Unification of Physical Theories

Problems can never be solved with the same mindset,
that created them.

Albert Einstein (1879 - 1955)

Over the past 500 years, the science of physics has limited itself to recording what happens in nature using measuring instruments and describing the relationships found using mathematical formulae. Today's physics can therefore answer neither the question of the nature of things nor the question of the genesis of physical laws. Both questions are considered taboo.

To be fair, the question of the nature of things has many pitfalls that have entangled many thinkers, while the focus of modern science on understanding phenomena through measurement has indeed led to enormous advances in knowledge. On the other hand, it must be said today that the progress of knowledge in physics seems to have come to a standstill with the transition to the 21st century. The great theories of the 20th century - the theory of relativity, quantum mechanics and the standard model of elementary particles - seem to agree brilliantly with empirical observations and experimental results. Experimental physicists have little to offer to cast fundamental doubt on today's accepted theories.

On the other hand, there is a certain unease among theoretical physicists because the fundamental physical theories stand side by side like solitary blocks and contain numerous arbitrary parameters. All the intensive attempts in recent decades to integrate gravity and quantum mechanics into a unified theoretical concept have been more or less unsuccessful. The fact that cosmology has to resort to hypothetical auxiliary assumptions such as dark matter and dark energy in order to reconcile astronomical observations with accepted physical theories is also unpleasant. The current situation in cosmology reminds those familiar with the history of science of the theory of epicycles, which was introduced by medieval astronomers in order to keep the observed orbits of the planets in line with the geocentric world view. Yet only a few outsiders, such as the non-fiction writers Alexander Unzicker and Sabine Hossenfelder, have voiced criticism, while the scientific community of active physicists is largely convinced that the generally accepted theories are correct.

During the transition from the 19th to the 20th century, physics found itself in a situation in which it seemed that progress in physical knowledge could only be made in matters of detail. At that time, the grand theories of the 18th and 19th centuries - classical mechanics and electrodynamics - had been excellently confirmed experimentally. Many physicists at the time believed that the body of physical theory was essentially complete and finalised. On the other hand, there were some experimental findings, such as the Michelson-Morley experiment, the photoelectric effect or the heat capacity of substances at low temperatures, which could not be reconciled with the established theories. These findings then became the starting point for the theoretical innovations of the early 20th century, the theory of relativity and quantum mechanics.

In analogy to the historical situation at the beginning of the 20th century, today's physicists hope that new experimental findings will provide the impetus to expand physical theories. But these are simply not forthcoming. They can therefore be excluded as a source of inspiration for the further development of physics. Hence, the starting point for the further development of physics must be the inadequacies of the theoretical framework. The fact that, despite many years of intensive effort, it has not been possible to further unify the major physical theories is a strong indication that we have been looking in the wrong place to find an approach to solve the unification problem. As Albert Einstein aptly remarked, you can never solve problems with the same kind of thinking that created them. It seems, therefore, that the time has come to critically examine some of the basic assumptions of physical theorising in order to change the usual frame of mind. Or as Anton Zeilinger says: 'It is time to ask the question about the nature of things again. We should therefore address the ontological question of what lies behind physical concepts.

Every theory inevitably contains fundamental concepts that cannot be directly deduced from observation. In today's physical theories, these are, for example, mass and energy, forces and fields. Contemporary physicists believe they have a good handle on some of these ontological concepts through measurements, including mass and energy. Other entities, such as the electromagnetic field or the wave function of quantum mechanics, are introduced as purely conceptual constructs that are neither accessible to direct observation nor are their essence defined in more detail.

The lack of reflection on the ontological foundations of the subject has led to the situation described at the beginning, where the individual parts of the physical theory building rest on completely different foundations. Any contradictions that arise between empirical observations and theoretical predictions have been bridged by ad hoc auxiliary assumptions such as dark matter or arbitrary parameterisations. As a result, the body of physical theory has become increasingly complicated over the last few decades. It therefore seems to be time to unpack Occam's razor and remove the beard that has grown. Simplification is the order of the day. Instead of making further additions to the theoretical edifice, we should look for inner connections.

But where does one start looking?

• One obvious starting point is the logical contradictions in the existing theories. You run into such contradictions, for example, when you try to interpret the spin of elementary particles or calculate the mass of the electron from the energy of its electric field. We hear about these well-known contradictions when studying physics, but are strongly warned not to delve any further into them, as this is futile and would only prevent us from productively dealing with other problems. We, on the other hand, think that you have to have the courage to ignore these intellectual stop signs if you want to resolve the troublesome contradictions in physical theory. We should do it like a woodpecker tapping on the rotten spots in the tree of knowledge. If a worm comes out and, after it has been removed, it turns out that, in addition to the directly affected area, other ailing areas also disappear, then you have landed a direct hit.
• Another suitable starting point for new theoretical approaches are previously unexplained anomalies and similarities. There are still plenty of these in physics. For example, nobody understands why there is practically no antimatter in our universe, even though every particle (matter) produces an antiparticle (antimatter) in all known transformation processes. Or why the electric charge and the spin of all elementary particles are in the ratio of small integers to each other, while the masses at rest show no correlation whatsoever. Or why the equations of motion in all areas of physics can be derived from the principle of extremal action.
• However, it is not possible to deduce logically what such peculiarities and similarities might be due to, but can only be guessed at to some extent. A powerful compass in this search can be overarching considerations that reveal what the structure of the theory we are looking for should be. However, one should be careful not to consider only those types of theory that have worked well in the past. It would be better to derive the cornerstones of the desired theories from general philosophical and methodological considerations.

Equipped with these tools, we want to set about unifying and simplifying the physical theory structure. We want to take today's physical knowledge seriously, especially the well-established empirical observations. However, we should be prepared for the fact that not all the currently accepted theories will fit together seamlessly as they stand today like pieces of a jigsaw puzzle. Instead, they will have to be cut at one point or another in order to fit.

Recommended reading:

• Richard P. Feynman, Robert B. Leighton, Matthew Sands: The Feynman Lectures on Physics, Addison-Wesley 1964/65 (Deutsche Millenniums-Edition im Verlag De Gruyter 2015). 
• Carl Friedrich von Weizsäcker: Der Aufbau der Physik, Hanser München 1985.
• Alexander Unzicker: Auf dem Holzweg durchs Universum – warum sich die Physik verlaufen hat, Hanser München 2012.
• Sabine Hossenfelder: Das hässliche Universum – warum unsere Suche nach Schönheit die Physik in die Sackgasse führt,  Verlag S. Fischer 2018.