Gravitational-Wave Interferometer (LIGO)

A Michelson interferometer so sensitive it can measure a change in arm length one ten-thousandth the diameter of a proton — registering the faint shudder of two black holes colliding a billion light-years away.

LIGO detects passing gravitational waves — ripples in spacetime — by measuring the differential change in length between two perpendicular 4-km vacuum arms. When a gravitational wave stretches one arm and compresses the other by a fractional amount (strain h = ΔL/L), the interference pattern of a precisely stabilised laser shifts by an amount proportional to h. At design sensitivity the instrument resolves strains smaller than 10⁻²³ /√Hz in its most sensitive band near 100 Hz, corresponding to a length change thousands of times smaller than an atomic nucleus. The first detection, GW150914 on 14 September 2015, registered a peak strain of about 10⁻²¹ from a binary black-hole merger 1.3 billion light-years away. With squeezed-light injection operating since the O3 science run, the quantum noise floor has been reduced by up to 3 dB in strain across the measurement band, increasing the detectable volume of the universe.

LIGO is a power-recycled, signal-recycled Fabry-Pérot Michelson interferometer. A 200-W Nd:YAG laser operating at 1064 nm is pre-stabilised and mode-cleaned before entering the power-recycling cavity, which coherently builds the intra-cavity power to about 5 kW at the beam splitter. Each 4-km arm contains a Fabry-Pérot cavity of finesse ~450, storing approximately 750 kW of circulating optical power — the high power is essential because photon shot noise scales as 1/√P and is the dominant noise source at high frequencies. The test masses are 40-kg fused-silica mirrors polished to angstrom-level surface quality, suspended from quadruple pendulums that attenuate seismic noise above about 10 Hz by more than 12 orders of magnitude. At frequencies above ~100 Hz, quantum radiation-pressure noise and photon shot noise dominate. Below ~20 Hz, residual seismic motion and Newtonian gravity gradients dominate. At intermediate frequencies, thermal noise in the mirror coatings and suspension fibres sets the floor. Since Observing Run 3 (O3, 2019–2020), a squeezed-vacuum state generated by an optical parametric oscillator is injected into the output port of the interferometer, redistributing quantum noise from the amplitude quadrature (where it drives radiation-pressure noise) to the phase quadrature (where it drives shot noise), reducing the quantum noise power spectral density by up to 3 dB across the detection band.

In plain terms

Imagine you held two rulers end-to-end, each four kilometres long, at right angles to each other. A gravitational wave squeezes one ruler a tiny bit and stretches the other — but only by about one ten-thousandth the width of a single proton. LIGO uses laser light bouncing back and forth inside its tunnels hundreds of times to detect exactly that impossibly small wiggle.

Also called: LIGO, Advanced LIGO, aLIGO, laser interferometer GW detector, Michelson-with-Fabry-Perot-arms.

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A short history

The idea of using light interferometry to detect gravitational waves was independently developed in the late 1960s and early 1970s by Rainer Weiss at MIT, and by Kip Thorne and Ronald Drever at Caltech. Weiss's 1972 internal MIT report (reproduced as a Quarterly Progress Report) provided the first quantitative noise budget for a laser interferometric GW detector, identifying shot noise, seismic noise, and thermal noise as the three fundamental limits — the same three that still dominate Advanced LIGO's noise curve. The Caltech group, led by Drever, pioneered Fabry-Pérot arm cavities and power recycling in the 1980s. The LIGO project was formally approved by the NSF in 1992, under director Robbie Vogt and later Barry Barish, who transformed it into a large scientific collaboration (the LIGO Scientific Collaboration, or LSC). Initial LIGO (iLIGO) operated from 2002 to 2010 without detecting any signals, setting upper limits. A five-year upgrade programme created Advanced LIGO (aLIGO), which achieved first lock in 2015. On 14 September 2015 — the first day of the first engineering run — GW150914 was detected, 1.3 billion light-years away. The Nobel Prize in Physics 2017 was awarded jointly to Weiss, Barish, and Thorne. Squeezed-light injection, developed at GEO 600 and demonstrated in small-scale LIGO tests, was incorporated into the full LIGO detectors in 2019 (O3) and upgraded to frequency-dependent squeezing for O4 (2023), further pushing the sensitivity frontier.

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