  We establish a relation between the solution of a relativistic bound state
equation in quantum mechanics and the field representation of a bound state
with the aid of creation and annihilation operators. We show that a bound
system can be represented by a gas of free constituents and a classical
effective field representing the countless quantum fluctuations generating the
binding potential. The distribution function of the internal momenta is given
by the projection of the free states on the solution of a relativistic bound
state equation in the rest frame of the bound system. In this approach Lorentz
covariance, mass-shell constraints and single particle normalizability of the
bound state function are simultaneously and explicitly satisfied. The
discussion is made for a two particle bound state and can be easily generalized
to the case of three or more particles.
