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]