  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.
