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This article comes from the official account of Wechat: SF Chinese (ID:kexuejiaodian), author: SF

Special relativity holds that the speed at which a person's time passes depends on the speed at which that person moves. Using this law, it is possible for us to design a time machine and go back to the past. One possible solution is to convert yourself into tachyons.

(Marcus Jon / tr. by Phil Newell)

Translation | Li Shijie

Editor | Chen Qiang

Time travel has always been a common plot in science fiction, but it is not just a fictional imagination. From a scientific point of view, time travel is also possible. Its theoretical basis is Einstein's theory of relativity.

The special theory of relativity published by Einstein in 1905 changed people's perception of time and space, all because of the unique nature of the speed of light. Einstein realized that nothing moves faster than light, and that the speed of light is the limit speed in our universe. He also proposed the principle of constant speed of light: no matter how fast you are and in which direction you move, the measured speed of light is always a fixed value.

On this basis, Einstein found that time is not absolute, but relative, and there is no universal clock that everyone agrees with. In other words, a moving clock will go more slowly than a stationary clock. This is the clock slow effect, and the closer the speed of motion to the speed of light, the more obvious the clock slow effect. If a person can move at the speed of light, time stops completely, which is impossible for massed objects, but possible for massless entities such as photons (the particles that make up light).

What happens if you exceed the speed of light? What happens if you move faster than the speed of light? According to special relativity, if you can do it, you will arrive at your destination before you leave. At this point, there is a headache for scientists-contrary to the theory of causality, because the result of the event precedes the cause.

For example, if you travel faster than light to the nearest star system, South Gate II (Alpha Centauri), a beam of light that leaves at the same time as you are still on the way when you get there. At this time, you can even look back at the earth and witness the launch of your spaceship.

Need to turn yourself into a tachyon? Special relativity holds that no mass object can move at the speed of light, so how can you exceed the speed of light?

Physicists have come up with a hypothesis that there is a hypothetical particle called tachyon. Tachyons are born to move faster than light, just as photons are born to move at the speed of light. If we can convert the atoms that make up our bodies into tachyons, launch them into space, and then change tachyons back into atoms, it is possible to travel faster than light. In this way, some observers will find that we are going back to the past from the present.

But there's a problem. In 1974, physicist Richard Goth of Princeton University discovered that tachyons radiate conical gravitational waves to the rear, just like sonic booms produced by airplanes flying at supersonic speeds. Because of the unusual characteristics of tachyons, something exactly happens to you: gravitational waves cause tachyons to lose energy and accelerate. Finally, it will collide with antiparticles of tachyons (antiparticles of tachyons) moving at the same high speed and annihilate. Although the speed of the collision tends to infinity, Gott believes that tachyons travel only slightly faster than the speed of light for most of their life. However, this will greatly reduce the efficiency of time travel.

This article was originally published in Science focus, No. 8, 2023, four possibilities of time Travel.

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