A type of radiation predicted in 1974 by Stephen Hawking to emerge continuously from black holes. Of pairs of particles produced by quantum effects in space near a black hole, one is absorbed by the black hole while the other is radiated. The theory predicts that black holes slowly evaporate into photons and other particles, finally expiring in a huge burst of gamma rays.
In physics, Hawking radiation is thermal radiation thought to be emitted by black holes due to quantum effects.
Overview
Black holes are sites of immense gravitational attraction into which surrounding matter is drawn by gravitational forces. Classically, the gravitation is so powerful that nothing, not even radiation or light (hence why it is black as no light is reflecting from it), can escape from the black hole. However, by doing a calculation in the framework of quantum field theory in curved spacetimes, Hawking showed that quantum effects allow black holes to emit radiation in a thermal spectrum. This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being "boosted" by the black hole's gravitation into becoming real particles.
A more precise, but still much simplified view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. By this process the black hole loses mass, and to an outside observer it would appear that the black hole has just emitted a particle.
An example
A black hole of one solar mass has a temperature of only 60 nanokelvins;
The Hawking radiation shows that the laws of black hole thermodynamics have a complete physical meaning.
Emission process
A black hole emits thermal radiation at a temperature
,in natural units with G, c, and k equal to 1, and where κ is the surface gravity of the horizon.
In particular, the radiation from a Schwarzschild black hole is black-body radiation with temperature:
where is the reduced Planck constant, c is the speed of light, k is the Boltzmann constant, G is the gravitational constant, and M is the mass of the black hole.
Black hole evaporation
When particles escape, the black hole loses a small amount of its energy and therefore of its mass (recall that mass and energy are related by Einstein's famous equation E = mc²).
The power emitted by a black hole in the form of Hawking radiation can easily be estimated for the simplest case of a nonrotating, non-charged Schwarzschild black hole of mass M. Combining the formulae for the Schwarzschild radius of the black hole, the Stefan-Boltzmann law of black-body radiation, the above formula for the temperature of the radiation, and the formula for the surface area of a sphere (the black hole's event horizon) we get:
where P is the energy outflow, is the reduced Planck constant, c is the speed of light, and G is the gravitational constant.
The power in the Hawking radiation from a solar mass black hole turns out to be a minuscule 10−28 watts.
Under the assumption of an otherwise empty universe, so that no matter or cosmic microwave background radiation falls into the black hole, it is possible to calculate how long it would take for the black hole to evaporate. The time that the black hole takes to evaporate is:
For a black hole of one solar mass (about 210 years—much longer than the current age of the universe.
In common units,
So, for instance, a 1 second-lived black hole has a mass of 2.28 × 10 J that could be released by 5 × 10 W.
Black hole evaporation has several significant consequences:
Black hole evaporation produces a more consistent view of black hole thermodynamics, by showing how black holes interact thermally with the rest of the universe. The simplest models of black hole evaporation lead to the black hole information paradox. The information content of a black hole appears to be lost when it evaporates, as under these models the Hawking radiation is random (containing no information).
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