lii

INTRODUCTION

TO

VOLUME

8

V-g

=

1, the

gravitational

field

of

a

point

mass

has

no energy.

Nordstrom had

to

derive this

result in

two different

ways

before

Einstein

accepted

it

(Docs.

382 and

393).

Nordstrom also

discovered that the

expression

for

tuv

given

in Einstein

1916g

(Vol. 6,

Doc.

32)

is

incompatible

with the

expression given

in Einstein 1916o

(Vol.

6,

Doc.

41).

This is

probably

what

alerted

Einstein to

a

serious

error

in

the

former

paper.

A

corrected

version

was published

as

Einstein 1918a

(Vol.

7,

Doc.

1).

In this

paper,

the

spurious gravitational waves are

eliminated

by showing

that

the metric

field

describing

them

is

simply

that

of

a

Minkowski

space-time

in

a

peculiar

coor-

dinate

system.

Einstein

apparently

neglected to

send the

corrected

version

of

the

paper to Mie, because,

later

in 1918,

Mie-whose

attempts

to convince Einstein

of

the need

of

privileged

coordinate

systems

had fallen

on

deaf

ears-confronted

Einstein with the

passage

in the

original

paper

proclaiming

the

privileged

nature

of

coordinates

satisfying

V-g

= 1

(Doc. 532).

IX

The

primacy

of

physical

over

mathematical

considerations

that

we

encounter,

for

instance,

in Einstein’s search for

gravitational

field

equations

and

in

his staunch

defense

of

his

gravitational energy-momentum pseudo-tensor

is also

very

much in

evidence in his

response to

early attempts

to find

a

unified

theory

of

gravitation

and

electromagnetism.

Einstein felt the

need for

such unification

(see, e.g.,

Docs.

400

and

565),

but his

role

at

this

stage was

restricted

to that

of

a

critic

of

the theories

proposed

by

others. He

quickly disposed

of

the

idea

suggested

to

him in

correspon-

dence in late 1917

by

Rudolf

Förster

to

construct

a

unified

theory on

the basis

of

a

nonsymmetric

metric

tensor,

with the

symmetric

and the

asymmetric parts repre-

senting

the

gravitational

and the

electromagnetic

fields,

respectively. Drawing

on

the

analogy

between

gravitational

and

electromagnetic waves

used in Einstein

1918a

(Vol.

7,

Doc.

1),

Einstein

pointed

out that

one

cannot

just

add

an electromag-

netic

field

to

a gravitational

potential (Doc. 439).

A few months

later,

Weyl

announced that

he had

found

a

unified

theory

based

on a generalization

of

Riemannian

geometry.[42]

The discussion

of

this

theory

dom-

inated the

correspondence

between

Einstein

and

Weyl

in

1918.[43]

Einstein imme-

diately objected

to

Weyl’s theory on

physical

grounds

and

never

wavered in his

conviction

that

the

theory

could not be

reconciled

with such basic

empirical

facts

as

the existence

of

sharp spectral

lines

(Doc. 512). Weyl quickly grew weary

of

the

reassurances

of

Einstein’s admiration

for

the mathematical

ingenuity

of

the

theory

that

accompanied

his harsh criticism

of

its

physical underpinnings, or

rather its lack

thereof. Einstein’s

admiration, however,

appears

to

have been

genuine,

as was

his

embarrassment when

Nernst,

after

learning

of

Einstein’s

reservations, objected

to