Electrons can be sent along two different paths and their phase difference measured, analogously to Einstein's clock Gedankenexperiment.

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