As I said, I had the whole series of papers that developed techniques for construction of these special BPS black holes. Then with the second string revolution, these BPS black holes became a hot subject. For the first time, with the advent of these D-branes, we had rigorous methods to connect the area of these black hole configurations to the underlying microscopic properties. The real breakthrough came with the work of Strominger and Vafa, who employed the quantum theory of D-branes to shed light on the microscopics—the internal quantum mechanical states—of this exact type of black hole that we had been working with all along. In other words, examples of this black hole that we had constructed provided prototypes for which we could now describe the microscopics.

Well, I also happened to be on sabbatical leave that fall at the Institute for Advanced Study. This was when we were all feeling this excitement about the unification of all these disparate string theories into what is now called M theory. The recognition that these non-perturbative objects, these BPS states, played an important role in understanding this unification came on top of everything else. But, yes, it was exciting the way physics just kind of turned my way.

You know, it was a blessing and a curse. It was exciting, but I also felt a certain pressure now that what I was doing was hot. Although, that pressure also motivated me. So those years, 1995 to 1997, were also my most productive years. The main prize went to Strominger and Vafa, but I did really good work and contributed.

I have to say I wish I had had more courage at the time. I may have been a little bit too conservative. I was aware that these black holes could be interpreted in terms of these D-branes; however Strominger and Vafa made the important insights into quantum interpretations of such D-branes. Had I been somewhat more courageous my work might have made even more impact. I had all these BPS black holes and yet I didn't really dare to apply the technique of counting D-brane microstates, which was similar to the counting employed with Arkady Tseytlin prior to the advent of D-branes. But nevertheless my programmatic effort I pursued still paid off. I was contributing to these microscopic interpretations, but my primary role was constructing those prototypes, really providing the right types of black holes with which we could say something important.

From 1996 through 1998 I continued to focus on aspects of these BPS black holes, particularly rotational solutions. I did some work with Finn Larson, who was a post-doc here, and who worked with Frank Wilczek on black hole microscopics prior to D-branes. It was a programmatic approach with the goal to push our understanding of the microscopics of these black holes beyond these BPS configurations, beyond even near-BPS configurations. Can we understand this for general black holes, not just these special BPS ones?

We were only semi-successful. The ultimate verdict on this program is that these microscopics can really only be understood for black holes that are close to the limit of BPS black holes, where you have this beautiful feature that the mass is precisely related to the charge. When one considers black holes far away from that limit, when the mass is an independent parameter, you have a huge class of other solutions for which we lose the microscopic understanding.

I would say we’re in a consolidation phase. In other words, there is no specific-focused direction. It’s a little bit like the early 1990s. People are working on different aspects of what we believe is the fundamental theory, but there is no unifying focus at the moment driving everybody in one direction.

I'm continuing to work on both gravitational and particle physics aspects of string theory. My focus is understanding explicitly the spaces on which we can compactify these multi-dimensional string theories. I'm trying to understand how much real particle physics we can get from string theory. Can we get standard model with three families of quarks and leptons? So I'm still trying to marry the gravitational aspects of string theory to the particle physics aspects.

People dismiss all that now because we can in principle find a huge number of four-dimensional solutions from string theory. So a lot of people now say, "Let’s not even bother to construct such solutions. We know there will be a huge number. Let’s try to study these solutions statistically and try to derive something from there." I see it differently. I believe we have to dirty our hands at some point. We have to really try to construct these things. Even if we know there are billions of possibilities, we have to demonstrate how far we can get to something that looks like the real world. It is not the approach to the ultimate prize, but it’s a way of seeing how far we can get. To make a long story short, I am personally excited about the recent constructions with intersecting D-branes, since they provide intriguing examples of the models with the semi-realistic particle physics feature. We have constructed a larger class of models, and they're more realistic and look more and more like the standard model, with fewer and fewer potential problems. And along the way we have advanced techniques to construct such solutions. Will I find the ultimate vacuum of string theory? Probably not. But understanding this class of solutions may play an important role in the effort. This is my main motivation at this point.

Mirjam Cvetic, Ph.D.
University of Pennsylvania
Philadelphia, PA, USA