V O L U M E 5 , D O C U M E N T 2 5 7 a 7
Vol. 5, 257a. To Vladimir Varic;ak
Zurich, 3 March 1911
Highly esteemed Colleague!
I thank you very much for your letter and the
I have now read the beau-
tiful study by Lewis and
but I cannot understand how you can draw from
this an support endorsement of your opinion. I want to justify my opposite opin-
Let S be a nonaccelerated frame of reference, in which there are clocks of the
same kind at rest with it. Let these be synchronized, e.g. by means of light rays, so
that they show the time of S. Let the rod AB be in uniform motion relative to S. Its
“real” length, i.e., the length measured by itself, be l. Then it follows from the rel.
theory in the well-known way that its length with respect to S is . This
means: if one determines those clocks in S, which show the same state of hands,
when the points A and B are passing by them, then the distance of these points mea-
sured in S is . The contraction is observable by measurement, hence
“real.” In order that you see that the contraction is not simply affected by the defi-
nition of simultaneity in S, i.e. of a purely conventional nature, I add: it is impossi-
ble, to reset the clocks in such a way, that even after this resetting the rod always
has the length with respect to S, if it has the velocity ±v measured by means of
the clocks. From this one can conclude with Ehrenfest that a rotation without elas-
tic deformation is excluded in the theory of relativity, if you assume in addition that
a transversal contraction does not take
One cannot ask whether one has to
conceive of the contraction as a consequence of the modification of the molecular
forces or as a kinematic consequence from the foundations of the theory of
Both points of view are justified side by side. The latter point of view
corresponds roughly to the one of Boltzmann, who treats the dissociation of gases
in a molecular-theoretic manner; this is completely justified, although one can de-
rive the laws of dissociation from the second law without
A principal
U2 U3
l 1
---- -–
l 1
---- -–
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