Black Hole

What is a black hole?

A black hole is not an empty hole punched through space or a cosmic vacuum cleaner that sucks in everything. It is an object in which enough mass has been concentrated into a very small region to curve spacetime profoundly. Around it lies a boundary from which no light or signal can travel outward. This boundary is called the event horizon. The term black hole does not refer to one particular object, but to the entire class of objects that possess such a boundary.

The event horizon is not a solid surface that someone could stand on. From far away, a black hole's gravity is no different from that of any other object with the same mass. Even if the Sun were hypothetically replaced by a black hole of equal mass, Earth would continue along nearly the same orbit. The difference becomes important only very close to the black hole, where even the paths of light and the passage of time are strongly distorted.

• Event horizon The boundary beyond which information cannot return to the outside
• About 3 km × solar mass A rule of thumb for the horizon radius of a nonrotating black hole
• Three observed scales Stellar-mass, intermediate-mass, and supermassive black holes
• An invisible object Revealed through nearby stars, hot gas, gravitational waves, and its shadow

Are the event horizon and the singularity the same thing?

The event horizon is the black hole's outer boundary, not its center. In the simplified case of a black hole with no rotation or electric charge, the horizon radius is proportional to its mass. It is about 3 km for each solar mass, so a black hole with ten times the Sun's mass has a horizon radius of roughly 30 km. Concentrating the same mass into a smaller space does not automatically make its gravity stronger at every distant location; what matters is that it becomes possible to approach regions that would previously have been inside matter.

Calculations based on general relativity produce a singularity at the center, where density and spacetime curvature tend toward infinity. Infinity, however, may be a sign that our present theory is no longer adequate under those conditions. Observations can test the region outside the event horizon, but we still lack a complete description of the state of matter inside it and of the center once quantum mechanics is included. Images that depict the singularity as a tiny black sphere are simplified illustrations of theory, not photographs.

What we have directly detected is not a “lump of black matter,” but stellar orbits, bent light, hot gas, and gravitational waves around an invisible and extremely compact region.

How are black holes classified by mass and size?

Stellar-mass black holes generally contain from a few to tens of times the Sun's mass. They can form when the core of a massive star exhausts its fuel and can no longer resist collapse under its own gravity. They may grow heavier by taking gas from a companion star or merging with another compact object.

Supermassive black holes range from hundreds of thousands to billions of solar masses or more and are found at the centers of most large galaxies. Sagittarius A* at the center of the Milky Way and the black hole at the center of M87 are representative examples. There is still no single accepted explanation for what seeds these objects began with in the early universe or how they grew so quickly.

Between these ranges are candidates for intermediate-mass black holes, estimated to contain hundreds to hundreds of thousands of solar masses. Some candidates have been identified through intense X-rays or stellar motions, and merger remnants above 100 solar masses have also been detected. Each candidate must nevertheless be assessed for alternative explanations, such as another kind of object or a cluster of stars. Primordial black holes, proposed to have formed in the early universe, remain a theoretical possibility and have not been confirmed.

How do black holes grow and make their surroundings shine?

A black hole itself neither emits nor reflects light. Nearby gas, however, does not simply fall straight down. Gas with angular momentum can circle the black hole and form a flat accretion disk. Friction and compression within the disk heat the material until it emits radio waves, visible light, ultraviolet radiation, and X-rays. The bright light we observe comes from matter that is still outside the event horizon.

Around some black holes, magnetic fields redirect part of the disk material into fast jets along the rotation axis. These jets do not escape from inside the event horizon; their material is diverted by the disk and magnetic fields before crossing it. Nor does every black hole always have a bright disk and jets. A black hole with little material to feed on may be extremely dark and detectable only through its gravitational influence on nearby objects.

How can we confirm an invisible black hole?

Is the dark circle imaged by the EHT the event horizon?

The Event Horizon Telescope (EHT) links radio telescopes across several continents so that they work like a virtual telescope the size of Earth. In 2019 it released the ring around the black hole at the center of M87, and in 2022 the ring around Sagittarius A* at the center of the Milky Way. The dark center is not a photograph of the event horizon's surface. It is the black hole shadow, made darker than its surroundings because strong gravity bends and captures some of the light.

The shadow appears larger than the event horizon. Light from behind the black hole can be bent toward us, while light on certain paths circles the object multiple times and creates a bright ring. The key result is that the size and shape of the rings observed by the EHT agree with predictions of general relativity calculated from the objects' masses. The images do not show the black hole's interior; they resolve the spacetime and hot plasma immediately outside the horizon.

Does a black hole swallow everything nearby?

Objects sufficiently far from a black hole can maintain stable orbits just as they can around any other mass. To fall inward, matter must lose energy and angular momentum through collisions with other material or friction in gas. That is why the many stars near a galaxy's central black hole orbit it instead of plunging straight in.

The danger at close range is the tidal force: the difference in gravity acting on the near and far sides of an object. A stellar-mass black hole has a small horizon, so this difference rises steeply on approach and can stretch an object apart before it reaches the horizon. A supermassive black hole has a much larger horizon, so the tidal force at the boundary itself can be weaker, but anything that crosses it is equally unable to return.

What do we know, and what remains unknown?

Stellar orbits, X-ray binaries, EHT images, and gravitational-wave observations provide evidence across very different mass ranges that extremely compact, dark regions behave like the black holes predicted by general relativity. The masses and spins of black holes and the motion of surrounding matter are being measured with increasing precision.

We still do not know how information and matter should be described inside the event horizon, how quantum theory should replace the singularity, or what seeds produced the first supermassive black holes. A black hole is neither an object about which we know nothing nor one for which every question has been settled. Its observable exterior is a subject of precision science, while its interior and origins remain at the frontier of modern physics.

Sources

• NASA Science — Black Holes
• NASA Science — Anatomy of a Black Hole
• NASA Science — Types of Black Holes
• NASA Science — How Do We Know There Are Black Holes?
• Event Horizon Telescope — First Image of M87*
• Event Horizon Telescope — First Image of Sagittarius A*
• LIGO Lab — GW150914 Detection Publications