Physics or Philosophy?

In a recent comment Charles has drawn my attention to an interview with Sheldon Glashow, a "Viewpoint on String Theory." I'm not sure when this interview was conducted, I think it was 2003. In any case I thought it might be of interest for you and it fits well with the theme of our recent workshop on Experimental Search for Quantum Gravity, so I want to use it to discuss some questions about our search for quantum gravity.

Glashow is very critical of string theory and does not hesitate to say so:

"[S]uperstring theory ... is, so far as I can see, totally divorced from experiment or observation. If not totally divorced, pretty well divorced. They will deny that, these string theorists.

[T]here ain't no experiment that could be done nor is there any observation that could be made that would say, "You guys are wrong." The theory is safe, permanently safe. I ask you, is that a theory of physics or a philosophy?

There is today a disconnect in the world of physics. Let me put it bluntly. There are physicists, and there are string theorists."

He is careful to then point out that string theory is not entirely useless, just that its use is unclear:

"[String theory] leads to many interesting ideas... It has had an impact on modern mathematics. They may even have a practical impact some day, these things that string theorists do. One never knows, just as number theory, the most useless of the mathematical sciences, has given us cryptography and has given us a secure way to encode information. The string theorist may also produce something equally useful. May. So it is science, it is physics, it is mathematics. It does stimulate ideas in related fields."

Glashow could, at the time the interview was conducted, not know of the more recent applications of string theory to heavy ion physics or condensed matter systems. Arguably, this is exactly the practical impact that he asks for. He ends on a conciliatory note, speaking of his string theory friends:

"[A]lthough I occasionally pick on the work string theorists do, I describe them as physicists. They are interested in the same problems that I am. They're approaching those problems in different ways, ways that they regard as somewhat more productive than I do. But they're not searching for a theory of everything. They're just trying to create better theories."

Indeed, I would agree that physicists will find different approaches promising, but in the end we're all - more or less - interested in finding solutions to the same problems. The reason why I work on the phenomenology of quantum gravity is not that I think it's the one and only right way to progress, but that it's one of the necessary contributions and one that presently not enough people work on.

In any case, the question that I would like to draw from this interview and pose to you is whether there's a balance between phenomenology and pure theory that is ideal for progress and if so, how that balance can be reached?

To add my two cents: During the last decade or so, maybe starting with string-theory-inspired extra-dimensional models, in the area of physics beyond the standard model one could clearly notice phenomenology come into more fashion. I certainly welcome this trend. The problem is however that many of the phenomenological models we've seen are little more than ad-hoc proposed parameterizations of not-yet-observed effects (and lets not forget that adding parameters typically allows a better fit of the data). It thus seems to me one of the essential factors needed is a healthy interaction between theory, phenomenology, and experiment. A hundred years or so ago, it would have been hard to imagine these areas being disconnected, but the increase of our communities has resulted in a specialization that brings the risk of negatively affecting these vital connections. In the beginning of his interview, Glashow also wonders what happened to this interaction.

It is very easy today to focus on ones' own community and not look right or left. To make matters worse, it might even be career-wise beneficial. One of the side-effects is then that people outside that community may wonder if it's physics or philosophy. This incidentally is nothing specific to string theory, it's just that there's so many string theorists that other people talk about it. I wouldn't be surprised if the same thing happens in other fields than physics as well. (Neuroeconomics - Science or Philosophy?) I'm having one of my more optimistic days today. Thus, I think the trend we've seen in the last years is a good one, and one that will eventually lead to the connection between quantum gravity theory and experiment that we are lacking so far.