Saturday, February 11, 2006


By Leonard Susskind

Little, Brown, $24.95, 403 pages


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.

Mr. Susskind makes it clear that string theory is so complicated that it is almost impossible to calculate anything with it. His enthusiasm for the theory comes from it providing a possible solution to another major mystery of the universe: Why it allows life to exist.

In short order after Albert Einstein produced his General Theory of Relativity in 1917, which connected the geometry of the universe with the distribution of energy and matter (which he had shown earlier was itself one manifestation of energy), it was shown that Einstein’s equations were consistent with three basic possible ways the universe as a whole could behave.

The universe would either always stay much the same, or would continue to expand forever, or would expand to some limit, then contract again to infinitesimal size (and, perhaps, continue cycling in this way forever). Einstein himself originally assumed the universe would stay in a stable state, and adjusted his equations to include a term (the “cosmological constant”) that would ensure this result.

Later, when Edwin Hubble discovered that the universe was expanding, Einstein removed the term, describing its introduction as “the worst blunder of my life.” However, as astronomers observed ever more distant regions of the universe, and physicists thought more deeply about the behavior of the universe, the cosmological constant would not go away.

The cosmological constant, Mr. Susskind explains, is not just a mathematical artifact. Quantum mechanics says there is no such thing as truly empty space. What appears to be a vacuum is not really a state of emptiness, but a caldron of activity where particles are continually being created and destroyed (“annihilated” is the preferred time) on an unimaginably short time scale. The cosmological constant is a measure of the energy contained in these “virtual particles.”

Physicists spent decades pulling out their hair trying to explain why this number should turn out to be zero. In the 1990s, they concluded that it was not exactly zero, but so close to zero (Mr. Susskind gives the value 10 to the power -120) that it seems ridiculous that it is not exactly zero.

Even more mystifying is the fact that if the cosmological constant had a slightly different value, the universe would have developed very differently, with no stars, planets or living creatures.

Furthermore, Mr. Susskind notes, a number of other numerical constants of nature also have values so sensitive that changing them by an almost imperceptible amount would foreclose the possibility of life. These remarkable relationships have led some scientists to formulate the “anthropic principle,” which says that the universe seems to be designed especially to allow life to exist.

This is a shocking idea to scientists who have spent the past 150 years painstakingly convincing themselves that the universe and everything in it are not products of design, but of the blind working of random processes.

Mr. Susskind tells us that he embraces string theory because it permits — or, better still, compels — the existence not just of one universe, but of a “cosmic landscape” consisting of an unimaginably large number (he suggests 10 to the power 500) of “pocket universes.”

These worlds, which are inaccessible to each other, were all produced in a very short period of rapid inflation following the big bang, and the laws of nature in each one are determined by a different set of constants of nature. The fact that the universe we inhabit is clearly ideally suited for human life is what Mr. Susskind calls in his subtitle “the illusion of intelligent design.”

To religious believers, the idea that the universe is designed by a Creator to allow the existence of human life is fundamental. To Mr. Susskind and those who think like him, that idea is so unacceptable that they are willing to abandon the idea that nature follows one set of laws, the principle upon which modern science was founded.

Jeffrey Marsh has written widely on scientific topics and public issues ranging from nuclear strategy to social policy.

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