In the novel "the Equation of Janus" writer Stephen Sproull examines one of the harrowing personal problems connected with time travel. In the center of the plot of the book is a brilliant mathematician, who set a goal to solve the mystery of time travel. He meets unusual beauty, they become lovers, but he knows nothing about her past. Tormented by curiosity, he tries to find out who is the mysterious lover. It turns out that it has changed its appearance with plastic surgery. And changed floor, also with the help of operations. In the end it turns out that actually "she" was a time traveler arriving from the future; not only that, actually, "she" is he himself, only of the future. It turns out that he was making love with himself. One can only guess what would happen if they have a child? And if that child went back into the past, would have grown there and became a mathematician (the same that appeared in the early history)? Can you be yourself and mother, and father, son, and daughter?
How to change the past

Time is one of the greatest mysteries of the Universe. The river of time takes us all, without exception, irrespective of our desire, and even against his will. In 400 ad, St. Augustine wrote a lot about the paradoxical nature of time: "how can these two tenses, past and future when the past is gone, and the future is not yet? And if present, always stayed real and not away in the past, it wouldn't be the time and eternity". If you continue logically thought of Augustine, it turns out that day at all, because the past is gone, the future does not exist, and now there is only a moment. (After this reasoning Augustine is set deep theological questions about how time affects of God, - matters which have not lost sense today. If God is omniscient and Almighty, Blessed Augustine wrote, be It over time? In other words, whether the God hurry, being late for an important meeting, as do we, mortals? Itself Augustine makes such a conclusion: God, the Almighty, and therefore cannot be limited by anything, including over time; therefore, he must exist outside of time. Although at first glance the concept of existence beyond time, seems absurd, it's one of those ideas that we still make sure that appear again and again in modern physics.)

Like St. Augustine, each of us at some point thought about the strange and mysterious nature of time and how much time is different from the space. If in the space, we can easily move in any direction, then why in time everything is different? Each of us thought about what will happen to mankind after us. The age of the individual is limited, but we all terribly interested in everything what happens in the future, after us.

The human desire to travel back in time were born, probably, at the same time with the man himself, but the first recorded history of time travel - Memoirs about the twentieth century" - is written by Samuel Madden and refers to 1733 it tells about the angel of 1997, which was transferred on 250 years ago, to transfer the British Ambassador documents describing the world of the future.

Later these stories have appeared many. In 1838, was the product of the anonymous author of "waiting for the coach: an anachronism"; his hero, waiting for the coach, unexpectedly transferred for a thousand years in the past. He met a monk of the ancient monastery and tries to tell him how to develop the story in the next thousand years, After a while it just as unexpectedly transferred back to the present; the only result, as his coach had left.

Time travel can be found in the most unexpected works - as, for example, in the novel by Charles Dickens "a Christmas Carol", written in 1843; hero of the novel, Ebenezer Scrooge, is transferred into the past and the future and show the world what he was before and what will be after his death.

In American literature of time travel for the first time appear in Mark TWAIN's novel 1889 "from Connecticut Yankee at the court of king Arthur". Yankee XIX century. transferred back in time and finds himself at the court of king Arthur in 528, from the birth of Christ. It captured and are going to burn in a fire, but resourceful Yankee announces that has the power to extinguish the sun, because he knows that in this day should be a solar Eclipse. When the moon covers the sun, the crowd is shocked; Yankee go and give graces, if only he returned them sunlight.

But the first serious attempt to investigate time travel in fiction has become a classic novel by H.G. wells, the time Machine; in it the main character goes to hundreds of thousands of years into the future. It turns out that in the distant future, mankind genetically split into two races - martial Morlocks, who look after dark underground machines, and carefree, like the children of Eloi who rejoice and dance at the top on the Sunny glades, not knowing and not thinking about waiting for their fate to be eaten them).

After wells travel in time become a common part of science fiction, from "Star trek" to "Back to the future". In the film "Superman" the main hero to learn of the death of Lois lane, in desperation decided to turn back the time; he begin to brood around the Earth and light ahead until time itself does not turn back. The earth slows the rotation stops and finally starts to rotate in the opposite direction - and all the clocks on the Earth begin the countdown. The waters of the flood roaring rush back, broken dam miraculously restored, and Lois lane back to life.

From the point of view of science can be said that time travel were strongly impossible Newtonian universe, where time was flowing uniformly and rectilinearly. Once the incident could not be changed under any circumstances. One second on Earth was equal to one second of the Universe. Einstein denied this concept and showed that the time is more like a meandering river, which crosses the Universe; winding between stars and galaxies, it speeds up and slows down. So one second on the Earth is not absolute; the time in different points of the Universe flows differently.

As I already told, according to the special theory of relativity time in the rocket is slowing, and the more, the faster it moves. Science-fiction writers like to talk about that, if you can overcome light barrier, it will be possible to go back in time. Actually this is not possible because in order to reach the speed of light, you have to get and infinite mass. The speed of light is insurmountable barrier for any rocket. The crew of "enterprise" in the TV series "Star trek IV: the Voyage home" kidnapped spaceship Clintons, dispersed its gravitational maneuver around the local sun, overcame a light barrier and was in San Francisco in the 1960s, In fact, it contradicts the laws of physics.

However, the journey to the future is possible, and it is experimentally confirmed already millions of times. Even the hero's journey "time Machine" in the distant future, in principle, possible. If astronaut will move at relativistic speeds, on the way to one of the nearest stars they may have, say, one minute. On the Earth when this will take place four years, but for him personally, the time will move just one minute more, because the ship will greatly slow down. It will mean that the astronaut will move in the future of the Earth for four years. (Generally speaking, our astronauts make the short journey into the future every time fly into space. While they fly around the Earth at a speed of 8 km/s, watch them go a little slower than the clock on Earth. You can calculate that during a year expedition on the space station by the time they return to Earth move into the future for a split second. World record in time travel is now owned by a Russian cosmonaut Sergei Andreeva, which for 748 days spent in orbit, moved into the future already 0.02 C.)

So, the time machine to travel to the future is not contradict the special theory of relativity. But as is the case with travel into the past?

If we could travel back in time, to study the history, it would be impossible. Would historian to write past events, how someone could go back in time and change it. Time machine not only would deprive historians of work, but would allow us to arbitrarily changing it. If, for example, someone went back in time, in the era of the dinosaurs, and accidentally crushed would be the first mammal is our common ancestor, he could erase from the face of the Earth, the whole human race. In the best case, the story would turn into endless crazy ride, when everywhere scurried would tourists from the future with cameras and tried to get to shoot a historic event.
Time travel: a Playground for physicists

Perhaps we can say that the more the rest of distinguished in mathematical jungle black holes and time machine cosmologist Stephen Hawking. Unlike other connoisseurs of relativity, which, as CRAvila, even at an early age manifest themselves in mathematical physics, Hawking in his youth was not an outstanding student. It was obvious that he is extremely smart, but teachers often noticed that he was not always focused on the study and do not work in full force. Turning to Hawking became 1962; after graduating from Oxford young physicist first began to notice the symptoms of amyotrophic lateral sclerosis (ALS, or disease Lou Gehrig). He was shocked by the news that suffers from incurable neurodegenerative disease that would deprive him of all motor functions and, most likely, will quickly kill. You can imagine how upset young man this news. What is the sense to receive the degree of doctor of philosophy, if you still going to die?

But later, having overcome the initial shock, Hawking focused on the job - maybe the first time in my life. Realizing that time, he had a little, he made a furious attack on some of the most complex problems of the General theory of relativity. In the early 1970s, Hawking published a remarkable series of scientific works and they showed that the singularity in the Einstein (the point where the gravitational field is infinite, as, for example, is in the center of a black hole or was happening in the moment of the Big Bang) are an essential part of relativistic picture of the world and cannot be simply ignored (as I thought Einstein). In 1974, Hawking also proved that black holes, generally speaking, not quite black; they slowly radiate what is now called Hawking radiation, because the radiation can soak even through the gravitational field of a black hole. This work was the first serious attempt to apply quantum theory to the theory of relativity, and it is the most famous work of Hawking.

As predicted by doctors, ALS gradually aroused Hawking paralysis of the arms, legs and even the vocal cords, but everything was much slower than they originally thought. As a result, he had already passed through many normal people, became the father of three children (now and grandfather), in 1991 he divorced his first wife, four years later married the wife of a man who had designed for him by a voice synthesizer, and in 2006 filed for divorce and this wife. In 2007, Steven again hit the headlines - he became a special passenger jet and traveled in zero gravity, thus fulfilling an old dream. His next goal is to fly to space.

Today Hawking is almost completely paralyzed, he moves around in a wheelchair and communicates with the external world through eye movements. But even in such a disastrous state, he manages to joke wrote scientific work, gives lectures and participates in the discussions. One mind it produces more scientific results than whole teams of scientists, it owns their bodies. (His colleague at Cambridge University, sir Martin Rees, whom the Queen appoints the astronomer Royal, once admitted to me that the disease does not allow the Hawking to do boring mathematical calculations required in big science. So instead, he concentrated on generating new ideas and calculations can do and his students.)

In 1990, Hawking has familiarized with work colleagues, which offered all sorts version of the time machine, and treated him very critical. Instinct told him that time travel is impossible, - otherwise, why we do not find tourists from the future? If you should go somewhere in the past it would be as easy as to arrange a Sunday picnic in the Park, we met each day would be on the streets of guests from the future, and they stuck to us asking to be photographed with them for the family album.

And Hawking threw the world of physics call. He said: there should be a law banning of time travel. In other words, he suggested that "the hypothesis on the protection of the chronology, which would eliminate travel time on the basis of the laws of nature and preserved the history for historians".

But something unexpected happened. As they tried, physics could not find a law that directly prohibits time travel. Apparently, they in no way contradict the known laws of nature. The Hawking, also unable to identify the ban, not so long ago changed my mind. He was back in the headlines, saying: "If time travel is possible, then they are not feasible".

Yes, if time travel were considered at best as a pseudo-scientific subject, now they suddenly turned into a favorite toy theoretical physicists. Physicist Kip Thorne of the California Institute of technology, writes: "once upon a time travel were the exclusive prerogative of science fiction writers. Serious scientists avoided them like the plague - even when he wrote under the pseudonym novels or secretly read them. How times have changed! Now in serious scientific journals, you can find the scientific analysis of time travel, owned by Peru prominent theoretical physicists... why such a change? Simply we physicists realized that the nature of time is too important a topic to give it out to the science-fiction writer".

The reason for all this fuss and confusion that the Einstein equations admit the existence of many different types of the time machine. (However, it is unclear that they will resist you before checking with the help of quantum theory.) Moreover, in Einstein's theory, we often see something called "closed primapagina curve"; this is the technical term for ways that allow travel into the past. If you follow along closed tramadolnow the curve, you can return from a trip before we went.

The first type of machine time involves the use of a mole burrows. The Einstein equations have many solutions, connecting two remote points of space. But since time and space in Einstein's theory is closely intertwined, the same cold snap can connect two points in time. Falling into the wormhole, you can navigate (at least mathematically) in the past. It seems you can then move back to the original point and found himself before the start. But, as we mentioned in the previous Chapter, safe passage in the center of a black hole is the road at one end. "I don't think the question is, can a human being into a black hole to get into the past, " says physicist Richard Gott. - The question is, will he be able to get out to boast of".

Another time machine can "work" in a rotating Universe. In 1949, the famous mathematician Kurt g?del found the first solution to the Einstein equations with respect to time travel. If the universe is rotating, then around her quickly enough, you can be in the past and to get to a starting point before you went. It turns out that travel around the Universe is also a journey back in time. When the Institute of advanced studies appeared astronomers, g?del often asked whether they have evidence that the universe revolves. To his disappointment, they were told that the universe just is expanding, but the total spin of the Universe, probably zero. (Otherwise, time travel, perhaps, would become habitual, and history in the form in which we know it would cease to exist.)

Third option: if you move around an infinitely long rotating cylinder, you will also be back before he set off. (This decision by Bill van Stockum found in 1936, before adequately, but the author, apparently, did not suspect that his decision allows you to travel in time,Here it turns out that if how to dance around a pole with ribbons at the may holiday, then you can accidentally end up in the previous April. (The problem, however, is that the cylinder must be infinite and rotate so fast that most of the materials will not endure and will fall to pieces.)

The latest version of time travel discovered in 1991, Richard Gott, Princeton. His decision is based on the discovery in the space of a giant cosmic strings (possibly left over from the Big Bang). For example, he suggested that two such cosmic strings are going to face. Now, if quickly to round these strings at the moment of collision, will get in the past. The advantage of this type of a time machine is that you don't need infinite rotating cylinders rotating universe, or even black holes. The problem, however, is that you have to first look in the space of these enormous cosmic strings, and then force them to face a certain way. Besides, the "road" in the past this opens for a very short period of time. Gott says, "Collapsing string loop, big enough to make it to round once and return at that, one year ago, on its mass-energy should exceed a half of the galaxy".

But the most promising scheme of the time machine - the so-called reversible wormholes mentioned in the previous Chapter. This hole in space-time where people can freely move forward and backward in time. Theoretically reversible wormholes is an opportunity not only to travel faster than light, but also to travel in time. The key to reversible krajowym the nor-negative energy.

Time machine for reversible mole burrows shall consist of two chambers; each chamber is of two concentric spheres separated by a tiny gap. If compress the outer sphere inside, towards the inside of the sphere, between the two spheres will have the Casimir effect and result in negative energy. Suppose a civilization III type is capable of maintaining wormhole between these two cameras (perhaps to build it from the space-timeauthorized foam). Next, take the first chamber and send it into space at relativistic speeds. Time in this cell is slowing, and the clock in two cameras lose synchronicity. Time in two chambers, the United Krotova hole, going at different speeds.

While in the second the camera, you can Krotova hole will jump to the first, which exists in an earlier time, and to be in the past.

The implementation of this scheme is associated with serious difficulties. So cold snap may be tiny, much smaller than the atom. And concentric spheres, you may need to crimp up to distances from the Planck scale to get enough negative energy. And the last. You will be able to go back in time just at the moment when was created the machine of time - because up to this point time in both chambers was in perfect unison!
Paradoxes and puzzles of time

Time travel generate many problems, both technical and social. Larry Dwyer raises all sorts of moral, legal and ethical issues; he says, "whether to charge the traveler in time, if he beat himself, only younger (or Vice versa)? If a time traveler to commit murder and disappear in the past, should be judged in the past for a crime he has yet to do? If he marries in the past, can we judge it for bigamy, if another wife to be, say, five thousand years?"

But perhaps the most trudnoreshaemyh problem is the logical paradoxes that arise when traveling in time. What happens, for example, if we kill their parents before his birth? It is logically impossible, so there is a paradox - it is sometimes called the " grandfather paradox".

There are three ways to resolve these paradoxes. First, it is possible that with the return of the past, you will simply have to re-live all the same, thereby restoring the history in its previous form. In this case, you are deprived of free will and forced to repeat the past as it once was implemented. In this situation, it turns out that if you go into the past to transfer to itself the secret of time travel, then that's exactly what was going to happen: the secret of time travel really was delivered from the future. Such is fate. (I must say, however, remains unclear where did the original idea.)

The second option. You have free will and, consequently, can change the past, but to a limited extent. Your Agency works as long as you do not create a time paradox. You should try to kill the parents before his birth, and a mysterious force will not give you pull the trigger. This position defends the Russian physicist Igor Novikov. (He makes it in the following way. There is, for example, the law of nature, not allowing us to walk on the ceiling, although we may want it. Why not to assume that there is a law that does not allow us to kill the parents before our birth? Just like that, unknown force will not allow us to pull the trigger.)

Finally, a third option. The universe splits into two. The people you killed, exactly like your parents, but actually they are not, because you are already in a parallel universe. It seems that this option corresponds to the quantum theory; I'll talk about this later when I talk about the Multiverse.

The second option discussed in the film "Terminator 3", where Arnold Schwarzenegger plays a robot from the future, which seized power aggressive machine. On the few survivors of the machine hunted like beasts; but the machines are not able to kill the leader of the resistance. Machine sent a series of robot killers in the past, in the time shortly before the birth of the leader, with the task of destroying his mother. But in the end, after an epic battle, the final film of the machine still destroy human civilization, as it was planned from the beginning.

The film "Back to the future" is considering a third solution. Dr. brown invented a machine that works on plutonium, on the basis of the old DeLorean car; this is actually a time machine to travel back in time. Marty McFly (Michael Fox) sits in the car, goes back in time and met there with his young mother, who then falls in love with him. There is a challenge. When a mother Marty will reject his future father and they will not get married, the hero of Fox's just not born.

The problem sheds some light Doc brown. He draws on the Board a horizontal line representing the time in our Universe, Then he draws a second line, which branches off from the first and represents a parallel universe; it occurs at the moment when you change the past. Thus, you should go back on the river of time, as it forks into two branches; one line of time becomes two. This approach is known as the concept of plurality of the worlds, and we will discuss it in the next Chapter.

This means that all the paradoxes of time you can allow. If you killed your parents before you were born, it simply means that you killed the people who are not really your parents - although genetically identical, have the same person and the same memories.

The idea of the plurality of the worlds solves at least one serious problem of time travel. For the physics of the problem, connected with the time travel (except for search of negative energy), is that the effects of radiation will be accumulated, and eventually it will happen one of two: or you'll drop dead when you try to log in to the machine, or meteor shower is cancelled when you're through it to pass. These radiation effects will be accumulated, because any emission caught in a time portal, go to the past; there's this radiation will come out and will have to wander around the Universe until today, when it will be time again to enter the portal. Because the radiation can enter the portal to an infinite number of times, within the portal it can achieve incredibly high level - it is enough to kill anyone who is going there. But if to speak about the version with "multiple worlds", this problem will solve itself. Radiation trapped in a time machine really is sent in the past, but gets into a new universe; it cannot enter into the portal again, again and again. This means that there are an infinite number of universes, each cycle has its own, and in each cycle at a time portal penetrates only one photon radiation is not infinitely many.

In 1997, when three physicists managed to finally prove that the intention Hawking once and for all to prevent time travel is incorrect in principle, issues slightly cleared. Bernard Kay, Marek our Radzikowski and Robert Wald showed that time travel is not contradict any known physical laws, with the exception of one point. When it comes to travel in time, all problems are concentrated on the event horizon (located near the entrance to the wormhole). But this horizon is the place where, according to modern concepts, the theory of Einstein gives way to quantum effects. The problem is that, in trying to calculate the radiation effects at the entrance to the time machine, we are forced to use the theory that combines the General theory of relativity and the quantum theory of radiation. But, no matter how we tried naive to combine these two theories, the result is inconclusive; in some places the answer is infinite, that makes no sense.

And here comes the so-called theory of everything. All travel through wormholes, torturing physicists (for example, the stability of the wormhole, life-threatening radiation, the collapse of the wormhole when you try to go through it), concentrated on the event horizon is exactly where loses the meaning of Einstein's theory.

Thus, the key to understanding the time travel is the understanding of the physics of the event horizon - and it can describe and explain only a theory of everything. That is why most physicists currently agree that the only way to resolve the issue of time travel is to develop a complete theory of gravity and space-time.

The theory of everything should unite four fundamental physical interactions of the Universe and allow us to mathematically calculate what happens when entering the time machine. Only the theory of everything would be able successfully to calculate radiative effects created Krotova Nora, and to clarify the question of how stable is safe passage when entering the time machine. But even after the creation of such a theory we may have to wait a few centuries or even longer before the first time machine will be able to experiment with its findings.

The laws of time travel is so closely related to the physics mole burrows that they themselves travelling, obviously, should be attributed to the II class of impossibility.