What Is a Black Hole?

A black hole is not made of a special substance. It is ordinary matter compressed into a small enough space that its gravity overwhelms every force that would normally hold it apart.

Squeeze anything far enough and it becomes a black hole. The Earth would need to be crushed to about the size of a marble. The reason this does not happen is that ordinary matter resists compression, and only the collapse of a massive star provides enough force to overcome that.

The event horizon is the boundary of no return. Outside it, an object moving fast enough can escape. Inside it, every possible path leads inwards, and nothing, not even light, can get back out.

The horizon is not a solid surface. Someone falling through it would notice nothing special at the moment of crossing, though from outside they would appear to slow down and fade as light from them stretches to longer and longer wavelengths.

Astronomers recognise several populations:

• •Stellar mass black holes, formed when a massive star collapses at the end of its life
• •Supermassive black holes, millions to billions of times the Sun's mass, sitting at the centres of galaxies
• •Intermediate mass black holes, a rarer in between class now being detected

Black holes cannot be seen directly, so astronomers watch their effects. Stars near the centre of our galaxy orbit an invisible object at enormous speed, and tracking those orbits over decades revealed a mass of about four million Suns in a tiny volume.

Gas falling towards a black hole also heats up and emits X-rays before it crosses the horizon. Since 2015, detectors have recorded gravitational waves from black holes merging, giving a completely independent line of evidence.

In 2019 the Event Horizon Telescope released an image of the black hole at the centre of the galaxy M87, produced by linking radio telescopes across the planet into one effective instrument the size of Earth. A second image, of our own galaxy's black hole, followed in 2022.

What the images show is not the black hole itself but the bright ring of hot material around it, with a dark shadow in the middle where the horizon blocks the light. The size and shape of that shadow matched predictions closely.

Stephen Hawking showed in the 1970s that black holes should not be entirely black. Quantum effects near the event horizon should cause them to emit a faint thermal radiation, now called Hawking radiation.

If correct, black holes slowly lose mass and will eventually evaporate. The timescales are absurd: a stellar mass black hole would take vastly longer than the current age of the universe. The prediction has never been observed, since the radiation is far too faint against the cosmic background, but it is theoretically important because it links gravity, quantum mechanics and thermodynamics in a single result.

A common picture shows black holes sucking in everything around them. In fact, far from the horizon, a black hole pulls exactly like any other object of the same mass. If the Sun were somehow replaced by a black hole of equal mass, Earth would carry on in the same orbit, although it would become very cold and dark. Stars and gas can orbit black holes stably for billions of years. Material only falls in if something, such as friction in a swirling disc of gas, removes the motion that keeps it in orbit.

Close in, the story changes because of tidal forces, the difference in pull between your head and your feet. Near a stellar mass black hole, that difference becomes so extreme well outside the horizon that a falling object would be stretched into a long thin strand, an effect nicknamed spaghettification. Surprisingly, a supermassive black hole is gentler. Its horizon is so much larger that the tidal stretching at the horizon is small, which is why someone falling into one could cross it intact. That is the case where crossing would feel like nothing special.

The idea is older than Einstein. In 1783 the English clergyman and scientist John Michell reasoned that a star massive and compact enough would have an escape speed greater than the speed of light, so its light could never reach us. Historians now call these imagined objects dark stars. The modern version began in 1916, when Karl Schwarzschild, serving in the German army during the First World War, found an exact solution to Einstein's new equations of gravity describing the space around a single mass. The name black hole caught on only in the late 1960s, popularised by the physicist John Wheeler.

Schwarzschild's solution sets the size of the event horizon, which grows in direct proportion to mass: about 3 kilometres of radius for every mass of the Sun. A black hole of ten solar masses would have a horizon only about 30 kilometres in radius, about the size of a large city. The one at the centre of our galaxy, about four million solar masses, has a radius of roughly 12 million kilometres, well inside the orbit of Mercury. M87's black hole, billions of times the Sun's mass, is wider than the orbit of Neptune. Strangely, its average density inside the horizon is lower than that of air.

On 14 September 2015, the two detectors of the LIGO observatory in the United States picked up a signal lasting a fraction of a second. Its pattern matched predictions for two black holes, roughly 36 and 29 times the mass of the Sun, spiralling together and merging about 1.3 billion light years away. The new black hole weighed about 62 solar masses. The missing three solar masses had been converted into gravitational waves, ripples in space itself, and for a brief moment the merger radiated more power than all the stars in the observable universe combined.

Detecting it required extraordinary precision. Each LIGO detector has two arms 4 kilometres long at right angles, with laser light bouncing back and forth along them. A passing gravitational wave stretches one arm while squeezing the other, and the change in length involved was far smaller than the width of a proton. The signal reached the two sites about seven milliseconds apart, consistent with a wave travelling at the speed of light. Its rising pitch, nicknamed a chirp, revealed the masses of the two objects, and only black holes could be that massive yet orbit so close together.

A black hole is matter compressed until its gravity traps light, bounded by an event horizon, detected through its effect on nearby stars and gas, and now directly imaged as a dark shadow ringed by glowing material.