How does Hawking radiation work ? My understanding is that particles and anti-particles form near the event horizon of a black hole, the anti-particle falls into the black hole, the particle doesn’t and eventually the black hole evapourates.
Why does are anti-particles more likely to fall into the BH, surely it would be 50-50 ?
It is 50-50. The black hole emits both sorts.
Then what causes the BH evapourate ?
Thanks.
The particles form in pairs to conserve energy, charge, and most importantly momentum. To conserve momentum, they must be moving in opposite directions from one another.
The case of interest is when they form on the horizon with one moving inward and the other outward. (which particle is in- or out-ward bound is random for any given pair). The one aimed outward escapes, the partner unfortunately does not. The hole has lost one particle (of random type).
In aggregate it loses a bunch of particles (roughly of 50-50 composition)
The black hole evaporates because it lost a net particle/antiparticle. A particle goes out into the universe, and since its partner virtual particle is now inside a singularity it can’t recombine back into nothing. Since nothing can come from nothing, the black hole has to pay for the free lunch the universe just got.
I’m still missing something. Doesn’t the BH only loose a particle if the anti-particle falls in ? So sometimes it looses a particle and sometimes it gains one, on average its mass is unchanged.
I think you are under the mistaken impression that anti-particles have negative mass/energy. Both particles and anti-particles have mass, thus if the black hole loses an anti-particle it loses some mass.
I am confused too about this explanation but I think I can clearly describe my confusion.
When the particle/anti-particle pair was created, where did it come from?
If it was created from energy/mass from the black hole, then it would be understandable that the loss of one particle would be “evaporation.” But without knowing how it was created, one might confuse this phenomenon with spontaneous creation of particles in a vacuum.
“Raaaaadiation! Get’cher radiation here! Fresh, hot radiation!”
I’m sorry…I’ll stop now.
You read my mind Hal, right down to the exact wording.
Is that a quote from something? 'Cause Google says it never heard of it.
Hal is hawking radiation, much like one might hawk any other product.
The answer to that is “no”. Virtual particles pop up all over the place, but the net energy of the particle-antiparticle pair is zero, and they promptly recombine back into nothing, so nobody gets too bothered by them. When one falls into a black hole, though, they can’t recombine, so the one that’s left can’t be virtual any more, so it has to have positive energy.
That’s the most common explanation, anyway. Personally, I much prefer the thermodynamic argument: The Second Law of Thermodynamics requires that black holes have entropy, since black holes are formed from other systems that have entropy. Everything that has an entropy has a temperature, so black holes have temperature. And everything that has a temperature radiates, so black holes radiate.
I think of it this way:
Virtual particles appear all the time. They’re always very close to each other, and always configured so that they can cancel each other other (no net spin, charge, etc.) In a pure, unadulterated vacuum, they form and disappear so quickly and with so little energy that we can’t prove they were ever there - and as long as we can’t prove it, the uncertainty principle lets them keep forming.
But a black hole is not unadulterated vacuum. It’s vacuum under an intense gravitational gradient. This gravity is able to spend energy to make the particles heavy/energetic enough to be real (that is, measurable) under the uncertainty principle, and I think it’s this separation that is really the source of the energy cost. The black hole reclaims some of the energy by pulling in one particle, but the particle that escapes represents a net outflow of energy. (If it didn’t have more energy than the other particle, it wouldn’t have been the one to escape).
I don’t know if that’s helping to make it more clear. Wikipedia’s phrasing for this is “This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being “boosted” by the black hole’s gravitation into becoming real particles.”
Didn’t Hawking come out and say the the black hole is not emitting radiation, and that it just sits above it?
I am being very simple here, but if we are talking about Hawking Radiation, we can’t talk about a black hole emitting it.
Well, it only occurs where there’s a black hole, and the net effect is that the black hole loses mass. Call it what you like, but that quacks like the hole itself emitting the radiation.
It seems to me that the black hole is inducing radiation by its influence rather than emitting it directly. Am I anywhere close to understanding it?
I would agree. “Emitting” is the problem word. Going forward, it might be best not to connotate it this way.
Old post of mine:
I just realized about all my post did was say the same thing that dracoi said.
As far as the emitting thing, you have to remember that the event horizon is basically a thermodynamic membrane, and therefore is most certainly radiating. Whether or not you call these radiated particles emitted or not seems to be a matter of semantics.