Why can’t time turn back?

　　Human beings live in four-dimensional space-time. We can also say that all life and all matter we observe exist in the four-dimensional space-time. What we call four-dimensional space-time is the sum of time and space, including three-dimensional space and one-dimensional time.
　　The difference between time and space has long been discovered. Space has three dimensions: up and down, front and back, and left and right. People can walk freely in each dimension of space. Time is different. Time has only one dimension, and time has a direction. From the experience of daily life, we can know that we can only go from the past to the future, but not from the future to the past.
　　Some people have always imagined that in time, it would be great if we could travel at will as if we were in space. It can go from the past to the future, or from the past to the past. There is a very popular sci-fi movie called “Creed” in 2020, and the theme is time travel. There is also a plot in the movie where a person goes back to the past, meets his past self, and ends up fighting with himself.
　　From a scientific point of view, can people really time travel?
　　According to the scientific theories we have now, time travel in the direction of “future” is possible, but time travel back to the “past” is not possible. Before the 20th century, people’s understanding of time and space was relatively simple, thinking that the so-called space was like a big empty box, and time was like a clock hanging in every position of space, no matter where you were State, where in the space, the time indicated by the clock is consistent. This kind of space-time view is the so-called “classical mechanics” space-time view.
　　But in the early 20th century, Einstein discovered special relativity. This theory combines our understanding of time and space, that is to say, the time we perceive and read is different in different states of motion. This is the so-called “clock slow scale” effect – for example, if a person leaves the earth for a few years at a speed close to the speed of light, and then returns to the earth at a speed close to the speed of light, then for this person, time is only Years have passed, and for Earth it may have been decades.
　　One can, so to speak, travel “faster” to the future than others through relatively rapid movement. Although we have no way to move at a speed close to the speed of light, every time we move quickly, such as flying, it is equivalent to arriving in the future “faster” than others, but the effect of this time travel is too weak, ordinary People can’t even notice it.
　　So why can’t time travel back in time? The reason is that scientists have discovered that time in our space-time has a direction, from the past to the future—this is commonly known as the “arrow of time”. Judging from the scientific theories we currently have, people have no way to reverse the direction of the arrow of time, they can only go from the past to the future. Why on earth does time have a direction? Scientists don’t know much about this issue, but so far, it seems that there are some physical factors that cause time to have a single direction. For example, physicist Stephen Hawking mentioned three possible reasons for the existence of the arrow of time in his famous book “A Brief History of Time”: First, the development of the entire universe has a direction. Our universe had a beginning in what is known as the “Big Bang” about 14 billion years ago. The entire universe expanded continuously from an extremely tiny scale, and developed into the grand universe we can observe now 14 billion years after the big bang. This process may have caused time to have a single directionality.
　　The second reason comes from thermodynamics. In the study of thermodynamics, a law has been discovered, that is, the degree of disorder of an independent system is always increasing. Described in terms of physics, the “entropy value” is getting bigger and bigger. Simply put, for example, if there is a room that no one cleans, and it gradually becomes more and more messy, we can say that its “entropy value” is getting bigger and bigger. This is also directional. There is also a third reason, which is human psychological factors. People’s feelings are also from the past to the future. Of course, this feeling may be determined by the first two factors.
　　It is precisely because we have discovered the existence of the arrow of time that we currently believe that time travel back in time is impossible. Time cannot be turned back. But what exactly is time? Our research on the nature of time can be said to have just begun. What is the real relationship between time and space? Why can we exist? More in-depth research is needed on these issues.