DOC.
47
257
respect to
the reference
system
S
that
is
at
first
at rest
relative
to
the
sun.
Let
then the reference
system
S
be
accelerated
by some
external
cause
for awhile, and,
finally,
let it
return
to
a
nonaccelerated
state. What
will
the physical laws look
like
when
the
processes are
referred
to
the
system
S
that is
now
in another
state
of
motion?
We now
make
the
simplest
possible
assumption,
which
is also
suggested
by
the
Michelson and
Morley
experiment:
The
physical
laws
are
independent of the
[19]
state
of
motion
of
the reference
system,
at
least
if
the
system
is
not
accelerated.
In the
ensuing
considerations,
we
will base ourselves
on
this
assump-
tion,
which
we
call "the
principle
of
relativity,"
as
well
as on
the
principle
of the
constancy
of the velocity of light
set
forth
above.
§2.
General remarks
concerning
space
and time
1.
We
consider
a
number
of rigid bodies in nonaccelerated
motion
with
equal
velocities
(i.e.,
at rest
relative
to
each
other).
In
accordance with
the
principle
of
relativity,
we
conclude that the
laws
according
to which
these bodies
can
be
grouped
in
space
relative
to each
other
do
not
change
with
the
change
of these bodies'
common
state of motion.
From
this it follows that
the
laws
of
geometry
determine the
possible
arrangements
of
rigid bodies
in
nonaccelerated
motion
always
in
the
same way,
independent of
their
common
state of
motion. Assertions about the
shape
of
a
body
in nonaccelerated
motion
therefore
have
a
direct
meaning.
The shape
of
a body
in
the
sense
indicated
we
will call its
"geometric shape." The
latter
obviously
does
not [20]
depend
on
the
state
of
motion of
a
reference
system.
2.
According
to
the definition of time
given
in
§1, a
statement
on
time
has
a
meaning
only
with reference
to
a
reference
system
in
a
specific
state
of
motion.
It
may
therefore
be
surmised
(and
will
be shown
in what
follows)
that
two
spatially distant
point events
that
are
simultaneous with
respect
to
a
reference
system
S
are
in
general not
simultaneous with respect
to
a
reference
system
S'
whose
state
of
motion
is different.
3.
Suppose
a
body
consisting
of material
points
P
moves
in
some manner
relative
to
a
reference
system
S.
At
time
t
of
S,
each material point
P