IAM INTERESTED in two things in the natural world: how things move and how the physics of interactions manifest themselves as collective phenomena.   These have led me to explore the evolution of astronomical systems from solar-system to groups of galaxies.  My style of work ranges from analytic perturbation theory to kinetic theory of N-body simulations to statistical inference from theoretical models and observational data.  I like to get to the bottom of things and attempt to understand the fundamental principles and apply these rigorously to astronomical problems.

Early on, in my thesis work, I studied the application of dynamical friction to secular evolution of galaxies with my mentor Scott Tremaine.  Then, I explored the evolution of globular cluster systems while at Cornell.  Dave Chernoff and I produced the first Fokker-Planck calculations with enough multimass species to see continuum effects and used these to constrain the evolution of the Galactic globular system.  At the same time, Stu Shapiro, Ira Wasserman and I examined the implication of collisions in field binary systems and the Nemesis hypothesis.  Ira and I went on to produce constrains on disk black holes from wide binaries.

While at the Institute for Advanced Study (IAS), I continued to think about binary and globular evolution while beginning to explore the modes and responses of stellar systems to perturbations.  I developed the theory of biorthgonal expansions to derive "dispersion relations" for stellar systems.  The first application was modes of a stellar sphere including a study of radial orbit instability.    This was followed up by N-body studies performed with colleagues at the IAS and additional fundamental theory, e.g. a new fundamental theory for adiabatic invariance in stellar dynamics, after moving to UMass in 1990.

I continued to investigate Galactic globular cluster evolution incorporating the new adiabatic theory and began applying the "dispersion relation" approach to a variety of secular evolution problems in barred galaxy evolution, tidal excitation, Magellanic cloud heating and Milky Way kinematics.  From 1994 through 2004, I was an active participant in and Science Team member of the 2MASS project.   My main contributions were methods of ensuring and diagnosing phometric uniformity including a procedure for a global photometric bootstrap calibration.  Sergei Nikolaev and I published several papers on the populations and structure Large Magellanic Cloud from 2MASS and this contiues to be an interest. 

All of this work is numerically intensive.  The perturbation theory, although formally analytic, demands numerical solutions.   To meet this need, I have developed three "suites" of parallel numerical codes for production work: 1) a perturbation theory package called Orbit; 2) an N-body code using the expansion approaoch which is well-suited to studying slow evolution over very long time scales; and 3) a Bayesian statistical package called the "Bayesian Inference Engine" (BIE).  This latter project grew out of the need to exploit 2MASS and other catalog data to contrain Milky Way and Local Group theory but is a stand-alone general Bayesian platform.