The event horizon

The line of no return

When the core of a collapsed star gets squeezed small enough, a boundary forms around it that not even light can escape. This boundary is called the event horizon. Its size depends on the mass. If the Sun became a black hole, its horizon would be about 6 km across. For Earth, it would be about 1.8 cm.

Why?To get out from inside the horizon, you would have to go faster than light. Nothing is faster than light, so nothing can come out. The funnel on the screen can only be drawn down to the horizon, its narrowest throat. What lies inside cannot be shown in this picture.

Learn moreSeen from far away, a clock right at the horizon seems to stop. The horizon is not a wall or a surface. It is simply a boundary you cannot come back from once you cross it. The funnel shape around it is drawn from a real calculation (Flamm’s paraboloid), with only its depth reduced to fit the screen.

Clock we are watching

A clock at the horizon (seen from far away)

How much slower

Appears to stop

Clock rate

0

seconds this clock ticks while a clock far away in empty space ticks one second · calculated with the formulas of relativity