5 0 8 A P P E N D I X C
With the simplicity of true genius, Prof. Einstein at once began a direct account of the
fundamental principle of his original special theory of relativity. The principle of relativity
itself, namely, that all rest or motion is relative to some material body which serves as a cen-
ter of reference or coordinates, is of course an established result of the classical mechanics
of Galileo and Newton. The novelty in Einstein’s theory is the new conception of time and
space, which makes it possible to combine this principle of relativity of mechanics with the
principle of the constancy of the velocity of light as fundamental to the modern science of
electro-magnetism.
The principle of the absolute constancy of the velocity of light in all directions of space
had previously been connected with the conception of a fixed ether, so that this constancy
was to hold good only with reference to fixed points in the ether. But the experiments of the
American physicists, Michaelson and Morley, showed conclusively that the constancy of
the velocity of light in all directions is also true on our moving earth. Experiment thus
shows that the principle of relativity is true in optics as well as in mechanics. Hence our
notions of time and space, which tell us that this is impossible, that light cannot travel with
the same velocity relative to different observers which are moving with reference to each
other, must be abandoned. If we accept these, the results of the experiment and the workable
conventions of science as to the measurement of time and space, and velocity, we must
regard our units of time and space and the simultaneity of events as different for different
systems that are in relative motion to each other. But that these differences do not conflict
with the existence of uniform laws of nature is shown by the Lorentz equations which
enable us to calculate the time and space of a moving system if we once know its dimen-
sions when at rest.
II. T
HE
D
EVELOPMENT OF THE
SPECIAL THEORY.
The second lecture dealt with the mathematical development of the original theory of
relativity. Prof. Einstein began with the Lorentz equations and went on to show how these
equations enable us to examine all the laws of nature and determine whether they are or are
not in harmony with the principle of relativity and the principle of the constant velocity of
light. He first took up the laws of electric and magnetic fields as formulated by Maxwell
and Lorentz and showed how these laws remain true for all systems moving uniformly with
reference to each other. He also showed how by means of the Lorentz equations the relation
between electric and magnetic fields could be seen in a new light and how it could be dem-
onstrated that electric energy has inertia.
He next took up the law of the conservation of energy and showed how if we assume that
the physical energy of a body is independent of our system of measurements, it follows all
energy has inertia, from this it follows that a body loses energy by radiating heat, light or
other radiant energy, thereby also loses mass or inertia.
The third or concluding portion of the lecture was devoted to an exposition of the elegant
and fruitful mathematical methods of treating the problems of relativity developed by the