A decade ago, Columbia University professor Brian Greene joined the ranks of an unlikely set of literary figures—physicists working in arcane and hyperspecialized fields who managed to transmute the base metal of mathematical theorems and conjectures into best-seller gold. Stephen Hawking, once known primarily for showing that black holes emit radiation, had lit the path with his 1988 book A Brief History of Time. The next year, eminent mathematician Roger Penrose mused on quantum theory, computation, and consciousness in The Emperor's New Mind. The success of these books was something of a surprise. Both Penrose and Hawking held positions high in the priesthood of science, but neither wrote the sparkling prose that might have made such difficult material understandable.
When Greene published The Elegant Universe, in 1999, he seemed to be toiling in still less fertile territory than Hawking or Penrose, if only because of the abstruseness of his specialty: string theory. Even among physicists, string theory has had a reputation for being out there; its harshest critics argue that the field is so divorced from everyday reality that it doesn't deserve to be labeled science. The real promise of string theory is that its mathematical language might help to fulfill the ultimate dream of physics: a single, consistent mathematical framework that describes all the matter and energy in the universe. Though string theory is not the only candidate for such a "theory of everything," for the past few decades it has arguably been the most credible one.
A Rhodes scholar who used his time at Oxford to earn a doctorate in physics, Greene first became known in the scientific community as an expert on the peculiar geometries of the higher-dimensional spaces that strings inhabit. Though he didn't have the stature of a Hawking or a Penrose, the enormous success of The Elegant Universe, which introduced the subject to a lay audience and was promptly turned into a PBS series, allowed Greene to position himself as the unofficial spokesman for string theory. His sequel, The Fabric of the Cosmos (2004), delved deeper into questions about the nature of space and plumped for a string-theory revolution, even as the theory itself was facing an increasingly stiff challenge from the physics community. Greene's latest, The Hidden Reality, answers the challenge and champions the string-theoretic cause by setting out arguments for a seemingly ridiculous concept: that our universe is one among many.
String theory is a tough subject for a pop-sci book. As Greene explained in The Elegant Universe, at its heart string theory posits that we inhabit a cosmos woven from tiny strings—or membranes, if you look at them in a slightly different manner. Every particle in the universe, every mote of matter that makes up our galaxy, every beam of light—all are composed of strings so small that we couldn't measure them even with a particle accelerator the size of the solar system. But the detail that would seem most unsettling to the lay reader is that these strings inhabit not the three- or four-dimensional universe we're familiar with, but instead a gnarled tapestry that warps and wefts in ten- or eleven-dimensional space. This seems hardly the stuff of an engaging book, but Greene came on the scene at a time when string theory was riding high on a set of mathematical breakthroughs. Bubbling with enthusiasm, he was able to transmit his community's excitement to his audience. Even if some readers didn't really understand the finer points of his arguments, Greene's confidence and facility with language cemented string theory's reputation as the ne plus ultra of cutting-edge physics—and Greene's reputation as its prophet.
However, behind the scenes, string theory was tangled up in a problem with the extra, imperceptible dimensions these strings live on. In the 1980s, scientists figured out that the mathematical properties of these dimensions imply that they themselves reside in tiny, curled-up, multidimensional shapes known as Calabi-Yau manifolds. The geometry of those shapes, in turn, determines the physical properties of the strings; in essence, the shape of the manifold dictates the precise nature of the universe. However, there were manifold manifolds that fit—meaning that the framework of string theory was describing lots and lots of different universes, each with its own Calabi-Yau manifold. String theory wasn't really a single theory at all, but a tremendous set of theories tailored for myriad cosmoses, only one of which we live in. This was the germ of what became known as the landscape problem.
The larger the landscape of alternate universes that string theory describes, the more bizarre the consequences of accepting it. In the late 1990s, when Greene wrote The Elegant Universe, string theorists were dimly aware of the problem this landscape of universes posed, but had little idea how serious it was. At the time, they thought that there were only tens of thousands of Calabi-Yau manifolds and thus universes. In mathematical terms, this many alternative universes wasn't a huge number, and Greene didn't worry too much about the consequences. "Although belonging to a club with tens of thousands of members might not sound very exclusive, you must compare it with the infinite number of shapes that are mathematically possible; by this measure Calabi-Yau spaces are rare indeed," Greene wrote.
But this was a vast mistake—the string community had unwittingly made a much more extreme underestimate than, say, calculating that the ocean contains a thimbleful of water. Physicists discovered in the early 2000s, much to their surprise, that there are at least 10⁵⁰⁰ Calabi-Yau manifolds out there. This is an ungodly—and unphysical—number. If you were to stick one hundred thousand manifolds on every single particle in the universe, you wouldn't even make a dent in the catalogue. String theory was describing an unimaginably large, and perhaps even infinite, number of universes. In the mid-2000s, critics, such as physicist Lee Smolin, attacked string theory, arguing that it had become so all-encompassing, so accepting of the enormous landscape of fictional universes, that it had completely lost whatever tenuous connection with physical reality it had once had. Instead of a "theory of everything," string theory had become a "theory of anything" and thus impervious to falsification. No matter what experiments might show, Smolin wrote in The Trouble with Physics (2006), "string theory cannot be disproved."
The critics, however, were unable to dent Greene's faith. If anything, his enthusiasm has grown stronger over the years. In The Hidden Reality, Greene answers naysayers by turning their most damning evidence against string theory into an asset. The panoply of universes described by string theory, argues Greene, isn't a failure of an overbroad mathematical framework. Instead, string theory is, in fact, tapping into a mind-blowing truth: that our cosmos is just one of a nearly uncountable panoply of cosmoses—that we inhabit a "multiverse" rather than a single universe.
If this seems like a drastic solution to the landscape problem, it is. This is not an elegant universe; it's a byzantine mess with enormous philosophical implications. For example, the inhabitant of a multiverse is shadowed by countless doppelgängers identical to her in every possible way, as well as infinite others who are subtly and bizarrely different. For example, there would be a copy of you reading this review in Fookborum right now—and stumbling across this sentence would cause you to scratch your head in amazement with your prehensile tail. "You might argue that the bizarre nature of where we've gotten—infinite copies of you and everyone and everything—is evidence of the faulty nature of one or more of the assumptions that led us here," Greene writes. Even though the consequences are indeed bizarre, he is rightly able to draw on the support of a large number of scientists who are now being driven to the same conclusion for reasons that have nothing to do with string theory. The most commonly accepted versions of the physical processes that took hold shortly after the big bang, for example, lead to the widely held belief that we live in one of countless bubble universes that are floating in an infinite cosmic plenum. As Greene writes, multiverses are an almost inevitable conclusion of our current understanding of the laws of physics.
And once you become willing to take on the philosophical baggage of a multifoliate universe (and aren't bothered by your countless identical twins), some of the deepest and most vexing problems about physics become easy to understand. All those nonsensical-seeming quantum-mechanical laws—that a particle can be in two places at once, that two objects can have a spooky connection that appears to transcend the laws governing space and time—instantly become explicable the moment you view our universe as one among many. And from Greene's point of view, the 10⁵⁰⁰ different cosmoses described by string theory have ceased to be an unwanted artifact of the theory's equations, instead becoming a factual description of universes that actually exist. Each of these universes is a bubble cosmos with its own cosmological constants, and as he says, "with some 10⁵⁰⁰ possibilities awaiting exploration, the consensus is that our universe has a home somewhere in the landscape." Which is to say, string theory can no longer be accused of describing a landscape of fictional universes; our universe is just one in a collection of cosmoses as real as our own, even if we're unable to see them.
Multiversism is a radical, ambitious, and frustrating argument that relies on many lines of evidence and modes of thought—cosmological reasoning about the nature of the big bang, quantum-mechanical reasoning about the nature of matter on the smallest scale, information-theoretic reasoning about the nature of black holes—and it can be bewildering. Furthermore, Greene argues for nine distinct varieties of multiverse, each of which approaches the issue from a slightly different direction. And since the majority of his readers are untutored in the mathematical formalism that physicists use to understand the underpinnings of a scientific theory, Greene must use the much less precise tools of metaphor and simile to do the intellectual heavy lifting. Unfortunately, he seems to confuse popularization with pop-culturization, repeatedly inserting gratuitous and homely allusions to television characters in an attempt to bring the cosmic down to earth. For example, he explains elements of inflationary cosmology with an image of South Park's Eric Cartman rolling down a snow-covered mountain. When Greene conjures Blue Man Group, he isn't helping his readers understand the famous "no hair" theorems about black holes (like the Blue Men, black holes have no external characteristics to tell them apart). Instead, his allusions obscure the material and, even worse, threaten to plunge fascinating and profound ideas into bathos.
But perfect clarity isn't the goal of The Hidden Reality. In fact, Greene encourages his audience to skip some of the more challenging portions of the text. The proper measure of success for one of Greene's books doesn't lie in the elegance of its prose, how well it teaches readers new physics, or even its novelty. (In the past decade, several authors less popular than Greene, including David Deutsch and Alex Vilenkin, have covered almost all of the material in The Hidden Reality.) Greene is admired because of his enthusiasm for a field that's perceived to be one of the most abstract and difficult in the entire range of human knowledge, and because he's willing to share that enthusiasm with a lay audience. Greene, like Hawking and Penrose before him, is an author who writes with the confidence and authority of one who has climbed Mount Nebo and has seen the promised land of cosmic truth. Readers flock to him not for understanding, but to be in the audience of a prophet.
Charles Seife's most recent book is Proofiness: The Dark Arts of Mathematical Deception (Viking, 2010).