When Heisenberg, Schrodinger and Dirac proposed the first quantum field theories (QFT) they did not obey Einstein's special theory of relativity (SR) :they were not invariant under the transform laws of special relativty. So Dirac develops a new equation for the wave function nature of the electon which did obey special relativity... But now he had to posit the existence of another particle, an antiparticle, the positron! So a QFT consistent with SR predicts a doubling of the number of particles: thanks a lot Dirac!! (The above perspective is derived from Charles Seife, THE STATE OF THE UNIVERSE, pages 158-162. (Seife says "Dirac found an unexpected result") What happens in a QFT which obeys SR to cause more particles to emerge? Do we have a modern interpretaton? Why does SR invariance, a geometric constraint, cause antiparticles to emerge in QFT?