Dale
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Sorry, I should have been clear. I want the one that is the unit scaling in units where c=1. I.e. so that the affine parameter has the same units as the units of space.PeterDonis said:Any affine parameter along a timelike worldline is proper time. Converting from one affine parameter to another is just rescaling the unit of time and/or picking a new "zero point". There is no one particular one that is "the" proper time; they all are.