630 DOC.

604 AUGUST

1918

Then

Weyl’s

basic

assumption

is not correct

on

the

molecular

level

in

any

case.

As

far

as

I

can

see,

not

a

single physical

[reason]

speaks

for

it

being

valid for

gravitational

fields. The

[argument

against

it], however,

is

that the

gravitational

field equations

become of

the fourth

order,[8]

for

which

there

has been

absolutely

no

indication

in

experience up

to

now,

also

that

a

somewhat

plausible

formulation

for

the

energy

law does

not

exist

as soon as

the

Hamiltonian of

the

gravit.

field

contains

derivatives

higher

than

of

the

first order for

the

guv’s.

This

leads

me

to

the

energy question.

Your

suggestion

reveals

to

me

that

you

also

are

of

the

opinion

that

an energy

tensor

for

gravitation

could be

dispensed

with.[9]

But then the

energy

law

immediately

loses

all value.

The

matter does

satisfy

[the]

“energy

law”

dxv

+

2

dx.

^

=

0.

But the

second term

causes

this

equation

to have

no

consequence

of

the

form

d_

dt

{fdV} =

0.

It

is

also

intuitively

immediately apparent

that without

a

stress tensor

for the

static

gravitational

field,

the

Newtonian forces cannot be derived from

an energy

tensor.

If

the

energy-momentum

conservation

concept

[is

not

also]

applied

to

the

guv-field,

it loses

all

physical

value.-

What

I

wrote

you

about

A

is

worthless.[10]

The

reasons

remain

the

following:

Either the

world has

a

center point,

has

on

the

whole

an

infinitesimal

density,

and

is

empty

at

infinity,

whither

all

thermal

energy eventually dissipates

as

radiation.

Or: All

points

are

on average

equivalent,

the

mean

density

is

the

same

throughout.

Then

a

hypothetical

constant

A

is

needed,

which indicates at which

mean

density

this

matter

can

be

at

equilibrium.

One

definitely gets

the

feeling

that the

second

possibility

is

the

more

satis-

factory

one, especially

since it

implies

a

finite

magnitude

for

the

world. Since

the

world

just

exists

as a

single

specimen,

it

is

essentially

the

same

whether

a

constant is

given

the

form of

one

belonging

within

the natural

laws

or

the

form

of

an “integration

constant.”-[11]

A

priori,

irreversible

elementary

laws

can

very

well be

expected.

But

closer

observations

up

to

now

do not

speak

for

it

(particularly

the

quantum

laws), no

more so

the fact

of

thermal

equilibrium. Judging

from all

I

know,

I

believe in

the

reversibility

of

elementary

events. All temporal

bias

seems

to

be based

on

“order.”

You

will

counter

with

radioactivity.

But

I

am

convinced

that

the

inverse

process

is

only impossible practically.

Warm

regards, yours,

Albert.