Black Holes, Explained

A black hole is a region of space where gravity is so strong that nothing can escape from it, not even light. That last detail is what makes it black, and it is the source of its strange power. Stephen Hawking spent much of his career studying these objects, not because they are exotic, but because they are the one place in the universe where our two best theories of physics are forced to confront each other.

What makes a black hole

To leave the surface of any object you have to travel faster than its escape velocity. On Earth that is about eleven kilometres per second. The more mass you pack into a small space, the higher that speed climbs. Squeeze enough mass into a small enough region and the escape velocity reaches the speed of light, which nothing can exceed. At that point, even light cannot get out, and you have a black hole.

The boundary around it is called the event horizon. It is not a surface you could touch; it is simply the line beyond which escape becomes impossible. Cross it, and every path leads inwards. The horizon marks the edge of what the rest of the universe can ever know about, which is why so much of black hole physics is really about its surface rather than its interior.

At the very centre, our equations predict a singularity, a point of infinite density where the known laws of physics break down. The honest position is that nobody knows what truly happens there; the singularity is a sign that general relativity has reached the limit of what it can describe.

How black holes form

The most common kind forms when a massive star runs out of fuel. For its whole life a star balances the inward pull of its own gravity against the outward push of the heat from nuclear fusion in its core. When the fuel is gone, gravity wins. The core collapses, and if the star was heavy enough, nothing can halt the collapse. The result is a stellar-mass black hole, a few times the mass of the Sun.

At the other extreme sit supermassive black holes, millions or billions of times the Sun's mass, which lurk at the centres of galaxies, including our own. Exactly how they grew so large is still an open question. Physicists also discuss intermediate-mass black holes and primordial ones that might have formed in the dense early universe, an idea Hawking himself explored.

How we know they are real

For most of the twentieth century black holes were a theoretical prediction. The evidence is now overwhelming, and it comes from several independent directions.

We can watch stars orbit something invisible and enormously heavy. At the centre of our galaxy, stars whip around an unseen object four million times the mass of the Sun, work that won a Nobel Prize in 2020. We can detect the radiation from gas heated to extreme temperatures as it spirals inwards. In 2015 the LIGO experiment detected gravitational waves, ripples in spacetime, from two black holes colliding more than a billion light years away. And in 2019 the Event Horizon Telescope released the first direct image of a black hole's shadow, in the galaxy M87, followed by an image of the one at the centre of our own galaxy.

The "no-hair" idea

One of the surprising results of black hole physics is how simple these objects are from the outside. No matter what fell in to make it, a black hole can be completely described by just three numbers: its mass, its electric charge, and its spin. Everything else about whatever formed it appears to be lost. Physicists summarise this by saying a black hole "has no hair." That apparent loss of detail is the seed of the deepest puzzle Hawking ever raised, the information paradox.

Why they mattered to Hawking

Hawking turned black holes from astronomical curiosities into a precise laboratory for fundamental physics. With Roger Penrose he proved that the singularities inside them are an unavoidable prediction of general relativity, in the singularity theorems. He proved that the area of an event horizon can never decrease, which hinted at a deep link to thermodynamics. And in his most famous result he showed that black holes are not entirely black at all but slowly glow and evaporate, in Hawking radiation.

A black hole, in his hands, became the place where gravity, quantum mechanics and the physics of heat and information all meet. That is why he kept returning to it for fifty years.

For the threshold that defines a black hole, see escape velocity; for a way energy can be drawn from a spinning one, see the Penrose process.

See how black holes compare in size with the black hole size comparison tool.