Beijing time on October 13 news, time travel has become an important theme of science fiction, is not without reason. Who doesn’t want to go back to the past, explore the mysterious history, or save the world from crisis? Time travel has tested the limit of human scientific theory, and scientists have also conducted in-depth research under the background of theoretical physics. If time travel is possible, it will have an impact on everything from the origin of the universe to the existence of free will. One of the core problems of time travel theory is that it can cause logical paradox, but some researchers believe that they have solved this thorny problem. < / P > < p > before we get to the details of their work, let’s talk about what time travel means in theoretical physics. In general relativity, there is no large cosmic clock representing the time or age of the universe. Time is not absolute, but relative. The time flow rate of one object depends on the way it moves relative to another. Each of us has a unique experience of time. This means that the position of an object is defined by its position in space and time. That’s why time is sometimes called the fourth dimension. Although we can’t understand it literally, four-dimensional space is a good way to imagine time and space. < / P > < p > when you move in space, you move in time. In physics, this means that we can draw a line in space-time to represent your path or motion in space-time. This is the so-called “time like curve”. From birth to death, your whole life can be traced back like a one-way street. It’s simple in special relativity, but it’s complicated in general relativity. In relativity, gravity is caused by the distortion of space-time. As the American theoretical physicist John Wheeler once said: matter tells how to bend space-time, and space-time tells how to move matter. Because of gravity, your time like curve can be bent or twisted. < / P > < p > therefore, if you can bend the time like curve to form a closed loop, it is a closed time like curve (CTV). This will mean that, in any case, your life path and life choices will inevitably bring you back to the place and time of your birth. Congratulations, you’re back in some way. In theoretical physics, if time travel is possible, it is also possible to create a closed time like curve. In fact, general relativity allows the existence of closed time like curves. In 1949, Kurt Godel, a mathematician and logician, designed a rotating universe with a closed time like curve, which was completely allowed by the theory of relativity. Although we don’t live in Godel’s universe, later studies have shown how closed time like curves can be created in our universe. Time travel is impossible in some aspects of particle physics and quantum theory. But as far as general relativity is concerned, time machines are still possible. < / P > < p > so let’s assume that we can build such a time machine. Now there’s a contradiction: Suppose someone goes back to 1955 and prevents his father from inviting his mother to a dance. It means that they will never get married, never have this first kiss. But if he wasn’t born, he couldn’t break up his parents. Doesn’t that mean he was born? Well, the plot in back to the future is just an example of the grandfather paradox. Because time travel allows you to disrupt your travel, there is no consistent timeline you can follow. < / P > < p > one of the ways to solve this contradiction is to assume that a series of events can always prevent the occurrence of paradox. Science fiction writer Larry Niven calls it “the law of conservation of history”; but in physics, it’s called “Novikov self consistency principle”, named after Russian physicist Igor Dmitry Novikov. His principle shows that in any case, you can always define a paradox free solution. One of the strange consequences of this principle is that you can produce a self induced effect in some way. For example, you can go back to the past and introduce your parents to each other; if you don’t go back to the past, you will never be born, but you go back, so now you are. < / P > < p > although the self consistency principle does solve this paradox, many people don’t like it because it means that you can’t change your mind and everything in the past has been determined. You were born, so you have to go back and introduce your parents; you can’t change your mind and decide not to go back. Your future, just like your past, has been determined. That’s all free will is. < / P > < p > but free will is an illusion, right? After all, our brains and bodies, like planets and stars, are made of the same material and follow the same laws of physics. We have no more free will than a thrown ball. The story of our life is not a diary we write every day, but a book that was bound by the physical laws of the universe and printed in advance before you were born. While this may be true, time travel may even prevent the illusion of choice, which seems strange. Given human nature, some time travelers will certainly try to create a paradox to see what happens. < / P > < p > so there’s this new paper. Jayne tolba and Fabio Costa of the University of Queensland in Australia did not focus on the physics of time travel, but explored the mathematical consistency of time travel. They studied whether we could have a closed class time curve and still have complex dynamics. In other words, you can go back to the past, but at least have the illusion of free will. They found that you can do it, but in an amazing way. < / P > < p > they found that the closed time-like curve allows the existence of complex systems, because the constraints of time closed loop cannot uniquely define a single path. On the contrary, there are many ways to form a closed time like curve. Therefore, time travelers always have many choices. If you were born because the “future you” arranged a date for your parents, you will be born; but you are also free to choose other things in your life. Maybe you’re back in your 50s, not your 20s. Maybe you tried to separate your parents, but failed. All kinds of things can happen. The point is that you have to close the curve and be free to make other choices. Even if time travel exists, it will not stifle free will. < / P > < p > of course, this work can not prove that we have the possibility of time travel, but can only prove that time travel is dynamically consistent. If you find yourself back in the past, you can rest assured that your life story is not over. (Ren Tian) < A= target=_ blank>After 12 years, “world class Super project” Shantou Bay Tunnel ushers in a historic breakthrough today