The Twin-Rod Thought Experiment

Einstein's 1911 twin-rod thought experiment is analyzed, and some of its implications for the physical status of length-contraction within the special theory of relativity are examined. Contrary to a widespread view, it is shown that the twin-rod thought experiment does not require that the two rods be travelling at equal speeds (with respect to a given inertial frame K) in order that the experiment serve its intended purpose. Since the equality or inequality of the speeds of the rods will depend upon the criterion of simultaneity adopted within the inertial frames of the special theory, the irrelevance of the speeds of the rods here indicates the synchrony-independence of this length-contraction effect within special relativity.