|Even space-time is stretched in gravitational fields.
Time in a reference system which is situated in such a field passes more
slowly compared to a reference system situated in weightlessness.
What this means is that the intervals of a regularly sent
out signal perceived by a distant observer becomes larger the nearer
to the source comes to the event horizon. Imagine if the astronaut were to wave
to the observeron his trip to the event horizon.
As he moves nearer to the horizon, his hand would move more and more slowly
until his waving is frozen in as a result of the Doppler effect.
|When the astronaut is accelerated to almost light
speed when he approaches the horizon, it looks for our observer as if the
astronaut was flattened like a pancake and spread on the horizon's surface.
The optical effect of flattening is created when a solid is accelerated
to almost 100% of light speed. Almost 100% because nothing except light
can have a speed of 100% of light speed. Einstein found
that the mass of the accelerated solid becomes larger as it
approaches the speed light. If one flew as fast as light, one
would be infinitely heavy - which is impossible.
|Here we recognize the importance
of the choice of a reference system in relativistic discussions: The astronaut,
as far as he is concerned, perceives the complete voyage as nothing special.
To him, in his own reference system, time passes normally. He keeps waving
even after having crossed the event horizon. Some time after he had passed
the horizon, and he approaches the singularity, its force of attraction
becomes so extreme that the astronaut is torn to pieces after his body
has already been stretched in his length.
|The astronaut's mass increases the black hole's mass,
hence its radius becomes larger. One could almost believe that the cosmic
vacuum cleaner doesn't have a choice but to become bigger and heavier by
the absorption of matter. Stephen Hawking however showed in 1974 that black
holes can lose mass - in the form of radiation, the so-called Hawking radiation.
|Two years earlier, Jacob Bekenstein, a teacher
of Wheeler's, had been of the following opinion: As black holes absorb
all the information about mass, electrical charge, and angular momentum
of every particle they swallow, there had to be a certain number of possibilities
Fto distribute this information in (or better on) the black hole.
|This number corresponded, so Bekenstein, to the entropy,
a measure for disorder within a volume of space. He believed that there
was a direct link between entropy and the size of the event horizon's surface.
But for Hawking the existence of entropy in a black hole was not possible.
He decided to disprove Bekensteins assumption - and failed. He had to admit
that black holes - rotating or not - possessed entropy.
|The laws of thermodynamics which describe the statistical,
random behavior of a great number of atoms, expressed that any solid possessing
entropy had autmatically a temperature. And when something has a temperature,
it had to send out radiation. So, was it possible that radiation could
escape from black holes?
|In 1974 Hawking proved that it was. He assumed that
spontaneously appearing pairs of particles - which are not determinable
directly but which had to exist as laws of quantum physics (the physics
of sub- atomic ranges) state - existed in the vicinity of black holes,
too. The energies of two suddenly created "virtual" particles
always add to an equilibrium. The sum of a positively and a negatively
charged particle always equals zero.
|When both materialize out of nothing, and part for
a brief moment and fuse again, the equilibrium of energy in space remains
constant. But if this process takes place near an event horizon, it is
possible that the black hole's gravitational field makes the virtual particles
become real particles. Then, both of them can exist independently of each
other and needn't join a common combination. Because of the process of
changing the particles' characteristics from virtual to real, the black
hole loses energy and hence mass - it shrinks.
|The two particles go own ways: one will fall into
the hole, the other one will escape from the gravitaional forces into space
in the form of radiation.
|Hawking could prove that the higher the temperature of
a black hole, the lower its mass. If the hole becomes smaller, the velocity
of the emission of particles raises. This follows that it loses mass and
becomes smaller, and so on, and so on. This happens until a black hole which is as heavy
as 10 suns evaporates, a process that takes almost 10^70 years. This amount of
time exceeds the age of our universe, which is 10^10 years, by about 10^60 times.
|Primordial black holes which were not created of
stars but during the big bang radiate more intensely. They were formed
when the pressure was high enough to compress comparably little matter
to singularities. Today processes that show lightweight samples die (10^12
kg, sun: 10^30 kg) should be observable. 300 primordial black holes are
supposed to be situated in every cubic light year. When they explode they
set free gamma rays, which is better known as gamma ray bursts. These holes
are hardly detectable as they are only as big as an atomic nucleus. Astronomers
can determine them when their satellite telescopes in earth's orbit perceive
suddenly appearing gamma bursts in the sky.