Like medieval knights in their quest for the Holy Grail, twentieth century physicists have been searching for a single theory that will explain all matter and forces in the universe. Albert Einstein, the first great quester, once described himself as working “like a man possessed” to discover the unified field theory that would combine gravity and electromagnetism. Wolfgang Pauli, Einstein’s colleague at Princeton’s Institute for Advanced Study, once commented on his friend’s fruitless search to reconcile the irreconcilable: “What God has put asunder, let no man join together.” Despite Einstein’s decades of passionate effort, he did indeed fail, but his failure did not end the quest, and in The Elegant Universe Brian Greene describes how physicists have developed string theory as an ultimate explanation of everything. This theory bridges the gulf between the laws of the very large (general relativity) and the laws of the very small (quantum mechanics).
The son of a composer and former vaudevillian, Brian Greene might at first seem an unlikely person to explain how string theory makes modern physics into a harmonious whole, but he was a child prodigy in mathematics, and he graduated in the top ten of his class at Harvard University and won a Rhodes scholarship to Oxford University, where he received his doctorate in 1987. After teaching at Cornell University, he became a professor of physics and mathematics at Columbia University in New York City, where he has also been able to pursue an avocation in acting (while simultaneously holding adjunct professorships at Cornell and Duke Universities). His talents as scientist and showman are on display in The Elegant Universe where he makes use of interviews, historical anecdotes, dramatic diagrams, insightful analogies, and revealing metaphors to put a friendly face on a forbidding subject.
Since Greene is convinced that everything in the universe arises from the vibrations of submicroscopically tiny strings, he often makes use of musical analogies to communicate his ideas. Furthermore, his book is structured, like the movements of a symphony, in five parts. Parts I and II introduce the basic themes: the two great theories of twentieth century physics—relativity and quantum mechanics, which were in conflict until string theory harmonized them. Part III, entitled “The Cosmic Symphony,” is both a history of string theories and an explanation of their principal motifs. Part IV plays on the ways in which string theory gives a quantum mechanical description of gravity that radically modifies Einstein’s original notions of space, time, and matter. Part V, the conclusion, contains speculations about the future of string theory and physics.
Traditionally, physicists simply accepted the changes in scientific laws when their attention shifted from the very big to the very small, but Greene and other string theorists have a deep aesthetic sense that sees ugliness in the incompatibilities between the gently curving forms of space described by general relativity and the writhing, roiling, and downright weird behavior of the miniworlds described by quantum mechanics. String theory’s beauty and elegance consist of its harmonization of the apparent tensions between relativity and quantum theories. Though the mathematics of string theory is mind-bogglingly complex, its basic idea is simple: All fundamental particles, both matter particles such as electrons and messenger particles such as photons, are composed of stringlike objects with effectively zero thicknesses and lengths of about a billionth of a trillionth of a trillionth of a centimeter. Strings may be open, with two loose ends, or closed into loops. The energies and patterns of a string’s vibrations correspond to the masses and charges of the familiar elementary particles. Furthermore, since matter’s mass determines its gravitational properties, string theorists posit a direct association between the pattern of a string’s vibration and its response to gravitational force.
Historically, the first convincing evidence that string theory might be the means of incorporating gravity into a quantum mechanical framework occurred when John Schwarz, working with Michael Green and others, used the ideas of string theory to modify Einstein’s general-relativity equations and make them compatible with the laws of quantum mechanics. To account for the mass and charge of fundamental particles, they had to situate their strings in a space with several more dimensions than the length, breadth, and depth of ordinary experience. It turned out that they needed six extra dimensions to determine the vibration patterns of the strings as they moved about.
When first confronted with string theory’s one temporal and nine spatial dimensions, the natural question is about the location of the extra dimensions, since our overwhelming experience demands one temporal and three spatial dimensions. The string theorists...
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