What Are Gravitational Waves? Ripples Measured Across a Thousandth of a Proton

General relativity describes gravity not as a force but as the curvature of spacetime, with mass telling space how to bend and the bending telling mass how to move. If mass moves, the curvature it produces must change, and the change propagates outward at the speed of light as a ripple in the geometry itself. The effect on anything the wave passes through is a stretching in one direction and a squeezing in the perpendicular direction, alternating as the wave goes by, and it acts on space rather than on the objects in it, so two free-floating mirrors change their separation because the distance between them changes. The waves are produced only by asymmetric acceleration of mass, so a perfectly spherical collapse radiates nothing, and the strength falls off with distance, which is why only the most violent events in the universe are detectable and why the amplitudes involved are so absurdly small.

The instruments are laser interferometers, and the principle is simple even though the engineering is not. A laser beam is split in two and sent down two perpendicular arms four kilometres long, bounced off mirrors at the ends, and recombined. If the arms are exactly equal, the returning beams cancel and no light reaches the detector; if a passing wave changes their relative length by even a fraction of a wavelength, light appears. Achieving the required sensitivity took forty years:

• •The mirrors, weighing forty kilograms each, hang from multi-stage pendulums that isolate them from ground vibration by a factor of around ten billion
• •The arms are held at a vacuum better than that of interstellar space, in one of the largest vacuum systems ever built
• •The laser light is recycled through the arms hundreds of times, giving an effective path length of over a thousand kilometres
• •Quantum squeezing manipulates the statistical properties of the light itself to reduce the noise floor below what ordinary photon counting allows
• •At least two widely separated detectors are required, because a real signal must arrive at both with a light-travel-time delay, which rejects local disturbances such as lorries, earthquakes and, on one recorded occasion, a raven pecking at a pipe

Einstein predicted the waves in 1916 and later doubted they existed, submitting a paper in 1936 arguing that they did not, which was rejected after peer review that he found so annoying he never published in that journal again; the reviewer was right. The first solid evidence was indirect and came from Russell Hulse and Joseph Taylor, who discovered a binary pulsar in 1974 and measured its orbit shrinking at exactly the rate predicted if it were radiating gravitational waves, which won the Nobel Prize in 1993. Direct detection was attempted from the 1960s with resonant bar detectors, and Joseph Weber's announcements of detections in 1969 could not be replicated and were eventually dismissed, an episode that damaged the field's credibility for years. The interferometer project was approved in 1992 against considerable scepticism about the cost, ran for a decade without detecting anything, was upgraded, and detected a signal within days of the improved instruments coming online in 2015, with the Nobel Prize following in 2017.

The field moved from a single detection to a catalogue within a few years, and around two hundred events have now been recorded. Most are black hole mergers, which has produced the surprise that black holes of intermediate mass, in the range of tens of solar masses, are far more common than expected and sometimes merge into objects of over a hundred solar masses. The single most consequential event was different: in August 2017 the detectors recorded a merger of two neutron stars, and a gamma ray burst arrived from the same direction 1.7 seconds later, after which roughly seventy observatories on the ground and in orbit followed the fading light across the whole electromagnetic spectrum. That one event confirmed that such mergers produce short gamma ray bursts, showed spectroscopically that heavy elements including gold and platinum are forged in them, confirmed that gravitational waves travel at the speed of light to within a fraction of a percent, and provided a new and independent way of measuring the expansion rate of the universe.

Ground-based detectors are limited at low frequencies by seismic noise, which caps what they can see, so the next instruments go elsewhere. LISA, a European-led space mission planned for the 2030s, will fly three spacecraft in a triangle two and a half million kilometres on a side, sensitive to much lower frequencies and therefore to supermassive black hole mergers at the centres of colliding galaxies. Pulsar timing arrays take a different approach, using millisecond pulsars scattered across the galaxy as clocks and looking for correlated timing deviations caused by waves with periods of years; in 2023 several such collaborations announced evidence for a background hum of exactly this kind, most plausibly from the combined signal of supermassive black hole pairs throughout the universe. The longer ambition is to detect primordial gravitational waves from the first fraction of a second after the big bang, which no other signal can reach, since the universe was opaque to light until 380,000 years in and has always been transparent to gravity.

Gravitational waves are ripples in the curvature of spacetime produced by asymmetric accelerating mass, stretching space in one direction while squeezing it in the perpendicular one, and they were predicted in 1916 and detected in 2015 by laser interferometers measuring a change of about a thousandth of a proton's width across four-kilometre arms. Around two hundred mergers have been recorded, a neutron star collision in 2017 was seen simultaneously in light and showed that gold is forged in such events, and space-based and pulsar-timing detectors will reach lower frequencies.