268 THEORY OF RELATIVITY

still controversial,

and often not

clearly distinguished

from

Lorentz's

electron

theory,

was

a major

topic

of

discussion

among leading German-speaking physicists.[92]

V

The

topic

"electrodynamics

of

moving

bodies,"

as

understood at the time

Einstein

wrote

his 1905

paper, usually

included not

only

the

microscopic

electron

theory

discussed

in

that

paper,

but also the

macroscopic theory, involving

conduction currents in

polarizable

and

magnetizable moving

media. The field

equations

of

such

a theory

could

either

be

postu-

lated

phenomenologically, as

Cohn

did;[93]

or they

could be derived

from

an

underlying

microscopic

theory,

as

in

Lorentz's

electron

theory.[94]

Einstein did not turn to the

mac-

roscopic

theory

until 1908. His work with Laub

on

that

subject

is

discussed

in the

editorial

note,

"Einstein

and Laub

on

the

Electrodynamics

of

Moving

Media,"

pp.

503-507.

Since the

theory

of

relativity grew

out

of Einstein's

long-standing

concern

with

electro-

dynamics,

and

his

applications

of

the

theory were

primarily

in this

field,

the

theory

was

often looked

upon

as

essentially

another

version

of Lorentz's

electron

theory (see

section

II,

pp.

256-257).

Einstein

soon

felt the need to make clear

the distinction between the

kinematical

results

of

the

theory,

deduced from the two

principles

of

the

theory,[95]

and

the

application

of

such kinematical results

to

the

solution

of

problems

in

the

optics

and

electrodynamics

of

moving

bodies,

to the derivation

of

the

equations

of

motion

of

a

charged

particle[96]-or

indeed

to

any physical theory.

He

pointed

out that

the

postulates

of

the

theory

do not constitute

a

"closed

system" ("abgeschlossenes System"),

but

only

a

"heuristic

principle,

which considered

by

itself alone

only

contains assertions about

rigid

bodies,

clocks,

and

light

signals" ("heuristisches

Prinzip,

welches

für

sich allein betrach-

tet

nur

Aussagen

über starre

Körper,

Uhren und

Lichtsignale

enthält").

Beyond

such

as-

sertions,

the

theory

could

only

establish "relations between otherwise

apparently

indepen-

dent

laws"

("Beziehungen

zwischen

sonst voneinander

unabhängig

erscheinenden

Gesetzmäßigkeiten")

of

physics.[97]

A few

months after first

publishing

the

theory

of

relativity,

Einstein discovered

a

rela-

tion that

particularly intrigued

him,

the relation between inertial

mass

and

energy.

He

wrote Conrad Habicht:

One

more consequence

of

the

electrodynamical paper

has also occurred

to

me.

The

principle

of

relativity, together

with

Maxwell's

equations, requires

that

mass

be

a

direct

measure

of

the

energy

contained in

a body; light

transfers

mass.

A noticeable decrease

of

mass

should

occur

in

the

case

of

radium. The

argument

is

amusing

and

attractive;

but

I

can't

tell whether the Lord

isn't

laughing

about

it

and

playing a

trick

on me.

[92]

For the

reception

of

the

theory

of

relativity

in

Germany

and several other

countries, see

Goldberg

1984, part II;

and Glick 1987.

[93]

See Cohn 1900,

1902, 1904a,

1904b.

[94] See, e.g.,

Lorentz 1904c.

[95]

See Einstein

1905r

(Doc. 23),

"Kinema-

tischer

Teil,"

pp.

892-907.

[96]

See Einstein

1905r

(Doc. 23),

"Elektro-

dynamischer

Teil,"

pp.

907-921.

[97]

Einstein

1907g

(Doc.

44), pp.

206-207.

The article is

a reply

to

Ehrenfest

1907.