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.