In nuclear physics, when two nucleons (e.g. a proton and neutron) fuse together, they release binding energy. There is a mass discrepancy between the newly formed deuterium nucleus and the initial mass of both nucleons. This mass defect is related to the binding energy by the famous equation E = mc^2. But is the loss in mass really converted to energy? I read that in a modification of special relativity, scientists have abandoned the concept of relativistic mass and opted for invariant mass instead. In this case, where does the mass defect come from?