This book is the fourth in a series of lectures of the S´ eminaire
Poincar´ e, whichis directed towards a large audience of physicists and
of mathematicians. The goal of this seminar is to provide up-to-date
information about general topics of great interest in physics. Both the
theoretical and experimental aspects are covered, with some historical
background. Inspired by the Bourbaki seminar in mathematics in its
organization, hence nicknamed "Bourbaphi", the Poincar´ e Seminar is
held twice a year at the Institut Henri Poincar´ e in Paris, with cont-
butions prepared in advance. Particular care is devoted to the
pedagogical nature of the presentations so as to ful?ll the goal of
being readable by a large audience of scientists. This volume contains
the seventh such Seminar, held in 2005. It is devoted to Einstein's 1905
papers and their legacy. After a presentation of Einstein's ep-
temological approach to physics, and the genesis of special relativity,
a cen- nary perspective is o?ered. The geometry of relativistic
spacetime is explained in detail. Single photon experiments are
presented, as a spectacular realization of Einstein's light quanta
hypothesis. A previously unpublished lecture by Einstein, which presents
an illuminating point of view on statistical physics in 1910, at the
dawn of quantum mechanics, is reproduced. The volume ends with an essay
on the historical, physical and mathematical aspects of Brownian motion.
We hopethatthe publicationofthis serieswill servethe community ofphy-
cists and mathematicians at the graduate student or professional level.
