Gravitational Waves

Gravitational waves are ripples in the fabric of spacetime itself, set off when very massive objects accelerate violently, such as two black holes spiralling into each other. As the waves pass, they stretch and squeeze space by a tiny amount. They were among the last major predictions of Einstein's general relativity to be confirmed, and their discovery opened an entirely new way of observing the universe.

What they are

In Einstein's picture, mass and energy curve spacetime. When massive objects move suddenly, that curvature does not adjust instantly everywhere; instead, the disturbance spreads outward at the speed of light, like ripples on a pond. These ripples are gravitational waves. Einstein predicted them in 1916, but suspected they would be far too faint ever to detect, because spacetime is extraordinarily stiff and the resulting stretching of space is almost unimaginably small.

How they were detected

He was nearly right about the difficulty. It took a hundred years and one of the most sensitive instruments ever built. In September 2015 the LIGO experiment, a pair of detectors with arms several kilometres long, registered a signal from two black holes that had merged more than a billion light years away. The passing wave changed the length of the detector's arms by a fraction of the width of a single proton. The result, announced in 2016, was the first direct detection of gravitational waves, and it earned a share of the 2017 Nobel Prize for the physicists who led the effort, among them Hawking's friend Kip Thorne.

The detection did several things at once. It confirmed Einstein's century-old prediction, it provided the most direct evidence yet that black holes genuinely exist and really do collide, and it gave astronomers a completely new sense, the ability to "hear" violent events that emit little or no light.

The connection to Hawking

There is a direct link to Hawking's own work. Back in 1971 he had proved the area theorem, which states that the total area of a black hole's event horizon can never decrease. When two black holes merge, the theorem demands that the area of the final horizon be at least as large as the two original horizons combined. In 2021, physicists analysed the very first gravitational-wave signal in detail and confirmed exactly that, an observational test, half a century later, of a theorem Hawking had proved with pen and paper. That area theorem was also the first clue that black holes possess entropy, the thread that led to Hawking radiation.

In 2021 these waves were used to confirm one of his predictions; see his legacy today.