In 1895, the English science fiction writer Gerber Wells published the novel The Time Machine. It set out an original idea: someday scientists will create a device that allows you to travel to the past and the future, perhaps even interfere with the course of history. Since then, this idea has haunted many. And it has already led physicists to a discovery that, without exaggeration, can be called sensational.
EVERYTHING IS RELATIVE
The first prerequisites for the study of the theoretical possibility of time travel appeared after Albert Einstein formulated the provisions of the Special and General Theories of Relativity. The first of them asserted the dependence of time on the speed of an object: the closer to the speed of light, the slower time flows. In the second - the dependence of time on gravity: near massive bodies, time again flows more slowly.
Einstein's theories revolutionized perception. It turned out that the time machine was "constructed" by nature itself! For example, if the ship is accelerated to near-light speeds, then its crew will be carried away into the distant future. To illustrate this effect, the "twin paradox" is usually used: an astronaut who returns from a flight to the stars will be younger than his twin brother who remained on Earth.
But the consequences of the General Theory of Relativity look much more spectacular. It turns out that the course of time can be changed by curving space in much the same way as gravity does. And if you contrive and create a special spatial formation, called today a "wormhole" (or "wormhole"), connecting distant points in space, then theoretically it becomes possible to break the causal relationship - and be at the exit from the "hole" before you went there …
Einstein himself denied the existence of "wormholes", since, in his opinion, they should immediately "collapse". However, later the American physicist Kip Thorne proved that the so-called “exotic matter” (theoretical material with negative energy density) can be used to stabilize "wormholes", but the impossibility of its existence in reality has not yet been proven by anyone. Moreover, Russian scientists Arkady Popov, Sergei Sushkov and Sergei Krasnikov have shown that "exotic matter" in theory can be generated artificially.
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THE PROBLEM OF CAUSE
It turns out that there are no serious theoretical obstacles to creating a time machine. And if it can be created in theory, sooner or later someone will definitely figure out how to implement it in practice. Why, until now, have we not seen a single time traveler who came to us from the future? Or is our era of little interest to them?
One of the hypotheses that explains the absence of such travelers is that time travel is impossible due to a violation of cause-and-effect relationships. The classic “murdered grandfather” paradox can be cited as an illustration. If, for example, a time traveler for some reason wants to kill his grandfather before the conception of his father and fulfills his plan, then he himself will disappear and … will not be able to kill the grandfather. The same can be said for any serious interference in the affairs of the past: disruption of causality will inevitably destroy the universe. Therefore, nature must impose restrictions on such violations. The same “murdered grandfather” paradox can be resolved provided that at some point something goes wrong with the aggressive time traveler and he will not be able to implement the murder plan.
Another hypothesis was put forward by the famous astrophysicist Carl Sagan. He believed that the creators of the time machine were powerful enough to track causal relationships, so although they are secretly among us, they manage to get by without interfering with the historical process.
HOKING TIME MACHINE
To prove the impossibility of building a time machine, American physicist Stephen Hawking conducted a funny experiment. He placed an advertisement in the newspapers urging all newcomers from the future to visit his home on the night of the coming Christmas. He proceeded from the assumption that in the future someone will definitely read his invitation and want to visit, using a personal time machine. But no one came to him at Christmas … What prevented him from making a trip through time? Stephen Hawking believes there are two possibilities.
The first option is pessimistic. The time machine will never be built, or they will build it and try to use it, as a result of which a kind of global catastrophe will occur, which will put an end to the development of this technology. And in fact: calculations show that to penetrate into the past according to the scheme proposed by Kip Thorne, energies are required that are comparable to the energy of the entire Universe and even exceed it. It is clear that it is unlikely that even in the distant future, the power of civilization will rise to the heights that allow control of such forces.
The second option is optimistic. The civilization of the future can change a certain local area of the Universe so that even with the use of minimal energies, time travel within this area will become real. Stephen Hawking calls such a machine “finite,” that is, finite both in space and in time. In this case, the first aliens from the future will appear among us only after Hawking's global time machine is built and launched. But we haven't done anything like that yet. Accordingly, you can calm down and not try today to look for aliens among us.
Lloyd's Time Machine
Perhaps the most surprising discovery was made by another American physicist, Seth Lloyd, who specializes in quantum computing.
He deduced the possibility of constructing a time machine by examining closed timelike curves - world lines that lead a material particle to its starting point. In a clever experiment, Seth Lloyd and his colleagues were able to use quantum teleportation to correct the closed curve so that the state of the photon was transmitted not in space, but in time.
First of all, an interesting detail became clear: it turns out that the quantum state of a photon transmitted into the past cannot in principle affect its current state, that is, the natural prohibition on the occurrence of the “murdered grandfather” paradox was proved by direct experience. The success of the experiment confirmed that quantum states, which, in fact, are unique information, can be transmitted to the past. Information time machine is built!
However, in this case, a new paradox arises, which is called the "unproven theorem" Seth Lloyd put it this way. Suppose someone reads a proof of a theorem in a textbook, and then sends this proof back to the mathematician who wrote the textbook, even before the theorem itself appeared. The mathematician includes the proof in a book that his counterpart will read in the future. Question: where did the proof come from? Lloyd resolves the paradox in a fantastic way: the quantum world is designed in such a way that it "contains" all possible proofs of the theorem, so the one that appears in the textbook is predetermined by the future, not the past.
How to transfer the laws of the quantum world to the space of large material objects, scientists do not yet know. However, there is no particular need for this. The main thing is that a way has been invented to overcome the barrier of time. And someday science will take the next step - create a transmitter that can communicate with the past.
Perhaps physicists from the future are already trying to contact us and tell us something important. Only we have not yet managed to build a receiver that can decode such messages. Will we be able to? Future will tell…
Anton Pervushin