Wednesday, 8 July 2009

Hawking Radiation

Hawking Radiation:
Hawking, an English theoretical physicist, was one of the first to consider the details of the behavior
of a black hole whose Schwarzschild radius was on the level of an atom. These black holes are not
necessarily low mass, for example, it requires 1 billion tons of matter to make a black hole the size of
a proton. But their small size means that their behavior is a mix of quantum mechanics rather than
Before black holes were discovered it was know that the collision of two photons can cause pair
production. This a direct example of converting energy into mass (unlike fission or fusion which turn
mass into energy). Pair production is one of the primary methods of forming matter in the early

Note that pair production is symmetric in that a matter and antimatter particle are produced (an
electron and an anti-electron (positron) in the above example).
Hawking showed that the strong gravitational gradients (tides) near black holes can also lead to pair
production. In this case, the gravitational energy of the black hole is converted into particles.

If the matter/anti-matter particle pair is produced below the event horizon, then particles remain
trapped within the black hole. But, if the pair is produced above the event horizon, it is possible for
one member to fall back into the black hole, the other to escape into space. Thus, the black hole can
lose mass by a quantum mechanical process of pair production outside of the event horizon.
The rate of pair production is stronger when the curvature of spacetime is high. Small black holes
have high curvature, so the rate of pair production is inversely proportional to the mass of the black
hole (this means its faster for smaller black holes). Thus, Hawking was able to show that the mini or
primordial black holes expected to form in the early Universe have since disintegrated, resolving the
dilemma of where all such mini-black holes are today.

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