Next year, NASA’s Europa Clipper will travel 1.8 billion miles to Jupiter’s icy Galilean moon. Engraved on the spacecraft will be a poem by U.S. Poet Laureate Ada Limón called “In Praise of Mystery: A Poem for Europa.” It may seem ironic, emblazoning a vessel on a fact-finding mission to outer space with an ode to mystery. Yet the vast puzzle of space remains exactly that. “I like a universe that includes much that is unknown,” the astronomer Carl Sagan wrote in a 1979 essay, “and, at the same time, much that is knowable.” This tension lies at the heart of all the sciences—perhaps, especially, the science of love. Since the 1980s, the study of romantic love and attraction has coalesced into a formal discipline. The interdisciplinary field of relationship science—which encompasses neuroscience, anthropology, psychology, and evolutionary biology—is currently experiencing a boom: A search of the National Library of Medicine’s PubMed database reveals that more than half of the papers written about romantic love since 1953 are from the past 10 years. Today, the findings …