(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?”

For starters, there is not one MWI, but many (perhaps fittingly), and none of them are compelling. That which they all presume, I would like to explain. Moreover, we have every reason to think we can explain those features of quantum mechanics that the MWIs merely assert, and in so doing we can really learn some physics.

MWI evangelism relies upon a set of tropes that become less engaging with increased exposure. The evangelist will often insist that his MWI, which he may equate with “the” MWI, is “simple”. This simplicity is belied by the dozens if not hundreds of pages that he is willing to write about his MWI; for he must establish its glory in detail, and if he admits there are others, he must prove his better than them all. “Simplicity”, rather like “elegance”, appears to be a burden of the beholder. (Nor is it clear that “simplicity”, however one might measure it, is necessarily a virtue. As Peter Shor asked, “Who promised us that Nature’s arcane rules / Would make sense to a merely human brain?”)

The evangelist is likely to say that “the” (his) MWI is a “natural” or perhaps even “inevitable” consequence of accepting that quantum physics is a correct theory. Stripped of rhetorical flourishes, the argument for naturalness becomes, “If wavefunctions are objective and always evolve unitarily, then wavefunctions are objective and always evolve unitarily.” It is a surprisingly empty business. Trying to add content to it, the evangelists disagree with one another; the “naturalness”, such as it was, is quickly used up. The topics of these disputes go by anodyne names, like “deriving the Born rule”. When their content is uncovered, they turn out to involve the very matter of how to wring meaningful probabilities out of the “interpretation”. In other words, they are about how to make the “interpretation” an actual scientific theory rather than a metaphysical ramble with no predictive traction. (For example, Sean Carroll’s preferred approach is based on an idea of Lev Vaidman; but Vaidman finds the key step in Carroll’s version “illegitimate”.) On this theme, I recommend essays by Adrian Kent, Huw Price and Carl Caves. I will cheerfully disagree with any of them on other points, no doubt, but their critiques have been, for me, the most stimulating. And I take no shame in recommending papers based on the enjoyable quality of their prose.

The MWI evangelist affects an air of audacity, but in more cases than not, his gospel betrays a deeply rooted ordinariness. His intuition finds no way for a theory to be “correct” unless its most obvious mathematical features are in blunt one-to-one correspondence with physical reality. Under his pose of radicality lies a trite obstinacy, a shallow imagination-like product that presents itself as depth and vision. Even when the evangelist is a scientist, during the encounter he conducts himself more like a fanboy for science than a seasoned practitioner of it.

(There is a certain emotional commonality between these conversations and those wherein a Silicon Valley type enthuses about biohacking, a trans woman replies, “Yes, check out my anti-androgen stockpile,” and the SV bro suddenly insists Not like that! Come to think of it, there may well be subreddits that provide all these varieties of STEM fanboyism simultaneously. And yes, my pronoun choices have been deliberate.)

Now, to be clear, I’ve been recounting my experiences with evangelism, which is qualitatively worse than endorsement or perhaps even a measure of advocacy. If you’re worried about being That Guy, you probably aren’t That Guy. Many physicists can hold this opinion or that with little consequence. The evangelist will go beyond, like a bad stand-up comedian still riffing on what was edgy in 1970, passing off a choice from the menu of lazy answers as daring and transgressive.

There is no “other branch of the wavefunction” where things turned out OK. There’s just this world, our world, full of careless actions and unintended consequences and science that, for all its power, always comes back to people.

Reading back over what I have written so far, it sounds more snarlish than I would like, but each individual piece says what I wanted, so I will offer my blueberry bread recipe and press on. Indeed, I might as well snarl about some other popular (or at least oft-mentioned) interpretational gambits while I’m here. Bohmian mechanics? More appealing to philosophers than to physicists; by now, sterile. Rovellian “relational quantum mechanics”? An exercise in transferring properties from the vertices of a graph to its edges, constantly backing away from its shot at grandeur. “The” Copenhagen interpretation? Again, there is not one of them, but a quarrelsome flock. Focusing on Bohr in particular, I found his philosophizing more subtle than he is often given credit for, yet ultimately unsatisfying even so. The venerable/hoary “Shut up and calculate”? As I wrote elsewhere, that is not a stable position, for it is vulnerable to perturbations by curiosity.

Even the most ascetic claim — the assertion to shut up and calculate with one mathematical formalism rather than another — is in some way a claim about the character of the world. Perhaps bound up with historical happenstance and social convention, but a claim about Nature nonetheless: Were the world a different way, would we not, after we shut up, calculate in a different fashion?

“Objective collapse models” might briefly be summarized as, wavefunctions are objective, and when they grow too big they fall over. I can sense a kind of second-hand intuitive appeal to this: In classical physics, we’re accustomed to dynamics becoming nonlinear when we push them hard. So, I can see how ideas in this general region might appeal to somebody else’s heuristics, even though to me, such schemes just modify an uninteresting idea of what quantum theory is about, and thus risk inheriting that dullness themselves. I do feel the romance in the possibility of gravity being the failure mode of quantum mechanics. If I were at a conference and a talk on that were on the program, I’d listen and maybe even think about ways I could turn their mathematics to my own nefarious purposes. Whereas if an MWI evangelist were next on the schedule, I’d skip the historical revisionism and leave the hotel to find a place that sells affordable coffee. Generally, I think the grand game of modifying quantum theory needs a better understanding of what principles quantum theory should be derived from — a project that is still very much in work.

Every job interview wraps around to the “What do you believe are your greatest weaknesses?” bit, and it would be only fair for me to talk about the objections to QBism. The Stanford Encyclopedia of Philosophy gives a decent third-party take on the complaints that have been put on the record. The most insightful comments, though, in the sense of potentially generating new ideas and being intellectually fruitful, have come in much less formal places — e-mails, chats in hotel bars and the like. It may be a rewarding exercise to try doing the dinosaur-bone thing and reconstructing how those exchanges have gone based on FAQBism. I caution the student, however, that many of the questions we hear most frequently are difficult to say anything interesting in response to, because they are interrogating their preconceptions rather than anything we have actually said. (A tell for this, one that I was initially surprised by but have since found dependable, is when the interrogator fails to see how little QBism has to do with “Bayesian inference”; see question 9.)

Just about the time I was wrapping up my PhD, I had enough cash on hand that I could do whatever I wanted for about a year, provided I kept living like a grad student while doing it. I decided that my version of backpacking around Europe would be investigating the theory of optimal quantum measurements. This had been a side interest during my thesis work, and turning it into my primary focus was appealing. There were lessons about reality in those calculations, I felt, lessons at least important enough to be worth another year of rice bowls for Friday dinner.

Eventually, on the strength of what I found, I was able to get funding to carry on with the work, so it is what I am still doing. Nowadays, of course, one’s justification for conducting pure research has an element of agony to it. The only remaining options in our world are anger and despair; the latter means paralysis, but can we draw strength from the former without, in technical terms, poisoning our souls? A few interludes of mathematical beauty, here and there a perspective shift that makes a bit of old intellectual history newly charming, a few chances to nurture wild new ideas with colleagues and friends — is that enough?

In other circumstances, publication would have seemed very premature. But in April 1940, who could be sure of a tomorrow? —André Weil

Yes, I definitely think we in general see things more clearly now. —Greta Thunberg