The term black hole is a very recent origin and was coined in 1969 by the American Scientist John Wheeler as a graphic description of an idea that goes back at least two hundred years, to a time when there were two theories about light:
It was composed of Particles which was favoured by Newton
It was composed of Waves
We now know that both theories are correct.By the wave/particle duality of quantum mechanics, light can be regarded as both a wave and a particle.
Under the theory that light is made up of waves, it was not clear how it would respond to gravity.
But if it is composed of particles one would expect them to behave similar to cannonballs, rockets and planets are.
At first people thought particles of light travelled infinitely fast, so gravity would not have been able to slow them down, but the discovery by Roemer that light travels at a finite speed meant that gravity might have an important effect.
On this assumption, in Cambridge John Michell, wrote a paper in 1973 in the Philosophical Transactions of the Royal Society of London in which he pointed out that a star that was sufficiently massive and compact would have such a strong gravitational field that light could not escape : any light emitted from the surface of the star would be dragged back by the star's gravitational attraction before it could get very far. Michell suggested that there might be a large number of stars like this. Although we would not be able to see them because the light from them would not reach us, we would still feel their gravitational attraction. Such objects are what we now call balck holes, because that is what they are: black voids in space.
A similar suggestion was made a few years later by the French scientist the Marquis de Laplace, apparently independently of Michell. Interestingly enough, Laplace included it in only the first and second editions of his book The System of the World, and left it out of later editions; perhaps he decided that it was a crazy idea.( Also, the partical theory of light went out of favor during the nineteenth century; it seemed that everything could be explained by the wave theory, and according to the wave theory, it was not clear that light would be affected by gravity at all).
In fact, it is not really consistent to treat light like cannonballs in Newton's theory of gravity because the speed of light is fixed.(A cannonball fired upward from the earth will be slowed down by gravity and will eventually stop and fall back; a photon, however, must continue upward at a constant speed. How then can Newtonian gravity affect light?) A consistent theory of how gravity affects light did not come along until Einstein proposed general relativity in 1915. And even then it was a long time before the implications of the theory for massive stars were understood.
To understand how a black hole might be formed, we first need an understanding of the cycle of a star. A star is formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction. As it contracts the atoms of the gas collide with each other more and more frequently and at greater and greater speeds-the gas heats up. Eventually, the gas will be so hot that when the hydrogen atoms collide they no longer bounce off each other, but instead coalesce to form helium. The heat released in this reaction, which is like a controlled hydrogen bomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction, and the gas stops contracting. It is a bit like a ballon-there is a balance between the pressure of the air inside, which is trying to make the ballon expand, and the tension in the rubber, which is trying to make the baloon smaller. Stars will remain stable like this for a long time, with heat from the nuclear reactions balancing the gravitational attraction. Eventually, however, the star will run out of its hydrogen and other nuclear fuels. Paradoxically, the more fuel a star starts off with, the soonoor it runs out. This is because the more massive the star is, the hotter it needs to be to balance its gravitational attraction. And the hotter it is, the faster it will use up its fuel. Our sun has probably got enough fuel for another five thousand million years or so, but more massive stars can use up their fuel in as little as one hundred million years, much less than the age of the universe. When a star runs out of fuel, it starts to cool off and so to contract. What might happen to it then was first understood only at the end of the 1920's.
In 1928 an Indian graduate sutdent, Subramanyan Chandrasekhar, set sail for England to study at Cambridge with the British astronomer Sir Athur Eddington, an expert on general relativity. (According to some accounts, a journalist told Eddington in the early 1920's that he had heard there were only three people in the world who understood general relativity. Eddington paused, then replied, "I am trying to think who the third person is.") During his voyage frm India, Chandrasekhar worked out how big a star could be and still support itself against its own gravity after it had used up all its fuel. The idea was this: When the star becomes small, the matter particles get very near each other, and so according to the Pauli exclusion principle, they must have very different velocities. This makes them move away from each other and so tends to make the star expand. A star can therefore maintain itself at a constant radius by a balance between the attraction of gravity and the repulsion that arises from the exclusion principle, just as earlier in its life gravity was balanced by the heat.
Chandrasekhar realized, however, that there is a limit to the repulsion that the exclusion principle can provide. The theory of relativity limits the maximum difference in the velocities of the matter particles in the star to the speed of light. This means that when the star got sufficiently dense, the repulsion caused by the exclusion principle would be less than the attraction of gravity. Chandrasekhar calculated that a cold star would not be able to support itself against its own gravity. (This mass is now known as the Chandrasekhar limit.) A similar discovery was made about the same time by the Russian scientist Lev Davidovich Landau.
This had serious implications for the ultimate fate of massive stars. If a star's mass is less than the Chandrasekhar limit, it can eventually stop contracting and settle down to a possible final state as a "white dwarf" with a radius of a few thousand miles and a density of hundreds of tons per cubic inch. A white dwarf is supported by the exclusion principle repulsion between the electrons in its matter. We observe a large number of these white dwarf stars. One of the first to be discovered is a star that is orbiting around Sirius, the brightest star in the night sky.
Landau pointed out that there was another possible final state for a star, also with a limiting mass of about one or two times the mass of the sun but much smaller even than a white dwarf. These stars would be supported even by the exclusion principle repulsion between neutrons and protons, rather than between electrons. They were therefore called neutron stars. They would have a radius of only ten miles or so and a density of hundrends of millions of tons per cubic inch. At the time they were first predicted , there was no way that neutron stars could be observed. They were not actually detected until much later.
Stars with masses above the Chandrasekhar limit, on the other hand, have a big problem when they come to the end of their fuel. In some cases they may explode or manage to throw off enough matter to reduce their masses below the limit and so avoid catastrophic gravitational collapse, but it was difficult to believe that this always happened, no matter how big the star. How would it know that it had to lose weight? And even if every star managed to lose enough mass to avoid collapse, what would happen if you added more mass to a white dwarf or neutron star to take it over the limit? Would it collapse to infinite density? Eddington was shoked by that implication, and he refused to believe Chandrasekhar's result. Eddington thought it was simply not possible that a star could collapse to a point. This was the view of most scientists: Einstein himself wrote a paper in which he claimed that stars would not shrink to zero size. The hostility of other scientists, paticularly Eddington, his former teacher and the leading authority on the structure of stars, persuaded Chandrasekhar to abandon this line of work and turn instead to other problems in astronomy, such as the motion of star clusters. However, when he was awarded the Nobel prize in 1983, it was, at least in part, for his early work on the limiting mass of cold stars.
Chandrasekhar had shown that the exclusion principle could not halt the collapse of a star more massive than the Chandrasekhar limit, but the problem of understanding what would happen to such a star, according to general relativity, was first solved by a young American, Robert Oppenheimer, in 1939. His result, however, suggested that there would be no observational consequences that could be detected by the telescopes of the day. Then World War II intervened and Oppenheimer himself became closely involved in the atom bomb project. After the war the problem of gravitational collapse was largely forgotten as most scientists became caught up in what happens on the scale of the atom and its nucleus. In the 1960's, however, interest in the large scale problems of astronomy and cosmology was revived by a great increase in the number and range of astronomical observations brought about by the application of modern technology. Oppenheimer's work was then rediscovered and extended by a number of people.
The picture that we now have from Oppenheimer's work is as follows:
The gravitational field of the star changes the paths of light rays in space-time from what they would have been had the star not been present. The light cones, which indicate the paths followed in space and time by flashes of light emitted from their tips, are bent slightly inward near the surface of the star. This can be seen in the bending of light from distant stars observed during an eclipse of the sun. As the star contracts, the gravitational field at its surface gets stronger and the light cones get bent inward more.This makes it more difficult for light from the star to escape, and the light appears dimmer and redder to an observer at a distance. Eventually, when the star has shrunk to a certain critical radius, the gravitational field at the surface becomes so strong that the light cones are bent inward so much that light can no longer escape.
According to the theory of relativity, nothing can travel faster than light. Thus if lightcannot escape, neither can anything else; everything is drgged back by the gravitational field. So one has a set of events, a region of space-time, from which it is not possible to escape to reach a distant observer. This region is what we now call a black hole. Its boundary is called the event horizon and it coincides with the paths of light rays that just fail to escape from the black hole.
In order to understand what you would see if you were watching a star collapse to form a black hole, one has to remember that in the theory of relativity there is no absolute time. Each observer has his own measure of time. The time for someone on a star will be different from that for someone at a distance, because of the gravitational field of the star. Suppose an intrepid astronaut on the surface of the collapsing star, collapsing inward with it, sent a signal every second, according to his watch, to his spaceship orbiting about the star. At some time on his watch, say 11:00, the star would shrink below the critical radius at which the gravitational field becomes so strong nothing can escape, and his signals would no longer reach the spaceship. As 11:00 approached, his companions watching from the spaceship would find the intervals between successive signals from the astronaut getting longer and longer, but this effect would be very small before 10:59:59. They would have to wait only very slightly more than a second between the astronaut's 10:59:58 signal and the one that he sent when his watch read 10:59:59, but they would have to wait forever for the 11:00 signal. The light waves emitted from the surface of the star between 10:59:59 and 11:00, by the astronaut's watch, would be spread out over an infinite period of time, as seen from the spaceship. The time interval between the arival of successive waves at the spaceship would get longer and longer, so the light from the star would appear redder and redder and fainter and fainter. Eventually, the star would be so dim that it could no longer be seen from the spaceship: all that would be left would be black hole in space. The star would, however, continue to exert the same gravitational force on the spaceship, which would continue to orbit the black hole.
This scenario is not entirely realistic, however, because of the following problem. Gravity gets weaker the farther you are from the star, so the gravitational force on our intrepid astronaut's feet would always be greater than the force on his head. This difference in the forces would stretch our astronaut out like spagetti or tear him apart before the star had contracted to the critical radius at which the event horizon formed! However, we believe that there are much larger objects in the universe, like the central regions of galaxies, that can also undergo gravitational collapse to produce black holes; an astronaut on one of these would not be torm apart before the black hole formed. He would not, in fact, feel anything special as he reached the critical radius, and could pass the point of no return without noticing it. However, within just a few hours, as the region continued to collapse, the difference in the gravitational forces on his head and his feet would become so strong that again it would tear him apart.
The work that Roger Penrose and I did between 1965 and 1970 showed that, according to general relativity, there must be a singularity of infinite density and space-time curvature within a black hole. This is rather like the big bang at the beginning of time, only it would be an end of time for the collapsing body and the astronaut. At this singularity the laws of science and our ability to predict the future would break down. However, any observer who remained outside the black hole would not be affected by this failure of predictability, because neither light nor any other signal could reach him from the singularity. This remarkable fact let Roger Penrose to propose the cosmic censorship hypothesis, which might be paraphrased as "God abhors a naked singularity." In other words, the singularities produced by gravitational collapse occur only in places, like black holes, where they are decently hidden from outside view by an event horizon.Strictly, this is what is known as the weak cosmic censorship hypothesis: it protects observers who remain outside the black hole from the consequences of the breakdown of predictability that occurs at the singularity, but it does nothing at all for the poor unfortunate astronaut who falls into the hole.
There are some solutions of the equations of general relativity in which
it is possible for our astronaut to see a naked singularity: he may be
able to avoid hitting the singularity and instead fall through a
"wormhole" and come out in another region of the universe. This would
offer greater possibilities for travel in space and time, but
unfortunately it seems that these solutions may all be highly unstable;
the least disturbance, such as the presence of an astronaut, may change
them so that the astronaut could not see the singularityuntil he hit it
and his time came to an end. In other words, the singularity would always
lie in his future and never in the past. The strong version of the
cosmic censorship hypothesis states that in a realistic solution, the
singularities would always lie either entirely in the future (like the
singularities of gravitational collapse) or entirely in the past (like
the big bang). It is greatly to be hoped that some version of the
censorship hypothesis holds because close to naked singularities it
may be possible to travel into the past. While this would be fine for
writers of sciece fiction, it would mean that no one's life would
ever be safe; someone might go into the past and kill your father or
mother before you were coceived!
The event horizon, the boundary of the region of space-time from which
it is not possible to escape, acts rather like a one-way membrane
around the black hole: objects, such as unwary astronauts, can fall
through the event horizon into the black hole, but nothing can ever
get out of the black hole through the event horizon. (Remember
that the event horizon is the path in space-time of light that is
trying to escape from the black hole, and nothing can travel faster
than light.) Anything or anyone who falls through the event horizon
will soon reach the region of infinite density and the end of time.
General relativity predicts that heavy objects that are moving will
cause the emission of gravitational waves, ripples in the curvature of
space that travel at the speed of light. These are similar to light
waves, which are ripples of the electromagnetic field, but they are
much harder to detect. Like light, they carry energy away from the
objects that emit them. One would therefore expect a system of massive
objects to settle down eventually to a stationary state, because the
energy in any movement would be carried away by the emission of
gravitational waves.(It is rather like dropping a cork into water: at
first it bobs up and down a great deal, but as the ripples carry away
its energy, it eventually settles down to a stationary state.) For
example, the movement of the earth in its orbit round the sun produces
gravitational waves. The effect of the energy loss will be to change
the orbit of the earth so that gradually it gets nearer and nearer to
the sun, eventually collides, with it, and settles down to a stationary
state. The rate of energy loss in the case of the earth and the sun is
very low - about enough to run a small electric heater. This means it
will take about a thousand million million million million
( 2x1029 ) years
for the earth to run into the sun, so there's no immediate cause for
worry! The change in the orbit of the earth is too slow to be observed,
but this same effect has been observed over the past few years occuring
in the system called PSR 1913 + 16 (PSR stands for "pulsar", a special
type of neutron star that emits regular pulses of radio waves). This
system contains two neutron stars orbiting each other, and the energy they
are losing by the emission of gravitational waves is causing them to
spiral in toward each other.
