DOC. 25
WHAT IS THE THEORY OF RELATIVITY 103
230
CONTRIBUTIONS TO SCIENCE
ciling
them
logically
by
a
modification of kinematics-i.e., of
the
doctrine
of the
laws
relating to
space
and time
(from
the
point
of view of
physics).
It became clear
that
to speak
of
the
simultaneity
of
two events
had
no
meaning
except
in
relation
to
a given
coordinate
system,
and that the
shape
of
measuring
devices and the
speed at
which clocks
move
depend
on
their
state
of
motion
with
respect
to
the coordinate
system.
But the old
physics,
including
the laws of motion of
Galileo
and
Newton,
did
not
fit
in with the
suggested
relativist
kinemat-
ics.
From the latter,
general
mathematical conditions
issued,
to
which natural laws had
to
conform,
if the above-mentioned
two principles
were
really to apply.
To
these,
physics
had
to
be
adapted.
In
particular,
scientists
arrived
at a new
law of motion
for
(rapidly moving) mass points,
which
was
admirably
con-
firmed in
the
case
of
electrically charged particles.
The
most
important
upshot
of the
special theory
of
relativity
concerned
the inert
masses
of
corporeal systems.
It turned
out
that the
inertia
of
a
system necessarily depends
on
its
energy-content,
and this led
straight
to
the notion
that
inert
mass
is
simply
latent
energy.
The
principle
of the conservation of
mass
lost
its
independence
and became fused
with
that of the conservation
of
energy.
The
special theory
of
relativity,
which
was
simply
a
systematic
development
of the
electrodynamics
of Maxwell and
Lorentz,
pointed beyond
itself,
however. Should the
independence
of
physical
laws
of the
state
of motion of the coordinate
system
be
restricted
to
the uniform
translatory
motion of
coordinate
sys-
tems
in
respect
to
each other? What has
nature to
do with
our
coordinate
systems
and
their
state
of motion? If
it
is
necessary
for the
purpose
of
describing nature,
to
make
use
of
a
coordinate
system arbitrarily
introduced
by
us,
then
the choice of
its
state
of motion
ought
to
be
subject to
no
restriction; the
laws ought
to
be
entirely
independent
of this choice
(general principle
of
relativity).
The establishment
of this
general
principle
of
relativity is
made easier
by
a
fact of
experience
that
has
long
been
known,
namely,
that the
weight
and the inertia of
a
body
are
controlled