Time travel in the physical world

one day in 1945, a mysterious baby girl appeared in an orphanage in Cleveland. No one knew who her parents were. She grew up alone and had nothing to do with her.

things didn't get better until one day in 1963, when she inexplicably fell in love with a homeless man. But the good times didn't last long, and unfortunate events happened one after another. First of all, when she found out that she was pregnant with a homeless child, the homeless man suddenly disappeared. Second, when she was giving birth in the hospital, the doctor found that she was bisexual. In order to save her life, the hospital gave her a sex change operation, and she became him. Most unfortunately, the little girl she just gave birth to was kidnapped by a mysterious man. After a series of blows, he never recovered, and finally took to the streets and became a homeless man.

until one day in 1970, he walked drunkenly into a small bar and told an older bar buddy about his misfortune. The barman sympathized with him and offered to help him find the tramp who made him pregnant and missing. The only condition is that he must join their time travel special forces. They got into the "time Flying car" together. When the car went back to 1963, the man let the homeless man out. The tramp inexplicably fell in love with a girl who grew up in an orphanage and got her pregnant. The man took the time Flying car for more than nine months, snatched the newborn baby girl from the hospital, took the baby girl back to 1945 and quietly put her in an orphanage in Cleveland. Then he brought the bewildered tramp forward to 1985 and asked him to join their time Travel Special Forces.

after the tramp has a formal job, his life is on the right track. And gradually gained a pretty good position in the special forces. Once, in order to accomplish a special task, his superiors sent him back to 1970, disguised as a bar boy, to pull a homeless man to join their special forces.

the introduction is the plot of the novelist Robert Heinlein's "you still dead". Rather confusing, in short, it is through the time machine that you get pregnant and give birth to yourself.

since Einstein put forward the theory of relativity in the early 20th century, time travel has gradually become a hot topic in science fiction and philosophy. No time travel can avoid a paradox that can be summed up as a "grandmother paradox" (for more information, enter grandmother paradox ): a person goes back to the past and kills his grandmother, so that there will be no future you without the grandmother, and it is impossible for you to go back and kill the grandmother without the future.

all science fiction and movies must use their brains to explain this paradox, such as the popular "Horror Cruises" and "Ring messengers" a few days ago. In our lives, everything must have an independent cause (things don't happen for no reason), like a chain, one link after another, until the first link (Aristotle called the first driving force of the universe, that is, God). The law of causality ensures that the chain can only be open and uncrossed, cannot be closed and cannot be retrogressed, which is in line with our logic (also known as thermodynamic arrowheads).

time travel forms a time closed loop, creating some incredible causal cycles that subvert our logic, such as the plot of the novel in the introduction. Such a closed loop has an independent cause at each point in time, but from the perspective of the whole cycle, there is no independent cause.

therefore, based on this paradox, some people say that time travel is impossible, while others try desperately to explain the paradox of the causal cycle in order to maintain the possibility of time travel. However, there are two questions that need to be clarified: first, is time travel a sufficient condition for the cycle of cause and effect? Second, if known knowledge cannot explain the causal cycle, does it mean that the causal cycle is impossible? For example, before we accepted the theory of relativity, the twin paradox was an obvious empirical paradox.

like the discussion of twin paradoxes, most people's discussion of time travel is completely divorced from science and becomes unfounded fantasy. Today's topic has nothing to do with the above. I just want you to understand how the concept of time travel has evolved in the scientific world.

Newtonian cosmology : physicists represented by Newton believe that space, time and motion are absolute, and the whole universe is a single, uniform inertial system. Time flows at a uniform speed and is an absolute existence independent of anything. But some scientists such as Leibniz, Berkeley and Mach oppose the concept of absolute time. Mach believes that change is more fundamental than time, and what we call the "passage of time" is actually the result of our attention to changes in the things around us. We invented time to quantify this change. This theory deeply influenced Einstein. In 1905, Einstein published a paper on special relativity. In absolute time and space, time travel is completely impossible.

Newton's law of universal gravitation can perfectly explain the trajectory of macroscopic celestial bodies

Special relativity : special relativity is based on two basic principles, the principle of relativity and the invariance of the speed of light. In short, the laws of physics are the same in any inertial system. The constant speed of light causes both absolute space and absolute time to become relative quantities in Newtonian mechanics. From then on, when we discuss time and space points, we have to declare what kind of inertial frame of reference, because the measurements of time and space are different under different frames of reference. In extreme cases, time is relative to anyone.Of.

in Minkowski's four-dimensional space-time, the great difference in relative time between different events

Special Relativity distinguishes the original time from the coordinate time. The original time refers to the measured values of the event, the observer and the clock in the same coordinate system, which is similar to the absolute time of the traditional Newtonian mechanics, while the coordinate time is that the event and the observer clock are in different coordinate systems. special relativity is used. In most cases, most people are in or approximately in the original time system, and our time can be regarded as an absolute quantity, so we can use the fixed frequency of the things around us to describe the motion quantitatively. The mechanical watch is made according to the natural frequency of quartz. We all live under the original norms, and when I ask you what time it is, I believe that the time you follow and the time I follow are in the same system. That's why we don't miss flights, appointments and so on.

Special relativity subverts our common sense of simultaneity. The observation of two simultaneous events in one frame of reference occurs successively in the other coordinate system. The direct consequence of this phenomenon is the empirical delay effect. If the relative velocities of the two reference frames are very different, there will be an obvious time difference. When you come back from a trip in a spaceship close to the speed of light, you find that everyone you know is older than you. This is time travel under special relativity. But this is not strictly time travel, because you are not going back to the past, but time is relatively slow.

General relativity : general relativity is based on the principle of equivalence, that is, gravity and acceleration are just different names describing the same phenomenon. An interesting phenomenon in general relativity is that light bends as it approaches an object. This is due to the distorted space-time structure caused by the gravitational field generated by mass, and the time passage of clocks near large masses (for example, black holes) is slower than that of clocks far away from large masses. In a very strong gravitational field, the bending of space-time will produce a closed time-like curve (CTC),. This kind of surface space not only means returning to the origin in space, but also to the origin in time. In other words, time has really lost its one-way in the practical sense here, and time travel has really become possible in theory.

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Classical closing time curve diagram

to put it simply, each event on the closing time curve is both the past and the future of another event. Tomorrow means the end of today.

Quantum Mechanics : quantum mechanics is a physical theory describing microscopic matter, which, together with the theory of relativity, is regarded as the two basic pillars of modern physics. In the micro world, many experimental phenomena can not be explained by common sense, but can only be explained by quantum theory. For example, in a cat that is both dead and alive, matter has a smallest quantized Planck constant.

in quantum mechanics, there is a quantum correlation phenomenon. After two particles are coupled to each other for a short time, disturbing either particle alone will inevitably affect the properties of the other, although the two particles may be a long distance apart. This effect is even 10000 times faster than the speed of light, which can be said to be instantaneous. Therefore, it can be said that particles have several possible states at the same time, like a "dead and alive cat".

two particle picture sources of quantum entanglement: Baidu Encyclopedia

many scientists use this feature in time travel. It is believed that when travelers go back to the past, any behavior that affects history will lead to the creation of a new parallel universe, which are completely uncrossed and unrelated.