236

COMMENT ON

EHRENFEST'S

NOTE

Doc.

44

COMMENTS

ON

THE NOTE

OF

MR. PAUL

EHRENFEST: "THE

TRANSLATORY MOTION

OF

DEFORMABLE ELECTRONS

AND

THE AREA LAW"

by

A.

Einstein

[Annalen

der

Physik 23 (1907):

206-208]

[1]

The

article

referred

to

above

contains

the

following

remarks:

"In

the

formulation

in which

Mr.

Einstein

published

it, Lorentzian

[2]

relativistic

electrodynamics

is

rather generally viewed

as a

complete system.

Accordingly,

it

must

also

be

able

to

provide purely

deductively

an answer

to

[3]

the

question

posed

by

transferring

Abraham's

problem

from

the rigid electron

to

the

deformable

one:

Granted

that there exists

a

deformable

electron that

[4]

has

some

nonspherical and

nonellipsoidal form

when

at

rest.

According

to

Mr.

Einstein,

this electron

undergoes

the

well-known

Lorentz

contraction

during

uniform

translation.

Well

then,

is it

possible

for this electron

to

undergo

[5]

force-free uniform translation in

every

direction,

or

is

it

not?"

Concerning

this

I have

the

following comments:

1.

The

principle of

relativity,

or,

more

exactly,

the

principle of

relativity

together

with the

principle of

the

constancy

of velocity of

light,

is

not to be

conceived

as a

"complete

system,"

in

fact,

not

as a

system

at

all,

but

merely

as a

heuristic

principle

which, when

considered

by

itself,

contains

only statements

about rigid bodies, clocks,

and

light signals.

It

is

only

by

requiring

relations

between

otherwise

seemingly

unrelated

laws

that

[6]

the

theory

of

relativity

provides

additional

statements.

For

example,

the

theory

of the

motion of

electrons arises in the follow-

ing

way.

One

postulates

the

Maxwell equations

for

vacuum

for

a

system

of

space-time

coordinates.

By

applying

the space-time

transformation derived

by

means

of the

system

of

relativity,

one

finds the transformation

equations

for

the electric

and

magnetic

forces.

Using

the

latter,

and applying

the

space–

time transformation

once

again,

one

arrives

at

the

law

for the acceleration

of

an

electron

moving

at

arbitrary

speed

from

the

law

for the

acceleration of

the

[7]

slowly

moving

electron

(which

is

assumed

or

obtained

from

experience).

Thus,

we are

not

dealing

here

at

all with

a

"system"

in

which

the individual

laws

are

implicitly

contained

and

from which they

can

be found

by

deduction alone,