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Daniel Thomas Gillespie (1938 - ) is a physicist who is best known for his derivation in 1976 of the stochastic simulation algorithm (SSA), also called the Gillespie algorithm. . The SSA is a procedure for numerically simulating the time evolution of the molecular populations in a chemically reacting system in a way that takes account of the fact that molecules react in whole numbers and in an essentially random way. Since the late 1990s, the SSA has been widely used to simulate chemical reactions inside living cells, where the small molecular counts of some reactant species often invalidate the differential equations of traditional deterministic chemical kinetics.

Gillespie's original derivation of the SSA began by considering how chemical reactions actually occur in a well-stirred dilute gas. Reasoning from physics (and not by heuristically extrapolating deterministic reaction rates to a stochastic context), he showed that the probability that a specific reaction will occur in the next very small time dt could be written as an explicit function of the current species populations multiplied by dt. From that result he deduced, using only the laws of probability, an exact formula for the joint probability density function p(τ,j) of the time τ to the next reaction event and the index j of that reaction. The SSA consists of first generating random values for τ and j according to p(τ,j), and then actualizing the next reaction accordingly. The generating step of the SSA can be accomplished using any of several different methods, and Gillespie's original paper presented two: the "direct method", which follows from a straightforward application of the well known Monte Carlo inversion method for generating random numbers; and the "first-reaction method", which is less straightforward but mathematically equivalent. Later workers derived additional methods for generating random numbers according to Gillespie's function p(τ,j) which offer computational advantages in various specific situations. It should be noted that Gillespie's original derivation of the SSA  applied only to well-stirred dilute gas systems. It was widely assumed/hoped that the SSA would also apply when the reactant molecules are solute molecules in a well-stirred dilute solution with many smaller solvent molecules. In fact it does, but that was not definitively established until 2009. The extent to which the SSA is valid under other typical cellular conditions, such as when the reactant molecules crowd each other or when they move by active transport mechanisms along physically confined pathways, remains to be seen.

The SSA is one component of stochastic chemical kinetics, a field that Gillespie played a major role in developing and clarifying through his later publications. His broader research has produced articles on cloud physics, random variable theory , Brownian motion  , Markov process theory  , electrical noise   , light scattering in aerosols  , and quantum mechanics.

Education
Born in Missouri, Gillespie grew up in Oklahoma where he graduated from Shawnee High School in 1956. In 1960 he received his B.A. (Magna cum Laude) with a major in physics from Rice University.

Gillespie received his Ph.D. from Johns Hopkins University in 1968 with a dissertation in experimental elementary particle physics under Aihud Pevsner. Part of his dissertation derived procedures for stochastically simulating high-energy elementary particle reactions using digital computers, and Monte Carlo methodology would play a major role in his later work. During his graduate student years at JHU he was also a Jr. Instructor (1960-63) and an Instructor (1966-68) in the sophomore General Physics course.

Career
From 1968 to 1971, Gillespie was a Faculty Research Associate at the University of Maryland College Park's Institute for Molecular Physics. He did research in classical transport theory with Jan Sengers. In 1971 he was also an Instructor in the University's Physics Department.

From 1971 to 2001, Gillespie was a civilian scientist at the Naval Weapons Center in China Lake, California. Initially he was a Research Physicist in the Earth and Planetary Sciences Division. There his research in cloud physics led to a procedure for simulating the growth of raindrops in clouds, and that prompted his paper on the SSA. In 1981 he became Head of the Research Department's Applied Mathematics Research Group, and in 1994 he was made a Senior Scientist in the Research Department.

Since his retirement from China Lake in 2001, Gillespie has been a private consultant in computational biochemistry, working under contract for various periods of time with the California Institute of Technology, the Molecular Sciences Institute (in Berkeley), the Beckman Institute (at Caltech), and the University of California, Santa Barbara. Most of this work has been, and continues to be, in collaboration with the Linda Petzold research group in the Computer Sciences Department of UCSB.

Books by Gillespie

 * Was in print from 1970 to 1986 by International Textbook Co., International Textbook Co. Ltd, Halstead Press, and Editorial Reverte (Spanish translation).
 * A short biography of radio and television comedy writer Tom Koch, focusing mainly on his work for Bob and Ray.
 * Scheduled to be published in October 2012.
 * Scheduled to be published in October 2012.