 The redshift formula for the frequency formula_17 is formula_18.When formula_19 is small, these results are consistent with the equation given above based on the equivalence principle.For an object compact enough to have an event horizon, the redshift is not defined for photons emitted inside the Schwarzschild radius, both because signals cannot escape from inside the horizon and because an object such as the emitter cannot be stationary inside the horizon, as was assumed above.