Exploring the Transparency of Black Holes: A Relativity Question

[stargazer3]

Hey, before people start throwing rotten vegetables at me, I'm not any good in general relativity!
So my question is: are black holes transparent to electromagnetic waves of wavelengths on the same order as the Schwarzschild radius of the BH?

 

[mfb]

They will scatter the electromagnetic waves and absorb some fraction. I would not call this "transparent" - there is no part of the wave going through the black hole itself.

 

[Naty1]

What is your observer? Different observers make different observations.

A hovering exterior observer is causally disconnected from all events INSIDE the horizon...while a free falling observer, in sharp contrast, can 'see inside'...something just ahead,not the singularity...if those two observers pass each other they observe very different things and, for example, time passes very differently for them. It's analogous, if not identical, to an Unruh [accelerated] observer making different observations than an inertial observer right alongside.


I surprised to find mfb thinks some of the radiation is 'scattered'...I'd like to know
more about that. As I understand it, free falling observers pass right into the black holes
observing nothing. Is electromagnetic energy different?? Or does this refer to the electromagnetic fields outside the BH horizon...Quoting from "Quantum Fields in Curved Space" by Birrell and Davies,
for outgoing radiation:

At first sight, black hole radiance seems paradoxical, for nothing can apparently escape from within the event horizon. However, the average wavelength of the emitted quanta is ~ M, i.e comparable with the size of the hole. As it is not possible to localize a quantum to within one wavelength, it is therefore meaningless to trace the origin of the particles to any particular region near the horizon. The particle concept, which is basically global, is only useful near [infinity]. In the vicinity of the hole, the spacetime curvature is comparable with the radiation wavelength in the energy range of interest, and the concept of locally-defined particles breaks down.

So saying the particles are emitted by the horizon, or asking what particles you see when you get near the horizon - both of these are meaningless.

 

[Naty1]

I found the description I could not recall:

A black hole is a region of spacetime from which nothing escapes. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, making it almost an ideal black body[43] (radiation with a wavelength equal to or larger than the radius of the hole may not be absorbed, so black holes are not perfect black bodies).

http://en.wikipedia.org/wiki/Black_body#Black_holes

So mfb is correct...but I still don't really understand why...

 

[Mordred]

This is an area I only know a little about, essentially a perfect blackbody never exists in nature, their is always some absorbtion, reflection etc.

I found on that same page with the references listed an article which provides some coverage

http://books.google.ca/books?id=IIIVHRirRgEC&pg=PA386&redir_esc=y#v=onepage&q&f=false

As I said I'm not familiar enough with it to make any detailed explanation

 

[Naty1]

Hey, before people start throwing rotten vegetables at me,...

Apparently you have been here long enough to know fools are not suffered lightly,
but perhaps not yet long enough to knew that pleas for 'love and understanding and
kindness' will go unheeded!

If that's what you are looking for, as they say in Washington, DC, Get a dog! [LOL]

 

[stargazer3]

Thanks for all the answers. It was sort of easy for me to imaging macroscopic black holes partially absorbing km radio waves, but when I tried to think about microscopic black holes in particle accelerators and shorter wavelengths, my imagination just didn't want to have part in it at all.

This is an area I only know a little about, essentially a perfect blackbody never exists in nature, their is always some absorbtion, reflection etc.

So should we call them grey holes instead?

Get a dog!

Well, the more I look at it, the less I get it!%) Apparently, I'm just not cut out for Washington, DC.

 

[mfb]

I surprised to find mfb thinks some of the radiation is 'scattered' [...] Or does this refer to the electromagnetic fields outside the BH horizon...

Right.
What is inside, cannot get out, but what is outside can go around the black hole, be scattered, get reflected, ...

 

[Mordred]

I'm not sure I'd go that far lol

again its not an area I'm too familiar with although I've been studying Hawking radiation I still have a considerable ways to go in my studies on various blackbody radiations. I have seen published papers on arxiv.com that describe black holes are more grey than black but I wouldn't consider it as mainstay regardless of its source

 

[mfb]

I am assuming an incident wave were some fraction would "miss" the black hole if we would not have gravity. With gravity, that fraction gets deflected, and everything gets complicated.

 

[Mordred]

Yeah I've yet to see a good article other than the one I posted covering transparent body, grey body, blackbody and white body. I know what each term means but not clear on the underlying mathematics as of yet. Those terms are in the link

 

[jartsa]

Hey, before people start throwing rotten vegetables at me, I'm not any good in general relativity!
So my question is: are black holes transparent to electromagnetic waves of wavelengths on the same order as the Schwarzschild radius of the BH?

A small piece of black cardboard hovering near the event horizon of a black hole has no problem emitting radiation that will have a very large wavelength when observed far away. (Redshift helps here)

Therefore it is possible to send, from far away, radiation with a very large wavelenght onto the same piece of cardboard. (we know this because there is such thing as time-reversal symmetry)

From that it follows that it is possible to send, from far away, radiation with very large wavelenth into the same black hole. (Because the black hole is larger than the small piece of cardboard)(I am less sure about small black objects away from large masses. The radiation they emit does not contain very large wavelengths??)

 

[ImaLooser]

Hey, before people start throwing rotten vegetables at me, I'm not any good in general relativity!
So my question is: are black holes transparent to electromagnetic waves of wavelengths on the same order as the Schwarzschild radius of the BH?

That would be an extremely long wavelength. I don't know. I feel certain that there would be some absorption. As to whether the fraction that is not absorbed passed through the black hole, it might or might not be possible to measure experimentally.

That inspired a question of my own. Can gravitational waves pass through a black hole?

 

[Mordred]

That would be an extremely long wavelength. I don't know. I feel certain that there would be some absorption. As to whether the fraction that is not absorbed passed through the black hole, it might or might not be possible to measure experimentally.

That inspired a question of my own. Can gravitational waves pass through a black hole?

I don't think anyone knows the wavelength of a gravity wave lol

I was thinking about this a bit more and read that link I posted in more detail, I can see how it could apply for microblackholes but I did not see how any of the examples of reflection and absorbtion apply with regards to a BH. No where did that article discuss the light paths generated from a BH twisting of spacetime .
The surface bouncing of waves I would think would be quite different than the cardboard example above. The other important factor is can a wavelength exceed the EH of the average size BH, I'm not sure that it does.
Anyways I would surmise that any reflection would occur at the outer portion of the ergosphere
most of the refelction I can see happening via the accretion disk but that's not the same as what's described by the OP on wavelength size.

 

[Naty1]

Can gravitational waves pass through a black hole?

If they do, they likely move to another universe...but the general answer remains:

'No mass-energy escapes a black hole'...especially in our lifetimes...

 

[mfb]

I don't think anyone knows the wavelength of a gravity wave lol

It is no problem to calculate the wavelengths of gravitational waves in free space. The formula is the same as for electromagnetic waves, all you need is the frequency.

 

[Mordred]

Yeah I know I have to calculate wavelengths for work regularly

any takers on the frequency of gravity lol

 

[ImaLooser]

Yeah I know I have to calculate wavelengths for work regularly

any takers on the frequency of gravity lol

Suppose a gravitational wave is generated by two mutually orbiting neutron stars. Then the frequency of the wave would be the frequency of the orbit (or maybe half of that frequency.) So the wave length could be extremely long. That's probably one reason they are so hard to measure.

As to what happens when such a wave encounters a black hole, I have no idea. I think it would be necessary to solve the equations to be sure.