Recently, after the release of the famous director Christopher Nolan's film "Creed", it has triggered a lot of discussion about the nature of time.
Nolan's "time trilogy" is in the same vein, in which the transformation of the time scale in "Inception" is very common in general relativity, and it is estimated that the idea of general relativity is used. Interstellar's treatment of the slowing down of time near black holes is consistent with modern science. (Presumably because of the hiring of scientific advisor Kip Thorne))
In "Creed", Nolan plays with the concept of counter-current time.
In previous science fiction films, the setting of returning to the past through time travel also appeared from time to time, such as "Contact", based on Carl Sagan's novel, which can be said to be the originator of this type of film. In this type of film, the tool against the flow of time is often a wormhole.
In fact, going back in time through wormholes is theoretically possible. According to general relativity, there are some strict solutions to Einstein's field equations, which are called wormholes, and the emergence of wormholes also allows for the existence of closed time-like curves in space-time, which is equivalent to going back to the past. But the conditions under which wormholes appear are harsh and generally require negative energy densities, which can only be obtained through quantum effects and are therefore not readily available in the real world. So, using wormholes to go back in time is also similar to a science fiction concept.
In the latest movie "Creed", Nolan uses another concept - entropy. The film's basic premise is that the direction of increased entropy is the same as the direction of time. From a physical point of view, is it possible to achieve such a time reversal? Now, let's take a closer look at some of the physics behind this.
What is time?
Time is an abstract and mysterious concept. Before entering the 20th century, some physicists believed that time had its way forward, and this direction was consistent with the direction of increased entropy in the local area. This is a trend of thought that began to rise in the scientific community after Clausius and others established the second law of thermodynamics. Of course, this trend of thought itself is flawed, because the direction of time discussed here coincides with the direction of increased entropy, but it does not clearly explain what "time" really is.
At that time, because Einstein's theory of relativity had not yet been born, the time in everyone's mind was similar to "Beijing time", which was a kind of public time. That is to say, it is believed that the time in New York and Paris is the same and can be unified; the speed with which time passes as the people on the train is the same as that of the people on the plane. Moreover, the time direction of different characters in different places above is the same.
With Einstein's theory of relativity, our understanding of time has changed qualitatively: different places have different speeds of time because of the different intensities of gravitational fields; different characters have different speeds and directions of time flow.
So in this sense, before the 20th century, the arrow of time was characterized by the direction of entropy increase, without the correction of relativity, which was a vague concept of time.
At the same time, it must be pointed out that the entropy of not all systems increases in one direction, and there are a large number of self-organizing systems whose entropy is reduced. For example, plants absorb sunlight for photosynthesis, and the entropy of this process does not necessarily increase. In 1944, Schrödinger, one of the founders of quantum mechanics, introduced the special concept of "negative entropy" directly in What Is Life, arguing that there is an increase in entropy in living systems on the one hand, and on the other hand, negative entropy is constantly obtained from the environment - for example, we humans eat vegetables in order to obtain materials that are negative entropy relative to us humans, so as to maintain our own low entropy state - of course, human excretion of feces is high entropy.
Turning back time with entropy reduction?
Entropy is a statistically physical concept whose essence is characterized by the formula s=klnw on Boltzmann's tombstone. where w is the number of microstates corresponding to the system. For example, there are three people standing in a row to take pictures, from the point of view of entropy, there is a "micro state" problem: the person standing in the c position has three possibilities, and the person standing on both sides also has a variety of possibilities, so that there are 6 kinds (that is, the factorial of 3) possibilities, corresponding to the micro state number w, so from here you can calculate the entropy of this photo.
In general, for a closed thermodynamic system, its entropy always tends to increase. Therefore, some people interpret this matter as an arrow of time. However, the direction of the increase in entropy is equivalent to the arrow of time, which is only a mathematical mapping, and whether this mapping is a real physical law or not needs further argument.
So, is it possible to use entropy reduction to achieve time reversal?
The air conditioner in our home is to turn the high entropy state into a low entropy state. (Of course, this process requires energy consumption) So if we think that the corresponding time counterflow from the high entropy state to the low entropy state is the time counterflow, then the air conditioner is also a kind of "time machine" that can reverse the flow of time. But it was clear that the air conditioning could not help us travel across the Qing Dynasty to meet Empress Dowager Cixi. From this, we can conclude that a machine that can produce a low-entropy state locally from a high-entropy state does not necessarily cause time reversal. Traveling through time and space requires a further physical understanding.
The relationship between gravity and entropy
In the movie "Creed", we can see Laura, a female scientist played by Clemans Posey, explaining the working principle of the "reverse bullet" to the hero who did not understand the "time and space reversal" at that time. This reverse bullet can be seen as an object whose time itself is counter-current — note that the time arrows of the bullet's surroundings are all pointing to the future, and only the time arrows of the bullet itself are pointing to the past.
Pointing to the future and pointing to the past is the argument in the theory of relativity, and we need to use a light cone. At each point in flat space-time, there are two light cones above and below: the tangent vector of the world line inside the upper light cone (that is, the time arrow) points to the future, and the light cone below points to the past.
When there is gravitational force, according to Einstein's theory of relativity, space-time is no longer flat, the structure of this time cone will be distorted and deformed, and the difference between the past and the future will become complicated. For example, in Gödel space-time, the light cone is slowly tilted by the rotation of space-time and finally reversed, and the world line can be a closed time-like curve - that is, there are people in Gödel space-time who can go back to the past.
So from a relativistic point of view, if you want to go back in time, it is best to let gravity help, because gravity can distort space-time. Relying entirely on entropy reduction to go back to the past is very unscientific. In Interstellar, Nolan has filmed scenes that rely on gravity to go back to the past, which is scientific.
However, some physicists have proposed new ideas to link entropy to gravity. In 2010, E. Verlinde proposed "entropy forces" to explain the direction of gravity in the direction of increased entropy (gravity has only one direction, that is, attraction, and gravity does not repel each other). Verlinde believes that there is a hypothetical surface between two objects, and when the distance between the two objects changes, the entropy on the surface of the object also changes. And the system always tends to increase, so gravity is reflected. In this theory, of course, gravity is not a fundamental interaction. So, if Nolan wants to make the physical logic of Creed clearer, he should probably hire Verlinde as a scientific advisor.
Counterflow along time equals antimatter?
If we consider quantum mechanics, the man who emerges from the revolving door of Creed moves against time, then this "anti-timer" is essentially made up of antimatter.
The antiparticles that make up the antimatter man (the anti-timer) have not only positrons, but also antiprotons. It is theoretically possible to form these antiparticles into a living organism. But the question is, how does such a contrarian behave in the real world?
The positron was predicted at a very young age by dirac, the master of quantum mechanics, that this particle has a positive energy, but the charge is opposite to that of ordinary electrons. A few years later, another great physicist, Feynman, used the "Feynman diagram" to calculate the interaction of photons with electrons, and he explained the positron as electrons moving against time, an explanation that was also recognized by the scientific community. Thus, in the general sense (space-time is considered flat because special relativity is considered), particles moving against time are antiparticles.
When the positrons in the body of the inverse timer meet the electrons in the real environment, they will be annihilated and become light; and the antiprotons in the body of the inverse timer collide with the positive protons in reality, just like the large hadron collider, producing a variety of particles.
Obviously, for such a contrarian, it is unrealistic to survive in the real world. Whether it is electrons or air in the environment, it will bombard the antiparticles of the human body and emit huge amounts of energy, causing the anti-timer to die in the explosion.
This is clearly considered in the film. People coming out of the revolving door time machine need to wear protective clothing (cannot come into contact with their past selves, because the annihilation of positive and negative matter occurs). Of course, the film makes a certain scientific concession: when the antimatter person comes into contact with other people or objects, he only needs to wear a mask, and there is no need to worry about the annihilation of the positive and negative matter.
In general, since antimatter is already available, there is no need to introduce the concept of entropy reduction, which would cause logical confusion.
Therefore, from the current physical theory, it is entirely possible to move against time, but the abuse of the concept of entropy requires vigilance. The relationship between entropy increase and time is already a relatively old physics, and perhaps the Theory of Relativity and Quantum Force Society gives us more enlightenment.
Author: Zhao Keyun
Editor: Wu Fei