l66 RELATIVITY, THE SPECIAL THEORY
all orders in v/c and also in which critical comments are made on the naive use
of simultaneity. It cannot be said, however, that the content of Einstein's June
1905 paper depends in any technical sense on these important remarks by Poin-
care. Others in Einstein's position might perhaps have chosen to mention Poincare
at the earliest opportunity. However, it does not seem to me that Einstein had
compelling reasons to do so in 1905. I shall return soon to what Einstein had to
say about Poincare in later years. Here I note that Poincare's name appears only
once in a paper by Einstein on relativity, to wit, in 'Geometric und Erfahrung,'
the text of a lecture he gave in 1921 on general relativity [E7] in which he praises
'der tiefe und scharfsinnige Poincare,' the deep and sharp-witted P., for his ideas
on non-Euclidean geometry—ideas which, incidentally, are found in Chapter 3
of La Science et I'Hypothese.
- Lorentz and the Aether. To Lorentz simplicity meant simple dynamics.
As an important example of the Lorentz style, consider his reaction to Kaufmann's
result of 1901-6 about the purely electromagnetic origin of the electron's mass*:
'With a view to simplicity, it will be best to admit Kaufmann's conclusion, or
hypothesis, if we prefer so to call it, that the negative electrons have no material
mass at all. This is certainly one of the most important results of modern physics
.. .' [LI]. I believe that Lorentz clung to the idea of a purely electromagnetic
electron mass for the rest of his life.
Lorentz's words about Kaufmann are found in his 1906 Columbia lectures, the
publication of which was held up for three years 'on account of my wish to give
some further development to the subject' [L2]. Despite this considerable delay,
'Einstein's principle of relativity [has not] received an adequate treatment' [L2].
This is indeed true. For example, Lorentz still opines that the contraction of rods
has a dynamic origin. There is no doubt that he had read and understood Ein-
stein's papers by then. However, neither then nor later was he prepared to accept
their conclusions as the definitive answer to the problems of the aether. With his
customary clarity, he stated his own credo in the course of lectures given at the
Teyler Foundation in Haarlem in 1913 [L3]:
'According to Einstein, it has no meaning to speak of motion relative to the
aether. He likewise denies the existence of absolute simultaneity.
'It is certainly remarkable that these relativity concepts, also those concerning
time, have found such a rapid acceptance.
'The acceptance of these concepts belongs mainly to epistemology.... It is cer-
tain, however, that it depends to a large extent on the way one is accustomed to
think whether one is most attracted to one or another interpretation. As far as this
lecturer is concerned, he finds a certain satisfaction in the older interpretations,
according to which the aether possesses at least some substantiality, space and time
can be sharply separated, and simultaneity without further specification can be
spoken of. In regard to this last point, one may perhaps appeal to our ability of
*See Section 7e.
