Stringtime in Pittsburgh

Yes, last weekend it was Stringtime here in Pittsburgh! (No, we don’t have the good taste to resist that pun.) Here’s the Part I of my report.

First, the background: Laura Ruetsche and Gordon Belot were the chairs of another scintillating workshop at Pittsburgh’s Center for the Philosophy of Science. Speakers included:

  • Richard Dawid on `String Theory and the Question of Theory Assessment’;
  • Nick Hugget, `(Again) A Philosopher Looks at String Theory’; and
  • Lee Smolin‘s `Remarks on the Reality of Time in Physics and Cosmology.’


We kicked the morning off with a few pots of coffee and a discussion with Richard about how we ought to assess String Theory. Richard has a background in theoretical physics, and has spent his fair share of time worrying about strings. After all, as Richard points out, it’s been over 30 years, and it’s still not clear that any of the popular candidate string theories are complete, verifiable, or predictive — even in principle. (The situation is nicely summarized here.) As a result, the legitimacy of string physics is hanging from… well, um… a precariously thin string.

Richard’s main claim is that both string theorists and their critics are talking past each other, because their arguments are being made from incompatible paradigms — and in particular, incompatible understandings of what science is. As an alternative, he suggests that string theory be judged `on its own terms,’ that is, by the standards of its own paradigm.

Lee Smolin was among the first to comment on Richard’s talk. As one of the more outspoken recent critics of string theory, Smolin suggested an alternative characterization of the debate. There are now several available approaches to quantum gravity, such as topological QFT, and Loop Quantum Gravity (which Smolin himself helped develop). Proponents of these theories are critiquing string physics precisely on its own terms, for example by criticizing the background dependence of string theory.

Still, such arguments take place at a very abstract level, often by emphasizing the past success of some particular a priori principle, or of unification in physics. In the face of this, John Norton couldn’t help but remind us that these kinds of arguments are uncomfortably analogous to another debate that happened over 100 years ago, which asked: what is the status of the luminiferous ether?

As for me, I just kept wondering: why should we evaluate a theory `on its own terms’? For example, think about 19th century electrodynamics. The `paradigm’ of this theory 1) included an ontology of the luminiferous ether, of which it 2) was incapable of determining the state of rest. It also 3) fixed the speed of light, independent of one’s inertial frame. Special relativity allowed for a new view of electromagnetism by rejecting 1, retaining 2 and 3, and deriving the relativity of simultaneity as a consequence. Einstein didn’t evaluate electrodymanics on its own terms — he presented a new paradigm, with a less bloated ontology, and it turned out work better!

After some cheese, crackers, and more coffee, we reconvened for Nick Hugget’s enjoyable look at underdetermination in String Theory, of the kind that arises out of T-duality. The basic idea is: there is a symmetry of string theory (called T-duality) in which two very different descriptions of the world are observationally indistinguishable. Nick’s suggestion was that it’s not clear how to interpret T-duality, but perhaps the right interpretation of the measurement problem will help.

Here are a few of the details. One route into String Theory is to add two new operators to the canonical momentum and position observables p and q. These are: the winding w (defined in terms of the winding number of a string around a cylinder), and its conjugate `momentum’ u. The eigenstates of w are then interpreted as possible windings. Interestingly, you can exchange these pairs of observables, and get back a dual theory that cannot be (string theoretically) distinguished from the original. In particular, if the first theory T describes a world in which one dimension is curled up into a ring of radius R, then the dual theory T’ describes a world in which that ring has radius a/R. So, Nick asked, which one is the right description of the world?

(`Neither,’ mumbled Balazs Gyenis, eliciting chuckles from the crowd.)

Nick’s point is this: we have here a pretty striking difference, that nevertheless turns out to be unobservable given the tools of string theory. What we need is a staisfying interpretation of this symmetry.

These theories T and T’ predict different radii, which might seem like a real physical distinction. If so, one wouldn’t want to just write this off as `pure gauge.’ So apparently we’re stuck with underdetermination. And as Nick rattled off various examples of underdetermination, it seemed clear that no classic example of underdetermination (such as the Poincare sphere or a Leibnizian view of metaphysics) is analogous. Perhaps, Nick suggested in a closing note, a solution to the measurement problem will choose one theory over its dual.

This launched John Norton and Tony Duncan into a discussion of whether or not an interpretation of quantum theory can actually do this job — or if there is always a `dual interpretation’ that would just give us underdetermination again. Balazs Gyenis pointed out that our discussion had left unclear how one can operationalize the notion of `length’ that is used to characterize the offensive radius and its dual; perhaps this might clarify the kind of underdetermination we are dealing with. In the end, all these winding strings left us a bit wound up, and so the group retired to tank up on lunch before returning for Lee’s talk.

Soul Physics is authored by Bryan W. Roberts. Thanks for subscribing.
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