Strings can account for all the known physical forces: gravity, electromagnetism and the strong and weak nuclear forces, which cause radioactive decay. No other hypothesis can claim as much. But to work their magic, strings must squirm inside additional dimensions of space beyond the three we can verify with our own eyes. These extra dimensions would be like tiny pockets in space, far too small to observe in any conceivable experiment. That in itself isn't disastrous; some indirect measurement could conceivably reveal the emanations of strings.
A bigger problem was made plain in recent years. In 1998 astronomers discovered an enigmatic "dark" energy pushing galaxies apart at an accelerating pace. (Before that they had assumed the spread of galaxies was decelerating.) Researchers suspect the phenomenon arises from quantum effects at work throughout spacetime, creating a sort of repulsive gravity. Simple calculations suggested this energy should be enormous--a clear sign that older ideas were not working when quantum and gravity mixed. Ideally, string theory should account for why dark energy is so much weaker than it could be. The only explanation string theory proponents have come up with is an unpalatable one to many physicists: assume that string theory is capable of describing an astronomical number of different universes, each with its own dark energy, and note that one of those universes is bound to look like ours. In other words, string theory "predicts" that dark energy can be anything you like--not exactly the kind statement that would win over doubters.
Some string theorists, taking their cue from Leonard Susskind of Stanford University, argue that these manifold universes of string theory may coexist, evolving from one to another in a way that now and then produces a universe like ours. Susskind has dubbed this concept the anthropic landscape of string theory.
Skeptics see the landscape as an abandonment of centuries-old scientific practice, in which a successful theory is one that ultimately describes only one universe: the one we see around us. In their eyes, string theorists are in the undesirable position of having to change the rules of science to make their theory work. Angst over the landscape has spilled from the academic world of preprints and colloquia into blogs, a number of recent popular science books, and even the pages of the New York Times Book Review. "I have been shocked to see the unwillingness of particle theorists to abandon string theory," says Peter Woit, a mathematical physicist at Columbia University. Woit maintains a blog called Not Even Wrong that is devoted to criticizing string theory and recently published a book by the same name. "The landscape spelled the end of string theory as a unified theory of particle physics," he asserts.
Not all string theorists are enthusiastic about the landscape either. "It's not that I find it implausible. It just doesn't feel like a deep explanation," says Brian Greene of Columbia University, author of The Elegant Universe. "I hold the ideal that we'll be able to explain things from a deeper place." Quantum physics took decades to hash out, he notes, and its pioneers had the benefit of actual experimental results to guide them on their way.
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