DOCS.

183,

184

JANUARY

1916 181

It

is important

that the

equations initially

be

given

in

a

generally

covariant

form because

only

in

this

setup

for the

equations

is

all arbitrariness avoided. For

if

you

confine

yourself

from

the

start

to

the

case

y-g

=

1,

then

a

factor

(V-g)n

could be added to scalar

G

without

disturbing

the

thus

restricted

covariance.

The

same

is

valid

analogously

to

the

equations covering

matter.

It

would

undoubtedly

be

a

step

forward if

later the

frame of reference could

be

specialized

even

further

in

a

natural

way. My

efforts in this direction have

been unsuccessful

up

to

now,

though.

In

any

case,

the

problem

must

obviously

be

arranged

so

that

dx1, dx2, dx3

is of

a

spatial nature throughout

and

dx4

of

a

temporal

one.

But

this

is

a

specialization solely by

means

of

inequalities,

not

equalities.

Your

remark

on

extinction

is

very

convincing.[8]

If

only light

were

finally

shed

on

the

absorption

process!

But the

proof

of

the

existence

of

zero-point

energy

shows

us

how far

away

we are

from

a genuine

understanding

here.[9]

In

thanking

you again

for

your burning

interest and

even more

for

your

inten-

tion

to

place your

efforts in

the

service

of

this

problem,

I

remain

with heartiest

greetings

to

you

and

yours,

in

friendship,

yours truly,

A.

Einstein.

Cordial

thanks

for

sending

your

lectures

on

statistical

mechanics.[10]

Feb.

23.

I

admire

very

much Tetrode’s

splendid analyses

on

the

entropy

constant.[11] I

recently gave a

report

on

it.[12]

184. To

Hendrik

A. Lorentz

[Berlin,]

19 January 1916

Dear and

highly

esteemed

Colleague,

Only

too

well

do

I

understand

your

attempt

also to derive

gravitation

from

the

field

equations

in

the

manner

of

Hamilton’s

principle.

I

myself

am

compelled

to

derive

the

Hamiltonian function

retroactively,

in

order

to

derive

the

expression

for

the

conservation laws

conveniently.[1]

Nevertheless,

I must

admit that

I

actually

do not

see

in Hamilt.

princip. anything

more

than

a means

toward

reducing

a

system

of

tensor

equations

to

a

scalar

equation

for which

the

conservation laws

are

always

satisfied and

easily

derived. I

definitely

believe

that it

is

possible

to

find

a

Hamiltonian form also for

the

generally

covariant form of

the

equations,

as

I

already

indicated

in

yesterday’s

letter.[2]

I

must

emphasize again

that

my

field

equations (2a) given

in

my

contribution,

“The Field

Equations of

Gravitation”[3]

Gim=k(Tim-1/2gimT)