η-ξ spacetime and thermo fields

In this paper an intuitive picture of η-ξ spacetime is given. It is shown why η-ξ spacetime can be regarded as a geometric background of field theory at finite temperature. In particular, a new way is put forth. We can get the real-time thermal Green's function with a 2x2 matrix by rotating the imaginary-time thermal Green's function from the Euclidean section to the Lorentzian section in η-ξ spacetime