Daniel Thomas Gillespie, a research physicist best known for his derivation of the Stochastic Simulation Algorithm (The Gillespie Algorithm) and innovator in the field of stochastic physics in biology, died peacefully at his home in Castaic, CA on April 19th, 2017. He was 78.

The Gillespie Algorithm has numerous applications, including cell biology, epidemiology, and cancer research. As is often the case in science, when Dan first presented his original work in 1976, it was called “completely wrong” by some prominent in the field, a pronouncement which was successfully refuted. More prescient commentators noted that future advances in computing power might someday render it useful. Indeed, Dan’s foundational publications would eventually garner over 11,000 literature citations, and he is credited with helping found the field of stochastic physics in biology.

Dan was born August 15th, 1938, in Springfield, Missouri. His father, Kenneth Gillespie, was an electrical engineer who ran the local appliance store in Shawnee, OK, and his mother, Betrix, was an English teacher. Dan graduated from Shawnee High School in 1956. Granted a full scholarship to Rice University, he received his B.A. in physics in 1960. Continuing his studies at Johns Hopkins University, he was awarded his Ph.D. in high energy physics in 1968.

Dan first came to China Lake in 1961, where he spent summers as a Junior Professional in the Chemistry Division. After getting his Ph.D. and completing a post-doc at the University of Maryland, Dan returned to the Naval Weapons Center in 1971, working for Pierre Saint-Amand as a Research Physicist. In 1981, Dan helped establish the Applied Mathematics Group, which he led until 1994. After 30 years, Dan retired in 2001 as a Senior Scientist in the Research Department.

Dan then embarked upon a second career consulting with scientists at Caltech and UC Santa Barbara. During this period, he and wife Carol traveled the world with Dan as an invited speaker at numerous scientific conferences in the US, Australia, Canada, and Europe.

In addition to publishing 97 scientific journal articles and letters, Dan wrote textbooks on quantum mechanics, Markov processes, and Brownian motion. His proudest achievement was the Small Voxel Tracking Algorithm, an extension of his original work, published in the Journal of Chemical Physics in 2014 with coauthors Effrosyni Seitaridou and wife Carol Gillespie.

Dan had two sons, Mark and Ewan, through his first marriage to Mariel Louise McEwan, daughter of Bill McEwan who headed the Chemistry Division. In 1976 Dan was remarried to Carol Ann Clarke. Carol was Dan’s best friend and partner in life over the next four decades.

Dan placed a high value on intelligence, moral integrity, and humor – although not necessarily in that order. Dan’s life – and that of those around him – was greatly enriched by his own keen sense of humor. Dan had an affinity for people who shared his love of Rocky and Bullwinkle, the movie Dr. Strangelove, and the comedy duo Bob and Ray. An enthusiast of old-time radio shows, Lone Pine film buff, and discriminating model train collector, Dan brought the same systematic approach to his hobbies that he put into his scientific endeavors. His tool cabinet was always organized. Dan was also an avid jazz fan – a love that he passed on to his family. After his second retirement in 2015, some of Dan’s greatest pleasures included listening to his grandson play saxophone and guitar, and watching his granddaughter dance ballet.

Dan died peacefully, surrounded by his family, after a 6-month fight with brain cancer. He is survived by his wife Carol; sons Mark and Ewan; their wives Chris and Yi; grandchildren Erica and Kevin; sister Jean Whitmarsh; and an extended-yet-close family. Dan leaves behind him a world better for his contributions – not to mention one fantastic collection of jazz albums.