Laser Interferometer Gravitational-Wave Observatory (LIGO)

A 4-kilometre ruler that measures the universe shrinking and stretching by less than one-thousandth the width of a proton.

LIGO detects gravitational waves — ripples in spacetime curvature — produced when massive objects accelerate. To date its primary sources have been merging pairs of black holes and neutron stars. Each detection encodes information about the objects' masses, spins, and distance that cannot be obtained by any electromagnetic telescope. LIGO has opened a new observational window on the universe: gravitational-wave astronomy.

Each LIGO interferometer is a power-recycled, signal-recycled Michelson interferometer with 4-km Fabry-Pérot arm cavities. A 200-W infrared laser (1064 nm) is injected into the system; a power-recycling mirror boosts the circulating laser power at the beam splitter, while the Fabry-Pérot cavities build up ~750 kW of optical power in each arm. The 40-kg fused-silica test mass mirrors are suspended on quadruple-pendulum chains that provide seismic isolation exceeding 10¹² above 10 Hz. The entire optical path resides inside a 10,000 m³ vacuum system maintained at 10⁻⁹ Torr, using 1.2-m-diameter beam tubes baked out to remove outgassed molecules. A passing gravitational wave differentially stretches one arm while compressing the other; the resulting differential length change shifts the interference fringe at the output. The fringe is held on the dark fringe by active feedback; the gravitational wave signal appears as the correction signal fed to the mirrors.

In plain terms

Imagine stretching a laser pointer beam to be four kilometres long, then bouncing it back and forth hundreds of times inside a mirror-lined tunnel. When a gravitational wave passes through — a ripple in the very fabric of space — the tunnel stretches by less than a trillionth of a millimetre, and the laser's interference pattern changes. That tiny flicker is the signal.

Also called: LIGO, Advanced LIGO, aLIGO, gravitational wave detector, GW observatory.

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A short history

Rainer Weiss at MIT outlined the laser interferometric gravitational wave detector concept in a 1972 Quarterly Progress Report, identifying the dominant noise sources and the path to sensitivity at 10⁻²¹ strain. Parallel prototype work by Ronald Drever's group at Glasgow and Caltech and by groups in Germany (Garching) and France/Italy (Virgo precursor) refined the cavity optics and seismic isolation through the 1980s. The formal LIGO proposal to NSF was submitted in 1989 by Drever, Raab, Thorne, Vogt, and Weiss; Congress approved funding in 1991. Construction of the Hanford site began in 1994 and Livingston in 1995. Initial LIGO (iLIGO) ran science operations from 2002 to 2010 without a detection. Advanced LIGO installation was completed in 2014, and on September 14, 2015 — during a pre-official engineering run of O1 — the two detectors recorded GW150914, the first gravitational wave detection, from a binary black hole merger at ~410 Mpc. The result was published in Physical Review Letters on February 11, 2016, and Weiss, Barish, and Thorne shared the 2017 Nobel Prize in Physics.

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