Phenomena · Universe scale
Event Horizon
The boundary around a black hole beyond which nothing — not even light — can return. At this surface, the escape velocity equals the speed of light. The Schwarzschild radius defines the event horizon for a non-rotating black hole.
Key fact
The event horizon of M87* — the first black hole ever imaged (2019) — has a radius of about 19 billion km, larger than our entire Solar System.
Black holes and the point of no return
A black hole forms when a massive star's core collapses under gravity at the end of its life, or when two neutron stars merge. The singularity — a point of theoretically infinite density — lies at the centre. Surrounding it is the event horizon: not a wall, not a surface, but a region of spacetime from which all possible paths lead inward. Once inside, even a photon travelling at the speed of light cannot escape.
The formula for the Schwarzschild radius is elegantly simple: r = 2GM/c². For a mass equal to the Sun (2 × 10³⁰ kg), this gives a radius of just 3 km. The supermassive black holes that lurk at the centres of most large galaxies are millions to billions of solar masses — their event horizons span the Solar System.
To understand how black holes are discovered via gravitational waves, see the gravitational wave entry. Black holes are a stage in the life of very massive stars — traced in our stellar life cycle guide.
Frequently asked questions
- The event horizon is not a physical surface — it is a mathematical boundary in spacetime. An outside observer watching someone fall toward a black hole would see the infalling person appear to slow down, redden, and dim as their signals take longer and longer to escape the deepening gravitational field. They would appear to freeze at the horizon — never actually crossing it, from the observer's perspective. However, the person falling in would not notice anything special as they cross the event horizon — there is no visible boundary, no sudden jolt. General relativity predicts they cross it in finite proper time and continue toward the singularity. What waits there is not yet described by any complete theory of quantum gravity.
- The Schwarzschild radius is the size to which any mass must be compressed to form a black hole — equivalently, it defines the radius of the event horizon for a non-rotating (Schwarzschild) black hole. The formula is r_s = 2GM/c², where G is the gravitational constant, M is the mass, and c is the speed of light. For the Sun, the Schwarzschild radius is about 3 km (the Sun would need to be squeezed to a 3 km ball to become a black hole). For Earth it is about 9 mm. The supermassive black hole at the centre of M87 (imaged by the Event Horizon Telescope in 2019) has a Schwarzschild radius of roughly 19 billion km — larger than our entire Solar System.
- Not directly — by definition, no light escapes from inside the event horizon. However, the Event Horizon Telescope (EHT) — a planet-spanning array of radio telescopes — captured the first image of the shadow of a black hole's event horizon in 2019. The target was M87*, the supermassive black hole (6.5 billion solar masses) at the centre of galaxy M87, 55 million light-years away. In 2022, EHT released the image of Sgr A*, the 4-million-solar-mass black hole at the centre of our own Milky Way. The dark shadow region is the area from which no light escapes — the event horizon's silhouette. The Event Horizon Telescope collaboration publishes all results.
- Yes, in theory. In 1974, Stephen Hawking showed that quantum effects near the event horizon cause black holes to slowly radiate energy — Hawking radiation. This happens because virtual particle-antiparticle pairs spontaneously form near the horizon; occasionally one falls in while the other escapes as real radiation, carrying energy away from the black hole. Over time, the black hole loses mass and eventually evaporates. For stellar-mass black holes, the evaporation timescale is enormously longer than the age of the universe (~10⁶⁷ years). Hawking radiation has never been directly observed — it is predicted to be undetectable with current technology — but it is widely accepted as a consequence of combining quantum mechanics with general relativity. The gravitational wave dictionary entry explains another way black holes are studied.
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