Theoretical physics is the search for a mathematical description of the world around us. For centuries, simplicity and elegance have been guiding principles in this search-time and again we have seen that the world is simpler than it could have been, and that complex phenomena can be captured by a small set of ideas whose mathematical formulation provides startlingly successful predictions of things waiting to be discovered. Despite these successes, many everyday things have eluded our grasp. Most obvious are the phenomena of life: from the decisions made by bacteria as they adapt to different environments to our own perceptions and memories, we have a whole universe of beautiful phenomena untamed by theory. Can we imagine a theoretical physics of biological systems? Can we reconcile the physicists' desire for unification with the obvious diversity of life's mechanisms? Could such theories engage meaningfully with the myriad details of particular systems, yet still be derivable from succinct and abstract principles that transcend these details? I am an optimist, and I think the answers to these questions are "yes," but they still keep me awake at night. In this talk I will try to convey my reasons for optimism, discussing a series of examples where I think we can see glimpses of more general principles. Along the way we'll look at problems ranging from the development of an embryo to the flocking of birds, with many stops in between.

William Bialek is the John Archibald Wheeler/Battelle Professor in Physics, and a member of the multidisciplinary Lewis-Sigler Institute, at Princeton University. He also serves as Visiting Presidential Professor of Physics at the Graduate Center of the City University of New York. An alumnus of the University of California at Berkeley, and a former KITP postdoctoral fellow, he was on the faculty at Berkeley and a member of the NEC Research Institute before moving to Princeton in 2001. Bialek's research interests include a wide variety of theoretical problems at the interface of physics and biology, from the dynamics of individual biological molecules to learning and cognition. Best known for contributions to our understanding of coding and computation in the brain, Bialek and collaborators have shown that aspects of brain function can be described as essentially optimal strategies for adapting to the complex dynamics of the world, making the most of the available signals in the face of fundamental physical constraints and limitations. More recently he has followed these ideas of optimization into the early events of embryonic development, and the processes by which all cells make decisions about when to read out the information stored in their genes. His hope is that these diverse biological phenomena may be understandable through some unifying theoretical principles, in the physics tradition