172 INERTIA
AND
ENERGY CONTENT
Doc.
24
DOES THE
INERTIA
OF
A
BODY DEPEND UPON
ITS
ENERGY CONTENT?
by
A.
Einstein
[Annalen
der
Physik
18
(1905):
639641]
The
results of
an
electrodynamic
investigation
published
by me
recently
in this
journal1
lead
to
a
very
interesting conclusion, which
shall
be
derived
here.
There I
based
myself
upon
the
MaxwellHertz
equations
for
empty
space
along
with Maxwell's
expression
for the
electromagnetic
energy
of
space,
and
also
on
the
following
principle:
The
laws
governing
the
changes
of
state of
physical
systems
do
not
depend
on
which
one
of
two
coordinate
systems
moving
in
uniform parallel
translation relative
to
each
other these
changes
of
state
are
referred
to
(principle
of
relativity).
Based
on
these fundamental
principles2, I
derived the
following
result,
among
others
(loc.
cit.,
§8):
Let
a
system
of
plane
waves
of
light, referred
to
the coordinate
system
(x,
y,
z), possess
the
energy
l;
let the
direction
of
the
ray
(the
wave
normal)
form
the
angle
p
with
the
xaxis
of
the
system.
If
we
introduce
a new
coordinate
system
(£,//,(),
which
is
uniformly
paralleltranslated
with
respect to
the
system
(x, y,
z),
and whose
origin
is
moving
along
the xaxis
with velocity
v,
then the abovementioned quantity of lightmeasured in the
system
(£,?7,)possesses the
energy
1

t?
cos
p
g*
=
£1,
1
,
where
V
denotes the
velocity
of light.
We
will
make
use
of
this
result in
the
following.
[1]
1A.
Einstein,
Ann.
d.
Phys.
17 (1905):
891.
2The
principle of the
constancy of
the velocity
of light used
there is
of
course
contained in
Maxwell's
equations.