Black Holes

A black hole is not an object so much as a region: a place where mass has been compressed inside its own Schwarzschild radius, and from which no path leads out. For decades they were inferred rather than seen — from the orbits of stars whipping around something invisible at the centre of the galaxy, from X-rays emitted by gas heated to millions of degrees on its way in. That changed twice in recent years.

In 2015 LIGO recorded two of them merging, measuring a distortion of spacetime smaller than a proton across a four-kilometre arm — a measurement so absurd that the instrument spends most of its engineering budget on not being fooled by a passing lorry. It is now a catalogue rather than an event: something approaching a hundred confirmed mergers, and with them a population nobody expected. Stellar-mass black holes were thought to top out around thirty solar masses; the very first detection was thirty-six and twenty-nine, and there are heavier ones since, in a range where the models say a star should blow itself apart entirely and leave nothing behind. Then in 2019 the Event Horizon Telescope photographed the shadow one casts on the glowing gas behind it, by combining radio dishes across the planet into a single instrument the size of the Earth — the only way to get an angular resolution equal to reading a newspaper in New York from Paris.

What remains unresolved is at both ends of the scale. Inside, general relativity predicts a singularity, which is not a physical answer but the theory reporting that it has stopped working; reconciling that with quantum mechanics is the oldest open problem in physics, and the information paradox — whether what falls in is destroyed or merely scrambled — is where the argument is sharpest. Outside, the supermassive ones are a timing problem. Millions to billions of solar masses, one at the centre of essentially every large galaxy, and Webb keeps finding them already assembled when the universe was a few hundred million years old. There has not been time to grow them by accretion at any rate anybody can justify.