Black Hole

The term black hole is of very recent origin. It was invented in 1969 by the American scientist John Wheeler as a graphic description of an idea that has existed for at least two hundred years, at a time when there were two theories of light: one, supported by Newton, was the light consists of particles, the other was that light consists of waves. now know that both theories are correct. By duality wave / particle in quantum mechanics that light can be seen both as wave and particle. In the theory held that light is made where it was not clear how it should respond to gravity. But if light consists of particles, one would expect them to be affected by gravity in the same way that cannonballs are affected cannon, rockets and planets. At first people thought that light particles travel at infinite speed, but Roemer's discovery that light moves at finite speed showed that gravity can have a significant effect. John Michell, a board member of Cambridge College, wrote, based on this assumption, in 1783, a paper in the journal Philosophical Transactions of the Royal Society of London, which showed that a star is sufficiently massive and compact would have a gravitational field so strong that light can not escape: any light emitted by the star's surface would be attracted back by the gravitational pull of the star before it can go very far. Michell suggested that there may be many stars as this. 

Although we could see that their light would not reach us, yet we could feel their gravitational attraction. These objects are now called black holes because that are black holes in space. A similar suggestion was made several years later by the French scientist the Marquis de Laplace, apparently independently of Michell: It is quite interesting that Laplace included it only in first and second edition of his book The World and took it out subsequent editions, perhaps decided that the idea was nonsense. (Also, corpuscular theory of light was not sustained in the nineteenth century, everything seems to be explained by wave theory and, as its light was not clear whether time affected by gravity.) Q8b10by

In fact, it is logical that light to be treated like cannonballs in Newton's theory of gravity because the speed of light is fixed. (A cannonball launched from the ground up will be slowed by gravity and will eventually stop and fall, however, a photon continues to move up to speed. Then how Newtonian gravity affect light? ) A consistent theory of how gravity affects the light has not appeared until Einstein proposed general relativity in 1915. And even then it took a long time until they were understood the implications of theory for massive stars.

To understand how they can form a black hole, we need first to understand the life cycle of a star. A star forms when a large amount of gas (mostly hydrogen) begins to suffer a collapse in itself, due to its gravitational attraction. When it contracts, gas atoms collide with each becoming more often and getting speeds higher gas heat. Finally, the gas will be so hot when hydrogen atoms collide they turn away from each other, but merge to form helium. The heat released in this reaction, which is like a controlled explosion of a hydrogen bomb, that makes the star shine. This extra heat further increases until the gas pressure is sufficient to balance the gravitational and the gas ceases to contract. Is there something like a balloon. balance the air pressure inside, trying to produce rubber balloon inflation and blood, trying to decrease ball. Stars will remain stable for a long time the heat generated by nuclear reactions balances the gravitational attraction. Eventually the star will not have hydrogen and other nuclear fuels. Paradoxically, the more stars have more fuel to start, the sooner it ends. This is because the more massive a star is, the more must be hotter to balance its gravitational attraction. and how is hotter, the faster the fuel consumed or. Our Sun probably has enough fuel for another five billion years, but more massive stars are running out of fuel just a hundred million years, far less than the age of the universe. When a star has no fuel, it begins to cool and such contracts. What can happen to him then was understood for the first time until the late '20s.

In 1928 an Indian student, Subrahmanyan Chandrasekhar, took ship for England to study at Cambridge with the British astronomer Sir Arthur Eddington, an expert in general relativity. (According to some reports, a journalist told Eddington in the early '20s he had heard that only three people in the world who understood general relativity. Eddington silent a while, then replied, "I'm trying to think who the third person ".) During his trip in India, Chandrasekhar calculated how big a star may exist and could maintain against its gravity after he consumed all the fuel. The idea was this: when a star goes down, matter particles get very close to each other and thus, according to the Pauli exclusion principle, they must have very different speeds. This makes them to move away from each other and tends to expand the star. Therefore, a star can be maintained at a constant radius by a balance between gravitational attraction and rejection that occurs because of the exclusion principle as gravity before it was balanced by heat.

Chandrasekhar, however, realized that there is a limit for rejection due to the exclusion principle. Relativity limits the maximum difference between velocities of the particles of matter from star to light speed. This means that when a star reaches quite dense, rejection caused by the exclusion principle would be less than the gravitational pull. (This table is now called Chandrasekhar limit.) A similar discovery was made almost simultaneously by Russian scientist Lev Davidovich Landau.

This has serious implications for the final fate of massive stars. If the mass of a star is less than the Chandrasekhar limit, it can-and eventually stop the contraction and stabilize to a final state enables a "white dwarf" with a radius of several thousand kilometers and a density of one hundred tons per cubic centimeter. A white dwarf is supported by repulsion, due to the exclusion principle, electrons from its material. We see a lot of these white dwarf stars.

One of the first discovered a star moving in orbit around Sirius, the brightest star in the night sky.

Landau has shown that there is another possible final state for a star, all weight limit about once or twice mass of the sun, but much smaller even than a white dwarf. These stars would be supported by rejection, due to the principle of exclusion, of neutrons and protons, electrons do not enter. They have been called, so neutron stars. They would have a radius of only about sixteen miles and a density of hundreds of millions of tons per cubic centimeter. When they were first predicted, there is a way of observing neutron stars. They were not detected, in fact, until much later.

On the other hand, above the Chandrasekhar mass stars have a big problem when they ran out of fuel. In some cases they may explode or fail to remove enough material to reduce its mass below and so avoid catastrophic gravitational collapse, but it was hard to believe that this always happens, no matter how big the star. How would she know that you have to lose weight? And even if every star managed to lose enough weight to avoid collapse, what would have happened if you had added weight to a white dwarf or a neutron star to exceed the limit? Would be collapsed to infinite density? Eddington was shocked by this implication and refused to believe the result of Chandrasekhar. Eddington believed that simply was not possible that a star has collapsed to a point. This was the view of many scientists, Einstein himself wrote a paper claiming that the stars will not restrict the size zero: Hostility other scientists, particularly Eddington, his former professor and an authority of first importance regarding the structure of stars, persuaded Chandrasekhar to abandon these for work and go to other astronomical problems, such as moving clusters of stars. However, when he was awarded the Nobel Prize in 1983, it was, in part at least, for his early limit on the mass of cold stars.

Chandrasekhar showed that the exclusion principle can not stop the collapse of more massive stars so Chandrasekhar limit, but to understanding what happens to a star such as general relativity theory, was solved for the first time by a young American Robert Oppenheimer, in 1939. But his results suggest that there would be no observable consequences which can then be detected by telescopes. Then occurred the second world war and Oppenheimer himself was involved in atomic bomb project. After the war, the problem of gravitational collapse was forgotten as most scientists were concerned about what happens at the atomic scale and nucleus. However, in the '60s, interest in large-scale problems in astronomy and cosmology was Reawakened a marked increase in the number and range of astronomical observations, determined by application of modern technology. When Oppenheimer's work was rediscovered and extended by many people.

The picture we now have in Oppenheimer's work is the following: the gravitational field of the star change trajectories of light rays from space trajectories for that would have been if the star there. Cones of light that indicate the paths followed in space and time the sparkle of light emitted from their tips curved inward near the surface of a star. This can be seen from distant starlight bending observed during an eclipse of the sun. When the star contracts, the surface gravitational field becomes stronger and cones of light curves and more inward. This makes it more difficult to stand out from the light and, to a distant observer, the light is weaker and redder. Finally, when the star has shrunk to a certain critical radius, the surface gravitational field becomes so strong that light cones are bent inward so much that light can not escape (Fig. 6.1). The theory of relativity, nothing can travel faster than light. Thus, if light can not escape, can not get anything else, everything is attracted by the gravitational field. There is therefore a set of events in a region of space-time which can not reach out to a remote observer. This region is called a black hole. Limit to be called event horizon and it coincides with the trajectories of light rays failed to leave the black hole.

To understand what you see when you look at the collapse of a star forming a black hole, we must remember that in relativity theory no absolute time. Each observer has its own measure of time. Time for one of the star will differ from one time to the remote, because the star's gravitational field. Suppose an astronaut out boldly on the surface of a collapsed star, which collapses and with it, sends a signal every second, according to the clock or the space ship, orbiting around the star. At a time indicated by the clock or, say 11:00, the star would shrink under critical radius at which the gravitational field becomes so strong that nothing can come out and its signals do not reach the ship. As 11:00 approaches his comrades, concerning the ship, would find that the intervals between successive signals issued by the astronaut would be getting longer, but this effect would be very small before 10:59:59 . They should expect very little more than a second signal from the astronaut at 10:59:58 and at 10:59:59 sent when the clock or look, but you should always wait for signal from 11:00 . Surface light waves emitted by the star between 10:59:59 and 11:00, after time astronaut, would be scattered over an indefinite period of time, as seen from the spacecraft. The time between arrivals of successive waves spacecraft would be getting longer, so that light from the star would appear increasingly red and getting worse. Finally, the star would be so dark that could not be seen from the spacecraft, all that remains is a black hole in space. But the star would continue to exert the same force of gravity on the spacecraft, which would continue to move in orbit around the black hole.

However, the scenario is not entirely realistic because of the following problems. Gravity gets weaker as you move away from the star, so the gravitational force on our feet cutezatorului astronaut would always be greater than the force exerted on the head. This difference between the forces would stretch our astronaut out like spaghetti or it would break before the star to contract the critical radius event horizon formed the car! However, we believe that there are much larger objects in the universe, as the central regions of galaxies, which may also suffer a gravitational collapse to form black holes, an astronaut out on one of them would be broken before the hole shape dark. In fact, he would not feel anything special when they reach the critical radius and could pass the point of no return without noticing it. However, only a few hours, as the region continues to suffer collapse, the difference between gravitational forces exerted on the head and his feet would become so large that, again, it would break into pieces.

The work that Roger Penrose and I did it between 1965 and 1970 showed, according to relativity theory, that a black hole must be a singularity of infinite density and infinite curvature of space-time. This is the Big Bang at the beginning of time, only that he would be an end time for the body and astronaut suffering collapse. At this singularity the laws of science and our ability to predict the future would not work. However, an observer outside the black hole still would not be affected by this failure of predictability, because neither light nor any other sign of singularity could not reach. This made the remarkable Roger Penrose cosmic censorship hypothesis to propose that can be paraphrased thus: "God hates a naked singularity." In other words, singularities produced by gravitational collapse occur only in places like black holes, where they are decently hidden A look outside the event horizon. Strictly, the weak cosmic censorship hypothesis is called: it protects observers who remain outside the black hole ability to predict the consequences of failure that occurs at the singularity, but does nothing for the poor unfortunate astronaut who falls into the hole.

There are some solutions of the equations of general relativity it is possible that our astronaut to see a naked singularity: he can avoid touching the singularity and instead fall through a "wormhole" and out in another region of the universe. It would provide great opportunities to travel in space and time, but unfortunately it seems that these solutions are all very unstable, the small perturbation, such as the presence of an astronaut, so you can change the astronaut could not see the singularity until not reach it and his time comes to an end. In other words, the singularity would always find its future and never in the past. Version of cosmic censorship hypothesis hard to say that a realistic solution, singularities would always find time entirely in the future (the singularities of gravitational collapse) or entirely in the past (the Big Bang). It is great to hope that a version of the hypothesis is valid censorship as close to naked singularities may be possible trip in the past. Although this would be great for writers of science fiction, it would mean nobody would have a safe life: anyone can come in the past and he can kill his father or mother before you be designed!

Event horizon limit where space-time region can not get out, acts as a membrane in one direction around the black hole: objects that can fall through imprudent astronauts into a black hole event horizon, black hole but not out nothing the event horizon. (Recall that the event horizon is the trajectory in space-time light trying to escape the black hole, and that nothing can travel faster than light.) Might say about the event horizon what the poet Dante said of entering the Hell: "I you enter here leave all hope." Whatever or whoever falls through the event horizon will soon reach the region of infinite density and end time.

General relativity predicts that moving heavy objects cause the emission of gravitational waves, where the curvature of space is moving at light speed. They are similar to light waves, which are where the electromagnetic field, but are more difficult to detect. As light from the objects they carry energy they emit. It would therefore be expected that a system of massive objects eventually reach a stationary state because the energy of any movement will be carried by gravitational wave emission. (It's like a cork in water falls: at first he moves pretty much up and down, but as the waves carry energy with them, he will eventually reach a stationary state.) For example, movement of earth in its orbit around the sun produces gravitational waves. As a result of loss of energy, the Earth's orbit will change so gradually he comes increasingly closer to the sun, bumping into him and reaching a steady state. Energy loss rate is very small, almost enough to put into operation an electric radiator. This means that it will take a thousand million million million million years before the earth will fall on the sun, so no need to worry now! Earth orbit change is too slow to be observed, but producing this effect was observed in the last few years in the system called PSR 1913 16 (PSR stands for "pulsar", a special type of neutron star which emits regular pulses of radio waves ). This system consists of two neutron stars moving in orbit around each other, and that lose energy by emission of gravitational waves makes them move on spiral toward each other.

During the gravitational collapse of a star when it forms a black hole, it moves much faster, so that energy is transported with a much higher rate. So, not long before it will reach a stationary state. Final state would look like this? It can be assumed that it would depend on all of the star complex which was formed not only on its mass and rotation speed, but also different densities of various parts of the star and complicated movements of gas from star. And if black holes would be as varied as the objects of which were formed after the collapse, can be very difficult to make predictions about black holes in general.

However, in 1967 the study of black holes has been revolutionized by Werner Israel, a Canadian scientist (who was born in Berlin, grew up in South Africa and completed his doctorate in Ireland). Israel showed that, in accordance with general relativity, black holes should not rotate very simple, they were perfectly spherical, their size depended only on their mass and any two such black holes with the same mass were identical. In fact, they could be described by a particular solution of Einstein's equations which was known since 1917, Karl Schwarzschild discovered shortly after the discovery of general relativity. At first, many people, including even Israel, have argued that because the black holes must be perfectly spherical, a black hole can be formed only by collapse of a perfectly spherical object. Any real star who would never be so perfectly spherical could suffer a collapse forming only a naked singularity.

There was however a different interpretation of the result obtained by Israel, which was supported mainly by Roger Penrose and John Wheeler. They claimed that rapid movements occurring during the collapse of a star would mean that it emits gravitational waves would make it even spherical and when they reach a stationary state, it would be precisely spherical. According to this view, any star that does not turn, no matter how complicated is the shape and internal structure, would end up after gravitational collapse of a perfectly spherical hole denies, whose size depends only on its mass. Further calculations confirmed this view and it was soon generally adopted.

The result of Israel treat only if black holes formed from bodies which do not rotate. In 1963, Roy Kerr in New Zealand found a set of solutions of general relativity equations describing rotating black holes. These black holes "Kerr" rotate at constant speed, size and their shape depends only on their mass and rotation speed. If rotation is zero, the black hole is perfectly round and the solution is identical to the Schwarzschild solution. If rotation is nonzero, the black hole at the equator bombeaza out or (as the earth or sun bombeaza due to their rotation) and how quickly it spins, the more it bombeaza more. Thus, to extend Israel's result to include rotating bodies, it was assumed that any rotating body that collapsed to form a black hole would eventually reach a stationary state described by Kerr solution.

In 1970 a research student and colleague of mine from Cambridge, Brandon Carter, the first step in demonstrating this hypothesis. He showed that if a rotating black hole has an axis of symmetry, as a top, size and shape would depend only on mass and its rotation speed. Then, in 1971, I showed that any stationary rotating black hole would indeed such a symmetry axis. Finally, in 1973, David Robinson of King's College London has used the results of Carter and mine to show that the hypothesis was correct: such a black hole must indeed be a Kerr solution. Thus, after the gravitational collapse of a black hole must reach a state where it can be rotated, but not pulsed. Moreover, the size and shape would depend only on mass and its rotation speed and not the nature of the body that has suffered a collapse forming. This became known by the maxim "A black hole has no hair". Theorem "bald" is of great practical importance because it restricts very much possible types of black holes. Therefore, it can develop detailed models of objects that may contain black holes, and predictive models can be compared with observations. This also means that when it forms a black hole loses a large amount of information on the body that suffered collapse, because only then can we measure the mass and rotation speed of the body. What this means is you see in the next chapter.

Black holes are one of very few cases in history of science in which theory was developed in great detail a mathematical model before there is any experimental evidence of its correctness. Indeed, it was the main argument of those against the black holes: how could anyone believe in the existence of objects for which the only evidence is the calculations based on the dubious theory of general relativity? However, in 1963, Maarten Schmidt, an astronomer at the Palomar Observatory in California, measured the redshift of an object as a star in the direction of low power radio waves called 3C273 (that is, source number 273 in the third Cambridge catalog of sources radio). He found that it was too large to be caused by a gravitational field: if it were a gravitational redshift, the subject should be so massive and so close to us that he would have disrupted the orbits of planets solar system. This suggested that the redshift was caused by expansion of the universe, which, in turn, means that the object was very far away. and to be visible from a distance so great, should be very bright object, namely to issue a huge amount of energy. The only mechanism that might thought to produce large amounts of energy just seems to be not only the gravitational collapse of a single star, but the entire central regions of galaxies. They found several "objects cvasistelare" similar, or quasars, all with large shifts to red. But they are all too far and too hard to notice so to provide reliable evidence for black holes.

Additional support for the existence of black holes in 1907 came the discovery by a student at Cambridge, Jocelyn Bell, objects in space that emit regular pulses of radio waves. First Bell or scientific leader, Anthony Hewish, thought they may have made contact with alien civilizations in the galaxy! indeed, the seminar in which they announced the discovery, I remember the first four sources have called him-4 found LGM, LGM meaning Small little green men "(Little Green Men). Eventually they and everyone else but concluded, less romantic, after which these objects were named pulsars were rotating neutron stars that actually emit pulses of radio waves due to a complicated interaction between their magnetic fields and surrounding material. This was bad news for writers of space westerns, but very promising for the few of us who believed in black holes at the time: was the first positive evidence that neutron stars exist. A neutron star has a radius of about sixteen miles, only a few times the critical radius at which a star becomes a black hole. If a star can undergo a collapse to a size so small, you can expect that other stars could suffer a collapse to a size and smaller and become black holes.

How can we hope to detect a black hole by definition if it does not emit any light? I would be like looking for a black cat in a dark cellar. Fortunately, there is a way. As shown by John Michell in his pioneering work in 1783, a black hole exerts gravitational pull on nearby objects. Astronomers have observed many systems where two stars orbit moving around each other, attracted towards each other by gravity. They have seen systems where there is only one star visible moving in orbit around an unseen companion. Of course, one can not conclude immediately that the companion is a black hole: is simply a star that is too weak to be seen. However, some of these systems, like the one called Cygnus Xl are also strong X-ray sources The best explanation for this phenomenon is that the visible star surface matter was thrown out. When she falls in the unseen companion has a spiral motion (as water drains from a bath) and becomes very hot, emitting X-rays For this mechanism to work, unseen object must be very small, like a dwarf white neutron star or black hole. Observed the star visible from orbit can determine the lowest possible mass of the object invisible. In the case of Cygnus Xl, it was six times the mass of the sun, which, according to Chandrasekhar's result is too great to be a subject of unseen white dwarf. He is also a table too large for a neutron star. Therefore, it seems to be a black hole.

There are other models that explain Cygnus Xl, which include a black hole, but they are a little forced. A black hole seems the only natural explanation of the observations. Despite this I did bet with Kip Thorne of the California Institute of Technology, in fact, Cygnus Xl does not contain a black hole! This is like an insurance policy for me. I worked hard on black holes and everything would have been a loss if it turns out that black holes do not exist. But in that case, I had the consolation that I won the bet, I would bring a four-year subscription to Private Eye magazine. If black holes exist, Kip will get a one-year subscription to Penthouse.