l I N T R O D U C T I O N T O V O L U M E 1 2
theory might allow for a regular, centrally symmetric solution that could represent
the electron. According to Grommer’s calculation, it did
not.[54]
Nonetheless, Ein-
stein deemed Kaluza’s theory “truly captivating” (Doc. 318). In his joint investiga-
tion with Grommer of the implications of Kaluza’s theory, Einstein reconsidered
his earlier rejection of Kaluza’s manuscript, and now renewed his original offer to
communicate a paper by Kaluza to the Prussian Academy (Docs. 270 and 281). He
presented a paper by Kaluza on the five-dimensional theory to the Academy on 8
December. The article contained a detailed discussion of the problems that the -
component caused in the equation of motion of a charged particle, and offered
some thoughts on how these may be avoided (Kaluza 1921, pp. 970–972; see also
Doc. 305). Yet Einstein remained skeptical because, as his work with Grommer had
shown, the theory failed to bring about the desired unification of field and matter
through a non-singular particle solution.
Theoretical research in general relativity did not only pertain to the exploration
of possible generalizations. In late 1921, Einstein was confronted with one of the
earliest intimations of the true nature of the Schwarzschild metric, namely, with
what is now called a black hole (see Docs. 314 and 302). The French mathemati-
cian and politician Paul Painlevé had discovered a new set of coordinates suitable
for describing the spacetime of a single point mass, known as the Schwarzschild
metric. The behavior of matter in this spacetime appeared to be different from that
seen in the usual Schwarzschild coordinates. In particular, an object falling toward
what was then seen as a singularity, but is now referred to as the event horizon of
the black hole, does not, in Painlevé’s coordinates, exhibit the bizarre deformations
associated with an approach to this point in space when the Schwarzschild metric
is described in the usual coordinates. Painlevé drew the conclusion that such radi-
cally different “measurements” of the same event in the same spacetime indicated
that there was something dubious about the theory’s claims of general covariance.
Einstein defended his theory in a spirited fashion against the implication that a
theory which predicted apparently contradictory measurements, depending on how
the calculation was performed, could not be consistent. However, like others at this
time, he did not perceive that Painlevé’s coordinate choice suggested anything
unphysical about the so-called Schwarzschild singularity.
V I
Einstein’s correspondence of the year 1921 provides ample evidence of a remark-
able variety of new and ongoing experimental investigations that he either directly
suggested, or that he followed with close interest. Not every idea or experimental
g55