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. Therefore today's physics cannot explain the nature of things nor the genesis of physical laws. Both issues are considered taboo. However, it will not be possible to avoid addressing these issues if we want to further unify physical theories.

To be fair, thinking about the nature of things bears tricky pitfalls that have entangled many thinkers, while the focus of modern science on understanding phenomena through measurement has led to enormous advances in mankind's knowledge. On the other hand, the progress of knowledge in physics seems to have come to a standstill. The great theories of the 20th century - Einstein's 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 what could cast fundamental doubt on today's accepted theories.

On the other hand, there is a certain unease among theoretical physicists because their 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 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. Only a few outsiders, such as Alexander Unzicker, voice criticism, while the community of active physicists is largely convinced that the generally accepted theories are correct.

At the end of the 19th century, physics found itself in a similar 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 corpus of physical theories was essentially complete. 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, namely 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. Therefore, we should abandon the hope that new experimental findings will serve as a source of inspiration for the further development of physics. Hence, we should look elsewhere for the starting point for the further development of physics. Personally I think, the inadequacies of the theoretical framework could provide a promising starting point. Despite many years of intensive effort, it has not been possible to further unify the major physical theories. This fact 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 mindset. 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 handling of some of these ontological concepts through measurements, among them are 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 physics has led to the current situation, where the individual theories rest on completely different foundations. Any contradictions that arose between empirical observations and theoretical predictions have been bridged by ad hoc assumptions such as dark matter or arbitrary parameterisations. As a result, the corpus of physical theories has become increasingly complicated over the last 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 corpus of theories, we should look for inner connections.

But where shall we start looking?

• One obvious starting point could be 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. I heared about these well-known contradictions when studying physics, but I was are strongly warned not to delve any further into them, as this is futile and would only prevent me from productively dealing with other problems. Meanwhile I am convinced that we have to ignore these intellectual stop signs if we want to resolve the troublesome contradictions in physical theories. 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 other ailing areas also disappear, then we have landed a full hit.
• Another suitable starting point for new theoretical approaches are previously unexplained anomalies and pecularities. There are still plenty of these in physics. For example, nobody understands why there is practically no antimatter in our universe. Or why the electric charge and the spin of all elementary particles are in a ratio of small integers to each other, while their masses show no correlation whatsoever. Or why the equations of motion in all areas of physics can be derived from the principle of stationary action.
• However, it is not possible to deduce logically what such peculiarities might be due to. This can only be guessed at. A powerful compass in this search might be overarching considerations that reveal the structure of the theory we are looking for. Instead of considering only those types of theory that have worked well in the past, we should 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 corpus of physical theories. We want to take today's physical knowledge seriously, especially the well-established empirical observations. However, we should not expect all the currently accepted theories to fit together seamlessly like pieces of a jigsaw puzzle. Instead, they will have to be cut at one point or another in order to fit together.

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.