Gravitational waves are ripples in the fabric of space-time, produced when massive objects accelerate. Einstein predicted them in 1916 but thought they would be too weak to detect. A century later, he was proven right on the first point and wrong on the second.
The first detection
On September 14, 2015, the two LIGO detectors in the United States recorded a signal lasting a fraction of a second: two black holes, about 36 and 29 times the Sun’s mass, merging 1.3 billion light-years away. The 2017 Nobel Prize in Physics went to Rainer Weiss, Barry Barish and Kip Thorne for this achievement.
How detectors work
LIGO uses laser beams traveling along two arms, each 4 kilometers long. A passing gravitational wave stretches one arm and squeezes the other by less than a thousandth of the width of a proton. Interference between the beams reveals this tiny change. Virgo in Italy and KAGRA in Japan join the network (see LIGO Caltech), which helps locate sources in the sky.
The main types
Compact binary coalescence. Pairs of black holes or neutron stars spiral together and merge, producing a “chirp” that rises in pitch. Almost all detections so far are of this type, see what black holes are and neutron stars.
Continuous waves. A spinning neutron star with a slight bump would emit a steady signal. None has been confirmed yet.
Bursts. Short signals from events such as a supernova in our galaxy, still awaited, see how supernovae happen.
Stochastic background. A random hum from countless unresolved sources, perhaps including the early universe. In 2023, pulsar timing arrays such as NANOGrav reported evidence of a low-frequency background, likely from pairs of supermassive black holes. They use millisecond pulsars as precise clocks spread across the galaxy.
What the detections have taught us
- Black hole pairs are common, and some are heavier than expected.
- In 2017, a neutron star merger (GW170817) was seen in both gravitational waves and light, confirming that such collisions produce heavy elements like gold.
- Gravitational waves travel at the speed of light, as general relativity predicts.
The future
ESA’s LISA mission, planned for the 2030s, will place three spacecraft millions of kilometers apart to detect lower-frequency waves, from supermassive black hole mergers, for example. See also how gravity bends light, another effect of mass curving space.
