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 5
ond relative to the sun and also relative to the body projected at 1,000 kilometers
per second. If this appears impossible, the reason is that the hypothesis of the
absolute character of time is false. One second of time as judged from the sun is not
equal to one second of time as seen from the projected body.
There is no audible tick-tock everywhere in the world that could be considered
as time. If physics wants to use time, it first has to define it. In this endeavor it is
apparent that this definition necessarily requires a body of reference, and that this
definition makes sense only with respect to this chosen body of reference. It turns
out that one can define time relative to this body of reference such that the law of
the propagation of light is obeyed relative to it. This definition of time can be real-
ized for bodies of reference in any state of motion. But it turns out that the times of
differently moving bodies of reference do not coincide. A more detailed justifica-
tion of this is found in my popular book about the theory of
relativity.[3]
If two
events occurring at different locations are judged simultaneous from a body of ref-
erence, then they are not judged so from a body of reference that is moving relative
to it.
Before I continue in this train of thought, I have to say something about the role
the body of reference plays in Galileo’s and Newton’s mechanics. In particular I
have to point out that the development of science knows only of a build-up, not of
a tearing-down. If a generation cannot build upon the achievements of its predeces-
sors, then there is no science proper. It would be sad if the theory of relativity would
have to topple the previous mechanics, somewhat like one tyrant toppling the other.
The theory of relativity is nothing but a step further in the centuries-old develop-
ment of our natural sciences, which preserves and deepens previously found con-
nections and adds new ones. The theory of relativity does not topple Newton’s and
Maxwell’s theories, just as the League of
Nations[4]
does not annihilate the states
that join it. They will have to accept some modifications of their laws but thereby
gain higher security.—
In everyday life we mostly use the surface of the earth as a body of reference
whose individual points can be repeatedly identified. Mathematical physics choos-
es as a body of reference (coordinate system) three mutually orthogonal straight
rods originating from one point. The position of a point relative to this system of
rods is described by three numbers (coordinates) that can be obtained by measuring
with rigid rods (measuring rods). For this procedure it is assumed that the laws of
orienting rigid bodies are correctly described by Euclidean geometry. All state-
ments of location made by physics hitherto are based upon this assumption.
Wherever a point may be located, one can always think of the system of rods and
the procedures of measurement to be completed such that eventually they reach up
to the point under consideration. This must be imagined like scaffolding at a
[p. 5]
[p. 6]