## What is the point of Quantum Foundations?

A couple of months ago, there was an interesting debate in the Quantum Foundations group here at PI, with the above title. Unfortunately, I missed it, but it is an interesting question given that QF is becoming increasingly popular amongst young physicists, whilst remaining a relatively obscure and controversial subject in most of the mainstream physics community. Here are 3 possible answers to the question:

1. The goal of QF is to correctly predict the result of an experiment for which the standard approach to QM gives the wrong result. That is, we are in the business of providing alternative theories, that will eventually superseed QM. Work on things like spontaneous collapse models or nonlinear modifications to the Schroedinger equation falls into this category.

2. The goal of QF is not to contradict QM within its domain of applicability, but it should suggest possible alterntive approaches in cases where we are currently uncertain how to go about applying quantum theory. The archetypal example of this is quantum gravity, although to be fair it is more common to hear foundations people give this response than to find them actually working on it. Notable exceptions are the work of Gell-Man, Hartle, Isham and collaborators, which draws on the Consistent Histories formalism, and the recent work of Lucien Hardy.

3. The goal of QF is not to contradict QM at all, but it should suggest a variety of different ways to conceoptualize the subject, suggesting new possible experiments and theory that would have been difficult to imagine without considerable insight from QF. The main example we have of this is the field of quantum information. David Deutsch arrived at quantum computing by thinking about the many-worlds interpretation and Schumacher compression bears some similarity to the frequentist justifications of the quantum probability rule that began in Everett’s thesis. More recently, the Bayesian viewpoint of Caves, Fuchs, Schack, et. al. leads to new ways of doing quantum tomography and new variants of the quantum de-Finetti theorem, which have applications in quantum cryptography.

4. The goal of QF is not to bother mainstream physics at all, but to come up with the most consistent and reasonable interpretation of QM possible, involving minimal unverifiable assumptions about the nature of reality.

In my view, all four points of view can be justified. However, I think it is very useful to spell out exactly what we are up to to the rest of the world. A large portion of the physics community is skeptical about QF and, in my opinion, this is probably because they think we are all doing 1 or 4. If this were the case, I think I would agree with them, since QM has withstood a vast array of experimental tests and most of the alternatives suggested under category 1 seem contrived at best to me. Also, it is difficult to see what 4 could ever contribute to the rest of physics. It is also a problem that is better left to philosophers, since they are better qualified to tackle it.

To me, 2 and 3 seem like the most promising avenues of research for physicists who are interested in the field.

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