DOC. 27

ON THE THEORY OF

GRAVITATION

291

Doc. 27

On

the

Theory

of Gravitation

by

A. Einstein

[Naturforschende Gesellschaft

in

Zürich.

Vierteljahrsschrift

59.

Part

2, Sitzungsberichte

(1914):

IV-VI]

[1]

Once

physicists recognized

the

superiority

of Maxwell's

theory

of

electromagnetic

phenomena

over

the earlier

action-at-a-distance

theories,

the conviction also

began

to

take hold that Newton's law of

gravitation represents only a

first

step

in the

understanding

of

gravitational phenomena.

One

can

hardly

dismiss the view that

we

have advanced

as

little in the

theory

of

gravitation

as

had

18th-century physicists

in

the

theory

of

electricity

when

they

knew

only

Coulomb's

law.

This realization

imposes

on us

the task of

completing

the

theory

of

gravitation

in such

a

way

that it also

encompasses

the

rapidly changing processes

and the

spatial–

temporal propagation

of

gravitational

effects. The

completion

of this task seemed

hopeless

at

first because of the arbitrariness

resulting

from the multitude of

possibilities.

However,

having

learned from the

theory

of

relativity

that time

enters

the laws

of

nature in

essentially

the

same

way

as

the

spatial

coordinates,

we

have

come

closer

to

the solution of the indicated

problem.

The theoretical

route

of march

is

almost

completely given

to

us

if

we assume

the

general

validity

of

a

fundamental

empirical law, namely

the

law of the

agreement

of the inertial and the

gravitational

mass

of

a

body.

Since the time of

Galileo,

we

know that the acceleration

of

falling

bodies is

independent

of the material of which

they are

made,

which

law

can

be

expressed as

follows: The

same

characteristic

constant

of

a

body

that determines

its

inertia also

determines

its

gravitational

action. This law

acquires

an even more

fundamental

significance by

the fact

that,

according

to the

theory

of

relativity,

there exists

a

general relationship

between the inertial

mass

and the

energy

of

a

body.

The

energy,

inertia,

and

gravity

of

a

body

are

thus reduced

to

one

another. The

equality

that exists

between inertia

and

gravitation

was

experimentally

demonstrated

by

Eötvös with such

accuracy

about 20

years ago

that relative deviations of the

gravitational

and the

inertial constant from each other of

the order of

magnitude

of 10-7

must

be ruled

out.

[2]

We have succeeded

in

establishing

two

theories that

satisfy

the above-indicated

demands,

that of Nordström and that

of

Einstein-Grossmann. The first of

these

theories

[3]

is

simpler

and

more

natural from the

point

of view of

the

original theory

of

relativity;

that

is to

say,

it adheres

to

the latter's fundamental

assumption

that

spatial-temporal

reference

systems

can

be chosen such that

light propagates everywhere

in the

vacuum

with the

same

velocity

c

(principle

of the

constancy

of the

velocity

of

light).