There is interest on behalf of our readers to know more on the discovery by Professor Stephen Hawking that black holes radiate away, or equivalently, lose mass over time.
Black holes are, well, black. They are hard to find and even harder to study. A black hole does have a surface which is called an event horizon. Unlike a surface of any planet, however, the event horizon is a point of no return. Any object that falls inside of this surface is lost forever.
The popular version of the story of Hawking radiation starts by reminding us that particles are created and destroyed in pairs constantly all over space. The plot takes shape when one of these particle-antiparticle pairs appears very very near the event horizon. The fate of such an unlucky couple is that if one particle wanders out of the event horizon then it will escape, leaving the other one imprisoned behind the event horizon.
The plot is resolved by saying that this action helps a black hole to lose its mass, although the quick come-back is to ask how can a black hole loses mass by gaining particles. What are we missing here?
There are a couple details that are not mentioned in this fair tale that make all the difference. First, the particle-antiparticle pairs are virtual and not real. They can move wherever they want or as we say, their momenta are unconstrained. Second, an attribute of this ghostly virtual couple is that one particle has positive mass while the other has negative mass, and it is always the particle with the negative mass that stays behind.
In this sense, on that fateful day when the positive particle is freed, the particle with the negative mass falls in and in doing so “takes away” mass from the black hole. This “lost” mass appears, equivalently, in the form of thermal radiation, or Hawking radiation.
This process makes black holes disappear by evaporating away its own mass, and it requires astoundingly long periods of time. For a black hole the mass of the Sun it will take an amount of time in years equal to a one with 67 zeroes after it, or 10,000 billion billion billion billion billion billion billion years!
All of this comes, of course, with the proviso that I have tried to take a well-known cartoon and offer some band-aids to make it right. I did this by a series of interviews with my favorite string theorist, my husband. In actual fact, unfortunately a cartoon cannot capture the quantum mechanical effects on which Hawking radiation is based.