ON THE IDEA OF TIME IN PHYSICS
Lightning has struck the rails on our railway embankment at two places
_A_ and _B_ far distant from each other. I make the additional
assertion that these two lightning flashes occurred simultaneously. If
I ask you whether there is sense in this statement, you will answer my
question with a decided Yes. But if I now approach you with the
request to explain to me the sense of the statement more precisely, you
find after some consideration that the answer to this question is not
so easy as it appears at first sight.
After some time perhaps the following answer would occur to you: The
significance of the statement is clear in itself and needs no further
explanation; of course it would require some consideration if I were to
be commissioned to determine by observations whether in the actual case
the two events took place simultaneously or not. I cannot be satisfied
with this answer for the following reason. Supposing that as a result
of ingenious considerations an able meteorologist were to discover that
the lightning must always strike the places _A_ and _B_ simultaneously,
then we should be faced with the task of 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 fulfilled in an actual case. We thus require a definition of
simultaneity 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 arrangement (_e.g._ two mirrors inclined at
90 ) which allows him visually to observe both places _A_ and _B_ at
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