|The cover is pretty cool|
Until not long ago, it sounded like physicists had found the cure to all the universe’s ills, and possibly to the ills of other universes as well. String theory had taken us to new dimensions, unveiled the ultimate laws of nature, and inspired some way cool computer graphics. The telegenic string theorist Brian Greene had hosted a successful NOVA miniseries and seemed poised for the leap to Hollywood stardom. (Correction, Oct. 13: Brian Greene is a Hollywood star.) Michio Kaku, another string theorist, had nailed one book contract after another, writing with an increasingly science-fictiony tone. Quotes about the successes of string theory — purported by Kaku and others to be the established theory of everything and the ultimate truth about the laws of nature — were often going unchallenged by reporters.
Much of the media’s changed attitude may have been inspired by a single man. Through his influential blog Not Even Wrong, Peter Woit has been for the last two years on a zealous, single-minded pursuit to deconstruct the string theory hype.
Woit’s anti-string theory fervor is now packaged in book form under the same name, “Not Even Wrong.” The appearance last month of its U.S. edition (the book came out in Britain first) coincided with the publication of another skeptical book, Lee Smolin’s “The Trouble with Physics,” and virtually every publication on Earth has by now reviewed one or the other, or both. (I just got Smolin’s book and will soon review it, too.)
Woit, a lecturer in the math department at Columbia University, in New York, makes an elaborate and often excruciatingly technical case trying to demonstrate that, in reality, string theory never even had a chance to explain the ultimate laws of nature, and that the hype around it is in fact having deleterious consequences on science.
I found Woit’s message alarming but not entirely convincing, and “Not Even Wrong” left me with more questions than answers. Here I will try to airdrop some of my dilemmas into the blogosphere, to see if the ecosystem shows any reaction.
When string theory became popular in the mid-1980s, it promised to give physics new beginnings. Three centuries earlier, Isaac Newton had established the context for classical physics, saying that space can be described by vectors, and that forces produce accelerations, and so on — the Newtonian laws of motion. Newton had also made another momentous discovery, with his law of gravitation. The two steps were logically distinct: nothing within the basic framework of Newtonian physics says that gravity between two bodies should be inversely proportional to the square of their distance — as opposed to, say, just their distance or its cube — but Newton figured that that the inverse-square formula was the one that fit the astronomical observations.
String theory was supposed to give new foundations to physics by changing both the framework and the laws governing the fundamental forces (which included not just gravity, but also the three forces of particle physics). The initial appeal of string theory lay in the hope that the two steps — the framework and the laws — would no longer be logically separable. Instead, both would be necessary mathematical consequences of the theory. The universe was supposed to make sense.
|The math dude|
Twenty years and thousands of research papers later, Woit explains, physicists haven’t even settled on what string theory is. Meanwhile, the hope that string theory would somehow naturally reveal the unique logically consistent set of fundamental forces and elementary particles is pretty much dead. String theory could not prove that the laws of nature were its logical consequences; worse than that, the opposite happened. Research showed that the string theory framework seems to allow for a virtually infinite variety of laws of nature (see for example my article in Symmetry magazine), and thus for a virtually infinite variety of possible universes. As Dennis Overbye once wrote in The New York Times, rather than being the theory of everything, string theory turned out to be the theory of anything. For reasons that would take me too far to go into here, this Amazon.com-like variety became known as the string-theory landscape.
|String theory stud|
Woit sees this as the ultimate proof of the failure of string theory. In the past few years, he and others have publicly engaged in what has become known as the string-theory landscape controversy (see for example my article in Physics News Update). But as Brian Greene pointed out at a round-table discussion in Tampa last year (an event organized by my AIP colleague Ben Stein during the American Physical Society’s April Meeting), this could be a case of string theory being a victim of its own hype. String theory is being judged by the standards of its original claims; but just because string theory did not live up to those expectations, Greene said, it does not mean that it is a failure. To go back to the example of Newtonian physics, the law of gravity does not follow logically from the laws of motion; within their framework, there are many different equations for the law of gravity that would make sense mathematically. Similarly, it is possible that physicists will some day nail down the right equations that describe our universe within the context of string theory.
For now, it’s hard to figure out what string theory — whatever string theory is — has to say about the real world, and whether it can be tested experimentally. But, Woit reminds us, scientific theories have to explain natural phenomena, or “make predictions.” He believes that string theory makes none, and that it never will.
I don’t completely agree that string theory doesn’t make predictions, or that it obviously never will. We may even not need to wait for string theorists to come up with new equations for the forces o nature. Again, one could make a comparison with Newtonian physics. Newton’s laws of motion make some very important predictions — for example, that “to every action there corresponds an equal and opposite reaction” — even if you know nothing about gravity or the other forces. Similarly, although string theory still doesn’t have an analogue for the law of gravity or for the other forces, it already has at least one very substantial fact to say about the world: that there are six more dimensions of space than those we can see. The reason why we haven’t seen them yet would be that they are too small for our current experiments to detect.
A new atom smasher due to start operating a year from now at CERN, in Geneva, might change that. Many physicists are hopeful that the Large Hadron Collider, as the atom smasher is called, might be powerful enough to “see” some signs of at least one extra dimension. “I would give it a 50-50 chance,” Joe Lykken, a theoretical physicist at Fermilab, said to me recently, even though, he said, those signs could be very subtle, “and may take another 20 years to figure out.”
You’re Not Falsifiable, but We Still Love You
Physicists disagree on whether seeing extra dimensions could be counted as a success for string theory. After all, other, less-exotic theories also predict the extra-dimensions signatures the LHC might see, and even say something about what those extra dimensions should look like. String theory can’t do that yet, and may never will. In particular, critics say, string theory doesn’t seem to have a way of predicting the size of the extra dimensions, so if none show up in a given experiment, string theorists could always say that the extra dimensions could have been too small for that experiment to detect. Some see this moving-target attitude as being close to pseudoscience. But isn’t that a bit unfair?
To be sure, the gold standard for a scientific theory is that its predictions should be falsifiable, as Woit reminds us. That means experiments should always be possible in principle that could show the theory to be wrong. A truly scientific prediction should tell us exactly how small the extra dimensions are, or at least give us a range.
|Stodgy old Mach|
But falsifiability, while desirable and ultimately inescapable, may be too strong a requirement for a young theory. Whether extra dimensions exist is one of the great questions of science, comparable to the question of whether matter is a continuum or it comes in discrete packets. By the mid 1800s there were strong hints that matter should be made of atoms, and the periodic table of elements even gave their empirical classification. But back then there was no mathematically developed explanation of atomic theory. Skeptics such as the Austrian physicist Ernst Mach regarded the atomic theory as unscientific, since atoms were not detectable experimentally. The word falsifiable was not yet en vogue at Mach’s time, but one could imagine him arguing that even if experiments of increasing precision had failed to discover the atom, proponents of atomic theory could have kept thinking they were right because (for a while) there seemed to be no limit to how small atoms could have been. But that doesn’t mean that asking the question was unscientific, or that trying to answer it didn’t lead to one of the greatest discoveries in history.
Eventually, it all boils down to how long you want to wait for a young, “promising” theory to live up to its promise. As an outsider and a non expert — albeit one who has done research in fields of math that are allegedly inspired by string theory — that is not a question for me to answer, but it seems like a point on which reasonable people could disagree. My modest prediction is that the next few years will decide if the media backlash against string theory will be followed a mood change in the physics theory mainstream. The LHC could see signs of extra dimensions or of new and exotic particles. Neither of those would prove that string theory is right, but either one would strongly boost the morale. But if the LHC sees none of that, the leaders of the field might start getting disaffected with string theory, and there could be a mass migration to other research interests.
Meanwhile, Woit argues, the public-relations successes of string theory have made it so that every physics department in the world that wants to be seen as being at the forefront of research will want to hire as many string theorists as they can find. Because there are a limited number of positions for theorists, other fields of theoretical physics end up being neglected. The process perpetuates itself, since prospective theorists who are now in graduate school will tend to choose the field where the jobs are — and become overspecialized in a subject that requires learning daunting amounts of prerequisites.
Woit even talks of an atmosphere of intimidation against young physicists who display an interest for other fields. Based on the anecdotal evidence I have seen, this might be true at some departments. One postdoc at a leading physics department once told me that he felt like he should do research in string theory rather than pursuing his interest in loop quantum gravity. “I didn’t want to bite the hand that feeds me,” he said. It may very well be true as Woit says that the community’s focus on string theory is choking other approaches to reforming physics.
However, Woit does not seem to have much faith in other approaches either. More generally, his book is very detailed in describing his qualms about string theory, but falls short when it comes to suggesting ways out of this impasse. Surprisingly, Woit is also reticent when it comes to pointing out one of the main shortcomings of string theory, mentioning it only in passing.
String theory is supposed to be the theory of everything. To claim that, a theory must at least be able to marry the two great physics theories of the 20th century: quantum theory and general relativity, Einstein’s theory of gravity. But in the church of strings, Einstein’s daughter would be marrying down. His theory would be reduced to a theory of the gravitational force, losing the ability to explain how galaxies bend light, how black holes form, and ultimately, how the universe evolves. General relativity predicts how the shape of space, and of the universe as a whole, can evolve in time, affected by the motion of matter and energy. String theory can’t do that — at least not yet. (Update October 20: Would String Theory Truly Realize Einstein’s Dream?)
Doing the math
The book has its strengths. Its highest moments are when Woit puts aside the professorial, sit-down-and-take-notes tone for that of the raconteur. Woit has a number of amusing anecdotes about mathematicians and physicists — as in the alledged ménage à trois involving Hermann Weyl and Erwin Schrödinger, or in the following quote attributed to David Hilbert, concerning the sexist discrimination by his colleagues against the mathematician Emmy Noether:
I do not see that the sex of a candidate is an argument against her admission as Privatdozent. After all, we are a university, not a bathing establishment.
|Institute for Advanced Study|
|Where did I put the physics again|
But the part I found most interesting personally was where the book documents the historical rise of the cultural split between physicists and mathematicians. Mathematicians are very well aware of it, but probably few of us could point to the point in history when math and physics started growing apart. Woit says that was with Weyl’s 1928 book “Group Theory and Quantum Mechanics,” whose formal, rarefied treatment turned off so many physicists. The situation was later exacerbated by the delirium of abstraction brought over by the Bourbaki movement. Nowadays most physicists seem to regard math as a supermarket. When they find something they like, they call it physics; the products they don’t buy are “just math,” even though they might come back some time later and use them as ingredients for their next recipe. Ironically, physicists will not call math with its name even when they are solving mathematical problems or inventing new mathematical concepts themselves.
In fact, Woit says, string theory may have been able to at least in part pull the two communities closer together again — perhaps the only positive outcome he is willing to credit it with. Mostly, it has been a case of mathematicians getting inspiration from new physics ideas, a trend that had started even earlier than the “string theory revolution.” (The nearly mystical aura of string theory-related research certainly pervaded mathematics departments throughout in the 1990s — when I was a student — though even those of us who worked on allegedly physics-inspired problems had little idea of what the physics was.) Unfortunately though, when Woit mentions landmarks such as Simon Donaldson’s studies on the topology of four-dimensional manifolds, it is only in passing and with little more than mentioning technical names. The reader is left wanting to know more about the supposed positive fallout of string theory on math. Hopefully somebody will write that book some day.