LIGO

Decades to build a detector

Rainer Weiss at MIT, Kip Thorne and Ronald Drever at Caltech, and later Barry Barish, who rebuilt the project as director, turned the idea of a laser interferometer into the Laser Interferometer Gravitational-Wave Observatory. Construction began at Hanford, Washington, in 1994 and at Livingston, Louisiana, in 1995. Initial LIGO began searching in August 2002 and found nothing in eight years; the detectors were then rebuilt as Advanced LIGO.

Two sites matter. A signal seen in both, a few milliseconds apart, is a signal from space, not from a passing truck, and the delay gives a direction.

GW150914

On 14 September 2015, just before the full Advanced LIGO observing run began, both detectors recorded a signal that rose in frequency over about a fifth of a second, from 35 to 250 hertz, the final chirp of two black holes, of about 36 and 29 times the mass of the sun, merging 1.3 billion light-years away. About three solar masses were radiated as gravitational waves in a fraction of a second. The detection was announced on 11 February 2016, a hundred years after Einstein predicted gravitational waves.

The change in arm length was about a thousandth of the width of a proton.

A new astronomy

In August 2017 LIGO and the Virgo detector in Italy recorded GW170817, the merger of two neutron stars, which was seen within two seconds by gamma-ray satellites and then by optical, radio and X-ray telescopes: the beginning of multi-messenger astronomy. Weiss, Barish and Thorne received the 2017 Nobel Prize in Physics. The detectors now work with Virgo in Italy and KAGRA in Japan, and have recorded hundreds of mergers; in 2025 the network reported GW250114, the clearest signal so far, used to test a theorem of Stephen Hawking about black hole horizons.

Instruments and technologies

• Michelson interferometer with Fabry–Perot arms A laser beam split into two four-kilometre arms, each with mirrors at both ends forming a cavity in which the light bounces hundreds of times before recombining.
• Seismic isolation Multi-stage pendulum suspensions and active platforms keep the 40-kilogram test mirrors still against the ground's vibrations.
• Squeezed light Quantum-squeezed states of light reduce the photon noise at the detector.

The people

• Rainer Weiss Physicist who conceived the interferometer design; Nobel Prize 2017.
• Kip S. Thorne Theorist and co-founder; Nobel Prize 2017.
• Barry C. Barish Director who built the observatory; Nobel Prize 2017.
• Ronald Drever Experimental physicist and co-founder; died in 2017.

Timeline

• 1994 Construction begins at Hanford (Livingston follows in 1995).
• 2002 Initial LIGO begins searching for gravitational waves in August.
• 2015 First detection, GW150914, on 14 September.
• 2016 Detection announced on 11 February.
• 2017 Neutron-star merger GW170817 also seen by gamma-ray and optical telescopes; Nobel Prize to Weiss, Barish and Thorne.

Key lessons for observation today

• Two sites are an instrument Coincidence in two distant detectors is what rejects local noise. Network design is part of the sensor design.
• Noise is the product Decades of work went into understanding and cancelling noise sources. A sensor's value is its noise floor.
• Share alerts openly Public alerts let other telescopes follow up within seconds. Real-time open alerting turns one observation into many.

Sources

• Nobel Prize in Physics 2017, press release
• LIGO Caltech, Press release on the 2017 Nobel Prize
• LIGO, Gravitational waves detected 100 years after Einstein's prediction
• Wikipedia, LIGO (secondary)