Gravitational waves

Predicted 1916; first detected 14 September 2015

Physics — general relativity and astrophysics

Gravitational waves are ripples in spacetime itself, radiated when massive objects accelerate violently — most spectacularly when two black holes or neutron stars spiral together and merge. Einstein predicted them in 1916, a year after general relativity, and then spent decades unsure whether they were real or a mathematical artefact. They were finally detected on 14 September 2015 by the twin LIGO instruments in Washington State and Louisiana, each a Michelson interferometer with 4-kilometre arms: the signal, named GW150914, came from two black holes of about 36 and 29 solar masses merging 1.3 billion light years away. The announcement came on 11 February 2016, and Rainer Weiss, Barry Barish and Kip Thorne shared the 2017 Nobel Prize in Physics. They opened an entirely new way of observing the universe, one that uses no light at all.

How to spot them

▸Ripples in spacetime, predicted in 1916 and not caught for another ninety-nine years
▸Detected by measuring a change in a 4-kilometre length smaller than a proton
▸The first signal, from two merging black holes, is labelled GW150914
▸Twin detectors in Washington State and Louisiana, 3,000 km apart, must see the same thing
▸A rising 'chirp' when converted to sound

Key facts

Ripples in spacetime, predicted in 1916 and first detected on 14 September 2015 (2015)

Einstein himself doubted for years whether they were physically real.

GW150914: two black holes of about 36 and 29 solar masses merging 1.3 billion light years away (2015)

Roughly three solar masses of material was converted into pure radiated energy in a fraction of a second.

Detected by twin 4-kilometre interferometers in Washington State and Louisiana

Both must record the same signal within the light-travel time between them.

Their measurement demands sensitivity to a length change of one part in 10²¹

Less than a thousandth of the width of a proton across a 4 km arm.

Indirect proof came from the orbital decay of the Hulse–Taylor binary pulsar (1974)

Its orbit shrinks at exactly the rate general relativity predicts for radiated energy; Nobel Prize 1993.

GW170817, a neutron-star merger, was seen in light as well and showed where gold is made (2017)

The first multi-messenger event: a gamma-ray burst and a kilonova followed within seconds and days.

The 2017 Nobel Prize in Physics went to Weiss, Barish and Thorne for detecting them (2017)

Weiss had sketched the laser-interferometer design as a student exercise in 1967.

Key ideas

Mass tells spacetime how to curve; when mass moves asymmetrically and fast, the curvature change propagates outward at the speed of light, stretching space in one direction while squeezing it at right angles.
An interferometer detects this because the stretch and squeeze change the relative lengths of its two perpendicular arms, shifting the interference pattern where the beams recombine.
Because the waves pass through matter almost untouched, they carry information from places light cannot escape — the insides of mergers and, in principle, the first instants of the universe.

Quick-fire facts

•Predicted by Albert Einstein in 1916, following his 1915 theory of general relativity
•First direct detection GW150914 on 14 September 2015; announced on 11 February 2016
•LIGO has two sites, Hanford in Washington State and Livingston in Louisiana, each with 4 km arms
•The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish and Kip Thorne
•Kip Thorne was the science adviser on the film Interstellar and famously bet Stephen Hawking about Cygnus X-1
•Indirect evidence came from the binary pulsar PSR B1913+16, discovered by Russell Hulse and Joseph Taylor, who won the 1993 Nobel Prize
•GW170817, on 17 August 2017, was the first neutron-star merger detected, and its kilonova confirmed that such collisions forge heavy elements including gold and platinum
•Other detectors include Virgo near Pisa in Italy, KAGRA in Japan, and the planned space mission LISA
•Pulsar timing arrays, including NANOGrav, reported evidence in 2023 for a background of nanohertz waves from supermassive black-hole pairs
•Converted to audio, an inspiral signal makes a rising 'chirp' lasting a fraction of a second

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Sources

LIGO Caltech — What are Gravitational Waves?
The Nobel Foundation — The Nobel Prize in Physics 2017
LIGO Scientific Collaboration — Gravitational Waves Detected 100 Years After Einstein's Prediction