6 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

construction site where one can reach every little spire and corner, no matter how

tall the building. In physics it is not even necessary that this scaffolding exists in

reality, provided one can imagine it being built indirectly (with rays of light etc.).

The fundamental mechanical laws of Galileo and Newton are such that they do

not claim validity relative to arbitrarily moving bodies of reference but only relative

to those of suitably chosen states of motion. Bodies of reference that are admissible

in mechanics are called “inertial systems.” Now, there is a theorem of mechanics:

if the body of reference K is an inertial system, then any other body of reference

moving uniformly in a straight line and without rotation relative to K is also an in-

ertial system. Phrased more simply: if the laws of mechanics hold relative to the

surface of the earth, then they also hold relative to a uniformly moving railroad car

that is taken as a body of reference.

What has been said previously about light can now be summarized in a simple

formula: relative to every inertial system—given the correct definition of time—the

theorem of the constancy of the speed of light in empty space holds true. More gen-

erally, one can express as a theorem of manifold experience: the laws of nature are

the same in all inertial systems. This theorem is called “principle of special relativ-

ity.”

That this theorem implies a novel method of research can be understood in the

following manner. Assume the universe or the individual events that constitute it

have been described relative to one inertial system, then the course of events seen

from a different inertial system is a different one, but nevertheless is also complete-

ly determined. The Dutch mathematical physicist calculated the general rules that

allow one to transform location and time from one inertial system into

another.[5]

Obviously, in this manner one can not only transform individual events but also

mathematically formulated laws of nature. The principle of special relativity de-

mands of these laws that they do not change under such transformation. If they do

not have this property, then they have to be rejected by the principle of special rel-

ativity. The laws of nature must be adapted to the principle of special relativity.

The need to modify Newtonian mechanics first emerged during the investiga-

tions that dealt with extremely fast movements, or more precisely, when motions

approached a speed that could no longer be treated as negligibly small compared

to the speed of light. Furthermore, it turned out that the inertia of a body is not a

characteristic constant of that body, but that inertia rather depends upon the energy

content. Mass and energy in essence are identical.

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