Contemporary Advances in Physics, XXXII 

 Particles of the Cosmic Rays 



By KARL K. D ARROW 



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. 



^XT'HEN a new member is admitted to a small and jealously- 

 ^ ' restricted club supposedly already filled for all time, the event 

 has a dramatic aspect. When a concept is formed in a nebulous way 

 and rapidly gains precision with the passage of the years, the story is 

 of philosophic interest. When physicists extend their knowledge into 

 ranges of energy heretofore unsuspected, and find them inhabited by 

 particles classifiable as electrons but in possession of powers ordinarily 

 unknown, and also by particles which must be put in a class by 

 themselves — when such things are available for telling, the tale has 

 scientific value. When evidence comes in the form of pictures so 

 striking as those which can here be shown, the science of lifeless matter 

 has an aesthetic splendor such as rarely embellishes it. All of these 

 features appear in the recent advances of the study of cosmic rays. 



The small and exclusive club consists of the subatomic particles, 

 long supposed to comprise only the negative electron and the proton 

 and other positive atom-nuclei. Into it the positive electron had 

 been forced in 1932, and the neutron in 1933; a vacant chair was 



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