Comparing the intensity of visible light from our Milky Way (Lm) with the intensity of light (Lw), that comes to us from the rest of the Universe, one obtains
Under the assumption that cosmic rays are not of local nature, one obtains for these the ratio of intensities, S, analogous to that in (11)
This follows from the practical absence of cosmic ray variations with sidereal time. As I discussed elsewhere (F. Zwicky, Phys. Rev., January 1933) the inequality (12) is difficult to understand because cosmic rays that originate at very large distance, would arrive at Earth with very reduced energy as a result of the redshift. If for example the redshift were consistently proportional to distance, then light quanta from a distance greater than 2000 million light years would reach us with zero energy. (This consideration implies by the way, that even in the presence of an infinite number of stars in the Universe, the light intensity would have a finite, well defined value everywhere.) Under reasonable assumptions about the type of reaction which produces cosmic rays, the inequality (11) is very hard to understand, and the main difficulty lies, as indicated, in the existence of the redshift (F. Zwicky, Phys. Rev., January 1933).
Finally I would like to point out, that the coexistence of the two inequalities (11) and (12) will pose great difficulties for certain recent opinions about the origin of cosmic rays. For example, G. Lemaitre has proposed that one may consider cosmic rays as remnants of certain super-radioactive processes that happened a long time ago. However, at the same time a correspondingly huge amount of visible and ultraviolet light must have been emitted. Since interstellar gases (as well as our atmosphere) absorb cosmic rays more than visible light, the coexistence of inequalities (11) and (12) is incomprehensible.
It is also important, in this context, to point out the following interesting fact. A belt of irregularly shaped boundaries which runs along the Milky Way and stretches from about -10° to about +10° Galactic latitude, completely blocks our view of extragalactic space, i.e. no extragalactic nebula can be observed in this belt. It is known that part of this absorption can be ascribed to very large, dense masses of dust. If cosmic rays were of extragalactic origin, one would actually expect them also to be absorbed along the Milky Way, i.e. one should observe on Earth a variation of cosmic ray intensity with sidereal time. Since such variation is not found, one is tempted to conclude that cosmic rays can not be of extragalactic origin. However, the density and extent of interstellar matter in the Milky Way must be examined more thoroughly.
The present work has emerged from numerous discussions with staff researchers at Mt. Wilson Observatory working in this area. I am particularly indebted to Dr. W. Baade for plenty of valuable advice.
California Institute of Technology, Pasadena.