  In 1904 Austrian physicist Fritz Hasenohrl (1874-1915) examined blackbody
radiation 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)mc^2, which in early 1905 he corrected to
E=(3/4)mc^2. Because relativistic corrections come in at order v^2/c^2 and
Hasenohrl'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/c^2 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 invariably attributed
Hasenohrl's mistake to neglect of the cavity stresses. We examine Hasenohrl's
papers from a modern, relativistic point of view in an attempt to understand
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 Hasenohrl's thought experiment, including a 1923
paper by Enrico Fermi, is misleading if not incorrect.
