Scientific research on the possibility of time travel has been considered by scientists from different countries for a long time. The fundamental impossibility of such travel has not yet been proven, but scientists agree that the practical problems associated with travel to the past or the future are so great that, most likely, it will never be realized.
The famous cosmologist Stephen Hawking believes that there must be some law that prohibits time travel. But failing to identify the law against time travel, Hawking later argued that if time travel is possible, then it is not feasible.
But there are other points of view on the problem of time travel. Some scientists have put forward their original hypotheses for the implementation of time travel.
1. The most famous of these is time travel using black holes. Very little is known about the very nature of black holes. It is believed that stars with a mass several times greater than the mass of the Sun, dying as a result of the combustion of their fuel, explode under the pressure caused by their own weight. As a result, black holes appear, in which such powerful gravitational fields are formed that even light cannot escape from this area. Any object that reaches the boundaries of black holes - the so-called event horizons - is sucked inward, and what is happening inside is absolutely invisible from the outside. Presumably, in the depths of black holes, at the singular point, somewhere in their center, the laws of physics cease to operate, and the temporal and spatial coordinates simply change places. It turns outthat travel in space turns into travel in time.
2. A time machine can operate in a rotating universe. In 1949 the famous mathematician Kurt Gödel found the first time travel solution to Einstein's equations. If the universe rotates, then, having circled it fast enough, you can be in the past and get to the starting point earlier than you went from there. It turns out that traveling around the Universe is at the same time traveling back in time. When astronomers appeared at the Institute for Advanced Study, Gödel often asked if they had evidence that the universe was rotating. To his disappointment, they replied that the Universe was expanding, but the total spin of the Universe is probably zero. (Otherwise, time travel might have become familiar, and history as we know it would have ceased to exist.)
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3. Another time travel option was discovered in 1991 by Richard Gott of Princeton. His solution is based on the discovery in space of giant cosmic strings (possibly left over from the Big Bang). Suppose, he suggested, that two such cosmic strings are about to collide. So, if you quickly go around these strings at the moment of collision, you will go back in time. The beauty of this type of time machine is that you don't need endless spinning cylinders, a spinning universe, or even black holes. The problem, however, is that you first have to find these very huge cosmic strings in space, and then make them collide in a certain way. Moreover, the "road" to the past will open for a very short period of time. Gott says: “Collapsing string loop,big enough to be able to go around it once and return one year ago, in terms of its mass-energy should exceed half of the galaxy."
4. You can build a time machine based on the Tipler cylinder. This hypothetical object was obtained from the exact solution of Einstein's equations, and in 1974 Frank Tipler discovered the possibility of the appearance of closed time-like lines in this solution. Stephen Hawking then showed that time travel with a Tipler cylinder is only possible if it has infinite length.
If it is enough to approach the surface of the cylinder, the space around which will be mostly deformed, and go around it several times, then you can move into the past. How far back in time depends on how many times you orbit the cylinder. Even if it seems to you that your own time is moving forward as usual while you circle the cylinder, outside of the distorted space you will inevitably move into the past.
The problem, however, is that the cylinder must be infinite and rotate so fast that most materials will break and fly to pieces.
5. Another promising time machine scheme is reversible wormholes. These are holes in space-time where a person can freely move back and forth in time. In theory, reversible wormholes are the ability to not only travel faster than light, but also travel in time. The key to reversible wormholes is negative energy.
The time machine for reversible wormholes should consist of two chambers; each chamber is made up of two concentric spheres separated by a tiny gap. If you squeeze the outer sphere inward, towards the inner sphere, then the Casimir effect arises between the two spheres and, as a result, negative energy. Suppose that some civilization is able to stretch a wormhole between these two chambers (perhaps it can be built from space-time foam). Next, we take the first camera and send it into space at near-light speed. The time in this chamber slows down, and the clocks in the two chambers lose synchronization. Time in the two chambers connected by a wormhole moves at different speeds.
Being in the second chamber, you can instantly move along the wormhole to the first one, which exists in an earlier time, and find yourself in the past.
The implementation of this scheme is associated with very serious difficulties. So, a wormhole can be very tiny, much smaller than the size of an atom. Concentric spheres may need to be compressed to Planck-scale distances in order to obtain enough negative energy. And the last thing. You will be able to go back in time only at the moment when this time machine was created - after all, up to that moment the time in both cameras was running perfectly synchronously!
6. It is possible to achieve the slowing down of the flow of time and the curvature of space using the powerful forces of gravity.
Based on this, the famous scientist Amos Ori came to the conclusion that if a curved space-time structure is given the shape of a ring or a funnel, it will be possible to travel in time - to the past. During such a movement, a person will go deeper and deeper into the depths of the past - with each new turn. For such a time machine, gigantic gravitational forces are required, which exist only near "black holes".
Despite the creation of a mathematical model proving the possibility of time travel, the technical means of our era do not allow us to make such a time machine.