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Tom Siegfried


‘QBists’ tackle quantum problems by adding a subjective aspect to science

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First of two parts

Alfred North Whitehead, mathematician-philosopher-theologian, was one of the 20th century’s most cerebral commentators on science, thought and humankind. In lectures compiled as a treatise called Science and the Modern World, he described the “century of genius” (the 1600s) during which modern science ascended, and humans began to grasp the rationality of the universe, ridding the world of “medieval fantasies.”

But that wasn’t the end of the story. Somewhere along the line, science took an unforeseen turn.

“The eighteenth century opened with the quiet confidence that at last nonsense had been got rid of,” Whitehead wrote. “Today we are at the opposite pole of thought. Heaven knows what seeming nonsense may not tomorrow be demonstrated truth.”

And that was in 1925.

Whitehead was reacting to the early 20th century surprises of relativity theory and quantum physics. Both signaled to him that scientific theory was “outrunning common sense.” He found it obvious that the established order of Newton’s laws of gravity and motion, complemented by Maxwell’s electromagnetism — the orthodox materialism at science’s heart — could not survive.

“There must be a reorganization,” Whitehead declared. In the nearly nine decades since, that reorganization, while vigorously pursued, has not been completely accomplished.

Sure, Newton’s neat deterministic world, cause following effect like the way a clock’s tock follows each tick, has been superseded by a new universe more like a casino than a clock. Quantum physics makes science an oddsmaker, foretelling probabilities the way meteorologists forecast rain. Just much more accurately. But nobody can tell a completely coherent story about why quantum theory works or what it means. The only consensus is that it is weird.

That weirdness is well documented. Particles are waves and waves are particles. No such thing as simultaneous location and velocity. Multiple locations at once for a single particle. Cats simultaneously dead and alive. Multiple realities, even, like satellite TV’s hundreds of channels. All nonsense to the 18th century mind. As well as to many minds still around today.

In fact, none of the minds around today have succeeded in making sense of all that stuff. Though not for lack of effort. “Interpreting” quantum physics, sometimes expressed as articulating its “foundations,” has become as popular as ridiculing Dennis Rodman. There are more proposed quantum interpretations than channels available on DirecTV, Platinum Package. Yet quantum physics remains a riddle. Nobody’s proposed interpretation succeeds to the satisfaction of anybody with a different interpretation.

Many physicists don’t care. If the math works, and it does, why worry about what it means, they reason.

But others believe deeply that science should seek to understand nature more deeply.

It’s an old issue — whether science should simply describe what happens in nature, or also explain why it happens. Some of the quantum interpretation problem is related to that dispute. But there’s a deeper division in scientific philosophy that also afflicts quantum interpretations. It’s about the part played by the scientist.

A remnant of the Newtonian approach persists in modern physics: regarding nature as the object of study with no role for a subject to study it. Some scientists insist that this view is shortsighted.

“Recognizing that science is about the subject (the user of science) and not just about the object (the world external to that user) can eliminate well entrenched confusion,” physicist David Mermin of Cornell University writes in a recent paper.

His view arises from a relatively recent interpretation of quantum mechanics called Quantum Bayesianism, or QBism for short. It’s an approach long championed by quantum physicist Christopher Fuchs, developed with collaborators Rüdiger Schack and Carlton Caves.

Their view is that quantum theory’s conundrums stem from banishing the subject — the scientist or whoever — from the facts about “objective reality.” When a physicist calculates the odds of finding an electron in various locations, or the probability that a radioactive atom will decay today, those probabilities are generally taken as objective facts about nature. So reality is like dice. Einstein didn’t like it. Nevertheless the world respects the probabilities that quantum mechanics provides, and never defies them.

But it’s not so obvious to some physicists that probabilities are physical facts. After all, probabilities are just numbers, calculated from a mathematical expression called the wave function, a formula describing the “quantum state” of things like electrons or photons. There has been much philosophical discourse about whether the quantum state is something real or merely mathematical.

QBists say the quantum state does not possess an independent objective physical reality. Rather it simply represents the information a scientist possesses about how to calculate probabilities in order to make bets on what will happen. And QBists adhere to one version of the Bayesian school of statistical inference, in which probabilities are subjective — a measure of a person’s degree of belief about what’s going to happen. Bayesian probabilities are computed by combining prior expectations with observable data. QBists view quantum mechanics as a method for calculating odds in a similar way.

“According to QBism, quantum mechanics is a tool anyone can use to evaluate, on the basis of one’s past experience, one’s probabilistic expectations for one’s subsequent experience,” Fuchs, Mermin and Schack wrote in a paper last year.

Much of the supposedly inexplicable quantum weirdness dissolves when this view is adopted, the QBists contend. When a measurement is made, for instance, multiple possibilities encompassed by the wave function all disappear, except for the one corresponding to the result of the measurement. This “collapse of the wave function” has elicited all sorts of philosophical hand-wringing about measurements “creating reality” instead of just recording it. But from the QBist perspective, a measurement is just an experience for an “agent” acting as an investigator of nature. What gets created is merely a new experience for the agent.

Any such agent can use the rules of quantum mechanics to calculate the odds for various measurement outcomes on a system described by a given quantum state. But those odds are the agent’s personal judgment. That means that the quantum state in the first place is also a personal judgment of the agent. “The notorious ‘collapse of the wave-function’ is nothing but the updating of an agent’s state assignment on the basis of her experience,” Fuchs, Mermin and Schack write.

Similarly, the mysterious “spooky action at a distance” that dismayed Einstein so much is not a problem for QBism. If Alice and Bob each possess one of two particles that are quantum-connected, or entangled, Alice’s measurement on one will guarantee the result that Bob will get on the other. Sounds spooky. But Alice doesn’t know what result Bob gets until she talks to him. Each result, Bob’s and Alice’s, is personal. The results correspond only when Bob and Alice compare notes via ordinary (slower than light) communication channels. No spooky magic quantum messaging between them is required.

This formulation differs in some respects from the views of Niels Bohr, a founding father of quantum physics. But some aspects of QBism resonate with Bohr’s emphasis on the futility of describing an objective quantum reality. Instead, science should focus on finding order in human experiences of nature. “In our description of nature the purpose is not to disclose the real essence of the phenomena but only to track down, so far as it is possible, relations between the manifold aspects of our experience,” Bohr averred.

Another quantum pioneer, Erwin Schrödinger, expressed a similar sentiment (even though he disagreed sharply with Bohr on other matters). Schrödinger once stressed that “because everything any of us knows about the world is constructed out of his or her individual private experience, it can be unwise to rely on a picture of the physical world from which personal experience has been explicitly excluded,” Mermin writes.

To a QBist, the personal experience of an agent is the most important thing about quantum physics. Quantum states cannot be objective elements of reality because they are subjective, personal information, used by an agent for calculating the odds of future experiences. In this respect QBism has something in common with science’s origins, as articulated by Whitehead.

“We cannot too carefully realize that science started with the organization of ordinary experiences,” Whitehead wrote in Science and the Modern World. “It was not asking for ultimate meanings. It confined itself to investigating the connections regulating the succession of obvious occurrences.”

But the way science started doesn’t necessarily have to restrict the freedom of its current practitioners to pursue deeper explanations. To some physicists, the quantum picture of the universe must stand apart from those who do the quantum calculations. People are not just the subjects who conduct quantum research, they are also part of the existence that quantum theory describes. A proper scientific explanation of the world should not depend on people evolving to do calculations, many experts assert, but should allow a high-level explanation for the actions of those agents and why they get the results they do. Many current interpretations of quantum physics purse that path instead of QBism.

Follow me on Twitter: @tom_siegfried

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