Hasenöhrl and the Equivalence of Mass and Energy


In 1904 Austrian physicist Fritz Hasenöhrl (1874-1915) examined blackbody ra-diation in a reflecting cavity. By calculating the work necessary to keep the cavity moving at a constant velocity against the radiation pressure he concluded that to a moving observer the energy of the radiation would appear to increase by an amount E = (3/8)mc2, which in early 1905 he corrected to E = (3/4)mc2. Because rel-ativistic corrections come in at order v2/c2 and Hasenöhrl’s gedankenexperiment evidently required calculations only to order v/c, it is initially puzzling why he did not achieve the answer universally accepted today. Moreover, that m should be equal to (4/3)E/c2 has led commentators to believe that this problem is identical to the famous “4/3 problem ” of the self-energy of the electron and they have invari-ably attributed Hasenöhrl’s mistake to neglect of the cavity stresses. We examine Hasenöhrl’s papers from a modern, relativistic point of view in an attempt to un-derstand where exactly he went wrong. The problem turns out to be a rich and challenging one with strong resonances to matters that remain controversial. We give an acceptable relativistic solution to the conundrum and show that virtually everything ever written about Hasenöhrl’s thought experiment, including a 1923 paper by Enrico Fermi, is misleading if not incorrect

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