Anyone who has ever strolled through the science section at a Barnes & Noble knows that popular books on quantum mechanics abound. Jim Baggott, a prolific science writer with a PhD in chemical physics, has himself already written several successful popular texts dealing with quantum theory. Seeing that Baggott has now written another book on the topic, Quantum Reality, one might ask: Why? What makes Baggott’s newest book—one more in a long list of attempts to introduce the public to philosophical issues within quantum mechanics—stand out?

Although the “Copenhagen interpretation” of quantum mechanics may be a postwar invention, the Danish capital was indeed a major site for intellectual exchange during the quantum revolution of the 1920s. Pictured here is the lakeside street of Sortedam Dossering, which features some of the oldest neon lights in the city.

Although the “Copenhagen interpretation” of quantum mechanics may be a postwar invention, the Danish capital was indeed a major site for intellectual exchange during the quantum revolution of the 1920s. Pictured here is the lakeside street of Sortedam Dossering, which features some of the oldest neon lights in the city.

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Fortunately, Quantum Reality quickly justifies its existence. Part 1 provides an entertaining yet compact introduction not just to the most important physical and formal features of quantum mechanics but also to the ways in which that physics opens itself—inevitably and irrevocably—to philosophical inquiry. Indeed, certain passages almost serve as tasty amuse-bouches for the entire field of philosophy of science, and in that way Baggott’s book achieves broader appeal than its competitors.

The necessity of engaging with philosophy during the scientific process is nicely illustrated by the book’s central metaphor: scientific theorizing as a ship navigating the treacherous waters between Charybdis, representing the shores of metaphysical reality, and Scylla, the isle of empirical reality. How one chooses to navigate that metaphorical strait says much about which interpretation of quantum mechanics one finds particularly appealing or explanatory.

Part 2 introduces the different interpretations of quantum mechanics. Baggott groups the interpretations with respect to whether their adherents consider quantum mechanics to be complete. He discusses the interpretations in relation to the sailing metaphor and measures them against several neatly articulated “realist propositions.” Which of those propositions one is inclined to take as axiomatic and how one answers the completeness question will indicate which interpretation—or class thereof—one finds most appealing. That is a new way of tackling a very old question, and it helpfully foregrounds the costs and benefits of kindred approaches vis-à-vis the twin criteria of realism and completeness.

Among the less realist views Baggott discusses are relational interpretations, consistent (or decoherent) histories, and information-theoretic views like quantum Bayesianism. Those are important approaches typically left out of popular accounts. The chapters progress toward increasingly realist views and focus on “completion attempts” like Bohmian mechanics, spontaneous-collapse theories, and even views incorporating the agency or consciousness of the observer. Everettian approaches get the last word, but somewhere in the mix Baggott takes time to discuss pivotal aspects of the interpretational debate that are too often neglected, including the role of decoherence, the differing types of probabilities at work in various views, and the array of interpretations fitting under Hugh Everett III’s umbrella.

I also appreciate the seriousness with which Baggott both motivates and explains less realist approaches. (I cringe at the term “antirealist.”) For although physicists have little patience for discussing such views and are quick to appeal to a comfortable realist stance, I suspect information-theoretic and relational views do a lot more work than they receive credit for and provide particularly useful ways of thinking about relativistic quantum theories—precisely the arena where the more familiar, more realist interpretations flounder.

The moments in Quantum Reality covering well-trodden terrain are relatively minimal. Unfortunately, they tend to be the same areas where popular accounts frequently go furthest astray: sections in which authors who are not credentialed in history nevertheless present as factual largely anecdotal narratives concerning the theory’s rich (and richly documented) historical and philosophical background. For example, Baggott—like many—continues to talk about the “Copenhagen interpretation.” But by now it should be well known that that approach was a postwar invention by Werner Heisenberg created for the express purpose of restoring his name and reputation after his dubious wartime activities.

Heisenberg reconstructed a history in which his early views aligned more closely with those of Niels Bohr, who, by contrast, emerged from the war as a scientist hero. Not only did Heisenberg, Bohr, and others disagree on key interpretational issues in those early years, but the points on which they did agree directly contradict two supposed features of the Copenhagen interpretation: the physical collapse of wavefunctions and the distinction between the classical and quantum domains. Bohr and Heisenberg—along with Grete Hermann, an early philosopher of quantum physics—were quite clear in their pre–World War II lectures, papers, and correspondence that an actual collapse cannot occur lest certain physical problems arise: A mechanism that instantaneously localizes a potentially broadly spatiotemporally diffused wavepacket, for example, will necessarily involve superluminal processes.

Heisenberg, Bohr, and other supposed Copenhageners were also adamant that there is no distinct classical world. Although at a certain scale one switches to classical mechanics for ease of calculation, events and apparatuses at those scales are still more accurately described with quantum mechanics. And the reverse is impossible: There is a point at which classical mechanics breaks down. It was thus widely appreciated by the early 1930s that certain assumptions in classical theories, such as the simultaneous testability of the full state of a system in complete isolation, were revealed by quantum mechanics to be in principle untenable.

Happily, those problematic bits are outshined by the aspects of Quantum Reality that set it apart from similar projects. In those moments, Baggott’s unique, smart-alecky-professor voice keeps you turning the pages, and you regret that the book wasn’t around when you were a precocious teenager grappling with the mysteries of physics.

Elise Crull is an associate professor of philosophy at the City College of New York and the Graduate Center of the City University of New York. She is a coeditor of the book Grete Hermann—Between Physics and Philosophy (2016) and a coauthor of the forthcoming book The “Einstein Paradox”: Debates on Nonlocality and Incompleteness in 1935.