170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
change from time to time. Newton’s Law of Universal Gravitation 
says that every particle of matter in the universe attracts every other 
particle. If forces of attraction cause matter to bunch up into aggre- 
gates of various sizes, why may not the various bunches themselves 
start coming together until eventually there results just one large, 
static bunch of matter floating quietly in an infinity of space? Such 
an end result seems logical, but it cannot happen until the kinetic 
energy of matter, the energy of motion, has been converted into 
radiation and transferred to infinity. Such a transfer of energy 
appears, in fact, to be going on. 
A study of the motions of the various aggregates may be expected to 
throw some light on this question. We start with the smallest par- 
ticles, electrons, for example. In addition to random motions caused 
by collisions with other particles, all electrons are supposed to spin. 
They may be thought of as being like tops which never run down. 
When an electron helps to form an atom, in addition to spinning 
it also revolves about the nucleus, just as the earth revolves about 
the sun. The aggregations of matter between atom and earth on 
the diagram of figure 1 may have various kinds of motion but when 
earth is reached we again have the spin about an axis and the revolu- 
tion about the sun. Our sun, together with all the other suns in its 
group, forms a nebula which spins with high speed about a central 
axis. The spin velocity is very high, but the size of our nebula is 
so great that it takes about 2 million centuries for it to make one 
revolution. As Shapley puts it, this is the time required to “click off 
one cosmic year.” 
The motion of the supernebula is not known in accurate detail. 
It is possible that some sort of gigantic spin is present here also, but 
so far such a spin has not been detected. Instead, a very surprising 
sort of motion has been discovered, a motion which is just contrary 
to what we expect if matter is to agglomerate into one big bunch. 
The supernebulae appear to be receding from us. The supernebula 
to which our galaxy belongs maintains its fixed dimensions, and be- 
haves more or less as a unit, but all the other supernebulae appear 
to be flying away from ours with high speeds. The farther away 
from us they are, the faster they seem to recede. There seems to be 
no good way of explaining such a phenomenon. One might assume 
that a primeval explosion started all matter out in all directions 
from an original concentration, but there are serious difficulties 
involved in such a theory. 
The whole question of the expanding universe is definitely con- 
troversial. The consequences of accepting or rejecting the theory 
are so great that it will be worth while to review briefly the evidence. 
