Every important theory of twentieth-century physics has a single underlying principle that sums up its basic idea. In special relativity, this underlying principle is the constancy of the speed of light. In general relativity, the underlying principle is the equivalence principle, which posits that accelerated motion and gravity are indistinguishable from each other. Even quantum mechanics has the uncertainty principle. But string theory, almost thirty years after its first incarnation, still lacks that unifying bottom line. Physicists are searching aggressively for the single principle that will make everything else fall into place, but so far, nothing has emerged.
For the moment, string theorists have only speculations and questions—a multitude of loose ends, so to speak. All theories about the physical universe presuppose the existence of space and time, but where did they come from? Is there such a thing as a fundamental ingredient for spacetime, something like a zero-brane?
Will string theory ever succeed in reformulating quantum mechanics so that it agrees with general relativity? Can both contradictory theories be modified to complement each other? Should string and M-theory have begun as a quantum mechanical framework? And, of course, can string theory ever be tested? If so, when? Physicists hope to locate superpartner particles within the next decade, but the “eureka” moment, as Greene puts it, still lurks on a distant horizon.
Greene ends the book with what is perhaps its most important question: are there limits to what science can explain? Can we ever know everything about our universe? Will we ever really be done?