Not so black after all

Twenty five years ago Stephen Hawking discovered that black
holes are not completely black after all, as viewed from outside.
He realized that a quantum process known as pair creation can
cause a black hole to leak its energy very slowly out into space. As
it does this, it will gradually shrink in size until it finally explodes
and nothing is left. It is therefore not true to say that what falls into
a black hole never comes out. It will, eventually, be radiated out
as tiny particles over a period that, for all intents and purposes,
can be considered to take forever!
This process is called Hawking radiation and requires a little
explanation.
In the subatomic world, two particles, such as an electron and
its antimatter particle9 (called a positron) can spontaneously pop
into existence out of complete nothingness. Quantum mechanics
says this is allowed as long as the two particles recombine very quickly in aprocess called pair annihilation, to give back the energy
they must have needed to form in the first place. I know this will
not make sense. In fact, it should not make sense, butmanystrange
things can and do happen in the world of quantum mechanics. You
may well ask where the energy to create the particles came from in
the first place. The answer is that the energy can be ‘borrowed from
nowhere’ and must be given back to ‘nowhere’ very quickly since
nature does not enjoy being in debt for long. Hawkingrealized that
when a pair of particles is created very close to the event horizon
of a black hole it could be that either the particle or its antiparticle
might fall into the hole while the other is able to escape. Since it can
no longer annihilate with its friend, it is allowed, like Pinocchio,
to become a real live electron or positron. The energy it has kept
and does not have to pay back has come from the black hole itself.
Hawking radiation must go on all the time just outside the
horizon of black holes, but only in a very small number of cases
will one of the particles escape. Most of the time both will fall into
the black hole. The reason one of the particles can ever get away is
due to tidal forces: the particle that is slightly closer to the horizon
is pulled much more strongly towards it.
For normal-sized black holes this process can be completely
ignored since far more particles will be sucked in from the space
surrounding the horizon than are ever lost through Hawking
radiation. The effect only becomes significant enough to be
interesting when a black hole has shrunk down to microscopic
size (after a time much longer than the life of the Universe) when
the region outside its horizon heats up and the radiation gets
very intense. This shrinking of a black hole is due to it obeying
Einstein’s famous equation E = mc2 which states that mass (m)
and energy (E) are interchangeable and one can be converted into
the other. This process is happening twice here. First, the escaping
particle has been endowed with mass (given substance) converted
from pure energy which has been extracted from the black hole.
Then, the hole itself, having lost this energy by being responsible
for the creation of a particle, must produce that energy by giving
up a minute fraction of its own mass. Viewed from afar we would
say that the black hole has spat out a particle. Its event horizon appears hot due to its radiation of these particles. But it is only
around very tiny black holes that this process is non-negligible.
It has been suggested that such mini black holes may actually
exist. They would have been created just after the Big Bang
and some might still be around today. As an example, if Mount
Everest could be squeezed down to the size of an atom it would
be converted into such a mini black hole. It would then give off
Hawking radiation at a very high rate. Even so, it would not
completely evaporate for billions of years. Now it just so happens
that the Universe is about 15 billion years old, so any mini black
holes created in the early Universe with an initial mass equal to
that of Mount Everest will just about have completely evaporated
away by now. Since they radiate at an increasing rate as they
shrink, they should end in a tremendous final explosion of high
energy radiation. Believe it or not, astronomers are on the look-out
for such telltale bursts of radiation.
It may be that the energy of black holes can be extracted
artificially too. In this case it is the rotational energy that is milked.
The English mathematical physicist, and long-time collaborator of
Hawking, Roger Penrose, had proposed, even before Hawking’s
evaporating idea, that if an object enters the ergosphere of a
rotating black hole and then splits into two parts with one falling
into the hole, then the other half can be thrown clear with more
energy than it came in with. The energy it acquires has come from
the hole and will slow its rotation down slightly.
It may be that an advanced civilization that comes across a
black hole could utilize this method to turn it into an energy source.