# Why does surface area of an Event Horizon increase?

1. Dec 17, 2014

### avito009

Stephen hawking came to know of a study that stated that surface area of an event horizon increases. So he said that since area has increased Entropy also had increased.

But why does surface area of an Event Horizon increase?

2. Dec 17, 2014

### martinbn

That's a theorem (proven by Hawking). Are you asking about the proof of the theorem?

3. Dec 17, 2014

### mathman

4. Dec 17, 2014

### EpicAll Rounder

It increases due to the black hole pulling more material into itself, therefore trapping it in the event horizon, enlarging it. The material would get trapped into an 'orbit' around the black hole. Never able to escape, neither get pulled into the black hole.

5. Dec 17, 2014

### Staff: Mentor

The event horizon is simply the distance from the center of the black hole where the curvature of space becomes so great that there are no paths through spacetime that can lead you back out of it. Just outside the event horizon the curvature of space is still extreme and the only paths available that don't take you into the black hole are the ones where your velocity is either the speed of light or very close to it. Since mass causes curvature of space, a more massive black hole will curve space more, causing the event horizon to be located further out, which means that the "surface" is larger.

6. Dec 18, 2014

### Staff: Mentor

The surface area of a black hole is given by $4\pi{R}^2$ where $R$ is the Schwarzschild radius. The schwarzschild radius increases as mass falls into the black hole, so the area also does.

7. Dec 18, 2014

### Staff: Mentor

yes.
That's not right. The trapped material falls through the event horizon and into the central singularity rather quickly; it's just that an observer outside the event horizon doesn't get to see it.

This question has been the subject of many threads over in the relativity forum, so you may want to try searching the topic there.