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A Universe Tuned for Life

John Peacock

The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. Leonard Susskind. xii + 403 pp. Little, Brown, 2006. $24.95

Does God exist? Most scientists would probably prefer to avoid confronting this question in their professional lives, but the growth of the "intelligent design" movement makes it harder for them to maintain such distance. Intelligent design appears to be a rediscovery of the 19th-century argument from design, which perceived the manifest hand of a Creator in complex biological mechanisms such as the eye. The fallacy in this view is well exposed in, for example, Richard Dawkins's The Blind Watchmaker, which details how evolution can yield living structures of almost arbitrary complexity from a basic set of building blocks. The genetic record essentially exhibits this process at work, including the sobering fact that you and I share more than 20 percent of our genes with worms. Nothing in life is certain, but our common genetic heritage with such primitive creatures is surely proof beyond reasonable doubt of the operation of evolution. The fact that this assertion can be disputed is a disturbing symptom of the growth of irrationality in the modern world.

Even if the evolution-deniers could be persuaded of their errors, however, the intelligent-design argument would still need to be confronted on a wider stage. A cursory search of the Internet yields many who think that the Earth is surprisingly well fitted for life and that this happy situation must result from the efforts of a designer. Most astronomers would say that, on the contrary, this match between our biological needs and the nature of the Earth is the result of a simple selection effect: There exist innumerable planets, and life will not be found on those that are too hot or too cold. The bias that arises from selection effects is often called "anthropic," although this is an unfortunate name because it apparently implies that the universe wants to create creatures like us, whereas the existence of life on Earth is a random outcome of the disinterested laws of physics and probability.

In an everyday analogy, winning the lottery does not prove that God exists (although it might feel that way to you). After all, there were many other ticket holders who were unsuccessful. In the same way, we could have inferred that the universe is probably full of planets at varying distances from their central stars long before we had the telescopes to detect them.

So far so good, but we are not finished. There are aspects of the laws of physics themselves that seem puzzling when the origin of life is considered. A good example is Fred Hoyle's discovery that stars very nearly fail to make carbon and that only a peculiar coincidence in excited energy levels permits this process to happen. More radically, cosmologists have established over the past decade that the energy density of the vacuum is not zero: Perfect emptiness still weighs something, impossible though that sounds. The energy density of the vacuum is tiny compared with the result of any attempt to predict it using particle physics, but this fact is a good thing: Galaxies, stars and planets would fail to form if the vacuum density were much higher than the observed level. This conclusion, which no scientist contests, sounds like intelligent design on the largest scale—the whole universe being carefully set up so that we could come into being. It seems as though, in Hoyle's words, "a superintellect has monkeyed with physics." Could this be true?

At first sight, this ultimate design argument does seem impressive. But a consideration of why Earth is amenable to life suggests a way out of the paradox. Recall that it is the diversity of planets that allows selection of the environments that best match life in general. The resulting conclusion is inevitable: If the laws of physics seem strangely tuned to permit the creation of life, there must exist an ensemble of universes with a variety of laws of physics.

The idea that many different laws of physics are possible has been the subject of speculation for years, but a firmer foundation has emerged recently from string theory, the "theory of everything" in physics. As a leading string theorist, Leonard Susskind is well placed to explain these developments. In 2003 (see, he coined the term landscape to summarize two related ideas. The first is that the mathematical edifice of string theory predicts many possible consistent laws of physics, not only the ones that we happen to observe. Furthermore, it is possible within modern inflationary cosmology to have a "multiverse" where all these possibilities actually exist in different regions. Not all string theorists accept this conclusion; if true, it represents the crushing of their dream that string theory would ultimately explain why nature has had to function in the way that it does.

Because the landscape idea has broad implications, it is good to see that in his new book, The Cosmic Landscape, Susskind has cast the arguments into a form suitable for a general readership. It is clear, though, that Susskind is not just intent on educating readers—he wants to pick a fight. The subtitle of his book is String Theory and the Illusion of Intelligent Design. This is not just entertainment for the scientifically curious lay reader but also opposition to the whole march of religious fundamentalism. Susskind lays out his position from the start, with a quote from Laplace about having no need for the hypothesis of God. This confrontational approach will probably do no harm to sales.

To present his case, Susskind has to outline particle physics as far as the Higgs field, introduce the main landscape idea by exploring the consequences of varying the Higgs field, explain why the Standard Model is too complicated to be fundamental, explain string theory and its history, and justify in terms of the topology of Calabi-Yau manifolds why there are so many possible incarnations of particle physics. On the way, he has to explain most of cosmology—how we know there is vacuum energy, why inflation was invented as a theory of initial conditions and how inflation allows for an ensemble of universes. Finally, he has to explain the general idea of the anthropic principle and show how observational selection can exploit the string/inflation ensemble of different universes with differing laws of physics, so that we find ourselves in a universe tuned for life, without the slightest need for a divine guiding hand.

Covering all of this territory in less than 400 pages is a huge challenge, one that Susskind meets extremely well. He uses helpful diagrams and vivid analogies to illuminate the concepts. And the structure of the book is engagingly quirky. Susskind mixes explanation with personal anecdote and interleaves different topics so that the reader doesn't become disheartened by having to follow a long and complex argument all in one go. This is not to say that the average reader will find the book simple to absorb, because there is a lot to take in—both in terms of concepts and of history. One of the many virtues of Susskind's presentation is that it gives some good insights into the history of string theory, starting with its origins as an attempt to model strongly interacting particles as rubber bands.

Nothing is perfect, of course. Some of the key points really need a longer explanation in order to make sense to an unprepared reader. I think anyone coming fresh to the cosmological constant here would struggle, for example, especially as the idea of this long-range repulsion is introduced before any of the other material about cosmology. Also, Susskind is sometimes happy to use more sophisticated concepts that may stump nonmathematical readers. An example is plotting a "two-dimensional" landscape as a surface with the Higgs field depending on x and y coordinates that are the electric and magnetic field strengths. This leap of casting the quantity of interest as an abstract space is automatic to a physicist, but a little more sympathy for the novice at such points might have helped.

Also, I was less than happy with some of the description of cosmology. Susskind seems to think that astronomical data have confirmed the reality of inflation, which is not so (although inflation is undoubtedly consistent with current observations). In terms of history, Edwin Hubble gets the usual overexposure, and it was amusing to read a detailed description of how Hubble measured redshifts, whereas in fact almost every redshift Hubble used was measured by Vesto Slipher, who had established the general tendency for galaxies to be redshifted by 1917 (compare Susskind's assertion that in 1917, "as far as [Einstein] or anyone else knew, the galaxies were stationary").

Finally, the language of the book will often seem a little cryptic to non-American readers: What is a BB ball? At least a "Rube Goldberg machine" is explained in the index, although all British readers will of course know that the inventor of absurdly elaborate machines was Heath Robinson, who was Goldberg's elder by nine years.

These obligatory small criticisms should in no way detract from Susskind's tremendous achievement. This book is a fine piece of popular science writing, but it is particularly significant for the timeliness of its message. Susskind emphasizes that the whole structure of the universe requires an active Creator no more than does the human eye or the temperature of the Earth. At a time when more and more people seem happy with a creation that took place 6,009 years ago, this lesson needs repeating.

In the end, however, good though this book is, I was left feeling that the argument was not carried to its logical conclusion. Despite his justified scorn for intelligent design, Susskind retains a hint of this worldview in his own attitude. It was Galileo who said that the book of Nature is written in mathematics, and almost all physicists subscribe to this view. When we contemplate the power and simplicity of constructions like general relativity, there is a temptation to carry intelligent design to an extreme in which God wrote the equations, from which all else follows. Frequently this perspective is quite explicit, as with Einstein (recall Bohr's admonition, "Stop telling God what to do!"). The landscape picture derails this thinking to some extent, but Susskind just transfers the quasi-religious awe to string theory, whose mathematical results he repeatedly describes as "miraculous."

But if life on Earth is a random accident in a universe where only chance yielded laws of physics suitable for life, why stop there? Perhaps string theory itself is nothing special and only part of a wider spectrum of possible prescriptions for reality. If the search for a unique and inevitable explanation of Nature has proved illusory at every step, is it really plausible that suddenly string theory can make everything right at the last? Reading Susskind's book should make you doubt that possibility, in which case we may have reached the end of the search for underlying simplicity that has driven physics since the beginning. A comment made by Steven Weinberg in his 1977 book The First Three Minutes sums things up well: "The more the universe seems comprehensible, the more it also seems pointless." Pointless to look for meaning in our existence in the universe, and also (according to Susskind) pointless to look for meaning in physics. To a physicist, this is a pretty depressing conclusion, but there is some consolation: The beauty we perceive in the laws of physics perhaps tells us as much about the human aesthetic response as it does about any fundamental design of the universe. In short, physics is a human creative art on the same level as painting and music, and that is reason enough to be proud of what the subject has achieved.

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