Gravitational Waves Detected (2015): LIGO Confirms Einstein

The first direct detection of gravitational waves took place on 14 September 2015, when the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States recorded a faint signal from two black holes merging over a billion light-years away. Announced on 11 February 2016, the discovery confirmed a key prediction of Albert Einstein’s general relativity a century after he made it. It also opened an entirely new way of observing the universe, through ripples in space-time rather than light.

Background: Einstein’s ripples

In 1915 Einstein published his general theory of relativity, which describes gravity as the curving of space and time by mass and energy. In 1916 he showed that the theory allowed for waves: when massive objects accelerate, they send out ripples in space-time that travel at the speed of light.

Einstein doubted the waves would ever be measured, because their effects are extraordinarily small. For decades physicists even argued over whether they carried real energy. A 1957 conference at Chapel Hill, North Carolina, helped settle the theory, and in the 1960s the American physicist Joseph Weber built large aluminium bar detectors. He claimed detections, but other laboratories could not reproduce them.

Strong indirect evidence arrived in 1974, when Russell Hulse and Joseph Taylor discovered a pair of neutron stars, one of them a pulsar, orbiting each other. Over years of observation, the orbit shrank at exactly the rate expected if the system were losing energy to gravitational waves. Hulse and Taylor received the Nobel Prize in Physics in 1993. Pulsars themselves had been found only a few years earlier, as told in the article on the discovery of pulsars.

Building LIGO

In the early 1970s Rainer Weiss at the Massachusetts Institute of Technology worked out in detail how a laser interferometer could catch gravitational waves. At the California Institute of Technology, the theorist Kip Thorne and the experimental physicist Ronald Drever, recruited from Glasgow, developed similar plans. The groups merged their efforts into LIGO, funded by the US National Science Foundation.

Each LIGO detector is L-shaped, with two arms 4 km long at right angles. A laser beam is split and sent down both arms, bounced back and forth by mirrors, and then recombined. A passing gravitational wave stretches one arm while squeezing the other by a tiny amount, which shows up as a change in how the two light beams interfere.

Two identical detectors were built about 3,000 km apart, at Hanford in Washington State and at Livingston in Louisiana. A real signal should appear in both within about 10 milliseconds, the time light takes to travel between them; local disturbances such as trucks or earthquakes would not. Barry Barish, who directed the project from 1994 to 2005, turned it into a large, professionally managed collaboration.

The initial LIGO detectors observed from 2002 to 2010 without finding a signal. They were then rebuilt with better lasers, heavier mirrors and improved seismic isolation. Advanced LIGO was roughly three to four times more sensitive and was preparing to begin its first official observing run in September 2015.

The detection of GW150914

On 14 September 2015 at 09:50:45 UTC, only days before the official start of observations, a signal swept through both detectors. It reached Livingston first and Hanford about 7 milliseconds later. The waveform rose in frequency and strength over about two-tenths of a second, a pattern physicists call a “chirp”, exactly as expected from two massive objects spiralling together.

Analysis showed that two black holes, of roughly 36 and 29 solar masses, had merged into a single black hole of about 62 solar masses. The missing three solar masses had been converted into gravitational-wave energy. For a brief instant the merger released more power than all the stars in the observable universe combined, though almost none of it as light. The event had happened about 1.3 billion years ago.

The first people to notice the event were not in the control rooms. An automated search flagged it within about three minutes, and the alert was spotted by Marco Drago, a postdoctoral researcher at the Albert Einstein Institute in Hannover, Germany, where it was still morning. Within hours, members of the collaboration around the world were discussing whether it could be real.

The signal was named GW150914, after the date. It was also the first direct evidence that pairs of black holes exist and can merge, and the first time black holes of this size had been observed.

Months of checking and the announcement

The collaboration had previously rehearsed with secret fake signals, called blind injections, to test its analysis. Team members first had to confirm that this was not one of them, and then spent five months ruling out instrumental faults and environmental noise. The statistical significance was very high: a false alarm of this strength was expected less than once in about 200,000 years.

On 11 February 2016 LIGO leaders announced the discovery at a press conference in Washington, D.C. The paper, published the same day in Physical Review Letters, listed around 1,000 authors from the LIGO Scientific Collaboration and the European Virgo Collaboration.

Recognition and what came next

In October 2017 the Nobel Prize in Physics was awarded to Rainer Weiss, who received half, and to Barry Barish and Kip Thorne, who shared the other half. Ronald Drever, a central figure in LIGO’s design, had died in March 2017; the Nobel is not given posthumously.

The discovery also confirmed predictions about how black holes behave. The shape of the signal, from the inspiral through the merger to the final “ringdown” of the new black hole, matched computer simulations based on general relativity, a strong test of Einstein’s theory in extreme gravity.

Detections soon multiplied. On 17 August 2017 the LIGO detectors and the Virgo detector in Italy recorded GW170817, the merger of two neutron stars. Telescopes around the world then saw light from the same event, including a short gamma-ray burst and a glowing “kilonova” that forged heavy elements such as gold. By the early 2020s, LIGO, Virgo and Japan’s KAGRA detector had reported about 90 events, and later observing runs added many more.

By the numbers

Timeline

• 1916: Einstein predicts gravitational waves from general relativity.
• 1974: Hulse and Taylor discover the binary pulsar that gives indirect evidence for the waves.
• 1990: The National Science Foundation approves construction of LIGO.
• 2002–2010: The initial LIGO detectors observe without a detection.
• 2015, 14 September: Advanced LIGO records GW150914 from two merging black holes.
• 2016, 11 February: The discovery is announced in Washington, D.C.
• 2017, 17 August: LIGO and Virgo detect merging neutron stars, also seen by telescopes.
• 2017, October: Weiss, Barish and Thorne receive the Nobel Prize in Physics.

Why it matters

The detection of gravitational waves confirmed one of the last untested predictions of general relativity and proved that black holes merge. More importantly, it gave astronomers a new sense with which to study the universe, much as the discovery of the cosmic microwave background and radio astronomy did in earlier decades. Gravitational-wave astronomy now allows scientists to observe events that emit little or no light.