Christopher Fuchs, one of the founders of QBism, believes the approach could help tame thorny issues like the measurement problem and nonlocality. Others aren’t so sure.
Even if you’ve never seen Hamlet performed, you likely know the famous “gravedigger” scene in Act 5, in which the prince contemplates the skull of the late court jester, Yorick. When Shakespeare wrote the play, quantum mechanics was still three centuries away from being invented – but, just for fun, we can describe the scene the way someone who had just sat in on an Intro to Quantum Mechanics class would: Prince Hamlet is the observer and the skull is the thing being observed.
In everyday life, there is no need to be so fussy in terms of dividing up the world in such a fashion – but in quantum mechanics, observations, or “measurements,” as they’re called, play a central role.
Let’s back up a moment. It’s been almost a century since quantum mechanics was introduced to the world. While Max Planck got the ball rolling in 1900 with the idea that energy is quantized, it was only in 1925 that Werner Heisenberg came up with a complete mathematical framework for the new theory.
By the late 1920s, with contributions from Erwin Schrödinger, Niels Bohr, Max Born, and Pascual Jordan, the project was pretty much complete: Physicists could use quantum mechanics to account for all manner of atomic phenomena. Over the ensuing decades, it took its place as one of the two great pillars of modern physics (alongside Albert Einstein’s theory of gravity, known as general relativity).
The meaning of quantum mechanics
There is no question that quantum mechanics works – but what it actually tells us about the world is far less clear. A hundred years later, physicists and philosophers are still locked in fierce debate over what the theory actually means.
For starters, there’s this peculiar emphasis on the act of observation: The theory says you can use a mathematical structure known as the wave function to work out the probability of measuring some particular result (for, say, the position or speed of a particle), but it says nothing at all about what the particle is doing up to that moment. Until the measurement is made, the theory says the particle is in a “superposition” of states. When a quantum system is observed, the wave function is said to “collapse” so that the particle is seen to be in just one state.
But what exactly do we mean by “measurement”? And where do we draw the line between the observer and the thing being observed? (For example, is the sweat on Hamlet’s palm part of the observer or part of the observed?) This is the famous “measurement problem,” and it has sparked endless debate ever since the development of quantum mechanics.
The measurement problem has long troubled Christopher Fuchs, a physicist at the University of Massachusetts in Boston – and he believes he has at least the seed of a solution. For more than 20 years, Fuchs has been the unofficial leader of a small group of theorists who have come to embrace a bold and controversial idea known as QBism.
“You’re going to run into trouble if you try to say there’s the system, and there’s a measuring device, and over here is the observer,” Fuchs explained recently over Zoom. “The middleman needed to be cut out, and that’s where QBism went.”
What is QBism?
So what exactly is QBism? The idea had originally been called “quantum Bayesianism,” referring to a statistical framework – Bayesian statistics – which interprets probability as expressing one’s degree of belief in a proposition. That degree of belief is based on prior knowledge about the proposition, but can be updated as more data becomes available.
The crucial outcome of taking a Bayesian approach to quantum mechanics is that the wave function is no longer seen as giving an objective description of some feature of the external world; rather, it is interpreted as giving an individual observer the information they need to place a bet on the outcome of a particular experiment. As a philosopher would put it, it’s an approach that favours epistemology (what we can know about the world) over ontology (what the world is actually made of) – and stands in stark contrast to the way physics has traditionally been done since the time of Galileo.
QBism also forces a re-think of what we mean by probability: In the QBist view, probabilities are completely subjective – they do not convey information about the world, only about our interactions with the world.
“There’s a presupposition that the world just is, and it’s our task to discover what it is,” says Fuchs. “QBism decided to take a different tack, and to say, no, quantum mechanics is not a theory of how the world is. It’s a very different thing. It’s a theory about how to better survive in the world.”
Fuchs first worked out the main principles behind QBism in the early 2000s, in collaboration with his PhD supervisor, Carlton Caves (now an emeritus professor at the University of New Mexico), and Ruediger Schack (now at the University of London). Over the years, they’ve picked up a few converts, including Cornell physicist David Mermin. Writing in Physics Today in 2012, Mermin called QBism “by far the most interesting game in town.” When he penned an essay about QBism in Nature in 2014, it ran under the headline “QBism puts the scientist back into science.”
How QBism tackles long-standing paradoxes
Among the attractive features of QBism is its apparent ability to solve many of the paradoxes of quantum mechanics. Consider Schrödinger’s now-famous thought experiment involving a cat. It suggested that, under certain conditions, superpositions could be scaled up from particle size to cat size – resulting in a feline that the theory pegs as partly alive and partly dead. Under QBism, however, there is no such weirdness: For a QBist, the wave function doesn’t describe the cat itself, but rather the observer’s expectations of what they’ll find when they open the box.
Another interesting example is the thought experiment known as “Wigner’s friend” (put forward by Hungarian-American physicist Eugene Wigner in 1961). Imagine an observer who performs a quantum measurement on a physical system (say, an electron that can be spin-up or spin-down). Wigner, meanwhile, is lurking outside the laboratory but cannot see inside it. Both Wigner and his friend use the rules of quantum mechanics to formulate a statement about the system.
In the traditional view of quantum mechanics, their statements will contradict each other. Wigner’s friend first declares that there’s a 50% probability of the electron being spin-up and a 50% probability of it being spin-down; when he actually makes the measurement, he finds the electron in just one of the two states. Wigner, meanwhile, tries to give a quantum mechanical description of the entire system (his friend plus his friend’s experiment). But, with no way to see inside the lab, he concludes the electron is still in a state of superposition. (Note that Wigner maintains this prediction even if he believes his friend has finished the experiment.)
So who’s right, Wigner or his friend? Mermin writes: “For the QBist, both are right: The friend assigns a state incorporating her experience; Wigner assigns a state incorporating his. Quantum state assignments, like probability assignments, are relative to the agent who makes them.”
QBism, nonlocality, and free will
QBism is said to tame other problems as well – like the peculiar issue of nonlocality. Ever since the early days of quantum mechanics, physicists and philosophers struggled to make sense of the notion that a measurement made here can depend on the outcome of a measurement made there, and vice-versa, as the theory seemed to suggest.
Some interpretations of quantum mechanics, like the de Broglie-Bohm theory (or pilot wave theory), take this nonlocality at face value; it’s just a fact about the universe that we need to get used to. But Fuchs sees nonlocality as a bug that needs to be squashed – and QBism does just that. For the QBist, the only information that an observer has is what she measures at her particular location; she can know nothing of what’s being observed somewhere else (at least, until somebody tells her). Locality is restored.
QBism may offer even more, especially for those who ponder the philosophical problems raised by modern physics. Fuchs has suggested that QBism can help make sense of the puzzle of free will, which has long stymied scholars from a multitude of disciplines.
In recent years, a number of physicists (and some neuroscientists and philosophers) have declared free will to be an illusion. Not so under QBism: In the QBist view, we push on the universe, and it pushes back; our actions matter. Schack, for example, has written that “the QBist vision is that of an unfinished universe, of a world that allows for genuine freedom, a world in which agents matter and participate in the making of reality.”
Chris Timpson, a philosopher at Oxford, says he does not consider himself a QBist, but likes the way QBism appears to shed light on foundational issues, particularly the measurement problem.
“For the founding fathers of quantum theory, there was a strong sense that the theory was teaching us that we had to deal with the relationship between subject and object differently; that we had to play a role,” Timpson says. “QBism certainly takes that idea and runs with it.”
QBism's skeptics
Others are more skeptical. The objection that has been raised most often is that QBism does away with a shared reality – an objective set of facts about a world that exists independently of humans and human observations.
“It would be crazy to deny that things look different to different agents,” says David Albert, a philosopher at Columbia University. “The question is whether those different appearances can all be somehow knitted together. Can it all be conceived of as following from an objective ‘God's eye view’ of the whole situation, including the physical situation of the agents? This was always the hope of physics.”
A theorist who gives up on this shared reality “strikes me as somebody who has their philosophical priorities deeply mixed up,” Albert adds.
Caves, in spite of his early enthusiasm for QBism, was eventually led to the same position. He praises QBism for its logical consistency, but says that this logical consistency comes “at the expense of really giving up on being physics.”
An “astonishing vision”
Fuchs is very familiar with these criticisms – but he insists they’re misguided. The way he sees it, QBism – in spite of its focus on observations made by individual agents – really can yield information about the actual, physical world.
Fuchs’ preferred analogy involves a single-cell organism known as Euglena. These diminutive creatures, no larger than a fiftieth of an inch long, can swim with the help of a tiny tail, called a flagellum, which lets them move about freshwater and saltwater ponds. Euglena’s interactions with the world are obviously limited – but, as it moves about its environment, it learns about its world. The way Fuchs sees it, QBism reveals the world to us just as Euglena learns with the help of its tail.
“It’s a tool. But that doesn’t mean that you can’t glean information about reality from it,” says Fuchs. “I think QBism views the quantum theory as a decision theory – but the decision theory is indirectly telling us something about reality. In that sense, it’s not anthropocentric.”
Not everyone is satisfied. “At the end of the day, they [the QBists] want to say, ‘That’s all there can be to it: There’s just me and my expectations for how things will be in the future,’” says Timpson. “But most people think that, actually, there’s more to explanation than expectation. This is one of the areas where I think the QBists still haven’t really faced up to the challenge that they have to meet.”
Still, he admires the QBists’ ambitions. The theory “holds out quite an astonishing vision,” says Timpson. “And if it could be made to work, that would be striking.”
So what exactly is the QBists’ view of reality? Fuchs admits he doesn’t yet have a satisfying answer to this question, and that the project is far from finished.
“You learn about something that’s larger than the individual. And what is that something that’s larger than the individual? I’d like to say in more detail,” says Fuchs. “I hope I can, in the last 20 years that I’ve got left in me. Maybe 25 – that would be better.”
Dan Falk (@danfalk) is a science journalist based in Toronto. His books include The Science of Shakespeare and In Search of Time.
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