288 RELATIVITY, THE GENERAL THEORY- Friedmann shows that Eq. 15.20 admits nonstatic solutions with iso-
tropic, homogeneous matter distributions, corresponding to an expanding universe
[Fl]. Einstein first believes the reasoning is incorrect [E45], then finds an error
in his own objection [E46] and calls the new results 'clarifying.' - Weyl and Eddington find that test particles recede from each other in
the de Sitter world. This leads Einstein to write to Weyl, 'If there is no quasi-
static world, then away with the cosmological term' [E47]. - Referring to the theoretical work by Friedmann, 'which was not influ-
enced by experimental facts' and the experimental discoveries of Hubble, 'which
the general theory of relativity can account for in an unforced way, namely, with-
out a A term' Einstein formally abandons the cosmological term, which is 'theo-
retically unsatisfactory anyway' [E48]. In 1932, he and de Sitter jointly make a
similar statement [E49]. He never uses the \ term again [E50]. - Einstein writes to a colleague, 'Von dem Mach'schen Prinzip sollte
man eigentlich iiberhaupt nicht mehr sprechen,' As a matter of fact, one should
no longer speak of Mach's principle at all [E51].
It was to be otherwise. After Einstein, the Mach principle faded but never died.
In the post-Einsteinian era of revitalized interest in general relativity, it has
become an important topic of research. At GR9, a discussion group debated the
issue, in particular what one has to understand by this principle. This question
can arouse passion. I am told that the Zeitschrift fur Physik no longer accepts
papers on general relativity on the grounds that articles on Mach's principle pro-
voke too many polemical replies. At stake is, for example, whether a theory is then
acceptable only if it incorporates this principle as a fundamental requirement (as
Einstein had in mind in 1918) or whether this principle should be a criterion for
the selection of solutions within a theory that also has non-Machian solutions.* It
must be said that, as far as I can see, to this day Mach's principle has not brought
physics decisively farther. It must also be said that the origin of inertia is and
remains the most obscure subject in the theory of particles and fields. Mach's prin-
ciple may therefore have a future—but not without quantum theory.
15f. Singularities; the Problem of MotionIn 1917 Einstein wrote to Weyl, 'The question whether the electron is to be
treated as a singular point, whether true singularities are at all admissible in the
physical description, is of great interest. In the Maxwell theory one decided on a
finite radius in order to explain the finite inertia of the electron' [E52]. Probably
already then, certainly later, there was no doubt in his mind (except for one brief
*For a detailed review of the various versions of the principle and a survey of the literature, see
[Gl].