#### Chapter 15: Prospects

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?