THE NEW KINEMATICS 1517.15, 7.16 as d(myx')/dt' — K'. The straightforward derivation of Eq. 7.23 via
the energy-momentum conservation laws of mechanics was not found until 1909
[L5].
Among other early papers on relativity, I mention one by Ehrenfest in 1907
[El7], in which is asked for the first time the important question: How does one
apply Lorentz transformations to a rigid body?
Planck was also the first to apply relativity to the quantum theory. He noted
that the action is an invariant, not only for point mechanics, (where it equals the
quantity \Ldt in Eq. 7.25), but in general. From this he deduced that his constant
h is a relativistic invariant. 'It is evident that because of this theorem the signifi-
cance of the principle of least action is extended in a new direction' [P9]—a con-
clusion Einstein might have drawn from his Eqs. 7.13, 7.18, and 7.19.
Not only the theoreticians took early note of the relativity theory. As early as
1906, there was already interest from experimentalists in the validity of the rela-
tinn*Von Laue had been on an alpine trip before coming to Bern. Einstein delivered himself of the
opinion, 'I don't understand how one can walk around up there' [S3].between the total energy and the velocity of a beta ray, as will be discussed in
Section 7e.
The publication of the 1905 papers on special relativity marked the beginning
of the end of Einstein's splendid isolation at the patent office. From 1906 on, vis-
itors would come to Bern to discuss the theory with him. Von Laue was one of
the first (perhaps the very first) to do so. 'The young man who met me made such
an unexpected impression on me that I could not believe he could be the father of
the relativity theory,' von Laue later recalled [S2].* Other young men came as
well. From Wiirzburg Johann Jakob Laub wrote to Einstein, asking if he could
work with him for three months [L6]; the ensuing stay of Laub in Bern led to
Einstein's first papers published jointly with a collaborator [El8, El9]. Rudolf
Ladenburg, who became a close friend of Einstein in the Princeton years, came
from Breslau (now Wroclaw). Yet in these early years the relativists were few in
number. In July 1907 Planck wrote to Einstein, 'As long as the advocates of the
relativity principle form such a modest-sized crowd, it is doubly important for
them to agree with one another' [P10].
Then, in 1908, came the 'space and time' lecture of Herman Minkowski. In
1902, Minkowski, at one time Einstein's teacher in Zurich, had moved to the
University of Goettingen. There, on November 5, 1907, he gave a colloquium
about relativity in which he identified Lorentz transformations with pseudorota-
tions for which