In what has to be one of most poorly kept secrets of modern science, on 11 February 2016 a team of over 1000 scientists working on the Laser Interferometer Gravitational-Wave Observatory (LIGO) project announced that they had indeed detected gravitational waves, as predicted 100 years ago as a consequence of Einstein’s general theory of relativity.
The LIGO project
The LIGO project is a large physics experiment to detect gravitational waves, namely ripples in space that are generated by distant cataclysmic events such as the explosive merger of two black holes. LIGO was founded by famed physicist Kip Thorne (who consulted extensively with the producers of the recent movie Interstellar), together with Ronald Drever of CalTech and Rainer Weiss of MIT. The researchers recalled that in the 1980s, when they first tried to explain the project to potential funders, proposing to experimentally detect length changes of one part in a billion trillion, everybody thought we were out of our minds.
The original LIGO system began operations in 2002, but as of 2010 it had not succeeded in detecting any gravitational waves above background noise. It was then upgraded, in a USD$200 million overhaul, replacing its detectors with significantly more sensitive detectors and including an extensive system to shield the system and compensate for the ever-present disturbances of autos, airplanes, winds and, according to one report, even hand clapping in the control room. Partly to help reject such local disturbances, the current LIGO system consists of two detectors — one in Washington State and one in Louisiana (since it is extremely unlikely that both detectors would detect a similar local event at nearly the same instant).
Each of the two systems is a large and exceedingly sensitive interferometer — an L-shaped vacuum chamber with arms 4.1 km (2.5 mi) long. A laser beam enters the system near the junction, splits and goes down each branch, bounces off mirrors at the end of the branches, and then recombines at the junction. If the two light beams cancel each other out, no light emerges, but if a gravitational wave has distorted one or both branches, then a faint signal is seen — see this diagram. Each system is astoundingly sensitive — it can detect a change in the length of one of the arms as small as 0.1 attometre, which is one ten-thousandth the diameter of a proton.
The discovery
The event in question is a “chirp” — a brief sound of steadily increasing pitch that quickly rises to middle C (you can listen to it here) — first detected in Louisiana and then, a few milliseconds later, at the Hanford, Washington facility. It is already being compared to Alexander Graham Bell’s “Mr. Watson, come here” and the first beeps of Sputnik. The chirp was the result, the scientists conclude, of the merger of two back holes, one 36 times as massive as the sun, and the other 29 times.
The detection occurred on 14 September 2015. The elated researchers were not very good in keeping it secret, since rumors of the discovery have been floating for several months. As Gabriela Gonzalez of Louisiana State University, one of the five principal figures in the project, exulted, “We are all over the moon and back. … Einstein would be very happy, I think.”
Unlike some other recent announcements, such as the 2014 report of evidence for the big bang inflation, which were later retracted, this one has been quite carefully checked, and it is a five-sigma statistical result. Moreover, the announcement accompanied peer-reviewed publication of the results. In any event, additional experiments with other facilities are planned, and so any remaining uncertainty should be resolved in the coming months and years.
Einstein vindicated
Although this is not the first vindication of Einstein’s general relativity (from its inception 100 years ago general relativity explained the anomalous perihelion of Mercury), it is the first-ever direct detection of gravitational waves, which were predicted by Einstein’s general theory of relativity. Secondly, it is probably the most compelling evidence we have that black holes are real. We leave it to philosophers to explicate what real means.
In 1916, Einstein told Schwarzschild, who first mathematically deduced the existence of black holes based on general relativity, that he was not sure that the gravitational waves existed. As late as 1936, Einstein and a research assistant set out to debunk the notion of the waves, before finally changing their minds.
The discovery is thus a dramatic example of what Eugene Wigner described in 1960 as “the unreasonable effectiveness of mathematics in the natural sciences.” After presenting several examples, Wigner concluded
It is difficult to avoid the impression that a miracle confronts us here, quite comparable in its striking nature to the miracle that the human mind can string a thousand arguments together without getting itself into contradictions, or to the two miracles of the existence of laws of nature and of the human mind’s capacity to divine them.
Recently physicist Max Tegmark proposed his own solution to this conundrum — mathematics is unreasonably effective in explaining the universe, because, at its foundation, the universe is a mathematical structure.
Whatever the explanation, it is remarkable indeed that scribblings on a chalkboard by a mathematician in the early twentieth century can be seen, 100 years later, as prescient to one of the most sophisticated scientific experiments our modern technological world has ever performed. Einstein would be very pleased at the outcome.
For additional details, see articles in Scientific American, Quanta, Science, the New York Times, the Washington Post and The Conversation. Some of the above was paraphrased from these sources.