In 1970, a young physicist named Leonard Susskind got stuck in an elevator with Murray Gell-Mann, one of physics' top theoreticians, who asked him what he was working on. Susskind said he was working on a theory that represented particles "as some kind of elastic string, like a rubber band." Gell-Mann responded with loud, derisive laughter.
Within a few years, however, many physicists saw string theory as a promising line of research (and Gell-Mann had apologized to Susskind, one of the theory's co-founders). String theory -- which posited the existence of unimaginably tiny, vibrating strands of energy -- evolved into "superstring theory" and then "M theory" (and expanded to include not just strings but wider "membranes"). String theory, broadly defined, became and remains the most prominent candidate to unify the physical world's diverse particles and forces into a single mathematical framework.
Susskind, now a professor at Stanford University, has written a book that is certain to stir up controversy, not only about the merits of string theory but over the charged question of whether the natural world displays evidence of an intelligent designer. The Cosmic Landscape: String Theory and the Illusion of Intelligent Design (Little, Brown) provides a valuable explanation and history of string theory, and has much of interest to say about cutting-edge physics and cosmology. However, Susskind's conclusions, including on matters of intelligent design, are debatable.
The variety of intelligent design with which Susskind is concerned consists of claims that the laws of physics were deliberately fine-tuned for the benefit of life. For instance, such arguments note that nuclear reactions in stars are well-suited to produce the carbon on which Earth life depends. Many such examples are quite ambiguous. (The "tuning" of carbon actually may not be that "fine"; the element could exist in a range of scenarios.) One feature of the universe, however, does appear to be strikingly fine-tuned; commonly referred to as the "cosmological constant," it is an energy level that infuses empty space. This energy level is extremely small -- which is fortunate because if it were even slightly larger, the universe would have expanded too rapidly to form galaxies, stars and planets.
String theory, according to Susskind, presents a compelling explanation of why the cosmological constant is so small, without invoking an intelligent designer. The answer lies in what Susskind calls "the Landscape," which is the set of all possible universes that are compatible with string theory. The Landscape can be thought of as having various locations, corresponding to different values of the cosmological constant and other parameters. In Susskind's estimate, the Landscape contains 10500 types of possible universes -- a stupendously large number far bigger than a googol (which is 10100).
According to Susskind, we live in a "megaverse" (also known as a "multiverse"). The Landscape, as he puts it, is "populated." That is, the universe we observe is just one pocket of a far larger cosmos, in which different pockets exhibit the highly diverse conditions possible under string theory. As space expands, such pockets proliferate as bubbles, an implication of the theory of cosmic inflation (which states that our own universe expanded extremely rapidly in its earliest moments). There are innumerable uninhabited bubbles, but some patches of the megaverse have life-friendly features such as a tiny cosmological constant; not surprisingly, we live in such a place.
Susskind may be right. However, he exhibits a confidence in his position that seems unwarranted, given the speculative nature of the material. For one thing, nobody knows if string theory is true. It does not, as yet, have experimental or observational evidence to support it. Rather, string theory has gained the enthusiasm of many physicists because of the intriguing nature of its mathematics; string theory offers a way to reconcile general relativity, which describes gravity and large objects, with quantum mechanics, the physics of the extremely small. Oddly, though, string theory requires extra dimensions of space (which are assumed to be unseen because they are very small or "compactified").
Some scientists regard string theory as an unjustified and over-hyped speculation. Peter Woit, who teaches mathematics at Columbia University, has a blog and upcoming book criticizing string theory as "Not Even Wrong." Moreover, contrary to longstanding hopes, string theory has not provided a concise formula -- something like Einstein's E=mc2 -- giving a deep mathematical explanation for why the cosmos is as it is. Instead, string theory increasingly has seemed compatible with diverse universes. That's something celebrated by Susskind but disturbing to some of his fellow string theorists; and to critics such as Woit, it's a sign the theory makes no sense.
Furthermore, The Cosmic Landscape presents an overly stark dichotomy between string theory and intelligent design. A reader may come away from the book thinking that if string theory (as elaborated by Susskind) is wrong, the evidence points to fine-tuning by a supernatural agent. Not really. For one thing, cosmic inflation and other possible mechanisms for producing multiple universes are not dependent on string theory. For another, any claim of fine-tuning relies on enormous guesswork as to the types of life and types of universes that are possible. Nor for that matter does intelligent fine-tuning necessitate a supernatural entity; one highly speculative theory suggests the universe was fine-tuned by its own inhabitants.
On the flip side, some enthusiasts of intelligent design suggest that a scenario of multiple universes with varying parameters is itself suggestive of a designer. In this view, a fantastic proliferation of universes was the designer's way of producing one or more universes compatible with life. Such a method seems remarkably inefficient, but we have little basis for saying what a designer would or would not do. If excessive flexibility is a problem for string theory, it is an even greater problem for intelligent design.