Kachru and Silverstein Join KITP

Eva Silverstein (l.) and Shamit Kachru. Photo by Nell Campbell.

Shamit Kachru and Eva Silverstein joined the Kavli Institute for Theoretical Physics in the fall of 2009, jointly filling one KITP permanent member position. Each holds half of two full-time positions. The other is as a visiting professor in the UCSB Department of Physics, while on leave from Stanford University, where they have served as faculty members for 12 years.

Kachru and Silverstein have broad interests in theoretical physics, with contributions to cosmology, particle physics, and string theory and its applications.

String theory is an ambitious theoretical enterprise that posits that the most fundamental constituent of physical reality is a vibrating string. Different vibrations of the string are thought to give rise to the different particles, such as the photon, electron, and quarks of the Standard Model of particle physics.

Among their diverse research efforts, both Silverstein and Kachru, working with collaborators, have found mechanisms connecting string theory to primordial cosmology, “a subject that has some sensitivity to the short-distance structure of quantum field theory and gravity through small fluctuations in the cosmic microwave background radiation,” according to Silverstein.

Both researchers “motivate their work with physical questions, as opposed to particular techniques,” said Silverstein. “We are interested in some of the questions that string theory set out to answer, such as: How does gravity work at very short distances? How does cosmology work at very early times? Is there a useful top-down formulation of particle physics? We are interested in those questions, and want to pursue whatever techniques answer those questions.”

“One of the most exciting things about theoretical physics today,” said Kachru, “is that answers to several of the most timely questions (the nature of electroweak symmetry breaking, the mechanism which explains early universe inflation, and the theory behind high temperature superconductivity, for example) may well involve interesting new strongly coupled quantum field theory dynamics of the sort that recent advances in string theory allow us to study qualitatively and, perhaps eventually, even quantitatively.”

Kachru and Silverstein express “deep appreciation for the theorists at KITP and UCSB,” with whom they have “enjoyed extensive interaction and collaboration.” For instance, one of the primary papers on the “Landscape” of string vacua was a 2001 article that Kachru authored with UCSB physics professors Steve Giddings and Joe Polchinski. The latter is also a KITP permanent member.

Polchinski and Silverstein recently reported progress on the long-standing problem of providing a complete (i.e., “non-perturbative”) formulation of four-dimensional string vacua. Silverstein also maintains a very fruitful collaboration with UCSB physics professor Gary Horowitz on basic problems of gravitational physics.

Kachru and Silverstein have also collaborated on mathematical aspects of string theory with Sergei Gukov and David Morrison, UCSB professors of mathematics and physics.

What Kachru and Silverstein find especially attractive about the KITP are its panoply of physics programs and their accompanying promise of rich intellectual stimulation. Said Silverstein, “A special feature of the KITP is the way it naturally brings together many of the strongest contributors at a given time to a given problem in theoretical physics. Before a subject is fully understood, there are often competing views that are important to explore (or eliminate, as appropriate), and the programs here accomplish this very effectively.”

“I get bored easily,” Kachru said. “The prospect of being exposed to a constant stream of new ideas at KITP, and perhaps often finding new directions for research that interest me more than what I’m already doing, has great appeal.”

For instance, Kachru enjoyed the summer 2009 mini-program on “Quantum Criticality and the AdS/CFT Correspondence,” which explored newly realized convergences between condensed matter theory and string theory.

“Nobody is thinking,” said Kachru, “that a condensed matter system is literally captured by strings in anti de Sitter [i.e., ‘AdS’] space. But there is a different way to compute quantities of interest in a field theory by using this duality, and that may be as good a starting point as the ones people have more normally used to model quantitatively new phenomena in these systems.”

Both Kachru and Silverstein were also intrigued by the discussions in the other summer 2009 program on “The Physics of Higher Temperature Superconductivity.”

“Here is Anderson talking,” said Kachru, referring to Nobelist Phil Anderson of Princeton University. “And Fisher [i.e., Matthew Fisher, who recently moved from UCSB to Caltech] disagrees with what Anderson says. Subir [i.e., Subir Sachdev of Harvard] believes that the central issue is a zero-temperature quantum critical point, but various eminent people disagree with Subir. So,” said Kachru, “what you hear here at the KITP are different competing points of view. They are all well represented by strong exponents. So, somebody like me watching and listening to the discussions realizes there are competing views. I understand that what I am hearing are opinions rather than facts. That is the great thing about tuning into these presentations at the KITP, the opportunity to experience dueling ideas.”

Said Silverstein, “I enjoyed this workshop tremendously precisely because I was not working in this area. It was all new to me. ”Such a program affords," she said, “a very easy mechanism for hearing about the problems of the subject in a way that gets you up to speed very fast."

“One of the real benefits of being a theorist,” said Silverstein, “is that you can so readily think about all these different things. It is a trade-off, of course: there are obvious downsides to the speculation intrinsic in theory, so we should make use of its flexibility and freedom from programmatics.”

Experimentalists encumbered by the necessity of large investments in laboratory facilities and long commitment for the conduct of experiments cannot so easily afford the same wide-ranging work style. For those so inclined, such as Kachru and Silverstein, the strong appeal of the KITP is that it facilitates and enhances intellectual versatility.

Silverstein and Kachru majored in physics at Harvard. Though undergraduates there at roughly the same time (he graduated in 1990; and she, in 1992), they didn’t really get to know each other until graduate school in physics at Princeton University, where both had the same Ph.D. advisor, Edward Witten, of the Institute for Advanced Study. The two got married in 1999.

After spending time as a prestigious Junior Fellow at Harvard in 1994-95, Kachru along with Silverstein joined the theory group at Rutgers University in New Jersey. And both were appointed members of the Institute for Advanced Study in Princeton in 1999, the same year that Silverstein was named a John D. and Catherine T. MacArthur Foundation Fellow.

Appointed assistant professor at the Stanford Linear Accelerator Center (SLAC) in 1997, Silverstein was promoted in 2001 to associate professor (jointly in the Stanford physics department and SLAC) and promoted again in 2006 to full professor.

Kachru, moving from Berkeley, joined the Stanford faculty in 1999 and was promoted to a tenured joint appointment between the physics department and SLAC two years later. He also was promoted to full professor in 2006. Both express great admiration for close colleagues at Stanford and SLAC, many of whom also participate heavily in KITP programs.

Between them, Kachru and Silverstein have been awarded honors too numerous for individual mention in an article announcing both their appointments.

Said KITP Director David Gross, “We really could not have done better than appointing two of the very brightest lights of their generation. They are spectacular physicists. We are fortunate to have Eva and Shamit as colleagues, but I think that this opportunity for great intellectual stimulation is very good for them at this stage of their careers. I am delighted too that they are sharing a permanent member position at KITP — a first for us!”

KITP Newsletter, Winter 2009