From classical mechanics with doubled degrees of freedom to quantum field theory for nonconservative systems

The 2×2-matrix structure of Greenʼs functions is a common feature for the real-time formalisms of quantum field theory under thermal situations, such as the closed time path formalism and Thermo Field Dynamics (TFD). It has been believed to originate from quantum nature. Recently, Galley has proposed the Hamiltonʼs principle with initial data for nonconservative classical systems, doubling each degree of freedom [1]. We show that the Galleyʼs Hamilton formalism can be extended to quantum field and that the resulting theory is naturally identical with nonequilibrium TFD.