# QBism: The Most Radical Interpretation of Quantum Physics

Quantum mechanics, the most powerful theory physicists have developed, doesn’t make sense. What I mean by this statement is that quantum mechanics – which was developed to describe the small world of molecules, atoms and subatomic particles – leaves its users without a logical picture of what it describes. Filled with what seem like paradoxes and mysteries, quantum physics, for most scientists, requires an explanation: a way to understand its mathematical formalism in terms of a concrete description of what is out there in the world and how we interact with it. Unfortunately, a century later, only a handful of “quantum explanations” have been proposed. Which is correct? Who among them clearly understands what quantum physics has been trying to tell us over the past 100 years?

In light of these questions, I’m going to start a series that explores the most extreme quantum interpretation, one that I think gets it right, or at least points in the right direction. It’s a relative newcomer to the scene, so you may not have heard of it. But he’s gained a lot of attention lately because he’s not only asking us to reimagine how we see atoms; He asks us to reimagine the process of science itself.

## Welcome to the world of “QBism”

The term “QBism” was short for “Quantum Bayesian” when this idea/theory/explanation was first proposed in the late 1990s and early 2000s. The name hits a nail in the head because “Bayesian” is such a radical way of interpreting probabilities. The Bayesian approach to what we mean by probability is very different from what you were taught in school about coin tosses, dice rolls, and how often a given outcome appears. Since probabilities lie at the heart of quantum mechanics, QBism focused on a key aspect of quantum formalism—one that other explanations have missed or been swept under the rug—because it focuses squarely on how we interpret probabilities. We’ll dig deeper into all of this as we move forward in this series, but since today’s column is supposed to be the introduction, let’s start with a 10,000-foot view of what’s at stake in the great “quantum wars.” So we can see where QBism fits.

The most radical departure that quantum physics makes from its predecessor in classical physics is its treatment of what is called a “state”. To be clear, let’s consider a particle of matter. Classically speaking, the state of a particle refers to its position and momentum (think “velocity”). In classical physics, we also have “dynamical” equations, such as Newton’s laws, which describe how the position and momentum of a particle (state) change with time. In this view, state is a property that a particle possesses independently of anything else (such as the person making measurements on the particle). Characteristics are self-existent and “objective”. In addition, classical physics says that particles can have exactly one state at any point in time, and only the dynamical equation determines how that state changes. Objective states and the rule of dynamical equations are what classical physics is all about.

However, things are completely different in quantum mechanics. Quantum states can be “superposed”, which means that a particle can have many values of position and momentum at the same time (such as a cup of coffee in many places at once). Even worse, the dynamical equation of quantum mechanics (called the Schrödinger equation) doesn’t describe the particle all the time. Instead, at the exact moment the measurement is made, the Schrödinger equation gets a pink coupon. At that moment, the state is determined not by the deterministic dynamic equation but by the so-called Born rule and its specification of the probabilities of different outcomes in the superposed state.

Many quantum interpretations have recoiled in horror at this situation. Their goal is to try to preserve the classical view that the equations of physics are somewhat like “ideas in God’s mind.” These interpretations take *my presence* View the quantum state, including its superpositions. The quantum state is really real. It is “there”, as a real thing in the real world, independent of us. But given superpositions, there is a price to pay for this kind of existential commitment: adding things to the universe for which there is no evidence, like parallel universes separating each time a quantum measurement is made. Parallel universes sound great for sci-fi movies, but they’re actually a heavy price to pay for sticking to the metaphysical preferences of classical physics.

## Radical conclusion

QBism takes a completely different stance. He looks at the changes that the inventors of quantum mechanics had to make and draws a truly radical but also radical conclusion: the quantum state, with its superimposed simultaneous possibilities, is not something that exists by itself. The state is not something that the particle has as a property, just as a house has the property of being painted blue. Instead, quantum states relate to our knowledge of the world. They are descriptions that encode our interactions with particles. QBism might say it’s not the case of the particle – it’s your case *on *particle. QBism leads not with ontology—a story about what essentially exists independently of us—but with epistemology: a story about our information about the world. This change makes all the difference. By refusing to force us to hold onto an ancient philosophy that came preloaded with classical physics no matter the cost, QBism does not have to force us to accept science fiction stories about parallel realities (or other unobservable “entities”) in science. . Instead, QBism is led by experience. What really happens when humans do quantum physics, he wonders?

The answer produced by QB theory is as radical as it is mundane. By departing from an impossible (and paradoxical) theistic view of the universe, QB theory puts humans squarely in the middle of the scientific enterprise. In this way, I think it “understands” what quantum mechanics has been trying to tell us since its invention a century ago. Doing physics is not about gaining a mythical and transcendent perspective, but about watching people (people like you and me) gain knowledge about the world. What’s more, and more exciting than that over-the-top mythic view, is that understanding quantum mechanics means understanding how *We and the world are always woven together as an inseparable whole*. Deconstructing this perspective is at the core of QBism’s ambitious research program, which we will dismantle as this series progresses.

(Just a note: I’ll be filling out this series over the next few months, so stay tuned!)