N E W Y O R K C I T Y C O L L E G E L E C T U R E S 5 1 1
the complexity of all the different mechanical models or explanations of the ether has led
physicists to drop all the mechanical properties of the ether. In the latest theory, that of
Lorentz, the only mechanical property of the ether left, is the negative one of immobility.
But on the restricted theory of relativity, the notion of a fixed ether must disappear. For if
the ether be at rest with reference to any one system of co-ordinates, it will be in motion in
all these systems moving uniformly relative to it. This, however, does not necessarily deny
all existence to the ether. It only denies that we can ascribe to the ether any mechanical
properties of rest or motion. But on the general theory of relativity, space itself has definite
metrical properties in different regions which determine mechanical as well as electromag-
netic phenomena. Space is, therefore, not entirely vacuous and may well be called the ether,
provided we do not attack mechanical properties to it and recognize that its properties de-
pend on the presence of matter in it. Einstein thus gets back to a certain extent to Newton’s
use of absolute space to give rotation physical reality (as illustrated in the famous experi-
ment of rotating a bucket of water). On this view, space, or the gravitational ether, is more
fundamental that [than!] electro-magnetic force. Prof. Einstein, however, prefers to leave
open the question of the relation between the ether and the structure of matter. Against those
who, like Weyl, would make a premature synthesis of the equations of gravity and electro-
dynamics, Einstein protests that the phenomena of radiation may prove insuperable obsta-
cles.
In the second part of his lecture, Prof. Einstein, then, turned to the phenomena of radia-
tion. He commented humorously on the fact that in this field, physicists use on one hand
the notion of discreet pulses of energy “pills,” and on the other hand radiation that is con-
tinuous, “like a soup.” Nor do the physicists let the right hand know what the left hand is
doing. Not only are they inconsistent, but they fail to explain the facts. Thus, according to
the classical view of photochemical action, the radiant energy of light breaks up the mate-
rial molecule. In fact, however, the action will take place no matter how weak the light and
will not take place at all, no matter how intense the light, unless this light is of the proper
frequency.
Prof. Einstein then sketched Planck’s theory of quanta, according to which energy is ra-
diated only in discreet pulses. (Others usually refer to this theory as the Planck-Einstein
theory.) This theory avoids logical inconsistency and is more in harmony with the facts. It
involves the application of the laws of probability to physical systems; and Prof. Einstein
sketched some of the leading ideas in this field which enable us to determine the conditions
of equilibrium and the direction in which phenomena must take place.
It was very interesting and instructive to note how Prof. Einstein’s treatment of the doc-
trine of relativity differed from his treatment of the doctrine of quanta. In the former we had
a very logical exposition of the fundamental principles of a theory whose main outlines
were simple and definitely established in Einstein’s own mind. In the treatment of the doc-
trine of quanta, however, we saw a great mind grappling with problems not yet definitely
solved, but determined to try certain modes of attack as most promising. Incidentally, it also
illustrated the difference between a physics which proceeds by generalization from experi-
ence—by what Rankine called the abstractive method—and a physics which proceeds from