Activities in 1996-1997

From the Report for Academic Year 1996-1997
of the Institute for Advanced Study

PIET HUT continued his research on dynamics of dense stellar systems, such as star clusters and galactic nuclei, with an emphasis on large-scale computer simulations. Together with Jun Makino, from Tokyo University, and Steve McMillan, from Drexel, he has refined and implemented algorithms for the treatment of simultaneous local interactions in the dense cores of such systems. These algorithms were subsequently applied on the GRAPE-4, a special-purpose computer developed at Tokyo University, and at a speed of 1 Teraflops one of the fastest computers in the world.

For a wider class of dynamical problems, Hut developed a new class of time symmetrization meta-algorithms, together with Yoko Funato from Tokyo University, and Makino and McMillan. As an example, they demonstrated how the standard fourth-order Runge-Kutta method can be symmetrized to yield a highly improved accuracy. For cosmological applications, a new group-finding algorithm, HOP, was developed in collaboration with Daniel Eisenstein, member of the School of Natural Sciences. This algorithm offers an efficient, adaptive, and coordinate-independent way to identify galaxies and clusters of galaxies in large-scale simulations of the origin of structure in the Universe.

Realistic astrophysical simulations of star clusters should include some approximate form of stellar evolution, in addition to gravitational dynamics. In a joint project with Simon Portegies Zwart from Amsterdam University, Frank Verbunt from Utrecht University, and Makino and McMillan, dynamical models were constructed with an correspondingly evolving Hertzsprung-Russell diagram (for an illustration, see the demonstrations at web site http://casc.physics.drexel.edu).

Professor Hut organized a workshop on `Smooth Particle Hydrodynamics: Models, Applications, and Enabling Technologies', sponsored by NASA's Goddard Space Flight Center, and held at IAS in June 1997. This workshop provided additional guidance for the design of the next-generation special-purpose computer, the GRAPE-6, planned to operate at a speed of 1 Petaflops. Together with Jeffrey Arnold and Thomas Sterling, from the Jet Propulsion Laboratory and the California Institute of Technology, Hut, Makino and McMillan held a smaller preparatory workshop in April, also at IAS, in order to study the feasibility of using reconfigurable hardware to extend the current GRAPE design.

During a visit at the Santa Fe Institute, Hut continued his work in the general area of the study of limits to scientific knowledge through a collaboration with mathematician David Ruelle, from Institut des Hautes Etudes Scientifiques, and computer scientist Joseph Traub, from Columbia. He also extended his ongoing collaboration with cognitive psychologist Roger Shepard, from Stanford, to include philosopher of science Bas van Fraassen, from Princeton University, physicist Arthur Zajonc, from Amherst, and writer Steven Tainer, from Berkeley. With van Fraassen, he published a dialogue entitled `Elements of Reality', in which they discussed the central role of experience with respect to science and to human values.