Mass defect - where does it go to?

[Michio Cuckoo]

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?

 

[Nugatory]

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?

That's not a "modification" of special relativity, it's just a different way of talking about the same phenomenon. You can either speak in terms of relativistic mass and say that E=mc$^{2}$, or you can speak in terms of the rest mass and say that E$^{2}$=p$^{2}$c$^{2}$+m$^{2}$c$^{4}$. The latter is more generally used these days, but the former is still convenient for problems like your deuterium atom.