  It is easy to check that both algebraic equation Det (hat p - m) =0 and Det
(hat p + m) =0 for u- and v- 4-spinors have solutions with p_0= pm E_p = pm
sqrt bf p^2 +m^2. The same is true for higher-spin equations. Meanwhile, every
book considers the equality p_0=E_p for both u- and v- spinors of the
(1/2,0)+(0,1/2)) representation only, thus applying the
Dirac-Feynman-Stueckelberg procedure for elimination of the negative-energy
solutions. The recent Ziino works (and, independently, the articles of several
others) show that the Fock space can be doubled. We re-consider this
possibility on the quantum field level for both s=1/2 and higher spin
particles.
