In science fiction movies, the mainstream way to travel through time and space is to use a time machine, or to use time tunnels such as wormholes as in the movie Interstellar. The purpose of crossing is nothing more than that the villain wants to influence the present or the future by changing the past. Tenet's future earth environment has become very bad. Some human beings are dissatisfied with the status quo and want to destroy the past world and change the future.
Because time travel is still a science fiction concept, time machines are often involved in movies and will not be introduced too much. The earliest analysis of this detail originated from the film "Back to the Future" series in the 1980s, and was also the originator of science fiction movies. In this movie, the hero travels through time and space in a specially modified car. When the speed reaches a limit, he can travel through time and space Now it seems that it is obviously impossible to travel through time and space in this way, and the way Iron Man and others enter the quantum field to travel through time and space in Reunion 4 is more reliable.
There is also a time machine in the movie Tenet, but this time machine is a little different, because it does not rely on time jumping, but on reversing the entropy of matter, so that these substances can travel back through time and space.
The relationship between entropy and time
The concept of time is both familiar and unfamiliar to everyone. Since the birth of the universe, everything is in constant movement and change, and the essence of time is movement. If everything is absolutely static, there is no concept of time. Humans record time through periodic movements, such as clocks and watches.
Entropy usually describes the degree of chaos of a system. The higher the degree of chaos, the higher the entropy value of the system. This concept was first put forward by Clausius in 1865 and applied to thermodynamics. Thermodynamics has four basic laws, and the second is the law of entropy increase. For an isolated system, if it does not exchange energy with the outside world, the entropy value of the system will increase continuously, which is the law of entropy increase.
Why does entropy increase?
Because matter is always in an eternal state of motion, in an isolated system, the degree of chaos will inevitably increase with time (or motion). Unless the external force can change the motion state of matter in an isolated system, that is, exchange energy with the external environment, it is possible to reduce the entropy value in the system.
It is easy to break a glass, but it is a thousand times more difficult to restore it. The universe may be an isolated system, because things in nature always seem to develop in a state of disorder (entropy increase).
Entropy and time are linked through the movement of matter, and time has the important feature of one-way, so the direction of entropy increase of things is considered as the direction of time passing.
Since the increase of entropy represents the forward flow of time, the decrease of entropy represents the backward flow of time. The time travel in the movie "Justice" is based on this idea. In the distant future, scientists invented a kind of black technology, which can reverse the entropy-increasing property of matter, thus transmitting the future matter or information back to the past.
When objects or people in normal space-time are reversed, they are in the reversed space-time. In this space-time, not only the flow direction of time has changed, but also the law of causality has reversed, and all physical laws are contrary to the positive space-time. It is also very important that the time of countercurrent and downstream is the same, and the speed of passage is the same.
The biggest problem in traveling through time and space? Grandmother paradox
This big problem will be encountered in all time-crossing scenarios. If the protagonist goes back to the past and accidentally kills his grandmother before his father is born, then the protagonist will not be born, so how can he come back from the future and kill his grandmother?
In essence, crossing behavior may lead to the destruction of causality. In this regard, scientists have given an explanation, that is, parallel universes.
Parallel universes come from quantum theory, which holds that there are countless universes parallel to us in this world. In the quantum world, the motion of matter is random, and every non-zero possibility will occur in different parallel universes, forming countless timelines.
If they are on the same time line, from a quantum point of view, any kind of behavior crossing the past will have an impact on the future, and this impact may be further amplified by the butterfly effect. According to the theory of parallel universe, the so-called time-space crossing actually means going to another parallel universe, or creating another parallel universe because of one's own crossing behavior.
In this way, the violation of the law of causality due to crossover is avoided. But it also confirms the fact that the past cannot be changed, and what has happened will happen eventually.
In Tenet, the protagonist reverses time and space, but he is still in the same timeline of the same universe, but his own time has changed and he has not gone to a parallel universe. Two people with different time flows are still in the same time and space, which leads to a two-way time attack war between the red and blue sides against the villains, that is, the time clamp war mentioned in the film.
In the movie, because the story happened on the same timeline, the whole timeline was designed as a closed loop in order to avoid the phenomenon similar to grandma's paradox, which led to the reversal of causality. Cause and effect, cause and effect, just like the name of the film purpose, everything is symmetrical. Both the beginning and the end of the story happened on the same day. Everything is doomed and the past cannot be changed.
Now the theoretically feasible time travel is to travel to the future.
If you want to travel to the future, relativity gives a feasible solution, that is, to change the delay speed of your own time and space.
When an object is in a state of high-speed motion or in a strong gravitational field, its time is slower than that in a low-speed weak gravitational field. It is for this reason that the atomic clock on the GPS satellite in space should be calibrated synchronously with the ground at regular intervals. If you fly for a long time, you may live a few tenths of a second longer than people on the ground.
If you fly a spaceship near the speed of light, or move around a black hole at high speed, when you leave your space-time environment, you will find yourself crossing into the future because of the relativity of time.
The future is uncertain, and crossing behavior will not destroy the law of causality. And when you reach that point in the future, you will become a part of the future history. Obviously, this is a one-way trip.
As for going back to the past, it is still a fantasy.
However, this way of going back to the past through time and space in the movie Purpose is really novel, and I have to admire Nolan's brain hole. In the first half of the film, some details are foreshadowed, which will not be gradually revealed until the second half. I don't understand, maybe it's because the characters are reversed repeatedly, which makes everyone dizzy.
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