340

DOC. 448

JUNE

1913

448. To

Ernst Mach

Zurich, 25

June

1913

Highly

esteemed

Colleague,

You

have

probably

received

a

few

days ago my

new

paper

on

relativity

and

gravitation,[1]

which

is

now finally completed

after

unceasing

toil and

tormenting

doubts.

Next

year,

during

the solar

eclipse,[2] we

shall

learn whether

light rays

are

deflected

by

the

sun,

or

in other

words,

whether

the

underlying

fundamental

assumption

of the

equivalence

of the acceleration of the reference

system, on

the

one

hand,

and the

gravitational field,

on

the other

hand, is

really

correct.

If

yes,

then-in

spite

of Planck's

unjustified

criticism-your

brilliant

investigations

on

the foundations of

mechanics will have received

a

splendid

confirmation.[3]

For

it

follows

of

necessity

that

inertia has its

origin

in

some

kind

of

interaction

of the

bodies,

exactly

in

accordance

with

your argument

about Newton's bucket

experiment.[4]

You will find

a

first

consequence

in this

sense on

the

top

of

page

6

of the

paper. [5]

Beyond

that,

the

following

results

have

been

obtained:[6]

1.

If

one

accelerates

an

inertial

spherical

shell

S,

then,

according

to

the

theory,

a body

enclosed

by

it

experiences

an

accelerating

force.

2.

If

the

shell

S rotates

about

an

axis

passing

through

its

center

(relative

to

the

fixed stars

("Restsystem"),

then

a

Coriolis field

arises

inside the

shell, i.e.,

the

plane

of

the

Foucault

pendulum

is

being

carried

along

(though

with

a

practically

immeasurably

small

velocity).

It

gives

me

great

pleasure

to

be

able

tell

you

about

this,

all

the

more

so

because

Planck's

criticism

always

seemed

to

me

to

be most

unjustified.

With

kindest

regards,

I remain

very respectfully yours,

A.

Einstein

Thank

you sincerely

for

sending

me

your

book.