Please carefully consider the following two experiments: Experiment 1 In flat space-time, two completely identical small test probes with built-in rockets and computerized navigation equipment separated by an initial distance l accelerate with a constant proper acceleration a for a proper time interval t. Each probe records the proper duration of the acceleration. An observer fetches both records and compares the durations as was recorded. Experiment 2 In flat space-time, one end of a metal rod of a length l is accelerated away from the direction of the rod, in other words the other end is trailing, with a constant proper acceleration a for a proper time interval t. Two completely identical small test probes with a built-in synchronized ideal clock, ideal accelerometer and computerized navigation equipment were placed at each end of this rod. However the probe in the trailing rod only records, its rocket engine is disabled. Each probe is individually programmed to record both the time when the acceleration started, the proper duration of the acceleration and the proper acceleration profile. An observer fetches both records and compares the start times as well as the proper durations and the proper acceleration profiles as was recorded. It seems that there is no problem with Experiment 1, each record will show an identical duration and each probe has recorded the same path curvature, one of constant proper acceleration. But what about Experiment 2? My question is what is the difference between the recorded proper times (if any) between the two probes and what is the proper acceleration profile for the probe on the trailing end of the rod.