Painting a QBist Picture of Reality

No, says Christopher Fuchs with the weary irritation of someone who has heard the accusation once too often, he is not a solipsist—someone who thinks the world is only a figment of his imagination. As a quantum physicist, he definitely believes that there is an external reality that can be described by science. It’s just that in his view, reality comes with a caveat: "There is more to it than can fit in any single description. What is left out of the pre-quantum view are our individual private contributions to reality."

It is this unorthodox philosophy of "participatory realism" that has led Fuchs, now at the University of Massachusetts Boston, to develop QBism: a radical interpretation of quantum theory that brings the role of the observer to center stage. Working mainly in collaboration with physicist Ruediger Schack at Royal Holloway, University of London, UK, Fuchs challenges the near-universal assumption among scientists that we can aspire to an overarching ’God’s eye view’ of how nature operates.

"Participatory realism says that no, there is no God’s eye view; the universe is too unwritten for that," says Fuchs.

Unorthodox or not, however, Fuchs’ participatory realism approach has already led to new practical insights in quantum cryptography—the field that exploits quantum features to securely transmit data. Now, along with philosopher Christopher Timpson, of Oxford University, UK, and with the aid of an FQXi grant of almost $141,000, Fuchs will investigate how QBism stacks up against rival quantum interpretations.

Quantum Lawlessness

Fuchs first became hooked on physics as an undergraduate at the University of Texas in Austin, in the 1980s. There, he met the physicist John Archibald Wheeler—a veteran of the Manhattan Project whose fertile imagination was still constantly probing the foundations of his science. One of Wheeler’s many fascinations was quantum theory, which describes the atomic realm in which nuclei fall apart without warning and particles can seemingly be in many different places at once—or even spin clockwise and counterclockwise simultaneously. Wheeler saw in the lawlessness of quantum measurement outcomes a hint that the universe as a whole might be lawless in a deep sense and more malleable to human action than previously imagined. (See "Reality’s Neverending Story.")

Since then, Fuchs has made it his life’s work to figure out where all this contradictory quantum behavior comes from. Physicists had long since learned how to describe quantum theory’s here-there-everywhere, clockwise-counterclockwise weirdness with a mathematical expression called a wavefunction, which had the virtue of obeying equations they could solve. Prior to making a measurement of a particle’s spin, say, the wavefunction encompasses its clockwise-counterclockwise duality. Yet when you actually try to observe this feature, you end up making the wavefunction collapse like a house of cards, until the only thing still standing is a single possibility—say, clockwise. Furthermore, there is no predicting which possibility that will be. All the wavefunction can tell you in advance is the probability of this or that happening, not what is going to happen.

It has also never been clear exactly what the wave function is—a convenient calculating tool or something real. In practice, those quantum probabilities are so useful that most young physicists soon give up worrying about the nature of the wavefunction and how quantum probabilities arise and just get on with their work. It’s an attitude that Cornell University physicist David Mermin likes to call, "Shut up and calculate!"

But Fuchs couldn’t let it go. Instead, he has been thinking deeply about the mathematics of probability in a quantum context. The probabilities that lie at the heart of quantum theory are usually interpreted as how frequently something will happen if you repeat an experiment zillions of times. This is how we’re taught to think about probability in school, and serves us well in simple situations. For instance, if you throw a pair of dice 36 million times, say, you will come very close to getting a million double-sixes. This reflects a seemingly objective 1 in 36 probability of that outcome; given fair dice, both you and I will calculate this same probability.

Degrees of Belief

But even in everyday life, calculating probabilities can be a lot messier and more subjective. Think about how two pollsters can make opposite predictions for the outcome of an election, due to biases in the samples of people they surveyed. The predictions of the winner also need to be constantly revised and updated on election night as partial results from different voting regions are revealed. In such cases, it is more fruitful to think of probabilities as an individual observer’s degree of belief that an outcome will occur—and to think of experiments that yield new and relevant data as a process for updating that belief.

This interpretation of probability goes back at least to the 18th century, when the English clergyman Thomas Bayes first wrote down a formula showing how to update beliefs based on new evidence. Fuchs and Schack realised that this more sophisticated interpretation of probabilities could also be applied to the quantum realm. They dubbed the resulting mathematics ’Quantum Bayesianism’—QBism, for short (C. A. Fuchs, arXiv:1003.5209).

According to QBism, the wavefunction is no longer to be thought of as an objective measure of the probability of getting an outcome of a quantum experiment that two observers will necessarily both agree upon. Instead, it hinges on the beliefs of a single observer, based on what he or she has experienced so far, while the outcome of a measurement can only be thought of as a new experience that the observer ought to take into account in any new beliefs.

QBism remains a niche interpretation, but it is gaining interest. Timpson notes that he was initially a skeptic, but as he delved into it, he began to realize that the ideas were surprisingly consistent and rigorous. One strength is that its mathematical framework is completely equivalent to that of standard quantum theory. This is an issue that Timpson, Fuchs and their students will systematically dig into, with the help of their FQXi grant. They plan to study similar work on quantum foundations being carried out by other physicists, and try to understand how all their many approaches to realism are related. Then they will try to thrash things out with a genuine debate. "There is this internal dialectic that’s one of the significant strengths of the project," says Timpson. "I’ve always been much more of a traditional realist, while Chris has always been wanting to challenge tradition."

Their participatory perspective has already offered new insights, adds Fuchs: In 2002, Fuchs and Schack, along with colleague Carlton Caves, used the QBism formulation to prove a theorem that has now found use in the field of quantum cryptography (R. Renner, arXiv:quant-ph/0512258). "What’s…interesting is whether a new interpretation causes someone to think in new directions, and results in new questions and experiments," Fuchs says.

Mermin has also become a late convert to QBism. He had been dubious until 2012, when he happened to spend six weeks at a workshop that Fuchs and Schack had organized in South Africa. "Sometime in week five, they persuaded me that this stuff ought to be taken seriously," says Mermin. It resonated strongly with his own empiricist views: "We have no access to the world except through our own experience," he says. And once you accept that, "then the question becomes, how do we arrive, with our fellow human beings, to a common understanding of the world? It’s that shared reality that corresponds to what we call the objective world—what we call science."