The book "The Great Space Journey"

The basis of this book was the Princeton University course, which was taught to the humanities by three famous astrophysics - Neil DeGrasse Tyson, Michael Strauss and John Richard Gott. They talk about what they love most, and tell them that the most complex theories become understandable to non-specialists.

Astronomers are not used to complicate things that can be simple. Big red stars are red giants. Little white stars are white dwarfs. If the star pulsates, it is called a pulsar. Even the beginning of all space, time, matter and energy that exist in space can be called just two simple words: the Big Bang.

What do we know about the universe? Our universe is great. Our Cosmos is much more than it seems. He's hotter than you think. Denser than you think. Faster than you think. Whatever you think of the universe, the reality will still be incredible.

Here we present an excerpt from the book "Cosmic Strings, Mole Holes and Time Travel."

Since I explore the problem of time travel within the framework of the general theory of relativity, neighbors' children think that I have a time machine in my garage. Once I visited a cosmology conference in California and went there in a turquoise sports jacket. My colleague Robert Kirchner, who at that time headed the astronomy department at Harvard, came up and said: “Rich, you must have bought this jacket in the future and brought it here, because you haven’t invented that color yet!” jacket from the future "and always come in it to read lectures on time travel.

I usually enter such a lecture in this turquoise jacket and carry a brown briefcase. I put the briefcase in the locker and hurry out. Then come back in a t-shirt. I explain to the audience that I have another meeting scheduled, and I arranged for a guest speaker to read this lecture instead of me, then I’m going out again. I come back again, already in a turquoise jacket, and I tell everyone that this is a “jacket from the future”. I explain that I could not give a lecture because I was in a hurry to another meeting, which was scheduled for the same time as the lecture; but since I have a time machine, after the meeting I was able to just go to the future, buy a jacket there and come back to give a lecture, but after I had had time to grow old.

Then I notice that I forgot to bring an outline of the lecture on time travel. What to do? Since I have a time machine, I think, it is possible to take a synopsis the next day (after the lecture) and come back here, leave a briefcase with the lecture sheet somewhere in the audience beforehand. I look around, but I don’t see them anywhere. So maybe I hid them. Where? Yes, even in this locker. I open the locker, find the briefcase in it, open it - hurray! There lies the abstract of a lecture on time travel.

Consider these events, tracing their world lines on the space-time graph. Space is deposited on the abscissa axis, time is plotted on the ordinate axis, and the future is at the top. The audience in which I lecture is a vertical line in the center. This is what my world line looks like (fig. 21.1).


On the space-time graph, I leave the audience, while I have a white T-shirt on. I return to the audience for a while, saying that I cannot give a lecture, because I am in a hurry to meet. I leave, I have time for a meeting, and then I fly to the future, where I buy a jacket. From this moment on, my world line becomes light gray. I come back, I go back to the audience and give a lecture there. When the lecture is finished, I need to go back in time shortly before the lecture begins, just to have time for the briefing with the outline of this lecture. I will enter the audience, and then I will quickly leave, after which I will enter it (slightly younger), in a T-shirt. Further I will continue to live strictly in the direction of the future. I got a difficult world line.

What about the world line of the briefcase? He finds himself in my hands as soon as I find him in the locker. If I just keep him, he will go on a chronological journey with me, and I will bring him to the audience before I find him in the locker. The world line of the briefcase will be circular. The line of the briefcase is strange - it has neither beginning nor end. My world line has a beginning (I was born) and an end (I died), but the world line of the briefcase is a closed loop. The briefcase in this case can be compared with the genie particle. Indeed, it, like the genie, arises from nowhere.

Briefcase is on my mind all the time. He does not get to the factory briefcases. Physicists who study travels in the past are forced to take into account genie-particles when they talk about quantum effects. What if a slight scuff appears on my briefcase when I take him with me from the lecture? Igor Novikov believes that such wear, which the genie-particle undergoes, must at some moment “be corrected” so that it returns to its original state, and my briefcase is no exception. At the same time, the laws of entropy are not violated, since a briefcase is not an isolated system; the energy needed to repair it comes from outside.

Information can also act as a genie. Suppose I arrive in 1915 and give Einstein the correct field equations for general relativity. He can write them down and publish them. Where did this information come from? I learned it from Einstein's article, and Einstein recognized it from me. It turns out the ring world line.

The laws of physics allow for the existence of genie-particles — simply their appearance is unlikely — and the more massive and more complex such particles are, the less likely they are to appear. A similar story could have happened if I had found a piece of paper on the floor in a streaming audience and took not a briefcase with me, but this piece. In this case, a piece of paper would become a genie, and carrying it with you is easier than a briefcase. Even easier - I could simply take an electron and carry it with me, and then return it to the past, to a lecture audience. It’s just not as likely to find such a massive object as a briefcase for this purpose, and also with notes to the lecture on time travel. I think such complex genies are possible, but they will arise only rarely.

The journey into the past takes place in the presence of a world line that is looping back into the past. The usual situation is shown in fig. 18.1: the world lines of the Earth and other planets spiral along the world line of the Sun. Nothing can move with superluminal speed, and all world lines are directed to the future. In fig. 21.2 shows the situation when we travel in the past. The traveler’s world line closes in on itself, and he becomes a witness to some events from his own past. The traveler begins his way down (in the past) and rises up until he meets a matured self, who says to him: “Hello! I am you in the future! I flew back in time to say hello to you. ” He replies: “Really?” And goes back through the loop to the past. Then he meets himself in his youth and says: “Hi! I am you in the future! I flew back in time to say hello to you. ” The young “twin” replies: “Really?” The time traveler participates in this meeting twice: as a young man and as a more adult person, but the situation itself develops only once. You can compare it with a four-dimensional sculpture, on which world lines are applied. It never changes: this is what the whole picture looks like. If you are wondering how such a situation would feel, just drag your finger along the world line and see what other world lines will intersect with it.

Here one of the varantas of the decision of the famous grandmother's paradox opens: what would happen if I went to the past and accidentally killed my own grandmother before she gave birth to my mother? In this case, she will not give birth to my mother, and mother - me, and then I will not be, I can not go to the future and kill my future grandmother. This, in turn, means that she will give birth to my mother without any problems, and my mother to me. Paradox. The traditional solution to the grandmother's paradox is this: time travelers cannot change the past. They are always part of the past. Yes, you could go back in time and drink tea with cookies in Grandma’s company when she was still a young girl, but you couldn’t kill her, because she had to give birth to your mother, who would give birth to you. The decision must be self-consistent. Kip Thorne, Igor Novikov and their colleagues formulated a series of thought experiments from the field of time travel, pushing billiard balls; they tried to show that you can always find self-consistent solutions that do not contain paradoxes.


Do not worry about changing the story: no matter how hard you try, nothing will come of it. If you go back to the Titanic and warn the captain about the iceberg, the captain will ignore your words, just as he did not attach importance to all other iceberg reports, because we know that the ship will sink. You will see that it is impossible to change the course of events. The chronopaths in the movie “The Incredible Adventures of Bill and Ted” are built on the same principle of self-consistency.

An alternative solution to the grandmother's paradox is Everett's many-world theory of quantum mechanics. Opinions of physicists about this theory differ, but let's first understand how it works. In many-world theory, it is believed that many parallel worlds can coexist, like railway tracks in a shunting park. We see one story, as if our train is on a specific path. The events that we observe are similar to the stations that replace each other. Here is the Second World War ... here are people landing on the moon and so on. But there are many parallel worlds. There is a world where the Second World War did not occur. Such a theory is based on the approach to quantum mechanics as the sum of the many stories proposed by Richard Feynman. Feynman realized that in order to calculate the probability of any outcome of a future experiment, it was necessary to take into account all possible stories that could lead to it. Some people think that this is just another strange rule of calculations in quantum mechanics, but supporters of the multi-world model believe that all these stories are real and interact with each other. David Deutsch believes that the time traveler can go back in time and kill her grandmother when she was still a young girl. In this case, a new branch of history will arise: in this version of the story there will be a live chronopravoshennik, and his dead grandmother. That way, where the chronopraveller was nevertheless born and his grandmother alive, also continues to exist. The traveler still remembers part of his biography, which happened before turning to a new path. Both ways exist.

Now we have two adequate solutions to the grandmother's paradox. The first is conservative, where space-time is represented as a single, self-consistent four-dimensional sculpture, essentially unchanged. The second is a more radical, multi-world interpretation of quantum mechanics. Both solutions work.

Further, if you go back to the world time traveler line, which is looped into itself in the past, then you can see one flaw in it. The light on this circuit flies at an angle of 45 °. When the chronopulete travels over the summit to begin a return to the past, at some point the slope of its world line relative to the time axis must be greater than 45 °. This means that at some point it must exceed the speed of light. In fact, crossing over the top of the circuit, it moves with infinite speed. The phenomenon of the interconnection of flights with superluminal speeds and time travel is captured in the following Limerick AG R. Bouller:

Very smart Miss from Dakota
She said: “Einstein is something!
Once I flew somewhere
Above the speed of light
And returned the day before departure! "

The problem is that (as Einstein demonstrated in his special theory of relativity) it is impossible to construct a rocket that would develop superluminal speed. If your speed does not exceed the speed of light, then your world line will never be located at an angle of more than 45 ° relative to the time axis, and you will not be able to return to the past. However, in Einstein’s general theory of relativity, where space-time is considered to be curved, one can outrun a ray of light by cutting off the path if one moves either through the wormhole or (as will be discussed below) along the cosmic string. If you overtake a ray of light, then, like our brisk miss, you can fly away into the past.

Suppose we have a sheet of paper on which one spatial dimension is laid out horizontally, and time - vertically (Fig. 21.3). In this case, your world line is the green bar in this picture. You are lazy and just sit at home, so your world line runs smoothly from the bottom to the top of the sheet. However, in the curved space-time, other rules apply. Let's roll the sheet horizontally into the cylinder, gluing the sheet together with tape. Now your world line is a circle going back in time.

All the time you are moving forward into the future, but in reality you are turning into the past. The same thing happened with the expedition of Magellan: the sailors sailed all the time to the west, circled the whole Earth and returned back to Europe. This would never happen if the surface of the earth were flat. Likewise, the chronopraveller moves all the time into the future, but if the space is curved enough, he can return to the events that happened in his past.

»More information about the book can be found on the publisher's website.
» Table of Contents
» Excerpt

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