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The passage around this formula

The proper-time loop $\mathcal L_{\rm P}$ is where mass stops being a fixed label and becomes something an object can carry a little more or less of depending on what it is doing internally. A composite system with an internal excitation of energy $\Delta$ E above its ground configuration carries additional rest mass $\Delta$ E/$c^2$ , by ordinary mass–energy equivalence — no redefinition of either the relativistic or the everyday meaning of mass, simply 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…

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