Science › Space, time & gravity
Gravitational waves (LIGO, 2015)
2015· Discovery / experiment
ProvenTested many times; part of working physics.
Ripples in spacetime itself, made by accelerating masses, first directly detected by LIGO on 14 September 2015.
See it on the Science map →How it works
When massive objects like black holes orbit each other, they shake spacetime, sending out waves at light speed that stretch and squeeze space as they pass. LIGO uses two 4-kilometer laser arms at right angles; a passing wave changes their lengths by less than a thousandth of a proton's width. The 2015 signal came from two black holes, about 36 and 29 times the Sun's mass, merging 1.3 billion light-years away.
Why it matters
It opened a new way to observe the universe, by "hearing" collisions of black holes and neutron stars that emit little or no light.
How we know
Hulse and Taylor's binary pulsar (1974) lost energy exactly as waves predict. LIGO's 2015 detection won the 2017 Nobel Prize. Hundreds of events detected since, including a 2017 neutron-star merger also seen with telescopes.
Still a mystery
Background waves from the early universe, and what they reveal about the Big Bang, have not yet been clearly detected.
Connections (3)
▸General relativity: gravity is curved spacetime predicted Gravitational waves (LIGO, 2015)Proven
The relationship
In general relativity, spacetime is not rigid; it can ripple. When heavy objects accelerate, especially two black holes or neutron stars spiraling together, they send out waves that travel at light speed and alternately stretch and squeeze distances. The waves carry energy away, so the orbiting pair slowly spirals inward until it merges.
How the link was found
Einstein predicted the waves in 1916. In 1936 he briefly argued they did not exist, then corrected himself. LIGO's direct detection came on 14 September 2015.
Why it's proven
The Hulse-Taylor binary pulsar's orbit, tracked from 1974, shrinks exactly as wave energy loss predicts. LIGO's signals match computer simulations of merging black holes.
Open questions and mysteries
Waves from the very early universe, and the low-frequency background hinted at by pulsar-timing arrays in 2023, are not yet fully understood.
▸Gravitational waves (LIGO, 2015) were first caught from merging Black holesProven
The relationship
When two black holes orbit each other, they shake spacetime, sending out ripples that carry away energy. The pair spirals closer, faster, then merges, producing a characteristic rising "chirp" followed by a ringing-down as the new black hole settles. LIGO's detectors measure stretches of space smaller than a thousandth of a proton's width, revealing the masses and spins of the merging holes.
How the link was found
LIGO detected the first signal, GW150914, on September 14, 2015: two black holes of about 36 and 29 solar masses, 1.3 billion light-years away. Weiss, Barish and Thorne shared the 2017 Nobel Prize.
Why it's proven
Two detectors 3,000 km apart saw the same signal 7 milliseconds apart, matching relativity's predicted waveform. Hundreds of mergers have been detected since, with Virgo and KAGRA.
Open questions and mysteries
Some detected black holes have masses that stars should not be able to produce. How such pairs form and meet is debated.
▸Gravitational waves (LIGO, 2015) from inflation would leave B-modes: hunting inflation's gravitational wavesOpen question
The relationship
A gravitational wave stretches space one way and squeezes it the other. At 380,000 years, an electron inside such a wave saw the light around it slightly hotter in some directions, and scattering polarized that light in a twisting pattern density ripples cannot make. Today these waves would be billions of light-years long, far beyond LIGO's range; only the afterglow can record them.
How the link was found
In 1997 Uroš Seljak with Matias Zaldarriaga, and Marc Kamionkowski with Arthur Kosowsky and Albert Stebbins, showed that gravitational waves leave B-modes. The DASI telescope first detected the afterglow's polarization in 2002.
Is it proven?
The polarization physics is textbook, and lensing B-modes behave as predicted. What is missing is the signal: after dust and lensing are removed, nothing primordial remains at today's sensitivity.
Open questions and mysteries
Galactic dust and lensing must be subtracted with extreme precision. Other early-universe sources, such as cosmic strings, could also make B-modes, complicating interpretation.