Figure 1. Also consult the entry on quantum theory: identity and individuality. In other words the state space of an elementary system shall have no internal structure with respect to relativistic transformations. This makes non-localizability within the formalism of QFT a much severer problem for a particle interpretation. Now with the particles or quantum particles, something quite interesting happens because it has a wave of probability which spreads over the space. Neither QM nor its immediate relativistic extension with the Klein-Gordon and Dirac equations can describe systems with a variable number of particles. Fraser, D.