In today’s blog we talk to Dr. Alyssa Ney, PhD from Brown University and Professor of Philosophy at UC Davis, author of “The World in the Wave Function: A Metaphysics for Quantum Physics”.

David: Is it correct to think of the version of quantum theory described in your book as similar in character to Bohm’s and Everett’s approaches?

Alyssa: I view Bohm and Everett's work as providing solutions to the measurement problem - the problem of reconciling the quantum formalism with the fact that measurements have determinate results. Bohm did this by adding additional variables to the quantum formalism (determinate positions). Everett did this by explaining how the quantum formalism could be interpreted not as saying that systems lack determinate values, but instead that they have multiple determinate values (relative to different observers). The disagreement between Bohmians and Everettians is a disagreement about what quantum theories should look like. Do they need additional variables to capture the fact that our measurements have determinate results or not?

In my book, I discuss wave function realism, which is not a solution to the measurement problem, but an answer to a different question, what kind of world (or worlds) do our quantum theories describe. To answer this question, we start from the various solutions to the measurement problem (Bohm's theory, Everettian quantum mechanics, collapse theories) and then ask what kind of entities, what kind of space or spacetime, what kinds of causal relationships are there, according to these theories. The wave function realist thinks these theories are all fundamentally about the wave function, and this wave function is a field on a very high-dimensional space.

Many philosophers and physicists today, including those who are sympathetic to Bohmian mechanics or Everettian quantum mechanics, do not think the wave function is a physical thing. Although some Bohmians still like the pilot wave picture, other Bohmians believe it is particles or fields alone that are the matter of the theory, and the wave function is something more like a law that guides the matter. Everettians have a variety of views about the role of the wave function in quantum theories, but the wave function realism defended in the book is just one approach.

So the short answer to your question is that Bohm and Everett were addressing a different question than the central question I address in my book, and their theories don't have to be interpreted as saying the wave function is a physical thing.

David: Would it make sense to say that Bohm’s guiding wave approach is the least abstract among realist approaches in the sense that it adopts structures that live in ordinary physical space?

Alyssa: As Bell pointed out when discussing the pilot wave theory in his "Quantum mechanics for Cosmologists" paper, in Bohm's theory, the guiding wave, if it is to be interpreted physically as a wave, must be understood as a wave not in 3-space but 3N-space.

David: Is the physical nature of the wavefunction less clear or more abstract in other approaches?

Alyssa: Again, the question about the nature of the wave function is not really one of Bohm vs. Everett vs. collapse theories. One can start from any of these solutions to the measurement problem and then ask, what should we think about the status of wave function representations according to these theories. Do they describe a physical field? If so, is it a field on our ordinary 3-dimensional space or spacetime, or is it a field on some higher-dimensional space? Is it rather a vector in a Hilbert space? Or is it something else entirely - something like a law, or maybe a set of relations among particles? Or should we adopt an epistemic attitude toward wave function representations, taking them not to describe anything objective, but just a description of one's state of knowledge about physical systems? I think these are all legitimate and clear ways of interpreting wave function representations. In my book, however, I want to see how far one can go in making sense of and motivating the view that wave functions are physical fields in a high-dimensional space (wave function realism). The motivation is that this view allows for a separable and local explanation of what seem (in the low-dimensional image of the world) to be correlations between spatially distant objects (nonlocality).

David: From an outsider’s perspective, one gets the impression that the versions of quantum theory are increasing, which does not seem to bode well for progress. From an insider’s perspective, how do you gauge progress over the last two decades?

Alyssa: I take it your question is more about the proliferation of different versions of quantum theory, rather than about the proliferation of different metaphysical interpretations of the nature of the wave function. So the question is what do I think about the fact that now there isn't just one version of quantum mechanics, but rather different versions: Everettian quantum mechanics, quantum mechanics with additional variables (like Bohm's theory), collapse theories, etc.

Here I would recommend everyone interested in this question check out Adam Becker's book "What is Real? The Unfinished Quest for the Meaning of Quantum Mechanics." I think there was a good reason for physicists to develop different versions of quantum mechanics in the second half of the twentieth century. The measurement problem was real. But at this point, I also think there isn't great evidence for collapse of the wave function or a need for hidden variables, so I think it is worth sticking with versions of quantum mechanics that lack such adornments. That is to say, I prefer Everettian quantum mechanics. I think in the past two decades, physicists and philosophers have done a lot of work making better sense of the Everettian picture and this is something that needed to be done. (See David Wallace's "The Emergent Multiverse" and Sean Carroll's "Something Deeply Hidden" for a less technical overview.)

I should say this is an issue I am completely neutral on in my book, since my project in "The World in the Wave Function" is not to defend one way of solving the measurement problem over the others, but instead to start from the fact that there are several theories that provide adequate solutions to the measurement problem, and then ask what kind of world these theories describe.

David: Does the existence of many possible ontologies of quantum theory suggest to you that we are still far from a deep understanding of quantum theory?

Alyssa: I definitely think we are far from knowing what is the one true ontological framework for quantum theories. All of the available options (wave function realism, primitive ontology views, holism, structuralism, spacetime state realism) have interesting motivations, but also each faces its own problems. I hope that my book can at least help us get clearer on what the various options are and how the higher-dimensional strategy may be viable.

David: Thank you Professor!