Greene begins The Elegant Universe with an analysis of the puzzling incompatibility between the two “foundational pillars” of twentieth-century physics, Einstein’s general relativity and quantum mechanics. General relativity deals with the universe on the large scale—stars and galaxies—while quantum mechanics attempts to explain the universe on the small scale—molecules, atoms, and subatomic particles. Right now, general relativity (laws of the large) and quantum mechanics (laws of the small) contrast with each other in bewildering, complex ways. For most of the twentieth century, physicists chose to study either general relativity or quantum physics and pretended that the other simply didn’t exist.

Then string theory came around. String theorists believe that general relativity and quantum mechanics, which seem to be opposing principles, actually function within one larger cosmic system. The primary objective of string theory is to describe the smallest ingredients of matter in the universe.

For centuries, scientists pondered the bizarre properties of the motion of light. It was Einstein who first overturned Isaac Newton’s widely accepted hypothesis that space and time were simply static concepts. Einstein proved that space and time are actually ever-changing constructs that depend on one’s state of motion. Space and time do not simply form a motionless backdrop for the events of the universe; they are, instead, crucial agents in the events.

Einstein’s thrilling reformulation of how space and time work created problems when it came to the development of quantum mechanics, and the incompatibility between Einstein’s theory of relativity and quantum mechanics remains the central problem of modern physics, as Greene reiterates every chapter.

When the Greeks coined the term atom to describe the building blocks of the universe, they assumed that atoms were the smallest units of matter. Scientists have since discovered that atoms consist of protons, neutrons, and electrons. Then, in 1968, physicists confirmed that protons and neutrons are themselves composed of three smaller particles called quarks. It was originally thought that two types of quarks exist: the up-quark and the down-quark. Scientists subsequently discovered still more fundamental particles: the ghostly neutrino and a much heavier particle called a muon. Even more recently, physicists have found more fundamental ingredients—four more types of quarks; a cousin of the electron, called a tau; and two particles similar to neutrinos. All of these particles have corollary antiparticles. Together, these matter particles are grouped into three families, each of which contains two of the quarks—an electron or one of its cousins—and one of the neutrinos.

To complicate matters further, the forces of nature come into play, of which four varieties exist: the gravitational force, the electromagnetic force, the weak force, and the strong force. Greene fully explains these forces later in the book, but in this first chapter he simply lays out their basic characteristics. Gravity is measured by the mass of an object. The electric charge of a particle determines how the particle can behave electromagnetically (the same impact that mass has on gravity). As for the lesser-known forces of nature, physicists have, over the last century, identified two features that strong and weak forces share: they all have a particle that is the smallest bundle of the force, and they are all endowed with varying amounts of strong charge and weak charge. After a discussion of other ways in which these forces interact, Greene poses one of the central questions of this book: why does the universe have these properties?

Greene is a string theorist, so his answer—as yet unconfirmed by science—goes something like this: if we could examine these elementary particles with the utmost precision, we would find tiny vibrating loops, or strings. This theory stands in contrast to classical physics, which holds that matter is composed of indivisible, zero-dimensional point-particles with no size or internal structure.

String theorists like Greene believe that string theory may resolve the clash between quantum mechanics and general relativity that has tormented physicists for so many years. String theory would brilliantly resolve Einstein’s failed thirty-year quest to uncover a unified field theory of the universe.

The search for the underlying coherence of the universe has become something of a Holy Grail in physics today. String theory offers a framework for understanding everything in the universe, from the big bang to the tiniest constituent of an atom. In today’s scientific community, string theory is both thrilling and extremely controversial, especially because its predictions have yet to be proven experimentally. From this standpoint, string theory is still at a very early stage. Its mathematical underpinnings are still so complex that, as yet, only approximations of equations and their answers exist. The most recent studies suggest that string theory belongs within an even larger framework, which is named M-theory.