Phys.org
Event horizon
The boundary around a black hole past which nothing gets out — not a surface, but a point of no return.
The event horizon is not a thing, it is a place: the distance from a black hole at which the escape velocity reaches the speed of light. Cross it and every path leads inward, including the paths light would take. It has no substance and nothing special happens to you as you pass it — for a large enough black hole you would not notice — but from outside, the crossing takes forever, because the light you emit as you fall takes longer and longer to climb out. What the Event Horizon Telescope photographed in 2019 was not the horizon itself but its shadow: the dark circle it cuts out of the glowing gas behind it.
Where it has come up lately
Related terms
Accretion disc
The flat, ferociously hot spiral of matter falling onto a compact object.
Singularity
The point where our description of gravity stops producing meaningful answers.
Schwarzschild radius
How small you would have to squeeze something to make it a black hole.
Supermassive black hole
Millions to billions of solar masses, one at the centre of essentially every large galaxy.
Questions
What is Event horizon?
The boundary around a black hole past which nothing gets out — not a surface, but a point of no return.
Why does it matter?
It has no substance and nothing special happens to you as you pass it — for a large enough black hole you would not notice — but from outside, the crossing takes forever, because the light you emit as you fall takes longer and longer to climb out. What the Event Horizon Telescope photographed in 2019 was not the horizon itself but its shadow: the dark circle it cuts out of the glowing gas behind it.