NEW YORK CITY—Amid a panel discussion about string theory and other candidates for the theory of everything—the long-sought system that would unify the four forces of physics—Brian Greene said something that sounded a bit curious. "If you asked me, 'Do I believe in string theory?'" began Greene, one of string theory's most famous proponents. "My answer today would be the same as it was 10 years ago: No."

Greene, a Columbia University physicist and author of the new book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos (Knopf, 2011), explained that he only believes ideas that make testable predictions—an area where string theory has fallen short. But that does not mean string theory is without value, Greene was quick to point out. "Do I believe that string theory is one of the best approaches we have [for unifying physics]?" he continued. "I do." He noted the "enormous progress" in string theory in the past decade or so and its continued promise in unifying quantum mechanics and gravity, which are independently well understood and rigorously tested but refuse to mesh cleanly.

The occasion was the March 7 Isaac Asimov Memorial Debate at the American Museum of Natural History in New York City, an annual event that covers a different topic each year. This year's debate, about theories of everything, was meandering and a bit abstract—for instance, it was not until the post-debate Q&A that Greene actually gave a brief description of what string theory is.

Abstraction is perhaps par for the course when a group of theorists assemble on stage. ("I'm a particle astrophysicist, and I'm not used to being the pragmatist of the group," joked Katherine Freese of the University of Michigan at Ann Arbor.) But the subject of experimental evidence did arise several times.

Extra dimensions of space, for instance, which are predicted in many forms of string theory (a variant called M theory requires 10 spatial dimensions rather than the familiar three), could be accessible at the Large Hadron Collider (LHC), said Barnard College physicist Janna Levin. In the LHC's unprecedentedly high-energy experiments, some products of particle collisions could go missing, having vanished into those extra dimensions.

Lee Smolin of the Perimeter Institute for Theoretical Physics noted that some forms of quantum gravity predict certain asymmetries—one direction of polarization might be favored over another—that could be imprinted in the cosmic microwave background (CMB), a faint echo of radiation from the early universe. And, if the signature is there, the new European Planck satellite could see it. "If it's there, it could knock our socks off," Smolin said.

Greene noted that string theory, if correct, could have left a similar detectable imprint on the CMB, although it appears unlikely. "I'm excited by this possibility, but I do consider it a long shot," he said.

But compared with the complexity of the real world, experiments are always somewhat crude. What if, having settled on an experimentally verified theory of everything, a better experiment comes along that throws a spanner in the works? That is the worry of Marcelo Gleiser of Dartmouth College. "What bothers me is saying that is the answer, that is the theory of everything," Gleiser said. "For me physics is a work in progress, a narrative. You make your knowledge grow, like an island, but then the shores of ignorance increase."