Question

An old joke says that Einstein boards a train in Dusseldorf and asks the conductor, “What time does Berlin stop at this train?” Which feature of the theory of special relativity does this joke not take into account?

Answer 1

Einstein and Conductor both are in train. In there train frame Berlin is moving towards them. When Einstein ask “What time does Berlin stop at this train?” he forget to mention the clock. Whether he is interested to know time regarding earth/station clock or the clock of train. Two clocks will differ when Berlin stop at there train. This is due to time dilation. Time measured in train clock is proper time and time measured by station clock is improper/dilated time.

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