  Apparently no one had, after 1929, relooked at Einstein’s objection  until I did in 1983. The result is interesting and deserves perhaps  to be a footnote in the history of science: Let us take Einstein’s  Gedankenexperiment …. When the two clocks come back, because  of the insertion of the factor \(-i\), they would not have  different scales but different phases. That would not influence their rates of time-keeping. Therefore, Einstein’s original objection  disappears. But you can ask a further question: Can one measure their phase difference? Well, to measure a phase difference one must do an  interference experiment. Nobody knows how to do an interference  experiment with big objects like clocks. However, one can do  interference experiments with electrons. So let us change Einstein’s  Gedankenexperiment to one of bringing electrons back along two  different paths and ask: Can one measure the phase difference? The  answer is yes. That was in fact a most important development in 1959  and 1960 when Aharonov and Bohm realized—completely  independently of Weyl—that electromagnetism has some meaning  which was not understood   before.[103] 