255
See also: Curves in spacetime 154–55 ■ The life cycles of stars 178 ■
The heart of the Milky Way 297 ■ Michell (Directory) 335T
he mathematical theory of
black holes was pioneered
in the 1910s by the German
physicist Karl Schwarzschild.
The object described by
Schwarzschild was a non-rotating
mass concentrated at a point of
infinite density, called a singularity.
At a distance from this, known as
the Schwarzschild radius, was an
imaginary spherical surface called
the event horizon. The gravity on
the singularity side of this surface
was so great that nothing—not even
light—could escape. In subsequent
decades, black hole theory evolved
in various ways, but black holes
continued to be regarded as
entirely black, emitting no light.Virtual particles
In 1974, a big change occurred in
black hole theory. British physicist
Stephen Hawking proposed that
black holes emit particles, known
today as Hawking radiation.
Hawking maintained that black
holes are not completely black,
since they emit radiation of some
sort, if not necessarily light.Quantum theory predicts that,
throughout space, pairs of “virtual”
particles and their antiparticles
should continually appear out of
nothing, then annihilate (cancel
out back to nothing). One of each
pair has positive energy, the other
negative energy.
Some of these particle–
antiparticle pairs will appear just
outside the event horizon of a black
hole. It is possible that one member
of the pair could escape—observed
as an emission of (positive) radiation
energy—while the other falls into the
black hole. In order to preserve the
same total energy in the system,
the particle that fell into the black
hole must have had negative energy.
This causes the black hole to slowly
lose mass-energy—a process called
black hole evaporation.
Hawking radiation remains a
theoretical prediction. If it proves
correct, it means that black holes
do not last forever, which has
implications regarding the fate
of the universe, since it had been
thought that black holes would be
among the last objects in existence. ■NEW WINDOWS ON THE UNIVERSE
BLACK HOLES
EMIT RADIATION
HAWKING RADIATION
IN CONTEXT
KEY ASTRONOMER
Stephen Hawking (1942–)
BEFORE
1916 Karl Schwarzschild
provides a solution to the field
equations of general relativity,
allowing him to describe the
gravitational field around a
black holelike object.
1963 New Zealand
mathematician Roy Kerr
describes the properties
of a rotating singularity.
1965 British mathematician
Roger Penrose shows that the
gravitational collapse of a giant
star could result in a singularity.
1967 American physicist John
Wheeler coins the term “black
hole” for the types of objects
described by Schwarzschild,
Kerr, Penrose, and others.
AFTER
2004 Stephen Hawking retracts
an earlier claim that any object
entering a black hole is
completely lost to the universe.