Symbol Δτ

The passage around this formula

…the standard bookkeeping in which a system’s total rest energy M$c^2$ includes whatever internal energy $H_{\rm int}$ it holds: M=$M_0$+$H_{\rm int}$/$c^2$ . If the same object is sent along two paths that differ in proper time by $\Delta$$\tau$ , and its internal state differs between the two paths — ground on one, excited on the other — then the two branches’ free-particle phases differ by $\Delta\phi_{\rm clock}=-\frac{\Delta E}{\hbar}\,\Delta\tau$. The shared $M_0$$c^2$$\Delta$$\tau$/$\hbar$ term, common to both arms, is an overall phase and drops out of any interference…

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