Symbol phi

The passage around this formula

…so two such clocks compare frequencies as $\nu_2$/$\nu_1$ = U($Z_1$)/U($Z_2$) to all orders — the same exact redshift formula an accelerating rocket produces for light climbing or falling inside it. The node potential is $\phi(Z)$ = $\ln$ U(Z) = $\ln(1 + gZ/c^2)$ , and its first derivative at Z=0 is $\phi$' = g/$c^2$ : a nonzero, entirely real, measurable slope. A network of clocks bolted to this elevator’s walls would report exactly the nonzero edge data a network in a genuine gravitational field reports at the same order.…

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