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DOC. 42 SPECIAL AND GENERAL RELATIVITY
NINE
The
Relativity
of
Simultaneity
Up
to
now our
considerations have been referred
to
a
particular body
of
reference,
which
we
have
styled
a
“railway
embankment.” We
suppose
a
very long
train
travelling along
the
rails
with the
constant
velocity v
and
in
the
direction indicated
in
Fig.
1.
People travelling
in
this train
v
M'-
v
Train
A M
B
Embankment
Fig.
1
will
with
advantage
use
the train
as a rigid
reference-body
(co-ordinate
system); they regard
all
events
in
reference
to
the
train.
Then
every event
which takes
place along
the line
also
takes
place at
a particular
point
of the train.
Also
the definition
of
simultaneity can
be
given
relative
to
the train
in
exactly
the
same way
as
with
respect to
the embankment.
As
a
natural
consequence,
however,
the
following question
arises:
Are
two events
(e.g.
the
two
strokes of
lightning
A
and
B)
which
are
simultaneous
with
reference
to the
railway
embankment
29