WE ARE ALL DESIGNIST:

The Q in QBism came from Quantum, of course, and the B originated with "Bayes". In the wide spectrum of ways to think about probability, "Bayesianism" encompasses a variety of schools of thought which hold that a probability is a value that an agent asserts, a quantitative expression of a degree of belief. Probabilities encode expectations, and without someone around to do the expecting, there would be no probabilities. Before there were weather forecasters, there were no forecasts, even though the world had plenty of weather. In the proto-QBist days, around the turn of the millennium, the idea was just that the probabilities in quantum physics could be understood in a Bayesian way. But "Bayesian" is a broad label, and those early attempts were not very good at narrowing it down; nor, as further investigation revealed, were they internally self-consistent. That early "Quantum Bayesianism" took several more years to mature into QBism.

QBism regards quantum mechanics as a "user's manual". In this interpretation, quantum mechanics is about what happens at the interface between an agent and the rest of nature. Most of the mathematical entities employed in the theory, like the "wavefunctions" of which so much has been said, boil down to being bundles of expectations. Whose expectations? Yours, or mine, or those of whoever has picked up the user's manual and is trying to benefit from its guidance. Expectations for what? For the consequences of the user's own actions. What kind of actions? Any kind, in principle. Very often, physicists think of a "quantum measurement" as something that requires a laboratory to pull off. But in principle, the act of smelling a rose has every right to be considered a "quantum measurement". It is only that roses are big and we manipulate them clumsily, so one's expectations about a rose will be too fuzzed out for invoking quantum mechanics to be worthwhile in practical terms.

QBism has elements that are radical --- perhaps subversive, even --- while at the same time showing how some things we do as part of "weekday physics" are philosophically respectable after all.

Following the genre conventions of information-theory books, let's name our agent Alice. She has a system of interest before her --- perhaps an atom, perhaps an ion-trap quantum computer or a rose or a loaf of sourdough bread. She contemplates the possible actions she might take upon the system. By using quantum mechanics, she can assign probabilities to the possible consequences of each action, in a self-consistent way. Then she makes a choice and reaches out, taking action and experiencing the result. She can then update her expectations for future experiences in accord with this measurement outcome --- with the new fact that she, in synergy with the system, has brought into being. Prior to the measurement, Alice's uncertainty was not due to ignorance of an outcome already there, waiting to be uncovered, but rather her recognition that the fact of the outcome did not yet physically exist. It is this last realization, the principle that measurement outcomes aren't just waiting to be read off but instead require participation to elicit, that opens up the radical new possibilities of quantum physics.

This is at least an internally coherent narrative about the mathematics, which is the first thing we ask of an interpretation. But what does it say about nature that quantum mechanics is such a good user's manual for you or me or Alice to employ? Why this particular sage advice for swimming in the madness and salt of the world? Here we move into the realm of speculation; it is one thing to provide a narrative and another to successfully extract a lesson from it.

Consider again Alice measuring an atom. Her wavefunction for the atom encodes her personal expectations for future experiences, and her changing her wavefunction for it upon obtaining a novel experience is a transformation within her. But both Alice and the atom participate in the measurement event; both partake in the creation of a new fact for the pair of them. And if the atom can participate in such ongoing acts of creation when the other player is an agent, surely it can do so when the other player is not. That is to say, whatever fundamental capacity for creation the atom brings to an event, it should bring whether or not the other participant is a conscious agent, let alone a trained quantum mechanic. As one of our papers said, "Certainly QBism has creation going on all the time and everywhere; quantum measurement is just about an agent hitching a ride and partaking in that ubiquitous process."

It is this last realization, the principle that measurement outcomes aren't just waiting to be read off but instead require participation to elicit, that opens up the radical new possibilities of quantum physics.

This kind of imagery has predecessors. It's not unlike Karen Barad's notion of "intra-actions", or Alfred North Whitehead's "actual occasions" and "throbs of experience". William James wrote of "new being com[ing] in local spots and patches". And John Archibald Wheeler went all in. For him, the generation of a measurement outcome was the "elementary quantum phenomenon". "Is the entirety of existence," he would ask, "rather than being built on particles or fields of force or multidimensional geometry, built upon billions upon billions of elementary quantum phenomena, those elementary acts of 'observer-participancy,' those most ethereal of all the entities that have been forced upon us by the progress of science?" He would say, "In some strange sense the quantum principle tells us that we are dealing with a participatory universe."

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