History of quantum field theory
The third thread in the development of quantum field theory was the need to handle the statistics of many-particle systems consistently and with ease. In 1927, Pascual Jordan tried to extend the canonical quantization of fields to the many-body wave functions of identical particles using a formalism which is known as statistical transformation theory; this procedure is now sometimes called second quantization. In 1928, Jordan and Eugene Wigner found that the quantum field describing electrons, or other fermions, had to be expanded using anti-commuting creation and annihilation operators due to the Pauli exclusion principle (see Jordan–Wigner transformation). This thread of development was incorporated into many-body theory and strongly influenced condensed matter physics and nuclear physics.
Gian Carlo Wick
While in the United States, Wick made fundamental contributions to quantum field theory, such as the Wick theorem in 1950, which showed how to express calculations in quantum field theory in terms of normally-ordered products and thus derive Feynman rules. He also introduced the Wick rotation, in which computations are analytically continued from Minkowski space to four-dimensional Euclidean space using a coordinate change to imaginary time He developed the helicity formulation for collisions between particles with arbitrary spin, worked with Geoffrey Chew on the impulse approximation, and worked on meson theory, symmetry principles in physics, and the vacuum structure of quantum field theory.