Laser Interferometer Gravitational-Wave Observatory

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

On 14 September 2015, both arms of LIGO's twin observatories registered the same brief chirp, a signal rising in pitch and volume over about a fifth of a second before cutting off. It was the sound, translated into an audio frequency, of two black holes, each tens of times the mass of the Sun, spiraling into each other and merging more than a billion light years away. Einstein had predicted gravitational waves in 1916 as a consequence of general relativity, and had at times doubted they would ever be detectable, since by the time such a disturbance in spacetime reached Earth it would stretch and compress a four kilometer detector arm by a distance thousands of times smaller than a proton. Rainer Weiss worked out in the early 1970s how a laser interferometer, patient enough and isolated enough from every other source of vibration, could in principle see a wave that small. Kip Thorne supplied the theoretical picture of what such a detector would actually see from a black hole merger, and Barry Barish, brought in to direct the project in 1994, rebuilt it from a promising but underpowered pair of instruments into the large, disciplined international collaboration capable of building Advanced LIGO and running it long enough to catch a real event. All three shared the 2017 Nobel Prize in Physics for the result. The detection did more than confirm a hundred year old prediction. It opened an entirely new way of observing the universe, one that does not depend on light at all, and within a few years LIGO and its sister observatories had recorded dozens of further mergers, including the collision of two neutron stars watched simultaneously by telescopes across the electromagnetic spectrum.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

LIGO's detectors are, at heart, the same instrument Albert Michelson first built alone in 1881 to look for the Earth's motion through a hypothesized aether, then refined with Edward Morley by 1887, scaled up by a factor of roughly a million and aimed at a different question. Michelson split a beam of light down two perpendicular paths, bounced each half off a mirror, and recombined them, reasoning that if the Earth moved through a stationary aether, light traveling with that motion and light traveling across it would take very slightly different times to return, shifting the interference pattern the recombined beams produced. He and Morley found nothing in 1887, a null result so clean it helped clear the way for special relativity instead of confirming the aether it was built to detect. LIGO uses the identical geometry, a beam split down two arms at right angles and recombined, but looks for a different kind of disturbance: not the observatory's motion through space, but a passing gravitational wave stretching one arm and compressing the other by an amount smaller than the width of a proton, over arms four kilometers long rather than Michelson's few meters. Where the Michelson-Morley experiment is remembered for a beautifully executed absence, LIGO's version of the same instrument was built to wait, for decades if necessary, for the one difference in that interference pattern that would mean something had actually happened. In September 2015 it did.