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30 Years of String-Theoretical Noodling Around, a.k.a. "The Emperor Has No Threads"

In the latter class I made friends with a couple of the graduate students from the UM theoretical-particle-physics group (I use "group" in a very loose sense here, not to give any impression of monolithicity or singleness of vision) who made me acquainted with the history, then-current state of and prospects for the Standard Model of particle physics. At the time, the Standard Model had just experienced its latest huge successes, the discovery of the weak-nuclear-force-carrying W and Z-bosons (discovered in 1983 at CERN), and despite the still-over-the-horizon discoveries of the Top Quark (discovered in 1995 at Fermilab) and the Higgs Boson (the elusive "one particle to mass them all" predicted by the Standard Model, as yet undiscovered in any direct sense but hoped for when CERN's mammoth Large Hadron Collider goes online sometime in late 2007), there was already a sense of restlessness among many physicists, of the inevitable kind that arises whenever the "end of an era" whiff begins to hang in the air. (Whether that appellation is objectively justified or more in the subjective views of the beholders.) Talented people with the kinds of restless intellects that gave us the Standard Model are generally not content to be doing hard work that appears to be little more than dotting the I's and crossing the T's of a theory, no matter how spectacularly successful the theory has been. Even restlessness aside, theorists rightly wanted to find a way to bring gravity into the fold, and of course there were the usual tantalizing rumors-of-new-physics-just-over-the-next-ridge go around. Not completely coincidentally, it was around this same time that String Theory began to come back into fashion, by way of the so-called first superstring revolution.

In any event, one the aforementioned physics buddies made two points that stuck in my mind. Firstly, that despite the incompleteness of the Standard Model (mainly, that it did not include any kind of quantum theory of that most stubbornly politically incorrect of the physical forces, gravity) and the fact that precise calculations (or any calculations at all) become extraordinarily difficult when it comes to the high energy levels where as-yet-incompletely-understood phenomena such as quark-gluon and other strong-nuclear-force-dominated interactions occurr, it is still overwhelmingly the best thing going as far a coherent theory that appears to explain all or nearly all of the known phenomena (quantum gravity aside, as that is outside its purview) and actually allows one to make any kind of reasonable computations and testable predictions about the rest (e.g. the mass of the Higgs.)

His second point was that while the desire to bring gravity under the same single unified theoretical umbrella as the rest of the fundamental interactions made perfect scientific sense, the various resulting subjective biases about what form such a theory should take were just that - subjective biases, rather than science. Supersymmetry (which arose out of an early version of string theory in the 1970s) had a mathematically appealing aesthetic but no real-world evidence in its favor (but to its credit did and does make some verifiable/refutable predictions, many of which should fall within the energy range of the LHC); string theory had an even more-debatable aesthetic (unless you call "the Standard Model is incomplete, so let's ditch it in favor of something completely speculative" an aesthetic) and to date has made no verifiable scientific predictions whatsoever. Even at that, time string theory had a religious-cultish aspect to it. This was noted in a 1986 article in Physics Today by particle-physics luminaries Paul Ginsparg and Sheldon Glashow, titled "Desperately Seeking Superstrings?" - they noted:

In lieu of the traditional confrontation between theory and experiment, superstring theorists pursue an inner harmony where elegance, uniqueness and beauty define truth. The theory depends for its existence upon magical coincidences, miraculous cancellations and relations among seemingly unrelated (and possibly undiscovered) fields of mathematics. Are these properties reasons to accept the reality of superstrings? Do mathematics and aesthetics supplant and transcend mere experiment? Will the mundane phenomenological problems that we know as physics simply come out in the wash in some distant tomorrow? Is further experimental endeavor not only difficult and expensive but unnecessary and irrelevant? Contemplation of superstrings may evolve into an activity as remote from conventional particle physics as particle physics is from chemistry, to be conducted at schools of divinity by future equivalents of medieval theologians. For the first time since the Dark Ages, we can see how our noble search may end, with faith replacing science once again. Superstring sentiments eerily recall “arguments from design” for the existence of a supreme being. Was it only in jest that a leading string theorist suggested that “superstrings may prove as successful as God, Who has after all lasted for millennia and is still invoked in some quarters as a Theory of Nature”?

The trouble began with quantum chromodynamics, an integral part of the standard model that underlies the quark structure of nucleons and the nuclear force itself. QCD is not merely a theory but, within a certain context, the theory of the strong force: It offers a complete description of nuclear and particle physics at accessible energies. While most questions are computationally too difficult for QCD to answer fully, it has had many qualitative (and a few quantitative) confirmations. That QCD is almost certainly “correct” suggests and affirms the belief that elegance and uniqueness — in this case, reinforced by experiment – are criteria for truth.

No observed phenomenon disagrees with or demands structure beyond the standard model. No internal contradictions and few loose ends remain, but there are some vexing puzzles: Why is the gauge group what it is, and what provides the mechanism for its breakdown? Why are there three families of fundamental fermions when one would seem to suffice? Aren't 17 basic particles and 17 tunable parameters too many? What about a quantum theory of gravity? Quantum field theory doesn't address these questions, and one can understand its greatest past triumphs without necessarily regarding it as fundamental. Field theory is clearly not the end of the story, so something smaller and better is needed: Enter the superstring.

And they note ironically:

The trouble is that most of superstring physics lies up at the Planck mass — about 1019 GeV – and it is a long and treacherous road down to where we can see the light of day. A naive comparison of length scales suggests that to calculate the electron mass from superstrings would be a trillion times more difficult than to explain human behavior in terms of atomic physics. Superstring theory, unless it allows an approximation scheme for yielding useful and testable physical information, might be the sort of thing that Wolfgang Pauli would have said is “not even wrong.” It would continue to attract newcomers to the field simply because it is the only obvious alternative to explaining why certain detectors light up like video games near the end of every funding cycle.

This mystical/religious aspect of the field seems to have only gotten worse with the passage of time.

Anyway, at the time my friend described string theory as not a scientific theory at all, as it sought to explain no actual facts (except for the fact that the Standard Model didn't include gravity), and made no testable predictions. At the time it was widely hoped that with further work and refinement of "the" theory (actually more a collection of abstruse mathematics) the latter defect might be remedied. Alas, over 2 decades later, in that respect nothing has changed. What *has* changed, however, is that not only has string theory come to dominate much of theoretical particle physics research (and a proportional share of the finding pie for same), the leading ST acolytes, instead of trying to make the field more scientific, are instead trying to change the definition of "science" in a way that suits them. This would all be laughable in a similar way to the bogus "scientific" credentials of the Intelligent Design movement (another faith-based initiative pretending to be otherwise) except that string theory gets a lot more government funding than ID, and attracts many of the nation's best scientific minds.

Fortunately, the tide at long last seems to be turning once again back in the direction of healthy skepticism. At least partly due to the success of the recent string-theory-critical books by insiders (or at least, former insiders to the field) Peter Woit and Lee Smolin, even mega-string-theory-proselytizers like Brian "El Elegante" Greene are apparently feeling a tad defensive. Greene recently wrote a lengthy guest editorial for The New York Times, titled "The Universe on a String". (Which I reprint below, in case the original NYT link goes dead or is moved to subscribers-only status.) I found the following quote by Professor Greene to be particularly telling:

"Exploring the unknown requires tolerating uncertainty."

But, at least for string theorists, the problem seems to be that their way of doing things, while admittedly (some might say "distressingly") tolerant of uncertainty, is not tolerant of *dissent* - check out what percentage of String Theory conferences only invite fellow ST cultists (I don't know if there's a secret password or Freemason-style initiation rites). It's somewhat reminiscent of how Niels Bohr's "Copenhagen Interpretation" doctrinal school tried to crush any dissent of their preferred interpretation of quantum mechanics in the first half of the twentieth century, but with one huge difference: quantum mechanics actually *worked*. String "theory", on the other hand, isn't a scientific theory at all - it's a large collection of interesting mathematics which purports to have relevance to the physical world, but which has yet to provide any compelling evidence that is indeed the case. There are at present so many variants of and tunable parameters in "the" theory that one can make it fit pretty much any selected subset of physical criteria one wants - only to run into paradoxes and a further wild proliferation of possibilities elsewhere.

My main concern is that the wildly popular and by-now self-reinforcing bandwagon that is string theory has come to dominate research in theoretical physics to such an extent that other potentially promising avenues of exploration are withering on the vine. It's basic human nature, having devoted a large portion of one's career to a particular avenue of exploration, to not want to engage in harsh (but scientifically necessary) self-examination, to address the issue (especially in the more speculative areas of research, which, make no mistake, string theory is, in spades) of: "But what if it's all just a bunch of hooey?" Even if the string theorists themselves are unwilling to ask themselves that, the funding agencies have a responsibility to do so. It would be unconscionable if, several decades and thousands of talented students' careers and many millions of precious research-funding dollars hence, string theory indeed proved to be just that: a bunch of hooey.

• Peter Woit's excellent and lively Not Even Wrong weblog.

The Universe on a String

By BRIAN GREENE
Published: October 20, 2006

Seventy-five years ago this month, The New York Times reported that Albert Einstein had completed his unified field theory - a theory of nature's forces into a single, tightly woven mathematical tapestry. But as had happened before and would happen again, closer scrutiny revealed flaws that sent Einstein back to the drawing board. Nevertheless, Einstein's belief that he'd one day complete the unified theory rarely faltered. Even on his deathbed he scribbled equations in the desperate but fading hope that the theory would finally materialize. It didn't.

In the decades since, the urgency of finding a unified theory has only increased. Scientists have realized that without such a theory, critical questions can't be addressed, such as how the universe began or what lies at the hart of a black hole. These unresolved issues have inspired much progress, with the most recent advances coming from an approach called string theory. Lately, however, string theory has come in for considerable criticism. And so, this is an auspicious moment to reflect on the state of the art.

First, some context. For nearly 300 years, science has been on a path of consolidation. In the 17th century, Isaac Newton discovered laws of motion that apply equally to a planet moving through space and to an apple falling earthward, revealing that the physics of the heavens later, Michael Faraday and James Clerk produce magnetic fields, and moving magnets can produce electric currents, establishing that these two forces are as united as Midas' touch and gold. And in the 20th century, Einstein's work proved that space, time and gravity are so entwined that you can't speak sensibly about one without the others.

This striking pattern of convergence, linking concepts once thought unrelated, inspired Einstein to dream of the next and possibly final move: merging gravity and electromagnetism into a single, overarching theory of nature's forces.

In hindsight, there was almost no way he could have succeeded. He was barely aware that there were two other forces he was neglecting - the strong and weak forces acting within atomic nuclei. Furthermore, he willfully ignored quantum mechanics, the new theory of the microworld that was receiving voluminous experimental support, but whose probabilistic framework struck him as deeply misguided. Einstein stayed the course, but by his final years he had drifted to the fringe of a subject he had once dominated.

After Einstein's death, the torch of unification passed to other hands. In the 1960's, the Nobel Prize-winning works of Sheldon Glashow, Abdus Salam and Steven Weinberg revealed that at high energies, the electromagnetic and weak nuclear forces seamlessly combine, much as heating a cold vat of chicken soup causes the floating layer of fat to combine with the liquid below, yielding a homogeneous broth. Subsequent work argued that at even higher energies the strong nuclear force would also meld into the soup, proposed consolidation that has yet to be confirmed experimentally, that there is no fundamental obstacle to unifying three of nature's four forces.

For decades, however, the force of gravity stubbornly resisted joining the fold. The problem was the very one that so troubled Einstein: the disjunction between his own general relativity, most relevant for extremely massive objects like stars and galaxies, and quantum mechanics, the framework invoked by physics to deal with exceptionally small objects like molecules and atoms and their constituents.

Time and again, attempts to merge the two theories resulted in ill-defined mathematics, much like what happens on a calculator if you try to divide one by zero. The display will flash an error message, reprimanding you for misusing mathematics. The combined equations of general relativity and quantum mechanics yield similar problems. While the conflict rears its head only in environments that are both extremely massive and exceptionally tiny 0 black holes and the Big Bank being two primary examples - it tells of a fissure in the very foundations of physics.

Such was the case until the mid-1980's, when a new approach, string theory, burst onto the stage. Difficult and complex calculations by the physicists John Schwarz and Michael Green, who had been tolling for years in scientific obscurity, gave compelling evidence that this new approach not only unified gravity and quantum mechanics, as well as nature's other forces, but did so while sweeping aside previous mathematical problems. As word of the breakthrough spread, many physicists dropped what they were working on and joined a global effort to realize Einstein's unified vision of the cosmos.

String theory offers a new perspective on matter's fundamental constituents. Once viewed as point-like dots virtually no size, particles in string theory are minuscule, vibrating, string-like filaments. And much as different vibrations of a violin string produce different musical notes, different vibrations of the theory's strings produce different kinds of particles. And electron is a tiny string vibrating in one pattern, a quark is a string vibrating in a different pattern. Particles like the photon that convey nature's forces in the quantum realm are strings vibrating in yet other patterns.

Crucially, the early pioneers of string theory realized that one such vibration would produce the gravitational force, demonstrating that string theory embraces both gravity and quantum mechanics. In sharp contrast to previous proposals that cobbled gravity and quantum mechanics uneasily together, their unity here emerges from the theory itself.

While accessibility demands that I describe these developments using familiar words, beneath them lies a bedrock of rigorous analysis. We now have more then 20 years of painstaking research, filling tens of thousands of published pages of calculations, which attest to string theory's deep mathematical coherence. These calculations have given the theory countless opportunities to suffer the fate of previous proposals, but the fact is that every calculation that has ever been completed within string theory is free from mathematical contradictions.

Moreover, these works have also shown that many of the prized breakthroughs in fundamental physics, discovered over the past two centuries through arduous research using a wide range of approaches, can be found within string theory. It's as if one composer, working in isolation, produced the greatest hits of Beethoven, Count Basie and the Beatles. When you also consider that string theory has opened new areas of mathematical research, you can easily understand why it's captured the attention of so many leading scientists and mathematicians.

Nevertheless, mathematical rigor and elegance are not sufficient to demonstrate a theory's relevance. To be judged a correct description of the universe, a theory must make predictions that are confirmed by experiment. And as a small but vocal group of critics of string theory justly emphasize, string theory has yet to do so. This is a key point, so it's worth serious scrutiny.

We understand string theory much better now than we did 20 years ago. We've developed powerful techniques of mathematical analysis that have improved the accuracy of its calculations and provided invaluable insights into the theory's logical structure. Even so, researchers worldwide are still working toward an exact and tractable formulation of the theory's equations. And without that final formulation in hand, the kind of detailed a definitive predictions that would subject the theory to comprehensive experimental vetting remain beyond our reach.

There are, however, features of the theory that maybe open to examination even with our incomplete understanding. We may be able to test the theory's predictions of particular new particle species, of dimensions of space beyond the three we can directly see, and even its prediction that microscopic black holes may be produced through highly energetic particle collisions. Without the exact quotations, our ability to describe these attributes with precision is limited, but the theory gives enough direction for the Large Hadron Collider, a gigantic particle accelerator now being built in Geneva and scheduled to begin full operation in 2008, to search for supporting evidence by the end of the decade.

Research has also revealed a possibility that signatures of string theory are imprinted in the radiation left over from the Big Bang, as well as in gravitational waves rippling through space-time's fabric. In the coming years, a variety of fabric. In the coming years, a variety of experiments will seek such evidence with unprecedented observational fidelity. And in a recent, particularly intriguing development, data now emerging from the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory appear to be more accurately described using string theory methods than with more traditional approaches.

To be sure, no one successful experiment would establish that string theory is right, but neither would the failure of all such experiments prove the theory wrong. If the accelerator experiments fail to turn up anything, it could be that we need more powerful machine; if the astronomical observations fail to turn up anything, it could mean the effects are too small to be seen. The bottom line is that it's hard to test a theory that not only taxes the capacity of today's technology but is also still very much under development.

Some critics have taken this lack of definitive predictions to mean that string theory is a protean concept whose advocates seek to step outside the established scientific method. Nothing could be further from the truth. Certainly, we are feeling our way through a complex mathematical terrain, and no doubt have much ground yet to cover. But we will hold string theory to the usual scientific standard: to be accepted, it must make predictions that are verified.

Other detractors have seized on recent work suggesting that one of string theory's goals beyond unification of the forces - to provide an explanation for the values of nature's constants, like the mass of the electron and the strength of gravity - may be unreachable (because the the4ory may be compatible with those constants having arrange of values). But even if this were to prove true, realizing Einstein's unified vision would surely be prize enough.

Finally, some have argued that if, after decades of research involving thousands of scientists, the theory is still a work in progress, it's time to give up. But to suggest dropping research on the most promising approach to unification because the work has failed to meet an arbitrary time-table for complete success is, well, silly.

I have worked on string theory for more than 20 years because I believe it provides the most powerful framework for constructing the long-sought unified theory. Nonetheless, should an inconsistency be found, or should future studies reveal an insuperable barrier to making contact with experimental data, or should new discoveries reveal a superior approach, I'd change my research focus, and I have little doubt that most string theorists would too.

But this hasn't happened.

String theory continues to offer profound breadth and enormous potential. It has the capacity to complete the Einsteinian revolution and could very well be the concluding chapter in our species' age-old quest to understand the deepest workings of the cosmos.

Will we ever reach that goal? I don't know. But that's both the wonder and the angst of a life in science. Exploring the unknown requires tolerating uncertainty.

Brian Greene, a professor of physics and mathematics at Columbia [University], is the author of "The Elegant Universe" and "The Fabric of the Cosmos."