  A new synthesis of the principles of relativity and quantum mechanics is
developed by replacing the Poincar\'e group for the de Sitter one. The new
relativistic quantum mechanics is an indefinite mass theory which is reduced to
the standard theory on the mass shell. The charge conjugation acquires a
geometrical meaning and the Stueckelberg interpretation for antiparticles
naturally arises in the formalism. So the idea of the Dirac sea in the second
quantized formalism proves to be superfluous. The off-shell theory is free from
ultraviolet divergences, which only appear in the process of mass shell
reduction.
