STRUCTURE OF THE UNIVERSE—HEAPS 179 
that the event occurs. We may well say that something peculiar is 
going on in the microscopic field. Something is happening which is 
foreign to our ordinary experience. 
Technically this phenomenon is known as pair production by a 
photon. The reverse process, conversion of matter into radiation, 
can occur when an electron and a positron come together under proper 
conditions. They disappear and two photons of radiation are shot 
out with the speed of light in opposite directions. 
Matter and energy can now be thought of as practically synony- 
mous. It thus becomes possible to make certain grand inferences 
with the object of saving the universe from running down. Millions 
of suns are slowly but surely converting their matter and their 
energy into radiation and this radiation is constantly escaping into 
infinity. Perhaps somewhere in space radiation may be changed back 
into matter. Perhaps the universe is engaged in a reversible cycle, 
instead of an irreversible one, as is commonly supposed. 
As an illustration of what Darrow calls an “adventurous excursion” 
of an observer we may take the Dirac theory of the positron. Dirac 
is a brilliant young Englishman, a mathematician who has demon- 
strated a high degree of daring and originality in his handling of 
theoretical physics. 
His theory of the positron starts out with two peculiar assumptions. 
First, a particle may have a negative kinetic energy. Second, all 
space is filled with particles of negative kinetic energy. There is a 
distribution of electrons of infinite density everywhere in the world. 
A perfect vacuum is a region where all the states of positive energy 
are unoccupied and all those of negative energy are occupied. 
When an electron, by some means or other, gets knocked out of 
this state of negative energy into a state of positive energy, it is 
observed as an ordinary electron; the hole which was left is a 
positron. This hole may wander around for a short time, but there 
are so many more electrons in the universe than holes that it is not 
long before some electron drops into the hole and both hole and 
electron disappear from the view of normal people. The very short 
life of the positron is thus explained, as is also the phenomenon of 
pair production and the conversion of matter into radiation. 
I have given this hasty outline of the theory, not that I expect 
anyone to understand it—it is hardly to be expected that negative 
energy can be understood—but because it illustrates the lengths to 
which a theorist has to go in creating physical explanation in this 
field. In the microscopic range of sizes a quite perfect explanation 
of things is given by a specialized type of mathematics called wave 
mechanics. It is only when this mathematical symbolism is explained 
in terms of physical symbolism that we call it an adventurous 
