For decades, the story went that Bohr’s anti-realism triumphed over Einstein’s quest for a deeper reality. Physicists were told to "shut up and calculate" and metaphysical debates were dismissed as distractions. But today, realist interpretations of quantum mechanics constantly emerge and the anti-realist position is far from the mainstream – so if Bohr won, why does almost no one take his side? University of Massachusetts Boston physicist Jacques Pienaar argues that we’ve been wrong all along – Bohr was not an anti-realist after all, and Einstein's apparent realism is not on sure-footing.

One grey morning in Vienna in 2014, I paused on the steps of the historic Strudlhofstiege. Its emblem, a fish-head that spouted water into an algae-stained basin, stared back at me with melancholy century-old eyes. I passed this spot every day on my way to work at the quantum physics institute of the University of Vienna, and found that gazing at the fountain and listening to the gentle splashing of water often helped me work through difficult physics problems. Today, a veil of fine rain dampened my face as I wondered: just what was the Copenhagen interpretation?

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According to the usual telling of the story, Einstein's case for ‘realism’ lost, and the ‘anti-realist’ Copenhagen interpretation, championed mainly by Bohr and Heisenberg, emerged victorious.

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As an undergraduate, I was enthralled by the atmosphere of drama and mystery surrounding the famous debates between Bohr and Einstein, which for me was perfectly captured by the iconic grainy photo of the two great scientists reclining side by side, Bohr laughing as Einstein shares some joke whose punchline we will never hear, his hand raised in an enigmatic gesture. For a long time, I was sure he was holding a cigar, until one day I examined the photo carefully and found his fingers empty.

According to the folklore, during their intense but convivial discussions in public and in private, Einstein argued that quantum mechanics had to be an incomplete description of reality, while Bohr argued that he was missing the point: quantum theory was never meant to describe reality, only predict what could be observed in experiments and communicated in terms we could understand, a function for which the theory was entirely sufficient. According to the usual telling of the story, Einstein's case for ‘realism’ lost, and the ‘anti-realist’ Copenhagen interpretation, championed mainly by Bohr and Heisenberg, emerged victorious.

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Like all good stories, it has a few grains of truth. During the war years and long afterwards, young physicists were frequently discouraged by their advisors from working on ‘metaphysical’ questions, on the grounds that Bohr had cleared up the confusion and there was nothing more to be said[1]. Quantum theory was a tool for calculating probabilities for observable events, and inquiring into the metaphysical causes of such events was a dead end. Fortunately, not all took this advice to heart. Looking back on the situation, physicist David Mermin said, “If I were forced to sum up in one sentence what the Copenhagen interpretation says to me, it would be “Shut up and calculate!”[2]. Einstein's own assessment was even more scathing: “The Heisenberg-Bohr tranquilizing philosophy – or religion? – is so delicately contrived that, for the time being, it provides a gentle pillow for the true believer from which he cannot very easily be aroused. So let him lie there.”[3]