rotation is non-zero, the black hole bulges outward near its equator (just as the earth or the sun bulge due to their
rotation), and the faster it rotates, the more it bulges. So, to extend Israel’s result to include rotating bodies, it was
conjectured that any rotating body that collapsed to form a black hole would eventually settle down to a stationary state
described by the Kerr solution. In 1970 a colleague and fellow research student of mine at Cambridge, Brandon Carter,
took the first step toward proving this conjecture. He showed that, provided a stationary rotating black hole had an axis
of symmetry, like a spinning top, its size and shape would depend only on its mass and rate of rotation. Then, in 1971, I
proved that any stationary rotating black hole would indeed have such an axis of symmetry. Finally, in 1973, David
Robinson at Kings College, London, used Carter’s and my results to show that the conjecture had been correct: such a
black hole had indeed to be the Kerr solution. So after gravitational collapse a black hole must settle down into a state
in which it could be rotating, but not pulsating. Moreover, its size and shape would depend only on its mass and rate of
rotation, and not on the nature of the body that had collapsed to form it. This result became known by the maxim: “A
black hole has no hair.” The “no hair” theorem is of great practical importance, because it so greatly restricts the
possible types of black holes. One can therefore make detailed models of objects that might contain black holes and
compare the predictions of the models with observations. It also means that a very large amount of information about
the body that has collapsed must be lost when a black hole is formed, because afterward all we can possibly measure
about the body is its mass and rate of rotation. The significance of this will be seen in the next chapter.
Black holes are one of only a fairly small number of cases in the history of science in which a theory was developed in
great detail as a mathematical model before there was any evidence from observations that it was correct. Indeed, this
used to be the main argument of opponents of black holes: how could one believe in objects for which the only
evidence was calculations based on the dubious theory of general relativity? In 1963, however, Maarten Schmidt, an
astronomer at the Palomar Observatory in California, measured the red shift of a faint starlike object in the direction of
the source of radio waves called 3C273 (that is, source number 273 in the third Cambridge catalogue of radio sources).
He found it was too large to be caused by a gravitational field: if it had been a gravitational red shift, the object would
have to be so massive and so near to us that it would disturb the orbits of planets in the Solar System. This suggested
that the red shift was instead caused by the expansion of the universe, which, in turn, meant that the object was a very
long distance away. And to be visible at such a great distance, the object must be very bright, must, in other words, be
emitting a huge amount of energy. The only mechanism that people could think of that would produce such large
quantities of energy seemed to be the gravitational collapse not just of a star but of a whole central region of a galaxy. A
number of other similar “quasi-stellar objects,” or quasars, have been discovered, all with large red shifts. But they are
all too far away and therefore too difficult to observe to provide conclusive evidence of black holes.
Further encouragement for the existence of black holes came in 1967 with the discovery by a research student at
Cambridge, Jocelyn Bell-Burnell, of objects in the sky that were emitting regular pulses of radio waves. At first Bell and
her supervisor, Antony Hewish, thought they might have made contact with an alien civilization in the galaxy! Indeed, at
the seminar at which they announced their discovery, I remember that they called the first four sources to be found
LGM 1 – 4, LGM standing for “Little Green Men.” In the end, however, they and everyone else came to the less
romantic conclusion that these objects, which were given the name pulsars, were in fact rotating neutron stars that were
emitting pulses of radio waves because of a complicated interaction between their magnetic fields and surrounding
matter. This was bad news for writers of space westerns, but very hopeful for the small number of us who believed in
black holes at that time: it was the first positive evidence that neutron stars existed. A neutron star has a radius of about
ten miles, only a few times the critical radius at which a star becomes a black hole. If a star could collapse to such a
small size, it is not unreasonable to expect that other stars could collapse to even smaller size and become black holes.
How could we hope to detect a black hole, as by its very definition it does not emit any light? It might seem a bit like
looking for a black cat in a coal cellar. Fortunately, there is a way. As John Michell pointed out in his pioneering paper in
1783, a black hole still exerts a gravitational fierce on nearby objects. Astronomers have observed many systems in
which two stars orbit around each other, attracted toward each other by gravity. They also observe systems in which
there is only one visible star that is orbiting around some unseen companion. One cannot, of course, immediately
conclude that the companion is a black hole: it might merely be a star that is too faint to be seen. However, some of
these systems, like the one called Cygnus X-1 Figure 6:2, are also strong sources of X-rays.
Figure 6:2
The best explanation for this phenomenon is that matter has been blown off the surface of the visible star. As it falls
toward the unseen companion, it develops a spiral motion (rather like water running out of a bath), and it gets very hot,
emitting X-rays Figure 6:3.
Figure 6:3
For this mechanism to work, the unseen object has to be very small, like a white dwarf, neutron star, or black hole.
From the observed orbit of the visible star, one can determine the lowest possible mass of the unseen object. In the
case of Cygnus X-l, this is about six times the mass of the sun, which, according to Chandrasekhar’r result, is too great
for the unseen object to be a white dwarf. It is also too large a mass to be a neutron star. It seems, therefore, that it
must be a black hole.
There are other models to explain Cygnus X-1 that do not include a black hole, but they are all rather far-fetched. A
black hole seems to be the only really natural explanation of the observations. Despite this, I had a bet with Kip Thorne
of the California Institute of Technology that in fact Cygnus X-1 does not contain a black hole! This was a form f
insurance policy for me. I have done a lot of work on black holes, and it would all be wasted if it turned out that black
holes do not exist. But in that case, I would have the consolation of winning my bet, which would bring me four years of
the magazine Private Eye. In fact, although the situation with Cygnus X-1 has not changed much since we made the bet
in 1975, there is now so much other observational evidence in favor of black holes that I have conceded the bet. I paid
the specified penalty, which was a one-year subscription to Penthouse, to the outrage of Kip’s liberated wife.
We also now have evidence for several other black holes in systems like Cygnus X-1 in our galaxy and in two
neighboring galaxies called the Magellanic Clouds. The number of black holes, however, is almost certainly very much
higher; in the long history of the universe, many stars must have burned all their nuclear fuel and have had to collapse.
The number of black holes may well be greater even than the number of visible stars, which totals about a hundred
thousand million in our galaxy alone. The extra gravitational attraction of such a large number of black holes could
explain why our galaxy rotates at the rate it does: the mass of the visible stars is insufficient to account for this. We also
have some evidence that there is a much larger black hole, with a mass of about a hundred thousand times that of the
sun, at the center of our galaxy. Stars in the galaxy that come too near this black hole will be torn apart by the
difference in the gravitational forces on their near and far sides. Their remains and gas that is thrown off other stars, will
fall toward the black hole. As in the case of Cygnus X-l, the gas will spiral inward and will heat up, though not as much
as in that case. It will not get hot enough to emit X rays, but it could account for the very compact source of radio waves
and infrared rays that is observed at the galactic center.
It is thought that similar but even larger black holes, with masses of about a hundred million times the mass of the sun,
occur at the centers of quasars. For example, observations with the Hubble telescope of the galaxy known as M87
reveal that it contains a disk of gas 130 light-years across rotating about a central object two thousand million times the
mass of the sun. This can only be a black hole. Matter falling into such a supermassive black hole would provide the
only source of power great enough to explain the enormous amounts of energy that these objects are emitting. As the
matter spirals into the black hole, it would make the black hole rotate in the same direction, causing it to develop a
magnetic field rather like that of the earth. Very high-energy particles would be generated near the black hole by the
in-falling matter. The magnetic field would be so strong that it could focus these particles into jets ejected outward along
the axis of rotation of the black hole, that is, in the directions of its north and south poles. Such jets are indeed observed
in a number of galaxies and quasars. One can also consider the possibility that there might be black holes with masses
much less than that of the sun. Such black holes could not be formed by gravitational collapse, because their masses
are below the Chandrasekhar mass limit: stars of this low mass can support themselves against the force of gravity
even when they have exhausted their nuclear fuel. Low-mass black holes could form only if matter was compressed to
enormous densities by very large external pressures. Such conditions could occur in a very big hydrogen bomb: the
physicist John Wheeler once calculated that if one took all the heavy water in all the oceans of the world, one could
build a hydrogen bomb that would compress matter at the center so much that a black hole would be created. (Of
course, there would be no one left to observe it!) A more practical possibility is that such low-mass black holes might
have been formed in the high temperatures and pressures of the very early universe. Black holes would have been
formed only if the early universe had not been perfectly smooth and uniform, because only a small region that was
denser than average could be compressed in this way to form a black hole. But we know that there must have been
some irregularities, because otherwise the matter in the universe would still be perfectly uniformly distributed at the
present epoch, instead of being clumped together in stars and galaxies.
Whether the irregularities required to account for stars and galaxies would have led to the formation of a significant
number of “primordial” black holes clearly depends on the details of the conditions in the early universe. So if we could
determine how many primordial black holes there are now, we would learn a lot about the very early stages of the
universe. Primordial black holes with masses more than a thousand million tons (the mass of a large mountain) could
be detected only by their gravitational influence on other, visible matter or on the expansion of the universe. However,
as we shall learn in the next chapter, black holes are not really black after all: they glow like a hot body, and the smaller
they are, the more they glow. So, paradoxically, smaller black holes might actually turn out to be easier to detect than
large ones!
.
PREVIOUS NEXT
.
.
CHAPTER 7
BLACK HOLES AIN’T SO BLACK
.
Before 1970, my research on general relativity had concentrated mainly on the question of whether or not there had
been a big bang singularity. However, one evening in November that year, shortly after the birth of my daughter, Lucy,
I started to think about black holes as I was getting into bed. My disability makes this rather a slow process, so I had
plenty of time. At that date there was no precise definition of which points in space-time lay inside a black hole and
which lay outside. I had already discussed with Roger Penrose the idea of defining a black hole as the set of events
from which it was not possible to escape to a large distance, which is now the generally accepted definition. It means
that the boundary of the black hole, the event horizon, is formed by the light rays that just fail to escape from the black
hole, hovering forever just on the edge Figure 7:1. It is a bit like running away from the police and just managing to
keep one step ahead but not being able to get clear away!
Figure 7:1
Suddenly I realized that the paths of these light rays could never approach one another. If they did they must
eventually run into one another. It would be like meeting someone else running away from the police in the opposite
direction – you would both be caught! (Or, in this case, fall into a black hole.) But if these light rays were swallowed up
by the black hole, then they could not have been on the boundary of the black hole. So the paths of light rays in the
event horizon had always to be moving parallel to, or away from, each other. Another way of seeing this is that the
event horizon, the boundary of the black hole, is like the edge of a shadow – the shadow of impending doom. If you
look at the shadow cast by a source at a great distance, such as the sun, you will see that the rays of light in the edge
are not approaching each other.
If the rays of light that form the event horizon, the boundary of the black hole, can never approach each other, the area
of the event horizon might stay the same or increase with time, but it could never decrease because that would mean
that at least some of the rays of light in the boundary would have to be approaching each other. In fact, the area would
