How Black Holes Work

Overview

A black hole is a region where gravity is strong enough that no path leads back out, not even for light. It is described entirely by general relativity, and — remarkably — an astrophysical black hole is characterised by just its mass and spin.

• The event horizon is not a surface or a wall; it is the boundary beyond which every future path leads inward.
• A black hole of a given mass has a fixed horizon size: about 3 km per solar mass for a non-rotating one.
• Almost everything we observe about black holes comes from the matter and spacetime around them, not from the hole itself.
• Two independent lines of evidence — gravitational waves and direct horizon-scale imaging — confirmed the predictions within five years of each other.

What a black hole actually is

In general relativity, mass and energy curve spacetime, and objects follow the straightest available paths through that curved geometry. A black hole is a solution in which the curvature becomes so extreme within a certain region that every future-directed path from inside that region leads further in. There is no trajectory, and no signal at any speed up to light, that escapes.

The boundary of that region is the event horizon. It is a one-way causal surface rather than a physical object — an infalling observer crossing the horizon of a very massive black hole would not necessarily notice anything locally dramatic at the moment of crossing.

Size scales with mass, and nothing else

For a non-rotating black hole the horizon radius is proportional to mass: roughly 3 kilometres per solar mass. A 10-solar-mass stellar remnant has a horizon about 30 km in radius; the four-million-solar-mass black hole at the centre of the Milky Way has one about 12 million kilometres in radius, comfortably inside Mercury's orbit; the most massive known are billions of solar masses.

The no-hair result of classical general relativity says an isolated, settled black hole is fully described by mass, angular momentum, and electric charge. Astrophysical black holes are expected to carry negligible net charge, so in practice two numbers — mass and spin — describe them completely. Everything else about whatever fell in is not accessible from outside.

How they form and grow

• Stellar-mass black holes form when the core of a sufficiently massive star collapses at the end of its life and no remaining pressure source can halt it.
• Supermassive black holes, millions to billions of solar masses, sit at the centres of most large galaxies. Their formation history is not settled: proposed routes include growth from stellar-mass seeds, direct collapse of very massive gas clouds in the early universe, or runaway mergers in dense clusters.
• Black holes grow by accreting gas and by merging with other black holes. Mergers are now directly observed as gravitational-wave events.
• Intermediate-mass black holes, between roughly a hundred and a hundred thousand solar masses, are the least well-characterised population and an active observational target.

Why they are bright

A black hole emits essentially nothing itself, yet accreting black holes are among the most luminous objects in the universe. The energy comes from the infalling matter. Gas with angular momentum forms an accretion disc; friction and magnetic stresses transport angular momentum outward and let material spiral inward, converting gravitational potential energy into heat and radiation with an efficiency far exceeding nuclear fusion.

Some systems also launch relativistic jets along the spin axis, extending in the largest cases for hundreds of thousands of light-years. Active galactic nuclei and quasars are supermassive black holes in this accreting, radiating state.

How we know they are there

• Stellar orbits: decades of precise tracking of stars orbiting the centre of the Milky Way reveal a compact object of about four million solar masses in a volume far too small for any star cluster. This work was recognised with the 2020 Nobel Prize in Physics.
• Gravitational waves: LIGO's first detection in 2015 recorded the merger of two stellar-mass black holes, with a waveform matching general relativity's prediction for inspiral, merger and ringdown.
• Direct imaging: the Event Horizon Telescope published a horizon-scale image of the black hole in M87 in 2019 and of Sagittarius A* in 2022, showing the bright ring and central shadow the theory predicts.
• X-ray binaries: compact objects too massive to be neutron stars, accreting from a companion star.

The parts that are not settled

General relativity predicts a singularity at the centre — a point where the theory's own equations stop giving meaningful answers. That is generally read as a signal that a quantum theory of gravity is needed there, not as a physical description.

Hawking's 1974 result that black holes should radiate thermally, and slowly evaporate, is theoretically well-motivated but has never been observed: for any astrophysical black hole the predicted temperature is far below the cosmic microwave background, so they absorb far more than they emit. The question of what happens to the information carried by infalling matter remains genuinely unresolved.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Compact objectsBlack holes, neutron stars, and white dwarfs as a catalogued population.
• Multi-messenger astronomyGravitational waves, neutrinos, and coordinated detections.
• Fundamental physicsRelativity, quantum theory, and the tests that constrain them.
• GalaxiesThe galaxies whose centres host supermassive black holes.

Frequently asked

What happens if you fall into a black hole?

Tidal forces — the difference in gravitational pull between your head and your feet — stretch an infalling object. For a stellar-mass black hole those forces become lethal well outside the horizon; for a supermassive one they are mild at the horizon and an observer could cross it without any local sensation. Either way, once inside, every future path leads inward, and general relativity offers no description of the endpoint beyond the breakdown of its own equations.

Can a black hole pull in the Earth?

Only by being close enough, and no known black hole is. A black hole's gravity at a given distance is exactly the same as that of any other object of the same mass: if the Sun were somehow replaced by a black hole of one solar mass, Earth's orbit would be unchanged and the immediate problem would be the loss of sunlight, not tidal capture.

Has anyone actually seen a black hole?

The Event Horizon Telescope published horizon-scale images of the supermassive black holes in M87 (2019) and at the centre of the Milky Way (2022). What is imaged is the bright ring of emission from hot material and the dark central shadow cast by the horizon — the hole itself emits nothing to see.

Do black holes last forever?

Classically yes; with quantum effects included, no. Hawking radiation implies a very slow evaporation, but the predicted temperature of any astrophysical black hole is far colder than the cosmic microwave background, so at present they absorb more than they emit and grow rather than shrink. No evaporation has ever been observed.