Illustration: Dina F. Graser

Physics has a special place among the sciences. The revolution in physics during the early part of the 20th century defined subsequent decades by spawning both the transistor and the atomic bomb. And physics has always been viewed as the hardest of all sciences, a domain of mathematically rigorous theories coupled with unambiguous experiments. But now, physicist Lee Smolin argues in his new book, physics is in danger of losing its way.

Smolin, based in Toronto, says in The Trouble With Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next that part of the problem is it is now almost impossible to conduct an experiment that cannot be accounted for within the framework of the Standard Model of particle physics. That model has reigned supreme for 30 years despite its incompatibility with that other great bastion of ­modern ­physics: general relativity. As a result, physicists have been starved of new ­experimental data that could clearly point the way to a fundamental theory that ­reconciles quantum mechanics and general relativity. In that experimental vacuum has risen string theory, which postulates that matter is not composed of pointlike particles but rather of structures consisting of 11-dimensional strings and membranes.

The other reason physics is in danger is sociological, and Smolin says this part of the problem facing physics should act as a warning to researchers in every field of science and engineering—that individuals and departments can make what appear to be reasonable decisions about whom to hire and what research to pursue, but ultimately those decisions can unbalance or distort a field as a whole.

Stephen Cass, IEEE Spectrum senior associate editor, talked with Smolin about the challenges facing physics.

Is there a crisis in physics?

The crisis is not in physics overall but in that part of physics whose goal is to deepen our understanding of the laws of nature. Since the middle 1970s, there has been no definitive progress, no match between a new experimental result and a new theoretical prediction. That’s worrying.

Many physicists say we have made great progress due to string theory. What’s wrong with string theory?

One of the things that’s wrong is the impression that you have been getting. The real situation is that there are several competing approaches to the problem of unifying quantum mechanics and general relativity to create a theory of quantum gravity. But string theory came to dominate the perception of the public and the American scientific academy. There are compelling reasons for interest in string theory, but there are also compelling reasons for interest in other approaches. Science would be stronger were there not an overemphasis on one approach.

On top of that, there has been overclaiming and overpromising on the part of some advocates of string theory. First of all, there was a hope, which was advocated very strongly, that string theory would lead to unique predictions about the universe. If true, that would have been a powerful reason for choosing it over the competing approaches, but it was just simply not true, and we knew from 1986 that it was not true.

String theory is not a theory in the sense that Newtonian mechanics or quantum mechanics is. It’s not defined by the statement of two or three principles that are expressed in the basic equations of the theory—which are then solved to yield examples and predictions. Instead, there are several approximation procedures and approximate arguments that describe an infinite number of cases, which are all conjectured to be solutions of a fundamental theory that has never been written down.

If string theory doesn’t have the normal characteristics of a theory, does it run the risk of undermining what it means for something to be a scientific theory?

It need not, but it has, because of the poor judgment by some advocates of string theory with regard to the question of whether the theory generates any predictions that might be tested. Once it became undeniable that string theory comes in an infinite number of different versions, which all give different predictions, some people, like Leonard Susskind and Steven Weinberg, unfortunately began to argue that, on the basis of other reasons—the mathematical beauty of the theory, et cetera—the theory was so compelling that we should consider this a situation where the rules of science should be modified.

Now, let me say that, for example, Brian Greene [author of The Elegant Universe, the best-selling 2003 book on string theory] urged me to emphasize that these people are a minority of string theorists. He believes the majority of string theorists agree with me that a scientific theory must make falsifiable predictions and that if string theory ultimately fails to do so, it will fail to be a scientific theory. But I don’t think this is a time for other renowned scientists to be calling for a weakening of the distinction between science and nonscience.