BOOK REVIEW
Unraveling Space and Time
Lee Smolin
The Fabric of the Cosmos: Space, Time, and the Texture of
Reality. Brian Greene. xii + 569 pp. Knopf, 2004. $28.95.
Unlike the anonymous peer reviews in which scientists evaluate one
another's papers and grant proposals, book reviews are not required
in principle to be unbiased. They are usually signed, which helps
ensure some degree of fairness. But there is also a great tradition
of bitchy book reviews written by rival authors, spurned
lovers—and even mothers: When The Early Universe, a
book on cosmology by Rocky Kolb and Michael Turner, was published, a
pair of humorous reviews appeared that were attributed to their
mothers. "Mrs. Dorothy B. Kolb" kvetches about the drab illustrations:
My son told me that it is not a "trade" book, but
a serious book intended for graduate students and professional
scientists. "Big deal," I told him. "You think you
are too smart to put in a few pretty pictures."
Book reviewers must, of course, fully disclose their possible
conflicts of interest. So I should confess that I am the author of
two books that might be seen as competing with Brian Greene's
The Fabric of the Cosmos (one has a similar title and
the other covers similar territory). I am also one of the inventors
of a theory, loop quantum gravity, that is generally seen as the
main rival to string theory, which Greene advocates in this and a
previous book. Even worse, I get mail from readers who complain that
I am not as good-looking as Greene, even though I write better.
(Fortunately, I am reassured by she-who-matters that the opposite is
true.) I should hasten to add that I know Greene and like him
personally, and I feel a great solidarity with him as a fellow
native New Yorker trying to make it in the harsh world of science.
Having said all this, let me begin by noting that this is a
wonderful book for the lay reader who wants to get a glimpse of what
we theoretical physicists are thinking about. It is extremely well
written. To prepare the reader to understand the theories that
currently animate the frontiers, Greene gives an introduction to the
main ideas of 20th-century physics—relativity, cosmology,
quantum theory and particle physics—and he does it very well.
As someone who, like Greene, has struggled—twice—with
the problem of how to write a fresh and compelling book that
introduces the reader to these topics, I commend him for his success
here. He is a master expositor and popularizer, and these parts of
the book really shine.
The Fabric of the Cosmos also has a feature that is missing
from too many popular books in this area: It is for the most part
fair and evenhanded. Although Greene has an agenda—he is,
after all, a string theorist—he does give space both to the
current mainstream theories and to their rivals. Although he
indicates his own preferences and hunches, he does this openly, with
a tone that makes it clear he respects those who disagree. His
chapters on quantum mechanics introduce the various competing views
as to the meaning and the adequacy of quantum theory. Each view is
presented sympathetically, but its weak points are also mentioned.
One comes away understanding why it is that smart people have
reached no consensus on the problem.
Similarly, his discussion of inflation, a hypothesis about the very
early universe, strikes exactly the right tone. Greene brings the
reader to marvel at how closely recent observations fit the
predictions of the inflation theory. But he also contrasts it with a
rival theory put forth by cosmologists Neil Turok and Paul
Steinhardt, pointing out the strengths and weaknesses of both theories.
When it comes to my own area of quantum gravity, I find far less to
gripe about than I had expected after reading an unsigned review in
The Economist, which claims that the book gives
"short, if civil, shrift" to loop quantum gravity. In
fact, Greene gives pride of place to loop quantum gravity: In his
closing section he praises it and proposes that the next revolution
in string theory will come about when it incorporates loop quantum
gravity. Although I might put it the other way around, I can't
complain much, as Greene's vision is not substantially different
from the future I argued for in the closing of my own book on the
subject. Moreover, he clearly embraces the main ideas behind loop
quantum gravity: that space and time are relational rather than
absolute and that all properties of space and time are the result of
dynamical evolution. As Greene says, this is the main lesson of
Einstein's theory of general relativity, and it is realized in loop
quantum gravity but not in string theory.
There are, however, two grounds on which I believe the book falls
short: It is far too uncritical of Greene's own subject, string
theory. And it offers little for the reader who remembers that
science is based on experiment and who may therefore wonder how it
is that all these beautiful, exciting ideas are to be tested against
the harsh light of reality.
A careful reader may wonder what is behind Greene's admission, late
in the book, that all is not well in stringland:
Even today, more than three decades after its initial
articulation, most string practitioners believe we still don't have
a comprehensive answer to the rudimentary question, What is string
theory? . . . [M]ost researchers feel that our current formulation
of string theory still lacks the kind of core principle we find at
the heart of other major advances.
Indeed, string theory has been "promising" for more than
30 years. And like those perpetual students one meets in the cafes
of any college town, it has yet to grow up and earn a serious
living. That is to say, it has yet to make any unambiguous
prediction that could be tested up or down in doable experiments. It
is fun to speculate about many unseen dimensions and particles, but
they have remained just that: unseen. There is, as Greene discusses,
a small possibility that string theorists are wrong about the sizes
of strings, in which case perhaps they will be observed in the Large
Hadron Collider now under construction at CERN, which will be the
world's most powerful particle accelerator when it is switched on in
2007. But the most likely scenario is that strings, if they exist,
are more than 10 orders of magnitude too small to be seen directly
by any accelerator built with known technology.
It is, then, worrying that many of the claims made in the book for
string theory are exaggerated. Greene describes clearly the
beautiful results that inspire hope among those who study string
theory. But a presentation for the general public of a speculative
and untested theory should be honest about which problems remain
open. There are a number of cases in which conjectures widely
believed by string theorists have remained unproven despite many
years of hard work by very smart people. In several key cases Greene
nevertheless presents those conjectures as facts.
For example, he clearly describes how strings, by having a finite
size, suppress the quantum fluctuations of the gravitational field,
perhaps leading to a consistent unification of gravity and quantum
theory. As Greene explains, the result is that the theory gives
finite answers, an obviously necessary condition, but one hard to
satisfy. What he does not say is that the suppression has been
demonstrated only in a certain low-order approximation. In
fact, determined efforts by many physicists and mathematicians over
a period of more than 20 years have failed to produce a proof of the
finiteness or consistency of string theory. The failure has been so
complete that I am aware of only two people, out of a community of
more than a thousand string theorists worldwide, who are still
trying to solve this problem.
This is the kind of thing that a truthful exposition for the general
public should include. It is, unfortunately, not the only instance
of important information going unmentioned. One of the strongest
motivations for believing in string theory, originally, was that it
was thought that there could be only one theory that unified all the
forces. If string theory did that, it had to be the one right
theory. Thus it was worrying when initially five different string
theories were discovered. These were followed by many more,
corresponding to hundreds of thousands of different ways to curl up
the unobserved dimensions. Understandably, there was a great sense
of relief when in 1995 Edward Witten conjectured that these theories
were part of a single theory. Here is how Greene describes this:
Witten . . . uncovered a hidden unity that tied all five
string theories together. Witten showed that rather than being
distinct, the five theories are actually just five different ways of
analyzing a single theory. . . . [A] single master theory
links all five string formulations. The unifying master theory has
tentatively been called M-theory. . . . String theory,
Witten demonstrated . . ., is a single theory.
Greene's exposition fully communicates the excitement that greeted
the announcement of Witten's conjecture. But one does not learn from
Greene that it was—and almost 10 years later still is—an
unproven conjecture. In fact, that "single,"
"unifying master" theory has never been written down. No
one has ever produced a convincing proposal for its fundamental
principles or its mathematical formulation. What Witten and others
did was to uncover mathematical evidence that the different theories
might be related and to assert that this might be explained by the
existence of a single unified theory. This is important, but it is
not what Greene says. Conjecture is not proof, and conjectures about
the existence of unknown theories are not theories.
There are several other examples of the same kind of exaggeration,
in which conjectures are reported as truths, and the phrase
"convincingly argued" is used to imply that something is
proven. This overstatement greatly weakens the value of the book for
the public. It also raises questions. Brian Greene knows that the
proofs of these assertions are incomplete. At the same time, his
fairness in other matters shows that he is a person of integrity. So
I am sure that there is no intent here to deceive. What is
happening, I suspect, is a phenomenon much more troubling. I'm
afraid that it is simply inconceivable to him, as it is to many
members of the string theory community, that these conjectures could
be wrong.
Among the uncomfortable developments not mentioned by Greene is that
18 months ago estimates for the number of different string theories
increased, to incredible numbers such as 10100. And none
of these appear to be involved in the conjectured unification of the
five original theories proposed by Witten under the name of
M-theory. If this is right, then there may never be an experiment
that could disprove string theory, for there will be a vast number
of string theories that all agree with any possible result of any
future experiment.
What some of us worry about is the possibility that, in the absence
of experimental tests, a community of even the smartest and
best-educated people in the world can convince themselves of the
truth of a theory, to a degree far beyond what an objective reading
of the actual evidence would justify. We have recently seen this
type of self-delusion in other domains. For example, it appears that
a vast majority of members of the international intelligence
community were convinced that Iraq had weapons of mass destruction.
Similarly, although greed certainly played a role in the accounting
scandals of recent years, I suspect that many of the people
involved, having gotten to the top of their professions by hard work
and superior intelligence, were simply confident that their version
of economic reality would prevail.
It is because of the tendency of even brilliant, well-educated
people to fool themselves—especially collectively—that
the progress of science has always required an interplay of theory
and experiment. What is most troubling about Greene's book is that
there is no evidence that he sees the "experimental
correction" coming. Indeed, the last few years have brought
several surprising experimental developments that have the potential
to dramatically challenge our understanding of fundamental physics.
It turns out to be simply false that the Planck scale, where gravity
and the quantum meet, is inaccessible to experiment. New
experimental results, in cosmic-ray physics and in observations of
gamma-ray bursts, and new discoveries concerning the properties of
dark matter and energy appear to offer unexpected windows into the
Planck scale. It may be that Greene does not mention these
experiments because they are so new, and it is certainly too soon to
draw definitive conclusions from them. But they are, very possibly,
the beginning of a new chapter in fundamental physics, one driven by
surprises forced on us by experiment rather than by the fantasies of
theorists. Perhaps they will be the subject of Greene's next book.