154 RELATIVITY, THE SPECIAL THEORY- A comment on the possibilities for determining the quantity (1 — v^2 /c^2 )
in beta-ray experiments [E21]. - A remark on the detectability of the transverse Doppler effect [E5].
- Brief remarks on Ehrenfest's query concerning rigid bodies: 'To date both
the dynamics and the kinematics of the rigid body. .. must be considered
unknown' [E22]. - Earlier Einstein had derived the expression mc^2 (y — 1) for the kinetic
energy. Now he introduces the form ymc^2 for the total energy. Furthermore, the
transformation of energy and momentum in the presence of external forces (i.e.,
for open systems) is derived.* Further ruminations about the rigid body: 'If rela-
tivistic electrodynamics is correct, then we are still far from having a dynamics for
the translation of rigid bodies' [E23]. In this paper Einstein also expresses an
opinion concerning the bearing of his recent light-quantum hypothesis on the
validity of the free Maxwell equations. It seemed to him that these equations
should be applicable as long as one deals with electromagnetic energy amounts or
energy transfers which are not too small, just as—he notes—the laws of ther-
modynamics may be applied as long as Brownian-motion-type effects (fluctua-
tions) are negligible. - The review paper [E3]. This is the transitional paper from the special to
the general theory of relativity. Among the points discussed and not mentioned in
the foregoing are (1) the remark that the total electric charge of a closed system
is Lorentz invariant, (2) comments on the beta-ray experiments of Kaufmann, a
topic to be discussed in the next section and, (3) a discussion of relativistic
thermodynamics.
1908-10. Papers with Laub on the relativistic electrodynamics of ponderable
media [E18, E19] (see [PI] or [P2], Sections 33, 35).
A further comment on this subject appeared in 1909 [E25]. In 1910, Einstein
published a brief note on the nonrelativistic definition of the ponderomotive force
in a magnetic field [E26].
This concludes the brief catalog of Einstein's later contributions to special rel-
ativity. (I have already mentioned that in 1935 [E12] and again in 1946 [E13] he
gave alternative derivations of E = me^2 .) In later years he reviewed the special
theory on several occasions, starting with the first lecture he gave at a physics
conference [E27], and again in 1910 [E28], 1911 [E29], 1914 [E30], 1915 [E31],
and 1925 [E32]. Special relativity is, of course, discussed in his book The Mean-
ing of Relativity [E33]. The first newspaper article he ever wrote deals largely
with the special theory [E34]; he wrote reviews of books bearing on this subject,
in praise of writings by Brill [E35], Lorentz [E35], and Pauli [E36].
•See [PI] or [P2], Section 43.
**For a discussion of the early contributions to this subject, see [PI] or [P2], Sections 46-49; see
also [E24]. For a subsequent severe criticism of these papers, see [O2]. Since this subject remains
controversial to this day (see, e.g., [L7]), it does not lend itself as yet to historic assessment.