THE ELEMENTARY PARTICLES OF PHYSICS! 
By Cart D. ANDERSON 
California Institute of Technology, Pasadena 
The idea of elementary particles of matter, of small, discrete, in- 
divisible particles out of which all matter in the universe is consti- 
tuted, is as old as recorded history. The Greeks in their philosophical 
speculations discussed at length the question of the ultimate nature of 
matter. They realized that there were only two possible choices open 
to them; either matter must be thought capable of being divided into 
smaller and smaller units without end, or else it must consist of small 
units which are themselves wholly indivisible. Many of the Greek 
philosophers experienced a philosophical difficulty in trying to conceive 
of infinite divisibility, whereas others found it equally difficult to think 
of a particle as being truly indivisible. The difficulty is closely akin to 
that which one experiences when contemplating the limits of the uni- 
verse, and trying to decide in his own mind whether it pleases him 
more to think of the universe as unbounded and extending to infinity, 
or to imagine a finite universe with definite bounds beyond which there 
is nothing, not even space. The idea of the existence of indivisible ma- 
terial particles, however, seems to have had more appeal to the Greeks, 
and the atomic hypothesis was expounded and developed in the fifth 
century B. C., chiefly by Thales, Leucippus, and his distinguished 
pupil, Democritus, until in many respects it resembled the views which 
are held today. 
The views of Democritus were prominent for 500 years but began 
to wane after the beginning of the Christian Era and by about A.D. 
200 had almost wholly disappeared from European philosophical 
thought. The idea of material atoms did not really appear again in 
Europe until about the middle of the seventeenth century, a time 
marking the beginning of the great era of scientific experimentation 
which has continued with an ever increasing tempo up to the present. 
During this period, through scientific research based on experimen- 
tation, the atomic theory of matter slowly developed. Highlights in 
1 Based on material presented in the Sigma Xi Annual Address, A. A. A. S. Centennial, Washington, 
September 1948. Reprinted from American Scientist, vol. 17, No. 2, April 1949, by permission from The 
Society of the Sigma Xi. 
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