What is Hawking Radiation

[INRM]

I've heard of virtual particles... but I thought in order for a particle to pop out of nowhere, another particle somewhere's gotta disappear?

Am I wrong?

 

[pauldmin]

If you are referring to Hawking Radiation, then that is the theory that black holes will eventually lose energy.
I think what happens is that a particle and anti-particle both appear across the event horizon. One particle gets free and the other gets sucked in.

With quantum theory, there is apparently a small amount of time where particles are allowed to exist before disappearing again. ( In true Discworldian fashion, the universe eventually 'catches on' and sorts the problem out ).

I'm not an expert in this area, so perhaps some Physicists can clarify?

 

[trvlr2]

Try reading here:

http://www.math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html

 

[andyandy]

I started a short OP on Hawkings not too long back...

there's a great Horizon documentary on Hawkings here

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and this is given as the radiation from a Schwarzschild black hole, with temperature

[latex]$$T=\frac{\hbar c^3}{8\pi GMk}[/latex]

where [latex]$ \hbar[/latex] is the reduced Planck constant, c is the speed of light, k is the Boltzmann constant, G is the gravitational constant, and M is the mass of the black hole.

In the mid 1970s, Stephen Hawking showed that black holes eventually evaporate away in a steady stream of featureless radiation containing no information. But if a black hole has completely evaporated, where has the information about it gone? This long standing question is known as the black hole information paradox.

Now, Professor Braunstein and Dr Pati have ruled out the possibility that information might escape from the black hole but be somehow hidden in correlations between the Hawking radiation and the black hole’s internal state. Braunstein and Pati’s result demonstrates that the black hole information paradox is even more severe than previously believed.

Dr Pati said: "Our result shows that either quantum mechanics or Hawking’s analysis must break down, but it does not choose between these two possibilities."

Professor Braunstein said: "The no-hiding theorem provides new insight into the different laws governing classical and quantum information. It shows that there’s got to be new physics out there."

http://www.sciencedaily.com/releases...0227105134.htm

So, the below equation is referred to as leading to Hawkings' theory re the information paradox - ie that information could be lost in a black hole.

[latex]$$ S=\frac{kc^3A}{4\hbar G} $$[/latex]

Where S= entropy of a black hole k=Boltzman constant c=speed of light [latex]$\hbar$[/latex]=reduced planck's constant G=gravity constant A=surface area 4=a good old bog standard number

 

[wahrheit]

It seems that Google has pulled the video, "We're sorry, but this video may not be available."

[nitpick]
The professor's name is Hawking, not Hawkings.
[/nitpick]

 

[andyandy]

It seems that Google has pulled the video, "We're sorry, but this video may not be available."

oh well...it was a great documentary

[nitpick]
The professor's name is Hawking, not Hawkings.
[/nitpick]

is it? *checks* Oh yes! Well, Hawkings sounds more natural. He should change it

 

[Schneibster]

The basic idea is that a black hole has a temperature greater than absolute zero. If it has a temperature greater than absolute zero, it must radiate energy, as all things that have a temperature greater than absolute zero must. The temperature equation andyandy gave above is the temperature of the black hole's event horizon. If you're curious enough (and I get enough time), I'll show how he came to that conclusion; it is a fairly compelling chain of logic.

The virtual pair hypothesis, that is, the hypothesis that virtual pairs form near the event horizon and one member escapes while the other gets sucked in, is the proposed mechanism by which the radiation is supposed to occur; however, that is separate from the radiation hypothesis and was worked out later.

 

[Orangutan]

This is probably a good time to ask my question. I never understood exactly what particles are appearing and disappearing. Photons? Quarks? Something more elementary. I may have a follow on question but I think the answer to the above might help first.

(I have read "a brief history...", the "elegant universe" and the "fabric of the cosmos", but I am really a layman so as simple as you can please!


EDIT:
Disregard I was confusing Virtual particles with particle-antiparticle pairs.

My first question still stands. Is there a difference between the particles in a virtual particle pair?

 

[Orangutan]

Let me rephrase my question.

What kind of particles are created due to quantum fluctuations. What is the force that makes them normally annihilate each other?

 

[Big Al]

Let me rephrase my question.

What kind of particles are created due to quantum fluctuations. What is the force that makes them normally annihilate each other?

This is tied up with Heisenberg's uncertainty principle. Absolutely flat spacetime would violate this, so particle / antiparticle pairs pop into existence and mutually annihilate each other to "preserve the uncertainty". Mass-energy is conserved in this process. However, near a black hole, you may have a situation where the antiparticle is just inside the event horizon and the particle isn't.

The black hole snatches the antiparticle, losing that amount of mass, and the particle escapes. To all intents and purposes, it's as if the black whole has radiated a particle.

 

[Big Al]

particle / antiparticle pairs pop into existence and mutually annihilate each other to "preserve the uncertainty".

I don't want to imply teleology (that the universe has a purpose) here. It's just that there is inherent uncertainty which is normally undetectable. However, a very hard vacuum (and you don't get much harder vacuum than near a black hole) is seething with particles popping into and out of existence.

 

[Orangutan]

OK I dug my copy of "A Brief History" Out. I see that he talks about the ability for a virtual particle with negative energy to fall into the black hole. I don't understand why it is always the negative energy particle that has to fall in though? Why can't the positive energy one fall in and the negative energy one escape?

 

[INRM]

This is tied up with Heisenberg's uncertainty principle. Absolutely flat spacetime would violate this, so particle / antiparticle pairs pop into existence and mutually annihilate each other to "preserve the uncertainty". Mass-energy is conserved in this process. However, near a black hole, you may have a situation where the antiparticle is just inside the event horizon and the particle isn't.

The black hole snatches the antiparticle, losing that amount of mass, and the particle escapes. To all intents and purposes, it's as if the black whole has radiated a particle.

If the particles appear and then annihilate each other then disappear, does that mean two particles had to disappear somewhere else in order to pop up?

Okay, I'm trying to say this... if a particle pops into existance, doesn't that mean another particle has to disappear somewhere else? Isnt' there a conservation of mass rule? Or is there not?

Do the particles really pop up out of NOWHERE or do they pop in from another location-- Another universe?

 

[RecoveringYuppy]

The black hole is losing mass in this process. That conservation law is maintained in the "long" run.

 

[RecoveringYuppy]

The black hole snatches the antiparticle, losing that amount of mass, and the particle escapes. To all intents and purposes, it's as if the black whole has radiated a particle.

Doesn't really matter (NPI) whether it's the particle or the anti-particle that escapes does it?

 

[INRM]

RecoveringYuppy,

Are you sure that virtual particle thing actually does conform to the conservation of mass or not? Because some people claim that the universe started as a virtual particle that appeared out of nowhere then expanded into the universe as we know it.

That strikes me as totally being against the law of conservation of mass.

When one particle pair appears, does another particle disappear somewhere?


Tony,

 

[RecoveringYuppy]

Are you sure that virtual particle thing actually does conform to the conservation of mass or not?

Yes, to the highest degree that I'm sure about anything.

When one particle pair appears, does another particle disappear somewhere?

I don't think that particles necessarily appear in pairs. But in the case we are talking about, I think a particle pair appears near the event horizon and one of pair then falls back in to the black hole and the other leaves the black hole. What they do beyond that depends on exactly what particular particles they are.

 

[Cuddles]

Let me rephrase my question.

What kind of particles are created due to quantum fluctuations. What is the force that makes them normally annihilate each other?

Any particle can be created. However, the more massive it is, the less likely it is to occur, so electrons and positrons are by far the most common, being the lightest particles (excluding neutrinos).

OK I dug my copy of "A Brief History" Out. I see that he talks about the ability for a virtual particle with negative energy to fall into the black hole. I don't understand why it is always the negative energy particle that has to fall in though? Why can't the positive energy one fall in and the negative energy one escape?

Because they both have negative energy. The uncertainty principles can be expressed in several forms, one of which relates energy to time, so the bigger the uncertainty in energy, the shorter the time it can vary in. Essentially what happens with pair production is that the particles "borrow" energy, but for such a short time that the universe doesn't notice. Normally this is fine, and the universe continues not noticing, but when you put an event horizon nearby things can change. A black hole gains the energy of anything that crosses its event horizon. However, since these produced pairs have only "borrowed" energy they effectviely have negative energy, and therefore the black hole gains negative energy, which is actually a reduction in mass. Of course, this is very much a simplification, but hopefully its still accurate enough to be understandable.

Schneibster's explanation about black-body radiation is also a useful way of looking at it without worrying about particle production at all. All matter must have a temperature above absolute zero, and anything with a temperature above zero must emit radiation. Therefore black holes must emit radiation. Finding a method for them to do so is just the icing on the cake. Again, a simplification, but a useful one.

If the particles appear and then annihilate each other then disappear, does that mean two particles had to disappear somewhere else in order to pop up?

Okay, I'm trying to say this... if a particle pops into existance, doesn't that mean another particle has to disappear somewhere else? Isnt' there a conservation of mass rule? Or is there not?

Do the particles really pop up out of NOWHERE or do they pop in from another location-- Another universe?

They pop out of nowhere. There is conservation of mass, but this is where the uncertainty principles come in. All conservation laws can be violated, but only within the bounds of the uncertainty principle, so energy can appear out of nowhere, but only for a very short period of time. It is only when you start throwing in things like black holes that things start getting really weird.

I don't think that particles necessarily appear in pairs.

Yes, they must always appear in pairs, a particle and its anti-particle.

 

[RecoveringYuppy]

Yes, they must always appear in pairs, a particle and its anti-particle.

Why? What conservation law would be broken if a single photon appeared as a virtual particle?

 

[INRM]

So you're saying a particle can appear out of nowhere, and then expand into the universe as we know it?

Tony L,
BTW: How do you know the particle appears out of nowhere and not from another universe?