On String Theory

— Ansar Fayyazuddin

Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law
by Peter Woit
New York: Basic Books, 2006. $26.95 paper

WHATEVER ELSE MAY be said about Not Even Wrong, it is one of the bravest popular science books that I have read.

Its author, Peter Woit, over the last few years, has carried on a lonely, public, almost Quixotic battle against a particular subset of theoretical particle physics research known as “string theory.” Briefly, string theory is a theory of one-dimensional objects or “strings,” that among other things posits dimensions of space that have not been observed as yet due to their presumptive smallness.

This book, a summary of Woit’s main critique addressed to a wider audience, poses an antidote to the uncritical popularization of string theory, which has successfully brought string theory into the popular realm in a way that must be unprecedented. The “string theorist” has seriously challenged the position of the “rocket scientist” and “brain surgeon” in the popular iconography of intelligence.

The title is borrowed from an expression of a 20th century physicist, Wolfgang Pauli who, never shy about his opinions of theories, coined “Not Even Wrong” to express his contempt for theories that were so muddled as to be untestable.

The bravery of this book lies, first, in its willingness to critique the dominant research program in theoretical particle physics without promoting an alternative program, except in the most general outline. It is much easier and sexier to present answers than to raise questions, to promote than to dismantle, to sell than to undo a sell.

Woit’s bravery also lies in the willingness to accept the possible wrath that will be unleashed by your scientific colleagues. While some criticisms of string theory by more established scientists, such as Sheldon Glashow and Richard Feynman, have been dismissed as the geriatric arguments of those beyond their mental prime, Woit is the target of vicious personal attacks which dismiss his arguments as the ravings of a bitter second rate mind whose failure as a professional physicist has led him to his current stance.

Yet in his willingness to question the dominant paradigm, Woit has also found allies in the scientific community who agree with or take seriously his critique. His blog Not Even Wrong is a popular site that attracts many scientists and mathematicians on both sides of the controversy.

The Advertising of Science

Popular science, with few exceptions, takes its cues from Madison Avenue. Research programs are brought to the public not with the nuance and critical distance that one would expect from a publishing industry that respects its readership, but instead with the sledgehammers of advertising hype.

Popular science in the form of an ad no longer informs the public, no longer gives it control of information, but resorts instead to the idolization of scientists as super-smart individuals whose research is inaccessible except as sound bites of incomprehensible jargon reduced to crude analogies.

This model is particularly evident in the popularization of research in genetics and its ludicrously oversold applications to human health. But it is also evident in the promotion of string theory, where a highly speculative approach is treated as a description of our world. The more bizarre and incomprehensible the latest speculative twist in string theory, the more rapt the audience is kept in uncomprehending awe.

Not Even Wrong follows a different model of popular science — one that avoids the impulse to provide grand answers where none exist and throws critical light on a popular, over-advertised idea. String theory is a useful program as I discuss below, but not in the form that drives the advertising machine.

Particle Physics

The first part of the book gives a meticulous description of our current understanding of particle physics. There are two aspects of this part that are striking. The first is the careful description of the experiments that led us to our current understanding of particle physics as well as the status of current and future experiments in the field. Although this section is a sketch of the actual history, it serves to place the development of particle physics theory very much in the context of experimental discovery.

The emphasis on experiment is important for another reason: It supplies the counterpoint to the more or less completely theoretical preoccupations of string theory. The preoccupation with theory sans experiment is what motivates the title of the book and its central argument: string theory does not make any experimental predictions. This lack of experimental prediction is what makes string theory “not even wrong:” One cannot disprove the theory by finding experimental evidence against predicted results, nor can one find evidence in favor of it by accumulating corroborating results.

Second, while emphasizing the critical importance of experiment to particle physics, Woit has a very positive view of the interplay of mathematics and physics. He spends a considerable amount of time discussing how the two fields have enriched each other.

One of the arguments leveled at string theory is that it relies on what is considered by some to be abstruse mathematics. Woit does not see this as a problem and is in fact very sympathetic to discovering mathematical organizing principles in physics. The book outlines the far from uniform interaction between the two fields. The history is anecdotal and certainly not comprehensive, but it serves to emphasize the friendly attitude that Woit has towards the interaction of mathematics and physics.

This first part of the book culminates in a summary of our current knowledge about particle physics. Our state of the art theory — the so-called Standard Model — was formulated more or less completely in the early to middle 1970s. It is a remarkably robust and predictive model that has withstood the test of time — annoyingly so at times.

Among the successes of the standard model are the predictions of certain particles know as quarks (the “Charm” and “Top” quarks for instance) necessary for the consistency of the model, as well as the existence of what are known as the “W” and “Z” particles which are responsible for what are called Weak Interactions.

The experimental discovery of these particles at the giant particle physics accelerator at CERN in Geneva, Switzerland in the late seventies was an important event that led to the acceptance of the standard model as a correct theoretical description of particle physics.

The standard model requires the introduction of approximately 19 experimentally determined parameters as well as a particle, called the Higgs particle, which has yet to be observed and is perhaps the least understood particle in the standard model.

The Higgs is introduced to execute what is called “electroweak symmetry breaking” which is responsible for taming the high-energy behavior of the weak interactions.

Without the Higgs particle (sometimes called “the God particle” in another manifestation of Madison Avenue science marketing), the standard model breaks down at energies that will be accessible shortly in particle physics experiments. Precisely what happens at these energies is the subject of new experiments that will hopefully soon settle the question of the Higgs particle’s existence.

The book’s section on the standard model is important in setting the stage for string theory. It gives a historical and theoretical context in which the questions string theory attempts to address can be understood.

While it is an honorable attempt at explaining these ideas, I am not sure how successful it is in achieving its goal. But even if it doesn’t completely succeed in giving the lay reader grounding in particle physics, it does give an inkling of the field and therefore a point of departure for the general reader to grapple with the questions addressed in the second half of the book.

Confronting String Theory

The second half of the book deals directly with string theory. It explains its origins as a model of strong interactions and its transformation from this more modest goal to a “theory of everything” or, more accurately, “The Theory of Everything.”

This half of the book is a carefully constructed argument against the claims of string theory. It addresses, point by point, the motivations for taking string theory seriously, and the failure of string theory to live up to its promise. The central argument, as mentioned above, is its failure to come up with conclusive predictions about our world.

The key stated motivations for string theory derive from programmatic commitments in particle physics that started in the 1970s. The idea that all forces must be unified in some sense is a powerful one that dates back to Maxwell’s unification of electricity and magnetism in the mid-19th century, and gained further credence through the unification of electromagnetism and the weak interactions in the standard model.

In this view all forces derive from a single underlying force, manifested differently in everyday life (i.e. at large distance scales). What has frustrated these efforts at unification is on the one hand the lack of an experimentally viable theory that unifies the electroweak and strong interactions, and on the other the complete failure to include gravity in this unification program.

The theory of gravity, from its first successful formulation by Newton to its revolutionary transformation as a geometric theory by Einstein, has provided surprising instances of experimental success.

Straightforward attempts at making gravity compatible with quantum mechanics — the physics of short distance scales — have largely failed for reasons that are not entirely well understood. Gravity then could not be included in the unification program, simply because it could not be formulated on the same footing as the other inherently quantum theories of the electroweak and strong interactions.

String theory, as stated above a theory of one-dimensional objects or “strings,” can accommodate a wide range of forces that includes gravity. There are no obvious problems with the compatibility of string theory with quantum mechanics. String theory is therefore a possible unification of all the forces of nature, and most spectacularly it is a quantum theory of gravity. These two features — unification and quantum gravity — are often stated as the main motivations for studying string theory.

String theory has to be formulated in 10-dimensional space-time and requires the existence of a symmetry known as “supersymmetry.” These two predictions — the dimension of space-time and existence of supersymmetry — are not experimentally observed, and one has to find ways to avoid obvious experimental contradictions. In addition, the allowed forces in string theory are beyond the forces observed in nature and again incompatibility with experiment is very difficult to avoid.

One of the main conceits of string theory as a theory of everything is its uniqueness. But as Woit points out, this uniqueness in 11 dimensions results in virtually an infinite number of possible four-dimensional worlds. These distinct four-dimensional worlds arise as a result of treating the remaining six dimensions of space differently.

The failure to come up with a convincing explanation of our four-dimensional universe (i.e. three spatial dimensions plus time) lead some string theorists to explore possibilities that are literally beyond the realm of experimental verification. The idea currently in fashion in some circles is that our universe is one of many different realizations of string theory.

All of these other possible universes may exist, but they are experimentally inaccessible to us. By circular reasoning that goes under the rubric of “the anthropic principle,” we live in a universe that is favorable to life. These ideas concerning the multiplicity of universes (or the “multiverse” in the current jargon) and the anthropic principle are inherently untestable.

Grandiose Claims

As a practicing string theorist, I feel that there is an abyss separating the stated aims and motivations of string theory and its actual practice. The goals of string theory are often stated in grandiose terms. String theory is supposed to be a “theory of everything,” by which one means a theory describing all the interactions that can take place between fundamental objects.

In this sense, it is very much in the reductionist tradition of particle physics — it conceives of the world as composed of some irreducible set of objects and their interactions. In ordinary particle physics, these fundamental objects are some finite number of particles. They interact with each other and everything in the world can be understood as a (sometimes impossibly difficult) problem in this theory (at least in the extreme reductionist point of view).

String theory posits strings as the fundamental objects of the universe, and sees our current, experimentally confirmed, view of particle physics as an approximation that emerges as a limit of string theory.

In reality, there are a very small number of string theorists who attempt to relate string theory to the world we live in. Those who engage in this program do so in a very broad sense — they attempt to find models within low energy string theory, which approximately look like the standard model. These attempts, to the extent that they are successful, find possible string theory scenarios compatible with extensions of the standard model, but never the standard model itself.

The attempt at embedding an extended version of the standard model and cosmology in string theory remains a minority activity that does not appear to be leading anywhere. The majority of string theorists are engaged in a much more fruitful program that completely ignores the grand goals of the project. It is here that string theory is most successful and really has provided us with a number of insights.

String theory, the way I view it and in the everyday practice of string theorists, is an interesting playground for ideas inspired by the physics of the real world but not capable of describing the actual world with any accuracy. There is a long and venerable tradition within physics to learn lessons from models that capture some aspect of a true description of our world. In this sense string theory has allowed us to study some very interesting aspects of physics, aspects that are otherwise difficult to study.

For instance, there are some properties of realistic black holes that arise in Einstein’s theory of gravity, general relativity, which are very difficult to investigate when quantum effects are important. Some of their properties, however, are shared by not so realistic black holes in string theory. Some of our guesses about real black holes have been confirmed recently for these less realistic ones in string theory. These results give new credence to ideas that occupied a much more conjectural status.

String theory has also allowed the scientific community to find new ways of calculating quantities in certain model quantum theories of particle physics that were previously inaccessible or difficult by other techniques. One can draw up a respectable list of such achievements of string theory.

It is these successes that ultimately justify the efforts of string theorists. The results, if modest in comparison with the grandiose aims, are hopefully important steps in our understanding of the features of a true theory of quantum gravity. The surprising relationships uncovered by string theory between disparate theories may lead us to deeper insights into realistic theories of physics in the future.

As far as I can see, Woit does not disagree with me on this question. He does, however, object to the disingenuous promotion of string theory as a theory of our world. The book, despite its combative title, is remarkably measured in its evaluation of string theory and respectful of its practitioners as individuals and scientists.

The book derives its passion from an ethical engagement with physics as a practice. Its imperative is to restore a semblance of accountability to the public promotion of scientific research, to speak the truth and to warn against what it perceives to be a silencing of alternative approaches to important problems.

ATC 129, July–August 2007