A quantum-mechanical theory applied to systems that have an infinite number of degrees of freedom. In quantum field theories, particles are represented by fields that have quantized normal modes of oscillation. For instance, quantum electrodynamics is a quantum field theory in which the photon is emitted or absorbed by particles; the photon is the quantum of the electromagnetic field. Relativistic quantum field theories are used to describe fundamental interactions between elementary particles. They predict the existence of antiparticles and also show the connection between spin and statistics that leads to the Pauli exclusion principle (see spin–statistics theorem). In spite of their success, it is not clear whether a quantum field theory can give a completely unified description of all interactions (including the gravitational interaction).