Black Hole Radiation: Types, Perfect Blackbody & Collapsing Bodies

[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?

 

[russ_watters]

Google "Hawking radiation"

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.

 

[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
http://en.wikipedia.org/wiki/Blackbody_radiation

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...

 

[skeptic2]

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

Not even faster than c neutrinos? ;-)

 

[wolram]

Not even faster than c neutrinos? ;-)

When were faster than c neutrinos discovered?

 

[skeptic2]

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

 

[wolram]

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

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

 

[phinds]

Hawking radiation would be emitted when the universe cools below the temperature of a black hole.

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.

 

[Naty1]

phinds..thx for picking that up!

Hawking radiation would be emitted when the universe cools below the temperature of a black hole.

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.

 

[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.

 

[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?

 

[WannabeNewton]

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?

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.

 

[phinds]

... 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 ...

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.

 

[DaveC426913]

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.

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.

 

[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?

 

[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.

 

[khil_phys]

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.

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

 

[George Jones]

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

In fact this argument also does not correspond in any clear way to the actual computation. Or at least I've never seen how the standard computation can be transmuted into one involving virtual particles sneaking over the horizon, and in the last talk I was at on this it was emphasized that nobody has ever worked out a "local" description of Hawking radiation in terms of stuff like this happening at the horizon. I'd gladly be corrected by any experts out there... Note: I wouldn't be surprised if this heuristic picture turned out to be accurate, but I don't see how you get that picture from the usual computation.

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.

http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad

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.

 

[DaveC426913]

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.

http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad

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.

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

 

[George Jones]

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

 

[khil_phys]

But that surely doesn't classify as electromagnetic radiation, does it?

 

[George Jones]

Yes, electromagnetic radiation is part of the radiation.

 

[1mmorta1]

My goodness, there is an awful lot of confusion on this matter. Hawking radiation is so difficult to discribe because its...its doesn't happen the way you normally envision radiation occurring.
I don't know how to explain it clearly, and it doesn't look like anyone else does either. Kil phys, since the addition of negative energy/mass is equivalent to the subtraction of positive energy/mass, maybe it would help you if you stopped thinking of it as "radiation" and try to picture it as a black hole "sucking in" negative energy and mass. If you can wrap your mind around that, you'll be picturing Hawking radiation.

 

[khil_phys]

I can. I have understood Hawking radiation. My question is pretty straightforward: Does a BH radiate electromagnetic radiation? If it does, how?

 

[Chronos]

It does, but, this radiation must originate outside the event horizon - for obvious reasons. Hawking reasoning on black hole radiation was brilliant. He began with an unsettled feeling about black holes being eternal - which is inconsistent with the laws of thermodynamics. He kept playing with the math until finally satisfied that, as usual, nature abhors infinities. There are still a few doubters out there on Hawking radiation, but, few is the operative term.

 

[DaveC426913]

I can. I have understood Hawking radiation. My question is pretty straightforward: Does a BH radiate electromagnetic radiation? If it does, how?

Didn't we just answer this, several times, in several ways?

The method by which the EM energy is generated is via Hawking radiation. Regardless of how it is generated, the energy that escapes the BH is quite real.

 

[bapowell]

http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad

I found Carlip's explanation to be rather clear and closer to what probably actually happens as well; however, I am unsatisfied by this statement:

Note that this doesn't work in the other direction -- you can't have the positive-energy particle cross the horizon and leaves the negative- energy particle stranded outside, since a negative-energy particle can't continue to exist outside the horizon for a time longer than h/E. So the black hole can lose energy to vacuum fluctuations, but it can't gain energy.

I agree that negative energy particles don't exist, but according to this description of the Hawking effect, they should still be produced. Is the suggestion that black holes simply don't eat positive energy particles?