A post mortem for string theory

So string theory is dead. But why? What went wrong causing its demise? Or more importantly, why did it not succeed?

We don’t remember those theories that did not succeed. Perhaps we remember those that were around for a long time before they were shown to be wrong, like Newton’s corpuscular theory of light or Ptolemy’s epicycles. Some theories that unsuccessfully tried to explain things that we still don’t understand are also still remembered, like the different models for grand unification. But all those different models that people proposed for the electro-weak theory are gone. We only remember the successful one which is now part of the standard model.

Feynman said at some point that he does not like to read the literature on theories that could not explain something successfully, because it may mislead him. However, I think we can learn something generic about how to approach challenges in our fundamental understanding by looking at the the unsuccessful attempts. It is important not to be deceived by the seductive ideas of such failed attempts, but to scrutinize it for its flaws and learn from that.

Previously, I have emphasized the importance of a guiding principle for our endeavors to understand the fundamental aspects of our universe. I believe that one of the reasons why sting theory failed is because it has a flawed guiding principle. It is based on the idea that, instead of particles, the universe is made up of strings. Since strings are extended objects with a certain scale (the Planck scale), they provide a natural cut-off, removing those pesky infinities.

The problem is, when you invent something to replace something else, it begs the question that there is something to be replaced. In other words, did we need particles in the first place? The answer is no. Quantum field theory, which is the formalism in terms of which the successful standard model is formulated does not impose the existence of particles. It merely requires localized interactions.

But what about the justification for extended objects based on getting rid of the infinities? I’ve written about these infinities before and explained that they are to be expected in any realistic formulation of fundamental physics and that some contrivance to get rid of them does not make sense.

So, the demise of a theory based on a flawed guiding principle is not surprising. What we learn from this post mortem is that it is important to be very careful when we impose guiding principles. Although such principles are not scientifically testable, the notions on which we base such principles should be.

Transcending the impasse, part V

Beauty as a guiding principle

Proceeding with the series on Transcending the impasse in fundamental physics, I like to address some of the issues that has been proposed as reasons for the current impasse. One such issue is the methods by which theorists come up with their theories in fundamental physics. Sabine Hossenfelder, for example, feels strongly that one should not use beauty in the mathematics as a guide to what could be a potential theoretical explanation for fundamental phenomena.

What am I talking about? Perhaps the idea that beauty can have anything to do with fundamental physics sounds ridiculous anyway. Well, beauty, as they say lies in the eyes of the beholder. To a theoretical physicist, the notion of beauty may refer to a different experience than to an artist or a lover. Potential salient aspects of the concept of beauty that would be relevant for all those that experience beauty may include things like symmetry, balance, consistency, etc.

However, it is not my intention here to philosophize about beauty and what it is. The fact of the matter is that physicist do sometimes use their notion of beauty to guide them in how they construct their theories, or in what they consider to be the correct theory. One example that springs to mind is the relativistic equation of the electron of Paul Dirac. It is said that Dirac was guided in its derivation by the beauty in the mathematics.

Paul Dirac, who apparently used beauty as a guide to derive the relativistic electron equation

The issue of whether one should use beauty, or for that matter anything else, as a guide in the construction of fundamental theories reveals a deeper issue at stake here. First, we need to identify a difference between fundamental theoretical physics and other fields of physics. I hasten to add that this is not to be interpreted as a distinction between what is inferior and what is superior.

Other fields of physics usually have some underlying scientifically established physical theory in terms of which investigations are (or can be) done. For example, in classical optics, the fundamental theory is electromagnetism. If all else fails, one can always start with electromagnetism and derive the theoretical description of a phenomena rigorously from Maxwell’s equations for electromagnetism. If the phenomenon includes quantum effects, one may need to fall back on quantum electrodynamics (QED) for this purpose.

In fundamental physics, one does not have this commodity. In most cases one can be lucky to have some experimental results to work with. Sometimes, the only guide is a nagging feeling that the current theories are not adequate. This is the case with quantum gravity. There are some conceptual arguments why general relativity cannot explain everything, but there are no experimental observations showing that something is missing.

How does one approach such a problem? One needs some form of inspiration. Different people tend to use different forms of inspiration. Some use the beauty in mathematics as their inspiration. Perhaps too many theorists have done that and ended up with unsuccessful theories. Hence, the reaction against it.

The point is, we need to remember what it takes to arrive at a scientifically established physical theory. Regardless of what method or form of inspiration or guiding principle one uses, the resulting theory can only become a scientific theory once it has survived experimental testing. In other words, the theory must be able to make predictions that can then be compared with actually observations and then be shown to agree with such observations.

So, in the end, whatever method theorists use to produce their theories is of no consequence, as long as it can succeed as a scientific theory. To put restrictions on the guiding principles, be it beauty or whatever else, makes no sense. Instead, one should allow the diversity of perspectives and freedom in thought to come up with potential theoretical explanations, and leave it to the rigors of the scientific method to sort out the successful theoretical descriptions from those that are to be discarded.

I do not believe that the use of beauty as a guiding principle is responsible for the current impasse in fundamental physics. That dubious honor belongs to a much more inimical phenomenon. But that is a topic for another day.

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