Warped Passages - Lisa Randall [147]
Elsewhere, however, physicists were convinced that all of the questions about superstring theory would soon be solved, and that string theory was the physics of the future (and of the present). Superstring theory was in its early stages. Some believed that with enough man-hours devoted to it (and they were primarily man-hours), string theorists would ultimately derive known physics. In the 1985 paper about the heterotic string, Gross and his colleagues wrote, “Although much work remains to be done there seem to be no insuperable obstacles to deriving all of known physics from the…heterotic string.”* String theory promised to be the Theory of Everything. Princeton was in the vanguard of this effort. Physicists there were so certain that string theory was the road to the future that the department no longer contained any particle theorists who didn’t work on string theory—a mistake that Princeton has yet to correct.
Today, we can’t say whether or not the obstacles facing the theory are “insuperable,” but they are certainly challenging. Many major unanswered questions remain. Addressing the unresolved problems of string theory appears to require a mathematical apparatus or a fundamental new approach that goes well beyond the tools that physicists and mathematicians have so far developed.
Joe Polchinski, in his widely used string theory textbook, writes that “string theory may resemble the real world in its broad outline,”† and so it does in some respects. String theory can include the particles and forces of the Standard Model, and can be reduced to four dimensions when other dimensions are curled up. However, although there is tantalizing evidence that string theory could incorporate the Standard Model, the program for finding the ideal Standard Model candidate is nowhere near completion after twenty years of searching.
Physicists initially hoped that string theory would make a unique prediction for what the world should be like, one that would be borne out by the world that we see. But there are now many possible models that can arise in string theory, each containing different forces, different dimensions, and different combinations of particles. We want to find the set that corresponds to the visible universe and the reason that this set is special. Right now, no one knows how to choose among the possibilities. And in any case, none of them look quite right.
For example, Calabi-Yau compactification can determine the number of generations of elementary particles. One possibility is indeed the three generations of the Standard Model. But there is not a unique Calabi-Yau candidate. Although string theorists originally hoped that Calabi-Yau compactification would single out a preferred shape and unique physical laws, they were quickly disappointed. Andy Strominger described to me how within a week of discovering a Calabi-Yau compactification and thinking it was unique, his collaborator Gary Horowitz found several other candidates. Andy later learned from Yau that there were tens of thousands of Calabi-Yau candidates. We now know that string theories based on Calabi-Yau compactification can contain hundreds of generations. Which Calabi-Yau compactification, if any, is correct? And why? Even though we know that some of string theory’s dimensions must curl up or otherwise disappear, string theorists have yet to determine the principles that tell us the size and shape of the curled-up dimensions.
Moreover, in addition to the new heavy string particles arising from waves that oscillate many times along the string, string theory contains new low-mass particles. And we would expect that if they existed and were as light as string theory naively predicts, those particles would be visible to experiments in our world. Most models based on string theory contain many more light particles and forces than we observe at low energies, and it is not clear what singles out the right ones.
Getting string theory to match the real world is an enormously complicated problem. We have yet to learn why the