No-Go on Seeing a Photon Cross the Sky

This paper states a narrow observational no-go: no photon has ever been seen in flight. Every observation of light is a registration at a detector, never an observation of a photon between emission and absorption. Within standard photodetection theory, detecting a photon means registering an interaction, typically absorption, at a detector. Any attempt to observe a photon at an intermediate point would itself constitute a new registration, terminating the prior relation rather than revealing an in-flight photon. The paper introduces no new equations, no new observables, and no modification to special relativity, general relativity, quantum mechanics, quantum field theory, Maxwell’s equations, or standard optics. Its claim is observational only: the familiar image of light crossing the sky as a visible thing in flight is not something physics has ever recorded. What is observed are registrations, scattering events, emissions, absorptions, and detector records. No photon has been seen in flight.