INTRODUCTION
TO VOLUME
8
xliX
principle
no longer
sounds
like
a
generalization
of
the familiar
relativity principle
for uniform
motion in
any readily recognizable
form. Einstein had
accepted
Erich
Kretschmann’s
argument
that
a
theory
does
not
relativize
motion
simply by
virtue
of
being
cast
in
generally
covariant form. He also
accepted
that
general
relativity
does not
satisfy any relativity principle
of
the sort
proposed
in Kretschmarin
1917.[26]
Rather
than
giving
up
the
relativity
principle
altogether,
he
changed
its
formulation,
elevating a
consideration
he
had earlier
used
as an argument
in
sup-
port
of
the
relativity
principle-identified
at
that
time
as
the freedom to choose
arbitrary
coordinate
systems[27]-to
the
expression
of
the
relativity principle
itself.[28]
The relevant
consideration,
the
so-called
point-coincidence argument,
made its first
appearance
in Einstein’s
correspondence
in December
1915,
where it
was
used
to
help explain why, contrary
to Einstein’s earlier
emphatic
claims,
the
“hole
argument”
does
not
rule
out
generally
covariant
field
equations.[29]
The Machian line
of
thought along
which
Einstein
had
sought
to relativize
non-
uniform
motion in 1914 turned out to be
problematic
as
well. One
of
the virtues
Einstein had claimed for the “Entwurf”
theory was
that
it
would allow the
interpre-
tation of
inertial forces in
rotating
frames
of
reference
as gravitational
forces
due
to
rotating
distant
masses.[30]
This claim
was
in
error
on
at least two counts.
First,
as
Einstein discovered
to
his
dismay
in
September
1915
(Doc. 123),
the metric field
of
a
Minkowski
space-time
in
a
rotating
frame
of
reference
is not
a
solution
of
the
“Entwurf” field
equations.
Second,
this metric field is not
equivalent
to the metric
field
near
the
center
of
a
rotating
hollow
sphere
(which
can
be used
to
represent
the
rotating
distant
masses).
The
conflation
of
these
two
metric fields
can
still be found
in
correspondence
with Besso in 1916
(Doc.
245)
and in
correspondence
with Hans
Thirring
in
1917.[31]
By
the
summer
of
1917,
Thirring
had started the calculations
of
the
metric field
inside
a
rotating
hollow
sphere,
which
were
published
in
Thirring
1918. He
was
puzzled
by
the fact that this metric field
was
not
simply
that
of
a
Minkowski
space-
time in
a rotating
frame
of
reference. As
Thirring
pointed
out in the
published
paper,
this
is
not
surprising
since Minkowskian
boundary
conditions
are
used
in the
approximative
calculation
of
the
metric field. Given the central role
of
the
problem
of
boundary
conditions in the debate with De
Sitter
and in the
argument
of
Einstein
1917b
(Vol. 6,
Doc.
43),
one
would have
expected
Einstein
to set
Thirring straight
as soon
as
the
problem
was
put
to him in 1917.
Einstein, however,
accepted
Thirring’s premise
that
the
metric field
he
calculated had to be
equivalent
to the
metric
of
a
Minkowski
space-time
in
a rotating
frame of
reference
(Doc. 405).
In Einstein
1918f
(Vol.
7,
Doc.
4),
there
is
no
longer
any explicit
reference to the
equivalence
of
inertial effects and
gravitational
effects attributed to
distant
matter,
neither
in the formulation
of
Mach’s
principle
(“the
[metric field]
is
completely