### Einstein the Physicist

Einstein's worldwide fame and his well-publicized advocacy of various causes after World War I depended entirely upon what came first his remarkable achievements as a physicist

1905: Einstein's "Annus Mirabilis" (miraculous year): While working as a clerk in the patent office in Bern, Switzerland, Einstein composed five papers that revolutionized 20th century physics. His paper on the light quantum opened the door to quantum mechanics. Two papers on the atomistic hypothesis considered Brownian motion and helped establish once and for all the reality of atoms. Two papers on the theory of special relativity rendered our notions of time relative and posited the interrelationship of energy and mass.

The birth of quantum statistics: Einstein struggled all his life against the probabilistic interpretation of quantum mechanics: hence his assertion that "God is not playing dice with the universe." Yet he himself made fundamental contributions to quantum mechanics through his use of statistical physics. Those contributions began with his 1906 study of the specific heat of solids and extended through his 1925 prediction of the Bose-Einstein condensation, verified experimentally in the 1990s.

"The happiest thought of my life": Einstein later described with these words his realization in 1907 of the equivalence between accelerated systems and systems within a gravitational field, a breakthrough which enabled him to incorporate gravity within his general theory of relativity. Between 1913 and 1917 he folded into the theory a Riemannian geometry of curved surfaces. Although classical physics predicted the precession of the elliptical orbits of planets around the sun, it could not explain the exact degree of that precession. In 1916, when Einstein included the relativistic contribution within his latest calculations of the precession of the planet Mercury's orbit, he found to his utter delight that his results agreed perfectly with astronomical observations. In 1919, two British expeditions measuring data from an eclipse of the sun proved that Einstein had successfully and precisely predicted a second phenomenon which classical mechanics could not explain: the deflection of light in the presence of a gravitational body. Newspapers reporting that Einstein had overthrown Newton's concepts of space and gravity soon made him a worldwide celebrity.

The quest for a "unified field theory": The last thirty years of his scientific life Einstein dedicated to his relentless search for the unification of general relativity and quantum mechanics, still an unsolved problem today. Alternately optimistic and frustrated, he once quipped in response to a query, "Don't worry about your difficulties in mathematics. I can assure you mine are still greater."