Hawking Radiation?

In this thread about the growth of black holes some esteemed board members mention that black holes radiate something called Hawking Radiation.

I thought black holes were so massive that no energy or matter could escape, and hence the “black” affixed to the hole in their name. Can anyone fill me in (using small words please :slight_smile: ) on how a black hole can radiate and what Hawking Radiation is?

Hawking figures that as particles of matter separate near or at the event horizon, some positively charged particles will escape (i.e., not get sucked in), while the negatively charged ones don’t. The black hole winds up eating all these negatives, so to speak, losing (radiating) mass over time. I’m sure this is the simple version, and I’m not sure if it’s 100% accurate, but you get the gist. You can find better explanations using any search engine.

I believe a key element in flyboy’s explanation is to recognize that the particle pairs being formed are so-called virtual particles.

Virtual particle pairs form everywhere, even in a vacuum. Normally they disappear back into nothingness so quickly that no conservation laws (eg. energy) are violated.

If a particle pair is created at the boundary of a black hole (the event horizon), it is possible for one of the particles to fall into the hole, while the other escapes. We see this escaping particle as radiation being emitted from the black hole.

Conservation of energy/mass is said to be fulfilled in this scheme by the fact that the black hole loses the mass of the escaped particle. In such a way, a black hole can eventually lose sufficient mass to no longer be a black hole! It evaporates.

In case you’re wondering how some of those virtual particles make it outside the event horizon when that’s not supposed to happen, the answer is that only in classical physics is nothing allowed to come back out of a black hole. Generally speaking, it is a property of quantum mechanics that particles always have a small probability of going places that would be forbidden in an analogous classical system. This phenomenon is called quantum tunneling, and my explanation glosses over all sorts of details, but that’s basically what’s happening–Hawking radiation consists of virtual particles that happened to tunnel outside the event horizon.

Ah, cool! Thanks for your explanations. Is Hawking Radiation actually detectable or is it too weak?

It’s considered to depend on the effective temperature of the black hole, which in turn is inversely proportional to, IIRC, the third power of the mass (there’s a proportionality constant, but I definitely don’t remember what it is, and I’m not going to go hunting for it at this hour).

What this means that a black hole with the mass of, say, the Sun, would be far too “black” for us to notice the feeble radiation coming from it (in fact, since its effective temperature would be far below the mean temperature of the universe, it would capture more photons from the background radiation than it emitted through the Hawking process, and thus increase in mass). A black hole with the mass of a small asteroid would radiate noticeably, however, and “evaporate” via the Hawking process over the lifespan of the universe to date (thus putting paid to Hawking’s earlier notion that there would be “quantum black holes” left over from the Big Bang. They might have been created, but they’ve also since gone away). A very small black hole would, from our perspective, vanish almost immediately in a burst of (very) hard radiation (at one time, it was thought that this explained so-called “X-ray bursters”, but this is now considered to be wrong).

good explanations. here are some more details…