BOOK REVIEW
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
http://arxiv.org/abs/hep-th/0302219), 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.