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COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
SCIENTIFIC AMERICAN 79 DON FOLEYAccording to string theory, the laws of physics that we see operating in the world depend on how extra dimensions of space are curled up into a tiny bundle. A map of all possible configurations of the extra dimensions produces a “landscape” wherein each valley corresponds to a stable set of laws. The entire visible universe exists within a region of space that is associated with a valley of the landscape that happens to produce laws of physics suitable for the evolution of life. O V E R V I E W The theory of strings predicts that the universe might occupy one random “valley” out of a virtually infinite selection of valleys in a vast landscape of possibilities THE STRING THEORY LANDSCAPE THEORETICAL LANDSCAPE populated with an array of innumerable possible universes is predicted by string theory. The landscape has perhaps 10500valleys, each one of which corresponds to a set of laws of physics that may operate in vast bubbles of space. Our visible universe would be one relatively small region within one such bubble. By Raphael Bousso and Joseph Polchinski COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC.
the geometry of space and time, which combine to form space- time. Any massive body leaves an imprint on the shape of spacetime, governed by an equation Einstein formulated in 1915. The earth’s mass, for example, makes time pass slight- ly more rapidly for an apple near the top of a tree than for a physicist working in its shade. When the apple falls, it is actu- ally responding to this warping of time. The curvature of spacetime keeps the earth in its orbit around the sun and drives distant galaxies ever farther apart. This surprising and beau- tiful idea has been confirmed by many precision experiments. Given the success of replacing the gravitational force with the dynamics of space and time, why not seek a geometric ex- planation for the other forces of nature and even for the spec- trum of elementary particles? Indeed, this quest occupied Ein- stein for much of his life. He was particularly attracted to work by German Theodor Kaluza and Swede Oskar Klein, which proposed that whereas gravity reflects the shape of the four familiar spacetime dimensions, electromagnetism arises from the geometry of an additional fifth dimension that is too small to see directly (at least so far). Einstein’s search for a uni- fied theory is often remembered as a failure. In fact, it was pre- mature: physicists first had to understand the nuclear forces and the crucial role of quantum field theory in describing phys- icsan understanding that was only achieved in the 1970s. The search for a unified theory is a central activity in the- oretical physics today, and just as Einstein foresaw, geomet- ric concepts play a key role. The Kaluza-Klein idea has been resurrected and extended as a feature of string theory, a promising framework for the unification of quantum me- chanics, general relativity and particle physics. In both the
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