What is a black hole, and what would happen if you fell in? (for a curious adult who has not studied physics). Cover, in this order: What is a black hole?; How can gravity be strong enough to trap light?; How does a black hole form?; What is the event horizon?; What happens if you fall into a black hole?; If nothing escapes, how do we see black holes?; Do black holes really suck everything in?; Can a black hole die? — page 4: The event horizon

Nib asked for

The event horizon

the short answer

The event horizon is not a surface you could land on but an invisible line in empty space, the exact place where escape becomes impossible.

6 steps, drawn by Nib. Read it here, or watch him draw it. Updated 16 Oct 2026.

1escape speed

From page 2, escape from any object needs a minimum speed, and near a black hole that speed climbs as you get closer. Far away it is modest. Closer in it keeps rising, until at one particular distance it reaches the speed of light.

Closer to the center, the speed needed to escape keeps rising.

2the horizon

At the distance where the required escape speed equals the speed of light, nothing can get out, because nothing travels faster than light. That distance, measured from the center, forms a sphere around the black hole. This sphere is the event horizon.

The event horizon is where escape speed equals light speed.

3in or out

Light aimed outward just outside the horizon can still get away, though it loses energy doing so. Light aimed outward just inside cannot. Even light pointed straight away from the center gets dragged back, so the horizon is a one-way boundary for everything.

Outside the line, escape is possible; inside, it is not.

4no surface

The horizon is not a material thing. There is no wall, skin, or crust there, only empty space with the property that crossing inward is a one-way trip. A traveler passing through would find nothing to touch, which is why it is a threshold rather than a surface.

The horizon is a boundary in empty space, not a solid surface.

5size and mass

The horizon's size depends only on the mass. A black hole with the mass of our Sun would have a horizon about six kilometers across, smaller than a city. More mass means a bigger horizon, in direct proportion.

More mass means a larger horizon, in direct proportion.

6why black

This also fixes what 'black' means. The hole is not a dark object but a region from which no light returns, so we see nothing from inside the horizon. Anything that happens beyond that line is hidden from the outside universe, which is why this boundary matters so much.

The horizon hides everything inside it from the outside universe.

So a black hole's edge is not a surface at all, just the invisible line past which escape ends, and next we cross it.