  We review Bohr's reasoning in the Bohr-Einstein debate on the photon box
experiment. The essential point of his reasoning leads us to an uncertainty
relation between the proper time and the rest mass of the clock. It is shown
that this uncertainty relation can be derived if only we take the fundamental
point of view that the proper time should be included as a dynamic variable in
the Lagrangian describing the system of the clock. Some problems and some
positive aspects of our approach are then discussed.
