  The similarity between the energy spectra of relativistic particles and that
of quasi-particles in super-conductivity BCS theory makes us conjecture that
the relativistic physical vacuum medium as the ground state of the background
field is a super fluid medium, and the rest mass of a relativistic particle is
like the energy gap of a quasi-particle. This conjecture is strongly supported
by the results of our following investigation: a particle moving through the
vacuum medium at a speed less than the speed of light in vacuum, though
interacting with the vacuum medium, never feels friction force and thus
undergoes a frictionless and inertial motion. The profound and intrinsic
relationship between the super fluid property of the relativistic physical
vacuum medium and the energy-momentum conservation law as well as the
relativistic energy-momentum dispersion relation or the principle of
relativity, can be established.
