutron-rich bodies promotes this process.

The arrival of gravitational-wave astronomy also shows the virtues of patience. Einstein, when he published general relativity, said that he did not expect that such waves would ever actually be detected. Many modern-day physicists make similarly pessimistic noises about some current ideas, such as string theory, saying these will remain forever in the realm of the hypothetical. Perhaps another century will prove them wrong, too.

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