What is a black hole?

What is a black hole?

People say a black hole is a place where "not even light escapes." But escapes from what? We usually think of gravity as something you can climb away from if you push hard enough. What would it take for gravity to become a trap with no exit?

Toss a ball upward and it falls back; throw it hard enough and it could, in principle, leave entirely — there is a certain speed that wins free. Now ask: what if the pull were so strong that the speed needed to escape rose past the fastest thing there is? What is the fastest thing there is?

Before reaching for some monstrous new kind of object, look at what escape speed depends on. Two things only: how much mass is pulling, and how far you stand from the middle of it. The second is worth dwelling on. The Sun's grip on us out here is gentle — about 42 kilometres a second carries you clear of the solar system. Stand on the Sun's own surface and the price is 618. Nothing about the Sun changed; only your distance from its centre did.

So run it the other way round. Hold the mass fixed, let the surface shrink inward, and escape speed climbs — with nothing in that climb knowing to stop politely short of light. Is there, then, a radius at which the price of leaving reaches exactly light speed?

There is, and it is oddly easy to state. Compress the Sun's mass into a sphere about three kilometres across and its surface sits precisely at that threshold; do it to the Earth's mass and the figure is roughly nine millimetres, a marble. That distance is the Schwarzschild radius, and notice it is simply proportional to the mass. One honest footnote: the argument as told is Newtonian, and Newtonian mechanics has no business describing light at all. It lands on the right number partly by luck — eighteenth-century writers reasoned their way to "dark stars" like this — and what makes the number stick is that general relativity, arguing quite differently about the shape of spacetime, arrives at the same radius.

That proportionality has a consequence you could in principle feel. What tears a falling body apart is not gravity but the difference in it between your head and your feet, and that difference falls away steeply with distance from the centre. Since a heavier hole's boundary sits further out, the stretching there is milder: fall toward a hole of a few solar masses and you are drawn into a thread well before reaching the boundary, while crossing the boundary of a supermassive one you would feel nothing whatsoever. The point of no return announces itself in no local way at all.

Then how do we know any of it is real, if the thing is by construction unseeable? By watching what it does. At the centre of our galaxy a star called S2 loops around an unseen four million solar masses every sixteen years, and that orbit fixes both how much mass is there and how small the region holding it must be. Gas spiralling inward heats and blazes in X-rays. In 2015 the LIGO detectors caught the gravitational waves from two holes of roughly 36 and 29 solar masses merging. And the Event Horizon Telescope has imaged the bright rings around the holes in the galaxy M87 and at our own centre — pictures of glowing gas and of the shadow it is cast against, since the hole offers nothing to photograph.

That line is not a solid surface; it is a boundary of no return. Outside it, things can still leave. Step across, and because nothing outruns light, nothing can send even a signal back out. The "blackness" is simply that no light can climb the hill to reach us.

It is worth being stubborn about the word surface, because almost every mental picture smuggles one in. There is no membrane at the horizon, no shimmer, no jolt as you pass. It is defined by where light rays eventually get to — the boundary of the region from which no ray ever reaches the outside universe — and "eventually" makes it a statement about the whole future of the spacetime, not something you could measure locally on your way by. What changes at the crossing is not what you feel but what you can do afterwards: no signal you send will get out, and the centre stops being a place ahead of you and becomes a time ahead of you.

Two corrections follow. Replace the Sun with a black hole of the same mass and Earth's orbit would not shift at all; it would go dark and cold while we kept circling, because gravity at a distance depends on the mass, and the mass did not change. And "spaghettification" is not something the horizon does — it is the tidal stretching above, wherever that stretching gets severe, which for the largest holes lies well inside the boundary rather than at it.

One theoretical clause, flagged as such: Hawking argued in 1974 that quantum effects should give a horizon a faint thermal glow, cooler the heavier the hole, so that black holes very slowly evaporate. It is widely accepted, and it has never been observed.

• Why an outside observer sees an infalling clock slow and redden without ever quite arriving.
• How a star's mass decides whether it ends as a white dwarf, a neutron star, or a hole.

Every term the collection defines is gathered in the glossary.