DOC. 42 SPECIAL
AND GENERAL RELATIVITY
271
26
Relativity
testing
whether
or
not
this theoretical result
is in
accordance
with the
reality.
We
encounter
the
same
difficulty
with
all
physical statements
in
which the
conception
“simultaneous”
plays
a
part.
The
concept
does
not
exist
for
the
physicist
until
he
has
the
possibility
of
discovering
whether
or
not
it
is
ful-
filled
in
an
actual
case.
We thus
require
a
definition
of simul-
taneity
such
that
this
definition
supplies us
with the method
by
means
of
which,
in the
present
case,
he
can
decide
by
experiment
whether
or
not
both the
lightning
strokes occurred
simultaneously.
As
long
as
this
requirement
is
not
satisfied,
I
allow
myself to
be deceived
as a
physicist
(and
of
course
the
same
applies
if
I
am
not
a
physicist),
when
I
imagine
that
I
am
able
to
attach
a
meaning to
the
statement
of
simultaneity.
(I
would ask the reader
not to
proceed
farther until he
is fully
convinced
on
this
point.)
After
thinking
the
matter
over
for
some
time
you
then offer
the
following suggestion
with which
to
test simultaneity.
By
measuring along
the
rails,
the
connecting
line
AB
should be
measured
up
and
an
observer
placed at
the
mid-point
M of the
distance
AB.
This
observer should be
supplied
with
an
ar-
rangement
(e.g.
two
mirrors
inclined
at 90°)
which
allows
him
visually
to
observe both
places
A
and
B
at
the
same
time.
If
the observer
perceives
the
two
flashes of
lightning at
the
same
time,
then
they
are
simultaneous.
I
am
very pleased
with this
suggestion,
but
for all
that
I
cannot regard
the
matter
as
quite
settled,
because
I
feel
con-
strained
to
raise the
following objection:
“Your definition
would
certainly
be
right,
if
only
I
knew that the
light
by means
of
which the observer
at
M
perceives
the
lightning
flashes
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