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]