DOCUMENT 527 APRIL 1918 743

pp.

478-480,

Weyl argued

that the notion

on

which Einstein’s

objection was

based,

namely

that rods

and clocks

directly measure

the line

element,

is

problematic even

in

ordinary general relativity.

In

par-

ticular,

he

argued,

it is not at all

clear

whether

a

clock continues

to

measure

the

length

of

its

worldline

when

it

is not

moving on a geodesic,

such

as

in the

presence

of

a strong

and

fluctuating

electromag-

netic field.

Citing

both

Weyl

1918c,

pp.

182-184, and Kretschmann

1917,

he added that clocks and

rods

can

be

dispensed

with

altogether

in the formulation

of

general relativity

and that

light signals

suffice

as

the

means

of

measuring

the line

element

and the

components

of

the metric

tensor

up

to

a

constant factor

reflecting

the freedom

to

choose

a (global)

unit

of

length. Weyl

then

pointed

out that

to

a very good approximation

the line element in his

theory

would almost

always

be

equal

to

the line

element

in

general relativity.

Even

if

it

is

assumed,

he

continued,

that clocks

measure

the

integral

of

the line

element

along

their

worldlines

(which

in

Weyl’s theory, as

Einstein had

pointed

out,

would

make the

rate

of

a

clock

path dependent),

the

theory

would still be

compatible

with the existence

of

identifiable

spectral

lines

(the empirical

evidence

Einstein

had cited

against

the

theory),

because,

as

Weyl claimed,

"the influence

of

any stormy prehistory

will

quickly

fade

away"

("der

Einfluß einer

etwaigen

stürmischen

Vorgeschichte

wird rasch

abklingen,"

Weyl

1918b, pp.

479-480).

Einstein’s

response

to

Weyl’s argument can

be found in

a

letter

to

Walter Dällenbach about six weeks later

(see

Doc.

565).

[2]See

Doc.

512 for

an

account

of

the difficulties that Einstein’s

objection

to

Weyl’s theory

had

caused two

weeks

earlier in

getting

Weyl’s

paper

accepted

for

publication

in the

proceedings

of

the

Prussian

Academy.

In the

end,

both

Einstein’s

objection (in

the formulation

given

in Doc.

512)

and

Weyl’s

rejoinder

were appended

to the

paper (Einstein

1918h

[Vol.

7,

Doc.

8],

Weyl

1918b, pp.

478-

480).

[3]Walther

Nernst

had

protested

in the Prussian

Academy against

silent

acceptance

of

Weyl’s

theory (see

Doc.

512).

[4]The

phrase

that Einstein had used

more

than

a

week

earlier

to

express

his admiration for

Weyl’s

theory

from

a

purely

theoretical

point

of

view while

rejecting

it

as a description

of

physical reality

(see

Doc.

512).

527. From Marga

Planck[1]

[Berlin,]

30.

April

1918.

Sehr

geehrter

Herr

Professor!

Haben Sie

herzlichen

Dank

für

die

Zusendung

Ihrer

"Predigt",

die wir

uns ge-

stern

nochmals

vorgelesen

haben.[2]

Wir

freuten

uns

wieder

an

Ihren

Gedanken;

und sie

nun

als

Erinnerung

an

den schönen

Abend

zu

besitzen.[3]

Und ich

selbst

freue mich noch

ganz

besonders-das möchte

ich Ihnen heut

zum

Ausdruck brin-

gen-,

daß mein Mann in Ihnen einen

so warmen

Freund

gefunden

hat!

Mit

bestem

Gruß Ihre

Marga

Planck.

ALS.

[19

274].

[1]Planck,

née

von

Hoesslin.

[2]Einstein

1918e

(Vol. 7,

Doc.

7),

delivered at

a special meeting

of

the Deutsche

Physikalische

Gesellschaft

on

26

April

in

honor of

Planck’s sixtieth

birthday

(on

23

April).

For

a published

account

of

the

festivities, see

Swinne

1918.

[3]The

participants

assembled in

a

restaurant

in the Friedrichstraße after the celebration

(see

Moritz

Schlick

to

Gerda

T.,

4

May

1918,

excerpt

in

Stargardt

auction

catalogue

659 [16-17 March

1995],

lot

496).