178 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
from which we can get really old light is from the distant nebulae, 
so our chances of establishing by experiment a new principle of 
physics like this seems at present to be involved in a vicious circle 
from which there is no escape. 
It appears, therefore, that our knowledge of the structure of the 
universe at the limits of the astronomical range is unsatisfactory. 
We have to recognize that there are discrepancies between theory and 
experimental observations. Hubble says that “a choice is presented, 
as once before in the days of Copernicus, between a strangely small, 
finite universe, and a sensibly infinite universe plus a new principle 
of nature.” 
We may now go back once more for a comprehensive view of what 
we have called the linear lay-out of the universe in figure 1. The 
three components, or variables, were assumed quite simply to be space, 
matter, and time. At the right-hand end of the scale we have become 
embroiled in some rather questionable speculations regarding the 
nature of space and the behavior of light. In this region, where a 
light-year is the unit of distance and a nebula the unit of mass, we 
have good reason for suspecting that the mechanics of the universe 
cannot be described or explained in such a simple way as in the region 
of miles and mountains. 
Peculiarly enough, if we go from the enormously great region to 
the extremely small region, the region of the electron and the posi- 
tron, we encounter similar difficulties. You will remember that Dar- 
row characterized the microscopic region as unique because “of the 
adventurous excursions of the observers,” and “the grandeur of the 
inferences.” One or two of these inferences and excursions may be 
cited here, and it will appear that the simple concepts of space and 
matter have suffered in the microscopic field in much the same way 
that they have suffered in the astronomical field. As the result of 
investigations in the field of the small particles it has become neces- 
sary to broaden our ideas as to the nature of matter. Cloud-chamber 
pictures have allowed us practically to see two particles of matter 
created in space from the energy contained in radiation. 
The thing that happens is that a photon, an atom of radiant 
energy traveling with the speed of light, somehow gets itself into a 
peculiar situation in a microscopic field of:some kind. The result 
is that the photon changes into two particles with electric charges, 
a positron and an electron. 
In the macroscopic size range an equivalent phenomenon would be 
for a quantity of sunshine, passing by an iron ball, to change sud- 
denly into a couple of buckshot. 
Needless to say, no one has ever seen anything like this happen. It 
is only when sizes become so small as to prevent direct observation 
