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See Inside April 2006

Beyond the Standard Model

Theoretical physicists are not yet at the end of their string
UNWEAVING



DUSAN PETRICIC

Warped Passages: Unraveling The Mysteries of the Universe's Hidden Dimensions
by Lisa Randall
Ecco (HarperCollins), 2005

The Cosmic Landscape: String Theory and the Illusion of Intelligent Design
by Leonard Susskind
Little, Brown and Company, 2005

Hiding In the Mirror: The Mysterious Allure of Extra Dimensions, From Plato to String Theory and Beyond
by Lawrence M. Krauss
Viking, 2005
What are theoretical physicists up to these days? Judging from the titles of the popularizations they are turning out, one might be forgiven for thinking that they are eating psychedelic mushrooms or chewing on lotus leaves. Warped Passages, Hiding in the Mirror, The Cosmic Landscape--sounds like a trilogy by Carlos Castaneda.

Well, what should theoretical physicists be up to? It's been three decades since the so-called Standard Model was hammered out. Thanks to this remarkable achievement, the behavior of all known particles can be described with exquisite precision, right down to the 11th decimal place. The only force the Standard Model leaves out is gravity, and that is handled very nicely by Einstein's theory of general relativity. Considering that the Standard Model and general relativity together account for any conceivable observation we might make, further theorizing can, without prejudice, be dismissed as metaphysics--which, of course, literally means "after physics."

Needless to say, the authors of these books--accomplished theoretical physicists, all three--don't buy this characterization. Theory has a future, they think, and it will probably involve a fairly extravagant addition to what we think of as reality. Beyond that, however, they don't see eye to eye.

Lisa Randall of Harvard University takes a cautious "bottom-up" approach: she wants to keep her theorizing firmly rooted in the actual observation of elementary particles. And what worries her most is a rather specific flaw in the theoretical status quo: our inability to explain why gravity is so absurdly weak compared with the other forces of nature. To solve this "hierarchy problem," she and her collaborator, Raman Sundrum, have posited the existence of a hitherto unnoticed dimension of space--not a little curled-up dimension, like those of string theory, but a large, possibly infinite dimension. As she explains in Warped Passages, our 3-D world may inhabit this 4-D space the way a shower curtain inhabits a bathroom. The particles in our world are like the water droplets running over the surface of the shower curtain. Only gravitons can escape the curtain into the larger space, and such leakage may account for the observed weakness of gravity compared with the other forces.

Leonard Susskind of Stanford University is very much a "top-down" fellow. In fact, he is one of the creators of string theory. In the four decades of its existence, string theory has not generated any novel predictions that can be tested. Nor have the legions of physicists who pursue string theory been able to find a mathematically unique version of it. At present, there is an enormous "landscape" of theoretical possibilities, numbering something like 1 followed by 500 zeros. Each version corresponds to a different universe with what might be called its own local weather: vacuum energy, number of dimensions, elementary particle masses, coupling constants, and so on. In The Cosmic Landscape, Susskind tries to make a virtue of this seeming embarrassment. Invoking inflationary cosmology, he submits that each of these theoretical possibilities has actually bubbled into existence as a "pocket universe." Together all these pocket universes make up what he calls the "megaverse" (other physicists prefer "multiverse"). Among this enormous multiplicity of pocket universes, Susskind further contends, there is bound to be one with just the right vacuum energy to permit the emergence of intelligent beings. No wonder we observe ourselves to be living in such a miraculously fine-tuned universe, he concludes, unabashedly embracing the notorious anthropic principle.

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