What it is
A black hole forms when enough mass is packed into a small enough space. Its boundary, the event horizon, is the point beyond which escaping would require going faster than light, so nothing comes back out.
Once a strange prediction of general relativity, black holes are now observed directly: by the gravitational waves of their collisions, by stars orbiting the one at the centre of our galaxy, and by radio images of their shadows.
Anatomy of a black hole
Names the parts of a non-spinning black hole and their sizes relative to the event horizon.
- Event horizon 1 × horizon radius
- Escape would need more than light speed
- Photon sphere 1.5 × horizon radius
- Light can orbit here, unstably
- Innermost stable orbit 3 × horizon radius
- Closest a stable disc can reach (non-spinning)
- Light rays blue
- A ray passing close is bent around the hole; a closer one crosses the horizon and is lost.
Source: NASA Science
A model to explain the idea; not drawn to scale.
Ideas to carry
- 1
The event horizon
Not a surface you could touch, but a one-way boundary in spacetime.
- 2
A shadow, not a hole
What telescopes image is the dark region outlined by light bent around the black hole.
- 3
Edges of knowledge
At the central singularity our current physics stops giving reliable answers.
Established What we know
- At a black hole’s event horizon the escape speed exceeds the speed of light, so nothing, including light, can escape.[1]
- Reinhard Genzel and Andrea Ghez shared the 2020 Nobel Prize for discovering a supermassive compact object at the centre of our galaxy.[2]
- LIGO’s first detection of gravitational waves, announced in 2016, came from two black holes merging over a billion years ago.[3]
Interpretation The older lens · Kabbalah
Kabbalah calls the boundless source before creation Ein Sof, “without end”. A singularity is physics’ own edge of the knowable.
Reading
- 1916Solution to Einstein’s field equationsKarl Schwarzschild
- 1994Black Holes and Time WarpsKip Thorne
- 2022Image of Sagittarius A*Event Horizon Telescope
Event horizon
The universe’s strict no-returns policy
Connected subjects, projects and writing
- AstronomySubject · Cosmos
Planets, stars and galaxies, measured from Earth and from space, and the old light that carries them to us.
- RelativitySubject · Cosmos
Einstein’s account of space, time and gravity, and why your phone’s map needs it to work.
Sources
- Black Holes NASA Science. Checked 5 Nov 2026.
- The Nobel Prize in Physics 2020 Nobel Prize, 2020. Checked 5 Nov 2026.
- Gravitational Waves Detected 100 Years After Einstein's Prediction LIGO Caltech, 2016. Checked 5 Nov 2026.