A Private View of Quantum Reality


Amanda Gefter profiles Christopher Fuchs in Quanta Magazine (Katherine Taylor for Quanta Magazine):

Christopher Fuchs describes physics as “a dynamic interplay between storytelling and equation writing. Neither one stands alone, not even at the end of the day.” And indeed Fuchs, a physicist at the University of Massachusetts, Boston, has a radical story to tell. The story is called QBism, and it goes something like this.

Once upon a time there was a wave function, which was said to completely describe the state of a physical system out in the world. The shape of the wave function encodes the probabilities for the outcomes of any measurements an observer might perform on it, but the wave function belonged to nature itself, an objective description of an objective reality.

Then Fuchs came along. Along with the researchers Carlton Caves and Rüdiger Schack, he interpreted the wave function’s probabilities as Bayesian probabilities — that is, as subjective degrees of belief about the system. Bayesian probabilities could be thought of as gambling attitudes for placing bets on measurement outcomes, attitudes that are updated as new data come to light. In other words, Fuchs argued, the wave function does not describe the world — it describes the observer. “Quantum mechanics,” he says, “is a law of thought.”

Quantum Bayesianism, or QBism as Fuchs now calls it, solves many of quantum theory’s deepest mysteries. Take, for instance, the infamous “collapse of the wave function,” wherein the quantum system inexplicably transitions from multiple simultaneous states to a single actuality. According to QBism, the wave function’s “collapse” is simply the observer updating his or her beliefs after making a measurement. Spooky action at a distance, wherein one observer’s measurement of a particle right here collapses the wave function of a particle way over there, turns out not to be so spooky — the measurement here simply provides information that the observer can use to bet on the state of the distant particle, should she come into contact with it. But how, we might ask, does her measurement here affect the outcome of a measurement a second observer will make over there? In fact, it doesn’t. Since the wavefunction doesn’t belong to the system itself, each observer has her own. My wavefunction doesn’t have to align with yours.

More here.