QBism and the philosophical crisis of quantum mechanics

What’s really the problem with quantum mechanics? Why, after 100 years of the most profound success describing nature, are people still arguing over what that description means — that is, how to interpret quantum mechanics? Even more to the point, if we can understand that essential problem, could we tell which of quantum physics’ multiple interpretations addresses the problem best?

In the first post of this series, I introduced the notion of quantum interpretations and QBism (Quantum Bayesianism) as one of the options. Now, we are going to dive a bit deeper into the structure of quantum mechanics itself to see exactly what quantum physicists are arguing about.

Classical physics vs. quantum mechanics

Quantum physics is the science of the nanoworld: molecules, atoms, and subatomic particles. At the turn of the 20^th century, physicists tried to bring concepts from what we now call “classical physics” (that is, Newtonian physics) to bear on experiments probing this nanoworld. The concepts underlying classical physics, however, spectacularly failed to explain the results from those experiments.

In particular, the classical concept of the “state” of a system no longer made sense. The state is a core concept in physics, which represents the description of a system’s properties. In classical physics, those properties — such as position and velocity — were assumed to exist independently of anything else, particularly anything involving us. The properties were seen as real attributes of the system, which could be anything from a particle to a planet. Philosophically, we would say the properties and the system were ontological. (Ontology is the branch of philosophy having to do with “being.”)

Quantum mechanics, however, changed the nature of the state. Instead of just a single set of numbers associated with a particle’s properties, the quantum state allowed for a range of possible numbers that existed simultaneously. Imagine we had a particle with the property “color” that could take values white (W) or black (B). The quantum mechanical state for that particle, before anyone made a “color” measurement, would look like this:

|Ψ > = a |W> + b |B>

Here, Ψ is the Greek letter “psi” and |Ψ> is the symbol physicists use for the quantum state. In this equation, |W> represents the particle having the color white, and |B> represents the particle having the color black. The terms a and b are associated with the probability that once a measurement is made the result will either be white or black.

So why is all this a problem? According to the equation above, before a measurement is made, the particle is neither white nor black — nor is it not-white or not-black. The quantum state |Ψ> seems to imply that before a measurement is made, the particle doesn’t have the property of color at all. Instead, the particle is in a weird “superposed” state whose philosophical implications are very muddy. It’s only after the measurement, when we see either a white particle or a black particle, that we can say the particle has the property of having color.

Now you can see why physicists feel like they need an “interpretation” to understand the most basic element of quantum physics: the quantum state |Ψ> (also called the state vector or the state function).

Psi-ontology vs. Psi-epistemic

In general, there are two different quantum interpretations. One prominent class is called psi-ontology. (Whoever came up with that deserves a medal.) These interpretations reify the mathematics. They make the state function something real, existing “out there” in the Universe in the same way classical physicists thought the classical state was something real and independent. 

The Many Worlds Interpretation (MWI) is a famous example of a psi-ontological interpretation.  To preserve the reality of the quantum state |Y>, fans of the MWI argue that, when a measurement is made, the Universe splits off into two separate parallel branches. In one branch, the particle has the color white, and in the other branch, the particle has the color black. For real systems, you end up with a multitude (maybe an infinite number) of unobservable branching universes for every quantum measurement. If that seems like a steep price to pay to keep the metaphysical biases of classical physics, then I’m with you.

The second class of quantum interpretations are classified as psi-epistemic. While ontology is about what really and truly exists, epistemology is about our knowledge. For those who favor a psi-epistemic perspective, the quantum state is not about stuff floating around out there in some perfect, platonic Universe. Instead, it’s a description we have about the particle as we interact with the world. As Joe Eberly, a senior quantum researcher in my department puts it, “It’s not the electron’s state vector; it’s your state vector.”

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The famous Copenhagen Interpretation favored by the founders of quantum mechanics is most definitely psi-epistemic. Niels Bohr, Werner Heisenberg, and others saw the state vector as being related to our interactions with the Universe. As Bohr said, “Physics is not about how the world is; it is about what we can say about the world.”

QBism is psi-epistemic

QBism is also definitively psi-epistemic, but it is not the Copenhagen Interpretation. Its epistemic focus grew organically from its founders’ work in quantum information science, which is arguably the most important development in quantum studies over the last 30 years. As physicists began thinking about quantum computers, they recognized that seeing the quantum in terms of information — an idea with strong epistemic grounding — provided new and powerful insights. By taking the information perspective seriously and asking, “Whose information?” the founders of QBism began a fundamentally new line of inquiry that, in the end, doesn’t require science fiction ideas like infinite parallel universes. That to me is one of its great strengths.

But, like all quantum interpretations, there is a price to be paid by QBism for its psi-epistemic perspective. The perfectly accessible, perfectly knowable Universe of classical physics is gone forever, no matter what interpretation you choose. We’ll dive into the price of QBism next time.