Evaporating black holes?

Can the concepts of relativistic quantum field theory be carried over to curved space-times - the kind of space-times with gravitational sources described by general relativity? The answer is a cautious "yes". The most notable step in this direction was taken by the British physicist Stephen Hawking in the 1970s.

Hawking examined a model of quantum particles not in the gravity-free environment of special relativity, but in a space-time containing a black hole. The result was surprising: The mere presence of the black hole means that, even if at early times not a single particle was present, at late times, there is a steady stream of them escaping to infinity. More succinct and less abstract: a black hole emits quantum particles! This hypothetical radiation is nowadays known as "Hawking radiation"

The greater the mass of the black hole, the lesser the temperature and intensity of Hawking radiation. The following table shows a few black hole masses, corresponding Schwarzschild radii (a measure for the size of a spherical black hole) and the temperature, measured in Kelvin, of the radiation it emits. Each entry has as a background the characteristic colour of thermal radiation with the given temperature:

As the colours show: Astrophysical black holes, such as stellar and supermassive black holes, are indeed black. Black holes lighter than about a hundredth of the mass of the earth's moon, however, glow in the dark, and even lighter ones, a five-thousandth the mass of the moon, are white-hot objects, and look the part. For holes that are lighter still, most radiation is given of as UV radiation, X-rays or even highly energetic gamma radiation.

The table doesn't indicate intensities. In fact, the black holes shown are rather dark. For black holes with lesser masses, however, significant fractions of mass and energy are radiated away - the smaller the mass, the greater the power. This leads to a runaway process and to a final, gigantic flash of energy in which the black hole evaporates.

If, in our universe's fiery youth, "mini black holes" of very little mass had formed, some might now have reached the stage where such violent evaporation occurs. So far, though, no astronomical evidence for highly energetic evaporation processes has been found, and Hawking radiation remains purely theoretical.

Calculations like Hawking's initial derivation of radiating black holes describe matter in quantum terms, but use the concepts of classical general relativity to describe their space-time environment. However, there are situations when this semi-classical treatment is insufficient. To fully understand our universe, these situations indicate, it is necessary to formulate a quantum theory of gravity, with space and time subject to the laws of the quantum world as well.

Next page: Border regions of gravity