FIELD THEORIES OF GRAVITATION: THE FIRST FIFTY YEARS 231
since c is not a constant. Hence
or, approximately,
which is Einstein's equation of 1911 (see Eq. 11.6). No use is made of Eq. 13.2
in this derivation. The latter equation looks invariant with respect to special rel-
ativity, but of course it is not, since c is variable. Abraham commented on this in
his next communication: 'The variability of c implies that the Lorentz group holds
only in the infinitesimally small' [A4], a statement that was almost at once dis-
proved by Einstein [E5].
A debate began in the Annalen der Physik which, from Abraham's side, lacked
style and substance. In a first comment [A5], Abraham noted that relativity was
threatening the healthy development of physics since 'it was clear to the sober
observer that this theory could never lead to a complete world picture if it were
not possible ... to incorporate gravity.' He added that Einstein had given 'the
death blow to relativity' by discarding the unconditional validity of Lorentz invar-
iance.* 'Someone who, like this author [A.], has had to warn repeatedly against
the siren song of this theory will greet with satisfaction [the fact] that its originator
has now convinced himself of its untenability.' Abraham acknowledged the cor-
rectness of Einstein's technical objections to his work. In a later paper [A6], he
unveiled his 'second theory': 'I would prefer to develop the new theory of gravi-
tation without entering into [a discussion of] the space-time problem.' Abraham
now gives up Lorentz invariance altogether and introduces an absolute reference
system (see also [Al], p. 488).
Einstein shot right back, though in measured language: '[Special] relativity has
a wide range of applicability [and is] an important advance; I do not believe it has
impeded the progress of physics.... There is not the slightest ground to doubt the
general validity of the relativity principle [for uniform motion]' [E6]. He
expressed his own views about the difficult and as yet unsolved problem of gravity
by making a comparison: 'In my opinion, the situation [regarding gravity] does
not indicate the failure of the [special] relativity principle, just as the discovery
and correct interpretation of Brownian motion did not lead one to consider ther-
modynamics and hydrodynamics as heresies.' He added that he himself did not
yet understand how the equivalence principle was to be implemented in general.
Abraham did not give up and published a rebuttal [A7]. It adds nothing sub-
stantially new and is vicious: '[Einstein] craves credit for the future theory of rel-
ativity.' In reply, Einstein published a five-line statement in which he declared
*Note that these comments preceded the publication of the Einstein-Grossman paper [El].