STELLAR BLACK HOLES
Astronomers
sense the presence of black holes throughout the universe. These fascinating bodies sit at the centers
of many galaxies, including our own Milky Way.
A black hole is pure gravity. The surface of a black hole, the “event
horizon,” is defined solely by a fierce gravitational attraction that traps
everything, including light. For an
object at the surface of a black hole the escape velocity is the speed of
light, therefore, nothing can escape a black hole’s event horizon. A black hole’s intense gravity bends light
beams and slows clocks.
Even though black holes seem
mysterious or bizarre, most professional astronomers embrace the concept of a
black hole. They see black-hole
formation as a natural and inevitable consequence of the deaths of massive
stars, and they readily appeal to supermassive black holes to explain the
extraordinary luminosities of quasars.
There is, however, a serious problem
with black holes, one that leaves some scientists sceptical about their
existence. The mystery lies hidden in
the hole’s center. Einstein’s general
theory of relativity predicts that we will find there an object more massive
than a million Earths and yet smaller than an atom. So small, in fact, that its density approaches infinity. Physicists dislike the idea of such an
infinitely dense point, which they call singularity, because no conceivable
theorem can describe what occurs there.
Observations of quasars, the centers
of nearby normal galaxies, and the nucleus of our own Milky Way suggest that
black holes really exist. The best
available evidence, however, comes from studies of three X-ray binary stars,
systems composed of a normal star and a supercompact, X-ray-emitting companion
in close orbit.
The first black-hole candidate, Cygnus
X-1 (the first X-ray source discovered in Cygnus), became famous the moment it
was identified in 1971. Anne Cowley and
her colleagues found the second probable black hole, LMC X-3, in the Large
Magellanic Cloud in 1983. More
recently, Ronald Remillard and Jeffrey McClintock discovered a third candidate:
A0620 – 00. The “A” refers to Ariel 5,
the British satellite that discovered the X-ray star, and the numbers indicate
the star’s position in the constellation Monoceros. It has a faint visual counterpart known as V616 Monocerotis.
The great outpouring of X-rays,
coupled with rapid variations in X-ray brightness, proves beyond doubt that
each of these three systems contains a compact object that is either a neutron
star or a black hole. Only such a tiny,
massive object can produce the power of 10,000 Suns and also flicker in a split
second.
The X-rays are produced when gas
captured from the normal star falls toward the collapsed companion, thereby converting
its gravitational potential energy into kinetic energy. Because the stars are in orbit about their
common center of mass and angular momentum must be conserved, the gas cannot
move directly from one to the other; instead, it forms an accretion disk around
the compact star. Collisions between
gas atoms gradually convert the ordered energy of orbital motion into the
distorted energy of heat and cause the material in the disk to spiral slowly
inward. Near the neutron star or black
hole the gas temperature rises to millions of degrees, and this heat energy is
radiated as X-rays.
Because these compact stars are in
binary systems, it has been possible to determine their masses by measuring the
velocities and orbital periods of their visible companions. This procedure, a cornerstone of 20th-century
astronomy, has been applied to thousands of ordinary binaries and is valid
regardless of the nature of the unseen star.
By this means, astronomers have convinced themselves that each of these
three X-ray stars is more massive than three Suns, the accepted upper limit or
a neutron star. In fact, the most
likely mass of each is closer to 10 Suns.
In summary, the argument for black
holes in Cygnus X-1, LMC X-3, and 0620 – 00 is the following: prodigious, flickering
X-rays reveal the presence of an extracompact star; the rapid orbital motion of
the visible companion implies that this X-ray star overweighs three Suns; and
relativity theory rules that nothing can keep such a massive, compact star from
collapsing to form a black hole.
The observational evidence is of high
quality, but the argument is indirect; it is not based on observations of
super-strong gravity effects like extreme light bending and the freezing of
time, phenomena peculiar to black holes alone.
No theory, not even general
relativity, can prove by itself that stellar black holes or any other kind
exist. General relativity does,
however, allow that they may exist, and it strongly favours their formation by predicting
a definite limit to the outward pressure a star can gather to prevent
gravitational collapse. Also, we have
found stars that are almost certainly black holes if general relativity is
valid.
Despite all this, there is good cause
for scepticism. That’s because the
argument sketched has a flaw: general relativity has been tested only in weak
gravity, gravity one millionth as strong as that at the surface of a black
hole. Therefore our reasoning is
circular: we presume that Einstein’s theory correctly describes strong gravity
when we argue that certain X-ray stars are black holes, yet, at the same time,
these alleged black holes are the test of Einstein’s theory of strong gravity.
There are two possibilities: general relativity is right about black holes, or it is wrong. It is entirely possible that Einstein’s theory is wrong, that it breaks down near the surface of a black hole. For 70 years Einstein’s theory has passed all weak-gravity tests, but it is only the simplest of many possible theories of curved space-time. This would pose quite a challenge, because it would leave physicists with the task of formulating a new gravitational theory to describe massive X-ray stars and quasars. Even more important, cosmological models of the evolution of our universe must be founded on a correct description of strong gravity. Of course, Einstein may have been right about black holes. That too would leave us with a considerable difficulty: the dreaded singularity. If he was right, it will demonstrate that purely deductive thought can lead us anywhere.
BIBLIOGRAPHY
Lasota, Jean-Pierre. “Unmasking Black Holes.” Scientific American. May 99, 40. McClintock, Jeffrey. “Do Black Holes Exist?” Sky and Telescope. Jan. 88, 28.
