DOCS.

182,

183

JANUARY

1916 179

obviously

since

the

energy

tensor should

surely

enter

linearly.

So

that the

outlined

consideration

yields

the

conservation

law of matter

((2)

with

Aa

=

0),

A

=

-1/2

must

necessarily

be chosen. Otherwise

a

contradiction arises.

It

was

precisely

this

that

I

had

not noticed in

my

first

communication.[13]

De Sitter

has asked

me

for

a copy

of

my

last

year’s

paper.[14]

Unfortunately

I

have

none

left. Please lend it

to

him and

relay

my

best

regards.

Imagine my delight

at

realizing

that

general

covariance

was

feasible and

at

finding

out

that

the

equations yield Mercury’s perihelion

motion

correctly.

I

was

beside

myself

with

joy

and excitement for

days.

Is

Tetrode at

Leyden?

His

papers on

the

entropy

constants

are superb.[15]

Congratulate

him for

me,

if

you

see

him.

Recently

I

gave

a

presentation

on

it

at

the

German

Phys.

Society.[16]

Accept,

with

your

wife

and

the little

ones,

my warm

greetings, yours,

Einstein.

Greetings

to Fokker.[17]

183. To Hendrik A. Lorentz

[Berlin,]

17 January 1916

Dear and

highly

esteemed

Colleague,

I

am

in

possession

of

your

three letters

and

very happy

about

your

concur-

rence,

especially

since

I

see

that

you

have considered

the

principal

parts of

the

theory thoroughly

and have taken to

the

idea

that

all

of

our

experience

in

physics

relates

to coincidences.[1]

This

point

of view

quite

consequently requires

the for-

mulation

of

generally

covariant

equations.

I

had taken

this

view

together

with

Grossmann

already

three

years ago

but

had then

come

to

the

false notion

that

it

was

in

contradiction

to

the

requirement

of

unique

causal

dependence.

I

had hit

upon

this

notion,

which

corresponds

to

the

standpoint

held

by you

in

the

first of

your

letters,[2]

after

all

my

efforts at

that

time

to find

a

link between covariant

gravitation equations

and Newton’s

theory

had failed.

My

series of

gravitation

papers

are a

chain of

wrong

tracks,

which nevertheless did

gradually

lead closer

to

the

objective.

That

is

why

now

finally

the

basic formulas

are

good,

but the

derivations

abominable;

this

deficiency

must still be eliminated.

In

both

your

letters

you presented

the

sense

of

the

general

covariance

require-

ment in

an

exemplarily

clear fashion.

It

would

certainly

be of tremendous benefit

to

the

issue if

you

made

your

considerations available to

other

physicists

as well,

by

writing

an

article

on

the

foundations of

the

theory,

as

you kindly proposed