As I understand it, Hawking radiation is caused when pairs of virtual particles appear near the event horizon of a black hole. Normally these particles would annihilate each other a short time later, but in this case, one of the pair passes through the event horizon while the other passes off into space theoretically detectable as radiation seemingly emitted from the hole. If that is the case, and I am not saying it is, how can a black hole evaporate via Hawking radiation? It would seem to be gaining mass. What is the nature of the particle pair anyway? Does one have negative mass? Do they come from a photon or do they just appear from nothing? If it is the latter, how do they annihilate (under normal circumstances) without emitting radiation? Wouldn’t we expect half of the particles to be of one type and the other half to be their corresponding antiparticles?
Until someone who knows more than me comes along (i.e. knows what they’re talking about), I’ll note that the virtual particles have come into existence by “borrowing” energy from the vacuum. Normally, they give back that energy as they disappear out of existence. But, when one flies off, that’s not possible. So, the energy debt comes from the black hole instead (and energy = mass, so the black hole gets smaller). I think.
When the pair pops into existence, one is a particle and the other is an antiparticle. The idea is that the antiparticle goes into the black hole, effectively annihilating some of the mass of the black hole, while the particle flies away. From the outside, it looks awfully like a particle has left the black hole.
Why it’s more likely for the antiparticle to enter the event horizon is beyond me.
It doesn’t matter whether the particle or anti-particle enters the black hole, it’s still losing mass. Anti-particles have positive energy.
Also, the photon is its own anti-particle, and most of the Hawking radiation is photons, not massive particles. (At least until you’ve got a small black hole. Smaller black holes give off more Hawking radiation than larger ones.)
Black holes should emit “normal” particles and antiparticles in about equal numbers (any given particle emitted is equally likely to be either). And if neutrinos have a low enough mass, then you’ll actually get more neutrinos emitted than photons, but I’d say that seems unlikely. Gravitons are also a significant component of Hawking radiation for plausibly-sized holes, but like photons, they’re their own antiparticle.
If you had a small enough black hole, though, such as might be left over from the beginning of the Universe, it could emit electrons and positrons (and eventually, protons and antiprotons), and would emit the same number of each. This is true regardless of whether the hole originally formed from normal matter, antimatter, some combination of both, or something that can’t be neatly pigeonholed into either category. Black holes “have no hair”, which means that most properties stuff can have, black holes don’t care about (in particular, the property of being matter or antimatter).
What about the idea that antimatter that crosses the event horizon annihilates matter inside the black hole which leads to a net loss of mass? Is that accurate?
Also, a couple of tangential questions: if photons are their own antiparticle, how can we get light from the Sun? Also, are gravitons and Higgs bosons the same thing?
Because there’s nowhere for the mass to have come from. At the end of the day, the books have to balance: If there’s a particle with mass flying away from the hole, then the hole had to lose mass.
Not in the slightest. If there even is anything inside the hole that can be called “matter”, it can’t be described as “normal matter” or “antimatter”: The hole is identical either way. Nor does matter-antimatter annihilation change the mass of a system (both have positive mass), nor is it only antimatter that goes in in Hawking radiation.
Why wouldn’t we be able to?
No, not at all. A graviton is a massless particle similar in many ways to a photon, and there’s no hope of us ever detecting individual gravitons. A Higgs boson has a very high mass, and we should probably detect them within a year or so at the LHC.
No, though that’s more intuitive than what actually happens. If a particle and antiparticle annihilate, their combined mass is released as energy. That energy can’t escape from the black hole, either, and it is equivalent to mass. From outside the black hole, we can’t tell the difference. From Chronos’ posts, I gather that matter does not even exist as such in a black hole–only mass. If you had a black hole formed entirely from mass X of matter, and dropped mass X of precisely opposed antimatter into it, you would end up with a black hole of mass 2X.
My understanding (which is admittedly somewhat dubious, and I hope someone more knowledgeable will correct me if I go astray) is more like this:
As Karl said above, virtual particle pairs form by “borrowing” energy from local space. (I prefer to think of it as condensation, but the “borrowing” concept works better for this.) The net effect on local gravitation is nil, since the energy was already present; it has merely changed forms. If the virtual particles form right at the event horizon of a black hole, they effectively “borrow” two particles’ worth of energy from the black hole. If tidal forces prevent the pair from annihilating, causing one particle to fall into the black hole while the other is flung away, the black hole only regains one particle worth of mass. That means it suffers a net loss of mass.
I suppose role of tidal forces in separating the pairs is the reason why smaller black holes evaporate faster.
After years of hearing about this, I’ve just realized one thing.
This effect assumes(or at least the simple explanation requires) that the event horizon itself is a razor sharp boundary. Or at least significantly more razor sharp than what you would expect of pair particle-antiparticles forming from vacuum energy and the distances they form/can get apart.
Given nothing else in physics is “razor” sharp at this scale, it makes me wonder…
The event horizon is a sharp line I suppose (figuratively speaking) where light cannot possibly escape.
However, I think you could get stuck “in” a black hole further away if you do not have enough energy to extract yourself. So particles with varying energy and (presumably) trajectories may or not make it away from the black hole. As such I suppose the boundary where they fall in might be a little fuzzy.
But both particles originated outside the hole, didn’t they?
Because presumably each pair of photons streaming away from the sun would annihilate. Is it because those photons came (ultimately) from the binding energy of various nuclei and as such they don’t have to destroy each other to conserve mass?
I know a photon of sufficient energy can spontaneously form an electron-positron pair which shortly annihilate one another to form a photon of equivalent energy to the original photon. But in the case of Hawking radiation-type particle appearance (whatever it’s called) the two particles come from nothing, Is it the case that photons from a star can combine into a single photon whose energy is the sum of the originals, but that photons created by this “boiling out of space” phenomenon combine to make nothing? Wouldn’t one of them have to have negative energy or mass?
Matter-antimatter annihilation doesn’t mean that the particles disappear without a trace. If you put together, say, an electron and a positron, what you’ll get out is a pair of photons. If, on the other hand, you put together a photon and an antiphoton (which is to say, another photon), then what you’ll get out is a pair of photons.
Two photons of enough energy can combine to become an electron-positron pair. A single photon can’t do that by itself no matter what its energy, because it wouldn’t be possible to conserve both energy and momentum. Similarly, two photons couldn’t combine into one, or one split into two, for the same reason.
The math is way beyond me, so I have to trust that all the people who do understand it and have checked his math and say he got it right are right about that fact. Also, since all the black holes we can observe are so far away, and so big that their Hawking Radiation is trivial, no-one has ever observed it. What evidence the theory has is weak and indirect, so the theory hasn’t really been well established yet. The next generation of big telescopes are hoped to be able to observe directly the event horizon of at least some of the black holes we know about, so maybe they will give us some good evidence one way or the other.
For other commenters: Gravitational fields have energy, and energy is mass, therefore high gravitational fields are slightly stronger than Newton predicts. Thats what causes Mercury’s orbit to precess, and light just grazing the sun from other stars during an eclipse to appear bent more than Newton predicts. The energy of the very high gravity field near the event horizon is what forms virtual particles. If one is sucked into the black hole, the other can’t recombine, and so becomes a real particle, which takes energy away from the black hole’s gravitational field, which therefore takes away some mass from the black hole.
In case you’re wondering about my qualifications, I’m an amateur astronomer, and I read about this stuff all the time. Take what I say with a pinch of salt as the understanding of an interested layman, but apparently I’m the best you have on this thread until and unless a pro comes along.
I’m pretty sure Chronos is a pro (a physics pro, not a prostitute pro, though what he does in his spare time to make ends meet is really none of my business, and what’s wrong with being a male prostitute anyway? A perfectly acceptable second job for a physicist.)
There might be cases in which the pair actually forms directly across the event horizon, but generally the pairs under discussion would manifest outside the horizon. They don’t move out of the hole, they appear near it (and everywhere else, constantly, according to the theory). Their appearance is a quantum mechanical effect involving lots of math that I don’t remember how to do. A new pair represents a certain amount of energy; that energy has to come from somewhere, and in pairs that have the potential to contribute to Hawking radiation, that somewhere is the black hole.
How that works exactly, I’m not sure. I’ve seen it explained as the creation of virtual particles leaving an equal-but-opposite negative mass/energy thingy, which then falls into the black hole, offsetting its mass (as opposed to annihilating it, which doesn’t work, as we discussed above). I don’t find this very satisfying, except perhaps as a metaphor, but I’m not a physicist. The point is that there is a quantum-mechanical migration of energy across the event horizon from inside, and sometimes part of that energy escapes.
Bear in mind that we don’t have a consistent theory of quantum gravity, and that quantum mechanics and general relativity become inconsistent with each other near singularities. As far as I know, we have no direct evidence that Hawking radiation exists, though there’s some circumstantial evidence for some of the underpinnings of the theory (specifically, the Unruh Effect, under certain conditions).
Oh, and Chronos is definitely a pro, to whom I defer on all physics matters. I’m just trying to translate stuff into terms slightly more comprehensible to the rest of us, hopefully without screwing it up too badly.
Well, I’m a physics graduate student, at least, and my specialty is relativity. Take that for whatever it’s worth. And nothing that Cheshire Human just said is incorrect, though I should stress (lest anyone get the wrong impression) that we’re nowhere near having telescopes capable of observing Hawking radiation directly. Also, although Hawking radiation has not been observed directly, there are enough different independent derivations of it that we can be pretty confident that it exists (or at least, if it doesn’t, then absolutely everything we know about black holes and/or quantum mechanics is wrong).