An Observer moving in the 3° K Radiation Field

Abstract

IF an observer is situated in a radiation field and observes that the radiation is black-body radiation of temperature T, then a second observer moving with a velocity v relative to the first will observe a different spectrum, and the spectrum will depend on the direction in which he looks. It is generally assumed¹ that the second observer, using the same radiometer as the first, would receive more power from the direction in which he is moving, but, according to a defective derivation by Condon and Harwit² based on photon counts, a detector receives less power from an approaching source than from a receding one, if hv/kT < 1.60. This also implies that the spectrum no longer appears as a black-body spectrum.