ELEMENTARY PARTICLES OF PHYSICS—ANDERSON 207 
and others, were made to remove it, although all were unsuccessful. 
If even one physicist in the world had taken the theory of Dirac 
seriously, he would have had an admirable guide leading directly to 
the discovery of the positron. Had this happened, the positron 
would almost certainly have been discovered by 1930 rather than in 
1932. However, after the positron was shown actually to exist, then 
it was a very short time indeed until many of its properties were 
understood in terms of the Dirac theory. 
ELEMENTARY PARTICLES AND RADIATION 
The discovery of the positron represented the first instance in which 
it was recognized that an elementary particle of matter may have only 
a transitory existence. In ordinary matter, for example, the average 
life span of a positron is only a few billionths of a second, for when 
a positron and a negative electron come close to one another they 
mutually annihilate one another—the two particles disappear and in 
their place one finds only radiation. The whole of the material sub- 
stance constituting the particles is spontaneously transformed into 
radiant energy. Measurements show that this process is quantita- 
tively in accord with the now famous Einstein equation H=mc?, which 
relates mass and energy. ‘The process which is the inverse of the 
annihilation of material particles also occurs, namely, the production 
of particles out of radiation. If radiation of sufficiently high energy 
is passed through matter, electrons and positrons are generated. In 
this process the material substance of the two particles is actually 
created out of the energy represented by the radiation, and again in 
conformity with the Einstein equation H=me’. 
In the light of these happenings one must change basically his con- 
cept of the elementary particles of matter; these particles are no longer 
to be thought of as permanent objects which always preserve their 
identity, and which serve only as building blocks of matter by joining 
together in groups to form the more complex chemical atoms. One 
must recognize instead the possibility of the creation of material 
particles out of radiation, and the annihilation of material particles 
through the production of radiation. Such a possibility as this, of 
course, was completely inconceivable to the Greeks in their long 
philosophical discussion on the indivisibility of matter versus the 
divisibility of matter. 
A further step toward a realization of the great complexity inherent 
in the relationships among the elementary particles of matter came in 
1935 with the discovery of the positive and negative mesotrons, or 
positive and negative mesons as they are now often called. This dis- 
covery was also made in investigations of the high-energy phenomena 
occurring when cosmic rays are absorbed in their passage through 
matter. 
