Black Holes Explained: Horizons, Singularities and Evidence

A black hole is a region of space where gravity is so strong that nothing, not even light, can escape once it crosses a boundary called the event horizon. Black holes are not holes in the usual sense. They are extremely compact masses, and the region around them is simply the part of spacetime that has been curved too sharply for light to climb back out. Despite their reputation, they are common and they are also measurable.

What makes a black hole

Escape velocity and the idea of a dark star

The idea of an object so massive that light could not leave it is older than general relativity. In the eighteenth century, John Michell and Pierre-Simon Laplace worked out that a sufficiently dense star would have an escape velocity greater than the speed of light. Their calculation used Newtonian gravity and light treated as particles. Albert Einstein's general relativity, published in 1915, gave the modern description, in which a black hole is a region where spacetime itself is curved so strongly that all future-directed paths lead inward.

The Schwarzschild radius

The size of the event horizon depends only on mass. Karl Schwarzschild found the solution in 1916, and the radius is written as Rs = 2GM/c², where G is the gravitational constant, M is the mass and c is the speed of light. The formula scales linearly with mass, so doubling the mass doubles the radius.

The table below shows the approximate Schwarzschild radius for several familiar objects. The numbers are rounded.

Object	Mass (Sun = 1)	Schwarzschild radius
Earth	0.000003	about 9 millimetres
Sun	1	about 3 kilometres
Stellar black hole	10	about 30 kilometres
Sagittarius A*	about 4 million	about 12 million kilometres
M87*	about 6.5 billion	about 19 billion kilometres

The Earth would have to be squeezed to about 2 centimetres across to become a black hole. The Sun would need to shrink to roughly 3 kilometres. Real stars do not compress that far, which is why black holes form only from very massive stellar cores.

Horizon and singularity

The event horizon is a surface, not a physical wall. An observer falling through it would notice nothing special at the moment of crossing, if the black hole is large enough that tidal forces are gentle there. The horizon is a one-way boundary in the sense that every possible future path from inside it ends at smaller radius.

General relativity also predicts a singularity at the centre, a point where the calculated density and curvature become infinite. Most physicists regard the singularity as a sign that the classical theory breaks down there, and that a full description needs quantum gravity. No observation has yet confirmed what happens at the singularity.

How black holes form

Stellar black holes form when the core of a very massive star collapses after its fuel is spent. A core above roughly 3 solar masses cannot be held up by neutron degeneracy pressure, so it keeps collapsing past the neutron star stage. The outer layers of the star may explode as a supernova, or they may fall back and add to the black hole, depending on the details of the collapse.

Two other formation routes are discussed but are less certain. Some black holes may form in the very early universe from dense regions of gas. Supermassive black holes, with masses from about a million to several billion solar masses, are thought to grow over cosmic time through accretion of gas and through mergers with other black holes, although the exact growth history is still an active research question.

Strange effects near the horizon

Gravity changes the flow of time. An observer far away would see a clock near the horizon run very slowly, and would see light from falling material become increasingly red-shifted as it climbs out. A falling observer, by contrast, would see nothing unusual at the horizon itself.

Tidal forces are the real danger for a falling object. For a stellar black hole, the difference in gravity between the head and feet of a person becomes lethal well before the horizon. Physicists call the resulting stretching spaghettification. For a supermassive black hole the horizon is so large that the tidal forces there are much weaker, so an observer could cross the horizon with less immediate harm.

Detecting black holes

Black holes do not emit light themselves, so astronomers find them by their effects on nearby matter and spacetime. There are four main methods.

Accretion and X-rays

Gas falling toward a black hole forms a disk that heats to millions of kelvin. This hot gas emits X-rays. Cygnus X-1, discovered in the 1960s, is a bright X-ray source with an orbiting companion star. Measurements of the star's orbit indicate a compact object of roughly 20 solar masses, which is too heavy to be a neutron star.

Stellar orbits

The motion of stars close to the centre of a galaxy reveals hidden mass. Stars orbiting Sagittarius A*, the compact radio source at the centre of the Milky Way, move at very high speed on tight orbits. Those orbits, tracked over decades, give a mass of about 4 million solar masses packed into a region that is tiny compared with the galaxy around it.

Gravitational waves

When two black holes merge, they radiate ripples in spacetime called gravitational waves. The first direct detection, GW150914, was made by the LIGO detectors in 2015. The signal came from two black holes of about 36 and 29 solar masses that merged into one of about 62 solar masses. About 3 solar masses of energy was radiated as gravitational waves in a fraction of a second.

Imaging the shadow

In 2019 the Event Horizon Telescope collaboration published the first image of a black hole's shadow, the dark region surrounded by light bent by gravity, in the galaxy M87. The image showed a ring of emission around a dark centre, consistent with general relativity predictions for a black hole of about 6.5 billion solar masses.

Hawking radiation

In 1974 Stephen Hawking showed that quantum effects near the horizon cause black holes to emit a faint thermal radiation, now called Hawking radiation. The temperature is inversely proportional to mass, so stellar and supermassive black holes are far colder than the cosmic microwave background and lose mass extremely slowly. Hawking radiation has not been observed directly, and it remains a theoretical prediction that links gravity, thermodynamics and quantum mechanics.

Key takeaways

A black hole is a region where gravity bends spacetime so strongly that escape becomes impossible inside the event horizon. Its size is set by its mass through the Schwarzschild radius, which makes stellar black holes only tens of kilometres across and the largest supermassive ones can be larger than the orbit of Neptune.

Black holes are detected indirectly through X-ray emission, stellar orbits, gravitational waves and imaging of the shadow around them. Each method supports the same conclusion: these objects are real and they behave as general relativity predicts. The deeper questions about the singularity and Hawking radiation remain open for future research.

A useful simplification is the no-hair idea. Physicists found that a stationary black hole is described completely by three numbers: its mass, its spin and its electric charge. Two black holes with the same three values look identical from outside, whatever fell into them. Astronomers hope to test this prediction by watching the shape of shadows and the motion of gas near the horizon.