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