translational motion relative to each other. Relative to other
reference-bodies _K_ the law is not valid. Both in classical mechanics
and in the special theory of relativity we therefore differentiate
between reference-bodies _K_ relative to which the recognised laws of
nature can be said to hold, and reference-bodies _K_ relative to which
these laws do not hold.
But no person whose mode of thought is logical can rest satisfied with
this condition of things. He asks: How does it come that certain
reference-bodies (or their states of motion) are given priority over
other reference-bodies (or their states of motion)? _What is the reason
for this preference?_ In order to show clearly what I mean by this
question, I shall make use of a comparison.
I am standing in front of a gas range. Standing alongside of each other
on the range are two pans so much alike that one may be mistaken for
the other. Both are half full of water. I notice that steam is being
emitted continuously from the one pan, but not from the other. I am
surprised at this, even if I have never seen either a gas range or a
pan before. But if I now notice a luminous something of bluish colour
under the first pan but not under the other, I cease to be astonished,
even if I have never before seen a gas flame. For I can only say that
this bluish something will cause the emission of the steam, or at least
_possibly_ it may do so. If, however, I notice the bluish something in
neither case, and if I observe that the one continuously emits steam
whilst the other does not, then I shall remain astonished and
dissatisfied until I have discovered some circumstance to which I can
attribute the different behaviour of the two pans.
Analogously, I seek in vain for a real something in classical mechanics
(or in the special theory of relativity) to which I can attribute the
different behaviour of bodies considered with respect to the reference
systems _K_ and _K _.[15] Newton saw this objection and attempted to
invalidate it, but without success. But E. Mach recognised it most
clearly of all, and because of this objection he claimed that mechanics
must be placed on a new basis. It can only be got rid of by means of a
physics which is conformable to the general principle of relativity,
since the equations of such a theory hold for every body of reference,
whatever may be its state of motion.
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