String theory

THE COSMIC LANDSCAPE: STRING THEORY AND THE ILLUSION OF INTELLIGENT DESIGN

By Leonard Susskind

Little, Brown, $24.95, 403 pages

REVIEWED BY JEFFREY MARSH

The greatest triumphs of 20th century physics were the revolutionary theories of quantum mechanics and relativity. The former brought unparalleled accuracy to the description of matter on the smallest scale, while the latter provided an explanation of the behavior of the universe on the largest scales of space and time. The greatest disappointment for physicists during most of the century, from Albert Einstein down, was their inability to unify the two theories. In the 1980s, though, a new approach called string theory seemed to offer a way to overcome the obstacles and provide a unified picture of nature.

In “The Cosmic Landscape,” Leonard Susskind, a physics professor at Stanford who was one of the originators of string theory, presents as comprehensible an account as a layman can imagine of this mathematically complex subject, and explains why he is willing to accept the bizarre picture it presents of the universe.

Mr. Susskind is a very entertaining writer, and further enlivens his breezy presentation of seriously challenging topics with many personal asides describing both his own history and his lively debates with other physicists, including numerous Nobel laureates and world-famous figures.

He begins with an account of the so-called standard model of elementary particles, which deals with the three forces that describe the behavior of atoms and nuclei. The first force is electromagnetism, which binds electrons to atomic nuclei and underlies chemistry and biology. Electromagnetism was tidied up in the 1940s by a theory called quantum electrodynamics (QED).

In the 1960s and 1970s, the protons and neutrons that inhabit the nucleus were dissected by a theory called quantum chromodynamics (QCD) into smaller building blocks called quarks. Unlike protons and neutrons, quarks cannot be examined individually, because the force that holds them together, called the strong force, binds them ever more tightly as attempts are made to pull them apart.

Rounding out the standard model is the imaginatively titled “weak force,” which is needed to explain how neutrons can become protons, and vice-versa, with the surplus energy carried off by almost undetectable particles called neutrinos.

The standard model excludes the force of gravity, whose effects are far smaller on the atomic and nuclear scales than the other three forces, but of overwhelming importance on the large scale behavior of the universe.

String theory, developed by Mr. Susskind and others in the 1970s and 1980s, elaborated on traditional quantum mechanics by regarding the tiniest constituents of matter not as points, but as tiny elastic strings (or, in another variant, loops.)

By mathematical magic, the gravitational force emerged naturally from this picture, where the various forces and different types of particles that exist in the universe demonstrated different ways the strings could vibrate. Complicating matters considerably, the world in which all this vibration took place did not simply have three dimensions of space plus one of time, as does the world we observe.

String theory, and its even more complicated elaboration, known as M-theory, has six or seven extra space dimensions of which we are unaware because they are “rolled up” tightly on a scale far too small for us to perceive.

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