Electromagnetic Measurement and the Observer-Invariance of Light Speed

Einstein's second postulate asserts that the speed of light in vacuum is the same in all inertial frames. The mathematical framework guaranteeing this — the Lorentz covariance of Maxwell's equations — has been secure for over a century. What has not been provided is a physical account of why measurement apparatus, which is physically different in different frames due to time dilation and length contraction, consistently yields the same numerical result. This paper provides that account. We show that the observer-invariance of measured light speed is a physical necessity arising from the electromagnetic constitution of measurement apparatus. All precise spatial and temporal measurement depends on electromagnetic processes — atomic transitions, signal propagation, electromagnetic binding — governed by the same constant c that determines the propagation speed of the radiation being measured. When an observer moves, the internal dynamics of their instruments change, but these changes are precisely correlated: both temporal and spatial measurement standards are electromagnetic, both transform under the same Lorentz transformation, and their ratio — which constitutes the speed measurement — is preserved. We call this the variance–invariance principle: the instruments are variant, and their variance produces the invariance of the measured value. We apply this principle to reinterpret the Michelson–Morley null result as physically inevitable rather than merely consistent with relativity: electromagnetic apparatus is constitutionally incapable of detecting variations in electromagnetic propagation speed. We connect the measurement argument to Energetic Time Theory, in which c is the speed of all energy — unrestricted and restricted alike — and the variance of instruments arises from energy redistribution between internal and external modes. The ETT connection deepens the argument but is not required by it; the central result stands on established physics alone.