Further Contents of Einstein's E = mc^{2}
  The energy-mass content of Einstein's E = mc^{2} is well known. For a fixed value of mass, E = mc^{2} is an energy-momentum relation which takes the form E = \sqrt{m^{2} + p^{2}}. This relation was formulated in 1905 for point particles. Since then, particles have become more complicated. They have internal space-time structures. Massive particles carry the package of internal variables including mass, spin and quarks, while massless particles have the package containing helicity, gauge variables, and partons. The question then is whether these two different packages of variables can be unified into one single covariant package as E = mc^{2} does for the energy-momentum relations for massive and massless particles. The answer to this question is YES. 
Y. S. Kim