274 THEORY OF RELATIVITY

point.[140]

It follows from this

equality

that there

is

no

observable difference between

the

behavior of

a

mechanical

system

in

a gravitation-free uniformly accelerating

reference

frame

and

the

behavior of

the

same system

in

an

inertial frame in which there

is a

uniform

gravitational

field.

Generalizing

this mechanical

equivalence,

Einstein

postulated

"the

complete

physical

equivalence

of

a gravitational

field and

a

corresponding

acceleration

of

the reference

system"

("die

völlige

physikalische Gleichwertigkeit

von

Gravitationsfeld

und

entsprechender

Beschleunigung

des

Bezugssystems"),

which allows

one

"to

replace

a homogeneous gravitational

field

by

a uniformly

accelerated reference

system"

("ein

homogenes

Gravitationsfeld durch ein

gleichförmig beschleunigtes

Bezugssystem

zu

ersetzen").[141]

This

equivalence

enabled him to draw conclusions about the effects

of

a

uniform

gravitational

field

on physical processes

from the

analysis

of

such

processes

in

a

uniformly

accelerated reference

system.

He thus deduced that

spectral

lines from the

sun

show

a

shift toward

longer wave lengths

when

compared

to the

corresponding

terrestrial

lines

(an

effect

now

called the

gravitational

red

shift)

and that

a light ray

passing through

a gravitational

field

is

deflected from

a straight-line path;

but he noted

that

the

effect

is

too

small to be observed in the

earth's

gravitational

field.

Although

his

correspondence during

the

intervening years

contains several references to the

problem

of

gravitation,

Einstein

did not

publish

again on

this

topic

until

1911

(see

Volume

4,

the editorial note,

"Einstein

on

Gravitation

and

Relativity").[142]

[140]

In

Einstein

1934

(first

published

in

En-

glish

in

Einstein

1933),

Einstein stated

that he

was

convinced

of

this

equality even though

not

yet aware

of

the

experiments

of

Eötvös

(see

Eöt-

vös

1890).

[141]

Einstein

1907j

(Doc.

47), p.

454.

[142]

See

Einstein

1911g.