4 D O C . 4 4 a M A I N I D E A S O F R E L A T I V I T Y
sun in a distinct direction. According to the law just stated, this ray travels a
distance of c per second. Now imagine the sun later hurls a body into space such
that it flies with a velocity of 1,000 kilometers per second in the same direction as
the ray of light. This is easy to imagine. We now can similarly imagine this project-
ed body as an alternative body of reference and ask ourselves: what is the propaga-
tion velocity of light in the judgment of an observer who does not sit on the sun but
rather on the projected body? The answer seems simple. When the hurled body runs
after the light at 1,000 kilometers per second, the ray of light advances against it by
only 299,000 kilometers per second. The same situation would prevail if the ray of
light were not sent by the sun but rather by the projected body, because we know
that the velocity of light does not depend upon the state of motion of the light
source.
This result raises suspicion. Should light when judged from the projected body
really propagate differently than when judged from the sun? Should the laws of the
propagation of light depend upon the state of motion of the body of reference? Then
there would have to exist in the universe something like absolute rest, because one
could argue like this. Relative to arbitrarily moving bodies of reference (here the
projected body), light propagates with a velocity different from c that depends on
the direction. Then there would exist a body of reference of a very distinct state of
motion for which light propagates in all directions with the same velocity c. Such
bodies of reference could justifiably be said to be at absolute rest (in our example,
the sun). Does such absolute rest really exist in a physical sense? Do the laws of
nature really depend upon the state of motion of the observer, i.e., on the system of
reference, as suggested by the argument above about the propagation of light?
Experience contradicts this. When we travel in a railroad car that is free of vibra-
tion we do not notice the motion of the car. All experiments of physics in this car
show the same success as found in a house resting on earth. The physical experi-
ments done on earth show no effects of the movement of the whole earth with all
its objects on it. In general: the laws of nature are independent of the state of motion
of the body of reference. This statement is called, for short, the “principle of rela-
tivity.” But we believed that the consideration given above forced us to conclude
that with respect to the law of light propagation the principle of relativity was not
valid; what is the truth? More than 30 years ago Michelson, an American, proved
through his famous optical experiment that the principle of relativity would be also
valid in a case where theory allowed to predict the influence of the earth’s move-
ment upon the
experiment.[2]
Therefore, the considerations given above must have contained an error. The law
of light propagation is the same, whether the sun or the projected body is chosen as
the body of reference. The same ray of light travels at 300,000 kilometers per sec-
[p. 3]
[p. 4]