Abstract
Einstein’s 1909 application of fluctuation theory to Planckian radiation is challenged by the fact that radiation within a completely reflecting cavity is not in thermal equilibrium and therefore should not qualify as a candidate for analysis by Einstein’s theory. We offer an alternative interpretation wherein Planck’s function, to which Einstein applied his theory, represents the source function in the wall material surrounding a real, partially reflecting cavity. The source function experiences thermal fluctuations and radiation within the cavity (which originates in the wall material and has an intensity equal to the source function) fluctuates in concert. That is, blackbody radiation within a real cavity exhibits the thermal fluctuations predicted by Einstein, but the fluctuations have their origin in the wall material and are not intrinsic to radiation.

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