Optical EngineeringOptical measurement with direct traceability to the primary standards of length and time: toward a system of metrology based entirely on the properties of the photon
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It is proposed that the primary standards of length and time have now reached a sufficient level of maturity that they may be removed from the standards laboratories and used directly for measurement calibration with minimal recourse to the use of intermediate secondary standards. In particular, if a measurement can be configured to give a time-related output such as decay time, time of flight (TOF), frequency, or phase shift, then direct traceability to the primary atomic clock standard can be realized through use of the LORAN C or global position satellite systems. Twelve illustrative examples are considered covering a wide range of optical and spectroscopic measurements. This approach is then extended to mass, the remaining primary standard that is not currently photon based. An optical definition of mass is realizable in terms of length and time through the angular momentum properties of the photon measured using the torsion balance. The constant 2π/h gives the circularly polarized photon flux required to produce a torque of 1 N . m from which mass may be derived in terms of the mechanical moment of inertia. The need to distinguish the sign as well as the magnitude ofthe photon angular momentum then suggests that the unit of time should also have a sign. This sign distinguishes the roles of left and right circularly polarized light for matter and antimatter.