The arguments employed by Einstein that led him to the conclusion that “we cannot attach absolute meaning to the concept of simultaneity” are subjected to critical analysis in light of experiments that have subsequently been carried out with atomic clocks. The physical significance of time dilation is a key element in this discussion. It is pointed out that the fact that two clocks at rest in a laboratory disagree on the elapsed time for a given event is normally interpreted to mean that they are not properly synchronized or that they run at different rates, not that the event in question did not occur simultaneously for them. An experiment using GPS technology is outlined to test whether events really do not occur simultaneously for all observers or if instead that their timing results only differ because the clocks employed by them run at systematically different rates. An example of two light pulses moving in opposite directions shows that one can come to a different conclusion about simultaneity depending on whether the Lorentz transformation (LT) of space-time coordinates or the associated relativistic velocity transformation (RVT) is employed as justification. Finally, it is shown that it is possible to satisfy Einstein’s two postulates of relativity while still maintaining the principle of absolute simultaneity of events by introducing an alternative (Global Positioning System) Lorentz space-time transformation (GPS-LT) that is also perfectly consistent with the RVT.
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