A new interpretation of quantum mechanics sees agents as playing an active role in the creation of reality. Blake Stacey outlines the case for QBism and its radical potential.

The pandemic shut down our university when I was in the middle of giving a lecture. We had been anticipating the possibility for a few days, but it was still impeccable timing. I finished my spiel, out came the phones, and suddenly we weren't going to see each other post-spring break after all. For the rest of the term, I did what so many teachers found themselves doing: gamely trying to soldier on. I scrounged and borrowed a whiteboard, easel and webcam, set myself up in the nicest light the house had to offer, and did my best to convey graduate-level physics to an audience of tiny rectangles. And like so many other teachers, I learned there's nothing like a radical change of circumstances for driving one to re-evaluate what the essential ideas of a subject must be. In my case, this was complicated by the minor detail that the course I was teaching involved a lot of quantum mechanics, and the physics profession hasn't yet figured out what exactly the essential ideas of quantum mechanics are.

Oh, we know how to do the calculations. Nobody could have designed a laser or a computer chip if we didn't know that much. But the story that our textbooks tell is implicitly a tale of defeat, in a subtle way. They drop a chapter or three of mathematical arcana upon the poor student, not out of cruelty, but because we can't yet do any better. Complex numbers, matrix algebra, partial differential equations, spectral theory --- not only do the topics grow intimidating rather quickly, they also (if we are scrupulously honest) look rather arbitrary. Out of all the mental contrivances that the Mathematics department can serve up, why does quantum physics rely upon such a particular selection, and why do we employ those tools in the way that we do?

It is difficult to avoid turning philosophical about such matters. Questions like "What is the relation between our mathematical abstractions and physical reality?" feel as though they ought to be followed by a "like, dude". The history of attempts to answer such questions is complicated and contentious and written in no one place. Sometimes, the ideas themselves seem as if they are retreating from clarity. At other times, one wonders if philosophers and physicists wish to write as though clarity were the enemy.

I first started to care about the "interpretation" of quantum physics several years after I began using it. Many physicists don't care about such things at all, or they grow out of it rather than into it. It does indubitably feel strange that we would have to "interpret" a scientific theory; such language seems like it would belong more to critics of free verse or abstract art. ("What I feel the sculptor is trying to say...") But sometimes, when we're trying to develop the theory in new directions, or find the clearest way to teach it to the next generation, or we've just had one too many late nights wondering what it all means, we have to get our fingers philosophical.

Sometimes, the ideas themselves seem as if they are retreating from clarity. At other times, one wonders if philosophers and physicists wish to write as though clarity were the enemy.