The idea that photons are massless is not solely based on experimental measurements but is deeply rooted in the theoretical framework of modern physics. The understanding of photons as massless particles comes from the theory of quantum electrodynamics (QED), which is a well-tested and highly successful theory describing the electromagnetic interactions of particles.
In the framework of QED, photons are considered gauge bosons associated with the electromagnetic force. The theory predicts that photons must be massless in order to maintain the underlying symmetry of the theory and to ensure the consistency of the equations.
Experimental measurements have been conducted to determine the upper limit of the photon mass, and these measurements consistently find that the photon mass must be extremely small, if not exactly zero. These measurements place an upper bound on the mass of the photon, which is many orders of magnitude smaller than the current experimental sensitivity.
Moreover, the masslessness of photons is consistent with numerous experimental observations and phenomena. For example, the behavior of light in various experiments, such as diffraction, interference, and the photoelectric effect, can be accurately explained and predicted by considering photons as massless particles.
While it is true that our measurements have limitations and uncertainties, the evidence from both theoretical considerations and experimental observations strongly supports the idea that photons are indeed massless particles. The current understanding of physics, supported by a vast body of experimental data and theoretical consistency, leads to the widely accepted conclusion that photons have zero mass.