Seeing the lighter side of quarks
Michael C. Birse, Judith A. McGovern · Physics World · 1995
Look up any table of fundamental constants and you will find listed the masses of electrons, protons and neutrons. Although for most practical purposes we can regard masses as universal constants, modern theories of particle physics predict that this is not the case. Instead these theories tell us that particles' masses arise from their interactions, and this implies that they should depend on their environment. Indeed, we are starting to find evidence that the protons and neutrons that are bound within nuclei have masses that are smaller than the familiar 938.3 or 939.6 MeV given in the tables.