A Question on Black Holes

[Dajath]

Been reading A Brief History of Time, and I have a few questions I want to sort out.

Hawking talks about the event horizon and says a few things that I think I've read right:

1. The event horizon is a set of events that it is impossible to escape to a large distance from.

2. The event horizon forms the boundary of the black hole, and is formed by the rays of light that nearly escape the black hole, but not quite.

3. These light rays must be parallel, otherwise they would eventually touch and cause each other to fall into the black hole.

4. Due to the way time works within a black hole, this cannot happen as then they wouldn't be able to be at the event horizon.

5. This means that the event horizon can never decrease in area, or else the light rays would cease to be parallel and would eventually touch etc.

 

But then they discovered Hawking radiation. So the event horizon can decrease in size (presumably with the black hole), and indeed can decrease to 0.

 

I understand that this proves Hawking wrong on the nondecreasing nature of the event horizon, but what about the parallel light rays? If the event horizon does decrease in area, do the light rays touch, then fall into the black hole, though that apparently can't happen? I'd really appreciate it if someone could clear this up. Thanks

[Spyman]

The Event Horizon of a Black Hole is not a real physical object, it is a mathematical calculated boundary in space, of where the escape velocity is equal to the speed of light in Newton mechanics and in Einstein relativity where spacetime is so curved that the future of infalling objects is towards the center.

 

 

1. General Relativity thrumps Newton mechanics, once at the Event Horizon or below it is no longer possible to escape even shortly to a small distance. To do so would require moving faster than the speed of light.

 

2. The Event Horizon is formed by the gravity of the Black Hole, further out there is a Photon Sphere consisting of orbiting lightrays, which also is caused by the Black Holes gravity.

 

3. An Black Hole is constantly trapping lightrays inside its Event Horizon and it doesn't matter whether two lightrays are parallel or not, if one or both pass the Event Horizon then they are caught inside. Also lightrays don't have the possibility to push each other towards a Black Hole.

 

4. A photon might be able to hover at the exact radius of the Event Horizon for a very short while, if it is going straight out from the center, but as soon as the Black Hole consumes anything the Event Horizon will grow surpassing the photon and it will be lost inside.

 

5. According to Newton mechanics and General Relativity once a Black Hole has formed it will only continue to grow, but with Quantum mechanics there are other possibilities. I don't think Hawking radiation is confirmed yet, but even so, a normal sized Black Hole would absorb far more cosmic microwave background radiation than it emits.

 

 

I think you might be mixing up parallel lightrays with the particle-antiparticle radiation mentioned in the Hawking radiation process.

 

 

Here are some Links with further reading:

 

http://en.wikipedia.org/wiki/Event_horizon

 

http://en.wikipedia.org/wiki/Black_hole

 

http://en.wikipedia.org/wiki/Hawking_radiation

 

http://en.wikipedia.org/wiki/Photon_sphere

Edited July 8, 2010 by Spyman
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