Photo by Dorothy Littell Greco "String theory seems to be a theoretical structure that you can't escape," says physicist Andrew Strominger of Harvard University. "It bangs you on the head wherever you turn."

   When superstring theory arrived in physics in 1984 as a potential theory of the universe, it was considered by mainstream physicists as little better than religion in terms of constituting a viable, testable theory. In string theory, the fundamental particles were string-like, rather than point particles; the universe had 10 or 11 dimensions, rather than four; and the theory itself existed at an energy so far from earthly energies that it took a leap of enormous faith to imagine the day when an experiment could ever test it. Quite simply, string theory seemed an excessively esoteric pursuit, which it still is. But the last five years have seen the theory undergo a series of major breakthroughs–theoretical ones, at least–while simultaneously entering the mainstream of the field. Last summer's string theory conference in Santa Barbara was attended by 350 physicists. And universities have taken to holding bidding wars to recruit the best string theorists–with Harvard, Stanford, and Princeton lately leading the way.

   Among the hottest physicists driving the string theory revolution is Harvard University's Andrew Strominger, who over the past five years has led the way in merging the study of quantum-mechanical black holes–a pursuit popularized by Cambridge University's Stephen Hawking–with that of string theory to advance understanding in both fields. During the spring and summer of last year, Strominger's 1996 paper in Physics Letters B, "Microscopic origins of the Bekenstein-Hawking entropy," written with Harvard colleague Cumrun Vafa, routinely appeared in Science Watch’s Physics Top Ten, taking the top spot in the May/June 1998 issue. That paper has now been cited nearly 500 times (see table on the next page, paper #1). In a field in which many researchers have taken to publishing their papers only electronically, Strominger has published more than 25 papers that have each attracted more than 100 citations, while his 1985 paper on "Vacuum configurations for superstrings, " written with Phil Candelas, Gary Horowitz, and Ed Witten, has garnered well over 1,500 citations (see table, paper #2).

   Strominger, 43, graduated from Harvard University in 1977. He completed a Master's degree at the University of California at Berkeley before earning his doctorate at the Massachusetts Institute of Technology in 1982 with Roman Jackiw. Strominger spent the next five years at the Institute for Advanced Study in Princeton before joining the faculty of the University of California at Santa Barbara. In 1997, Strominger moved back to Harvard University, where he is now a professor of physics.

Strominger spoke to Science Watch correspondent
Gary Taubes over a dinner in Aspen, Colorado.

Your paper with Cumrun Vafa involves both black holes and string theory. Could you explain how these two come together?

   Strominger: The problem we set out to solve is that of understanding the Bekenstein-Hawking entropy, named for Jacob Bekenstein and Stephen Hawking. Historically, there are two ways that physicists have thought about entropy. The first was in the 18th century, when they discovered experimentally that all thermodynamic systems have some kind of entropy associated with them, and that there is a set of laws–the laws of thermodynamics–in which the entropy plays a key role. Then, in the 19th century, the Austrian physicist Ludwig Boltzmann derived those laws from more fundamental laws: he showed that if you take a gas, and if you have a model for the gas as a collection of molecules bouncing around, you can apply statistical reasoning and actually derive the laws of thermodynamics from the more fundamental microscopic laws of the fundamental constituents of the gas, namely the molecules. This is the kind of progress that physicists want to achieve–they want to have fewer laws. One interesting thing to keep in mind is that at the time Boltzmann derived the laws of thermodynamics, the theory of molecules was extremely controversial, and it wasn't until 50 years later–with Brownian motion and so on–that people really began to believe that molecules actually exist.
   In this century there has been a parallel development in the subject of black holes. In the 1970s, Stephen Hawking discovered that black holes had to have both a temperature and an entropy. That's this famous Bekenstein-Hawking entropy. Black holes were found to obey a set of laws which are perfectly analogous to the laws of thermodynamics. But what was missing back in the 1970s was a derivation of the laws of black holes from some fundamental principals–in other words, the analogue of Boltzmann's derivation of the thermodynamics of gasses. Many people wanted to derive the black hole laws from a fundamental set of principles. In particular, the central goal was to derive the Bekenstein-Hawking entropy by counting the number of quantum microstates of a black hole, in the same sense that Boltzmann derived the entropy of a gas by counting the quantum microstates of the gas.

Since when do black holes have quantum microstates?

   Strominger: Well, that was suggested by the entropy formula. We know that, in general, entropy counts the number of quantum microstates for everything besides black holes. It would be a deep and unnerving asymmetry if the relation between entropy and the number of microstates was valid for every system in nature except a black hole.

So what are these quantum microstates?

   Strominger: That was the problem we had to solve. In order to count microstates, you need a microscopic theory. Boltzmann had one–the theory of molecules. We needed a microscopic theory for black holes that had to have three characteristics: One, it had to include quantum mechanics. Two, it obviously had to include gravity, because black holes are the quintessential gravitational objects. And three, it had to be a theory in which we would be able to do the hard computations of strong interactions. I say strong interactions because the forces inside a black hole are large, and whenever you have a system in which forces are large it becomes hard to do a calculation.
   The old version of string theory, pre-1995, had these first two features. It includes quantum mechanics and gravity, but the kinds of things we could calculate were pretty limited. All of a sudden in 1995, we learned how to calculate things when the interactions are strong. Suddenly we understood a lot about the theory. And so figuring out how to compute the entropy of black holes became a really obvious challenge. I, for one, felt it was incumbent upon the theory to give us a solution to the problem of computing the entropy, or it wasn't the right theory. Of course we were all gratified that it did. continued