(a friendly warning for police violence, transphobia and philosophy of physics)

The way I see it, the two big Why? questions about quantum mechanics are, first, why do we use the particular mathematical apparatus of quantum theory, as opposed to any alternative we might imagine? And second, why do we only find it necessary to work with the full perplexities of quantum physics some of the time? These two questions are related. In order to understand how imprecise measurements might wash out quantum weirdness, we need to characterize which features of quantum theory really are fundamentally weird. And this, in turn, requires separating deep principles from convenient conventions and illuminating the true core of the physics. My own research has focused on the first question, but the second is never too far from my mind.

Of course, I have a lot on my mind these days, but I don’t think I’m special in that regard.

If you ask me, a “quantum system” can be any part of nature that is subject to an agent’s inquiry. A “quantum measurement” is, in principle, any action that an agent takes upon a quantum system. The road between Boston’s City Hall and the Holocaust Memorial is a quantum system. When the police use their bicycles as battering rams against queer kids and street medics, running towards the trouble is a quantum measurement. Being threatened with pepper spray, while secoondhand exposure already stings the eye and throat, one human thrown to the pavement in the intersection in front of you while another arrest happens on the sidewalk just behind you, is an outcome of that measurement. Unsurprisingly, textbooks provide little guidance on casting that event into the algebraic formalism of density matrices, and in the moment, other types of expertise are more immediately useful.

I first encountered quantum physics in a serious way during the spring of my second year at university — 2003, that would have been. I did not particularly care about the conceptual or philosophical “foundations” of it until the summer of 2010. The interval in between encompassed six semesters of quantum mechanics and subjects dependent upon it, along with my first attempts to find a research problem in the area. Once my curiosity had been provoked, it took the better part of a year to find an “interpretation” of quantum mechanics that was at all satisfying, and longer than that to make the transition from “this is how a member of that school would answer that question” to “this is what I declare myself”. Part of that transition was my discovery that I could put my own stamp on the ideas: The concepts and the history provoked new mathematical questions, which I could approach with a background that nobody else had.

The interpretation I adopted was the QBism of Chris Fuchs and Rüdiger Schack, later joined by N. David Mermin.

QBism is

an interpretation of quantum mechanics in which the ideas of agent and experience are fundamental. A “quantum measurement” is an act that an agent performs on the external world. A “quantum state” is an agent’s encoding of her own personal expectations for what she might experience as a consequence of her actions. Moreover, each measurement outcome is a personal event, an experience specific to the agent who incites it. Subjective judgments thus comprise much of the quantum machinery, but the formalism of the theory establishes the standard to which agents should strive to hold their expectations, and that standard for the relations among beliefs is as objective as any other physical theory.

That’s how we put it in the FAQ. Any physicist who is weird enough to endorse an interpretation of quantum mechanics will naturally get inquiries about it. Many of these, we get often enough that we try to compile good answers together into a nicely portable package — with the proviso that the quantum is a project, and some answers are not final because if physics were easy, we’d be done by now.

There’s a question which seems particularly suited to answering in the blog format, though: “Why don’t you believe in the Many Worlds Interpretation?”
Continue reading On Being a Quantum Physicist in Autumn 2019 →