This is not problematic in pr e-Einstein i an physics, where arbitrarily fast signals could be
used in place of light. As faster and faster signals are used, the interval between departure
and return shrinks toward a unique instant, which then must also be the time that is
simultaneous with the arrival of the infinitely fast signal atB. In SR, however, nothing can
travel faster than light.
2
In fact, if standard simultaneity is adopted in every inertial frame
of reference, then it follows in SR that every instant in the open interval between the
departure of the light ray from A and its return is simultaneous with its arrival at B in some
inertial frame (where A and B still refer to fixed spatial locations in the original frame).
The question is which instant at A in the interval between the departure and return
of the light ray is simultaneous with the ray’s arrival at B.