The Critical Test between Alternative Interpretations
The distinguishing feature between the two pictures is the recession. So the observer concentrates on the recession. He inquires if there is an empirical test for determining whether or not a luminous body is rapidly receding. The answer is yes; in principle, at least, a test does exist. A rapidly receding nebula should appear fainter than a stationary nebula at the same momentary distance. If we had previous knowledge of the distances of nebulae, an examination of their apparent faintness would tell us at once whether or not they are receding.
The test is valid in principle but, unfortunately, there is a catch in its application to the particular problem. Our only information concerning great distances is derived from the same apparent luminosities which we desire to test. If a nebula appears abnormally faint because of recession, the fading would merely introduce an error in our estimation of distance, and the two effects could not be distinguished. Perhaps later, if apparent diameters can be calibrated as reliable measures of distance, or new criteria can be formulated, it may be possible to apply the test directly. At present, however, the direct investigation ends in a vicious circle, and the persistent observer is forced to consider a possible indirect attack on the problem.