Contemporary Advances in Physics, XXXII, Particles of the Cosmic Rays

Even after fifteen years of intensive research following on two decades of more desultory study, the cosmic rays are still a store of new and remarkable data. The question of their ultimate origin, though by no means extinct, has been set aside by many physicists in favor of a fuller inquiry into their qualities. The distinctive mark of the cosmic-ray particles is the immensity of their energies; for, great by all previous standards as are the energy-values which physicists now can impart in their laboratories, those manifest in the cosmic rays are greater by factors not of thousands merely, but often of millions. To this remote and exalted energy-range belong the penetrating particles capable of cleaving through a metre of lead, and the wonderful and beautiful phenomenon of cosmic-ray showers. It is not to be wondered at that with energies so high, particles so familiar as electrons and photons should be invested with unfamiliar powers. So evidently they are; but some of the charged corpuscles of the cosmic rays have properties such that their strangeness cannot be ascribed to high energy alone, but apparently must be based upon some fundamental difference (perhaps a difference of mass) from all the particles thus far identified.