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“When I take an action on the world, something genuinely new comes out.”

That might sound like a deep remark you’d expect from a practitioner of Zen Buddhism. In fact, it was uttered by Christopher Fuchs, a quantum physicist, during the opening talk of the third “Phenomenological approaches to physics” meeting in Linköping, Sweden, in June. Fuchs, who is based at the University of Massachusetts Boston, said the statement was the “noble truth” needed to make sense of quantum mechanics.

Fuchs is the principal promoter of an interpretation to quantum mechanics known as “QBism”. Coined in 2010 (not by Fuchs), the term was originally short for “quantum Bayesianism” but has since lost that connection and is now self-standing. According to QBism, experimental measurements of quantum phenomena do not quantify some feature of an independently existing natural structure. Instead, they are actions that produce experiences in the person or people doing the measurement.

For the likes of Fuchs, quantum mechanics is thus not about an already existing world being measured – that’s the “noble truth” part – but is a theoretical guide for predicting what we will experience in future events.

It was as if the door between physicists and philosophers – slammed shut for perhaps a century – had suddenly collapsed and we found ourselves in the same room.

By putting experience at the heart of laboratory work, QBism has seized the attention of a group of philosophers known as “phenomenologists”, who examine the different ways that experience gives rise to everything humans know, and can know, about the world. The Linköping conference brought together these physics-versed phenomenologists, such as myself, with philosophically sensitive physicists, in roughly equal numbers. It was as if the door between physicists and philosophers – slammed shut for perhaps a century – had suddenly collapsed and we found ourselves in the same room, dazed and amazed, with the two groups sometimes speaking a little awkwardly with each other.

Back story

For nearly a century, the mathematical formalism of quantum mechanics has been clear and conclusive, yet its meaning has been opaque. In trying to work out what quantum mechanics says about the world, some interpretations suggest that quantum theory does not describe the world outright but is simply a tool for making predictions about it. Those are “epistemological” interpretations.

Other interpretations of quantum mechanics, however, are “ontological”. They consider what happens once we uncover more about the quantum world (when we find variables that are still “hidden”) or once we accept that some of its structures (such as the wave function) aren’t the ones we’re familiar with. When that occurs, we’ll see that its fundament, or “ontology”, is more or less like ours.

QBism is different. It is agnostic about whether there is a world that is structured independently of human thinking. It doesn’t assume we are measuring pre-existing structures, but nor does it pretend that quantum formalism is just a tool. Each measurement is a new event that guides us in formulating more accurate rules for what we will experience in future events. These rules are not subjective, for they are openly discussed, compared and evaluated by other physicists.

QBism therefore sees physicists as permanently connected with the world they are investigating rather than somehow getting “behind” it. Physics, to them, is an open-ended exploration that proceeds by generating ever new laboratory experiences that lead to ever more successful, but revisable, expectations of what will be encountered in the future.

Phenomenologists like myself find this obvious. We see QBism as simply stating that physicists form their ideas about the world the way the rest of us do: through experience. Humans are pre-connected with the world, and experience comes first. As Laura de la Tremblaye – a philosopher from the University of Geneva – said at the Linköping meeting: “QBism is a phenomenological reading of QM.”

Overlapping thoughts

These remarkable overlaps between QBism and phenomenology made physicists at the conference feel they needed to study phenomenology – and the phenomenologists to study physics. Fuchs himself explained how he’d once driven 75 miles through Boston traffic to pay $1600 for a complete set of the works of William James, the 19th-century American philosopher and proto-phenomenologist. Meanwhile, Delicia Kamins – a philosophy student at Stony Brook University who also spoke at Linköping – last year used her Fulbright fellowship to bone up on quantum mechanics at the University of Bonn.

For phenomenologists, experience is always “intentional” – i.e. directed towards something – and these intentionalities can be fulfilled or unfulfilled. Phenomenologists ask questions such as: what kind of experience is laboratory experience? How does laboratory experience – in which physicists are trained to see instruments and measurements in a certain way – differ from, say, emotional or social or physical experiences? And how do lab experiences allow us to formulate rules that anticipate future lab experiences?

Another overlap between QBism and phenomenology concerns the nature of experiments.

Another overlap between QBism and phenomenology concerns the nature of experiments. They don’t magically beam physicists into a special, more fundamental world. Instead, as I have long argued, experiments are performances. They’re events that we conceive, arrange, produce, set in motion and witness, yet we can’t make them show us anything we wish. That doesn’t mean there is a deeper reality “out there” – just as, with Shakespeare, there is no “deep Hamlet” of which all other Hamlets we produce are imitations. In physics as in drama, the truth is in performance.

The critical point

In the final session of the June conference, the question arose as to whether QBism is an “interpretation” of quantum mechanics – i.e. a new perspective on it – or simply a “reconstruction”, a reassembly with new pieces. That led to a heated, insightful, productive (if occasionally technical) discussion among the philosophers and physicists present as to what these terms mean. I had always dreamed that this sort of debate would occur. I just didn’t think it would happen in my lifetime.

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