Read Stephen Hawking's Brief History of the Universe. Stephen Hawking A Brief History of Time

Stephen Hawking

A BRIEF HISTORY OF TIME:

FROM THE BIG BANG TO BLACK HOLES

© Stephen Hawking, 1988, 1996

© AST Publishing House LLC, 2019 (design, translation into Russian)

Preface

I did not write the preface to the first edition of A Brief History of Time. Carl Sagan did it. Instead, I added a short section called “Acknowledgments,” where I was encouraged to express my gratitude to everyone. True, some of charitable foundations, who supported me, were not very happy that I mentioned them - they had a lot more applications.

I think that no one - not the publisher, not my agent, not even myself - expected the book to be such a success. It made it onto the London newspaper's bestseller list. Sunday Times as many as 237 weeks - this is more than any other book (naturally, not counting the Bible and the works of Shakespeare). It was translated into about forty languages ​​and sold in huge quantities - for every 750 inhabitants of the Earth, men, women and children, there is approximately one copy. As Nathan Myhrvold of the firm noted Microsoft(this is my former graduate student), I have sold more books on physics than Madonna has sold books on sex.

The success of A Brief History of Time means that people are very interested in fundamental questions about where we came from and why the universe is the way we know it.

I took advantage of the opportunity presented to me to supplement the book with newer observational data and theoretical results that were obtained after the release of the first edition (April 1, 1988, on April Fool's Day). I've added a new chapter on wormholes and time travel. It appears that Einstein's general theory of relativity allows for the possibility of creating and maintaining wormholes—small tunnels connecting different regions of space-time. In this case, we could use them to quickly move around the Galaxy or to travel back in time. Of course, we have not yet met a single alien from the future (or maybe we have?), but I will try to guess what the explanation for this might be.

I will also talk about what has been achieved so far Lately progress in the search for “duality,” or correspondence between seemingly different physical theories. These correspondences are serious evidence in favor of the existence of a unified physical theory. But they also suggest that the theory may not be formulated in a consistent, fundamental way. Instead, in different situations one has to be content with various “reflections” of the fundamental theory. Likewise, we cannot depict the entire earth's surface in detail on one map and are forced to use different maps for different areas. Such a theory would be a revolution in our ideas about the possibility of unifying the laws of nature.

However, it would in no way affect the most important thing: the Universe is subject to a set of rational laws that we are able to discover and comprehend.

As for the observational aspect, the most important achievement here, of course, was the measurement of fluctuations of the cosmic microwave background radiation within the framework of the project COBE(English) Cosmic Background Explorer –"Cosmic Background Radiation Researcher") 1
Fluctuations, or anisotropies, of the cosmic microwave background radiation were first discovered by the Soviet Relict project. – Note scientific ed.

And others. These fluctuations are essentially the “seal” of creation. We are talking about very small inhomogeneities in the early Universe, which was otherwise quite homogeneous. Subsequently, they turned into galaxies, stars and other structures that we observe through a telescope. The forms of fluctuations are consistent with the predictions of a model of the Universe that has no boundaries in the imaginary time direction. But in order to prefer the proposed model to other possible explanations for CMB fluctuations, new observations will be required. In a few years it will become clear whether our Universe can be considered completely closed, without beginning and end.

Stephen Hawking

Chapter first. Our picture of the Universe

Once a famous scientist (they say it was Bertrand Russell) read public lecture in astronomy. He talked about how the Earth moves in orbit around the Sun and how the Sun, in turn, moves in orbit around the center of a huge collection of stars called our Galaxy. When the lecture ended, a small elderly woman in the back row of the audience stood up and said: “Everything that was said here is complete nonsense. The world is a flat plate on the back of a giant turtle." The scientist smiled condescendingly and asked: “What is that turtle standing on?” “You are a very smart young man, very smart,” the lady answered. “A turtle stands on another turtle, which stands on the next one, and so on ad infinitum!”

Most would consider it ridiculous to attempt to pass off our Universe as an infinitely tall tower of turtles. But why are we so sure that our idea of ​​the world is better? What do we really know about the Universe and how do we know all this? How did the Universe originate? What does the future hold for her? Did the Universe have a beginning, and if so, what came before it? What is the nature of time? Will it ever end? Is it possible to go back in time? Some of these long-standing questions are being answered by recent breakthroughs in physics, thanks in part to the advent of fantastic new technologies. Someday we will find new knowledge as obvious as the fact that the Earth revolves around the Sun. Or maybe as absurd as the idea of ​​a tower of turtles. Only time (whatever that is) will tell.

A long time ago, 340 years BC, the Greek philosopher Aristotle wrote a treatise “On Heaven”. In it, he put forward two convincing proofs that the Earth is spherical and not at all flat, like a plate. First, he realized that the reason lunar eclipses– the passage of the Earth between the Sun and the Moon. The shadow cast by the Earth on the Moon is always round in shape, and this is only possible if the Earth is also round. If the Earth were shaped like a flat disk, the shadow would typically be elliptical; It would be round only if the Sun during an eclipse was located exactly under the center of the disk. Secondly, the ancient Greeks knew from the experience of their travels that in the south the North Star is located closer to the horizon than when observed in areas located to the north. (Since the North Star is located above the North Pole, an observer at the North Pole sees it directly overhead, and an observer near the equator sees it just above the horizon.) Moreover, Aristotle, based on the difference in the apparent position of the North Star during observations in Egypt and Greece, was able to estimate the circumference of the Earth at 400,000 stadia. We do not know exactly what one stade was equal to, but if we assume that it was about 180 meters, then Aristotle's estimate is about twice the currently accepted value. The Greeks also had a third argument in favor of the round shape of the Earth: how else to explain why, when a ship approaches the shore, first only its sails are shown, and only then the hull?

Aristotle believed that the Earth was motionless, and also believed that the Sun, Moon, planets and stars revolved in circular orbits around the Earth. He was guided by mystical considerations: the Earth, according to Aristotle, is the center of the Universe, and circular motion is the most perfect. In the 2nd century AD, Ptolemy built a comprehensive cosmological model based on this idea. At the center of the Universe was the Earth, surrounded by eight nested rotating spheres, and on these spheres were located the Moon, Sun, stars and the five planets known at that time - Mercury, Venus, Mars, Jupiter and Saturn (Fig. 1.1). Each planet moved relative to its sphere in a small circle - in order to describe the very complex trajectories of these luminaries in the sky. The stars were fixed to the outer sphere, and therefore their relative positions remained unchanged, the configuration rotating in the sky as a single whole. Ideas about what was located outside the outer sphere remained very vague, but it was certainly located outside the part of the Universe accessible to humanity for observation.

Ptolemy's model made it possible to quite accurately predict the position of the luminaries in the sky. But in order to achieve agreement between predictions and observations, Ptolemy had to assume that the distance from the Moon to the Earth is different time could differ twofold. This meant that the apparent size of the Moon sometimes had to be twice as large as usual! Ptolemy was aware of this shortcoming of his system, which nevertheless did not prevent the almost unanimous recognition of his picture of the world. The Christian Church accepted the Ptolemaic system because it found it consistent with Scripture: there was plenty of room for heaven and hell beyond the sphere of the fixed stars.

But in 1514, the Polish priest Nicolaus Copernicus proposed a simpler model. (However, at first, for fear of being accused of heresy by the church, Copernicus disseminated his cosmological ideas anonymously.) Copernicus proposed that the Sun was motionless and located in the center, and the Earth and planets moved around it in circular orbits. It took almost a century for this idea to be taken seriously. Two astronomers, the German Johannes Kepler and the Italian Galileo Galilei, were among the first to publicly speak out in favor of the Copernican theory, despite the fact that the trajectories of celestial bodies predicted by this theory did not coincide exactly with those observed. The final blow to the world system of Aristotle and Ptolemy was dealt by the events of 1609 - then Galileo began observing the night sky through the newly invented telescope 2
The telescope as a spotting scope was first invented by the Dutch spectacle maker Johann Lippershey in 1608, but Galileo was the first to point a telescope at the sky in 1609 and use it for astronomical observations. – Note translation

Looking at the planet Jupiter, Galileo discovered several small moons orbiting around it. It followed that not all celestial bodies revolve around the Earth, as Aristotle and Ptolemy believed. (One could, of course, continue to consider the Earth stationary and located at the center of the Universe, believing that the satellites of Jupiter move around the Earth in extremely intricate trajectories so that it is similar to their revolution around Jupiter. But still, Copernicus’ theory was much simpler.) Approximately at the same time, Kepler clarified the Copernican theory, suggesting that the planets do not move in circular orbits, but in elliptical (i.e., elongated) orbits, thanks to which it was possible to achieve agreement between the predictions of the theory and observations.

True, Kepler considered ellipses only as a mathematical trick, and a very odious one at that, because ellipses are less perfect figures than circles. Kepler discovered, almost by accident, that elliptical orbits described observations well, but he could not reconcile the assumption of elliptical orbits with his idea of magnetic forces as the reason for the movement of planets around the Sun. The reason for the motion of the planets around the Sun was revealed much later, in 1687, by Sir Isaac Newton in his treatise “Mathematical Principles of Natural Philosophy” - perhaps the most important work on physics ever published. In this work, Newton not only put forward a theory describing the movement of bodies in space and time, but also developed a complex mathematical apparatus necessary to describe this movement. In addition, Newton formulated the law universal gravity, according to which every body in the Universe is attracted to any other body with a force, which is greater, the greater the mass of the bodies and the smaller the distance between the interacting bodies. This is the same force that causes objects to fall to the ground. (The story that Newton's idea of ​​the law of universal gravitation was inspired by an apple falling on his head is most likely just a fiction. Newton said only that the idea came to him when he was "in a contemplative mood" and was "under the impression from the fall of an apple.”) Newton showed that, according to the law he formulated, under the influence of gravity, the Moon should move in an elliptical orbit around the Earth, and the Earth and the planets should move in elliptical orbits around the Sun.

The Copernican model eliminated the need for Ptolemaic spheres, and with them the assumption that the Universe had some kind of natural external boundary. Since the “fixed” stars did not show any movement other than the general daily movement of the sky caused by the rotation of the Earth around its axis, it was natural to assume that these were the same bodies as our Sun, only located much further away.

Newton realized that according to his theory of gravity, stars must attract each other and therefore, apparently, cannot remain motionless. Why didn’t they get closer and accumulate in one place? In a letter to another prominent thinker of his time, Richard Bentley, written in 1691, Newton argued that they would converge and cluster only if the number of stars concentrated in a limited region of space was finite. But if the number of stars is infinite and they are distributed more or less evenly in infinite space, then this will not happen due to the absence of any obvious center point, into which the stars could “fall through.”

This is one of those pitfalls that occur when thinking about infinity. In an infinite Universe, any point can be considered its center, because on each side of it there is an infinite number of stars. The correct approach (which came much later) is to solve the problem in the finite case where stars fall on each other, and study how the result changes when adding stars to the configuration that are located outside the region under consideration and are distributed more or less evenly. According to Newton's law, on average, the additional stars in the aggregate should have no effect on the original stars, and therefore these stars of the original configuration should still fall rapidly into one another. So no matter how many stars you add, they will still fall on top of each other. Now we know that it is impossible to obtain an infinite stationary model A universe in which the force of gravity is exclusively “attractive” in nature.

It says a lot about the intellectual atmosphere before the beginning of the 20th century that no one then thought of a scenario according to which the Universe could contract or expand. The generally accepted concept of the Universe was either that it had always existed in an unchanged form, or that it had been created at some point in the past - in the form in which we observe it now. This could, in part, be a consequence of the fact that people tend to believe in eternal truths. It is worth remembering at least that the greatest comfort comes from the thought that although we all grow old and die, the Universe is eternal and unchanging.

Even scientists who understood that, according to Newton's theory of gravity, the Universe could not be static, did not dare to suggest that it could expand. Instead, they tried to adjust the theory so that the gravitational force becomes repulsive over very large distances. This assumption did not significantly change the predicted movements of the planets, but allowed an infinite number of stars to remain in a state of equilibrium: the attractive forces from nearby stars were balanced by the repulsive forces from more distant stars. Now it is believed that such an equilibrium state must be unstable: as soon as the stars in any region get a little closer to each other, their mutual attraction will intensify and exceed the repulsive forces, as a result of which the stars will continue to fall on each other. On the other hand, if the stars are only slightly further away from each other, the repulsive forces will prevail over the attractive forces and the stars will fly apart.

Another objection to the concept of an infinite static universe is usually associated with the name of the German philosopher Heinrich Olbers, who published his reasoning on this matter in 1823. In fact, many of Newton's contemporaries drew attention to this problem, and Olbers's article was by no means the first to present strong arguments against such a concept. However, it was the first to receive widespread recognition. The fact is that in an infinite static Universe, almost any ray of vision should rest on the surface of some star, and therefore the entire sky should glow as brightly as the Sun, even at night. Olbers' counter-argument was that the light from distant stars must be attenuated by absorption by matter between us and those stars. But then this substance would heat up and glow as brightly as the stars themselves. The only way to avoid the conclusion that the brightness of the entire sky is comparable to the brightness of the Sun is to assume that the stars did not shine forever, but “lit up” some specific time ago. In this case, the absorbing substance would not have time to heat up or the light of distant stars would not have time to reach us. Thus, we come to the question of the reason why the stars lit up.

Of course, people discussed the origin of the universe long before this. In many early cosmological ideas, as well as in Jewish, Christian and Muslim pictures of the world, the Universe arose at a certain and not very distant time in the past. One of the arguments in favor of such a beginning was the feeling of the need for some kind of first cause that would explain the existence of the Universe. (Within the Universe itself, any event that occurs in it is explained as a consequence of another, earlier event; the existence of the Universe itself can be explained in this way only by supposing that it had some kind of beginning.) Another argument was expressed by Aurelius Augustine, or St. Augustine, in the work “On the City of God”. He noted that civilization is developing and that we remember who committed this or that act or invented this or that mechanism. Consequently, man, and perhaps the Universe, could not exist very for a long time. St. Augustine believed, in accordance with the Book of Genesis, that the Universe was created approximately 5000 years before the birth of Christ. (Interestingly, this is close to the end of the last Ice Age - around 10,000 BC - which archaeologists consider the beginning of civilization.)

Aristotle, as well as most ancient Greek philosophers, on the contrary, did not like the idea of ​​​​the creation of the world, because it came from divine intervention. They believed that the human race and the world have always existed and will exist forever. The thinkers of antiquity also comprehended the above-mentioned argument about the progress of civilization and countered: they stated that the human race periodically returned to the stage of the beginning of civilization under the influence of floods and other natural disasters.

Questions about whether the Universe had a beginning in time and whether it is limited in space were also raised by the philosopher Immanuel Kant in his monumental (though very difficult to understand) work “Critique of Pure Reason,” published in 1781. Kant called these questions the antinomies (that is, contradictions) of pure reason because he felt that there were equally compelling arguments for both the thesis - that is, that the Universe had a beginning - and the antithesis, that is, that the Universe has always existed . To prove his thesis, Kant cites the following reasoning: if the Universe had no beginning, then any event should have been preceded by an infinite time, which, according to the philosopher, is absurd. In favor of the antithesis, the consideration was put forward that if the Universe had a beginning, then an infinite amount of time must have passed before it, and it is not clear why the Universe arose at any specific moment in time. In essence, Kant's justifications for thesis and antithesis are almost identical. In both cases, the reasoning is based on the philosopher's implicit assumption that time continues indefinitely into the past, regardless of whether the Universe has always existed. As we will see, the concept of time has no meaning until the birth of the Universe. St. Augustine was the first to note this. He was asked, “What did God do before he created the world?” and Augustine did not argue that God was preparing hell for those who asked such questions. Instead, he postulated that time is a property of God's created world and that before the beginning of the universe, time did not exist.

When most people considered the universe as a whole to be static and unchanging, the question of whether it had a beginning was more a matter of metaphysics or theology. The observed picture of the world could equally well be explained both within the framework of the theory that the Universe has always existed, and on the basis of the assumption that it was set in motion at some specific time, but in such a way that the appearance remains that it exists forever. But in 1929, Edwin Hubble made a fundamental discovery: he noticed that distant galaxies, no matter where they are in the sky, are always moving away from us at high speeds [proportional to their distance] 3
Here and below, the translator's comments clarifying the author's text are placed in square brackets. – Note ed.

In other words, the Universe is expanding. This means that in the past, objects in the Universe were closer to each other than they are now. And it seems that at some point in time - somewhere 10-20 billion years ago - everything that is in the Universe was concentrated in one place, and therefore the density of the Universe was infinite. This discovery brought the question of the beginning of the Universe into the realm of science.

I mastered Stephen Hawking's book "The Brief History of Time". The author himself became familiar to many - this is the same brilliant physicist confined to a wheelchair.

The book is interesting, well written and accessible. What especially struck the imagination in my summary:
1) If you lay on geographical map using a ruler to draw a straight line between two points, then this straight line will not be the shortest distance between two points. The shortest curve will be in the form of an arch, the radius of which is equal to the radius of the Earth.
2) In the presence of matter, four-dimensional space-time is distorted, causing curvature of the trajectories of bodies in three-dimensional space. Although it is difficult to visualize, the mass of the Sun bends space-time in such a way that the Earth, following the shortest path in four-dimensional space-time, appears to us to be moving in a nearly circular orbit in three-dimensional space.
3) The general theory of relativity declares that the passage of time is different for observers located in different gravitational fields. If one of the twins lives on top of a mountain and the other by the sea, the first will age faster than the second.
4) If we knew the state of the system at a given moment and knew the laws of development of the system, we could predict the position of the system at any time. So, the Heisenberg uncertainty principle generally states that no matter how much we puff ourselves up, we absolutely cannot determine the state of the Universe at the present moment. And this is not related to the level of development of science. This is closer to a philosophical principle - we, in principle, cannot know the position of any system at any particular moment. We know at any moment either the speed of the particle or its location. Exactly one of the two, but not both values ​​at once.
Therefore, accept it - any prediction in our Universe is impossible in principle. From a purely philosophical point of view. Any.
5) If we send an electron into the wall, and put two slits for it to pass through, then, damn it, it will pass through both slits at once. Pause for reflection. In general, an electron can be in all possible positions at the same time. Because the creature is so small, he is not only a particle, but when he wants, he is also a wave. The binding of an electron to specific orbits of an atom is precisely due to the fact that it is in these orbits that the electron does not interfere with itself, i.e. does not extinguish itself. Once again, an electron, flying from one point to another, flies along all possible trajectories at once. He is essentially capable of being in all points of space at the same time, and only there he is not, where he interferes with himself.
6) Purely theoretically, time travel to the past is possible. Solving the equations of the theory of relativity shows that yes, this is so. One thing is true: to travel back in time, you must move faster than the speed of light. And vice versa - movement faster than the speed of light is impossible without simultaneous movement into the past.
Those who know that you cannot move faster than the speed of light breathe a sigh of relief. But there is one more problem - purely, again, hypothetically, traveling faster than the speed of light is also possible. Possible in the case of the existence of wormholes in space-time. And the damn equations show that yes, such holes can exist. And if they can, then they exist somewhere.
7) The newest theory, which simply amazingly describes the latest discoveries in science and anticipates them, is string theory. Nothing special, just everything that is predicted by this theory is later confirmed by one-to-one experiments. And this is really annoying. It’s annoying, because string theory takes as an assumption one small statement - we live not in a four-dimensional world, but in a 26-dimensional one. Moreover, 4 dimensions are expanded, and we can move along them, and another 22 are collapsed into a point. Physicists would happily abandon this theory, but they have not yet come up with anything more intelligible in terms of mathematics, and experiments continue to perfectly coincide with the predictions made on the basis of this theory.

In general, it seems to me that our Universe, like that electron, is capable of being in all states at the same time, with the exception of those states in which it interferes with itself. And now I am simultaneously in Krasnodar and Moscow and on Alpha Centauri. And at the same time, there is no me at all. But the idea of ​​the ent is clearly worthy of chewing on in a separate abstruse philosophical book.

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Leonard Mlodinow

A Brief History of Time

© Stephen Hawking and Leonard Mlodinow, 2005

© AST Publishing House LLC, 2017 (translation into Russian)

* * *

Preface

The title of this book differs from the title of the first one, published in 1988, by just one word. A Brief History of Time remained on the London newspaper's bestseller list The Sunday Times over the course of 237 weeks - approximately one copy was sold for every 750 inhabitants of the Earth, men, women and children. This is an incredible achievement for a book dealing with one of the most difficult aspects of modern physics. But the most difficult thing is always the most interesting, because we're talking about about important, fundamental questions: what do we really know about the Universe? How do we know this? How did the Universe come into being and what fate awaits it? These questions are at the heart of A Brief History of Time, and they remain the focus of this book. In the years since A Brief History appeared on the shelves, I have received many letters from all over the world from readers of all ages and backgrounds. One of the most common requests is to write a new “Brief History”, keeping the essence of the previous one, but presenting the main ideas more clearly and slowly. Of course, one could call this book “A Slightly Less Brief History of Time,” but, as I understand it, hardly anyone would want to get an impressive volume that resembles a university course on cosmology.

So, a few words about the nature of this book. In writing A Brief History of Time, we followed the logic of the first edition, but expanded it, keeping in mind that A new book should be easy to read and not too long. The story was indeed shortened, since I excluded some overly complex, technical aspects, but this was more than compensated for by an in-depth approach to the material underlying the book.

We also took the opportunity to expand the publication by including new observational and theoretical data. A Brief History of Time describes the recent achievements of theoretical physicists struggling with a unified theory of all physical forces. In particular, we are talking about the progress of string theory, as well as dualism, or mutual correspondences between seemingly completely different physical theories, which can be considered as evidence of the existence of a single theory - the foundation of all physical science. The book also presents important new observations made by the satellite. COBE(English) Cosmic Background Explorer– “CMB Explorer”) and the Hubble Space Telescope.

About forty years ago, Richard Feynman said: “We are very lucky to live in an age where we are still making discoveries. It's like discovering America - you only do it once. The time in which we live is the era of discoveries of the fundamental laws of nature.” Today we are closer than ever to understanding the nature of the Universe, and in these pages we want to share with the reader the delight of getting acquainted with these discoveries and new picture the world they are shaping before our eyes.

Chapter 1. Reflections on the Universe

We live in a strange and wonderful Universe. It takes a remarkable imagination to understand and appreciate her age, size, violent temperament and beauty. And it seems that we occupy a very insignificant place in this vast cosmos, and we want to understand it and realize our role in the Universe. Several decades ago, a famous scientist (said to be Bertrand Russell), giving a public lecture on astronomy, explained how the Earth moves in orbit around the Sun and how the Sun in turn moves in orbit around the center of a huge collection of stars called the Galaxy. When the lecture ended, a small elderly woman at the very end of the room said: “Everything they said here is complete nonsense. The world is a flat plate on the back of a giant turtle." The scientist smiled condescendingly and asked: “What is the turtle standing on?” “Well, you’re a very smart young man,” said the elderly woman, “a turtle stands on another turtle, and that turtle stands on the next one, and so on until the end!”

Nowadays, most would consider the picture of the Universe in the form of an endless tower of turtles to be ridiculous. How do we know that our view of the world is better? Let's for a moment forget everything we know or think we know about space and just look at the night sky. Well, what can we say about these luminous points? Maybe these are small lights? It is actually difficult for us to imagine their true essence because it is far beyond our everyday experience. If you like to look at the stars, you may have noticed a blurry point of light near the horizon during twilight. This is the planet Mercury, but it is not at all like our Earth. A day there lasts two-thirds of the local year. The temperature of the part of the planet’s surface illuminated by the Sun reaches 400°C and above, and on the night side, not illuminated, it drops to –200°C. But despite all its differences from our own planet, Mercury has even less in common with a typical star, which is a gigantic furnace, where billions of kilograms of matter are burned every second, and the temperature in the core reaches tens of millions of degrees.

It is also very difficult to imagine how far planets and stars are from us. IN Ancient China They built stone towers in the hope of getting a closer look at the stars. It is quite natural to imagine stars and planets located much closer than they actually are - after all, in ordinary life we do not have to deal with colossal cosmic distances. They are so large that there is no point in trying to measure them in meters and centimeters, as is the case with most distances and lengths in our everyday life. Cosmic distances are usually measured in light years. A light year is the distance that light travels in one year. In one second, a beam of light travels about 300,000 kilometers. So a light year is a very long distance. The closest star to us after the Sun is Proxima Centauri (also known as Alpha Centauri C) - located at a distance of about 4 light years. This is so far that the fastest one actually designed spaceships It will take at least 10,000 years to overcome the space separating us.

People in ancient times tried very hard to understand the structure of the Universe, but they did not yet have modern mathematics and in general modern science. We now have very powerful thinking tools at our disposal, such as mathematics and scientific method, and technical means like computers and telescopes. Thanks to this, we were able to learn a lot about space. But what do we really know about the Universe and how do we know it all? How did the Universe originate? What does the future hold for her? Did the Universe have a beginning, and if so, what came before it? What is the nature of time? Will it ever end? Is it possible to go back in time? Some of these long-standing questions are being answered thanks to recent breakthroughs in physics, thanks in part to the advent of new technologies. Someday we will find these answers as obvious as the fact that the Earth revolves around the Sun. Or maybe as ridiculous as the idea of ​​a tower of turtles. Only time (whatever that is) will tell.

Chapter 2. Our picture of the Universe yesterday and today

Although even in the time of Christopher Columbus many believed that the Earth was flat (and such people still exist today), the foundations of modern astronomy were laid back in Ancient Greece. Around 340 BC, the Greek philosopher Aristotle wrote his treatise On Heaven. In it, he laid out a lot of evidence that the Earth is spherical and not flat like a plate.

One such consideration is based on the observation of lunar eclipses. Aristotle realized that the cause of these eclipses was the passage of the Earth between the Sun and the Moon. At the same time, the Earth casts a shadow on the Moon, and we see it as an eclipse. Aristotle noticed that the Earth's shadow always has a round shape, which is natural if the Earth is spherical. But, of course, this would not be the case if the Earth had the shape of a flat disk. In this case, the shadow would be round only if during the eclipse the Sun is located exactly under the center of the disk. With any other arrangement, the shadow would be elongated, in the shape of an ellipse (an elongated circle).

The ancient Greeks had other arguments in favor of the sphericity of the Earth. If the Earth were flat, then a ship heading towards the shore should first look like a small, barely noticeable point. Then, as the ship approaches, individual details could be distinguished on it - the sails and the hull. But in reality, everything is completely different. When a ship appears on the horizon, at first we only see its sails. And only then the body appears. The fact that the tops of the ship's masts, located high above the hull, are the first to appear over the horizon indicates the spherical shape of the Earth.

Appearing above the horizon. The earth has the shape of a ball. Therefore, when a ship approaches us, first we see its masts and sails above the horizon, and only then its hull appears

The Greeks also paid attention to the starry sky. By the time of Aristotle, they had already been studying the movements of lights in the night sky for many hundreds of years. They noticed that although thousands of lights move across the sky as one, five luminaries, not counting the Moon, move differently from the rest. They sometimes turn off the beaten path from east to west and even sometimes even move backwards. These luminaries were called planets from the Greek word meaning “wanderers.” The Greeks saw only five planets because they were the only ones visible to the naked eye: Mercury, Venus, Mars, Jupiter and Saturn. Now we know why the planets move across the sky in such an unusual way: the movement of stars relative to ours solar system almost imperceptibly, but the planets orbit around the Sun and therefore write out much more complex trajectories against the background of distant stars.

Aristotle believed that the Earth was motionless, and also believed that the Sun, Moon, planets and stars revolved in circular orbits around the Earth. He thought so based on mystical considerations, believing that the Earth is the center of the Universe and movement in a circle is most perfect. In the 2nd century AD, Greek scientist Ptolemy built a complete model of the sky based on this idea. Ptolemy was a passionate explorer, it is not for nothing that he wrote the words: “That I am mortal, I know, and that my days are numbered, but when in my thoughts I tirelessly and greedily track down the orbits of the constellations, then I no longer touch the Earth with my feet: at the table of Zeus I enjoy ambrosia, food of the gods."

In Ptolemy's model of the world, we are surrounded by eight nested rotating spheres like a nesting doll, and in the center of all these spheres is the Earth. Ideas about what was outside the largest sphere were the foggiest, but in any case it was outside the Universe observable by man. Thus, the outermost sphere represented a kind of boundary of the Universe. The stars were fixed on this sphere, and therefore, when it rotated, the relative positions of the stars remained unchanged - exactly as we observe it in reality. The planets were located on the inner spheres. Unlike stars, they were not attached to their spheres, and each planet moved relative to its sphere in a small circle called an epicycle. Very complex non-circular visible trajectories of planets in the sky could be explained by a combination of movement along the epicycle and rotation of the sphere.

Ptolemy's model. In Ptolemy's model, the Earth was at the center of the Universe, surrounded by eight spheres containing all the celestial bodies known at that time

Ptolemy's model made it possible to quite accurately predict the position of the luminaries in the sky. But in order to achieve agreement between predictions and observations, Ptolemy had to assume that the distance from the Earth to the Moon could change by half! This meant that the apparent size of the Moon should sometimes be twice as large as at other times! Ptolemy was aware of this shortcoming of his system, which, however, did not prevent the (almost) universal recognition of his picture of the world. The Christian Church accepted the Ptolemaic system because it found it consistent with Scripture: there was plenty of room for heaven and hell beyond the sphere of the fixed stars.

But in 1514, the Polish priest Nicolaus Copernicus proposed a different model. (True, at first, for fear of being accused of heresy by the Church, Copernicus disseminated his ideas anonymously.) The revolutionary nature of Copernicus’ idea lay in the assumption that all celestial bodies revolve around the Earth. Copernicus believed that the Sun is motionless and located in the center of the solar system, and the Earth and planets move around it in circular orbits. The Copernican model turned out to be no worse than the Ptolemaic model, but it still did not accurately predict the observations. It was much simpler than the Ptolemaic model, so it could be expected that people would accept it. However, it took almost a century for this idea to be taken seriously. Two scientists were among the first to publicly speak out in favor of the Copernican theory: German astronomer Johannes Kepler and Italian astronomer Galileo Galilei.

In 1609, Galileo began observing the night sky with a telescope he had just invented. Looking at the planet Jupiter, Galileo discovered several small moons orbiting around it. It followed from this that not all celestial bodies revolve around the Earth, as Aristotle and Ptolemy believed. Around the same time, Kepler refined the Copernican theory by suggesting that the planets move not in circular orbits, but in ellipses, thanks to which it was possible to achieve agreement between the theory's prediction and observations. All this finally finished off the Ptolemaic world system.

Although the assumption of elliptical orbits made Copernicus's model more accurate, Kepler considered this only a mathematical trick, since his ideas about the structure of nature were not based on observations. Like Aristotle, Kepler considered ellipses to be less perfect figures than circles. The very idea that planets could move on such imperfect trajectories seemed too ugly to be true. In addition, Kepler did not like the fact that the assumption of elliptical orbits did not agree with his idea of ​​​​magnetic forces as the cause of the movement of planets around the Sun. He was, of course, wrong about magnetism, but we must give him credit for the very idea that the motion of the planets must be caused by some force. The correct explanation for the motion of the planets around the sun was given much later in 1687 by Sir Isaac Newton in his treatise, The Mathematical Principles of Natural Philosophy, perhaps the most important work on physics ever published.

In this work, Newton formulated the law according to which a body at rest remains at rest unless acted upon by some force, and also described how the motion of a body changes under the influence of a force. So why do planets move around the Sun in elliptical orbits? According to Newton, a very specific force is responsible for this - the same one that makes the released (dropped) body fall to the ground and not remain at rest. He called this force gravity and developed a mathematical apparatus that allowed him to calculate how bodies react to a force applied to them, such as gravity, and also solved the corresponding equations. Thus, Newton was able to show that, under the influence of the Sun's gravity, the Earth and other planets should move in elliptical orbits exactly as Kepler predicted! Newton assumed that his laws were valid for everything in the Universe, from a falling apple to stars and planets. For the first time in history, the movements of planets and the movements of bodies on Earth could be explained as a consequence of the same laws, and this was the birth of modern physics and modern astronomy.

In the absence of the Ptolemaic spheres, there was no longer any need to assume that the Universe had some kind of external boundary. Moreover, since the stars did not show any movement other than the general daily movement of the sky caused by the rotation of the Earth, it was natural to assume that these were the same bodies as our Sun, only located much further away. Thus, scientists not only abandoned the idea of ​​the central position of the Earth in the Universe, but also the idea of ​​​​the uniqueness of our Sun and the entire Solar System. The new view of the world marked a fundamental change in human thinking, the beginning of a new modern scientific understanding of our Universe.

Chapter 3. The nature of scientific theory

Before discussing the nature of the Universe and answering questions about whether it had a beginning and whether there is an end, we should form a clear idea of ​​what scientific theories are. We will stick to simple glance to a theory - as a model of the Universe or any part of it, together with a set of rules connecting the parameters of this model with our observations. It exists only in our consciousness and does not really exist in any other way (whatever that means). A theory is considered good if it satisfies two requirements. First, it must correctly describe a large class of observations based on a model with a small number of arbitrary elements, and second, it must be able to predict the results of future observations with reasonable certainty. For example, Aristotle believed in Empedocles' theory, according to which everything in the world consists of four elements: earth, air, fire and water. It was a fairly simple theory, but it did not allow for any precise predictions. On the other hand, Newton's theory of gravity was based on an even simpler model, in which bodies attract each other with a force proportional to a quantity he called mass, and inversely proportional to the square of the distance between the bodies. And at the same time, Newton’s theory allows very high accuracy predict the movements of the Sun, Moon and planets.

Any physical theory is by its nature provisional in the sense that it is just a hypothesis that cannot be proven. No matter how many experiments confirm this theory, you can never be sure that the next result will not contradict it. On the other hand, to refute a theory, a single observation is sufficient, the results of which contradict its predictions. As the philosopher of science Karl Popper noted, a good theory is one that makes many predictions that can in principle be falsified or, as Popper calls it, falsified by observation. With each new experiment, the results of which are consistent with the predictions of the theory, the degree of our confidence in it increases, and the theory itself becomes stronger. However, the very first observation that contradicts the theory is grounds to reject or significantly change it.

In any case, this should be the case ideally, although, of course, one can always question the qualifications of the observer or experimenter.

In practice, a new theory is often an extension of a previous one. For example, very precise observations of the planet Mercury have revealed small discrepancies between the observed motion and the predictions of Newton's theory of gravity. The planet's motion, as calculated by Einstein's general theory of relativity, differed slightly from what Newton's theory predicted. The agreement of the motion of Mercury predicted by Einstein's theory with observations, in the absence of such agreement for Newton's theory, became one of the key confirmations of the new theory. Nevertheless, we still continue to use Newton's theory for most practical problems, because in the situations we usually encounter, its predictions differ very little from those of general relativity. (Besides, Newton's theory is much simpler than Einstein's theory!)

The ultimate goal of science is to create a unified theory to describe the entire Universe. But in reality, the approach of most scientists comes down to dividing the problem into two parts. First, there are laws that govern how the universe changes over time. (If we know the state of the Universe at a certain moment in time, then such physical laws allow us to determine what it will look like at any other moment.) The second question is the initial state of the Universe. Some believe that science should deal only with the first problem, and that the question of the initial state rather falls within the purview of metaphysics or religion. They believe that God, being omnipotent, could create the universe in any way he desired. This may be true, but then God could also have caused the universe to evolve in a completely arbitrary manner. However, it seems that God wanted the universe to develop according to clearly defined laws. And therefore it seems quite reasonable to assume that the initial state of the Universe also obeyed clearly defined laws.

Creating a theory that describes the entire Universe at once turned out to be very difficult. Instead, scientists have divided the problem into many parts and built many partial theories. Each of these partial theories describes and predicts a certain limited class of observations, neglecting the influence of other factors, or representing them as simple sets of numbers. It is quite possible that this approach is fundamentally wrong. If everything in the Universe is fundamentally interdependent, then it is, of course, impossible to obtain a complete solution by studying the problem in parts in isolation from the whole. Nevertheless, until now this approach has ensured the progress of science. Again classic example Newton's theory of gravity can serve, according to which the force of mutual attraction of bodies depends only on the numerical characteristic inherent in each of the bodies - its mass - and does not depend at all on what these bodies are made of. Thus, the orbits of planets can be calculated without going into details of their structure and internal structure.

Currently, two fundamental partial theories are used to describe the Universe: general relativity and quantum mechanics. These are two great intellectual achievements of the first half of the 20th century. General relativity describes gravity and the large-scale structure of the Universe, that is, its structure on scales from a few kilometers to a million million million million (one followed by twenty-four zeros) kilometers - the size of the observable Universe. On the other hand, quantum mechanics deals with phenomena on extremely small scales, such as a millionth of a millionth of a centimeter. But, unfortunately, these two theories are known to be incompatible with each other and therefore cannot both be correct. One of the main directions of research in physics today and main theme This book is to develop a new theory that would combine both special cases - the quantum theory of gravity. There is no such theory yet, and perhaps we are still far from creating it, but we already know many of the properties that it should have. And as will be seen in subsequent chapters, we already know quite a few inevitable predictions of the quantum theory of gravity.

From atoms to galaxies. In the first half of the 20th century, physicists, making assumptions about the structure of the world, tried to cover not only the familiar world of Isaac Newton: theories appeared that described extremely large and extremely small objects

So if we assume that the Universe is not structured in an arbitrary way, but is subject to certain laws, it will be necessary to eventually combine particular theories into one comprehensive theory that can describe everything in the Universe. But the search for such a complete unified theory is associated with a fundamental paradox. The view of scientific theories described above assumes that we are intelligent beings who are free to observe the universe in the ways we wish and draw logical conclusions from what we see. In such a scheme, there is reason to believe that we can move closer and closer to the laws that govern our Universe. But if a complete unified theory really existed, then it would most likely also determine our actions themselves, that is, including the result of our search! And why should it follow from it that we will come to the correct conclusions based on the data obtained? But wouldn’t it follow from the theory that we would come to erroneous conclusions? Or will we get no conclusions at all?

The only way to solve this problem is based on Darwin's principle of natural selection. The idea is that individuals in any population of self-replicating organisms will inevitably differ in their genetic material and upbringing. This means that some individuals will be better than others at making correct inferences about the world around them and acting accordingly. They will be more likely to survive and reproduce, so their behavior and thoughts will become dominant. Of course, in the past, intelligence and scientific discoveries more than once they have become an advantage for survival. It is not entirely clear that this is still the case: after all, our scientific discoveries may well completely destroy us all, and even if this does not happen, a comprehensive unified theory may not be particularly important for our chances of survival. However, if the Universe evolves in a regular manner, then we can expect that the data given to us natural selection rational abilities will also manifest themselves in our search for a comprehensive unified theory and therefore will not lead us to wrong conclusions.

Since existing partial theories are sufficient to make accurate predictions in all but the most extreme situations, the search for a definitive theory of the Universe is difficult to justify on purely practical grounds. (Note, however, that similar arguments could be made for the theory of relativity and quantum mechanics, and thanks to these theories we have mastered nuclear energy and revolutionized microelectronics.) So the construction of a complete unified theory is of particular benefit to the survival of us as a species It may not be, and it may not have any effect on our way of life. But already at the dawn of civilization, people did not want to be content with perceiving the world as a set of unrelated and inexplicable events and phenomena. We sought to understand the underlying order of the universe. And today we want to understand why we are here and where we come from. Humanity's deep desire for knowledge is sufficient justification for our continued quest, and our goal is nothing more and nothing less than Full description The universe in which we live.

The telescope as a spotting scope was first invented by the Dutch spectacle maker Johann Lippershey in 1608, but Galileo was the first to point a telescope at the sky in 1609 and use it for astronomical observations.

This is not entirely true. Internal structure gravitating bodies can be neglected only if the density distribution in them is spherically symmetric (that is, depends only on the distance to the center of the body). In the case of the planets and the Sun, this is strictly speaking not the case - these bodies are at least slightly flattened at the poles. For example, the oblateness of the Sun is one of the reasons for the precession of Mercury's perihelion. Terrestrial planets also have other inhomogeneities in their density distribution. Studies of the gravitational field of the Earth and other celestial bodies are the subject of a separate field of science - gravimetry.

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10. A Brief History of Time

The idea of ​​writing a popular science book about the Universe first came to me in 1982. Part of my goal was to earn money to pay school fees for my daughter. (In fact, by the time the book was published, she was already in her senior year.) But main reason The reason for writing the book was that I wanted to explain how far I think we have come in understanding the Universe: how close we may already be to creating a complete theory that describes the Universe and everything that is in it.

Since I was going to put the time and effort into writing a book like this, I wanted as many people as possible to read it. Before that, my purely scientific books were published by Cambridge University Press. The publisher did a good job, but I felt that it would not be able to reach as wide an audience as I would like. So I contacted literary agent Al Zuckerman, who was introduced to me as the son-in-law of one of my colleagues. I gave him a draft of the first chapter and explained my desire to make a book like those sold in airport kiosks. He told me there was no chance of that. Scientists and students, of course, will buy it, but such a book will not break into the territory of Jeffrey Archer.

I gave the first version of the book to Zuckerman in 1984. He sent it to several publishers and recommended that they accept the offer from Norton, an elite American book company. But contrary to his recommendations, I accepted the offer of Bantam Books, a publishing house aimed at the general reader. Although Bantam did not specialize in publishing non-fiction books, its books were widely available in airport bookstores.

Perhaps Bantam became interested in this book because of one of its editors, Peter Guzzardi. He took his work very seriously and forced me to rewrite the book so that it could be understood by non-experts like himself. Every time I sent him a revised chapter, he responded with a long list of shortcomings and issues that he felt needed to be clarified. At times I thought this process would never end. But he was right: the book turned out much better as a result.

My work on the book was interrupted by pneumonia, which I contracted at CERN. It would have been completely impossible to finish the book if it had not been for the computer program. It was quite slow, but I was thinking leisurely at the time, so it was quite suitable. With her help, prompted by Guzzardi, I almost completely rewrote the original text. One of my students, Brian Witt, helped me with this revision.

Cover of the first edition of A Brief History of Time

I was very impressed by Jacob Bronowski's television series The Ascent of Man. (Such a sexist name would not be allowed to be used today.) It gave a sense of the achievements of the human race and its development from the primitive savages that it was only fifteen thousand years ago to our modern state. I wanted to evoke similar feelings regarding our movement towards a full understanding of the laws that govern the Universe. I was sure that almost everyone is interested in how the universe works, but most people cannot understand mathematical equations. I don't really like them myself. Partly because it’s difficult for me to write them, but the main thing is that I don’t have an intuitive sense of formulas. Instead, I think in visual images, and in my book I tried to express these images in words, using familiar analogies and a small number of diagrams. By choosing this path, I hoped that most people would be able to share with me the admiration for the successes that physics has achieved as a result of its amazing progress over the past fifty years.

And yet some things are difficult to understand, even if you avoid mathematical calculations. The problem I was faced with was: should I try to explain them at the risk of misleading people, or should I just sweep the garbage under the rug, so to speak? Some unusual concepts, such as the fact that observers moving at different speeds measure different lengths of time for the same pair of events, were irrelevant to the picture I wanted to paint. So I felt I could just mention them without going into detail. But there were also complex ideas that were essential to what I was trying to convey.

There were two concepts that I thought were particularly important to include in the book. One of them is the so-called summation by history. This is the idea that the universe has more than one history. Instead, there is the totality of all possible histories of the universe, and all of these histories are equally real (whatever that means). Another idea needed to make mathematical sense of summation over histories is imaginary time. I now realize that I should have put more effort into explaining these two concepts because they were the parts of the book that people had the most difficulty with. However, it is not at all necessary to understand exactly what imaginary time is; it is quite enough to know that it differs from what we call real time.

When the book was about to be published, the scientist who was sent an advance copy to prepare a review for the magazine Nature, was horrified to discover a huge number of errors in it - incorrectly placed photographs and diagrams with incorrect signatures. He called Bantam, they were also horrified and on the same day they recalled and destroyed the entire circulation. (Surviving copies of this actual first edition are now likely to be highly prized.) The publisher spent three weeks of hard work revising and correcting the entire book, and it was ready in time to hit stores in time for its announced April Fool's release date. April Fool's Day. Then the magazine Time published a biographical note about me with it on the cover.

Despite all this, Bantam was surprised by the demand for my book. It remained on the bestseller list The New York Times for 147 weeks, and on the London bestseller list Times - in a record 237 weeks, was translated into 40 languages ​​and sold over 10 million copies worldwide.

I originally titled the book From the Big Bang to Black Holes: A Short History of Time, but Guzzardi swapped the title and subtitle and replaced it with “short.” to "brief" This was brilliant and must have contributed greatly to the success of the book. Since then, many “brief histories” of this or that and even “A Brief History of Thyme” have appeared. Imitation is the sincerest form of flattery.

Why did people buy this book so much? It is difficult for me to be confident in my objectivity, and I would rather quote what others have said. It turned out that the majority of reviews, even if approving, did not clarify much. Basically they are built according to the same scheme: Stephen Hawking suffers from Lou Gehrig's disease(term used in American reviews), or motor neuron disease(British reviews). He is confined to a wheelchair, cannot speak and only moves N fingers(Where N ranged from one to three, depending on how inaccurate the article about me that the reviewer had read). And yet he wrote this book about the greatest question of all: where did we come from and where are we going? The answer Hawking proposes is that the universe was not created and will never be destroyed - it simply is. To express this idea, Hawking introduces the concept of imaginary time, which I(i.e. reviewer) I find it somewhat difficult to understand. However, if Hawking is right and we do find a complete unified theory, then we will truly understand God's design.(During the proofreading stage, I almost removed the last phrase from the book about us understanding God's plan. If I had done that, sales would have dropped by half.)

I think the article in the London newspaper is much more insightful The Independent, where it is said that even such a serious scientific book as “A Brief History of Time” can become a cult. I was very flattered by its comparison with the book “Zen and the Art of Motorcycle Maintenance.” I hope that, like it, my book gives people a sense that they should not dismiss great intellectual and philosophical questions.

Undoubtedly, human interest in the story of how I managed to become a theoretical physicist, despite my disability, also played a role. But those who bought the book just for this were disappointed, since my condition was mentioned only a couple of times. The book was intended to be the story of the universe, not my story at all. This did not protect the Bantam publisher from accusations that it was shamelessly exploiting my disease and that I was indulging them by allowing my photograph to be placed on the cover. In fact, according to the contract, I had no right to influence the design of the cover. I did, however, manage to convince the publisher to use a better photo for the British edition than the crappy, outdated photo that was in the American version. However, on the American cover the photo remained the same because, as I was told, the American public identified this photo with the book itself.

It was also suggested that many people bought this book to display it on their bookshelf or coffee table without actually reading. I'm sure this was the case, although I don't think it was any more so than with numerous other serious books. And yet I know that at least some readers must have made it through, because every day I receive a stack of letters about this book, many of them containing questions or detailed comments, which indicates that people are reading this book. read it, even if they didn’t fully understand it. People also stop me on the street and tell me how much they liked it. The frequency with which I receive such expressions of public recognition (even though I am, of course, a very different, if not the most excellent, author) seems to me to reassure that a certain proportion of the people who bought the book actually read it.

After A Brief History of Time, I wrote several more books to bring scientific knowledge to a wider audience. These are “Black Holes and Young Universes”, “World in nut shell" and "Higher Design". I think it is very important that people have a basic knowledge of science that will enable them to make informed decisions in a world where science and technology play more and more roles. In addition, my daughter Lucy and I wrote a series of books for children - tomorrow's adults. These are adventure stories based on scientific concepts.

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What is Stephen Hawking's A Brief History of Time about?

From open sources

Today, March 14, the famous English theoretical physicist Stephen Hawking died at the age of 77. the site publishes a synopsis of his popular science book “A Brief History of Time: From the Big Bang to Black Holes” (1988), which became a bestseller

The book by the outstanding English physicist Stephen Hawking, “A Brief History of Time: From the Big Bang to Black Holes,” is dedicated to finding the answer to Einstein’s question: “What choice did God have when he created the Universe?” Warned that every formula included in the book would halve the number of buyers, Hawking lays out in accessible language the ideas of quantum theory of gravity, an as-yet-unfinished branch of physics that combines general relativity and quantum mechanics.

The book begins with a story about the evolution of human ideas about the Universe: from the celestial spheres of the geocentric system of Aristotle and Ptolemy to the realization of the fact that the Sun is an ordinary yellow star of average size in one of the arms of a spiral galaxy - among hundreds of billions of other galaxies in the observable part of the Universe. The discovery of the redshift of the spectra of stars in other galaxies meant that the Universe was expanding, and this led to the big bang hypothesis: ten or twenty billion years ago, all objects in the Universe could be located in one place with an infinitely high density (singularity point).

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The Big Bang serves as the beginning of time. There is no answer to the question of what happened before the Big Bang, since scientific laws stop working at the point of singularity; the ability to predict the future is lost, and therefore, if something happened “before”, it will not affect current events in any way. After the Big Bang, two scenarios are possible: either the expansion of the Universe will continue forever, or at some point it will stop and go into a compression phase, which will end with a return to the singularity - the Big Bang. It is unclear which option will be realized - it depends on the distances between galaxies and the total mass of matter in the Universe, and these quantities are not precisely known.

Singularities can exist in the Universe even after the Big Bang. A star, having used up nuclear fuel, begins to shrink, and with a sufficiently large mass it cannot resist gravitational collapse, turning into a black hole. So, the English mathematician and physicist Roger Penrose showed that the volume of the star tends to zero, and the density of its matter and the curvature of space-time tend to infinity. In other words, a black hole is a singularity in space-time.

By reversing the direction of time, Penrose and Hawking proved the claim that if general relativity (GR) is true, then the Big Bang point must exist. So the big bang hypothesis became a mathematical theorem, and general relativity itself turned out to be incomplete: its laws are violated at the singularity point. This is not surprising - after all, GTR is a classical theory, and in a small region of space near the singularity, quantum effects become significant. Thus, the study of black holes and the early Universe requires the use of quantum mechanics and the creation of a unified theory - the quantum theory of gravity.

Dealing with the phenomena of the microworld, quantum mechanics developed independently of general relativity. Quantum physics has accumulated some experience in unifying various types interactions. Thus, it was possible to combine electromagnetic and weak interactions into one theory. Namely, it turned out that the carriers of electromagnetic interaction (virtual photons) and the carriers of weak interaction (vector bosons) are realizations of one particle and become indistinguishable from each other at energies of about 100 GeV. There are also grand unification theories, that is, the unification of the electroweak and strong interactions (however, to achieve the energies of the grand unification and test these theories, an accelerator the size of the Solar System is needed).

All these theories do not include gravity, since it is very small for elementary particles. However, at the point of singularity, gravitational forces, together with the curvature of space-time, tend to infinity, so that the joint consideration of quantum mechanical and gravitational effects becomes inevitable. This leads to the following surprising results.

According to the Penrose–Hawking theorem, falling into a black hole is irreversible. But, as is known, every irreversible process is accompanied by an increase in entropy. Does a black hole have entropy?

Hawking notes that the area of ​​the event horizon of a black hole does not decrease with time (and when matter falls into a black hole, it increases), that is, it has all the properties of entropy. His American colleague Bikenstein proposes that the area of ​​a black hole's event horizon be considered a measure of its entropy. Hawking objects: having entropy, a black hole must have a temperature and therefore radiate - contrary to the very definition of a black hole! - but later he himself discovers the mechanism of this radiation.

The source of radiation turns out to be a vacuum near a black hole, in which particle-antiparticle pairs are born due to quantum fluctuations of energy. One member of the pair has positive energy, the other has negative energy (so the sum is zero); a particle with negative energy can fall into a black hole, and a particle with positive energy can leave its vicinity. The flow of positive energy particles is the radiation of the black hole; particles with negative energy reduce its mass - the black hole “evaporates” and disappears over time, taking the singularity with it. Hawking sees this as the first indication of the possibility of eliminating the singularities of general relativity using quantum mechanics and asks the question: will quantum mechanics have a similar effect on the “big” singularities, that is, will quantum mechanics eliminate the singularities of the Big Bang and the Big Bang?

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The classical general theory of relativity leaves no choice: the expanding Universe is born from a singularity, and the initial conditions are unknown (GTR does not work at the “moment of creation”). At the initial moment, the Universe could be ordered and homogeneous, or it could be very chaotic. The further process of evolution, however, significantly depends on the conditions at this boundary of space-time. Using Feynman's method of summing over various "trajectories" of the development of the Universe, Hawking, within the framework of the quantum theory of gravity, obtains an alternative to singularity: space-time is finite and does not have a singularity in the form of a boundary or edge (it is similar to the surface of the Earth, but only in four dimensions) . And since there is no boundary, there is no need for initial conditions on it, that is, there is no need to introduce new laws that determine the behavior of the early Universe (or resort to the help of God). Then the Universe "...would not have been created, it could not be destroyed. It would simply exist."

The theme of God is present throughout the book; Essentially, Hawking is having a discussion with God. Here is a quote that kind of sums up this discussion.

“From the idea that space and time form a closed surface, very important consequences also follow regarding the role of God in the life of the Universe. In connection with the successes achieved by scientific theories in describing events, most scientists have come to the conviction that God allows the Universe to develop in in accordance with a certain system of laws and does not interfere with its development, does not violate these laws. But the laws do not tell us anything about what the Universe looked like when it first appeared - winding the clock and choosing the beginning could still be the work of God. we think that the Universe had a beginning, we can think that it had a Creator. If the Universe is truly completely closed and has no boundaries or edges, then it should have neither a beginning nor an end: it simply is. , and that’s it! Is there still room for the Creator then?”

Here is the answer to Einstein’s question: God did not have any freedom to choose the initial conditions.

By summing over Feynman trajectories in the absence of space-time boundaries, Hawking finds that the Universe in its current state is highly likely to expand equally quickly in all directions - in agreement with observations of the isotropic background of the CMB. Further, since the origin of time is a smooth, regular point in space and time, then the Universe began its evolution from a homogeneous, ordered state. This initial order explains the presence of a thermodynamic arrow of time, indicating the direction of time in which the disorder (entropy) of the Universe increases.

In the final part of the book, Hawking describes string theory, which claims to unify all physics. This theory does not deal with particles, but with objects like one-dimensional strings. Particles are interpreted as vibrations of strings, emission and absorption of particles - as breaking and joining of strings. String theory, however, does not lead to contradictions only in 10-dimensional or 26-dimensional spaces. Perhaps, during the development of the Universe, only four coordinates of our space-time “unfolded,” while the rest turned out to be folded into a space of negligibly small sizes.

Why did it happen? Hawking gives the answer from the standpoint of the so-called anthropic principle: otherwise the conditions for the development of intelligent beings capable of asking such a question would not have arisen. In fact, in the case of a smaller dimension of space, evolution is difficult: for example, every through passage in the body of a two-dimensional creature divides it into two parts. In spaces of higher dimensions, the law of gravitational attraction will be different, and the orbits of the planets will become unstable (“we would then either freeze or burn”). Of course, other universes are also possible, with a different number of unfolded coordinates, “... but in such areas there will be no intelligent beings who could see this variety of operating dimensions.”

Hawking is optimistic about the prospects for creating a unified theory that describes the Universe. Having taken away the act of creation from God, he assigns God the role of the creator of its laws. When a mathematical model is built, the question remains why the Universe, which obeys this model, exists at all. Unbound by the need to build new theories, scientists will turn to its research. “And if the answer to such a question is found, it will be a complete triumph of human reason, for then God’s plan will become clear to us.”

Summary of Stephen Hawking's book "A Brief History of Time" prepared by Igor Yakovlev