The most celebrated theory in modern physics faces increasing attacks from skeptics who fear it has lured a generation of researchers down an intellectual dead end.
In its original, simplified form, circa the mid-1980s, string theory held that reality consists of infinitesimally small, wiggling objects called strings, which vibrate in ways that yield the different subatomic particles that comprise the cosmos. An analogy is the vibrations on a violin string, which yield different musical notes.
Advocates claimed that string theory would smooth out the conflicts between Einsteinian relativity and quantum mechanics. The result, they said, would be a grand unifying "theory of everything," which could explain everything from the nature of matter to the Big Bang to the fate of the cosmos.
Over the years, string theory has simultaneously become more frustrating and fabulous. On the one hand, the original theory has become mind-bogglingly complex, one that posits an 11-dimensional universe (far more than the four- dimensional universe of Einstein). The modified theory is so mathematically dense that many Ph.D.-bearing physicists haven't a clue what their string- theorist colleagues are talking about.
On the other hand, new versions of the theory suggest our universe is just one of zillions of alternate, invisible -- perhaps even inhabited -- universes where the laws of physics are radically different. String buffs claim this bizarre hypothesis might help to explain various cosmic mysteries.
But skeptics suggest it's the latest sign of how string theorists, sometimes called "superstringers," try to colorfully camouflage the theory's flaws, like "a 50-year-old woman wearing way too much lipstick," jokes Robert B. Laughlin, a Nobel Prize-winning physicist at Stanford. "People have been changing string theory in wild ways because it has never worked."
Already, the split over string theory has caused tensions at some of the nation's university physics departments. "The physics department at Stanford effectively fissioned over this issue," said Laughlin, now on sabbatical in South Korea. "I think string theory is textbook 'post-modernism' (and) fueled by irresponsible expenditures of money."
The dispute could become explosive this year, with the publication of contrarily minded books by two of the best-known and most eloquent scientific popularizers of physics, string theorist Michio Kaku of City University of New York and astrophysicist-particle theorist Lawrence Krauss of Case Western Reserve University in Cleveland.
Skeptics have long mocked string theory as untestable, because experimental studies of it would require machines of huge scale, perhaps even as big as the solar system. In his new book "Parallel Worlds" (Doubleday), Kaku disagrees and argues that the first experimental evidence for string theory might begin to emerge within several years from experiments with scientific instruments such as a new particle accelerator, the Large Hadron Collider, which opens for business near Geneva in 2007.
Kaku, whose previous books include the acclaimed "Hyperspace" and equation-packed textbooks on string theory, also suggests that humans might eventually travel to those alternate universes, perhaps via hypothetical portals in space called wormholes.
Such claims dismay Krauss, a leading expert on cosmic dark matter and dark energy who is popularly known as author of a best-seller, "The Physics of Star Trek." In his book "Hiding in the Mirror: The Mysterious Allure of Extra Dimensions," to be published by Viking in September, Krauss argues that string theorists have produced no satisfactory explanations for anything.
Krauss believes continued research is worthwhile just in case it pans out. But he said that so far, string theorists have promised far more than they have delivered and have fostered the false impression that string theory is the only feasible way to explain cosmic mysteries.
Those who dabble in alternate-universe speculations might be just modern versions of "16th century theologians (who) speculated that spirits and angels emerge from the extra-dimensional universe," says Krauss, who is also an outspoken foe of creationist teaching in schools.
A great deal is at stake. Over the last two decades, a generation of brilliant young physicists -- the kinds of proto-Einsteins who historically have led intellectual revolution after revolution -- has flocked to string theory because their professors told them that's where the action was. Now many of them are reaching middle age and have gained tenured posts on prestigious campuses. They're also educating a whole new generation of fresh- faced wannabe string theorists who are thrilled by the publicity that string theory attracts, which has included several best-selling books and a special effects-packed TV extravaganza on PBS.
The dispute has split partly along subdisciplinary lines, and mirrors a timeless squabble in the philosophy of science: Which is more important for scientific innovation -- theoretical daring or empirical observations and experiments?
"Superstringers have now created a culture in physics departments that is openly disdainful of experiments. ... There is an intellectual struggle going on for the very soul of theoretical physics, and for the hearts and minds of young scientists entering our field," says physicist Zlatko Tesanovic of Johns Hopkins University.
String theorists and their foes can't even agree on what constitutes success or failure. For example, the most unexpected and counterintuitive discovery of recent science occurred in the 1990s, when astrophysicists at Berkeley and elsewhere realized the universe is expanding faster with time. The apparent reason: a mysterious dark energy pervades space and drives the accelerated expansion.
Critics mock superstringers because their so-called theory of everything failed to predict this colossal discovery. String theorists fire back that no one else predicted it, either, and besides, "string theory is the only approach that has the potential for explaining dark energy" based on pure theory, says John Schwarz, a pioneering string theorist at Caltech.
That's because string theory is the only existing hypothesis that holds serious promise of merging the two grandest branches of physics -- the theory of gravity, the basis of cosmological theory; and quantum mechanics, the science of the subatomic realm, Schwarz says.
Even so, "it's my impression that more and more physicists are starting to join Krauss as 'skeptical agnostics' about string theory," said mathematician Peter Woit of Columbia University, who offers comments on string- theory developments at his blog: www.math.columbia.edu/~woit/blog/.
One possible reason for the sea change is that string theory's ambitions have radically changed since the 1980s. Back then, theorists hoped to develop a string theory that would predict the existence of one universe -- ours, of course -- with its given physical forces and constants, such as the known intensity of gravity and the known electrical charge on the electron.
In later years, though, string theorists realized their theory predicted innumerable possible universes with widely varying physical forces and constants. As usual, superstringers and their critics viewed this development differently.
To critics, like Woit, it is a disaster for string theory because the sheer number of estimated universes -- equal to the number one followed by 500 zeroes -- is unimaginably large.
If true, it means that string theory is so flexible that it can be used to predict almost any kind of universe you want, no matter how crazy, and hence it predicts nothing specific enough to be scientifically interesting.
"A theory that can't predict anything is not a scientific theory," Woit says.
But what if the universe is unimaginably complex and as jammed with diverse universes as the seas are jammed with diverse fish? That's the thesis of Kaku, who compares the history of string research to "wandering around the desert and then stumbling on a tiny pebble. But when we examine it carefully, we find that it's actually the tip of a gigantic pyramid."
"But just as we are about to open the door," Kaku says, "some critics say that it's taking too much time, that the writings are too hard to understand, that (it) is draining resources from other projects, that it's getting too much publicity, that the script seems to be mutating as we go from floor to floor, et cetera, et cetera."
Opinions on the theory
In an informal Chronicle e-mail survey, the world's physicists expressed widely differing, sometimes emotional, opinions on the dispute over string theory:
-- "String theory is anything but a futile effort," said an e-mail from David Gross of UC Santa Barbara, who shared the Nobel Prize in physics last year. Among other accomplishments, it has enabled physicists "to understand, finally, many of the mysteries of black holes. ... I am convinced that string theory, as presently understood, is on the right path, but that this path is quite long, and (perhaps many) further breakthroughs are required."
-- "I agree entirely with Larry Krauss," says Nobel Prize-winning physicist Philip Anderson of Princeton University. In academia, "we from outside the (string) field are disturbed by our colleagues' insistence that every new semi-adolescent who has done something in string theory is the greatest genius since Einstein and therefore must occupy yet another tenure track. ... Our sciences are becoming increasingly infected with quasi-theology, a tendency which needs to be openly debated."
-- "To the considerable extent that string theory has been developed, it has turned out to be a logically consistent quantum theory of gravity," says string theorist Raphael Bousso of Lawrence Berkeley National Laboratory. "This is a very significant achievement utterly unmatched by any other approach to this problem -- and many have been tried over the past several decades."
-- "There has been, in recent years, a pernicious, uncritical hype of string theory," says Carlo Rovelli of the Centre de Physique Theorique in Marseille, France. While the theory is worth developing and is a "very interesting attempt to address the fundamental open problems of physics," he says, "so far it is only an attempt, (one) that has delivered less than what was expected some years ago," and "its uncritical promotion is damaging to science."
-- Krauss' charge that string theory "has probably been the least successful 'great' idea in physics" in a century is unfair and premature, replies string physicist Brian Greene of Columbia University, author of two acclaimed books on the topic, including "The Elegant Universe." "That's like someone going into Antonio Stradivari's workshop and complaining about the sound produced by one of his as yet unfinished violins."
Keay Davidson, Chronicle Science Writer