[Kim Byung-min's Science Village] People Dreaming of Entropy Inversion Like the Movie 'Tenet'

Byungmin Kim, Science Writer

When an empty magazine is attached and the gun is aimed at the wall, bullets embedded in the wall fly backward and load into the magazine with an explosion. Scattered smoke and ashes gather to create flames, but they absorb heat, causing the area around the fire scene to freeze. It is common knowledge that humans inhale oxygen and exhale carbon dioxide, but in this reversed world, they inhale carbon dioxide and exhale oxygen. Cars also suck in carbon dioxide, separate oxygen, combine the remaining carbon to make fuel, and run backward. This is a world where all physical and chemical movements and phenomena we know are reversed. Is such a world really possible?

This strange world is a scene from the recent film "Tenet" directed by Christopher Nolan, which has become a hot topic not only among the public but also among scientists. Nolan's films are famous for their complex scripts, to the extent that while some people may have never seen one, no one has seen just one.

The underlying theme of "Tenet" is "Entropy inversion." In science, "entropy" is one of the concepts that is difficult for the general public to understand. But since it is "inversion," it means this concept is flipped. Does flipping something difficult make it easier to understand?

The concept of entropy was first introduced by the German physicist Rudolf Clausius (1822?1888). He incorporated it into the laws of thermodynamics in the early 1850s. Later, entropy was defined in statistical physics and quantum mechanics to explain probabilistic distributions. It is even used effectively to explain black holes.

Entropy is addressed in the second law of thermodynamics. If there is a second law, there must be other laws as well. The first law is the "law of conservation of energy," which most people are familiar with.

Most people have a rough idea of what this law means. Within a certain scope, although the forms of energy may differ, the absolute total amount does not change. Sometimes this scope is metaphorically compared to life. The emotions of joy, anger, sorrow, and pleasure that people experience in life are just differences in magnitude, but their total amount does not change.

Everyone has different colors, but the interpretation that the total amount of pain and hope is the same often brings comfort. Of course, such metaphors can provide a broad understanding of thermodynamic laws. However, it is difficult to explain these scientific laws rigorously unless one deals with science or engineering.

Especially in thermodynamics, the concept of a "system" is dealt with under specific conditions. A system refers to the domain or scope that satisfies the conditions defining the thermodynamic laws. The "life" in the total amount of joy, anger, sorrow, and pleasure is a concept similar to a system. Therefore, the first law is strictly defined as "energy is conserved in an isolated system." In an isolated system, the surroundings outside the system are cut off from energy or matter exchange, so it is naturally defined that the energy inside the system is conserved.

What is Entropy Inversion?
Coffee particles spreading in hot water become disordered according to the entropy law, but they cannot return to coffee and sugar particles... Inversion means this concept is flipped.

The current pandemic situation resembling a disaster movie calls for broad interdisciplinary science and epidemiological investigation as the answer.

Now, let's look at entropy, the second law of thermodynamics. The term entropy is used as a physical quantity that indicates the "direction of change" occurring in nature. What does the direction of change mean? For example, instant coffee is dropped into a cup of hot water. Then, coffee and sugar granules spread in the water. Due to collisions with polar water molecules, the particles break apart and scatter evenly throughout the cup. We call this "dissolving." Over time, the hot coffee cools down. Heat exchange occurs with the external environment of the cup system, lowering the temperature.

This change is not strange at all in the world we live in; it is natural. In physics, the degree of this change is expressed as the "degree of disorder," which defines entropy. Disorder means a state where something well-organized or arranged becomes disrupted.

The term entropy is also used outside science and in everyday life. Physicists sometimes use the phrase "entropy increases" metaphorically to describe complex feelings or chaotic surroundings.

What is important in the entropy law is that the entropy of the entire isolated system increases or does not decrease. So, coffee particles spreading in hot water gradually become disordered and cannot return to the original simple coffee and sugar granules.

Theoretically, it is possible to revert to the original granules. To do so, enormous work must be done by inputting energy from outside the system. Water must be evaporated, and the remaining particles separated and recombined. Of course, making them exactly the same as the original granules before dissolving is nearly impossible. If this were possible, entropy within the cup system could be decreased. However, when including the external environment of the system, the larger isolated system, the energy required to do this causes the total entropy to increase even more.

In summary, the entire universe, which is the world we live in, has no change in total internal energy according to the first law of thermodynamics. The second law defines that all phenomena and processes occurring within the isolated system of the universe proceed in the direction of increasing entropy, like coffee particles spreading. Reversible processes, where entropy decreases or goes backward, rarely occur.

But what if we film all our daily lives and play the video backward? Well-mixed coffee separates back into particles, and the temperature of the cooled water rises again. Things that seem impossible could happen.

The only possible answer to making entropy decrease instead of increase might be "time." We just need to reverse the flow of time. Time travel with a time machine would do.

But can time be reversed? The concept of time is even difficult for physicists to explain. What humanity currently knows is that time flows in one direction. Or perhaps the flow of time is defined because our world moves. If everything were stopped, the flow of time would be meaningless.

Time travel is impossible because of the concept of entropy. However, in the movie dealing with entropy inversion, different entropy changes operate simultaneously in past, present, and future worlds like parallel theories. People and cars move backward as if the video is reversed, and birds in the sky fly backward. In the inversion state, the energy absorbed by fire freezes the surroundings, and all physical and chemical changes such as wind resistance and friction operate in reverse. This world, opposite to everything we know, is unfamiliar and strange. Of course, the movie is just a movie, and such phenomena are impossible in reality.

Citizens are waiting for COVID-19 testing at the Gangnam-gu Office screening clinic.
Photo by Yonhap News Agency

The movie screen went up, and stepping back into the world, the pandemic resembling a disaster movie was reality. This thought came to me: "Would someone not dream of entropy inversion?"

Recently, one lie led to a seventh wave of infection, resulting in over 80 confirmed cases. The quarantine authorities were devastated. Moreover, the proportion of so-called "dark patients," whose infection routes are unknown, increased, and quarantine efforts hit a limit. Information about infection routes is the life of people themselves.

The flow of time buries the present in the past. Lives trapped in the past cannot be seen, and information disappears. Furthermore, selfishness ignoring public interest leads to distortion of past information, breaking the chain of connections.

The increasingly uncertain infection routes can be seen as an increase in information entropy. If possible, I would want to reverse the world with entropy inversion like in the movie. Then, infection routes of dark patients could be easily found, and this difficult time could be overcome quickly.

Such time travel is impossible in reality. But there is one hope to reduce entropy within the isolated system of the pandemic. That is the epidemiologist. When we think of epidemiological investigation, we tend to think of tracing the patient's movements. However, it is actually an interdisciplinary science based on a comprehensive understanding of fields ranging from medical science and statistics to social sciences and humanities.

When an infection occurs, the epidemiologist is the first to rush to the scene, risking infection. We will surely get through this difficult time like in a movie. Because there are protagonists who make entropy inversion possible in reality.

Adjunct Professor, Nano Convergence School, Hallym University