In Dyson's book Advanced Quantum Mechanics , he said "These two examples (the discovery of antimatter and meson) are special cases of the general principle, which is the basic success of the relativistic quantum theory, that A Relativistic Quantum Theory of a Finite Number of Particles is Impossible." However, when we calculate the Feynman diagram of particles collision process, the number of particles should be finite.
So my question is why a relativistic quantum theory of a finite number of particles is impossible. What does it mean explicitly? Does it mean that we need to consider infinite number of harmonic oscillators when we quantize free field?