EINSTEIN IN COLLABORATION WITH

GROSSMANN

299

1912.[25]

Nordstrom's

theory

is

a special

relativistic,

scalar

theory

of

gravitation,

and

Einstein had earlier convinced himself that

in

such

a

theory

the

equality

between

gravitational

and

inertial

mass

cannot hold.[26]

Another

argument against

scalar

the-

ories,

involving

an

analysis

of

a

radiation-filled

box in

a

gravitational field,

is

put

forward

in

Einstein and Grossmann

1913

(Doc. 13), §7.

Einstein

and Nordström

discussed their

gravitational

theories

during

the

latter's

visit to

Zurich

in late

June

1913[27]

and

came

to the

conclusion

that

Einstein's

last

objection

could

be

avoided if

a

change

of

the

length

scale

as an

effect of

the

gravitational

field is

admitted

in

Nordström's

theory.

As

a

result of

the

discussions

with Einstein,

Nordström

developed

a

second version of

his

theory

which

also satisfies the

equivalence

of

gravitational

and inertial

mass.[28]

Einstein

extensively

discussed

this

version of

the

theory

in

his

Vienna

lecture, and

made

it

clear

that

Nordström's

theory

was

a

viable alternative

to

the

"Entwurf"

theory.[29]

His

main

objection

to it

was

that

it cannot

explain

the

inertia

of

a body by

the

gravitational

effects of

surrounding masses, as

Einstein believed

the

"Entwurf"

theory

promised

to do.

At

the

time, however,

a

decision in favor of

one

or

the

other

theory on empirical

grounds

was

impossible.[30]

In

December

1913

Einstein

once

again

returned

to the

Nordström

theory,

this

time

in

collaboration

with

Adriaan

Fokker.[31]

While Einstein had

failed to

develop

a gen-

erally

covariant formulation for

the

"Entwurf"

theory,

in his

joint paper

with

Fokker

a

generally

covariant formalism

is

presented

from which Nordström's

theory

follows

if

the

single assumption

is made

that

it is

possible

to

choose

preferred systems

of

[25]See

Nordström

1912.

For

a

historical discussion of Nordstrom's

contributions

to

the

understanding

of

gravitation, including

a

discussion of

the

connection between

his work

and

that of Max

Abraham,

see

Isaksson

1985.

For

a study

of

the

relationship

between

Nordström's

and Einstein's

theories,

see

Norton

1992b.

[26]For

evidence of

a

letter

by

Einstein

to

Nordström

in

which

he

pointed

this

out,

see

Nordström

1912,

p.

1129, and,

for

a

later recollection of Einstein's earliest

argument against

a

special

relativistic scalar

theory

of

gravitation,

see

Einstein

1979,

pp.

60-61.

A

similar

argument

is

made for

more

general special

relativistic

gravitational

theories

in

Einstein

1912h

(Doc. 8),

pp.

1062-1063.

[27]See

the entries for

this

period

in

Ehrenfest's

Diary

"I," NeLR,

Ehrenfest

Archive,

Note-

books,

ENB:4-15.

[28]See

Nordström

1913b.

Nordström

also

changed

the

source

term

of

his

earlier

theory

under

the influence of Laue

and

Einstein

(see

Nordström

1913b,

p. 538).

[29]See

Einstein 1913c

(Doc. 17), §3.

[30]Because Nordström's

theory

does

not predict

any

gravitational light deflection,

the

planned

solar

eclipse expedition

would make

an

empirical

decision

possible.

See

also Einstein

and

Besso's

manuscript

on

the motion of

the

perihelion

of

Mercury (Doc. 14),

[p. 53],

for

a

calculation of the

perihelion

motion of

a

planetary

orbit

in the

framework of

the

Nordström

theory.

[31]For

discussions of Fokker's

joint

work with

Einstein,

see

Pais

1982,

pp.

236-237,

and

Stachel

1989, sec. 5;

see

also the editorial

note,

"Einstein

and Stern

on

Zero-Point

Energy,"

p.

273,

for

an

account

of their collaboration

on a

different

subject.

For

a

discussion of

Fokker's

further work

on

general relativity,

see

Kox 1992.