How QBism Made the Wavefunction All About You

The most useful thing about QBism may have nothing to do with whether it is true. A century after quantum mechanics was written down, physicists who use it every day still cannot agree on what its central object is, and for most of that century the disagreement was polite enough to be ignorable. QBism made it unignorable by taking the least popular option and pushing it to its limit, and in doing so it forced everybody else to state, out loud and in print, what they think a wavefunction is. The claim itself is easy to state and hard to swallow. A quantum state is not a description of an electron. It is a description of a person, specifically of one agent’s personal expectations about what they will experience if they act on the world in a particular way. Move the observer and the state moves with them, because it was never sitting on the electron. Christopher Fuchs, who named the position, has described quantum theory in exactly those terms, calling it “a users’ manual that any agent can pick up and use to help make wiser decisions in this world of inherent uncertainty.” Whether you find that liberating or…