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Retinò, Spallicci and Vaivads test whether the photon truly has zero mass, since nearly everything known about the universe arrives by photons. They use data from the four Cluster spacecraft in the solar wind at 1 AU. They look for departures from Ampère's law, computing the magnetic field's curl from four-point measurements and the current from ion and electron velocities, which a single spacecraft could never do. The upper bound on the photon mass lies between 1.4 × 10^-49 and 3.4 × 10^-51 kg. These limits rest on fewer assumptions than earlier estimates, and this is the only solar wind test to treat experimental errors in detail.

Solar wind test of the de Broglie-Proca massive photon with Cluster multi-spacecraft data