A brief history of time by stephen w. hawking

Author: \"A Brief History of Time\" was written by Professor Stephen Hawking, who was born in Oxford, Great Britain, on 8th January 1942.
He studied physics at Oxford University and went on to pursue his graduate studies at Cambridge. In his early twenties he was diagnosed as having ALS (Amyotrophic Lateral Sclerosis), known in the UK as Motor Neurone Disease. He holds Newton\'s chair as Lucasian Professor of Mathematics at Cambridge and is widely considered to be the greatest scientific thinker since Newton and Einstein. In 1989 he received an Honorary Doctor of Science degree from Cambridge University and was made a Companion of Honour.


Published :
It´s a Bantam Books Book, published by Bantam Press. It was first published by Bantam Books in 1988. \"A Brief History of Time\" remained on The New York Times best-seller list for fifty-three weeks; and in Britain, as of February 1993, it had been on The Sunday Times list for 205 weeks. (At week 184, it went into the Guinness Book of Records for achieving the most appearances on this list.) The number of translated editions is now thirty-three.

Type of book:
\"A Brief History of Time\" is a book that tries to explain the main theories of today physics in a quite \"non-technical\" language so everybody can understand them. Stephen also explains also the basics to these theories so that the reader has to know almost nothing about physics to understand them. This book starts at the beginning of science with the Greek philosopher Aristotle and goes on until the youngest theories about our universe like the superstring-theory which needs 10dimensions.
The book is divided into 11 chapters and 3 epilogues about Einstein, Newton and Galileo Galilei. There is also a glossary that explains the main technical verbs that are used in this book.
\" This book marries a child\'s wonder to a genius intellect. We journey into Hawking\'s universe, while marvelling at his mind \" - Sunday Times


Subject:
We go about our daily lives understanding almost nothing of the world. We give little thought to the machinery that generates the sunlight that makes life possible, to the gravity that glues us to an Earth that would otherwise send us spinning off into space, or to the atoms of which we are made and on whose stability we fundamentally depend. Except for children few of us spend much time wondering why nature is the way it is; where the cosmos came from, or whether it was always here; if time will flow backward one day and effect precede causes; or whether there are ultimate limits to what humans can know. Was there a beginning of time? Could time run backwards? Is the universe infinite or does it have boundaries? These are just some of the questions considered in an internationally acclaimed masterpiece which begins by reviewing the great theories of the cosmos from Newton to Einstein, before delving into the secrets which still lie at the heart of space and time.
This book tries to answer this question, but a lot of these questions can not be answered now and so we can only follow the theories of Stephen Hawking, which are very well explained here.


The most important persons:

Albert Einstein: He is a German-born American physicist and Nobel laureate, best known as the creator of the special and general theories of relativity and for his bold hypothesis concerning the particle nature of light. He is perhaps the most well-known scientist of the 20th century.
Einstein was born in Ulm on March 14, 1879. At the age of 12 he taught himself Euclidean geometry.
In 1902 he secured a position as an examiner in the Swiss patent office in Bern. After 1919, Einstein became internationally renowned. He accrued honours and awards, including the Nobel Prize in physics in 1922, from various world scientific societies. His visit to any part of the world became a national event.
When Hitler came to power, Einstein immediately decided to leave Germany for the United States. He took a position at the Institute for Advanced Study at Princeton, New Jersey. Einstein died in Princeton on April 18, 1955.
He often said, only the discovery of the nature of the universe would have lasting meaning.
Galileo Galilei: Galileo was born near Pisa, on February 15, 1564. He was a Italian physicist and astronomer, who, with the German astronomer Johannes Kepler, initiated the scientific revolution that flowered in the work of the English physicist Sir Isaac Newton. Born Galileo Galilei, his main contributions were, in astronomy, the use of the telescope in observation and the discovery of sunspots, lunar mountains and valleys, the four largest satellites of Jupiter, and the phases of Venus. In physics, he discovered the laws of falling bodies and the motions of projectiles. In the history of culture, Galileo stands as a symbol of the battle against authority for freedom of inquiry.
In 1589 he became professor of mathematics at Pisa. Only the Copernican model supported Galileo\'s tide theory, which was based on motions of the earth. He discovered mountains and craters on the moon. He also saw that the Milky Way was composed of stars. By December 1610 he had observed the phases of Venus, which contradicted Ptolemaic astronomy and confirmed his preference for the Copernican system.
He died in 1642.
Sir Isaac Newton: He was born on January 4, 1643 at Woolsthorpe, near Grantham in Lincolnshire. He was an English mathematician and physicist, considered one of the greatest scientists in history, who made important contributions to many fields of science. His discoveries and theories laid the foundation for much of the progress in science since his time. Newton was one of the inventors of the branch of mathematics called calculus. He also solved the mysteries of light and optics, formulated the three laws of motion, and derived from them the law of universal gravitation.
Later, in the summer of 1661, he was sent to Trinity College, at the University of Cambridge. Newton received his bachelor\'s degree in 1665. He received his master\'s degree in 1668.
Newton is probably best known for discovering universal gravitation, which explains that all bodies in space and on earth are affected by the force called gravity. He published this theory in his book Philosophiae Naturalis Principia Mathematica in 1687. This book marked a turning point in the history of science. Newton died in 1727.


Plot synopsis:
In the introduction Stephen Hawking explains that he left out all but one equation, the most famous found by Albert Einstein because someone told him every equation would halve the sales of the book. He also describes how he got help from friends, who donated a communication programme and a speech synthesiser to him, which in combination with a small personal computer mounted on his wheelchair, allows him a better communication than before he lost his voice.
In the beginning Hawking tells an anecdote on how a well-known scientist once gave a public speech on astronomy and in the end an old lady got up and said that he´d talked rubbish, because in reality the world would be a flat plate supported on the back of a giant tortoise that stands on the back of another tortoise and so on. He points out that most people would find this picture rather ridiculous, but why do we think to know better?
Only recent breakthroughs in physics suggest answers to our questions about the history of the universe, which may seem as obvious as the earth orbiting our sun, or as ridiculous as a tower of tortoises. Only time (whatever that may be) will tell.
The Greek philosopher Aristotle was the first to point out that earth was a round sphere. Nevertheless the Greek still believed in the earth being the stationary centre of the universe. This idea was elaborated by Ptolemy into a complete cosmological model, which was generally accepted and adopted by the Christian church as the picture of the universe that was in accordance with the Scriptures, for it had the great advantage that it left lots of room outside the sphere of the fixed stars for heaven and hell.
A simpler model suggesting that the sun was stationary at the centre and the earth and the planets moved in circular orbits around the sun, was proposed by Nicholas Copernicus. Nearly a century passed before this idea was taken seriously by two astronomers, the German, Johannes Kepler and the Italian, Galileo Galilei. Galileo observed the moons of Jupiter with a just invented telescope, which was the deathblow to the old theory. An explanation was provided only much later, in 1687, when Sir Isaac Newton published his \"Philosophiae Naturalis Principia Mathematica\", probably the most important single work ever published in the physical sciences. In there, he postulated the law of universal gravitation and developed the complicated mathematics needed to analyse the motions of planets and calculus.
The beginning of the universe has been discussed long then, because according to religious traditions the universe started at a finite, and not very distant time in the past. Nowadays we take it for granted that we live in a lacy spiral disk galaxy and that there are many other galaxies more or less like it in the universe. But early in our century not everyone accepted this picture.
It was the American astronomer Edwin Hubble who, in the 1920s, showed that there are indeed many galaxies besides our own. It was Hubble again who showed that distant galaxies, wherever you look, are all moving away from us. In other words, the universe is expanding. The most helpful way to think of the expansion of the universe is not as things rushing away from one another but as space between them swelling. Imagine a balloon with dots on its surface being inflated. When the balloon swells, the dots move apart.
This discovery finally brought the question of the beginning of the universe into the realm of science. If galaxies move apart from each other, they used to be much closer together at some moment in the past, ten or twenty thousand million years ago. They all have been in exactly the same place. All the enormous amount of matter in the universe packed in a single point, infinitely dense and infinitesimally small. Such a situation is called the \"big bang\". One may say that time had a beginning at the big bang, in the sense that earlier times simply would not be defined.
But this is not the only possible history of an expanding universe.

The second chapter describes the non-existence of absolute rest and therefore the lack of an absolute position in space and time.
The fact that light travels at a finite, but very high, speed ( 186,000 miles per second) was first discovered by the Danish astronomer Roemer. He measured the motion of Jupiter and the eclipses of its moons. A better theory of the locomotion of light did not come until the 19th century when the British physicist Maxwell managed to unify the partial theories that till then had been used to describe the forces of electricity and magnetism. Maxwell\'s theory predicted that radio or light waves should travel at a certain fixed speed. In order to fit Newton\'s theories he introduced a substance called \"ether\" that was present also in empty space.
Finally in 1905 Albert Einstein came to the conclusion that the whole idea of an ether was unnecessary, providing one was willing to abandon the idea of absolute time. Based on this Einstein worked out first the special, then the general theory of gravity, presenting the famous equation and the law that nothing can travel faster than the speed of light.
The theory of relativity describes that any observer can work out precisely what time and position any other observer will assign to an event, provided he knows the other observer\'s relative velocity. Nowadays we use just this method to measure distances precisely, because we can measure time more accurately than length. In effect, one meter is defined to be the distance travelled by light in 0.000000003335640952 seconds, as measured by a caesium clock.
So, we must accept that time is not completely separate from and independent of space, but is combined with it to form an object called \"space-time\".
Einstein spent several years attempting to find a theory of gravity that would work with what he had discovered about light and motion at near light speed. In 1915 he introduced the theory of general relativity where he thinks of gravity not as a force acting between two bodies but in terms of the shape, the curvature, of four-dimensional space-time itself. In general relativity, gravity is the geometry of the universe. According to Einstein the curvature is caused by the presence of mass. Every massive body contributes to the curvature of space-time. Things going \"straight ahead\" in the universe are forced to follow curved paths. Imagine a heavy object, such as a cannon ball, representing the sun, being placed in the middle of a taut rubber sheet, creating a cone-shaped dent all around it.
Einstein argued that whenever something heavy bent space-time like this, it would naturally affect the path of anything lighter travelling nearby. If you now try to roll a smaller ball representing the Earth or one of the other planets across the stretched rubber sheet representing space-time, it will certainly change direction slightly when it meets the dent caused by the cannon ball sun.
It will probably do more than that: it may describe an ellipse and roll back in your direction. Something like that happens as the earth tries to continue in a straight line past the sun. The sun warps space-time as the canon ball warps the rubber sheet. The earth's orbit is the nearest thing to a straight line in warped space-time. At just the right speed, the small planet ball would be travelling fast enough not to fall right into the dent, but too slowly to escape it completely. With nothing else to stop it or slow it down, it would find its level on the \'side\' of the dent in space-time.
The speed of light in space time can be seen like the ripples that spread out on the surface of a pond when a stone is thrown in. The ripples spread out as a circle that gets bigger as time goes on. If one thinks of a three-dimensional model consisting of the two-dimensional surface of the pond and the one dimension of time, the expanding circle of ripples will mark out a cone whose tip is at the place and time at which the stone hit the water. Similarly the light spreading out from an event forms a three-dimensional cone in the four-dimensional space-time. This cone is called the future light cone of the event. In the same way we can draw another cone, called the past light cone, which is the set of events from which a pulse of light is able to reach the given event.
Einstein's theory predicts that not only planets, also Photons are affected by the warp of space-time. If a light ray is travelling from a distant star and its path takes it close to our sun, the warping of space-time near the sun causes the path to bend inward towards the sun for a few degrees. Perhaps the path of light bends in such a way that the light finally hits the earth. Our sun is too bright for us to see such starlight; except during an eclipse of the sun.
If we see it then and do not realise the sun is bending the path of the stars light, we would get the wrong idea about which direction the beam of light is coming from and where that star actually is in the sky. Astronomers make use of this effect. They measure the mass of objects in space by measuring how much they bend the paths of light from distant stars. The greater the mass , the greater the bending.
Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a consequence of the fact that space-time is not flat, as had been previously assumed: it is curved, or warped, by the distribution of mass and energy in it. Roger Penrose and Stephen Hawking showed that Einstein\'s general theory of relativity implied that the universe must have a beginning and, possibly, an end.

Chapter three begins with the observation that even fixed stars in fact change their position, and all visible to us are concentrated in one band, which we call the Milky Way. Our modern picture of the universe dates back to Hubble, who demonstrated that ours was not the only galaxy. There were in fact many others, with a lot of empty space between them. We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating. The stars in its spiral arms orbit around its centre about once every several hundred million years. Our sun is just an ordinary, average-sized, yellow star, near the inner edge of one of the spiral arms.
The characteristics of a stars, we get by observing the spectra, which not only tells us the temperature, but also the elements it consists of.
Friedmann started with two assumptions in his model: 1st: The universe looks much the same in whatever direction you look (except for nearby things like our Solar System and the Milky Way); 2nd: The universe looks like this from wherever you are in the universe.
Friedmann's first assumption is fairly easy to accept. The second isn't. We do not have any scientific evidence for or against it.
From these two ideas alone, Friedmann showed that we should not expect the universe to be static. In fact, in 1922, several years before Edwin Hubbell\'s discovery, Friedmann predicted exactly what Hubble found! In 1965 two American physicists at the Bell Telephone Laboratories in New Jersey, discovered the microwave background of the universe and thereby proved Friedmann´s theories.
Although Friedmann found only one, there are in fact three different kinds of models that obey Friedmann´s two fundamental assumptions. In the first which Friedmann found, the universe is expanding too slowly so that the gravitational attraction between the different galaxies causes the expansion to slow down and eventually to stop. The galaxies then start to move toward each other and the universe contracts until the big crunch. In the second kind of solution, the universe is expanding so rapidly that the gravitational attraction can never stop it, though it does slow it down a bit. Finally, there is a third kind of solution, in which the universe is expanding only just fast enough to avoid recollapse. However, the speed at which the galaxies are moving apart gets smaller and smaller, although it never quite reaches zero. Because of insufficient measuring methods and some uncertainty about dark matter we cannot exactly figure out which Friedmann model describes our universe. All the Friedmann solutions have the feature that at some time in the past, the distance between neighbouring galaxies must have been zero. At that time, which we call the big bang, the density of the universe and the curvature of space-time would have been infinite. Because mathematics cannot really handle infinite numbers, this means that the general theory of relativity (on which Friedmann´s solutions are based) predicts that there is a point in the universe where the theory itself breaks down. Such a point is an example of what mathematicians call a singularity.
In 1965 the British mathematician and physicist Roger Penrose discovered the existence of black holes, which also contain singularities.
The idea, that stars may end up and can become a black hole, is based on the gravitational effect of mass. Imagine a star that has ten times the mass of the sun. The star's radius is about 3 million kilometres, about five times that of the sun. Escape velocity is about 1,000 kilometres per second. Such a star has a life span of about a hundred million years. On one side the sun has to fight against gravity: the attraction of every particle in the star for every other. On the opposing side is the pressure of the gas in the star. This pressure comes from heat released when hydrogen nuclei in the star collide and merge to form helium nuclei what is called the fusion process. The heat makes the star shine and creates enough pressure to resist gravity and prevent the star from collapsing.
For a hundred million years this balance is held. Then the star runs out of hydrogen that it could convert into helium. Some stars then convert helium into heavier elements, but that gives them only a short reprieve.
When there's no more pressure to counteract gravity, the star shrinks. As it does, the gravity on its surface becomes stronger and stronger because the mass gets more and more compressed. It won't have to shrink to a singularity to become a black hole. When the 10-solar-mass star's radius is about 30 kilometres, escape velocity on its surface will have increased to 300,000 kilometres per second, the speed of light. And out of these facts rises the definition of a black hole: When light can no longer escape the star is a black hole.
Stars with less than 8 solar masses probably don't shrink all the way to form black holes. Such stars are then called brown dwarfs. The limit, beyond that a star can become a black hole, is called the \"Chandrasekhar limit\".
Whether our star goes on shrinking to a point of infinite density or stops shrinking just within the radius where escape velocity reaches the speed of light, gravity at that radius is going to feel the same, as long as the star's mass doesn't change. Escape velocity at that radius is the speed of light and will stay the speed of light. Such a border of a black hole is called \"event horizon\". Light coming from the star will find escape impossible. Nearby beams of light from distant stars may curl around the black hole several times before escaping or falling in.
A black hole, with its event horizon for an outer boundary, is shaped like a sphere, or if it is rotating, a bulged-out sphere, a convex lens. The event horizon is marked by the paths in space-time of rays of light that hover just on the edge of that spherical area, not being pulled in but unable to escape. Gravity at that radius is strong enough to stop their escape, but just not strong enough to pull them back in. We can\'t see them because the photons of those rays can't escape from that radius, they can't reach our retina.
General relativity predicts the existence of singularities, but in the early 1960s only a few took this prediction seriously. Until Hawking and Penrose showed that if the universe obeys general relativity, a star of great enough mass undergoing gravitational collapse must form a singularity.
Hawking realised that if he reversed the direction of time so that the collapse became an expansion, everything in the theory would still hold. If general relativity tells us that any star which collapses beyond a certain point must end in a singularity, then it also tells us that any expanding universe must have begun as a singularity therefore as a Friedman model.
With newly developed mathematical techniques and other technical conditions from the theorems that singularities must occur, Penrose and Hawking at last proved that there must have been a big bang singularity provided only that general relativity is correct and the universe contains as much matter as we observe. It is perhaps ironic that now, having changed his mind, Hawking actually is trying to convince other physicists that there was in fact no singularity at the beginning of the universe - as you will see later, it can disappear once quantum effects are taken into account.

The fourth chapter begins with the theory of determinism proclaimed by the French scientist the Marquis de Laplace at the beginning of the 19th century.
To avoid that a hot object, or body, such as a star, must radiate energy at an infinite rate, the German scientist Planck suggested in 1900 that light, X rays, and other waves could not be emitted at an arbitrary rate, but only in certain packets that he called quanta. 1926 another German scientist, Heisenberg, formulated his famous uncertainty principle. In order to predict the next position and velocity of a particle, one has to be able to measure its present position and velocity exactly.
This led Heisenberg, Schrödinger, and Dirac to reformulate mechanics into a new theory called quantum mechanics, based on the uncertainty principle. It predicts a number of different possible outcomes and tells us how likely each of these is. Quantum theory also led to the theory of wave-particle duality.

The fifth chapter deals with elementary particles and the forces of nature. Just thirty years ago, it was thought that protons and neutrons were elementary particles, but experiments in which protons were shot on one another at high speeds had shown that they were in fact made up of smaller particles which were named quarks. There are different types of quarks: there are thought to be at least six \"flavours\", which we call: up, down, strange, charmed, bottom, and top. Each flavour comes in three \"colours\", red, green, and blue. These colours and names are just a creative invention of physicist, quarks are much smaller than the wavelength of visible light and so do not have any colour in the normal sense.
A proton or neutron for instance, is made up of three quarks, one of each colour. We can create particles made up of the other quarks, but these all have a much greater mass and decay very rapidly into protons and neutrons.
The wave-particle dualism leads to a characteristic of particles, called spin. Since particles have no well-defined axis, the spin really tells us what the particle looks like seen from different directions. A particle of spin 0 is like a dot: it looks the same from every direction. A particle of spin 1 has to be turned round a full revolution to look the same, a particle of spin 2, half a revolution. But there are particles that do not look the same if one turns them through just one revolution: you have to turn them through two complete revolutions! Such particles are said to have spin ½. All the known particles in the universe can be divided into two groups: particles of spin ½, which make up the matter in the universe, and particles of integer spin which give rise to forces between the matter particles. The matter particles obey what is called Pauli´s exclusion principle. This was discovered in 1925 by an Austrian physicist, Wolfgang Pauli.
There exist just four groups of force-carrying particles. You can sort them according to the strength of the force they carry and the particles with which they interact. These four are: gravitational force, electromagnetic force, weak nuclear force, and strong nuclear force. The last three are combined into what is called a Grand Unified Theory (GUT), but these contain a number of parameters whose values cannot be predicted from the theory. That\'s the reason why this theory is not the ultimate theory up to now.
Till 1956 it was believed that the laws of physics obeyed each of three separate symmetries called C, P, and T. The symmetry C(charge) means that the laws are the same for particles and antiparticles. The symmetry P(parity) means that the laws are the same for any situation and its mirror image (the mirror image of a particle spinning in a right-handed direction is one spinning in a left-handed direction). The symmetry T(time) means that if you reverse the direction of motion of all particles and antiparticles, the system should go back to what it was at earlier times; in other words, the laws are the same in the forward and backward directions of time. However, in 1964 two Americans, J. W. Cronin and Val Fitch proved this believe to be wrong.

The sixth chapter gets a little bit more practical and explains the mystery of black holes. In 1969 the term \"black hole\" was put into the world by the American scientist John Wheeler as a graphic description of an idea at least two hundred years old. Roemer´s discovery that light travels at a finite speed meant that gravity might have an important effect on it, following the wave-particle duality of quantum mechanics. However, a consistent theory of how gravity affects light did not come along until Einstein proposed general relativity.
The possible final states are \"white dwarfs\", neutron stars or black holes. 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. He found that at this singularity the laws of science and our ability to predict the future would break down.
Anybody who remained far enough away of 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.
There are some solutions of the equations of general relativity in which occur so-called \"wormholes\". These are \"highways\" of transport through space. You can get from one region of the universe to another in no time at all. This would offer great possibilities for travel through space and time, but unfortunately, it seems that these solutions may all be highly unstable.
The extra attraction of a large number of black holes could also 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 centre of our galaxy.
As in the case of Cygnus X-1, a very possible candidate for a black hole, the gas will spiral inward and will heat up. 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 centre. 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 centres of quasars. 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, and though producing a magnetic field 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. Such jets are really observed in a number of galaxies and quasars.

The seventh chapter is about light rays in the event horizon. If the rays of light that form the event horizon 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 increase whenever matter or radiation fell into the black hole. The nondecreasing behaviour of a black hole\'s area was very significant for the behaviour of a physical quantity called entropy, which measures the degree of disorder of a system. It is a matter of common experience that disorder will tend to increase if things are left to themselves.
This idea combined with the second law of thermodynamics, leaded to a fatal law. If a black hole has entropy, then it ought also to have a temperature! But a body with a particular temperature must emit radiation at a certain rate. This radiation is required in order to prevent violation of the second law. So black holes ought to emit radiation. But by their very definition, black holes are objects that are not supposed to emit anything. It therefore seemed that the area of the event horizon of a black hole could not be regarded as its entropy.
But calculations showed that black holes in fact emitted radiation. The spectrum of the emitted particles was exactly that which would be emitted by a hot body, and the black hole was emitting particles at exactly the correct rate to prevent violations of the second law.
So there occurred a new question: How is it possible that a black hole appears to emit particles when we know that nothing can escape from within its event horizon? The answer, quantum theory tells us, is that the particles do not come from within the black hole, but from the \"empty\" space just outside the black hole\'s event horizon.
What we think of as \"empty\" space cannot be completely empty because that would mean that all the fields, such as the gravitational and electromagnetic fields, would have to be exactly zero. There must be a certain minimum amount of uncertainty, or quantum fluctuations, in the value of the field. One can think of these fluctuations as pairs of particles of light or gravity that appear together at some time, move apart, and then come together again and annihilate each other. These particles are virtual particles like the particles that carry the gravitational force of the sun: unlike real particles, they cannot be observed directly with a particle detector, but their indirect effects, like small changes in the energy of electron orbits in atoms, can be measured. And those measurements agree with the theoretical predictions with a high accuracy.
Heisenberg\'s uncertainty principle also predicts that there will be similar virtual pairs of matter particles, such as electrons or quarks. In this case one member of the pair will be a particle and the other an antiparticle. Out of the reason that energy cannot be created out of nothing, one of the partners in a particle/antiparticle pair will have positive energy, and the other partner negative energy. The one with negative energy is condemned to be a short-lived virtual particle because real particles always have positive energy in normal situations. It must therefore seek out its partner and annihilate with it. Normally, the energy of the particle is still positive, but the gravitational field inside a black hole is so strong that even a real particle can have negative energy there. If a black hole is present, it is possible for the virtual particle with negative energy to fall into the black hole and become a real particle or antiparticle. In this case it no longer has to annihilate with its partner. Now there are two possibilities what could happen: The second partner could also fall into the black hole and disappear forever, or it might also escape from the black hole as a real particle or antiparticle. To an observer at a distance, like us humans, it will appear to have been emitted from the black hole. And this is the radiation we can observe.
Because of this radiation, the black hole therefore reduces its mass, very slowly, but it does.
Moreover, the lower the mass of the black hole, the higher its temperature. So, as the black hole loses mass, its temperature and rate of emission increase, so it loses mass more quickly. What happens when the mass of the black hole eventually becomes extremely small, is not quite clear, but the most reasonable guess is that it would disappear completely in a huge final burst of emission, equivalent to the explosion of millions of H-bombs.

The eighth chapter is about the origin and fate of the universe as general relativity predicts it and when quantum effects are taken into account. Hawking first gives us an opportunity to think about the role the church had played in the picture of the universe, and then goes on with new theories science has uncovered.
We don´t yet have a complete and consistent theory that combines quantum mechanics and gravity, but we are fairly certain of some features that such a unified theory should have.
One is that it should incorporate Feynman´s proposal to formulate quantum theory in terms of a sum over histories. In this approach, a particle does not have just a single history, as it would in a classical theory. Instead, it is supposed to follow every possible path in space-time, and with each of these histories there are associated a couple of numbers, one representing the size of a wave and the other representing its phase. The whole thing works with probabilities of the sums of waves, associated with every possible history that a particle has.
To avoid technical problems, one must add up the waves for particle histories that are not in the \"real\" time that you and I experience but take place in what is called imaginary time. Imaginary time may sound like science fiction but it is in fact a well-defined mathematical concept. There are special numbers, called imaginary, that, unlike ordinary numbers, give negative numbers when multiplied by themselves. So for the purposes of the calculation of Feynman\'s theory, one must measure time using imaginary numbers, rather than real ones. This has an interesting effect on space-time: the distinction between time and space disappears completely.
A second feature that we believe must be part of any ultimate theory, is Einstein\'s idea that the gravitational field is represented by curved space-time. When we apply Feynman´s sum over histories to Einstein\'s view of gravity, the analogue of the history of a particle is now a complete curved space-time that represents the history of the whole universe.
In the classical theory of general relativity, there are many different possible curved space-times, each corresponding to a different initial state of the universe. If we knew the initial state of our universe, we would know its entire history!
Similarly, in the quantum theory of gravity, there are many different possible quantum states for the universe. Again, if we knew how the Euclidean curved space-times in the sum over histories behaved at early times, we would know the quantum state of the universe now and in the future.
In the classical theory of gravity, which is based on real space-time, there are only two possible ways the universe can behave: either it has existed for an infinite time, or else it had a beginning at a singularity at some finite time in the past. In the quantum theory of gravity, a third possibility arises. Because it is possible for space-time to be finite in extent and yet to have no singularities that formed a boundary or edge. Space-time would be like the surface of the earth, only with two more dimensions.

The ninth chapter discusses the arrow of time and its direction. There are at least three different arrows of time. First, there is the psychological arrow of time. This is the direction in which we feel time passes, the direction in which we remember the past but not the future. Then, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy increases. Finally, there is the cosmological arrow of time. This is the direction of time in which the universe is expanding rather than contracting.

Chapter ten speculates about the unification of physics. To remove infinities, one uses a process called renormalization, but this leads to many errors conflicting with observation. The introduction of \"supergravity\" caused problems, too. So, in 1984 there was a change of opinion in favour of string theories. In these theories the basic objects are not particles, which occupy a single point of space, but things that have a length but no other dimension, like an infinitely thin piece of string. These strings may have ends or they may join with themselves in closed loops. A particle occupies one point of space at each instant of time. Thus its history can be represented by a line in space-time, the \"world-line\". A string, on the other hand, occupies a line in space at each moment of time. So its history in space-time is a two-dimensional surface called the world-sheet. Any point on such a world-sheet can be described by two numbers: one tells the time and the other the position of the point on the string.
But, string theories seem to be consistent only if space-time has either ten or twenty-six dimensions, instead of the usual four. The suggestion is, therefore, that the other dimensions are curved up into a space of very small size.

The eleventh, final chapter, tries to draw a conclusion. The history of science (and time) is once again briefly summed up, and Stephen Hawking ends with the hope of finally gain understanding of everything that happens in our universe.


Ideas, opinions and comments:
I liked this book very much, for it is one of the very best I´ve ever read. Stephen Hawking points out the most complicated scientific facts in an easily understandable and very fascinating way. This book will attract the interest of any reader, and probably everyone willing to think about it will have no problems to understand it.
At the end I just want to mention that Stephen\'s book has sold 8 million copies world-wide and familiarised a whole generation with complex but intensely exciting scientific theories. I think that he has a very good ability to explain complicated things in an easy understandable way. I really loved to read this book and I can only strongly recommend this book to anyone!