Midway through his new book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos, Brian Greene pauses to consider what he calls “the soul of science.” That phrase (also a chapter subheading) is the clearest signal yet that this is no ordinary work of popular physics. It is, of course, an extraordinary example of that genre, thanks to the detailed analogies, careful contextualizing, and friendly, encouraging voice that Greene is known for. But The Hidden Reality, we begin to realize, is also a manifesto for a particular conception of science—one in which the possibility of other universes is worth investigating to the fullest, even if we can never experimentally detect, let alone visit, those realms. Not everyone agrees, to put it mildly. Up to this point in the book, Greene’s been showing us why the existence of multiple universes is anything but wild speculation. Over decades, researchers in the theories of relativity, quantum mechanics, and superstrings have repeatedly found their calculations pointing toward a perhaps infinite number of universes. The “multiverse” comes in a variety of configurations, depending on how one arrives at it. Assuming our own universe is infinite, for example, gives us the “Quilted Multiverse,” as conditions in an infinite space-time expanse will inevitably repeat themselves. The “Inflationary Multiverse” posits an eternally expanding universe, which in turn produces an infinite network of “bubble” universes. The “Brane Multiverse,” derived from string theory, suggests our universe exists on a three-dimensional “membrane,” one of many occupying a higher-dimensional expanse. The “Landscape Multiverse” combines string theory and inflation to give us bubble universes in many dimensions. There are other versions, each more jaw-dropping than the last. The “Holographic Multiverse,” in which worlds are, essentially, projections of fundamental laws existing elsewhere, is probably the most unsettling. The Matrix (though, one hopes, in a less gruesome form) might just be real. “Might,” however, is the operative word. As of now, there’s no experimental confirmation for any of these propositions. In fact, direct confirmation of some aspects may never be possible. Yet the mathematics tells us (or tells Greene, who tells us) that this mind-blowing notion is likely to be true. All of which leads to a fundamental question: “Is this science?” Attempts to come to an answer have turned the multiverse into “a battleground for the very soul of science.” Is it scientifically justifiable to speak of a multiverse, an approach that invokes realms inaccessible not just in practice but, in many cases, even in principle? Is the notion of a multiverse testable or falsifiable? Can invoking a multiverse provide explanatory power of which we’d otherwise be deprived? If the answer to these questions is no, as detractors insist is the case, then multiverse proponents are assuming an unusual stance. Nontestable, nonfalsifiable proposals, invoking hidden realms beyond our capacity to access – these seem a far cry from what most of us would call science. And therein lies the spark that makes passions flare. Greene, a professor of physics and mathematics at Columbia, is well versed in defending the scientific pursuit of the inaccessible. He is an influential contributor to string theory, a vastly ambitious attempt to find a unified explanation of the cosmos; The Hidden Reality includes a helpful overview of the theory’s progress according to various measures. Mathematically, it has succeeded in bringing together the previously inimical theories of relativity (which governs the very large, e.g. stars and galaxies) and quantum mechanics (whose purview is the very small, e.g. quarks and gluons). But in its 30-some years of existence—including the 12 years since Greene’s bestselling first book on the subject, The Elegant Universe—string theory has yet to yield a prediction that can be tested experimentally. This may well change with the Large Hadron Collider in Geneva, which has had some famous problems of its own, but some predictions will still lie beyond its capacity. That’s why many scientists have come out vociferously against string theory, and the multiverse(s) to which it gives rise—going so far as to compare the theory to a religion. Greene spends the rest of the central chapter answering specific objections to the multiverse as legitimate science. However, his overall point, here and throughout the book, is that you never know where the pursuit of any mystery is going to take you. “[Q]uestions aren’t floating in some preexisting realm in which the role of science is to pick them off one by one. Instead, today’s questions are very often shaped by yesterday’s insights. Breakthroughs generally answer some questions but then give rise to a host of others that previously could not even have been imagined.” In other words, questions—all kinds of questions—are good. Questions that beget more questions, especially about the nature of science itself, are even better. If this isn’t science, Greene seems to say, I don’t want to be a scientist. Still, it’s audacious to invoke a “soul” to make this particular case. In doing so, Greene is obviously tweaking those who call his approach religious. He’s confident enough in his own view of science’s mandate—to follow mysteries wherever they may lead—that he can borrow an especially nebulous religious term to characterize it. As a scientist he rejects the substitution of faith for rigorous method, the hallmark of religion in general; but his use of “soul” is not merely ironic. Rather, he’s reminding us that science is first and last a human endeavor. And the most human endeavors of all are those in which our reach exceeds our grasp. Let me now say that by this definition, I am being very human in undertaking this review. Though I’m enthralled by the many possible multiverses Greene describes, as I believe you will be, I have nowhere near the training to evaluate their scientific merits. I’m a writer, reader, and erstwhile scholar of literature—what is sometimes called a humanist. But this does qualify me, I think, to state that the humanity of The Hidden Reality is its most compelling aspect. In his 2004 book, The Fabric of the Cosmos, Greene describes a game he and his father played when he was little: One of us would look around, secretly fix on something that was happening—a bus rushing by, a pigeon landing on a windowsill, a man accidentally dropping a coin—and describe how it would look from an unusual perspective such as the wheel of the bus, the pigeon in flight, or the quarter falling earthward. The challenge was to take an unfamiliar description like “I’m walking on a dark, cylindrical surface surrounded by low, textured walls, and an unruly bunch of thick white tendrils is descending from the sky,” and figure out that it was the view of an ant walking on a hot dog that a street vendor was garnishing with sauerkraut. What a great game this is to play with a kid—or an adult, for that matter. It obviously affected Greene profoundly, encouraging him to consider the everyday world from radically different perspectives. It also, I suspect, helped make him a writer. The challenge he undertakes in all his books is to explain something very familiar to him—string theory, the probable existence of other universes—to someone completely unfamiliar with those concepts. It even seems to me (all right, this is a leap) that the voice in these books is some version of the father’s voice in this game. It’s enthusiastic, often playful, encouraging, and never condescending. It’s the way Greene was introduced to scientific thinking, and he wants us, whatever our age or background, to have that same experience. Greene never forgets that as writer and reader, we are in this thing together. In The Hidden Reality, he makes frequent asides like this one (regarding string theory’s many-dimensional “braneworld”): “You may find it difficult to picture this. I certainly do.” Or: “I’ll now explain [the] final step, but in case you’re reaching saturation and just want the punch line, here’s a three-sentence summary.” This is not just a scientist explaining complicated ideas to a lay person, but a human being talking to another, through the rather marvelous medium of the printed word. It’s true that humans are limited; we get tired and confused, and our minds are hopelessly mired in three dimensions. Yet by considering these theoretical accounts of parallel universes, we prove to ourselves that, like our universe, we’re not as limited as we might think. It may turn out that the multiverse is the true story of reality. But if it’s proven wrong, or its realms are permanently inaccessible, it may be a species of fiction. Still, let’s not call it “mere” fiction. As humanists know, great fiction builds new worlds that help us ask new questions. It shakes us up, making us rethink the world we build for ourselves every day—which is to say, our assumptions. It makes us pause, not because we’ve reached saturation, but in wonder.