Giving Black Holes a Temperature
1974: Stephen Hawking
Just as astronomers and physicists were starting to accept the possibility that black holes could exist, young British physicist Stephen Hawking stirred things up a bit.
The basic concept says a black hole has such powerful gravity that matter and energy can fall in, but nothing can ever come out. But Hawking found a loophole. According to quantum mechanics, black holes could emit a form of radiation. Eventually, they would evaporate in a blaze of energy and particles.
Hawking radiation relies on the idea that even the vacuum of space isn't completely empty. Instead, pairs of particles are always popping into and out of existence. Known as "virtual" pairs, they consist of a particle of matter and particle of its antimatter equivalent, an antiparticle.
Normally, the particles appear then ram together and annihilate each other in a tiny fraction of a second. Hawking, however, said that something different can happen if the particles appear just beyond a black hole's event horizon (the point of no return for anything that falls into the black hole). One of the particles falls into the black hole, while the other, given permanent form by the black hole's gravitational field, remains outside.
To balance the cosmic ledger, the black hole expends work by breaking the two particles apart. The only energy it has at its disposal is its mass. Thus, the black hole has to give up a little bit of its mass to allow the one particle to fly free. In other words, swallowing one virtual particle drained away some of the black hole's mass.
If Hawking radiation exists, then black holes actually produce a sort of glow. Like every other object in the universe, that would give them a temperature. The temperature is so low, however, and the amount of radiation so minuscule, that it's impossible to detect.
Over time, a black hole that doesn't ingest any other matter or radiation will slowly decrease in mass and size, eventually reaching a point where it is no longer stable. It suddenly turns itself inside out, evaporating in a blaze of particles and energy.
For the ordinary black holes in our universe — those with anywhere from a few to a few billion times the mass of the Sun — this process would take many times the present age of the universe. Theory predicts, however, that the early universe could have created mini-black holes that are no more massive than a mountain. These black holes could evaporate in just a few billion years, producing torrents of gamma-rays. An orbiting gamma-ray observatory is looking for such outbursts.
Hawking Radiation: Pairs of ‘virtual’ particles constantly pop into existence, then come together to annihilate each other (top). If they appear just outside a black hole’s event horizon, however, one particle can fall into the black hole while the other escapes. This process adds ‘negative’ mass and energy to the black hole, essentially causing it to evaporate.