May 1, 2016


A Private View of Quantum Reality : Quantum theorist Christopher Fuchs explains how to solve the paradoxes of quantum mechanics. His price: physics gets personal.  (Amanda Gefter, June 4, 2015, Quanta)

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 RĂ¼diger 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.

In a sea of interpretations of quantum weirdness, QBism swims alone. The traditional "Copenhagen interpretation" treats the observer as somehow standing outside of nature, imbued with mysterious wave-function-collapsing powers, governed by laws of physics that are different from those that govern what's being observed. That's all well and good until a second observer comes along to observe the first observer. The "many worlds" interpretation claims that the universe and all of its observers are described by a single, giant wave function that never collapses. Of course, to make that work, one must insist that at every fork in the road -- every coin toss, every decision, every moment -- the wave function branches and so do we, splitting into countless versions of ourselves who have collectively done and not done everything we'll ever do or not do. For those to whom a set of infinite parallel realities is too high a price to pay to avoid wave-function collapse, there's always the Bohmian interpretation, which seeks to restore a more concrete reality to the world by postulating the existence of a guiding force that permeates the universe and deterministically governs everything in it. Unfortunately, this new reality lies forever out of reach of scientific probing.

Those interpretations all have something in common: They treat the wave function as a description of an objective reality shared by multiple observers. QBism, on the other hand, treats the wave function as a description of a single observer's subjective knowledge. It resolves all of the quantum paradoxes, but at the not insignificant cost of anything we might call "reality." Then again, maybe that's what quantum mechanics has been trying to tell us all along -- that a single objective reality is an illusion.

It's a choice among homocentrist universes.
Posted by at May 1, 2016 9:40 AM