254

THE

RELATIVITY PRINCIPLE

The

first

two

parts

of the

paper

deal with the kinematic foundations

as

well

as

with their

application

to

the fundamental

equations

of the

Maxwell-

Lorentz

theory, and

are

based

on

the studies1

by

H. A.

Lorentz

(Versl. Kon.

[9] Akad.

v.

Wet., Amsterdam (1904))

and

A.

Einstein

(Ann.

d.

Phys.

16

(1905)).

In

the

first

section,

in

which

only

the kinematic foundations

of

the

theory

are

applied,

I

also discuss

some

optical

problems

(Doppler's

principle,

aberration,

dragging

of light

by moving

bodies);

I

was

made

aware

of

the

possibility of

such

a

mode

of

treatment

by an

oral

communication and

a

paper

[10]

by

Mr.

M.

Laue

(Ann.

d.

Phys.

23 (1907):

989),

as

well

as

a

paper

(though

in

[11]

need

of correction)

by

Mr.

J.

Laub

(Ann.

d.

Phys.

32

(1907)).

In the third part I

develop

the

dynamics

of the material point (elec-

tron).

In the derivation

of

the

equations

of

motion I used

the

same

method

as

in

my

paper

cited earlier.

Force

is defined

as

in

Planck's

study. The

reformulations of the

equations

of motion

of material

points, which

so

clearly

demonstrate the

analogy

between

these

equations

of motion

and

those of

classical

mechanics,

are

also taken

from

that

study.

The

fourth

part

deals with the

general

inferences

regarding

the

energy

and

momentum

of physical

systems

to

which

one

is

led

by

the

theory of

relativity.

These have been

developed

in

the

original

studies,

[14]

A.

Einstein,

Ann.

d.

Phys.

18

(1905):

639

and

Ann.

d.

Phys.

23 (1907):

371,

as

well

as M.

Planck,

Sitzungsber. d.

Kgl.

Preuss.

Akad.

d.

Wissensch.

XXIX

(1907),

but

are

here derived in

a new way,

which,

it

seems

to

me,

shows

especially

clearly the relationship

between

the

above

application

and

the foundations

of

the

theory.

I

also discuss here the

dependence

of

entropy

and

temperature

on

the

state

of

motion;

as

far

as

entropy

is

concerned, I

kept completely to

the

Planck

study

cited,

and

the

temperature of

moving

bodies

I

defined

as

did

Mr.

Mosengeil

in his

study

on moving

black-body

radiation.2

The most important

result

of the

fourth

part

is that

concerning

the

inertial

mass

of

the

energy.

This result

suggests

the

question

whether

energy

also

possesses heavy

(gravitational)

mass.

A

further

question

suggesting

[8]

1E.

Cohn's

studies

on

the subject

are

also pertinent, but

I

did

not make

use

of

them

here.

[15] 2Kurd

von

Mosengeil, Ann. d.

Phys. 22

(1907): 867.

[12]

[13]