To leave Earth for good, a rocket must reach about 11.2 kilometers per second. Squeeze the same mass into a smaller ball and the escape speed at the surface climbs. Squeeze Earth to the size of a marble, about 9 millimeters in radius, and the escape speed reaches the speed of light. Since nothing moves faster than light, nothing gets out. That is a black hole.

The English clergyman John Michell reasoned his way to such “dark stars” in 1783 using Newton’s physics. The modern version came in 1916, weeks after Einstein published general relativity, when the German astronomer Karl Schwarzschild solved the new equations for a single concentrated mass while serving with the army on the Russian front. His solution contained a critical radius at which time, as seen from outside, appears to stop. He died that May. For decades most physicists, Einstein included, treated the result as a mathematical curiosity that nature would somehow avoid.

A star spends its life in a standoff. Gravity pulls inward and the heat of nuclear fusion pushes back. When the fuel runs out, gravity wins, and what is left depends on mass.

A star like the Sun ends as a white dwarf, an Earth-sized ember held up by the pressure of its electrons. In 1930 a 19-year-old Indian student, Subrahmanyan Chandrasekhar, calculated on the boat to England that this support fails above about 1.4 solar masses. A heavier core collapses further into a neutron star, a city-sized ball with more mass than the Sun. Above roughly two to three solar masses nothing known can stop the collapse. The core falls inside its own horizon and a black hole is born, often during a supernova.

The name came later. John Wheeler began using “black hole” in lectures in 1967 and it stuck.

The event horizon is the surface of no return. It is not a physical skin, and an astronaut falling through a large black hole’s horizon would notice nothing special at that instant. It is a boundary in spacetime: inside it, every possible path leads inward.

The singularity is where the equations say the collapsed matter ends up, a point (or, for a spinning hole, a ring) of unlimited density. Most physicists read this as the place where general relativity stops working, to be replaced by a quantum theory of gravity nobody yet has.

The accretion disk is what we see. Gas spiraling inward is heated by friction to millions of degrees and glows in X-rays. Some black holes also launch narrow jets of particles at nearly light speed along their spin axes.

The shadow is the dark center in the famous images, about two and a half times wider than the horizon itself, where light rays are captured rather than bent around to the observer.

Black holes give off no light of their own, so they are found by their effects.

Cygnus X-1. A rocket-borne detector found a strong X-ray source in the constellation Cygnus in 1964. By the early 1970s astronomers had tied it to a blue supergiant star being whirled around every 5.6 days by an unseen companion far too heavy to be a neutron star. Stephen Hawking bet Kip Thorne in 1974 that it was not a black hole and conceded in 1990. Current measurements put the hole at about 21 solar masses.

The center of our galaxy. Beginning in the 1990s, teams led by Reinhard Genzel in Germany and Andrea Ghez in California tracked individual stars whipping around an empty spot in Sagittarius. One star, S2, completes an orbit in 16 years at speeds up to nearly 3 percent of light speed. The orbits require about 4.3 million solar masses inside a region smaller than our solar system. Genzel and Ghez shared the 2020 Nobel Prize in Physics with Roger Penrose, who had proved in 1965 that black holes are an unavoidable consequence of general relativity.

Gravitational waves. Since 2015 LIGO and its partner detectors have recorded black holes colliding, a method that needs no light at all. See Spacetime and Gravity.

On April 10, 2019, the Event Horizon Telescope collaboration released the first image of a black hole’s shadow. Eight radio observatories at six sites, from Hawaii to Spain to the South Pole, had observed together in April 2017, their data combined to act as one dish the size of Earth. The target was M87*, a 6.5-billion-solar-mass black hole 55 million light-years away in the galaxy Messier 87. Its horizon is about four times wider than the orbit of Neptune.

On May 12, 2022, the same team showed Sagittarius A, our own galaxy’s black hole, about 27,000 light-years away. It is 1,500 times lighter than M87 and its surroundings flicker within minutes, which made the image harder to build.

In 1974 Stephen Hawking applied quantum theory to the horizon and found that black holes are not perfectly black. They should leak a faint glow, now called Hawking radiation, and slowly lose mass. For a black hole of stellar mass the glow is far colder than the leftover heat of the Big Bang, so today such holes gain more than they lose. Given unimaginable spans of time, around 10^67 years for one solar mass, they would evaporate.

That raised a problem still unsettled: what becomes of the information about everything that fell in? Quantum physics says information cannot be destroyed, and Hawking’s calculation seemed to say it is. Work on this “information paradox” has driven much of theoretical physics for fifty years.

Sources and further reading