1. Sep 21, 2011

### khil_phys

What are the types of radiation that the black hole emits? Does it radiate like a perfect blackbody?
Is the information of the bodies collapsing into it released by the black hole?

2. Sep 21, 2011

### Staff: Mentor

Otherwise, a black hole emits no EM radiation itself - that's kinda the definition of "black hole". It can, however, cause matter falling toward it to emit radiation -- up to x-rays.

3. Sep 25, 2011

### Naty1

khil: you mean does it ABSORB like a perfect blackbody....it DOES do that. It sucks in everything within it's gravitational grasp.
see

There is no radiation from inside the event horizon of a black hole to outside where we might observe it. Nothing gets out.

We seek indirect observational evidence of black holes from their gravitational effects on nearby stars, via orbital eccentricities for example, that we can observe and from the radiation emitted by matter as it accelerates into the black hole.

Hawking radiation would be emitted when the universe cools below the temperature of a black hole....so there is no expectation that any radiation, Hawking or otherwise, can be currently observed which originated inside the event horizon. The universe is around 2.7K and I think black holes are expected to be maybe a few tenths of a degree...

4. Sep 25, 2011

### skeptic2

Not even faster than c neutrinos? ;-)

5. Sep 25, 2011

### wolram

When were faster than c neutrinos discovered?

6. Sep 25, 2011

### skeptic2

http://news.sciencemag.org/sciencenow/2011/09/neutrinos-travel-faster-than-lig.html [Broken]

Last edited by a moderator: May 5, 2017
7. Sep 25, 2011

### wolram

I will wait for confirmation of this result, i think it is far to early to give it any credence.

Last edited by a moderator: May 5, 2017
8. Sep 25, 2011

### phinds

There was a thread on this some time back. What you said is incorrect. Hawking radiation does not depend on the temperature of the rest of the universe.

What IS true is that the mass of a black hole will not likely decrease due to Hawking radiation because it will be so slow that it will be more than offset by even tiny amounts of infalling stuff.

9. Sep 27, 2011

### Naty1

phinds..thx for picking that up!!!

I've seen conflicting views about this, but I now have to agree the statement I posted above IS incorrect.

I checked in Kip Thorne's BLACK HOLES AND TIME WARPS where there is a nice discussion of how this understanding came about...via Stephen Hawking, Chapter 12.

The bottom line is that that according to Thorne, there IS a general consensus, spin or not, a black hole WILL radiate gravitational, electromagnetic, neutrino, etc radiation. At first this was not accepted. Whereas the spin energy lost via gravitational waves was stored in the swirl of space OUTSIDE the horizon, when that energy is dissipated, and spin ended, the energy continued to be lost and continues to come from the black hole interior! And the black hole gets hotter as it radiates.

Last edited: Sep 27, 2011
10. Sep 27, 2011

### phinds

Right. I'm aware that Hawking radiation took everyone by surprize and only gradually became accepted wisdom. I had not heard that there had ever been a part of the theory that said Hawking radiation required a low-temp U, but at any rate, if there ever was, there clearly isn't now.

11. Sep 30, 2011

### khil_phys

I understood form Hawking's "A Brief History of Time" that Hawking radiation is due the virtual particle-antiparticle pairs created outside the event horizon of the black hole. Then, the one that falls inside the black is said to have negative energy, which leads to black hole evaporation.

But, if a black hole sucks in EVERYTHING, how can it let out any sort of electromagnetic radiation, even due to its spin?

12. Sep 30, 2011

### WannabeNewton

It does not "suck" in anything in your sense of the word. The geometry of space - time outside the event horizon is like that of any schwarzchild space - time, kerr space - time etc. The "sucking in" is what happens past the event horizon where the time - like $\partial _{t}$ and space - like $\partial _{r}$ become space - like and time - like respectively and since time - like vectors are always future directed, $r = 0$ becomes inevitable and this is the "sucking in" part really. The vacuum fluctuations you are talking about happen outside the event horizon as you yourself stated.

13. Oct 1, 2011

### phinds

I think you have that backwards. Hawking radiation is when a virtual particle/antiparticle pair form INSIDE the EH and one part skitters outside the EH then the pair doesn't reunite and the BH loses that tiny amount of mass/energy.

If the pair formed OUTSIDE the EH and half went in, this would not be radiation FROM the BH, it would just be more stuff going INTO the BH.

14. Oct 1, 2011

### DaveC426913

No, I don't think so. He's got it right.

A virtual particle pair is created out of vacuum fluctuation, just outside the EH. When one falls into the BH and the other escapes, the escaper becomes real (since it can't recombine and wink out of existence anymore). In order to preserve total energy, the particle that fell into the black hole must have had a negative energy.

The net effect is that the BH has emitted a real particle and lost energy.

15. Oct 1, 2011

### phinds

Ah ... that DOES make sense, and clearly was not the way I understood it, so thanks for the clarification. Does the "negative energy" of the infalling half of the pair then mean that it will annahilate something inside the BH or does it just hang around as "negative energy" or do we just not know?

16. Oct 1, 2011

### khil_phys

The "negative energy" gets converted to "negative mass" by the equation E=mc2. Thus, negative mass gets "added" to the BH, and thus it loses mass, evaporates and fades out.

17. Oct 1, 2011

### khil_phys

I didn't quite get it. Why should a black hole radiate from inside the event horizon?

18. Oct 1, 2011

### George Jones

Staff Emeritus
Sometimes it is difficult to give accurate non-mathematical descriptions of processes that involve advanced physics. This is particularly true for Hawking radiation - it is very hard to see the correspondence between the non-mathematical description involving virtual matter-animatter pairs and the actual mathematical description of Hawking radiation.

Mathematical physicist John Baez and theoretical physicist Steve Carlip both try hard to make physical concepts clear, both for laypersons and for experts.

John Baez writes

http://www.obscure.org/physics-faq/Relativity/BlackHoles/hawking.htm
Hawking radiation does not come about because antimatter particles sometimes fall into black holes; it comes about because negative-energy particles (both matter and animatter) sometimes fall into black holes. Some popular-level treatments of black holes obscure this, and even sometime get this completely wrong.

Steve Carlip has written a non-mathematical virtual particle description of Hawking radiation which is more challenging than most non-mathematical descriptions, but which also is more accurate than most non-mathematical descriptions.

What happens, very roughly, is this. Energy is associated with time and spatial momentum is associated with space. When an matter-antimatter pair of virtual particles is created *outside* the event horizon, they can become a little bit separated in the time that the Heisenberg uncertainty principle allows them to live. Tidal forces caused by the curvature of spacetime help them to separate, and, sometimes, the negative-energy particle (which could be either matter or anitimatter) wanders over the event horizon and into the black hole. Inside the event horizon, the roles of time and space coordinates get interchanged. Thus, according to what I wrote above, the roles of energy and spatial momentum get interchanged. What was negative energy becomes a negative spatial component of a local (for an observer inside the horizon) momentum vector. Only a virtual particle can have negative energy, while any particle, real or virtual, can have a negative component of spatial momentum.

Bottom line: the whole process can become a real process. In this real process, an observer outside a black hole "sees" the black hole hole swallow a negative-energy particle while emiitting a positve energy particle (the other member of the matter-antmatter pair). The balck hole radiates.

Last edited by a moderator: Apr 26, 2017
19. Oct 1, 2011

### DaveC426913

That's what I said. Isn't that what I said?

Last edited by a moderator: Apr 26, 2017
20. Oct 1, 2011

### George Jones

Staff Emeritus
Yes, but I wanted to give more details, and I wanted to give links to what Baez and Carlip wrote.