Time and space travel is a dream for many people. We often fantasize about one day going to the future or going back to the past. Go back to the past and relive those wonderful times, and at the same time you can correct the mistakes you have made.
Now we have achieved half of it: humans have been able to travel to the future, but the "travel" here is not absolute, but relative. Because no matter what form you travel to the future, in your own reference system , the speed of time is always the same, and what is changed is only the speed of time passing by other people or other things.
This is also the principle of time expansion in special theory of relativity .
Special theory of relativity is proposed based on two premises, one of which is the principle of constant light speed . No matter how fast your moves , no matter how fast your moves, no matter in any reference system, you will always see light moving at a constant speed. The principle of
will bring a fascinating result: if a person is exercising relative to you, his time will slow down.
Imagine there is such a photon clock, a clock that light moves back and forth between two mirrors. The faster a person's movement speed, the farther the distance the light moves horizontally, and the person's clock will become slower.
Because movement is relative, your clock will also slow down in this person's eyes.
But who is the time slower? When you meet again, one of you will always get older. Is it you older or that person older?
In fact, this is the core question of the "Twin Paradox". Now we already know the answer: if you were in the same reference system at the beginning, such as on Earth, you stay on Earth, and another person travels in a spaceship, and then return to Earth, then his time will slow down and he will become younger, rather than you will become younger.
If his speed is very fast and is infinitely close to the speed of light, then when he returns to Earth, he will find that your time has passed, decades or even longer, and it is possible that you are no longer there by then!
He will even see the destruction of the earth and solar system, and even the destruction of the universe... It depends on how close he is to the speed of light!
In other words, sub-light speed flight can allow a person to go to the future.
But it seems harder to go back to the past than going to the future. The time expansion principle in special relativity allows us to go to the future, but if we want to go back to the past, we need the help of General Relativity .
General Relativity emphasizes that space-time is not absolute. Matter and energy can affect the surrounding space-time and space, distort, fold space-time, and even create a loop in the structure of space-time.
However, this is only a theoretical analysis, and it is more reflected in mathematical description, but mathematics is not the same as physics and reality. Manipulating the surrounding space and time requires great energy.
Moreover, even if we can go back to the past, it is difficult to avoid the emergence of the "grandfather paradox". Nature does not object to us to go to the future, but always try our best to prevent us from going back to the past!
