How does the String Theory Propeller behave as you approach the event horizon?

[Passionflower]

Yes, this is true, but this doesn't mean that we can't calculate what an observer would see on when on the event horizon.

Right.

If an observer, free falling at escape velocity, knows the mass of the non rotating black hole he can calculate how much time he is away from reaching the singularity and where he is wrt the event horizon by measuring the tidal acceleration between the floor and ceiling of his spaceship.

If he does not know the mass he can still determine how much time he has left until he reaches the singularity.

 

[Naty1]

From George:

For stationary black holes (e.g., spherical black holes and rotating black holes) these two horizons coincide. In general (for positive mass/energy), the apparent horizon is at or inside the event horizon.

Kip Thorne's BLACK HOLES AND TIME WARPS, page 415, Box 12.1 seems to have a slightly different description:

The absolute horizon is created at the star's center...well before the star's surface shrinks through the critical circumference. The absolute horizon is just a point when created but it then expands smoothly and emerges through the star's surface precisely when the surface shrinks through the critical circumference. It then stops expanding and thereafter coincides with the suddenly created apparent horizon.

He goes on to say
...

the areas of absolute horizons (but not necessarily apparent horizons) will increase not only when black holes collide and coalesce but also also when they are born, when matter or gravitational waves fall into them...

and so forth.

which may relate to George's description.

One thing I have never read is whether when an additional bit(s) is added to a black hole, does the absolute horizon area increase "smoothly" and the apparent horizon discontinuously. Why wouldn't the apparent horizon grow?? If there is a theoretical difference, what does it mean?

Thorne continues:

Hawking was well aware the choice of definition of horizon, absolute or apparent, could not influence in any way any predictions for the outcome of experiments...however, the choice of definition could influence the ease with which physicsts deduce...the properties and behaviors of black holes...

 

[George Jones]

See Figure 5.7 on page 134 (pdf page 150) and Figure 5.16 on page 155 (pdf page 171) of Eric Poisson's notes,

http://www.physics.uoguelph.ca/poisson/research/agr.pdf,

which evolved into the excellent book, A Relativist's Toolkit: The Mathematics of Black Hole Mechanics.

 

[nismaratwork]

See Figure 5.7 on page 134 (pdf page 150) and Figure 5.16 on page 155 (pdf page 171) of Eric Poisson's notes,

http://www.physics.uoguelph.ca/poisson/research/agr.pdf,

which evolved into the excellent book, A Relativist's Toolkit: The Mathematics of Black Hole Mechanics.

If I'm reading this correctly then the answer is: 'It's all inside the hole, so you don't get to see it anyway'... which is what you said without diagrams in the first place... right?

 

[kj30]

2. Suppose observer that A hovers at a great distance from a black hole, and that observer B hovers very close to the event horizon. The light that B receives from A is tremendously blueshifted. Now suppose that observer C falls freely from a great distance. C whizzes by B with great speed, and, just past B, light sent from B to C is tremendously Doppler reshifted. What about light from A to C. The gravitation blueshift from A to B is less that the Doppler redshift from B to C. As C crosses the event horizon, C sees light from distant stars redshifted, not blueshifted.

Thanks for the clarification! I guess my conceptual hangup was that C must pass through a point where a suspended observer, B, would see the whole future of the universe pass in a short time and get burned up in a flash of gamma radiation. But somehow C gets through that same point without suffering that experience. But by equivalence, I guess that's no different from the fact that I would not share the experience of a galactic traveler accelerating back and forth across the galaxy merely because he passes very close to me. He might see the entire future of the galaxy in a few seconds, and burn up from the radiation, and I would not. That fact that we were at the same point in space for an instant of time during which he was getting burned with gamma radiation is irrelevant. Is that a somewhat correct analogy?

Can I surmise that if a mirror were suspended very close to the event horizon, and I sent a flash of light towards the black hole, it might take a very LONG time for the reflection to come back to me? And in the local physics of the mirror, the flash of light would appear as high-end gamma radiation?

 

[Naty1]

George, unsure whether to thank you or cuss you for the reference in your post #73...
I think I'll "thank you" now, perhaps saving the cussing for later when I try to plow thru that book...

 

[kj30]

Just bumping up my unanswered question (my pervious post of Dec25-10). Either it stumped the experts, or it was too boring and sophomoric for the experts! Probably the latter, ;-)

Basically, if you suspend a mirror very near (but above) the event horizon, and you send a flash of light towards the black hole from a "safe" distance away from the black hole, it might take a very long time for the light flash to come back. Correct?

 

[George Jones]

Just bumping up my unanswered question (my pervious post of Dec25-10). Either it stumped the experts, or it was too boring and sophomoric for the experts! Probably the latter, ;-)

Basically, if you suspend a mirror very near (but above) the event horizon, and you send a flash of light towards the black hole from a "safe" distance away from the black hole, it might take a very long time for the light flash to come back. Correct?

Yes. For quantitative details, see

https://www.physicsforums.com/showthread.php?p=928277#post928277.