Many references of quantum field theory begin with an explanation of the necessity of a field theory as opposed to a single particle theory. Sometimes they use the argument that E=mc^2 implies particle creation. For example, in Peskin "the Einstein relation E=mc^2 allows for the creation of particle-antiparticle pairs" (page 13). How does Einstein's relation imply the possibility of creation of particles? Seems to me that you might as well say that the kinetic energy formula Ek=(1/2)mv^2 implies creation of particles.