QBism: The simplest interpretation of quantum physics

Quantum mechanics is both the most powerful and the strangest of our scientific theories. They are powerful because they provide exquisite control in the nanoworld of molecular, atomic and subatomic phenomena. It's strange, because although we have a perfect mathematical formalism, we physicists have been arguing for over a century about what that formalism means. In other words, unlike other physical theories, the mathematics of quantum mechanics has no clarity Explanation. This means that physicists and philosophers are left arguing about which explanation makes the most sense. Sometimes the idea of ​​”simplicity” is invoked to answer this question.

So, today, based on a fascinating discussion started around

The “simpler” explanation.

There are two main parts of quantum formalism. The first is the so-called dynamic equation. This section gives us a mathematical description of how systems remain unperturbed development. We physicists love our dynamical equations, things like Newton's equations for particles or Maxwell's equations for electromagnetic waves. In classical physics, the dynamical equation was pretty much the end of the story. Nothing else was needed, and we came to think of those equations as being “there.” They were timeless laws of physics and never required any reference to what physicists were doing.

But this is not the case for quantum mechanics. In non-relativistic quantum mechanics, the Schrödinger equation is the fundamental dynamical equation. The Schrödinger equation produces what is called a “wave function,” a mathematical object that describes the state of a system as it evolves over time. But here the monkey wrench is thrown into the story. The wave function is strange in several ways. In particular, it tells us that a system (e.g., a particle) can have many values ​​of its physical properties (e.g., position or spin) at the same time. Another way to say this is that the different values ​​overlap. It is the ambiguity about previously non-existent properties in the superposition that is the source of all the fuss over Schrödinger's famous cats, with their “death” and “non-death” at the same time. “Death” and “non-death” cannot occur in classical physics, as there is no doubt about the truth of the properties of the system other than experimental uncertainty. So, what determines the actual value of a particular property in quantum physics?

The role of measurement

The strange answer is the analogy itself. Together with the dynamical Schrödinger equation, quantum formalism adds the Born rule. Born's rule tells us that when a measurement is made, the dynamics of the Schrödinger equation are interrupted. The superimposed wave function “collapses” to a single value. In particular, the Born rule tells us that we should use probabilities extracted from the wave function itself to predict what will be measured.

The dynamical Schrödinger equation – the wave function with its strange superposition of properties and the Born rule: that is the quantum formalism. The century-long debate over quantum interpretation has revolved around how to understand these things in terms reality. But in the long history of this debate, most efforts have come from trying to work around the Born Rule. Because we physicists love our timeless dynamical laws so much, the Born rule was seen as a scar on a beautiful theory. For many researchers, the task has become to preserve this beauty and think your way out of the constraints of the Born Rule. This included explaining (or clarifying) the role of measurement and, implicitly, the role of measurement.

It's an effort that has led to some wild extrapolations. For example, the popular Many Worlds Interpretation (MWI) claims that every time a measurement is made, a new “world” appears. In MWI, there is one world for each part of the wave function. When the analogy is made, these worlds become separate from each other, existing side by side like ghosts (which is why I call it the “multiple ghost interpretation”). In the Within a certain type of logic, the question is logical. Those different (possibly infinite) superimposed parts of the wave function just come out of the Schrödinger equation. Why don't we treat them all as real in some way? (It's worth noting that Wright says he's not a supporter of MWI, he's just asking the question.)

My answer is that there is a simpler way to find out what's going on, which is exactly what QBism does. Instead of seeing the wave function as a holy of holies whose truth must be preserved at all costs, he focused instead on what was new in quantum formalism, namely the Born rule. Quantum mechanics has been screaming at us that measurement matters for 100 years. Hearing that and taking it seriously is exactly what QBism brings to the table.

Focus on the born base

So, how can QBism achieve this so simply? It treats the wave function as notational information We interacted With the world, not the world itself as seen from God's perfect point of view. As my colleague, the eminent quantum physicist Joe Eberle, puts it, “It's not the electron's wave function, it's your own.” Take this position and you won't need countless unobservable ghost worlds or other sci-fi entities. This sure seems much easier to me.

However, the thing about quantum mechanics is that every interpretation pays some price that takes you beyond the classical physics perspective that is easy to envision in your head. QBism price is a view of quantum mechanics that reveals fundamental aspects of We and the world togetherAnd not just the world. The way to do this is to continually focus on the Bourne Rule. Why does the Born rule exist? Where do you originate? How does it arise? What does it tell us about us as agents responding to the world as it resists us when we experiment?

Instead of trying to go beyond the Born Rule and seeing it as a scar, quantum mechanics theory sees it as a central and very beautiful puzzle of quantum mechanics. How this happens and what it might mean for science will be the next part in my QBism series.