3.1.1 Definition of the GREEN's Function

where is the ground-state of the interacting system in the HEISENBERG picture (Appendix B) and is the time-ordering operator defined in (B.21). The field operator in the HEISENBERG picture is given by

Inserting (3.3) into (3.2), the physical interpretation of the GREEN's function becomes obvious

If , the GREEN's function is the probability amplitude that a particle created at time at place moves to time and place . This follows from the definition of . At zero time the system is at the ground-state . The system then evolves to time with the operator . At this time creates a particle at place . Then, the system continues its evolution from to with the operator , after which annihilates the particle at place . The system returns to the initial ground-state with the operator . In a similar way, if , the field operator creates a hole at time , and the system then propagates according to the HAMILTONian . These holes can be interpreted as particles traveling backward in time [191]. The probability amplitude that a hole created at time at place moves to time and place is again just the GREEN's function for .

To calculate , a perturbation expansion is very useful. However, the definition of the GREEN's function in (3.2) does not allow a direct solution, since it involves the exact ground-states of the interacting HAMILTONian , which is one of the things to be calculated. In the interaction representation the HAMILTON ian is expressed in terms of the non-interacting and interacting parts, see the equation (3.1). The ground state of the non-interacting part, , can be calculated easily. Therefore, one tries to express the ground state of the interacting system in terms of the ground state of the non-interacting one . For that purpose, in equation (B.18) one adds to the operator a factor , which switches the interaction off at [189]. The non-interacting ground state is assigned to the system at and the connection to is formed by the GELL-MANN and Low theorem [192]

where the operator in defined in Appendix B.4. The traditional argument is that one starts from with a wave function which does not contain the effects of the interaction . The operator brings this wave function up to the present, [189]. Thus one has the wave function which contains the effects of the interaction , so that it is an eigenstate of . As , one gets

(3.6) |

One possible assumption is that must be related to . The system returns to its ground state for except for a phase factor [190] which implies that,

(3.7) |

An alternative to this assumption is discussed in Section 3.3.2.

Using the relation (3.5) for the ground-state, equation (3.2) becomes

where the short-hand notation is introduced to represent the expectation value over the ground-state of the non-interacting system at zero temperature. The transition from the first to the second line is achieved by using (B.13) for converting the HEISENBERG representation of operators into the interaction representation. The second step is obtained by taking account of the properties of the operators described in Appendix B.4 and the return of the system to its ground-state as . In the forth line the operator contains several time intervals , , and . The operator automatically sorts these intervals so that they act in their proper sequences. Replacing operator with its formal definition (see (B.24)) one gets