DOC.
60
385
This
path
leads
to
the so-called
theory
of
relativity,
of
whose
conse-
quences
I would
here
like
to mention
only
one,
because it
brings
about
a
certain modification
of
the basic ideas of
physics.
It
turns out
that the
inertial
mass
of
a body
decreases
by L/c2
when
the
body
emits the
radiation
energy L.
One can
arrive
at
this in the
following
way.
[16]
We
consider
a
motionless,
freely
floating
body
that emits in
two
oppo-
site directions
the
same
amount
of
energy
in the
form of
radiation.
The
body
remains
at
rest.
If
E0
denotes the
energy
of
the
body
before the emission,
E1
its
energy
after
the
emission,
and
L
the
amount
of radiation
emitted,
then
we
have,
according
to
the
energy
principle
E0
=
E1
+
L
.
We
now
observe
the
body,
as
well
as
the radiation it
emits, from
a
coordinate
system
relative
to which
the
body
moves
with
velocity
v.
The
theory of
relativity
then
provides
the
means
for
calculating
the
energy
of the
emitted radiation with respect
to
the
new
coordinate
system. The
value
obtained for it is
L'
= L
.
1
.
1-
c2
Since the principle of conservation of
energy
must
hold for the
new
coordinate
system
as
well,
one
obtains,
using
an
analogous
notation,
E'0
=
E'1
+ L
-
1
'
-c2
By
subtraction, and
omitting terms
of fourth
or
higher
order in
v/c,
we
get
(E'0
-
E0) =
(E'1
-
E1)
+
1/2
c2
v2
.
However,
E'0 -
E0
is
nothing
other than the kinetic
energy
of
the
body
before the emission
of
light,
and
E'1 - E1
is
nothing
other than its
kinetic
energy
after the emission of light. If
M0
denotes the
mass
of the
body