DOC.

56

357

Doc. 56

ON

THE

PRESENT

STATUS OF THE

RADIATION

PROBLEM

by

A.

Einstein

[Physikalische

Zeitschrift

10

(1909):

185-193]

This

journal has recently published expressions of opinion

by

Messrs.

H. A.

Lorentz1,

Jeans2, and

Ritz3

which

offer

good

insight

into the

[4]

present status

of this

extremely important

problem.

In the belief that it

would

be

of benefit if all those

who

have seriously

thought

about

this

matter

would communicate

their

views,

even

if

they

have

not

been

able

to

arrive

at

a

final

result, I

would

like

to

communicate

the

following.

1.

The simplest

form in which

we can

express

the

laws of

electro-

dynamics

established

so

far is that

presented

by

the

Maxwell-Lorentz

partial

differential

equations.

In

contrast to

Mr.

Ritz3, I

regard

the

forms

containing

retarded functions

as merely

auxiliary

mathematical forms.

The

[5]

reason

I

see myself

compelled

to

take this

view

is first

of

all that those

forms

do

not

subsume

the

energy

principle,

while

I

believe that

we

should

adhere

to

the strict validity of the

energy

principle

until

we

shall

have

found

important

reasons

for

renouncing

this

guiding star.

It is

certainly

true

that Maxwell's

equations

for

empty

space,

taken

by

themselves,

do not say

anything,

that

they only represent

an

intermediary construct;

but,

as

is well

known,

exactly

the

same

could

be

said

about Newton's equations

of motion,

as

well

as

about

any

theory

that

needs to be

supplemented

by

other theories in

order

to yield

a

picture for

a

complex

of

phenomena.

What

distinguishes

the

Maxwell-Lorentz differential

equations from

the

forms

that contain retarded

functions is the circumstance that

they

yield

an

expression

for the

energy

and

the

momentum

of the

system

under

consideration for

any

instant of time,

relative

to

any

unaccelerated

coordinate

system. With

a

theory

that

operates

with retarded forces it is

not

possible

to

describe the instantaneous

state

of

a

system

at

all without

using

earlier

states

of the

system

for this

description.

1H.

A.

Lorentz,

Phys.

Zeit.

9 (1908): 562-563.

2J.

H.

Jeans,

Phys.

Zeit.

9

(1908):

853-855.

3W.

Ritz,

Phys.

Zeit.

9

(1908): 903-907.

[1]

[2]

[3]