Black holes: what we actually know
Directly imaged, routinely detected in gravitational waves, and still hiding the interesting part.
4 min read · Published 27 August 2026
The short answer
A black hole is a region where gravity is strong enough that nothing, including light, can escape from within its event horizon. They are not hypothetical: two have been imaged directly, dozens of mergers have been detected in gravitational waves, and stars have been tracked orbiting the invisible mass at our galaxy's centre for decades. What remains unknown is what happens at the singularity, where general relativity stops giving sensible answers.
What is established
- They exist and have been imaged. The Event Horizon Telescope produced an image of the black hole in galaxy M87 in 2019 and of Sagittarius A* at our own galactic centre in 2022 — see the first image of a black hole.
- Stars orbit our galaxy's centre. Decades of tracking stellar orbits around Sagittarius A* established a mass of roughly four million Suns in a region small enough to exclude any alternative. This work won the 2020 Nobel Prize in Physics.
- Mergers are routinely detected. Gravitational wave observatories have detected many black hole mergers since 2015, matching relativity's predictions in detail.
- They come in size classes. Stellar-mass black holes from collapsed stars, supermassive ones of millions to billions of solar masses at galactic centres, and an intermediate class that is harder to find and now has some candidates.
The misconceptions worth dropping
They do not suck things in. A black hole's gravity at a given distance is exactly the same as any other object of the same mass. If the Sun were replaced by a black hole of identical mass, Earth's orbit would be unchanged — it would simply be dark. Black holes are not cosmic vacuum cleaners.
They are not infinitely dense objects in space. The singularity is a place where the mathematics breaks down, which is generally read as a sign the theory is incomplete rather than a description of something real. What is physically defined is the event horizon, a boundary rather than a surface.
They are not small. The event horizon of a supermassive black hole can be larger than the solar system. Density inside the horizon actually *decreases* with mass, and for the largest it is lower than water.
Falling in is not instant. From an outside viewpoint, an infalling object appears to slow and redshift, asymptotically approaching the horizon and never quite crossing. From the object's own frame it crosses in finite time and notices nothing special at the horizon itself.
What is genuinely unknown
What happens at the singularity. General relativity predicts infinite density, which is universally read as a failure of the theory rather than a physical fact. Resolving it requires a theory of quantum gravity, which does not yet exist.
The information paradox. Hawking showed black holes radiate and slowly evaporate. If they evaporate completely, what happens to the information about what fell in? Quantum mechanics says information cannot be destroyed; the naive picture says it is. This remains one of the sharpest open problems in theoretical physics.
How supermassive black holes formed so early. Extremely massive black holes are observed at very early cosmic times, and growing them that fast from stellar seeds is difficult. Direct collapse of enormous gas clouds is one proposed route.
Whether the interior description means anything. Since nothing can report back, the interior is beyond observational reach in principle rather than merely in practice.
Hawking radiation
In 1974 Stephen Hawking showed that black holes should emit thermal radiation through quantum effects near the horizon, and therefore slowly lose mass.
The temperature is inversely proportional to mass, so large black holes are extraordinarily cold — a stellar-mass one is far colder than the cosmic microwave background, meaning it currently absorbs more than it emits and grows rather than shrinks.
Evaporation timescales are vast, exceeding the current age of the universe by enormous factors for any astrophysical black hole. The prediction has never been observed and probably cannot be with current technology.
It is nonetheless taken seriously because it emerges from combining quantum field theory with general relativity in a regime where both should apply, and because it produced the information paradox that has driven decades of theoretical work.
Common questions
- Do black holes actually exist?
- Yes, and they are directly observed. Two have been imaged by the Event Horizon Telescope, stellar orbits around our galaxy's centre established a four-million-solar-mass object in a tiny region, and gravitational wave observatories have detected many black hole mergers.
- Do black holes suck things in?
- No. A black hole's gravity at a given distance is identical to any other object of the same mass. Replacing the Sun with a black hole of the same mass would leave Earth's orbit unchanged — the sky would simply go dark.
- What is the information paradox?
- Hawking showed black holes radiate and slowly evaporate. If a black hole evaporates completely, it is unclear what happens to the information about what fell in — quantum mechanics says information cannot be destroyed. It remains a major open problem.
- What happens if you fall into a black hole?
- From outside you would appear to slow and redshift, never quite crossing the horizon. From your own frame you cross in finite time. Tidal forces would destroy you well outside a stellar-mass black hole, but a supermassive one's horizon could be crossed without noticing.