P O P U L A R P R I N C E T O N L E C T U R E S 6 1 9
in the case of Newton’s laws. There is one difficulty, however, which the general theory of
relativity removes which has been a difficulty for over a hundred years in the Newtonian
theory of gravitation. That is in respect to the motion of the planet Mercury.
Instead of the planet Mercury describing an ellipse which remains a fixed ellipse with
respect to the fixed stars, the axis of this ellipse continually rotates in the direction of the
rotation of the planet about the sun. This rotation amounts to about forty-two seconds of an
arc in one hundred years, and has been for a long time a very puzzling difficulty in the or-
dinary dynamic theory. The general theory of relativity predicts in such a case a revolution
of almost exactly this same amount, forty-two seconds in one hundred years, the same
amount that has been observed, and that, of course, is a remarkable result of the general
theory.
There is one other result, however, of the general theory which is even more remarkable
for the reason that the result was predicted before the experimental fact was determined,
and that has to do with the bending of light when it travels near a heavy body, as in partic-
ular the light from a star which can be observed at the time of a total eclipse of the sun.
When this light passes near to the sun there is an extremely slight bending of the light, but
one which was found by the two English expeditions sent out to agree within about 10 per
cent of the amount predicted by the theory. And that, of course, is really the more remark-
able result of the general theory of relativity, in that it has predicted a result which was not
previously known.
There is a third consequence of considerable importance, because it has not yet been ver-
ified. That has to do with the change of time. If we had a clock in the sun, being in a differ-
ent gravitational field from a clock on the earth, we would expect from the general theory
of relativity that two clocks, the one on the sun and the one on the earth, even if they were
exactly alike, would go at different rates. We have such clocks in the vibrations, whatever
they are, which take place in the chemical atoms, and these clocks give us these indications
by the spectral lines emitted from them.
The consequence predicted by the general theory of relativity is that all spectral lines
coming from elements in the sun should be shifted extremely slightly towards the red end
of the spectrum. Whether or not this effect exists is still a matter of controversy. And the
extremely interesting remark that Prof. Einstein made near the end was that if it is definitely
proved that this effect does not exist, then his whole general theory of relativity falls down.
And his confidence in the theory is apparently such as to lead him to believe that such an
effect really will be found to exist.
That, then, is very briefly the content of the general theory of relativity, and perhaps the
most important consequence of it is the fact that our whole concept of space and time has
got to be altered. We can no longer think of space, time, and matter as independent con-
cepts, but they are interwoven upon each other.