Gravitational wave

A gravitational wave is a ripple in spacetime itself — a stretching and squeezing of the fabric of space that radiates outward at the speed of light when a massive object accelerates. Predicted by Albert Einstein in 1916 as a consequence of general relativity, they went undetected for a century — not because they were rare, but because they are unimaginably weak: by the time a wave from a cataclysm on the far side of the universe reaches Earth, it changes the length of a 4-kilometre detector arm by about a thousandth of the width of a proton. On 14 September 2015, the Laser Interferometer Gravitational-Wave Observatory heard one anyway — a chirp from two colliding black holes — and opened an entirely new sense for astronomy.^[1]

The prediction Einstein didn't quite believe

General relativity says mass curves spacetime, and when mass accelerates, the curvature propagates outward as a wave — analogous to how accelerating charges radiate light. Einstein derived the result in 1916, then nearly talked himself out of it, and the physics community spent decades arguing whether the waves were real or a coordinate artifact. The first proof came indirectly: in 1974, Russell Hulse and Joseph Taylor found a binary pulsar — two neutron stars orbiting each other — whose orbit was decaying exactly as fast as general relativity predicted it would lose energy to gravitational radiation. They never saw the waves; they watched them carry the energy away. It won them the 1993 Nobel Prize.^[2]

The chirp that broke the silence — GW150914

Direct detection took a machine of almost absurd sensitivity. LIGO's two detectors — in Hanford, Washington and Livingston, Louisiana — are 4-kilometre L-shaped laser interferometers: mirrors hung on vibration isolators, laser beams split and recombined so that a passing wave's strain (~10⁻²¹, a fractional length change of one part in a sextillion) shows up as an interference shift. On 14 September 2015 — days into a newly upgraded observing run — both detectors caught the same signal 7 milliseconds apart: two black holes, about 36 and 29 solar masses, spiraling into each other 1.3 billion light-years away, merging into a single ~62-solar-mass black hole and radiating the remaining ~3 solar masses as pure gravitational energy — briefly outshining every star in the observable universe in gravitational power alone.^[3]

The waveform — a rising "chirp" that sweeps up in frequency as the objects spiral faster — is unmistakable, and it can be listened to: the video above plays the real detection, the sound of spacetime being rung like a bell by a collision that happened when multicellular life was young on Earth. The discovery won the 2017 Nobel Prize for LIGO pioneers Rainer Weiss, Barry Barish, and Kip Thorne — and it launched gravitational-wave astronomy: a way to observe the universe that doesn't use light at all.^[4]

What gravitational waves see that light can't

Gravitational waves pass through everything — dust, plasma, the early opaque universe — carrying information no telescope can reach:

• Binary black holes — GW150914 opened the catalogue; dozens of mergers are now catalogued each observing run by LIGO, Virgo (Italy), and KAGRA (Japan)
• Neutron-star mergers — GW170817 (2017) produced the first multi-messenger event: gravitational waves plus a gamma-ray burst plus a kilonova that forged gold, platinum and uranium — direct evidence heavy elements are made in these collisions
• The stochastic background — pulsar-timing arrays (NANOGrav and partners) announced evidence in 2023 for a persistent hum of gravitational waves, likely from supermassive black hole binaries throughout the universe
• The first second of the Big Bang — light can't reach earlier than 380,000 years, but gravitational waves from inflation could, in principle, carry direct echoes of the universe's first instant — see Zeptoverse for how the smallest signals became the galaxies

What's next — the instruments

LIGO/Virgo/KAGRA continue coordinated observing runs, cataloguing mergers by the hundreds. LISA (planned for the 2030s) will move the technique to space — three spacecraft forming a triangle of 2.5-million-kilometre arms, sensitive to the low-frequency rumble of supermassive black hole mergers and galactic binaries. And proposed instruments like the Einstein Telescope and Cosmic Explorer aim for tenfold sensitivity — hearing nearly every stellar-mass black hole merger in the observable universe.^[5]

See also

• Zeptoverse — LIGO's strain is itself a zepto-scale signal
• Smallest things ever measured — the displacement record
• Neutrino — the other non-optical sense astronomy just gained
• History of physics

References

• Abbott et al. (LIGO/Virgo) — "Observation of Gravitational Waves from a Binary Black Hole Merger," Phys. Rev. Lett. 116, 061102 (2016)
• Hulse & Taylor — binary pulsar energy loss (1975); Nobel Prize 1993
• Nobel Prize in Physics 2017 — Weiss, Barish, Thorne, for LIGO/GW150914
• GW170817 multi-messenger papers (2017)
• NANOGrav collaboration — nanohertz background evidence (2023)