Time Travel

The Physics of Time Travel: A Scientific Exploration Beyond Popular Belief

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The definition of physics on time travel is different from the definition of ordinary people. It is said that the definition of ordinary people is time travel, reaching the past and the future. I need to explain the definition of ordinary people in detail. In this article, I will explain only from a purely physical point of view, without any imaginary events.

The future

Time travel to the future is theoretically possible and has been confirmed by numerous experiments. Theoretically, when an object travels at nearly the speed of light, its time flow will be slower than that of other objects. If you compare it to another object standing at that time, you can clearly see the time difference.

In theory, a 25-year-old man would ride aboard a rocket traveling at nearly the speed of light. Suppose there was a person of his age left in the world. In another 5 years, the man on the rocket will return to Earth. If you calculate, the person in the world will be 30 years old. But the man on the rocket is not yet 30 years old. This is the time digression of the theory of relativity. For the man on the rocket, 5 years of time passes slowly, so 5 years don’t take long for him. (2 years, 3 years may be wrong, but I won’t tell you anymore.) People on earth are passing through 5 years normally for 5 years because the flow of time is normal. If you think about it, if the man on the rocket traveled at almost the speed of light, he would reach the age of 30 much younger than that. It’s a kind of time travel to the future, isn’t it? This phenomenon is seen in muon particles in cosmic rays and has been proven many times in experiments in particle accelerators. (I will write about the muon problem later.)

Next is the effect of the gravitational field. A strong gravitational field slows down the flow of time significantly.

Regarding that, atomic clocks have been tested on airplanes. According to the experiment, an atomic clock will be placed on the ground. When we flew the plane and measured the time of the atomic clock, we found that the ground was faster than the atomic clock. Theoretically, the gravitational field at altitude is weaker than the gravitational field on the ground. (The higher you go, the weaker it is.) According to the relativity effect, the clock on the ground slows down more than the clock on the plane. Therefore, the clock on the ground can reach the future earlier than the clock on the plane and time travel can be done.

A similar event can be seen in the movie Interstellar. When the character lands on a planet near a black hole millions of times bigger than the sun, his time flow slows down. Because of this, even though he only spent a few hours on the planet, when he returned to the spaceship, years had passed. So if you want to time travel to the future, you need to spend time near a strong gravitational field like a black hole.

The past

Traveling back in time has many paradoxes and is not allowed by current physics at all. One of the most famous paradoxes is the Grandfather Paradox, which deals with the disruption of the time stream. Suppose a man could travel back in time and prevent his grandfather from being born. Because his grandfather was not born, his father could not be born. Because his father was not born, he could not be born either. Because he wasn’t born, he couldn’t invent a time travel machine. Since he couldn’t invent a time travel machine, he couldn’t go back to the past and bring his grandfather back. Then his grandfather was born, Because his grandfather was born…etc. This is the famous paradox of time-stream disruption.

It is necessary to solve this paradox because when you can time travel to the past, you will inevitably encounter such events. (Even small actions have a large effect on the flow of time.) Current physics cannot resolve this paradox. About that, I have seen a paper that says that the time travel cannot be changed by the actions of the time traveler. Even if we could go back in time and kill Hitler, we wouldn’t be able to put out the fires of World War II that Hitler had ignited. But this is still in the rough stage and has not yet been scientifically proven.

According to the second law of thermodynamics, everything in the world is always moving from disorder to disorder. You can never go from unplanned to unplanned. The egg will break after some time. The nails will decay over time. However, after a while, the broken egg cannot be returned to its original state, rusting, Decomposed nails cannot return to their original form even after a period of time. From this point of view, time flows only in one direction, the future, and there is absolutely no going back to the past.

So, the question arises whether it is possible to travel back in time according to the theory of relativity. According to the equations of relativity, when traveling faster than the speed of light, time becomes negative. It represents the past. So, if you can exceed the speed of light, you can say that you can travel back in time, but note that the theory of relativity itself is strictly limited by the fact that nothing can exceed the speed of light. Because this paradox can be caused, time travel is also impossible according to the theory of relativity. (If new physics doesn’t appear, I can’t tell.)

By the way, Hawking did an interesting experiment about time travel to the past. He held a party in the back garden with a few people. A few days after the event, I sent out invitations to people to attend the party. According to him, if there was a person who could time travel to the past, a person from the future who was invited would be able to attend his party in the present. (A few days later means the future. The party is held in the present time. If you get an invitation and attend the party, that person from the future can time travel to the past.)

But unfortunately, no one attended his party, so we can only conclude that no one can travel back in time.

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