This week, physicists are gathering on the German island of Helgoland to commemorate 100 years since the discovery of quantum mechanics. In the century following Werner Heisenberg’s theoretical masterstroke — made while convalescing from hay fever on that island — this theory has become ever more precise in its descriptions and successful in its applications. With technologies such as quantum computers, we stand at the cusp of a revolution that promises to bring the quantum world into many aspects of our lives.
Yet there is still no consensus about what it all means — about the deep lessons that quantum mechanics reveals about the fundamental nature of reality. Addressing this issue takes us from the domain of physics into the realm of philosophy. Although physicists are best placed to develop and extend the mathematical framework of quantum theory and design experiments to test it, philosophers of physics are needed to come to conceptual grips with the result: they have the tools to contextualize that framework and set it in a coherent account of reality.
Long-running disputes
Yet physicists and philosophers cannot agree about even the basics. A common refrain among philosophers of physics, and indeed some physicists, is that the textbook formulation of quantum mechanics has a ‘measurement problem’ — an unmanageable emphasis on the role of observers and their observations that makes it impossible to even begin to answer questions about the theory’s connection to the structure of reality. Many physicists beg to differ: some even argue that the emphasis on observers is the key to unlocking the deeper meaning of quantum mechanics.
How quantum mechanics emerged in a few revolutionary months 100 years ago
These two camps remain at loggerheads. But there might be a way to bring them together — by revisiting connections between ideas that have been around for more than half a century.
Disagreements about the meaning of quantum mechanics start with its description of physical systems through ‘wave functions’ that assign probabilities for the many possible values of a system’s measurable properties, such as its position. Repeated measurements of the same quantum system deliver a range of results in accordance with the probabilities encoded in the wave function. The standard explanation makes use of the Copenhagen interpretation, an approach championed by Heisenberg and the Danish physicist Niels Bohr in quantum theory’s early years. It says that physical objects fail to have definite values for their properties until they come into contact with an external observer or measuring device. This interaction between observers and quantum systems is essential to creating a definite reality. It is typically described as ‘wave function collapse’, although Bohr and Heisenberg were sceptical of interpreting this collapse as a genuine physical process.
The Copenhagen interpretation was controversial from the start. Bohr and Albert Einstein, two titans of physics, notoriously maintained a decades-long debate about its validity, lasting until Einstein’s death in 1955. Much later, physicist John Bell laid out1 a series of objections to interpretations of quantum mechanics in which measurement played a fundamental part. According to this take, observers and measurements are imprecise and subjective concepts that don’t correspond to fundamental physical entities; they arise, rather, from the complex behaviour of more-basic entities, and so shouldn’t be referenced in fundamental physical laws.
Over the decades, alternative interpretations of quantum mechanics have been proposed that remove reference to measurements or observers from the fundamental description of the quantum world. There are, for example, spontaneous-collapse theories, in which wave functions collapse into determinate realities independently of any observer or measurement process2,3. And the many-worlds interpretation, initially proposed4 by the physicist Hugh Everett in his PhD dissertation at Princeton University in New Jersey in the mid-1950s, suggests that observers do not collapse the wave function at the point of measurement. Instead, they pass into one of several parallel universes corresponding to the possible outcomes encoded in the wave function. By making a measurement, observers are not playing any fundamental part in creating reality, but merely choosing their route through it.
Why even physicists still don’t understand quantum theory 100 years on
This idea has many detractors, not least because a potentially infinite number of parallel universes seems a profligate solution to the measurement problem. But the dominant narrative among philosophers has become that only by starting from ‘objective’, observer-free mathematical frameworks, and escaping the clutches of the (in their view) confused and confusing Copenhagen interpretation, can we begin to draw concrete conclusions about what quantum mechanics tells us about the fundamental nature of the Universe (or, if the many-worlds theory is right, multiverse)5,6.
But many physicists working in quantum foundations espouse a counter-narrative: although there isn’t absolute truth in the Copenhagen interpretation, quantum mechanics does force a reinterpretation of the observer’s part in fundamental scientific theories. In this view, textbook formulations of quantum mechanics do not face a measurement ‘problem’. They make central use of observers and measurements — but so they must. The interesting question is not how to get rid of observers, but rather how exactly to understand their role, and what that implies for the relationship of quantum theory to the world beyond the observer.
Making the Universe
Several contemporary interpretations of quantum mechanics build on this idea. These include quantum Bayesianism, or QBism, which interprets quantum states in terms of the subjective probabilities that agents assign to events7, and the approaches of Anton Zeilinger8 and Časlav Brukner9, both quantum physicists at the University of Vienna, which stress the centrality of information. But this work is proceeding mostly without the support of philosophers of physics, who remain reluctant to embrace what they see as the central fallacy of Copenhagen and its offspring: a fundamental role for observers.
One common reason expressed for this reluctance6,10 is that such a move would equate to embracing instrumentalism: the idea that a theory doesn’t provide a true description of reality, but is a tool that allows observers to make better predictions about it. If the goal is to understand the fundamental nature of reality using quantum physics, any instrumentalist interpretation is thus a dead end.
Instrumentalism was promoted in the early days of the Copenhagen interpretation — it is explicit in Heisenberg’s work, for example. But physicists today are more likely to espouse realism, viewing quantum mechanics as an attempt to gain knowledge of the fundamental structure of reality. In the words of the Christopher Fuchs, a physicist at the University of Massachusetts in Boston and one of the creators of QBism, “what is at stake with quantum theory is the very nature of reality. Should reality be understood as something completely impervious to our interventions, or should it be viewed as something responsive to the very existence of human beings?”11. In this view, the privileged status of observers in creating reality is the distinctive claim that quantum mechanics makes.
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This position has little to do with instrumentalism. But it does go down the road of idealism — the philosophical position that reality is fundamentally composed of mental entities such as ideas. Contemporary philosophers of science are strongly disposed against this concept, and that position is not purely dogmatic. Idealism was the dominant position for most of the 2,500-year history of philosophy in the West, defining the work of influential thinkers from Plato in ancient Greece to George Berkeley, Immanuel Kant and Georg Wilhelm Friedrich Hegel in the eighteenth and nineteenth centuries. It was rejected only in the twentieth century, after a long march of breakthroughs in physics, chemistry, molecular biology and neuroscience that established the molecular nature of biological processes, demystified much of the operation of the brain and dispelled notions of the ‘vital forces’ that supposedly gave life to inert matter12.
Quantum physics has played a huge part in establishing this reductive, supposedly objective world view — the one also referenced in Bell’s critique1 — which makes plain that human observers are far from fundamental entities. So it might seem odd that increasing numbers of physicists working on the foundations of quantum theory are inclined to treat observers as such entities.
One towering figure of twentieth-century physics, John Archibald Wheeler, argued forcefully in defence of this approach. Wheeler’s work includes pioneering early studies with Bohr on nuclear fission and extensive contributions to the study of the general theory of relativity and black holes (a term that he popularized), but his contributions to quantum foundations have, in my view, not been adequately recognized. I further feel that the key to resolving the current impasse in the understanding of quantum mechanics could come from revisiting his idea of a ‘participatory universe’ — a concept in which reality is not static, but shaped by the perception of conscious observers — and his motivations for proposing it.
In 1978, Wheeler devised13 a seminal thought experiment on the nature of quantum reality, known as the delayed-choice experiment. In one version, he considers an iteration of the double-slit experiment, in which a stream of single electrons, photons, atoms or molecules — any object obeying the rules of quantum mechanics — is sent through a screen with two narrow apertures. If a detector is placed at the slits, each object seems to pass through one or the other, behaving like a particle. But if the objects pass through the slits and are instead measured on a second screen behind the first, over time, an interference pattern will emerge — as if each object went through both slits at once, diffracting and interfering with itself as a wave would.
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