Nonrelativistic quantum mechanics, useful as it is in the formulation of all fields of modern physics and in their applications, nevertheless has limitations. In particular, it is not generally applicable to the study of elementary particles because it cannot properly predict the dynamic behavior of systems evolving at high velocities and, in its usual formulation, cannot account for the phenomena of creation and annihilation of particles that regularly occur at high energies. Two concepts — special relativity and field — have crucially contributed to the creation of relativistic quantum field theory, which has unquestionably become the foundation of particle physics. It is then possible, for example, to have a natural explanation for the existence of spins of particles, to make the connection between particles and interactions, and to associate to each particle a charge conjugate particle.
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