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 constrained 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 travels along the length _A_ _M_ with
the same velocity as along the length _B_ _M_. But an examination of
this supposition 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 assumes absolutely nothing about
light. There is only _one_ demand 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 reference[7] (here the railway embankment). We
are thus led also to a definition of time in physics. For this
purpose we suppose that clocks of identical construction are placed at
the points _A, B_ and _C_ of the railway line (co-ordinate system) and
that they are set in such a manner that the positions of their pointers
are simultaneously (in the above sense) the same. Under these
conditions we understand by the time of an event the reading
(position of the hands) of that one of these clocks which is in the
immediate vicinity (in space) of the event. In this manner a time-value
is associated with every event which is essentially capable of
observation.
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