In a 1907 discussion of simultaneity, Einstein again considered identical clocks at rest in
some inertial frame. For the totality of the clock readings to give us ‘‘time’’, he wrote (as
quoted in translation by Jammer, p. 123), ‘‘we need a rule according to which these clocks
will be set relative to each other. We now assumethat the clocks can be adjusted in such a
way that the propagation velocity of every light ray in vacuum—measured by means of these
clocks—becomes everywhere equal to a universal constant c, provided that the coordinate
system is not accelerated. If AandBare two points at rest relative to the coordinate
system, which are equipped with clocks and are separated by a distancer, while tAis the
reading of the clock atAat the moment when a ray of light propagating through a vacuum
in the direction ABreaches pointA, and tBis the reading of the clock at Bat the moment
the ray reaches B, then we should always have r/(tB–tA)¼c, whatever the motion of the
light source emitting the light ray or the motion of the other bodies may be.’’