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This might be more of a philosophical question, But I have to put this out there. Photons as we know it have no mass but carry energy, yet gravity is able to affect light. Here is my whole dilemma. What if gravity doesn't directly effect the mass of the object, but gravity acts on the energy of an object. We don't really see this though because almost all equations dealing with energy whether it be .5(m)v^2 mgh E=mc^2 all have mass in them. So again this is just a crazy out there idea of mine.