I don’t think that they will find that light has mass. It makes intuitive sense that they can’t both have mass and travel at the speed of light. At that speed, length contraction will cause them to become 2 dimensional, and I can’t conceive that a 2 dimensional object can have mass which would effect 3 dimensional objects. So, just a hunch, I doubt they have any mass, and if they did, they’d become something other than a photon, and travel slower than c.
Well, are they then the ‘missing mass’ being looked for.
“length contraction will cause them to become 2 dimensional”
And I can’t conceive that ever making any sense.