Our understanding of the speed of light and many other things is predicated on the fact that light has no mass. As we can’t weight it directly like on a scale I am wondering (outside of mathematics) how we can test and prove this theory? Is it possible that light does have mass, it is just very very very small?
Further, if light has no mass, does it also have no energy? e=mc2 means energy for something massless would be 0. We know light has energy, so how does this equation work?
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PhD physicist here who survived Jackson E&M. The answer is we aren’t 100% sure that light has zero mass, but there have been experiments that place an upper bound on said mass to incredibly small values. I’ll refer you to the intro chapter of Jackson for more details.
Also, it turns out we’re not 100% sure of a lot of things we usually take for certain (because we might as well). People still look for Pauli exclusion violation, for example.
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