What are theoretical physics, experimental physics and computational physics?

Physics, like all natural sciences, is in constant evolution. The current classification of the sciences obeys to a historical moment where it was possible to distinguish the boundaries between one branch and another. For example, biology was related to the study of those aspects of nature related to life. Very early on, there was a more or less clear definition of what life was. When it began to be understood that the processes related to life are essentially chemical, the problem of defining between organic and inorganic chemistry arose. This raised the question of what was the threshold between organic and inorganic.

When talking, for example, about chemistry, at one time there was a consensus among the scientific community about what kind of phenomena are studied in this branch of science. With the passage of time, the problems that could be studied through new methods and technological advances arose systems where the boundary between Chemistry and Biology are indistinguishable. Thus, new problems to be solved and investigated were born, which are now grouped in what is called Biochemistry.

In the particular case of Physics, understood as the branch of science related to natural phenomena, we can see that it has a very close relationship with other sciences. The phenomena underlying chemical processes are essentially described by a sub-branch of Physics called quantum mechanics, applicable in natural systems of the size of an atom or smaller. For this reason, many people claim that Physics is one of the most fundamental sciences. However, although quantum mechanics is a fundamental description of nature, it is not practical for studying phenomena whose elements are larger.

More than 100 years ago, physics was understood as the study of the phenomena of nature, in particular mass, energy and their interactions with each other. To understand in more detail, incipiently some branches of this branch of science were being timidly developed, which over the decades have become the backbone of modern physics, such as quantum mechanics, statistical mechanics and relativity. In turn, these fields form the basis of other more sophisticated disciplines of science, such as quantum field theory, condensed matter physics or theories of gravitation. This is without neglecting the fields of classical physics, such as optics, thermodynamics, Newtonian mechanics or electromagnetic theory, among others.

As we were saying, if more than 100 years ago a physicist had been asked what field he or she was involved in, he or she might have said something like: "I study the electrical and magnetic phenomena that occur when rubbing a piece of cloth against a piece of quartz". At that point we would have assumed that he was doing his experiments in some laboratory of an educational institution or perhaps in a workshop or even at home. Furthermore, it is very likely that he also, if he had knowledge of the newly developed electromagnetic theory of the English scientist James Clerk Maxwell, tried to model the phenomenon by making use of mathematical equations (today known as Maxwell's equations).

In this context, the physicists of that time were either theoretical or experimental, and most probably both. It was years later that the use of computers opened the way to what we know today as computational physics; an approach that became relevant and that continues to be successfully applied and extended in each of them.

As can be seen, the process of studying the phenomena of nature by Physics requires people to specialize, not only by branch but also by the emphasis placed on certain skills, interests and motivations. Let us make a brief description (as we understand it) of what physicists are.

What about physicists?

Experimental physicists are those who study nature using direct or indirect observation. When they "see" nature, we mean that an experimental physicist relies on laboratory instruments, from a simple magnifying glass to very sophisticated equipment such as particle accelerators or electron microscopes to obtain data from which he/she makes graphs and statistical analyses in order to draw conclusions from the laboratory experiments. Sometimes he obtains qualitative information (based on experimental observations) and sometimes quantitative (using some mathematical model that gives him a specific number of a measured physical property).

Figure 1. An external view of laboratory experiments. Source: https://dream.ai/

The ultimate goal of a physicist or experimental physicist is reproducibility, i.e., that from the information obtained in the laboratory he can provide a kind of "recipe" to repeat the phenomenon under study. The functioning of electronic devices, the complex mechanics of an automobile and even the functioning of the screen from which you are reading this prose are examples of the benefits of reproducibility whose recipe was developed in a laboratory.

However, it is not all plain sailing, since the laboratory presents several obstacles that often limit the study of a vast group of phenomena. For example, the concept of "emptiness" as defined by set theory is a source of controversy when one tries to interpret it from observations and experimental models. Sometimes the number of experimental variables is so overwhelming that it ends up limiting reproducibility, as is often the case in the study of dissipative phenomena, which are those involving friction between surfaces.

Likewise, distance and time are usually extremely large or small that prevent a complete experimental analysis, as is often the case in various astronomical or nanometer-scale phenomena. This is why, over time, two branches of physics have emerged to complement the experimental part and often serve as extrapolation agents of the results derived from it: theoretical physics and computational physics.

Figure 2. A researcher conducting an experiment. Source: https://dream.ai/

Using one branch of science or another is not a matter of size since, as we know, classical physics describes very efficiently the motion and the forces that obey the bodies at distances of a few microns up to the motion of the stars. The use of one science or another is determined by the characteristics of the phenomena we want to study. Today we speak of transdisciplinary research, i.e. projects that now require different perspectives and methods developed in different branches of science.

In Physics, natural phenomena are described through the presence of forces acting between objects. This description is equivalent to understanding how objects move and why they move. In more technical words, we speak of the kinematics and dynamics of natural phenomena. A mechanical system is defined when we can establish these two aspects to describe a natural phenomenon.

That is why this branch of science can study a great variety of phenomena at many scales of distance; that is to say, study objects of many sizes. This is the reason why, in order to understand any area of the natural sciences, it is necessary to know something about physics. The depth of its concepts depends on the nature of what we are studying.

In physics, as in all natural sciences, the scientific method is used. This means that the task of understanding nature is divided into multiple different activities. The scientific method should not be thought of as an "algorithm" to be followed, it is not a recipe; rather, it is a philosophy of how we should approach the partial and certain truths that science can offer. The scientific method groups the tasks of research into general and sometimes complicated-to-define actions such as observation, hypothesis generation, and testing. The concrete form that these general activities take are diverse and each natural science has its particularities.

Figure 3. A theoretical physicist developing his equations and models of nature. Source: https://dream.ai/

There was a time when scientists could perform all the tasks involved in the study of a natural system from its observation, the generation of a hypothesis that explained the phenomenon to form a theory and finally test, through experiments or new applications, the predictions that the theories yielded.

As science progressed, and especially as the phenomena observed became more complex, the scientific community began to divide up the tasks that were necessary to understand nature. This meant that the construction of science became a social task.

The activities to carry out the scientific method can be grouped in many ways and, as with the classification of the natural sciences, it is difficult to say when one begins and when another begins. There is a consensus among the community of scientists, and particularly within physics, that the branches that group the tasks of research in this area are two: theory and experiment.

That is why there are theoretical physicists and experimental physicists whose only difference lies in the activities they carry out around the study of a phenomenon, but which together form a whole. There can be no theory without experiment and vice versa, everything is fundamental to understand nature.

There is another additional branch of Physics that was developed around the 50's of the last century. It is called computational physics and its arrival was due to the development of machines capable of doing more calculations than humans can do with pencil and paper. Computational physics began as a tool to employ the favorable computational advantages of electronic computing machines during World War II. These machines were successfully employed to decrypt encrypted messages and for the development of nuclear weapons. With the availability of computers for non-military uses, electronic computers allowed computational physics to appear as a tool in the different branches of this science.

Nowadays, the term computational physics is coined to a modality of physics that uses personal computers to perform mathematical calculations efficiently, allowing to simulate and describe the behavior of simple and complex physical systems that can be successfully contrasted with experiments that in many cases can be very expensive or impossible to perform in practice.

Among the many applications of computational physics are: simulation of molecular systems, drug design, numerical simulations of phenomena in the universe, machine learning, data science, simulations of living systems, meteorology and astrophysics.

Figure 4. A computational physicist developing programming codes. Source: https://dream.ai/

One of the most recent applications of computational physics was the creation of a high-resolution image of the vortex of light waves generated by the magnetic field surrounding a black hole. To achieve this image, a large amount of data had to be analyzed and perfectly organized. This organization could only be done by means of computational physics allowing to reveal how a significant portion of the light around a black hole is polarized due to the attraction of the magnetic field (See Fig. 5). As mentioned by researchers who participated in the project,"revealing this image in polarized light has required the use of computers and complex techniques for data acquisition and analysis". An unprecedented achievement for our current era.

Figure 5. Vortex of light waves escaping from the black hole at the center of the galaxy M87. Source: https://eventhorizontelescope.org

Today, computational physics continues to make successful inroads into countless sectors of science. For example, in medicine, the use of simulators that allow virtual operations has grown exponentially, and medical physics has allowed the design and simulation of mechanical parts that adhere to the organism. Also in the film industry, it has made it possible to generate and recreate increasingly realistic images of 3D animations and productions; for example, the film Pinocchio by Guillermo del Toro (see Fig. 6).

On the other hand, the development of liquid crystals from the computational point of view has allowed a breakthrough in the development of visual tools that respond to conformational changes of molecules and are used in LCD ( Liquid Crystal Diode) or OLED ( Organic Light-Emitting Diode) television screen technology.

Finally, the most recent application of computational physics was in the fight against the SARS-CoV-2 virus. In this case, the use of computational tools made it possible to analyze statistical and mathematical models to anticipate the future and support decision-making. In addition, molecular simulation made it possible to establish appropriate proposals for drugs to combat and treat the virus.

Figure 6. Image from the animated film Pinocchio by Guillermo del Toro. Source: https://www.vogue.mx/estilo-de-vida/articulo/diferencias-entre-pinocho-de-guillermo-del-toro-netflix-y-pinocho-de-disney

It is evident that each branch of Physics can be approached from three perspectives, all of which are complementary and allow us to learn more about the objects that surround us, from very small to very large scales, and which function as bridges that allow us to broaden our knowledge of nature.

By the way, several of the images that illustrate this article were made using an Artificial Intelligence called Dream by Wombo, another of the branches of science where Physics has impacted for the benefit of humanity.