In Part I of *The Elegant Universe*, “The
Edge of Knowledge,” Greene introduces the central problem of modern
physics: the incompatibility of Einstein’s theory of general relativity
with quantum mechanics. He then lists the physical curiosities—properties
of the motion of light, gravity, and the behavior of particles at
the subatomic level—that have prevented physicists from establishing
a single coherent theory for how the universe works. For most of
the twentieth century, physicists contented themselves with describing either
the most small-scale or the most large-scale workings of the cosmos,
but never both simultaneously. At the end of this section, Greene
describes both the objectives and the promise of superstring theory,
which attempts to merge the laws of general relativity with those
of quantum mechanics.

Without dismissing the importance of electrons and quarks, which
are the basis of quantum mechanics, superstring theory depicts the
smallest particles in the universe not as dots but as tiny strings
of energy. These strings are one hundred billion billion (a quintillion)
times smaller than a single atomic nucleus. They vibrate in different
patterns, which in turn produce different particle properties. But
because these strings are too tiny to locate with current scientific
tools, superstring theory is not yet predictable or testable. Therefore,
physicists like Greene must work with approximations of equations
until more information has been verified. Still, the promise of
string theory is tremendous. Only string theory’s conceptual framework
offers any possibility of unifying general relativity and quantum
mechanics into one complete understanding of how the universe works.

In Part II, “The Dilemma of Space, Time, and the Quanta,” Greene
reviews the basic precepts of the two competing theories—first,
Einstein’s special and general relativity, and then the “microscopic
weirdness” of quantum mechanics. General relativity presupposes
a smooth surface of space, but at an ultramicroscopic level (which
quantum mechanics has helped unveil), the spatial fabric is subject
to violent undulations known as “quantum foam.” Greene also discusses
the basic principle of quantum mechanics: the uncertainty principle.
The uncertainty principle predicts the impossibility of knowing
both the exact location and the velocity of a particle at any given
time. Greene goes over the four fundamental forces—the strong force,
the weak force, electromagnetism, and gravity—and he describes the
complexity of incorporating gravity into the standard model of the
first three. In the last chapter of this section, Greene stresses
the necessity of finding a new theory that revises both general
relativity and quantum mechanics. Like many of his colleagues, Greene
simply cannot accept that the universe is, at its core, divided
into two contradictory theoretical frameworks.

In Part III, “The Cosmic Symphony,” Greene discusses in
detail how superstring theory works. He employs many musical metaphors
to suggest how strings “harmonize” or bring together the most puzzling
aspects of the cosmos. After praising the elegance and economy of
the theory, Greene gives a brief history of its first incarnation
in the 1970s, when it was referred to as the bosonic string theory.
He also explains the subsequent revisions the theory underwent during
the first superstring revolution in 1984. Greene then describes
how supersymmetry— a concept that predicts the existence of superpartners
that correspond with all known particles—transformed string theory
into superstring theory.

With these basics covered, Greene proceeds to one of the
oddest claims of string theory: the theory that the universe contains
far more dimensions than we can perceive. In its current form, superstring
theory postulates the existence of *eleven* total
dimensions: ten of space and one of time. According to the theory,
equations of quantum theory can mesh beautifully with relativity
if we assume the existence of eleven dimensions. Greene concludes
“The Cosmic Symphony” by analyzing the central difficulty of string
theory—namely, the lack of experimental evidence for superstring
theory. He describes the efforts he and his colleagues have made
to develop string theory and refine its underlying mathematical
principles. He shows what a Calabi-Yau space (the six-dimensional
shape that physicists believe the additional, curled-up dimensions
of space will form) may look like.

Part IV, “String Theory and the Fabric of Spacetime,”
is the most complicated and involved section of the book. Greene
begins with an overview of quantum geometry and the new kind of
math that must emerge to explain the universe on an ultramicroscopic
scale. He argues that contrary to what physicists previously believed,
the fabric of space *can* be ripped and torn with
no catastrophic consequences. He also covers the second superstring
revolution, which shows that all five string theories are really
part of a single, unified framework called M-theory. It is fitting
that no one knows what the “M” signifies, because M-theory is one
of the most radical, incompletely understood theories ever to evolve.
M-theory proposes the unification of gravity with the three nongravitational
forces. It is an extension of string theory that suggests the elementary
particles of the universe might include, in addition to strings,
two-dimensional membranes and three-dimensional blobs of varying
size. Greene ends his discussion of the latest advances in superstring
theory by discussing its cosmological implications, what it might
reveal about the origins of the universe.

Part V, “Unification in the Twenty-First Century,” concludes
the book. It covers the prospects of string theory in the twenty-first
century. It describes the advances Greene and other string theorists hope
to make in unveiling a single theory to explain the entire universe.
For all his optimistic predictions, however, Greene never hesitates
to admit that because of its complexity, superstring theory might
not be fully understood for many years.

### The Fabric of the Cosmos

In 2004, Brian Greene published a simplified follow-up
to *The Elegant Universe* called *The Fabric
of the Cosmos*. He claims to have written *The Fabric
of the Cosmos *because his mother—to whom *The Elegant
Universe* is dedicated—told him that she put down the book
and never finished it. She said it was too difficult for the average
person to understand. *The Fabric of the Cosmos *is
the best first resource for someone having trouble with *The
Elegant Universe*. It is written in the same engaging, user-friendly
language, but the technical explanations are considerably easier
to understand. Chapter 12 of *The Fabric of the Cosmos*,
“The World on a String,” succinctly summarizes the history of string
theory, while Chapter 13, “The Universe on a Brane,” introduces
the findings of the second superstring revolution.