  A physical theory is proposed that obeys both the principles of special
relativity and of quantum mechanics. As a key feature, the laws are formulated
in terms of quantum events rather than of particle states. Temporal and spatial
coordinates of a quantum event are treated on equal footing, namely as
self-adjoint operators on a Hilbert space. The theory is not based upon
Lagrangian or Hamiltonian mechanics, and breaks with the concept of a
continuously flowing time. The physical object under consideration is a
spinless particle exposed to an external potential. The theory also accounts
for particle-antiparticle pair creation and annihilation, and is therefore not
a single-particle theory in the usual sense. The Maxwell equations are derived
as a straightforward consequence of certain fundamental commutation relations.
In the non-relativistic limit and in the limit of vanishing time uncertainty,
the Schr\"odinger equation of a spinless particle exposed to an external
electromagnetic field is obtained.
