272 DOC. 42
SPECIAL AND GENERAL RELATIVITY
On the
Idea
of Time
in
Physics
27
travels
along
the
length
A -»
M with the
same velocity as
along
the
length
B
-»
M. But
an
examination of this
suppo-
sition would
only
be
possible
if
we
already
had
at
our
disposal
the
means
of
measuring
time.
It
would thus
appear
as
though
we were
moving
here
in
a logical
circle.”
After further consideration
you cast
a
somewhat disdainful
glance at
me-and
rightly
so-and
you
declare:
“I
maintain
my previous
definition
nevertheless,
because in
reality
it
as-
sumes
absolutely nothing
about
light.
There
is
only one
de-
mand
to
be made of the definition of
simultaneity, namely,
that in
every
real
case
it
must supply
us
with
an empirical
decision
as
to
whether
or not
the
conception
that
has
to
be
defined
is
fulfilled.
That
my
definition
satisfies
this demand
is
indisputable.
That
light requires
the
same
time
to traverse
the
path
A
-
M
as
for
the
path
B
-
M
is
in
reality
neither
a
supposition nor a
hypothesis
about the
physical nature
of
light,
but
a stipulation
which
I
can
make of
my own
freewill in order
to
arrive
at
a
definition of
simultaneity.”
It
is
clear
that
this definition
can
be used
to give
an
exact
meaning not
only
to two events,
but
to
as many
events
as
we
care
to
choose,
and
independently
of the
positions
of
the
scenes
of
the
events
with
respect
to
the
body
of
reference1
(here
the
railway
embankment).
We
are
thus led
also
to
a
definition of
“time”
in
physics.
For this
purpose we suppose
1
We
suppose
further, that, when three
events A,
B
and C
occur
in
different
places in
such
a
manner
that
A
is
simultaneous with
B,
and
B is
simultaneous with
C (simultaneous in
the
sense
of the above
definition),
then the criterion
for
the
simultaneity
of the
pair
of
events A,
C is also
satisfied. This
assumption is
a
physical hypothesis
about the
law
of
propagation
of
light;
it
must
certainly
be fulfilled if
we are to
maintain the
law
of the
constancy
of
the
velocity
of
light
in
vacuo.