Across the Megaverse
Physicists are not like ordinary people, and string theorists are not like ordinary physicists. Even compared with their peers, crafters of the arcane model of reality that is string theory think in terms of sweeping explanations of nature's design. Leonard Susskind, a founder of the theory and one of its leading practitioners, brazenly lays out this no-boundaries attitude on the first page of his new book. His research, he declares, "touches not only on current paradigm shifts in physics and cosmology, but also on the profound cultural questions that are rocking our social and political landscape: can science explain the extraordinary fact that the universe appears to be uncannily, nay, spectacularly, well designed for our own existence?"
What troubles Susskind is an intelligent design argument considerably more vexing than the anti-evolution grumblings recently on trial in Dover, Pa. Biologists can point to unambiguous evidence that evolution truly does happen and that it can account for many otherwise inexplicable aspects of how organisms function. For those who take a more cosmic perspective, however, the appearance of design is not so simply refuted. If gravity were slightly stronger than it is, for instance, stars would burn out quickly and collapse into black holes; if gravity were a touch weaker, stars would never have formed in the first place. The same holds true for pretty much every fundamental property of the forces and particles that make up the universe. Change any one of them and life would not be possible. To the creationist, this cosmic comity is evidence of the glory of God. To the scientist, it is an embarrassing reminder of our ignorance about the origin of physical law.
Until recently, most physicists took it on faith that as they refined their theories and upgraded their experiments they would eventually expose a set of underlying rules requiring the universe to be this way and this way only. In "A Brief History of Time," Stephen Hawking recalled Albert Einstein's question "How much choice did God have in constructing the universe?" before replying that, judging from the latest ideas in physics, God "had no freedom at all." Like many leading physicists at the time, Hawking believed that scientists were closing in on nature's essential rules - the ones that even God must obey - and that string theory was leading them on a likely path to enlightenment.
Although string theory resists translation into ordinary language, its central conceit boils down to this: All the different particles and forces in the universe are composed of wriggling strands of energy whose properties depend solely on the mode of their vibration. Understand the properties of those strands, the thinking once went, and you will understand why the universe is the way it is. Recent work, most notably by Joseph Polchinski of the University of California, Santa Barbara, has dashed that hope. The latest version of string theory (now rechristened M-theory for reasons that even the founder of M-theory cannot explain) does not yield a single model of physics. Rather, it yields a gargantuan number of models: about 10500, give or take a few trillion.
Not one to despair over lemons, Susskind finds lemonade in that insane-sounding result. He proposes that those 10500 possibilities represent not a flaw in string theory but a profound insight into the nature of reality. Each potential model, he suggests, corresponds to an actual place - another universe as real as our own. In the spirit of kooky science and good science fiction, he coins new names to go with these new possibilities. He calls the enormous range of environments governed by all the possible laws of physics the "Landscape." The near-infinite collection of pocket universes described by those various laws becomes the "megaverse."
Susskind eagerly embraces the megaverse interpretation because it offers a way to blow right through the intelligent design challenge. If every type of universe exists, there is no need to invoke God (or an unknown master theory of physics) to explain why one of them ended up like ours. Furthermore, it is inevitable that we would find ourselves in a universe well suited to life, since life can arise only in those types of universes. This circular-sounding argument - that the universe we inhabit is fine-tuned for human biology because otherwise we would not be here to see it - is known as the Anthropic Principle and is reviled by many cosmologists as a piece of vacuous sophistry. But if ours is just one of a near-infinite variety of universes, the Anthropic Principle starts to sound more reasonable, akin to saying that we find ourselves on Earth rather than on Jupiter because Earth has the mild temperatures and liquid water needed for our kind of life.
Although Susskind's title and central motivation are drawn from this fascinating debate over design, most of "The Cosmic Landscape" is structured not around philosophy but around the nuts-and-bolts concepts of modern particle physics. Here Susskind's long years as a theorist and lecturer at Stanford University prove a mixed blessing. He is a good-humored and enthusiastic tour guide but he clearly does not know how baffling he sounds much of the time. He coaxes the reader along with rhetorical questions and charmingly corny allegories. Still, this isn't much help when it comes to material like "Let's suppose that the Calabi Yau manifold has a topology that is rich enough to allow 500 distinct doughnut holes through which the fluxes wind. The flux through each hole must be an integer, so a string of 500 integers has to be specified." Um, is this going to be on the exam?
Susskind's insider perspective also lends an air of smugness to the whole affair. He falls prey to the common error of Whig history: interpreting past events as if they were inevitable stepping stones to the present. He allows remarkably little doubt about string theory considering that it has, as yet, not a whit of observational support. "As much as I would very much like to balance things by explaining the opposing side, I simply can't find that other side," he writes in his concluding chapter.
Such braggadocio begs for an anthropic question of its own. Humans have been around in more or less their present form for about 150,000 years; detailed stories of the origin of the world run as far back as the first written languages and surely existed in oral form much earlier still. How likely is it that this generation, right now, is the lucky one that has discovered the final answer?
I'm not a physicist, but if I were putting money on the table, I wouldn't take those odds.
'The Cosmic Landscape'
http://www.nytimes.com/2006/01/15/books/chapters/0115-1st-sussk.html?pagewanted=all
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