Modern astronauts are known to already do this. True, they travel only to the future so far. And for a split second. But who knows - maybe one day we will be able to fly away for years or even centuries ahead, or, conversely, pat the forelock of our five-year-old grandfather? But let's start in order.
Stop for a moment
Time flies differently in the empty space of space and on Earth. This is known to every student. The stronger the gravity of an object, the slower time flows in its vicinity. This is due to the fact that gravity distorts the "fabric" of four-dimensional space-time. On the other hand, Einstein showed that the higher the speed, the greater the mass. Therefore, for all objects that move at very high speeds, time also slows down. The speed of the ISS is over 27 thousand km / h. Russian cosmonaut Sergei Krikalev, for example, spent a total of 803 days, 9 hours and 39 minutes in orbit. Thus, he lives in time, a full 1/50 of a second ahead of us.
Time Machine
The theory of relativity tells us that a time machine that takes us to the future can be created. You enter it, wait. Come out and find that centuries have passed on Earth. There are no technologies for this yet, but science knows that it is possible.
However, for this you have to accelerate to a speed close to the speed of light. Is it any wonder that a time machine is nothing more than a spaceship, because, according to the General Theory of Relativity, time and space are inextricably linked (the question of how, when accelerating to such a frantic speed, to keep your body and the ship intact, not yet). But can a person who has made such a journey return back to the past?
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The first hints that the laws of physics allow people to travel into the past appeared in 1949, when the mathematician Kurt Gödel found a new solution to Einstein's equations, and in fact - a new structure of space-time, which is quite acceptable from the point of view of general relativity. However, based on Gödel's equations, the universe should rotate as a whole, and not expand with acceleration - which, as it turned out since then, does not correspond to reality.
In recent years, scientists have suggested other avenues for potential time travel - the curvature of space-time. However, analysis of the microwave background and other data show that the universe has never been twisted enough to make such travel possible. However, there is also a workaround.
What is the past?
According to general relativity, there is not only a single universal measure of time for all observers, but under certain circumstances there is no need even for observers to agree on a single sequence of certain events. Let's say that time on Alpha Centauri moves at the same speed as on Earth (the planet on which the aliens live has the same mass and moves at the same speed). In 2014, the Sochi Olympics took place. Let us also assume that the opening of the Interplanetary Chess Tournament on Alpha Centauri will take place in 2015. Which event happened earlier?
From the point of view of earthlings - the Olympics. From the point of view of the "Centauri" - a tournament. After all, the light from the Earth to Alpha Centauri will take four years. Moving faster than light, you could go to the Olympics and fly to the tournament, and then return to Earth again … before the Olympics. Naturally, in theory - if you find a way to travel faster than the speed of light.
Moving faster than the speed of light, based on the theory of relativity, is known to be impossible. As you approach the "light barrier", more and more energy is required to accelerate an object. At one point - with the theoretical achievement of the speed of light - it would take an infinite amount. In addition, a body that would reach such a speed must acquire an infinite mass.
Wormholes
This is where a roundabout maneuver is possible. It lies in the potential to deform space-time. For example, so that a short path from the Olympics to the chess tournament was opened. You will not move faster than the speed of light - but you will travel faster in space.
In 1935, Albert Einstein and Nathan Rosen wrote a paper in which they argued that general relativity admits the existence of such space-time bridges, bows - "wormholes."
Einstein-Rosen bridges, the so-called "wormholes"
Alamy
Maintaining the integrity of a wormhole requires tremendous energy, and theory predicts that they cannot last long enough for a spacecraft or other macroscopic object to pass through. Such a bridge can "collapse", and the ship will disappear somewhere in the singularity.
True, scientists admit the idea that a technically advanced civilization could keep such a hole open for the right time. But how this can be achieved is still completely unclear.
It is worth saying here that all matter to which we are accustomed has a positive energy density, which gives space-time a positive curvature resembling a sphere. And for the deformation of space-time, which would allow us to travel into the past, we need matter with negative curvature - that is, with negative energy density. Quantum mechanics, as you know, allows the existence of such a negative energy density (provided that this "negativity" is compensated by "positiveness" in other areas), and allows the theoretical possibility of deforming space-time.
This is not easy to imagine. To do this, astrophysicists often use the hill example. If you dig a large hole and throw dirt out of it onto the edge of the hole, you end up with not only a hole, but a hill as well. In this case, the hill will be a metaphor for this positive energy, and the pit will be negative.
Black holes and more
Scientists are cautiously suggesting that black holes may be some kind of analogs of wormholes. The point is that bo? Most of spacetime is nearly flat. It is severely deformed only in black holes. The black hole distorts the space-time around itself so much that it forms a kind of "funnel", a "hole" of a conical shape.
The gravity in the immediate vicinity of the black hole is so huge that space-time in it, in fact, ceases to exist, or is distorted so much that time practically stops. In addition, some black holes rotate at near-light speeds. As a result, space-time "folds" in the hole, practically into a "tube". Perhaps, having penetrated into a black hole, we could pass through its narrow tunnel and find ourselves … in the past, or, for example, in another universe?
The most famous theoretical physicist of our time - Stephen Hawking - is sure that this is impossible. Even if the spaceship in some incredible way (overcoming the effect of enormous gravity unharmed) manages to get into the very center of the black hole, it will end up in a singularity and simply cease to exist.
However, many other scientists believe that once in a black hole, under certain conditions, you can still survive, and even look for ways to do it. Of course it looks eccentric. But the history of science knows many examples when such eccentrics invented an airplane or went to the moon.
The black hole in the constellation Unicorn and its companion B (e) -star as seen by the artist
ESA
Richard Gott, an astrophysics professor at Princeton University, is a time travel enthusiast. He creates his theoretical project for a time machine and even claims to have found a solution for traveling to the past. Following some astrophysicists, Gott believes that the natural time machine is the center of a rapidly rotating black hole. But he also understands how unreliable such a "transport" can be.
However, Gott found a potential less dangerous analogue of the center of a black hole - a phenomenon called cosmic strings. Cosmic strings are hypothetically existing folds of space-time, thin threads of energy left over from the Big Bang. Their width is less than the atomic nucleus, but they have a phenomenal density. Such a string, only 1 m long, would have more gravity than the entire Earth, and would create a tremendous curvature. Gott discovered that the interaction of such fast-moving strings could lead to a natural time machine.
Gott's calculations have not yet been confirmed by observations, but the scientist is trying to prove that these strings exist. However, even he says that it is almost impossible to find two such strings that would pass opposite each other. Therefore, Gott draws attention to another theoretical structure, to cosmic rings that could form closed strings. Despite the lack of evidence of their existence, there are no direct errors in Gott's theory. Besides that inside such a "ring" there will be … a black hole again. And in general, to control a system with such a grandiose gravity, the energy resources of entire galaxies would be needed.
Galaxy M83. A black hole found in galaxy M 83 has bypassed the theoretical brightness limit, refuting the fact that the Eddington limit is a fundamental law of nature
NASA
You can't stay back in the past
But does quantum theory allow time travel on our - macroscopic - scale? Stephen Hawking says he does, at first glance. This is evidenced by the Feynman integrals over trajectories (the essence of Feynman integrals is that they replace the definition of a unique, only possible trajectory of any elementary particle by the total sum of an infinite set of possible trajectories of its motion). After all, they cover all possible scenarios, and, therefore, allow for the existence of such a distortion of space-time, which is necessary for travel to the past. Therefore, it is impossible to say that such travels are impossible in principle.
Heavy particles that are accelerated in colliders at the European Center for Nuclear Research (CERN) or at the National Laboratory. Fermi in the United States reach speeds equal to 99.99% of the speed of light. However, no matter how much the power of the installation is increased, it will not be possible to exceed the light barrier.
Hey, guests from the future
But if so, why haven't guests from the future visited us yet? The popular point of view is that the civilization of the future is so "advanced" that it considers it inappropriate to reveal the secret of time travel to such unreasonable beings like us. What if an enthusiastic contemporary wants to go back in time and reveal to the Nazis the secret of the atomic bomb?..
Such a different story
It may turn out that history is a strictly fixed chain of events, so even if you return to the past, you will be doomed to do the same thing that you did before. Otherwise, returning to your future, you may even find that you … simply do not exist, or your loved ones do not exist, or there is no country in which you live, etc. A similar drama is well described in the famous science fiction story by Ray Bradbury "And Thunder Rocked", the main character of which, on a trip to the past, accidentally crushed a butterfly - a liberal is a dictator. In natural sciences, this term is called the butterfly effect:a small impact on a chaotic system can have large and unpredictable consequences elsewhere and at a different time.
Another possible way to solve the paradoxes of time travel can be described as the hypothesis of alternative history. When time travelers return to the past, they find themselves in alternate stories that are different from the one they know. Many scientists today are talking about the possible existence of a Multiverse, which can include all these - and an infinite number of others - variants of the past, branching in an infinite number of worlds …
At first glance, this hypothesis resembles Feynman's quantum mechanical equations. But there is also an insoluble contradiction between them. In Feynman integrals, each trajectory completely includes space-time and everything that is in it. And, as we found out, within the framework of such a view, a rocket could travel through curved space-time even into the past. But the rocket itself would have remained in the same "own" space-time, and, therefore, in the same history. Therefore, Feynman integrals, rather, speak in favor of the hypothesis of a fixed past.
Modern science doubts the possibility of travel to the past. But still, we do not advise you to argue with someone on this score: what if this someone is arguing, knowing the future in advance?..
Olga Fadeeva