Physics Hanging by a Thread Brian Greene, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. New York: Vintage Books, 2003. (First edition 1999) Peter Woit: Not Even Wrong: The Failure of String
Theory and the Continuing Challenge to Unify the Laws of Physics. London:
Jonathan Cape, 2006. Reviewed by John
Bedell In
1919 a German mathematician named Theodor Kaluza was playing
around with Einstein's new theory of gravity, now known as general
relativity,
when he made a remarkable discovery: formulated in five dimensions,
relativity produces the standard equations that describe electricity
and magnetism. Nobody knows what to make of this odd fact.
Einstein himself showed that Kaluza's work contains certain technical
problems that prevent its
being useful physics, but this discovery hints that our physical laws
are
expressions of some deeper law that would tie all of our theories
together.
Ever since then, physicists have been searching for that
underlying
unity, for a "grand unified theory" or a "theory of everything."
The
latest expression of this unifying dream is something called string
theory.
What is it, and why has it become so controversial? From the eighteenth century until the 1970s, scientists made continual
progress in understanding matter at its smallest scales. First through
chemistry and the atomic theory, then through the study of electricity,
and finally through subatomic physics, they gradually expanded our knowledge
of the most fundamental physical things. In the twentieth century
physicists brought forth one of humanity's strangest and most marvelous
creations, the quantum theory. Through arcane mathematics and ingenious
experiments they learned to understand things that seem at first completely
beyond comprehension. Protons and electrons do not follow the rules of the
familiar, macroscopic world, yet we have somehow stretched the human brain
to encompass the incredible strangeness of the quantum realm. Using
our own logic, we have come to comprehend a logic that turns everything
we thought we knew on its head. In the 1960s physicists refined quantum
theory into a set of "gauge theories" that describe three of the four basic
forces of the universe as mathematical fields. This "standard model"
is astonishingly precise and tells us almost everything we want to know
about electrons and quarks, the basic building blocks of the universe. Then,
some time in the 1970s Peter Woit gives the exact date of 1973
progress in particle physics slowed to a crawl. At that point the
standard model was completed, and since then nothing solid has been
added to our understanding of the subatomic world. The standard
model
is incredibly impressive, but nobody thinks it is complete or
finished. First, it ignores gravity, the most familiar of the four
basic forces;
when you try to combine quantum mechanics with general relativity you
get
mathematical nonsense. Second, it is in many ways
arbitrary. Some
of the equations have many possible solutions, and the only way we know
which one is correct is through experiment; physicists would much
prefer
laws that have only one possible solution and that therefore make
predictions
about the world that we can verify experimentally. Third, in its
simplest
and most elegant form it gives all particles a mass of 0. To give
things mass it requires the postulation of an entity called the "Higgs
field",
with an asyet unobserved particle called the Higgs boson (the only
particle
in the standard model that has not yet been observed), and many
physicists
find this construction dubious. Why
did progress in particle physics stop in the 1970s? Most people
would agree that the root of the problem is on the experimental
side. From Galileo to the 1970s, theoretical physics was closely
tied to experiment: experimenters discovered things, which
theoreticians explained; theoreticians made predictions, which
experimenters tested. Subatomic physics was driven throughout the post
World War II period by increasingly powerful atom smashers. However,
since 1970 the accelerator labs have produced few new discoveries, and
these have confirmed the standard model rather
than challenging it or helping us to get beyond it. The resolving
power of these quantum microscopes increases only with the square root
of
the instrument's power, and we have reached the point at which real
progress
seems to require an enormous leap in resolution. With the Cold War
behind us, nobody is interested in paying the cost for such an
instrument,
which would run into the tens of billions of dollars. Why should
governments
or taxpayers care, since, as we said, the standard model already
answers
almost every practical question about the subatomic world? The situation in theoretical physics is essentially this: we
have an incomplete, unsatisfying (although supremely useful) fundamental
theory, but no new data that would help us to go beyond what has already
been done. In earlier centuries, work on these questions might simply
have stopped, but we now have numerous research institutes and graduate physics
departments where work in theoretical physics is supposed to go on, and the
people who work in them have to do something. Besides, the curiosity
of physicists has been aroused by the power of the standard model, and many
of them feel in their hearts that a complete "theory of everything" is only
a few steps away. So work in theoretical physics goes on. And what is it that theoretical physicists do? They study string
theory. String theory in its various permutations is the only candidate
for a final theory of the physical nature of the universe that anyone takes
seriously. However, a lot of people don't take it seriously. String
theory tantalizes physicists and mathematicians with glimpses of a perfect
theory that explains everything about the universe in terms of its basic
geometry. Yet, despite an enormous amount of mental effort expended
by hundreds of the world's smartest people, string theory remains little
more than a set of calculation techniques. Its equations have never
been worked out, and it exists in several different forms that may be different
ways of expressing the same deep mathematical truth, but then again may not
be reconcilable with each other. It has yet to make any predictions
that can be tested with any conceivable experiment. It is so arcane
and difficult to understand that only people who devote their careers to
it have any chance of following the discussion, which means that only people
with a deep commitment to its success are qualified to speak about whether
it is really making progress. Some outsiders have always viewed the
whole edifice with alarm and disdain, and the debate has at times grown very
ugly. Thanks to the two books under review here, nonphysicists can now
get some understanding of what string theory is and what is at stake in
the controversy. Brian Greene is a string theorist who has recently
embarked on a second career expounding his vision of the universe to a broad
public, writing The Elegant Universe and another book and starring
in a NOVA episode. Peter Woit is a casualty of the string theory revolution;
he trained in theoretical physics but had no interest in strings, with the
result that he couldn't get a job and wound up as a lecturer in mathematics.
Let's take their two books together, and see what we come up with. The Elegant Universe is a marvelous book, and if you really
want to know something about string theory I recommend it highly. Brian
Greene has a knack for explaining mathematical arcana in ordinary language,
and I found his presentation enjoyable. He is likable and his enthusiasm
for physics is infectious. Yet Greene regularly says things that make
me uneasy. Greene several times says that string theorists have "discovered"
things, when he means that they have been produced mathematically. True,
mathematicians use the same language, but for physicists "discovered" usually
implies something more concrete. Greene is fascinated with mathematical
order and in particular with the property known as "symmetry." Symmetry simply
means that when you perform some operation on an object, it ends up the same
as when you started. Thus the facade of a classical building has "mirror
symmetry", because if you reflect it in a mirror it doesn't change. Some
of the basic symmetries we observe in nature are that the laws of physics
do not change over time, and they are the same in every place. Trying
to make the subatomic world as symmetrical as possible, physicists in the
1970s created "supersymmetry," which requires that each particle we know
have a supersymmetric particle of a much higher mass. (Thus the "super" in
"superstrings," since string theory is supersymmetric.) None of these
particles has ever been observed, so supersymmetry remains a speculation,
but Greene seems to regard it as already established because of its pleasing
completeness:
What is string theory? String theory assumes that subatomic
particles are not dimensionless points, as posited in the standard model,
but onedimensional strings with a length of about 10^{33}
meters. Most versions of the theory suppose that these strings are
actually loops. The strings can vibrate in different ways, and these
vibratory modes create all of the various subatomic particles. That
is, an electron is a string vibrating in one way, while a photon is a string
vibrating in some other way. In order to make the math work out, physicists
have had to assume that the universe has many more spatial dimensions than
the three we can sense; 10 is the current favorite number. According
to the theory, we can't sense the seven extra extra dimensions because they
are curled up into very tiny spaces. Imagine life on the surface of
a garden hose: this universe would have one extended dimension, the length
of the hose, and one dimension that is curled up, since if you walk around
the hose you very quickly end up where you started. The extra dimensions
of string theory are not curled into circles, but into intricate structures
called CalabiYau manifolds. (A 2dimensional rendering of one of
these 6dimensional forms is shown below.) Because the properties of
the strings depend very sensitively on the geometry of the particular CalabiYau
shape into which you fold the dimensions, string theorists spend a great
deal of time working on the geometry of CalabiYau manifolds.
String theory has certain appealing characteristics that keep people
working at it, even though the math is incredibly difficult and the results
meager. Analyzing the paths of tiny, vibrating loops through space
turns out to produce many of the key equations of modern physics. As
Greene writes, "numerous features of the standard model  features that
had been painstakingly discovered over the course of decades of research
 emerged naturally and simply from the grand structure of string theory."
Physicist Michael Green went further, saying that "the moment you
realize that almost all of the major developments of physics over the last
hundred years emerge  and emerge with such elegance  from such a simple
starting point, you realize that this incredibly compelling theory is in
a class of its own." String theory predicts the existence of gravity,
so it seems in principle able to reconcile relativity with quantum mechanics.
Using its tools one can make calculations about situations where the
standard model breaks down completely, such as the inside of a black hole.
(There's no way of knowing if the calculations are correct, but at
least string theory is in principle (again) able to make them.) While
most string theorists admit that their field has as yet made little contact
with experimental data, most say that they just need more time. The
mathematics is so difficult, they say, and the concepts so new, that it is
unfair to judge what the theory may become on the basis of what it is capable
of doing now. Peter Woit is unimpressed. Woit is one of those characters
who have hung around the edges of the academic world, never quite making
it in. From his position as a lecturer in mathematics he has watched
the progress of string theory, and his disgust seems to have grown with
each passing year. He includes in this book some numbers showing how grim
the academic job market is for theoretical physicists, and how tightly string
theorists have locked up the best jobs. Some of these passages reek
of bitterness, and I am sure that many people who make it that far in the
book will conclude from them that Woit is just a disgruntled loser. I
have a feeling, though, that Woit's reputation is in little danger on this
score, because I doubt many people will make it that far. The book
is written without equations, but it is full of math, so much so that I think
I understood about half of it. It seems to me that the book is too
technical for anyone other than mathematicians and physicists, but not technical
enough to satisfy them, leaving it in a very odd position. And yet, despite the contrast between Woit's bitterness and Greene's
infectious enthusiasm, I think Woit has by far the better argument. Woit
explains quantum field theory and the math on which it is based in some
detail (most of the book consists of this exposition), and using this base
he is able to show how limited and unimpressive string theory actually is.
As Woit would have it, string theory isn't even a theory, just a way
of making calculations based on the assumption that there is an underlying
theory that has certain characteristics. The way that theory is imagined
has evolved over time (the current version is called "M theory"), but no
one has been able to write down any of its equations. If Woit is right
that string theory is only a tantalizing dead end, then his picture of
the physics community is disturbing. According to Woit, the only
way for a theoretical physicist to get a job is to throw himself into string
theory and produce articles on 10dimensional geometry, and by the time
he has written enough to get tenure he has committed too much of his life
to string theory to ever get out and work on something else. Thus
all the best theoretical physicists are working on strings, and nobody is
looking for an alternative theory. Attacks from outsiders have turned
the string community into a sort of cult, intolerant of criticism and ready
to make personal and institutional attacks on anyone who publicly expressed
doubts about what they are doing. I find this troubling. I am extremely skeptical of string theory
because I don't think we are smart enough to figure out the laws of the
universe without experimental data. Modern science leapt far beyond
anything humanity knew before because scientists began to insist that theories
be based on carefully executed experiments or other precise observations.
Medieval and ancient science did not have this experimental basis,
which is why scientists before Galileo were wrong about almost everything.
Newton was not smarter than Aristotle, but he approached science in a different
way, with an insistence on agreement between theory and data. String
theory in this sense takes us back to the kind of airy speculation that
characterized "natural philosophy" before Galileo, and I can't see how this
is a good thing. Of course, we know much more physics than the ancient
Greeks, and we understand much better what mathematical form a fundamental
theory should take, but that doesn't mean we can dream up such a theory in
our heads. It may be true that much of modern physics "emerges naturally"
from the structure of string theory, but it is also possible that string
theorists have found those things because they already knew to look for
them, like psychics announcing their predictions at the end of the year.
As in the strange case of electromagnetism sprouting from the 5dimensional
form of general relativity, the remarkable results produced by string theorists
hint at some deeper truth, but what that truth is remains elusive. I have a feeling that the whole edifice of contemporary physics
is in for some serious shocks. In particle physics we have no new data
and attempts to expand our current, incomplete theories seem to be going
nowhere. Meanwhile, in cosmology there is an embarrassment of new
data, but it is not of a very comforting kind: unexplained entities (inflation,
dark matter, dark energy) are proliferating in the models like the epicycles
that medieval astronomers used to make their observations fit with an earthcentered
universe. I found it very interesting that some of our leading physicists
seem to share my bewilderment; both Edward Witten and Steven Weinberg told
Nova that we just may not be smart enough to understand the fundamental
laws of the universe. I rather suspect that we are, provided we have the
necessary data I think were are due for a new revolution in physics,
perhaps as profound as the quantum revolution of the twentieth century, and
that the frustrations of current physicists are only the prelude to that
new leap. Some form of string theory may well be part of that revolution,
but I doubt it. July 14, 2006 Further reading: Good article on strings at wikipedia. Nova page accompanying The Elegant Universe. 
From
the Commonplace Book God forbid that we should give out a dream of our own imagination for
a pattern of the world.
Departments Thoughts
Index Raymond Aron, JeanPaul Sartre, and the Limits of Being Right Susan Haack
& Intellectual Ba'al Hammon
and the Clearer proofs, in the discovery of secrets, and in the investigation
of the hidden causes of things, are afforded by trustworthy experiments
and by demonstrated arguments, than by the probably guesses and opinions
of the ordinary professors of philosophy. William Gilbert
Edward Witten
