Picturing Einstein's Train of Thought

Abstract

In a recent series of articles in this journal, Michael Cohen and I have had a fundamental disagreement over the construction of Einstein's Gedankenexperiment about the relativity of simultaneity that is found in his famous lay exposition of special and general relativity (hereafter SR and GR, respectively).' I should make it clear from the outset that it appears that we do not disagree significantly over matters of the consistency, coherence, and adequacy of SR and GR themselves as physical theories. Rather, our disagreement is simply about whether Einstein logically and adequately constructed his famous 'train and lightning-bolts' thought-experiment that has figured prominently in countless popular and technical expositions of relativity. Cohen maintains that Einstein did not so construct it-I argue that he did. Given that many secondary references to this thought experiment merely parrot Einstein's account, it does seem to be of some importance to know whether the use of such passages is vindicated. Rather than retrace the course of our particular disagreements, perhaps it would be most profitable to simply and literally see what Einstein says in the disputed passage. What I propose is to illustrate the principal remarks of Einstein's Gedankenexperiment on an appropriate spacetime diagram, and only then examine why Cohen seems to have thought Einstein's presentation to be inadequate. The thought-experiment runs as follows. (I shall attempt to summarize the relevant passage in question concisely but accurately. Readers should consult the original to verify that I have done this.) A moving train on an embankment is struck by two lightning bolts at points A (in the rear of the train) and B (in the front of the train) and which contains a stationary observer M' at a location 1 Albert Einstein, Relativity: The Special and the General Theory (New York: Crown Publishing, 15th edition, 1951), 25-26. Cohen's previous papers are, in order of appearance: Michael Cohen, 'Simultaneity and Einstein's Gedankenexperiment', Philosophy 64, (July 1989), 391-396; 'Einstein on Simultaneity', Philosophy 67, (October 1992), 543-548; 'Simultaneity: A Composite Rejoinder', Philosophy 70, (October 1995), 587-589. My respective replies are: V. Alan White, 'Cohen on Einstein's Simultaneity Gedankenexperiment', Philosophy 66, (April 1991), 245-246; 'Relativity and Simultaneity Redux', Philosophy 70, (July 1993), 401-404.