WHAT IS AN ELEMENTARY PARTICLE? 



By E. SCHBODINQEB 



Dublin Institute for Advanced Studies 

 Dublin, Ireland 



1. A PARTICLE IS NOT AN INDIVIDUAL 



Atomism in its latest form is called quantum mechanics. It has 

 extended its range to comprise, besides ordinary matter, all kinds of 

 radiation, including light — in brief, all forms of energy, ordinary 

 matter being one of them. In the present form of the theoiy the 

 "atoms" are electrons, jorotons, photons, mesons, etc. The generic 

 name is elementary particle, or merely particle. The term "atom" 

 has very wisely been retained for chemical atoms, though it has 

 become a misnomer. 



This essay deals with the elementary particle, more particularly 

 with a certain feature that this concept has acquired — or rather lost — 

 in quantum mechanics. I menn this : That the elementary particle is 

 not an individual; it cannot be identified, it lacks "sameness." The 

 fact is known to every physicist, but is rarely given any prominence 

 in surveys readable by nonspccialists. In technical language it is 

 covered by saying that the particles "obey" new-fangled statistics, 

 either Einstein-Bose or Fermi-Dirac statistics. The implication, far 

 from obvious, is that the unsuspected epithet "this" is not quite prop- 

 erly applicable to, say, an electron, except with caution, in a restricted 

 sense, and sometimes not at all. My objective here is to explain this 

 point and to give it the thought it deserves. In order to create a foil 

 for the discussion, let me summarize in sections 2-5 what we are usually 

 told about particles and waves in the new physics. 



2. CURRENT VIEWS: THE AMALGAMATION OF PARTICLES AND 



WAVES 



Our image of the material world had been made up of two kinds 

 of "fittings" — waves and particles. The former were instanced 

 mainly, if not exclusively, by Maxwell's waves of electromagnetic 

 energy, comprising such as are used in radio, light, X-rays, and 

 gamma-rays. Material bodies were said to consist of particles. One 



* Reprinted by permission from Endeavour, vol. 9, No. 35, July 1950. 



922758—51 13 183 



