Superstring theory may provide the long-sought unification of physics for which Einstein sought in vain. Here is a look at the current state of the quest. Greene (a professor of physics and mathematics at Columbia and Cornell) begins by pointing out the central problem of modern physics. Quantum mechanics and general relativity both work perfectly, and they cannot both be right. Relativity works for large, massive objects; quantum theory for tiny ones. Normally, the two realms can be kept separate. Yet increasingly, physics deals with phenomena such as black holes, where the conflicts are impossible to avoid. Out of the search for a more complete explanation came string theory. Its foundations were laid down some 30 years ago by Gabriele Venizano, who found that a two-century-old formula by Leonard Euler described subatomic particles more elegantly than existing theory. The relationships would make sense if elementary particles were not pointlike, but elongated and vibrating, like tiny musical strings—in one sense, a modern version of the ancient metaphor of the music of the spheres. It took a while for physicists to embrace string theory; for one thing, it seemed to predict things nobody had ever seen. And despite its formidable explanatory power, its mathematical expressions were often even more formidable—Greene describes some of the equations as nearly impossible to understand, let alone solve. Still, it has the right look about it, and two waves of enthusiasm (one in the mid-1980s, the other ten years later) have convinced many physicists of the theory’s probable validity. Greene deftly summarizes these findings, in areas from subatomic-particle theory to cosmology, with occasional forays into deeper waters such as the ten-dimensional structure of the universe, with several dimensions folded undetectably back into themselves. A final chapter forecasts that string theory will become the standard physical model in the next century. Entertaining and well-written—possibly the clearest popular treatment to date of this complex subject.