This book presents the first English translation of the original French treatise “La Physique d’Einstein” written by the young Georges Lemai^tre in 1922, only six years after the publication of Albert Einstein’s theory of General Relativity. It includes an historical introduction and a critical edition of the original treatise in French supplemented by the author’s own later additions and corrections. Monsignor Georges Lemai^tre can be considered the founder of the “Big Ba