What to read: Brief Answers to Big Questions is Stephen Hawking's latest book

2024 Author: Malcolm Clapton | [email protected]. Last modified: 2023-12-17 03:44

An excerpt from the work of the great scientist on whether time travel is possible.

Closely related to time travel is the ability to quickly move from one point in space to another. As I said earlier, Einstein showed that infinitely powerful jet thrust would be required to accelerate a spacecraft to near-light speed. So the only way to move from one part of the Galaxy to another in a reasonable period of time is to be able to fold space-time in such a way that a small tube, or "wormhole" is formed. It can link two parts of the Galaxy and act as the shortest path between them; you can fly back and forth and still catch all your friends alive. Such "wormholes" were seriously considered as an opportunity available to the civilization of the future. If you manage to move from one part of the Galaxy to another in a couple of weeks, then you can return through another "hole" - at the same time before you hit the road. Also, nothing will prevent you from traveling forward and returning to the past through one "wormhole" if both ends of it move relative to each other.

We can say that in order to create a "wormhole", it is necessary to bend space-time in the opposite direction to that into which ordinary matter bends it. Ordinary matter bends space-time towards itself, like the surface of the Earth. But creating a "wormhole" requires matter that bends space-time in the opposite direction, like the surface of a saddle. The same is true for any other curvature of spacetime to travel into the past, unless the universe is so curved that it already has time travel capabilities. Only in this case will you need matter with negative mass and negative energy density.

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

Classical laws make it impossible for us to bend the universe so that time travel becomes possible. But the classical laws are refuted by the quantum theory - the second after the general theory of relativity, the great intellectual revolution in our understanding of the Universe. Quantum theory is more flexible and allows overdraft in some cases. However, the bank should be kind enough to us. In other words, quantum theory allows for a negative energy density in some places, if you provide a positive density in others.

Quantum theory allows negative energy density because it is based on the principle of uncertainty. And he argues that some characteristics, such as the position and speed of a particle, cannot simultaneously have exactly measured values. The more accurately the position of the particle is determined, the higher the uncertainty about its velocity and vice versa. The uncertainty principle also applies to fields - for example, to an electromagnetic or gravitational field. He argues that these fields cannot have a null value even where we think there is empty space. The fact is that if their values are equal to zero, then this means that they must have a well-defined position, equal to zero, and a well-defined speed, equal to zero. And this is contrary to the principle of uncertainty. This means that the fields must have some minimum fluctuation. One can imagine the so-called vacuum fluctuations in the form of pairs of particles and antiparticles that suddenly arise, separate, then merge again and annihilate, mutually annihilating.

Such pairs of particles - antiparticles are considered virtual, because they cannot be directly detected using a particle detector. But an indirect effect can be observed. For this, the so-called Casimir effect is used. Try to imagine two parallel metal plates spaced a short distance from one another. The plates act as mirrors for virtual particles and antiparticles. This means that the space between the plates looks like an organ pipe, only it transmits light waves of a certain resonant frequency. As a result, it turns out that a certain amount of quantum fluctuations occurs between the plates, different from what happens behind them, where these fluctuations can have any wavelength. The difference in the number of virtual particles between the plates and outside means that the plates are under more pressure on one side than on the other. A small force arises, which brings the plates closer to each other. This force can be measured experimentally. So virtual particles exist in reality and have a real effect.

Since there are fewer virtual particles, or quantum fluctuations in a vacuum, between the plates, the energy density is also lower here than in the surrounding space. But the energy density of empty space at a great distance from the plates should be zero. Otherwise, space-time will be curved and the Universe will not be completely flat. This means that the energy density in the area between the plates must be negative.

Experimentally proven deflection of light indicates that spacetime is curved, and the Casimir effect confirms that curvature can be negative. And it may seem that as science and technology develop, we will be able to create "wormholes" or bend space and time in some other way in order to be able to travel into the past. But in this case, a number of questions and problems inevitably arise.

For example: if time travel becomes possible in the future, why has no one returned to us from the future and told us how to do it.

Even if there are good reasons to keep us in the dark, it is inherently difficult for humans to believe that no one wants to show up and reveal to us poor backward peasants the secret of time travel. Of course, some argue that guests from the future are already visiting us - they fly on UFOs, and governments are involved in a giant conspiracy to cover up these facts in order to use the scientific knowledge that the guests carry with them. I can only say one thing: if governments are hiding something, they are still not able to use the useful information received from aliens. I am very skeptical about "conspiracy theory" and I find more plausible "mess theory". UFO reports cannot be exclusively related to aliens because they are mutually contradictory. But if we admit that some of these observations are just errors or hallucinations, isn't it more logical to admit that they are, than to believe that we are visited by guests from the future or from another part of the Galaxy? If these guests really want to colonize the Earth or warn us of some kind of danger, then they are extremely ineffective.

There is a way to reconcile the idea of time travel with the fact that we have never met guests from the future. We can say that such travel will only become possible in the future. The spacetime of our past is fixed because we observed it and saw that it was not curved enough for us to be able to travel back in time. And the future is open, so someday we will learn to bend space-time and get the opportunity to travel in time. But since we will be able to bend space-time only in the future, we will not be able to return from it to 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 many paradoxes. Suppose there is an opportunity to fly in a spaceship and return before the start of the flight. What will prevent you from detonating a rocket at the launch site and thereby exclude the possibility of such a flight for yourself? There are other no less paradoxical versions: for example, to go back in time and kill your parents before you were born. There are two possible solutions to this.

One thing I would call a consistent historical approach. In this case, one can find a consistent solution to the physical equations - even though spacetime is curved to the extent that it is possible to travel into the past. From this point of view, you cannot prepare a rocket for travel into the past if you have not returned to it and have not been able to blow up the launch pad. This is a sequential picture, but it says that we are completely determined: we are not able to change our thoughts. This is too much for free will.

Another solution I call the alternative history approach. It was championed by the physicist David Deutsch, and was probably meant by the creators of Back to the Future. With this approach, in one alternative history there will be no return from the future before the launch of the rocket and, accordingly, there will be no possibility of detonating it. But when the traveler returns from the future, he finds himself in another alternate history. In it, the human race makes incredible efforts to build a spaceship, but before starting from another part of the Galaxy, a similar ship appears and destroys the constructed one.

David Deutsch prefers an alternative historical approach to the concept of plurality of histories, which was put forward by the physicist Richard Feynman. His idea is that, according to quantum theory, the universe does not have a unique and unique history.

There are all possible stories in the Universe, each with its own degree of probability.

There should be a possibility of a story in which there is a stable peace in the Middle East, but the likelihood of such a story is most likely low.

In some stories, spacetime is curved so much that objects such as rockets can return to their past. But each story is integral and self-sufficient, describing not only curved space-time, but also all the objects in it. Therefore, the rocket, returning, cannot get into another alternative history. It remains in the same story, which must be self-consistent. And I, unlike Deutsch, believe that the idea of a plurality of stories works in favor of a consistent historical rather than an alternative historical approach.

Apparently, we are not in a position to abandon the consistent historical picture. However, this may not address the issues of determinism and free will if there is a very small likelihood of stories in which spacetime is curved so that time travel is possible beyond the macroscopic scale. I call this the chronology security hypothesis: the laws of physics are designed to prevent time travel at the macroscopic level.

It looks like it looks like if space-time is curved almost enough to allow travel into the past, then virtual particles can become almost real particles moving along closed trajectories. The density of virtual particles and their energy increase significantly, which means that the likelihood of such stories is very small. Although this is becoming similar to the activities of a chronological protection agency that seeks to preserve the world for historians. But the theme of the curvature of space and time is still in its infancy. According to a unifying form of string theory known as M-theory, which we have high hopes for uniting general relativity and quantum theory, spacetime should have eleven dimensions, not the four we experience.

The bottom line is that seven of these eleven dimensions are rolled up into such a small space that we do not notice it. On the other hand, the remaining four dimensions are practically flat and represent what we call space-time. If this picture is correct, then it should be possible to somehow connect the four flat dimensions with the remaining seven highly curved, or distorted, dimensions. What will come of this, we do not yet know. But the opportunities are exciting.

In conclusion, I will say the following.

Our modern concepts do not exclude the possibility of fast space travel and return to the past. This can create huge logical problems, so let's hope there is some kind of Chronology Security Law that will prevent people from going back in time and killing their parents.

But science fiction fans shouldn't be upset. M-theory gives hope.

The last work of the world famous physicist Stephen Hawking, a book-testament, in which he summarizes and speaks out on the most important topics of concern to everyone.

Will humanity survive? Should we be so active in space? Is there a God? These are just a few of the questions that one of the greatest minds in history answers in his final book.