When two black holes spiral together, the last moments radiate more power than all the stars in the observable universe combined. That energy leaves as gravitational waves: alternating stretching and squeezing of space itself, travelling at the speed of light.
The number
By the time such a wave crosses Earth, it changes distances by roughly one part in 10²¹. Over LIGO's four-kilometre arms, that is a displacement of about 10⁻¹⁸ metres — around one thousandth of the width of a proton.
Measuring it anyway
LIGO is a Michelson interferometer. A laser is split down two perpendicular arms, bounced off mirrors at the far end, and recombined. Normally the two beams are arranged to cancel at the detector. A passing wave lengthens one arm and shortens the other, the cancellation breaks, and light appears where there should be none.
Several tricks make the impossible merely very difficult. The light bounces roughly 300 times before recombining, extending the effective path length to over 1,000 kilometres. The mirrors, weighing 40 kg each, hang from multi-stage pendulums that isolate them from ground vibration. The whole apparatus sits in one of the world's largest vacuum systems.
Why there are several detectors
A single interferometer cannot distinguish a real signal from a passing truck or a settling of the local rock. Two widely separated detectors — Hanford in Washington State and Livingston in Louisiana — must see the same waveform within the light-travel time between them, about ten milliseconds. Adding Virgo in Italy and KAGRA in Japan also allows the source to be triangulated across the sky.
The first confirmed detection, GW150914, arrived in September 2015 from two black holes about 1.3 billion light years away.
