# Thread: Long wavelength photon interacting with a black hole.

1. Hello everyone,

What would happen if a photon with a wavelength longer than the diameter of a black hole were to try to enter it? Would it partially enter the blackhole and partially increase its mass? Would it like 'shake' the horizon of the black hole temporarily, rebound, etc ... ?

Thank you,
Nic.

2.

3. Originally Posted by Nic321
Hello everyone,

What would happen if a photon with a wavelength longer than the diameter of a black hole were to try to enter it? Would it partially enter the blackhole and partially increase its mass? Would it like 'shake' the horizon of the black hole temporarily, rebound, etc ... ?

Thank you,
Nic.
Nothing escapes a BH. The wavelength of the photon is irrelevant.

4. But if the wavelength is longer than the diameter of the black hole, it can't enter completely, so what happens?

5. Originally Posted by Nic321
But if the wavelength is longer than the diameter of the black hole, it can't enter completely, so what happens?
You are thinking that the wavelength has anything to do with dimension, It doesn't.

6. I am not sure what you mean by this. The photon occupies a certain space with a certain probability distribution, so it may well be that its wave function doesn't fit completely inside a black hole.

7. Originally Posted by Nic321
I am not sure what you mean by this. The photon occupies a certain space with a certain probability distribution, so it may well be that its wave function doesn't fit completely inside a black hole.
But quantum mechanics is incompatible (at small scales) with GR. From the perspective of GR, a photon past the EH can never cross back, its trajectory will keep curving backwards from the EH, towards the singularity. You are trying to apply QM (distribution probability) to the problem, it doesn't work.

8. The front of the wave will enter the EH, but what about the rest of the wave? The wavelength would have to get shorter for the photon to fit inside.

Also, problems of compatiblity between GR and QM occur at the singularity. There is speculation as to what happens at the EH, maybe there is a problem there too ( firewall problem ), but it is not sure at this point.

Finally, I heard physicist Leonard Susskind in one of his internet lectures say that the photon would bounce back, although I got the impression it was not really settled yet.

9. Originally Posted by Nic321
The front of the wave will enter the EH, but what about the rest of the wave? The wavelength would have to get shorter for the photon to fit inside.
Photon is a dimensionless particle, so it "fits inside" any box.

Finally, I heard physicist Leonard Susskind in one of his internet lectures say that the photon would bounce back, although I got the impression it was not really settled yet.
See if you can find that lecture, I would be interested in hearing it.

10. Originally Posted by Howard Roark
Originally Posted by Nic321
The front of the wave will enter the EH, but what about the rest of the wave? The wavelength would have to get shorter for the photon to fit inside.
Photon is a dimensionless particle, so it "fits inside" any box.
I don't understand, the wave does have a length. What do you think it is dimensionless? A photon is a electromagnetic wave, but the electric and magnetic field are is space.

Finally, I heard physicist Leonard Susskind in one of his internet lectures say that the photon would bounce back, although I got the impression it was not really settled yet.
See if you can find that lecture, I would be interested in hearing it.
I remember him saying that in 2 videos if I am not mistaken. I am going to try to find it.

11. I looked for it I couldn't find it. He talks about it for like 30 seconds and there's like 5x2 hours videos to check. I will try to find it tomorrow again.

12. Originally Posted by Nic321
Originally Posted by Howard Roark
Originally Posted by Nic321
The front of the wave will enter the EH, but what about the rest of the wave? The wavelength would have to get shorter for the photon to fit inside.
Photon is a dimensionless particle, so it "fits inside" any box.
I don't understand, the wave does have a length. What do you think it is dimensionless?
The wave has length, the particle doesn't.

A photon is a electromagnetic wave, but the electric and magnetic field are is space.
I know what a photon is, you have this misconception that its "dimensions" equal its wavelength. It doesn't.

13. I believe this paper will be of interest :

http://arxiv.org/pdf/0905.3339v1.pdf

Basically, once the wavelength is of the same order as the black hole itself, you will get "back scattering", just as Susskind has indicated.

14. Thanks for the article Markus.

In the article they say:
We have presented graphs with accu- rate numerical results for massless ﬁelds of all spins, that is, scalar (s = 0), fermionic (s = 1/2), electromagnetic (s = 1) and gravitational (s = 2) ﬁelds. All non-zero spin massless ﬁelds have a vanishing scattering cross section in the backward direction (θ = π), whereas the scatter- ing cross section of the massless scalar ﬁeld has a local maximum.
So this means they are not talking about massive particle like fermions or W and Z bosons, right?

I remember Susskind say that the black hole behaves like a sort of mirror for wavelengths of the order the diameter of the black hole, although he didn't get into details. From reading the article there seems to have been quite a substantial amount of research on this issue and it seems relatively well established theoretically.

15. Originally Posted by Nic321
So this means they are not talking about massive particle like fermions or W and Z bosons, right?
Yes, it seems that way.

I remember Susskind say that the black hole behaves like a sort of mirror for wavelengths of the order the diameter of the black hole
Yes, this makes intuitive sense to me. On the other hand though intuition is dangerous when it comes to GR, so it would be handy to see his actual calculations, but I can't find them either. The above article is the best I could come up with on short notice.

16. As I said, he didn't give any detail in the lectures I watched. Hopefully I will find what he said but it won't give us a lot of information.

Do you think that the photon partially enters the black before it is reflected, or what? I don't understand the calculations in the article you posted to tell you the truth.

17. Originally Posted by Markus Hanke
I believe this paper will be of interest :

http://arxiv.org/pdf/0905.3339v1.pdf

Basically, once the wavelength is of the same order as the black hole itself, you will get "back scattering", just as Susskind has indicated.
Interesting paper. What it says is that there is scattering due to the interference between the ray passing the BH at angle with the ray passing at an angle . The graphs clearly show no interference for the case . I.e. any particle (regardless of spin and regardless of the ratio ) moving radially towards the BH, does not scatter. (it gets captured). This explains (in part) why no textbook mentions this effect despite being debated for the past 40 years.

18. As a photon approaches a gravity well, it shifts toward the blue end of the spectrum.

The leading edge of the wave is always in a region of space where time flows more slowly than it does in the region where the tail end is located. That causes the tail end to kind of "catch up" with the leading edge.

Or at least that's one possible explanation. Another explaination would be that gravity simply accelerates the photon, thereby shifting it toward the blue end of the spectrum.

Or another explanation would be "It shifts toward the blue end of the spectrum because: it just plain does" Or another explanation would be that it shifts toward the blue end of the spectrum because: "General Relativity predicts that it does."

Whatever works for you.

19. That causes the tail end to kind of "catch up" with the leading edge.
Couldn't the front of the wave simply oscillate more slowly than the tail?

What would happen near the singularity for the wave? Would it become very disformed or would it only become much more blue-shifted?

This makes me think of another question: what would happen if an observer inside the black hole tried to create a photon with a wavelength longer than the diameter of the black hole?

Also, if a long wavelength photon is absorbed by a black hole, what happens with the conservation of energy for the photon. Say a photon has a wavelength several orders of magnitude longer than the diameter of the black hole, if it get absorbed by the plack hole, wouldn't the squeezing increase its energy a lot?

20. [QUOTE=Nic321;596724]

Also, if a long wavelength photon is absorbed by a black hole, what happens with the conservation of energy for the photon. Say a photon has a wavelength several orders of magnitude longer than the diameter of the black hole, if it get absorbed by the plack hole, wouldn't the squeezing increase its energy a lot?

It would be exactly the way a laser amplifies the energy of the wave by "squeezing" it to shorter wavelengths. The black hole need to impart energy to the photon, so the "extra" energy comes from the black hole.

21. Originally Posted by Howard Roark
Originally Posted by Nic321

Also, if a long wavelength photon is absorbed by a black hole, what happens with the conservation of energy for the photon. Say a photon has a wavelength several orders of magnitude longer than the diameter of the black hole, if it get absorbed by the plack hole, wouldn't the squeezing increase its energy a lot?

It would be exactly the way a laser amplifies the energy of the wave by "squeezing" it to shorter wavelengths. The black hole need to impart energy to the photon, so the "extra" energy comes from the black hole.
Ok, but how can the black hole do that? It would have to lose energy somewhere else??

22. Originally Posted by Nic321
Originally Posted by Howard Roark
Originally Posted by Nic321

Also, if a long wavelength photon is absorbed by a black hole, what happens with the conservation of energy for the photon. Say a photon has a wavelength several orders of magnitude longer than the diameter of the black hole, if it get absorbed by the plack hole, wouldn't the squeezing increase its energy a lot?

It would be exactly the way a laser amplifies the energy of the wave by "squeezing" it to shorter wavelengths. The black hole need to impart energy to the photon, so the "extra" energy comes from the black hole.
Ok, but how can the black hole do that? It would have to lose energy somewhere else??
No, it doesn't, the energy of the BH decreases, the energy of the photon increases in the same amount, the energy of the closed system BH+photon stays the same. Are you trying to learn physics by taking random potshots at different fields? Learning physics doesn't work this way.

23. Well, that's exactly what I said...

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