Photons Inside the Event Horizon

[zapatos]

What is the behavior of a photon inside the event horizon? Is it able to travel at c from our perspective (if we could see it)? Since it cannot escape, does it simply orbit the black hole? If it is travelling on a path from the center of the black hole toward the event horizon, since we know that it cannot escape, does it lose velocity? Do photons even exist inside the event horizon?

[MigL]

I would think that any photon which 'crosses' the event horizon of a black hole, has as its ultimate fate, an encounter with the ( possible ) singularity very quickly as they are constrained to travel at only one speed.

The event horizon is the closest possible distance at which a stable orbit can possibly be acheived, and the orbital speed is the maximum allowed, the speed of light, c .

For light to 'escape' from this orbit requires moving faster, which is not allowed, so an outside observer will never see this light, it must originate outside the event horizon to be seen by external observers.

Any orbit inside the event horizon, by even the smallest amount, requires moving faster than c , which is, again, not allowed, and so the photon 'crashes' into the singularity in virtually no time at all.

If you look at a Penrose diagram for a Swartzchild ( non -spinning, non-charged ) black hole you will note that the diagonals represent the event horizon AND the speed of light ( like light cones ) and to avoid the singularity ( Hawking and Penrose came up with the singularity requirement in the 60s or 70s but have since changed their minds ) you need to travel faster than c . The only realistic black holes are, however, spinning and non-charged, so this may make a difference.

Edited September 9, 2012 by MigL

[zapatos]

Does this mean that a photon can never travel from its location inside the event horizon in a direction toward the event horizon, as travelling toward the event horizon requires velocity greater than light speed? For example, if a photon is generated inside the event horizon, will its path only be directly (or indirectly) toward the singularity, but never toward the event horizon?

[MigL]

It is my understanding that your interpretation is correct.

If you were to cross the event horizon ( of a massive black hole such that tidal effects are delayed ) you would see nothing but blackness coming from your foreward direction, ie no light is coming up towards you from the central region nearer to the singularity, and you would see an ever shrinking circle of light from the outside universe coming from your rearward direction.

[md65536]

It is my understanding that your interpretation is correct.

If you were to cross the event horizon ( of a massive black hole such that tidal effects are delayed ) you would see nothing but blackness coming from your foreward direction, ie no light is coming up towards you from the central region nearer to the singularity, and you would see an ever shrinking circle of light from the outside universe coming from your rearward direction.

 

You're speaking now of an apparent horizon [http://en.wikipedia....pparent_horizon], which is different depending on the observer's point of view.

 

From what little I understand, you (in a ship crossing the BH's absolute event horizon) would be able to see things (photons) that we on Earth couldn't see because they're inside the apparent horizon separating us from the BH. You would also have your own apparent horizon so the black hole singularity still looks like a (less massive) black hole to you.

 

Then if your ship sends out a probe to enter what to you is the apparent horizon, the probe will be able to see light that you're not able to see, and it will have its own apparent horizon as well. This can be repeated ad infinitum? Ie. if you were heading toward the singularity, it would always look like a black hole though you would continually be able to see more of it?

Edited September 9, 2012 by md65536