Physical Theories: An Overview

Now that I have described how modern physics was founded by Galileo Galilei as a new science, I will follow the path of physics in the following and show how the legacy of the pre-Socratics and ancient mathematicians has borne fruit. After a decline of the ancient culture one could, after about 2000 years, revive it and reach new heights of knowledge. The fact that it took so long may has many reasons – if one can speak of reasons at all in history. However, I do not wish to take part in such considerations.

First, I want to give an overview of the theories that have been developed and established in physics over the course of time. The fact that they always had to prove themselves in competition with other theories than the “better ones” in each case was explained, for example, in (Honerkamp, 2017, p. 111ff). In the next chapters I will show what role the questions raised by the pre-Socratics have played in the development of these theories. In particular, I will discuss the development of the theories and their relationships to each other. Especially I will pursue the path along which there have always been mergers or unifications of theories. Thus, today we are only talking about two great theories and are out to recognize these two as parts of a single “theory for everything”. This would answer the most noble question of the pre-Socratics, the question of a “One”, albeit in a completely different way than one could imagine at that time.

The development and establishment of a physical theory always involved the explanation of phenomena of a certain type. We have already seen that the phenomenon of “motion” was the first theme that interested people in antiquity and again in the Renaissance. It is the most original and probably also the most general phenomenon that we know. 

We encounter the phenomenon of “light” in a similarly direct way.  So, it is no wonder that at the beginning of modern physics not only the phenomenon of motion was dealt with, but also experiments with light were carried out, as we know them from Isaac Newton, for example. A century later, people began to study other, seemingly quite different phenomena, electrical or magnetic. Finally, at the turn of the 20th century, they discovered sorts of radiation which obviously differed greatly from light rays, and among these new rays there were also different types.

The space of phenomena

In short, the history of physical theories is a history of discoveries in a “space of phenomena” where there have always been certain objects in play. In this space, one can identify large areas in which there are phenomena that seem to be so similar to each other that one might be tempted to invoke the same reason for their explanation. Thus, over time, the idea of fundamental forces acting between planets and the sun, electrons and other found or discovered objects emerged. Over time, four such interactions were distinguished: Gravitational, electromagnetic, strong and weak interaction. This distinction is still very helpful for an overview of the set of the theories developed in the 400 years since Galileo.

There are, however, two other categories in respect of which one should distinguish the theories. It is not only the interaction or the force that can be decisive for a phenomenon. This can also be the range on a length scale. Thus, phenomena can be distinguished according to the scale of length on which the phenomenon appears, whether in the world of the smallest dimensions, the largest dimensions or the middle dimensions.

Finally, an aspect will become important that has to do with our cognitive abilities, namely the question of whether we must describe the phenomenon as a complex one, namely as one in which it is not sufficient for an understanding to consider only a few objects with few important properties. Many or very many objects can represent a system that has new properties that are not inherent to the individual objects themselves, but only “emerge” through the interaction of the objects. Water, for example, has the property of being liquid. However, this does not apply to its components, the H2O molecules.

With these two characteristics, spatial size and complexity, we can already consider aspects that allow us to have an overview in the form of a landscape of phenomena. If one enters the characteristic length of some objects, which play a role in physical theories, into a coordinate system, in which this size is plotted against the complexity of the objects, then one obtains e.g. Fig.4.2.

Fig.4.2: Rough classification of certain objects according to size and complexity. Objects like planets can appear at different places, depending on how much of their properties you want to consider (R stands for order of magnitude, N for number of degrees of freedom or complexity resp.).

On this figure we can show how far we have explored the space of natural phenomena today with our physical theories. It also makes it clear that in addition to the physics of fundamental interactions, there is also a very large area of complex systems for which areas of physics such as thermodynamics or statistical mechanics, solid state physics, etc. are responsible. This should be kept in mind, even if we do not deal with it here and we mainly consider the wide range of spatial scales in the area of “simple” systems – from 10-15 to 1020 m.

The world of the middle dimensions in the range of about 10-4 to 1010 m is most accessible to us intellectually, because we ourselves, as participants of this world, can have direct experiences with it. Thus, the phenomena of this world are also the subject of the earliest physical theories; they are also called classical theories. The exploration of the space of phenomena thus started in the world of the middle dimension.

At the beginning of the 20th century, phenomena of the world’s smallest dimensions were discovered. One had to state that the concepts of the world of the middle dimensions are no longer suitable here. A completely different concept, a “quantum”, replaced the concept of a material object and gave physics on this scale the name “quantum physics”. At the same time a modern cosmology and astrophysics began to emerge. Today we hear of particularly spectacular discoveries in this field of the largest dimensions.

Classical physics, quantum physics and cosmology: this is a classification that can also be described as physics of medium, smallest and largest dimensions. Cosmology today has not yet required its own conceptual apparatus, as quantum physics does, which is why it is also added to classical physics, if one wants to emphasize the methodological aspect.

With a distinction regarding the spatial size alone one has of course not yet exhausted the space of phenomena. There are other quantities which, measured by the conditions of our world of daily experiences, can be small or large. Particularly prominent in this context is speed; but the strength of fundamental forces, in particular of gravity, will also be significant for the nature of physical theories. The landscape sketched in Fig.4.2 must therefore only be imagined as a slice from the whole space of phenomena.

The exploration of this space of phenomena resembles the exploration of our earth in the time of the great discoveries in the 16th century. One spoke thereby of the discovery of the “world”, although it were only new ranges of the planet earth, which one discovered at that time gradually. Today one knows almost every corner of the earth and “reaches for the stars”. 

Thus, one also knows all laws of nature in the world of the middle dimensions, but only on the fundamental level. The more complex the systems are in these dimensions, the less familiar they are to us today. But the more we limit ourselves to the fundamental side, the further we have advanced into the world of the smallest and also the largest dimensions.

Even if it were possible in several years or decades to establish a theory for all fundamental interactions, physics would not be at its end. In the direction of complex systems there are still many questions waiting for an answer. The transition to chemistry, biology and cognitive science will be fluent. Also, in the exploration of life and consciousness one will not be able to ignore physical conditions.

The theories of classical physics

Classical physics is dominated by three large phenomenon areas: the phenomenon of motion and the two areas in which we encounter the fundamental forces of gravity and electromagnetism, respectively.

Phenomena from these areas have been known since ancient times. “Nothing is older than motion,” we may quote Galileo once again. For the pre-Socratics, motion or non-motion always played a role, Aristotle distinguished different types of motion and formulated a first kind of theory of motion. Even in the Middle Ages there were always natural philosophers who wanted to trace the nature of motion.

Gravity was also an everyday phenomenon. With Aristotle it was a quality that made all bodies of the sublunar world strive for the centre of the world. Also, the sphericity of the earth was later explained by such a “natural striving”.

We read less about magnetic and electrical forces in early sources, but magnetic and electrical phenomena were already known in ancient times. If you rubbed an amber, it would attract dust or shreds of wool. Iron was attracted by a magnetis stone and it was discovered that splinters of such stones always rotate in a north-south direction.

The physical theories, which today explain all basic phenomena from these three phenomenon areas, are

– for motion: Newton’s and Einstein’s theory of motion,

– for gravitation: Newton’s and Einstein’s theory of gravitation,

– for electrical and magnetic phenomena: Maxwell’s theory of electromagnetism.

Normally, Newton’s theory of motion and gravitation is subsumed under the name classical mechanics. Newton’s theory of gravity essentially consists of a law for the forces between two material bodies. Newton was able to use this law to explain the motion of the planets within the framework of his theory of gravity.

Einstein’s theory of motion is the special theory of relativity, Einstein’s theory of gravity is the general theory of relativity. Both represent extensions of the corresponding Newtonian theories to a larger range of phenomena: for motions to “higher” velocities, for gravitation to “higher” velocities and to “stronger” gravitational forces. It will still be necessary to make precise what the terms “higher” or “larger” mean in each case.

Maxwell’s theory of electromagnetism serves to explain all electrical and all magnetic phenomena as well as those phenomena in which electrical and magnetic effects are mutually dependent. It is the result of a unification of two earlier theories, one for electricity and one for magnetism.

Gravity and electromagnetic forces act over long distances. Otherwise, we wouldn’t have felt it all the time. They are therefore called long-range, in contrast to the short-range forces that were only discovered in the world of the smallest dimensions. They hold the world “together at its innermost” and their reach does not go beyond that. Of course, the long-range forces can affect even at short distances, electromagnetic forces are even quite important for understanding structure of atoms. However, gravitational forces at the level of atoms have not yet been registered. The masses of the building blocks of the atoms are obviously much too small.

The theories of quantum physics

The establishment of Maxwell’s theory, in particular by the discovery of electromagnetic waves in 1886, increasingly drew physicists to the question of how an electric current and how electromagnetic radiation can be generated in matter. In the end, the question of the structure of matter stood in the center of attention.

From a pre-Socratic Leukipp and his follower Democritus one knew the concept of an atom, a smallest indivisible particle (άτομος gr. indivisible). The chemists used this idea in the 19th century with great profit for the explanation of the laws in the reactions of different chemical elements. But there were also vehement opponents, because one had not yet “really seen” an atom and the idea of indivisibility only raised new questions.

But other questions also came to mind. In the “golden years of physics” from 1895 to 1898 further rays were discovered, such as X-rays, cathode rays, α-, β- or γ-rays. Finally, there were the heat rays, a phenomenon that had been known for some time: All bodies become red, light red and finally white-yellow with constant heating; and one feels that heat emanates from them.

The origin and nature of these rays had to be understood. It was a very fruitful time for physics, and during this time the idea of atoms should establish itself as building blocks of matter, but only as a milestone on the way to ever smaller building blocks. One could then obtain an explanation for all these rays and thereby gain a consistent picture of the structure of matter and the atom.

In an attempt to develop this picture into a consistent theory success was only achieved after daring to describe the discovered relationships between the experimental results with a completely different mathematical conceptual apparatus.

A test case for each approach of a theory for the structure of an atom was the calculation of the possible energy states of a hydrogen atom. The success or failure of a mathematical calculation thus now decided on the success of a theory in this world of the smallest dimensions. The building blocks of an atom such as electrons, protons or neutrons could then no longer be regarded as particles in the sense of classical physics and the living world. They were soon called “quanta”, like the energy packages Max Planck had talked about in a lecture on 14 December 1900 when he gave an explanation of thermal radiation. By the way, the date of this lecture is regarded today as the birthday of quantum physics.

The quantum theories responsible for all the fundamental phenomena of the world’s smallest dimensions are first of all

– quantum mechanics, to a certain extent the replacement for classical mechanics

– quantum electrodynamics, the continuation of electrodynamics on the atomic level.

An explanation of the origin and nature of α- and β-rays could only be achieved by introducing two completely new types of forces, the “strong” force, which is responsible for the binding of the building blocks of the atomic nucleus, and the “weak” force, which can cause the transformation of a neutron into a proton, but also serves to describe the decays of other later discovered “particles”. Thus

– theories of weak and of strong interaction

were created. These two theories were constructed according to the model of quantum electrodynamics. This soon led to the desire to describe these three interactions within a unified theory. As an intermediate step the

– theory of electroweak interaction,

a unification of the electromagnetic and weak interaction was found, and finally

– the unified theory of electrical, weak and strong interaction, the so-called standard model.

Today, this model is regarded as the basic quantum theory. Quantum mechanics now plays the role of a theory for a limited range of phenomena in which relativistic effects do not have to be taken into account and in which there is no decay and no generation of particles.

Figure 4.3 shows the development of the individual theories over the course of time.

Fig.4.3: Timetable for the emergence of physical theories of fundamental interactions