The theory that black holes emit radiation like any other hot
body. Virtual particle-antiparticle pairs are constantly being
created in supposedly empty space. Occasionally, a pair will
be created just outside the event horizon of a black hole.
There are three possibilities:
The first two cases are straightforward; the virtual
particle-antiparticle pair recombine and return their energy
back to the void via the uncertainty principle.
- both particles are captured by the hole;
- both particles escape the hole;
- one particle escapes while the other is captured.
It is the third case that interests us. In this case, one
of the particles has escaped (and is speeding away to infinity),
while the other has been captured by the hole. The escapee
becomes real and can now be detected by distant observers. But
the captured particle is still virtual; because of this, it has
to restore conservation of energy by assigning itself a
negative mass-energy. Since the hole has absorbed it, the hole
loses mass and thus appears to shrink. From a distance, it
appears as if the hole has emitted a particle and reduced in
The rate of power emission is proportional to the inverse
square of the hole's mass; thus, the smaller a hole gets, the
faster and faster it emits Hawking radiation. This leads to
a runaway process; what happens when the hole gets very small
is unclear; quantum theory seems to indicate that some kind
of "remnant" might be left behind after the hole has emitted
away all its mass-energy.