Hawking Radiation

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  • #26
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"One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy." (emphasis added)

Particle Creation by Black Holes
S. W. Hawking
Commun. math. Phys. 43, 199—220 (1975)
________

Without requiring virtual (pairs of) particles, perhaps Hawking is putting things in a way easy to view. Or perhaps he's hedging.
 
  • #27
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"One might picture this negative energy flux in the following way. Just outside the event horizon there will be virtual pairs of particles, one with negative energy and one with positive energy." (emphasis added)

Particle Creation by Black Holes
S. W. Hawking
Commun. math. Phys. 43, 199—220 (1975)
________

Without requiring virtual (pairs of) particles, perhaps Hawking is putting things in a way easy to view. Or perhaps he's hedging.
is there any other evidence for hawking radiation? i read back in october hawking radiation was observed.
 
  • #28
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does virtual particles traveling faster than light mean they can time travel to the past?
 
  • #29
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Post #24 is correct....Hawking himself supposedly explained the radiation via an intuitive approach using virtual particles.,,it reportedly did NOT come directly from his mathematical work. There is no "research" concerning virtual particles as a source of HAwking radiation. In fact Hawking radiation cannot occur until the universe becomes cooler than black holes, so it has NEVER been observed.

You can read a lot about Hawking radiation in Kip Thorne's BLACK HOLES AND TIME WARPS and Leonard Susskind'a THE BLACK HOLE WAR (between Susskind and Hawking).

Susskind (pgs 376- 380) explains a string version of Hawking radiation: If a short segment of string briefly and partially passes the horizon and crosses itself, string theory says a small percentage of the time a small loop can break free... This could be a photon, graviton or any other particle..... the remainder of the string remains inside the horizon.

These books are conceptual, not mathematical, intended for knowledgeable lay people.
 
  • #30
phinds
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... Hawking radiation cannot occur until the universe becomes cooler than black holes, so it has NEVER been observed.
I'm puzzled by this statement. Can you give a layman's explanation of it? What puzzles me is that the requirement that the universe reach a particular temperature seems to me to imply that the radiation is in some way thermal radiation, but thermal radiation does not totally depend on the surroundings. For example, my understanding is that a room-temperature object put into a 1000 degree kiln STILL radiates heat, it just radiates such a trivial amount relative to what the furnace is radiating into it (to say nothing of convection and conduction) that the result is irrelevant. BUT ... irrelevant is not non-existent.

Thanks
 
  • #31
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Hawking radiation cannot occur until the universe becomes cooler than black holes, so it has NEVER been observed.

I got that from reading Leonard Susskind's THE BLACK HOLE WAR,2008, PAGE 200:

...No astronomical black hole is currently evaporating. Quite the opposite, they are all absorbing energy and growing...The emptiest regions of instellar space are still far warmer than a stellar mass black hole......thermal energy always flows from hot to cold....so it follows that radiation from the warmer regions of space flow into the colder black holes...
 
  • #32
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Wikipedia says: Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter
http://en.wikipedia.org/wiki/Thermal_radiation

So we may have a semantics issue???

Under HAwking Radiation, Wikie says:

"Thus thermal radiation contains information about the body that emitted it, while Hawking radiation seems to contain no such information....."

but I'm not sure that is correct...now out of date...Susskind says that eventually at the end of the (cold dark) universe, all the information, though scambled, IS emitted....

I checked: Susskind bases that on work of Maldacena and Ed Witten. Apparently relativists (like HAwking) don't like such an idea (but I think he has accepted it) while string theorists like Strominger,Vafa,Polchinski and Horowitz accept it....

Thats a LOT more than I know....
 
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  • #33
phinds
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In fact Hawking radiation cannot occur until the universe becomes cooler than black holes, so it has NEVER been observed.
I still do not understand this statement and would appreciate knowing how/why you believe this. I can understand that if Hawking radiation is not thermal radiation or the equivalent, your statement might be true, but why would it be?

Thanks
 
  • #34
phinds
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I still do not understand this statement and would appreciate knowing how/why you believe this. I can understand that if Hawking radiation is not thermal radiation or the equivalent, your statement might be true, but why would it be?

Thanks
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  • #35
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I think the issue is that black holes put out such low amounts of radiation that they are drowned out by the background radiation. Given that the output is inverse to the mass, stellar black holes put out such little amounts that even in deep space the amount of energy absorbed from the background radiation is more than the black hole produces, so it would continue to grow because it is absorbing more energy than it is releasing.
 
  • #36
phinds
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I think the issue is that black holes put out such low amounts of radiation that they are drowned out by the background radiation. Given that the output is inverse to the mass, stellar black holes put out such little amounts that even in deep space the amount of energy absorbed from the background radiation is more than the black hole produces, so it would continue to grow because it is absorbing more energy than it is releasing.
Oh, that absolutely makes sense to me and is what I would expect. It was the statement that Hawking radiation
cannot occur until the universe becomes cooler than black holes
that I was questioning.

Thanks for the reply.
 
  • #37
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I think the issue is that black holes put out such low amounts of radiation that they are drowned out by the background radiation. Given that the output is inverse to the mass, stellar black holes put out such little amounts that even in deep space the amount of energy absorbed from the background radiation is more than the black hole produces, so it would continue to grow because it is absorbing more energy than it is releasing.
Interesting, though, this might not be the case for the "biggest" (strongest) of black holes (I'd guess the biggest one would radiate more than absorb from background radiation, if we imagine nothing else is around it to suck that in)?
 
  • #38
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Interesting, though, this might not be the case for the "biggest" (strongest) of black holes (I'd guess the biggest one would radiate more than absorb from background radiation, if we imagine nothing else is around it to suck that in)?
But there is radiation coming from distant stars (and from CMB), and this radiation will also be absorbed by the black hole. So again, it will absorb more than radiate.

In fact, if you want a black hole that radiates more than absorbs, then you need a SMALL black hole (not a big one), because smaller black hole has a larger temperature.
 
  • #39
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But there is radiation coming from distant stars (and from CMB), and this radiation will also be absorbed by the black hole. So again, it will absorb more than radiate.

In fact, if you want a black hole that radiates more than absorbs, then you need a SMALL black hole (not a big one), because smaller black hole has a larger temperature.
So, simulating this could be possible? (Aren't we creating miniscule black holes in particle accelerators?)
 
  • #40
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This is a great calculator for getting specifics relating to Hawking radiation and various sizes of black holes-

http://xaonon.dyndns.org/hawking/

as the last statement says, 'Clearly for a solar-mass hole the lifetime is essentially infinite. In fact, for a large enough hole (such that T < 2.726 Kelvin, or M > 0.75% the mass of the Earth) the hole will actually grow slightly by feeding on cosmic background radiation. Only when the universe cools below the hole's Hawking temperature will it start to shrink.'

So basically, based on the current temperature of the universe, only if a black hole is 0.75 times (or smaller) the mass of the Earth will it display Hawking Radiation.
 
  • #41
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So, simulating this could be possible?
In principle, yes.

(Aren't we creating miniscule black holes in particle accelerators?)
So far, there is no experimental evidence that we do. Theories that predict that we should involve some rather exotic assumptions, such as large extra dimensions.
 
  • #42
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Actually, I don't quite understand how can physicist know whether it is hawking radiation or not? The accretion disc is emitting a whole spectrum of light, so how to determine?
 
  • #43
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About analogue Hawking radiation, not about true Hawking radiation. Do you know what "analogue" means?
Sure, it means analog. :smile:
 
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  • #44
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Actually, I don't quite understand how can physicist know whether it is hawking radiation or not? The accretion disc is emitting a whole spectrum of light, so how to determine?
There's another complication that people, including many on here, just don't seem to get.

ALL black holes (and some neutron stars) have a very large layer of photons orbiting them. Since the probability of the creation of matter/antimatter decreases as the distance from the energy source increases, it is likely that essentially all Hawking radiation (if it does indeed exist) would begin it's journey within this photon zone.

Reference for the flamers who don't know simple physics:
http://www.deepastronomy.com/travel-to-black-holes.html
 
  • #45
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But there is radiation coming from distant stars (and from CMB), and this radiation will also be absorbed by the black hole. So again, it will absorb more than radiate.

In fact, if you want a black hole that radiates more than absorbs, then you need a SMALL black hole (not a big one), because smaller black hole has a larger temperature.
Exactly,Which is why smaller B.H's evaporate easily.
 
  • #47
phinds
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So basically, based on the current temperature of the universe, only if a black hole is 0.75 times (or smaller) the mass of the Earth will it display Hawking Radiation.
No, you are making exactly the same point made earlier, that I questioned when it was made, and which is not true. You are mistaking the NET RESULT with the actual radiation. Yes, it will not decrease in mass due to Hawking radiation because it will absorb more than it will radiate, but that does NOT at all say there there is no Hawking radiation. This is NOT semantics. Either there IS Hawking radiation or there is not, and the bottom line is, there IS.
 

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