How can the nucleus be lighter than its constituents?

Abstract

The fact that the nucleus is lighter than its constituents, seems rather strange. How can the whole have a smaller mass than its components? To get some intuition about how this is possible, one can look at a simpler more familiar system exhibiting the same phenomena; the hydrogen atom. It turns out that the same is true here; the hydrogen atom is a little bit lighter than the sum of its constituents. This difference corresponds to the ionisation energy of hydrogen. This observation allows a simple explanation for how this is possible; the destructive interference between the electric fields of the proton and electron causes a reduction in the energy of the electric field and hence a reduction in the contribution to the mass of the system from the electric field energy. The same explanation can be extended to the nucleus, thus giving some intuition about how this mass reduction happens. In the process, the idea of classical particles being ball-like objects with localised properties, is challenged.