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Mass defect - where does it go to?

  1. Apr 29, 2012 #1
    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?
  2. jcsd
  3. Apr 29, 2012 #2


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    Staff: Mentor

    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[itex]^{2}[/itex], or you can speak in terms of the rest mass and say that E[itex]^{2}[/itex]=p[itex]^{2}[/itex]c[itex]^{2}[/itex]+m[itex]^{2}[/itex]c[itex]^{4}[/itex]. The latter is more generally used these days, but the former is still convenient for problems like your deuterium atom.
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