Quantum Mechanics with Non-Unitary SymmetriesThis book shows how a minimal modification of postulates of non relativistic quantum mechanics to allow for non unitary representations of symmetry groups (Lorentz group in particular), leads to a fully relativistic quantum theory without any of the issues (like negative energies, etc.) that led to the second quantization and QFT. Relativistic treatment of bound states in momentum space is developed and used to calculate decay widths of various