132

DOC.

8

REPLY TO A COMMENT BY

M.

ABRAHAM

There exists

a

reference

system

K in which

every light ray propagates

in

vacuum

with the universal

velocity

c,

regardless

of

whether the

light-emitting body

is in

motion

or

at rest

relative

to

K.

From these two

principles

it

is

possible

to

develop

the

theory currently

known

as

the

"theory

of

relativity."

This

theory

is

correct to

the

extent to

which the

two

principles

upon

which it is

based

are

correct.

Since these

seem

to

be correct to

a great

extent,

the

theory

of

relativity

in

its

present

form

seems

to

represent

an

important

advance; I

do

not

think that it has

hampered

the further

development

of theoretical

[7] physics!

But what about the limits of

validity

of

the

two

principles?

As

I

have

already

emphasized,

we

have

not

the

slightest

reason

to

doubt the

general validity

of

the

principle

of

relativity.

On the other

hand,

I

am

of the view that the

principle

of

the

constancy

of

the

velocity

of

light can

be maintained

only

insofar

as one

restricts

oneself

to

spatio-temporal regions

of

constant

gravitational potential.

This is

where,

in

my opinion,

the limit of

validity

of the

principle

of the

constancy

of

the

velocity

of

light-though

not

that of the

principle

of

relativity-and

therewith the limit of

validity

of

our

current

theory

of

relativity

lies.

I

have

come

to

this

opinion

on

the

basis of the

arguments

indicated below.

One

of

the

most

important

results of the

theory

of

relativity

is

the realization that

every type

of

energy

E

possesses

an

inertia

(E/c2)

proportional

to it. Since,

as

far

as

our

experience goes, every

inertial

mass

is

at

the

same

time

a gravitational

mass,

we

cannot

avoid

also

ascribing

a

gravitational

mass E/c2

to

every type

of

energy E.1

From this it

follows

immediately

that

gravity

has

a

stronger

effect

on a

moving body

than

on

the

same

body

when it

is at

rest.

If the

gravitational

field

can

be

interpreted

in

terms

of

our present

theory

of

relativity,

this

can probably

be

done

in

only

two

ways.

One

can

conceive of the

[9]

gravitational

vector

either

as a

four-vector

or as a

six-vector. For each of these

two

[10]

cases one can

obtain transformation

formulas for the transition

to

a

uniformly moving

reference

system.

With the

help

of

these

transformation formulas and the transformation

formulas for the

ponderomotive

forces,

one can

then find the forces that

act

on

material

[8]

1In

a

conversation,

Mr.

Langevin

has

drawn

my

attention to the fact that

one comes

into conflict with

experience

if

one

does not make this

assumption. Namely,

since

great

quantities

of

energy

are

given

off

during

radioactive

decomposition,

the

inertial

mass

of

the

matter must

diminish in this

process.

If

the

gravitational mass were

not

to

diminish

proportionally,

bodies

composed

of

different elements would have to have

demonstrably

different

gravitational

accelerations in the

same

gravitational

field.