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ON THE RELATIVITY OF THE CONCEPTION OF DISTANCE

Let us consider two particular points on the train [8] travelling

along the embankment with the velocity v, and inquire as to their

distance apart. We already know that it is necessary to have a

body of reference for the measurement of a distance, with respect

to which body the distance can be measured up. It is the simplest

plan to use the train itself as reference-body (co-ordinate

system). An observer in the train measures the interval by

marking off his measuring-rod in a straight line (e.g. along the

floor of the carriage) as many times as is necessary to take him

from the one marked point to the other. Then the number which

tells us how often the rod has to be laid down is the required

distance.

[8] e.g. the middle of the first and of the hundredth carriage.