In his famous *Lectures on Physics*, Richard Feynman argued that nothing more is needed to get a solid grasp of the behavior of quantum objects than the simple double-slit experiment, in which electrons or photons are fired toward two thin openings cut in a screen. To Feynman, the double-slit experiment encapsulated quantum physics’s one and only mystery. Its results could be described but, he cautioned, could not be explained.

Despite Feynman’s warning, the past 60 years have seen an explosion of interpretations appealing to devices as diverse as pilot waves and parallel universes in the hope of elucidating quantum behavior. So far, no interpretation has proven fully convincing, leading many physicists to conclude that the theory’s mathematical formalism should be left uninterpreted and to demand a return to the “shut up and calculate” attitude that was prevalent among the physicists of Feynman’s generation.

Veteran science journalist Anil Ananthaswamy rejects this fatalistic perspective. On the contrary, he argues, a deeper understanding of the quantum world can only be achieved by embracing the diversity of interpretations available to us, a claim he persuasively defends in *Through Two Doors at Once*. In the book, he takes Feynman to task by offering a spirited introduction to the various quantum interpretations, examining their respective explanations of the supposedly inscrutable experiment.

Ananthaswamy starts his investigation with a description of the double-slit experiment that is so natural and elegant that one may forget that it took physicists close to 30 years to develop the mathematical framework needed to describe it adequately. He rapidly recaps this struggle, from Planck’s original suggestion that energy might sometimes be quantized to the Bohr-Einstein debates of the 1930s. Here, *Through Two Doors at Once* offers little more than the usual narrative, apart from an amusing detour through G. I. Taylor’s nonchalant approach to an early one-slit version of the experiment.

It is surprising to learn that the double-slit experiment played a minor role in the early development of quantum theory—that is, until Ananthaswamy explains that it was not performed in the laboratory until the 1960s. Until then, it was only a thought experiment.

Feynman might have obtained ideal data by imagining himself firing at the two fictive slits with a futuristic tungsten electron gun, but in real life, physicists wrestled for decades—relying on everything from spider silk to ingenious beamsplitters—to bring the experiment to life. But this is where the many iterations of the double-slit experiment really take center stage in the development of our understanding of quantum reality. Technological advances, we learn, prompted physicists to conduct ever-more-sophisticated versions of the experiment, which in turn fueled a greater variety of interpretations. This increased the need for ever-more-sophisticated experiments.

Step by step, *Through Two Doors at Once* reveals how physicists transformed, retooled, and repurposed the original double-slit setup to throw light on the fundamental principles of quantum physics. Each iteration of the experiment is, for Ananthaswamy, an opportunity to introduce readers to fundamental concepts, such as entanglement; to analyze iconic experiments, such as Aspect’s test of Bell’s inequalities; or more importantly, to examine the most prominent interpretations of quantum physics, from the Copenhagen interpretation of the 1920s to the more recent “many interacting worlds” hypothesis.

Ananthaswamy’s introduction of increasingly complex versions of the slit experiment proves extremely effective. Halfway through the book, even neophytes will likely find predicting the outcome of the delayed-choice quantum eraser experiment barely harder than figuring out the motions of a gear train. This approach also brings to the forefront the strengths and weaknesses of various interpretations, offering a perfectly balanced overview of each.

But Ananthaswamy carefully guards himself from offering any guiding principle that might help us decide which explanation is the best one. There is, he explains, no such thing as the “right” interpretation in good science. This does not mean, however, that we have to be mere instrumentalists and reject interpretations as misguiding fantasies. We have another, better option: We can decide to embrace the diversity of interpretations at our disposal because despite their respective flaws, each likely holds the key to at least one essential aspect of quantum behavior.

*Through Two Doors at Once* offers beginners the tools they need to seriously engage with the philosophical questions that likely drew them to quantum mechanics. But readers will also receive a more important lesson, one that Feynman would have approved: In science, a deep understanding is not achieved by limiting ourselves to a single perspective but by simultaneously exploring competing conceptions of reality.

#### About the author

*The reviewer is at the History of Science and Technology Program, University of King’s College, Halifax, NS B3H 2A1, Canada.*