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How does LIGO detect gravitational waves?
The short answer (tap to open)
LIGO splits a laser beam down two arms 4 kilometres long, bounces each off a mirror and brings them back together so they cancel. A passing gravitational wave stretches one arm and squeezes the other by far less than a proton's width. The cancelling breaks slightly, and light reaches the detector. Quantum tricks now make it even quieter.
What is a gravitational wave?
Einstein's general relativity, from 1915, predicts that heavy objects whirling around each other send out ripples in space itself. As a ripple passes, it stretches space one way and squeezes it the other, then reverses. The effect is tiny. Even from two black holes colliding, the stretch on Earth is about one part in a thousand billion billion.
How do you measure a change that small?
With interference. LIGO's laser light splits at the corner of an L. Each half runs 4 kilometres down an arm in a vacuum tube, bounces off a 40-kilogram mirror hanging from fine threads, and returns hundreds of times before the halves recombine. Normally they're set to cancel, so the detector stays dark. If one arm grows and the other shrinks, the cancelling slips and light leaks through.
The chirp is illustrative, drawn from a simple formula shaped like the 2015 signal.
The chirp, in words
The trace starts as a slow, small wiggle. Over a fraction of a second it gets faster and taller as the two black holes spiral together, then stops sharply when they merge. Played as sound, it's a rising whoop. The chirp is illustrative, drawn from a simple formula shaped like the 2015 signal.
What did it find?
On 14 September 2015 both LIGO detectors, in Washington and Louisiana, caught the same chirp seven milliseconds apart: two black holes, about 36 and 29 times the Sun's mass, merging 1.3 billion years ago. The collision turned about three Suns' worth of mass into gravitational waves. Rainer Weiss, Barry Barish and Kip Thorne shared the 2017 Nobel Prize in Physics. Detectors have since logged hundreds of mergers, including neutron stars whose crash also lit up telescopes.
Where's the quantum part?
Light comes in photons, and photons arrive with a little randomness, which adds noise. Since 2019 LIGO has fed in squeezed light, a quantum state that trades extra randomness in one property for less in the one that matters. It makes the detectors more sensitive without a bigger laser. It's one of the first large-scale uses of a deliberately engineered quantum state.
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The most sensitive ruler ever built is an interferometer.
LIGO measures a stretch smaller than a proton, and uses squeezed quantum light to do it.