[Note: this article is cross-posted on the Frontier Fields blog]
In November 1915, Albert Einstein published a series of papers that laid out the ideas, equations, and some astronomical applications of the general theory of relativity. While Isaac Newton described gravity as a force between two massive bodies, Einstein's general relativity re-interprets gravity as a geometric distortion of space and time (see my previous blog post "Einstein's Crazy Idea").
One example cited in those papers was that general relativity can explain the extra precession of Mercury's orbit that Newton's formulation does not explain. Another prediction, the bending of light as it passes a massive object, was tested and shown accurate less than four years later. This effect, called gravitational lensing, has been shown in tremendous detail by the Hubble Space Telescope (see my previous blog post "Visual 'Proof' of General Relativity"), and is one of the prime motivations behind the Frontier Fields project.
Last year, scientists celebrated the centennial of general relativity, which has been a resounding success in diverse astronomical situations. However, there was one major prediction that had not yet been tested: gravitational waves.
General relativity predicts that mass not only can create distortions in space-time, but also can create waves of those distortions propagating across space-time. In cosmology, the global expansion of space over time is a familiar concept. For a gravitational wave, space also stretches/shrinks, but that localized distortion moves across space at the speed of light.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is one of the projects designed to observe the minute distortions of gravitational waves. It consists of two detectors, one in Hanford, Washington, and one in Livingston, Lousiana. Each detector has two perpendicular arms, consisting of ultra-high-vacuum chambers four kilometers (two and a half miles) in length.
For the experiment, a laser light source is split and sent down and back each arm. By measuring how the laser light signals interfere with each other when recombined, extremely precise measurements of any change in distances can be made. The idea is that when a gravitational wave passes by, the minuscule stretch of one arm and shrink of the other will be observable.
The signal observed in the LIGO event GW150914
On September 14, 2015, both LIGO detectors observed an event (see the accompanying image). The pattern in the signal indicates that a series of gravitational waves passed through the detectors in about two-tenths of a second. It is extremely important that multiple detectors saw the same event so that local disturbances can be ruled out. Plus, the time delay between the detectors helps measure the speed of the waves.
To analyze the event, the LIGO team used computer simulations. The shape and duration of the event waveform matched that expected for the merger of two black holes. The amplitude of the detection helped determine how far away the black-hole merger took place. The best fit is a merger of a 36-solar-mass black hole with a 29-solar-mass black hole to form a 62-solar-mass black hole, about 1.3 billion light-years away.
The energetics of the merger are simply astounding. Recognizing that 36 + 29 = 65, one can see that three solar masses of material did not end up in the resulting black hole. Instead, it was converted in the energy that created the gravitational wave. Released in less than half a second, the peak wattage of the event was greater than the visible-light wattage from all the stars in the observable universe.
And yet, when detected on Earth, the measured space distortion was smaller than the size of a proton. The reason it took a century to find gravitational waves is because one has to measure subatomic displacements. Gravity is demonstrably the weakest of the four fundamental forces. It takes a tremendous amount of energy to produce a gravitational wave that can be seen at cosmic distances.
The title and abstract of the paper announcing the discovery of gravitational waves
There are several major results from this observation. The detection shows, for the first time, that both black-hole mergers and gravitational waves exist. The time delay between detectors, and analysis of the signal at different frequencies, demonstrates that gravitational waves travel at the speed of light. All the results are consistent with the predictions of general relativity.
This event marks the beginning of gravitational-wave astronomy. With more detectors coming online and planned improvements to current detectors, the field is burgeoning. Dozens to thousands of black-hole or neutron-star mergers, with more detail about each event, should be found in the next decade.
More than a billion years ago, two black holes merged in a distant galaxy, emitted a tremendous amount of energy, and created a gravitational ripple moving across space. Recently, the LIGO project detected this almost-infinitesimal motion of space, a deviation much smaller than the size of an atom. With that amazing observation, the last major prediction of general relativity was verified. A century later, Einstein still rules.