Why Did Einstein Put So Much Emphasis on the Equivalence Principle?
Robert J. Heaston
Einstein considered the discovery of the equivalence principle as ?die gl?ckichste Gedanke? in his life. Whether translated as ?the happiest thought?, ?luckiest thought,? or ?the most fortunate thought,? Einstein formulated the equivalence principle as the result of an epiphany he experienced in October or November 1907. He regarded the equivalence principle as the beginning step in adding gravitation to the special theory of relativity to create general relativity. Why? James Prescott Joule published a paper ?On the Mechanical Value of Heat? in 1850 and started an avalanche of interest in the equivalence of different forms of energy. Equivalence was the hot new addition to the 19th century physics paradigm. Heat was also considered then as a mode of motion that led to the kinetic theory of gases, statistical analysis, Avogadro's number, the motion of atoms and molecules and Einstein's paper on Brownian motion. Einstein could not help but be aware of the significant meaning of equivalence to physics. An overly simplistic interpretation of the equivalence principle is that ?gravitation is acceleration.? But there is much more to this interpretation because the equivalence principle is at the heart of the derivation of the field equations of general relativity. Various definitions of the equivalence principle in the literature are analyzed and put into perspective. Recognition of the overall importance of the equivalence principle leads to a dramatically new understanding of the general theory in the 21st century physics paradigm. TOPIC AREA: General Relativity.
