Since the Scientific Revolution, the period from roughly
1500 to 1700 during which modern science was born, physicists have worked
toward uncovering a single theory capable of uniting all the fundamental
forces into a single equation and explaining the basic nature of
matter and energy. This theory is sometimes called a Theory of Everything.
Establishing this theory has been a gradual process. Isaac Newton
contributed to that process in the late seventeenth century when
he proclaimed a universal theory of gravity, which led to his insights
into the nature of light. Almost 200 years later, James Clerk Maxwell
developed a set of four equations that successfully integrated electricity
and magnetism into a single force known as electromagnetism.

Twentieth-century physicists made the greatest contributions
to the Theory of Everything. In 1905, Albert Einstein revolutionized our
understanding of reality when he declared that the speed of light is
a constant. This theory led to many surprising deductions, including
the theory that space and time are not separate entities. With this theory,
the idea of “spacetime” was born. A decade later, Einstein toppled
Newton’s universal theory of gravity by defining gravitational force
as the curvature of space and time.

Einstein’s theories helped make sense of the largest aspects
of the universe—movement in space and time, and the speed of light. Another
group of physicists helped make sense of the smallest aspects of
the universe—tiny subatomic particles. With the discovery of the
strong force and the weak force in the 1930s, electromagnetism and
the laws governing subatomic particles were named *quantum
mechanics*.

In just thirty years, physicists had made gigantic leaps
toward an integrated understanding of the cosmos. But there was
one large problem: the laws didn’t match up. Physicists quickly
discovered that quantum mechanics was fundamentally incompatible
with Einstein’s general theory of relativity. Theories of the small
simply did not agree with theories of the large, which suggested
some massive defect in physicists’ formulation of the universe’s
laws.

Both quantum mechanics and general relativity had been
experimentally confirmed time and again, and for many years physicists could
make no sense of the inconsistency between them. Many scientists
had trouble believing that the universe operates according to two
separate, contradictory sets of laws, but all attempts to reconcile
quantum mechanics with general relativity met with frustration and
failure. Because combining the two sets of laws seemed impossible,
physicists tended to study one at the expense of the other. Pursuing
a divide-and-conquer-style course toward truth, they studied the
laws governing either the ultramicroscopic or the massive, but seldom
both. Finally, in the last thirty years, the development of string
theory and M-theory have given physicists a satisfactory way to
merge the large with the small.

String theory had existed for a decade in cruder forms
before it became popular in the mid-1980s when, in 1984, scientists
John Schwarz and Michael Green published a groundbreaking paper
that launched the first superstring revolution.

Brian Greene, author of *The Elegant Universe*,
was an instant convert to string theory. He was convinced that particle
physics was at an end. Despite the difficulty of proving the theory,
Greene believed, as did many of his colleagues, that the basic ingredient
of the universe was not zero-dimensional point particles, but rather tiny
one-dimensional strands of string that vibrate in different patterns.

Greene earned his undergraduate degree from Harvard. He
was a first-year graduate student and Rhodes Scholar at Oxford University
when Schwarz and Green published their groundbreaking paper. Greene
is now a professor of physics and mathematics at Columbia University,
where he is also the codirector of Columbia’s Institute for Strings,
Cosmology, and Astroparticle Physics (ISCAP). Greene’s *The
Elegant Universe* was phenomenally successful, selling three-quarters
of a million copies worldwide and becoming a Pulitzer Prize finalist
in 2000. In 2003, Greene hosted a three-part *NOVA* special,
also called “The Elegant Universe,” which drew twice the average
audience for a *NOVA* program and won a 2004 Peabody
Award for broadcast excellence. Greene’s follow-up to *The Elegant
Universe*, *The Fabric of the Cosmos: Space, Time,
and the Texture of Reality*, was on the *New York
Times* bestseller list for ten weeks. The *Washington
Post* has called Greene “the single best explainer of abstruse
concepts in the world today.”

To this day, physicists cannot experimentally test and
verify superstring theory’s predictions. The equations remain so
complex that physicists must content themselves with approximations. While
string theory has a long way to go, its promise will determine the
future of physics in the twenty-first century. In *The Elegant
Universe*, Greene shows how string theory offers a single
theory capable of synthesizing quantum mechanics and general relativity.