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A slick sales job with a large side of information
on January 4, 2006
Unlike the physicists who wrote the first two reviews, I don't know much 'bout string theory. Which is why I turn to books like this, or Greene's _The Elegant Universe_. Let me try to explain what this book is trying to do, and how, for one proverbial intelligent layman, it stacks up.
Susskind is a man with a mission. What he's describing here is not settled science, but his own view of the direction fundamental physics should be trying to go. In order to describe that properly, of course, he has to explain a good deal of settled physics along the way. He does this engagingly and fairly clearly, though he doesn't have the truly remarkable expository gifts of Brian Greene, and I strongly recommend that anyone who wants to tackle this book should read _Elegant Universe_ first.
The book has two tightly intertwined main theses. The first has to do with the Anthropic Principle: the observation that a large number of physical constants are required to fall within a surprisingly narrow range of values, in order for the apparatus of biology ever to appear. Slight tweaks to any of them would make galaxies, stars, atoms, chemical elements heavier than helium, to say nothing of carbon based life forms, impossible. Susskind's thesis here is that the AP is neither, as many theists would like to claim, evidence for a Designer who tailored the universe to make us possible; nor, as secular physicists would like to claim, an uninteresting tautology requiring no explanation. Rather, its explanation is to be found in the last decades' developments in string theory.
His second thesis is that these developments - especially the way in which string theory, which was originally hoped could prove mathematically that the various physical constants could have only one uniquely determined set of values, turned out to be a family of five, then many, then mind bogglingly many, distinct theories - are not the intellectual catastrophe some have felt them to be. Rather they are an argument in favor of the truth of string theory, because the innumerable variations in the laws of physics permitted by the various string theories provides a naturalistic explanation for the Anthropic Principle. To wit: cosmic inflation creates innumerable new universes all the time, each with its own set of physical constants, and it is not surprising that some of them should have laws (and in particular a value for Einstein's cosmological constant, which is more extremely constrained than any of the others) which permit life to arise. The collection of all these possible universes, by analogy with the "fitness landscapes" of evolutionary theory in biology, is what Susskind designates as "the cosmic landscape" of the title.
There are a lot of problems with this point of view. Susskind considers them, and argues enthusiastically, subtly and fairly that none of them is a show stopper. In the end, I felt he failed to close the sale. Until someone solves what he calls the "measure problem", the whole scheme is dead in the water. Further, we are never given a positive reason to believe in the truth of string theory, other than the fact that no other consistent theory unifying gravity and quantum mechanics has surfaced yet.
In his final chapter, Susskind tries to summarize the disparate attitudes of a dozen major living theorists toward this emerging Landscape picture. The two most telling criticisms come from physicist David Gross. The book gives them a pretty fair hearing , and doesn't claim to dispose of them.First, if we adopt the idea that the physical constants have randomly created values, the enterprise of trying to explain why they have the particular values they do comes to a dead halt - perhaps prematurely. "Quantum fluctuations did it" puts the kibosh on further inquiry as surely as "God did it" would. Second, we don't really know how wide a range of physical constants could produce life and intelligence in *some* form.
Those who are looking for a primer on string theory, or on the latest truths that scientists have learned and agreed on, won't find it here. But if you are interested in the Anthropic Principle, or in the ferment of controversies at the edge of the presently knowable, you won't have to agree with Susskind to take delight, as I did, in colorfully articulated, intriguing, and frequently illuminating read.