Event Calendar

The event horizon is the invisible boundary that defines a black hole. It is the ultimate point of no return: anything that crosses this boundary— matter, light, information—is irrevocably captured and can never escape. It is not a physical surface, but a mathematical boundary in spacetime.

Why can nothing escape?

At the event horizon, the escape velocity— the speed required to escape gravity—is equal to the speed of light. However, nothing can travel faster than light. Any particle that crosses this horizon, regardless of its speed, inevitably follows a trajectory that brings it back toward the center of the black hole. Even light, as it follows the geodesics of curved spacetime, is directed inward.

A Boundary Without a Wall

Contrary to what one might imagine, the event horizon is not a physical wall. An astronaut crossing it would not feel anything unusual at the moment of crossing—at least for a black hole with sufficient mass. He would not notice anything unusual locally. It is only when trying to turn back that he would realize the impossibility of the situation. To an outside observer, however, the astronaut would appear to slow down and come to a standstill at the event horizon—without ever crossing it, due to time dilation.

The Schwarzschild radius

The size of theevent horizon is described by the Schwarzschild radius, calculated by Karl Schwarzschild in 1916 using Einstein’s equations. It is proportional to the black hole’s mass. For a black hole with the mass of the Sun, this radius would be approximately 3 kilometers. For Sagittarius A*, the supermassive black hole at the center of our galaxy (4 million solar masses), it reaches about 12 million kilometers.

Did you know?

In 2019, the Event Horizon Telescope project captured the first image of the shadow of an event horizon— that of the black hole M87*, 55 million light-years away. The ring of visible light corresponds to the gas orbiting just inside this horizon.

Frequently Asked Questions

What happens if you cross the event horizon?

For a stellar-mass black hole, the tidal forces would be so intense that they would tear any object apart before it even reached the event horizon. For a supermassive black hole, the passage would be imperceptible locally, but the final destination would still be the central singularity.

Can the size of the event horizon change?

Yes. If a black hole absorbs matter or energy, its event horizon expands. Stephen Hawking demonstrated that, classically, the area of the event horizon can only increase— this is the area theorem, analogous to entropy in thermodynamics.

What is Hawking radiation?

It is a quantum phenomenon predicted by Stephen Hawking in 1974: near the event horizon, pairs of virtual particles are created, with one falling into the black hole and the other escaping. Over very long periods of time, this causes the black hole to slowly evaporate. This radiation has never been directly observed.