Unfortunately, physicists and philosophers disagree on what exactly the preferred basis problem is, what would constitute a solution, and how this relates (or subsumes) “the measurement problem” more generally. In my opinion, the most general version of the preferred basis problem was best articulated by Adrian Kent and Fey Dowker near the end their 1996 article “On the Consistent Histories Approach to Quantum Mechanics” in the *Journal of Statistical Physics*. Unfortunately, this article is long so I will try to quickly summarize the idea.

Kent and Dowker analyzed the question of whether the consistent histories formalism provided a satisfactory and complete account of quantum mechanics (QM). Contrary to what is often said, consistent histories and many-worlds need not be opposing interpretations of quantum mechanics ^{ a }. Instead, consistent histories is a good mathematical framework for rigorously identifying the branch structure of the wavefunction of the universe ^{ b }. Most many-world’ers would agree that unambiguously describing this branch structure would be very nice (although they might disagree on whether this is “necessary” for QM to be a complete theory).

In my opinion, the situation is almost exactly analogous to the question of whether an abstract formulation of classical mechanics (e.g. Lagrangian mechanics) is satisfactory in the absence of a clear link between the mathematical formalism and our experiences. I could write down the Euler-Lagrange equations very compactly, but it would not feel like a satisfactory theory until I told you how to link it up with your experiences (e.g. this abstract real scalar *x* = the position coordinate of a baseball) and you could then use it to make predictions. Similarly, a unitarily evolving wavefunction of the universe is not useful for making predictions unless I give you the branch structure which identifies where *you* are in wavefunction as well as the possible future, measurement-dependent versions of you. I would claim that the Copenhagen cook book for making predictions that is presented in introductory QM books is a correct but incomplete link between the mathematical formalism of QM and our experiences; it only functions correctly when (1) the initial state of our branch and (2) the measurement basis are assumed (rather than derived).

Anyways, Dowker and Kent argue that consistent histories might be capable of giving a satisfactory account of QM if only one could unambiguously identify *the* set of consistent histories describing the branch structure of our universe.^{ c } They point out that the method sketched by other consistent historians is often circular: the correct “quasi-classical” branch structure is said to be the one seen by some observer (e.g. the “IGUSes” of Murray Gell-Mann and Jim Hartle), but then the definition of the observer generally assumes a preferred set of quasi-classical variables. They argue that *either* we need some other principle for selecting quasi-classical variables, *or* we need some way to define what an observer is without appealing to such variables. Therefore, the problem of identifying the branch structure has not been solved and is still open.

I like to call this “Kent’s set-selection problem”. I consider it *the* outstanding question in the foundations of quantum mechanics, and I think of the preferred basis problem as a sort of special case.

The reason I say special case is that the preferred basis problem answers the question: “How does the wave function branch *when there is a preferred decomposition into system and environment* (or into system and measuring apparatus)?” However, the boundaries of what we intuitively identify as systems (like a baseball) are not eternally well-defined. (What happens as atoms are removed from the baseball one by one? When does the baseball cease to be a useful system?) In this sense, I say that the decoherence program as led by Zeh, Zurek, and others is an *improvement*—a monumental advance—but not a complete solution.

**Added**: See follow-up post here.

*[This post was prompted by someone on the Physics StackExchange asking whether or not the preferred basis problem is considered to be solved. The answer, of course is disputed, but it was worth it for me to write this down…]*

### Footnotes

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- Of course, some consistent historians make ontological claims about how the histories are “real”, where as the many-world’ers might say that the wavefunction is more “real”. In this sense they are contradictory. Personally, I think this is purely a matter of taste.↵
- Note that although many-worlders may not consider the consistent histories formalism the only way possible to mathematically identify branch structure, I believe most would agree that if, in the future, some branch structure
*was*identified using a completely different formalism, it could be described at least approximately by the consistent histories formalism. Consistent histories may not be perfect, but it’s unlikely that the ideas are totally wrong.↵ - Dowker and Kent argue that this set should be
*exact*, rather than approximate, but I think this is actually too demanding and most Everettians would agree. David Wallace articulates this view well.↵

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