Gravitational waves: listening to ripples in spacetime

At a glance

Gravitational waves are ripples in spacetime itself, produced when massive objects accelerate. They travel at the speed of light and pass through matter almost unhindered, which makes them a messenger that carries information light cannot. Predicted in 1916, they were first detected directly in 2015. The table collects the milestones and the numbers most often quoted.

Sources and dates: LIGO Caltech pages on gravitational waves and on GW170817 (16 October 2017); the GW150914 discovery paper (Physical Review Letters, February 2016); the NANOGrav 15-year paper (June 2023); the GWTC-4.0 summary (August 2025) and the GW250114 paper (September 2025). Masses are source-frame central values.

Sources: [1] [4] [5] [9] [6] [8]

What a gravitational wave is

In general relativity, mass and energy curve spacetime, and when mass moves the curvature changes. Einstein showed in 1916 that these changes do not appear everywhere at once: they spread outwards as waves at the speed of light. Because there is no negative mass, a body cannot radiate simply by pulsing in and out symmetrically; the leading source is a changing quadrupole, such as two stars orbiting each other or a spinning body that is not perfectly round. The strongest waves come from merging black holes and neutron stars and from supernovae.

A passing wave stretches space in one direction while squeezing it in the perpendicular direction, then reverses, in a pattern that repeats at twice the orbital frequency of the source. The size of the effect is described by the dimensionless strain h, the fractional change in length ΔL/L. Strains reaching Earth are extraordinarily small: the first detected wave changed distances by a part in 10²¹, and LIGO’s arm-length changes are thousands of times smaller than an atomic nucleus.

The first, indirect evidence came from the radio sky. In 1974 Russell Hulse and Joseph Taylor discovered a pulsar in a tight orbit with another neutron star, using the 300-metre Arecibo telescope. The orbit, under eight hours long, shrinks by about 75 millionths of a second per year, matching the energy loss that general relativity predicts for gravitational radiation. The pair received the 1993 Nobel Prize in Physics. Our neutron stars page describes pulsars in more detail.

Sources: [1] [2] [4]

How laser interferometers measure strain

LIGO, Virgo and KAGRA are laser interferometers, refined versions of the instrument Michelson used in the nineteenth century. A laser beam is split in two; each half travels down one of two perpendicular arms, reflects off a suspended mirror at the far end and returns. When the two beams recombine, the instrument is tuned so that they cancel and almost no light reaches the photodetector. A gravitational wave lengthens one arm and shortens the other, the beams fall slightly out of step, and light leaks out. That flicker of light is the signal.

The engineering challenge is sensitivity. Each LIGO arm is 4 km long and sits inside one of the largest vacuum systems in the world, about 10,000 m³. At peak sensitivity LIGO can register a change in mirror separation of about 1/10,000 of the width of a proton. The two LIGO observatories, at Hanford (Washington) and Livingston (Louisiana), are 3,002 km apart; a real signal must appear in both, with a delay of no more than about 10 ms, while local noise does not. Virgo, a Michelson interferometer with 3 km arms at Cascina near Pisa, and KAGRA in Japan complete the network and allow sources to be located on the sky.

Sources: [2] [3]

The chirp: inspiral, merger and ringdown

As two compact objects orbit, they lose energy to gravitational waves and spiral closer, which makes them orbit faster and radiate more strongly. The signal therefore rises in both frequency and amplitude, a pattern called a chirp. It has three phases: the long inspiral, well described by approximations to general relativity; the merger, which requires numerical simulations; and the ringdown, in which the newly formed black hole settles down by ringing at characteristic frequencies set by its mass and spin.

The first detection, GW150914, recorded on 14 September 2015 at 09:51 UTC, is the textbook case. Its frequency swept from 35 to 250 Hz and the strain peaked at 1.0 × 10⁻²¹. The signal reached Livingston 7 ms before Hanford. Analysis showed two black holes of about 36 and 29 solar masses, roughly 410 Mpc away, merging into one of about 62 solar masses; the missing three solar masses were radiated as gravitational waves. The detection, with a matched-filter signal-to-noise ratio of 24 and a significance above 5.1σ, was announced on 11 February 2016.

Sources: [4]

Gravitational-wave chirp simulator

Choose two masses and a distance, or load the GW150914 or GW170817 preset, and compare the curves. Heavier systems sweep through the detector band in a fraction of a second, while the GW170817 neutron stars spend minutes above 20 Hz; the reported strain shows why nearby sources matter.

The simulator uses the leading-order inspiral formula, so it stops at the innermost stable circular orbit; for the GW150914 preset that is roughly 70 Hz, whereas the real signal kept rising to about 250 Hz during the merger, which the simple formula does not describe. Its strain estimate assumes an ideal orientation and ignores redshift, as the method note explains. The preset distance of 440 Mpc lies within the uncertainty range of the discovery paper’s 410 Mpc estimate.

GW170817 and multimessenger astronomy

On 17 August 2017 LIGO and Virgo recorded two neutron stars spiralling together. About 1.7 seconds after the gravitational waves arrived, NASA’s Fermi telescope recorded a short gamma-ray burst from the same part of the sky. With localisations from the detectors and gamma-ray observatories, astronomers found a fading optical source in the galaxy NGC 4993, about 130 million light-years away. Some 70 of LIGO’s partner observatories followed the event across the spectrum, and LIGO describes it as the most broadly and intensely studied astronomical event in history.

GW170817 established that at least some short gamma-ray bursts come from neutron star mergers, confirmed that gravitational waves travel at the speed of light to extremely high precision, and opened a way to measure the expansion of the Universe using gravitational-wave “standard sirens”. The approach is explored in our multimessenger astronomy dossier, and the distance methods it complements are on the cosmic distance ladder page.

Sources: [5]

Catalogues and the fourth observing run

The LIGO–Virgo–KAGRA collaboration observes in runs separated by upgrade breaks. The fourth run, O4, began in May 2023 and ended on 18 November 2025. Its first segment, O4a (May 2023 to January 2024), was analysed in the GWTC-4.0 catalogue released in August 2025: 128 new confident signals, roughly doubling the catalogue to 218 candidates. Most are binary black hole mergers; two are likely black hole–neutron star systems, no new binary neutron star was found, and several have total masses above 150 solar masses. The record-setting GW231123 belongs to this period and is described on our black holes page.

As of October 2026 the detectors are in a period of upgrades and commissioning. The collaborations are planning a six-month observing run, designated IR1, targeting a mid-January 2027 start, with LIGO and Virgo taking part and KAGRA joining later; plans for the fifth run, O5, are still under discussion. Results from the later O4 segments are being analysed.

Sources: [6] [7]

The gravitational-wave spectrum

Like light, gravitational waves span a wide range of frequencies, and each band needs a different detector. Ground-based interferometers hear the last seconds to minutes of stellar-mass mergers, from tens of hertz upwards; below that, seismic and other noise on Earth dominates. Space interferometers can reach the millihertz band, where merging massive black holes and compact binaries in our Galaxy radiate. At nanohertz frequencies, with periods of years to decades, the “detector” is a galaxy-sized array of pulsars.

Pulsar timing arrays track the arrival times of radio pulses from millisecond pulsars. A passing wave changes those times in a pattern that depends on the angle between pulsars on the sky, the Hellings–Downs correlation. In June 2023 the NANOGrav collaboration reported evidence for this correlation in 15 years of data on 67 pulsars, at a significance of about 3.5–4σ, with a characteristic strain amplitude of about 2.4 × 10⁻¹⁵ at a frequency of one cycle per year. The most likely source is a population of supermassive black hole binaries, although other origins are still being tested.

Sources: [9] [11] [4]

Current research (2024–2026)

The loudest signal tests Hawking and Kerr. GW250114, recorded by the two LIGO detectors in January 2025 and published in Physical Review Letters in September 2025, reached a network signal-to-noise ratio of 80, the highest so far. Its two black holes, of about 33.6 and 32.2 solar masses and with small spins, merged into a remnant whose horizon area exceeded the sum of the initial areas to high credibility, as Hawking’s area theorem requires. The ringdown contained the fundamental mode and its first overtone, with frequencies matching a Kerr black hole to within about 30 per cent.

LISA. ESA adopted the Laser Interferometer Space Antenna on 25 January 2024. Three spacecraft will fly in a triangle with 2.5-million-kilometre sides, trailing Earth around the Sun, and are planned to launch on an Ariane 6 in 2035. NASA provides the lasers, telescopes and UV light sources. LISA will cover 0.1 mHz to 1 Hz, where merging supermassive black holes and compact binaries in the Milky Way are expected to be heard.

Next-generation ground detectors. Europe’s Einstein Telescope would be built several hundred metres underground, either as a triangle with 10 km sides or as two L-shaped detectors with 15 km arms. Candidate sites are Sardinia, the Belgian–German–Dutch border region and Saxony; bid books are due by the end of 2026 and a site decision is expected in 2027, with operations unlikely before 2035. In the United States, Cosmic Explorer proposes two L-shaped observatories with 40 km and 20 km arms for the 2030s. India approved LIGO-India at Aundha, Maharashtra, on 17 April 2023, with first observations expected by the end of the decade.

Sources: [8] [10] [11] [12] [13] [14]

Working with the data

Gravitational-wave data are unusually open. The Gravitational Wave Open Science Center (GWOSC) publishes calibrated strain time series for every catalogued event and bulk data from completed runs, with tutorials in Python. During observing runs, low-latency candidate alerts are distributed publicly through GraceDB, so that telescopes can search for counterparts. NANOGrav releases its pulsar timing data sets with each analysis. A good first exercise is to download the GW150914 strain from GWOSC, whiten it and band-pass filter it between roughly 35 and 350 Hz to see the chirp emerge.

Sources: [4] [9]

Known vs open questions

Established results

• Gravitational waves exist, travel at the speed of light and carry energy away from binary systems, as general relativity predicts.
• Binary black holes merge frequently enough that hundreds of candidates are now known.
• At least some short gamma-ray bursts are produced by merging neutron stars.
• Black hole ringdowns observed so far are consistent with the Kerr solution and with Hawking’s area theorem.

Open questions

• How do the heaviest and fastest-spinning black hole binaries form?
• Is the nanohertz background entirely due to supermassive black hole binaries?
• Will gravitational-wave sirens help resolve the tension between measurements of the Hubble constant?
• Will deviations from general relativity appear at higher precision, or signals from the early Universe?

Related on this site

• Research dossier: multimessenger astronomy
• Black holes
• Neutron stars
• Cosmic rays and neutrinos
• Cosmic distance ladder
• Research dossier: instruments and measurement
• Cosmic scales tool