Bachelier and his Times:
A Conversation with Bernard Bru ∗†‡
Murad S. Taqqu
July 9, 2001
Louis Bachelier defended his thesis “Theory of Speculation” in 1900.
He used Brownian motion as a model for stock exchange performance.
This conversation with Bernard Bru illustrates the scientific climate of
his times and the conditions under which Bachelier made his discove-
ries. It indicates that Bachelier was indeed the right person at the right
time. He was involved with the Paris stock exchange, was self-taught
but also took courses in probability and on the theory of heat. Not
beinga part of the “scientific establishment,” he had the opportunity
to develop an area that was not of interest to the mathematicians of
the period. He was the first to apply the trajectories of Brownian mo-
tion, and his theories prefigure modern mathematical finance. What
follows is an edited and expanded version of the original conversation
with Bernard Bru.
Bernard Bru is the author, most recently, of Borel, Lévy, Neyman,
Pearson et les autres . He is a professor at the University of Pa-
ris V where he teaches mathematics and statistics. With Marc Barbut
and Ernest Coumet, he founded the seminars on the history of Proba-
bility at the EHESS (École des HautesÉtudes en Sciences Sociales),
which bringtogether researchers in mathematics, philosophy and the
∗This article first appeared in Finance and Stochastics . This is a slightly expanded
version. It appears in French in .
†AMS 1991 subject classifications: 01A55, 01A60, 01A65, 01A70.
‡The work was partially supported by the NSF Grant ANI-9805623 at Boston Univer-
sity. c© Murad S. Taqqu.
M.T. : It took nearly a century for the importance of Louis Bachelier’s
contributions to be recognized. Even today, he is an enigmatic figure. Little
is known about his life and the conditions under which he worked. Let’s
begin with his youth. What do we know about it?
B.B. : Not much. Bachelier was born in Le Havre to a well-to-do family
on March 11