What to read: "Short answers to the big questions" - the last book of Stephen Hawking

With time travel is closely related to the ability to move quickly from one point in space to another. As I said earlier, Einstein showed that for the acceleration of the spacecraft to near light speed required infinitely powerful jet thrust. So the only way to move from one part of the galaxy to the other within a reasonable period of time - possibility to fold space-time so as to obtain a small tube or "mole Nora". It can link the two parts of the galaxy, and act as the shortest path between them; you will be able to fly back and forth and catch more alive all your friends. These "Wormholes" seriously considered as an option available civilization of the future. If you can move from one part of the galaxy to the other for a couple of weeks, then you can go back through another "hole" - in this case before you hit the road. Also, there is nothing to prevent you from traveling forward and back in time through a "wormhole", if both of its end move relative to each other.

We can say that for the creation of a "wormhole" is necessary to bend the space-time in a direction opposite to that in which it distorts the normal matter. Ordinary matter bends space-time itself, as the Earth's surface. But to create a "wormhole" is required matter, which bends space-time in the opposite direction as the surface of the seat. The same is true for any other curvature of space-time to travel to the past, if the universe is not there so curved that it is already possible to travel time. Only in this case it will require matter with negative mass and negative energy density.

Energy - the money. If you have a positive balance in the bank, you can use the money any way you like. However, according to the classical laws, which until recently were regarded as immutable, using the energy of the overdraft is not allowed.

Classical laws rule for us to bend the universe so that there was the possibility of time travel. But the classical laws of quantum theory denies - second only to the general theory of relativity, the great intellectual revolution in our ideas about the universe. Quantum theory is more flexible and allows you to avoid in some cases, overdraft. However, the bank has to provide us with such kindness. In other words, quantum theory allows for the existence in some places of negative energy density by ensuring positive density in others.

Quantum theory allows negative energy density, as it is based on the principle of uncertainty. He argues that some features, such as the position and velocity of the particle can not simultaneously be accurately measured values. The more accurately determined position of the particle, the greater the uncertainty as to its speed and vice versa. uncertainty principle also applies to the fields - for example, electromagnetic or gravitational field. He argues that these fields can not be zero, even where it seems to us, the empty space. The fact is that if their values ​​are equal to zero, it means that they must have a well-defined position of zero, and a well-defined speed equal to zero. And this is contrary to the principle of uncertainty. Hence, the field must have a certain minimum fluctuation. It is possible to introduce the so-called vacuum fluctuations in a particle-antiparticle pairs that suddenly appear, separated, then merged again and annihilate vzaimounichtozhayas.

Such a pair of particles - antiparticles are considered virtual because they can not be directly locked with the particle detector. But the indirect effects can be observed. For this purpose a so-called Casimir effect. Try to imagine two parallel metal plates that are a short distance from one another. The plates act as mirrors for the virtual particle-antiparticle. This means that the space between the plates look like organ pipes, but it allows light waves of a certain resonance frequency. As a result, it turns out that between the plates there is a number of quantum fluctuation different from what happens behind them, where these fluctuations may have any wavelength. The difference in the number of virtual particles between the plates and on the outside means that the one side of the plate experience a greater pressure than the other. There is a small force, which brings the plates together. This force can be measured experimentally. So virtual particles exist in reality and produce a real effect.

As between the plates is less than the virtual particles or quantum fluctuations in vacuo, and the energy density is lower than in the surrounding space. But the energy density of empty space at a great distance from the plate should be zero. Otherwise, the space-time would be curved and the universe - not quite flat. Hence, the energy density in the region between the plates must be negative.

Experimentally proved deflection of light indicates that the space-time curved and Casimir effect confirms that the curvature can have a negative value. And it may seem that with the development of science and technology, we are able to create "wormholes" or to bend space and time in some other way to get the opportunity to travel to past. But in this case inevitably raises a number of issues and problems.

For example, if in the future will be possible travel time, why nobody has returned until now to us from the future and did not tell how to do it.

Even if there is good reason to keep us in the dark, a man is by nature hard to believe, that no one wants to appear and reveal to us poor backward peasants secret travel time. Of course, some claim that we have already visited by guests from the future - they arrive on a UFO and the government involved in a gigantic conspiracy to conceal these facts to themselves to use scientific knowledge to bear with a visit. I can only say one thing: if the government something and hide, they will still not be able to take advantage of the helpful information provided by the aliens. I am very skeptical about "conspiracy theories"And I believe more plausible" theory Laje. " UFO reports can not be associated exclusively with the aliens because they are mutually contradictory. But if we recognize that some of these observations - a mistake or hallucinations, is not logical to admit that they are such, than to believe that we were visited by guests from the future or from another part Galaxy? If these guests really want to colonize Earth or warn us of any danger, they are extremely inefficient.

There is a way to reconcile the idea of ​​time travel with the fact that we have never met with guests from the future. It can be said that such travel will be possible only in the future. The space-time of our last fixed, because we have seen it and saw that it was not sufficiently twisted so that we had the opportunity to go back in time. And the future - open, so one day we will learn to bend space-time and get the opportunity to travel in time. But since we can bend the space-time, we can only in the future, then do not come back from it in our present, or even earlier.

Such a picture may well explain why we are not experiencing an influx of tourists from the future. But it still leaves room for a number of paradoxes. Suppose an opportunity to make a flight in a spaceship and go back to the beginning of the flight. What do you prevent the rocket to explode on the launch pad, and thus eliminate any possibility of such a flight? There are other equally paradoxical version, for example, go back in time and kill your parents before you were born. That there are two possible solutions.

One thing I would say in unison-historical approach. In this case it is possible to find an agreed solution to the equations of physics - even though that space-time is curved so that it is possible to travel back in time. From this perspective, you can not prepare the rocket for travel into the past, if you had not come back and were not able to blow up the launch pad. This is - a consistent picture, but she says that we are fully determined that we are not able to change your thoughts. It is too much for free will.

Another solution I call alternative-historical approach. His defended physicist David Deutsch, and it is likely to mean the creators of the film "Back to the Future». With this approach, one alternative history there will be no coming back from the future to launch rockets and, accordingly, there will be no possibility of its blow. But when the traveler returns from the future, he finds himself in another alternative history. In it the human race has made enormous efforts to build a spaceship, but before the start of another part of the galaxy appears like a ship and destroys built.

David Deutsch prefers alternative-historical approach stories multiplicity of concepts put forward by physicist Richard Feynman. His idea is that, according to quantum theory, the universe is not unique and the only history.

In the universe there are all possible stories, each - with its probability.

There should be a possibility of a history in which the Middle East - a stable world, but the probability of such a story is likely to be small.

In some stories, the space-time is curved so that objects such as missiles, able to return to his past. But each story - one-piece and self-sufficient, not only describes the curved space-time, but all the objects in it. Therefore a rocket back, can not go to another alternative history. It remains in the same story, which should be self-consistent. And I, unlike Deutsch, I believe that the idea of ​​a multiplicity of stories works rather in favor of a coordinated and historical, rather than the alternative-historical approach.

Apparently, we are unable to give up the concert and historical paintings. However, it can not deal with the problems of determinism and free will, if there is very little probability of stories in which space-time is curved in such a way that time travel is possible outside of the macroscopic scale. I call it a hypothesis about the vulnerability of history: the laws of physics are arranged so as to prevent a trip back in time at the macroscopic level.

It seems that everything looks as if the space-time is curved almost enough to travel opportunities in the past, the virtual particles can become almost real particles moving along closed paths. The density of the virtual particles and their energy is considerably increased, which means that the probability is very small such stories. Although it becomes similar to the activities of the Agency for the Protection of history, which tends to keep the peace for historians. But the theme of the curvature of space and time as yet in its infancy. According to the unifying form of string theory known as M-theory, which we have high expectations in terms of association general theory of relativity and quantum theory, space-time must have eleven dimensions instead of four, which we We feel.

The bottom line is that seven of the eleven dimensions are folded in such a small space that we do not notice it. On the other hand, the remaining four dimensions is almost flat, and are what we call space-time. If this picture is correct, it should be possible to somehow connect four flat measurement with the remaining seven severely crooked or distorted, measurements. What happens, we do not know yet. But the opportunities are very exciting.

In conclusion, I will say this.

Our modern ideas do not exclude the possibility of fast space travel and the return to the past. It can generate huge logical problems, so let's hope that there is a law about the vulnerability of history, which does not allow people to go back in time and kill your parents.

But fans of science fiction should not be upset. M-theory gives hope.

The last work of the world-famous physicist Stephen Hawking, the book-testament, in which he sums up the kind and speaks on the most important topics of concern to all.

Will mankind survive? Are we so actively implemented into space? Is there a God? These are just some of the questions that are answered in his final book, one of the greatest minds in history.

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