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5. Rigid rotation in the sense used here—i.e. absence of differential rotation from which energy could be extracted—implies that the body periodically returns to its original configuration. The Schwarzschild time is numerically the same whether measured in the strong field region or at infinity by way of light signals. For this reason constantω implies rigid rotation. A similar situation occurs in connection with the measurement ofΘ. The results below therefore take their simplest form in terms of Schwarzschild time, but they have an invariant, physical significance, independent of the special choice of time scale. (This special significance of the Schwarzschild time of course applies to measurement of the time taken byany process, when carried out in a Schwarzschild background.)