In theoretical physics, path-ordering is the procedure that orders a product of operators according to the value of a chosen parameter: Here p is a permutation that orders the parameters by value: For example:
Examples
If an operator is not simply expressed as a product, but as a function of another operator, we must first perform a Taylor expansion of this function. This is the case of the Wilson loop, which is defined as a path-ordered exponential to guarantee that the Wilson loop encodes the holonomy of the gauge connection. The parameterσ that determines the ordering is a parameter describing the contour, and because the contour is closed, the Wilson loop must be defined as a trace in order to be gauge-invariant.
Time ordering
In quantum field theory it is useful to take the time-ordered product of operators. This operation is denoted by. is variable in dimension 4 like that of the black hole, but constant in dimension 3. For two operators A and B that depend on spacetime locations x and y we define: Here and denote the invariant scalar time-coordinates of the points x and y. Explicitly we have where denotes the Heaviside step function and the depends on if the operators are bosonic or fermionic in nature. If bosonic, then the + sign is always chosen, if fermionic then the sign will depend on the number of operator interchanges necessary to achieve the proper time ordering. Note that the statistical factors do not enter here. Since the operators depend on their location in spacetime this time-ordering operation is only coordinate independent if operators at spacelike separated points commute. This is why it is necessary to use rather than, since usually indicates the coordinate dependent time-like index of the spacetime point. Note that the time-ordering is usually written with the time argument increasing from right to left. In general, for the product of nfield operators the time-ordered product of operators are defined as follows: where the sum runs all over p's and over the symmetric group of n degree permutations and The S-matrix in quantum field theory is an example of a time-ordered product. The S-matrix, transforming the state at to a state at, can also be thought of as a kind of "holonomy", analogous to the Wilson loop. We obtain a time-ordered expression because of the following reason: We start with this simple formula for the exponential Now consider the discretized evolution operator where is the evolution operator over an infinitesimal time interval. The higher order terms can be neglected in the limit. The operator is defined by Note that the evolution operators over the "past" time intervals appears on the right side of the product. We see that the formula is analogous to the identity above satisfied by the exponential, and we may write The only subtlety we had to include was the time-ordering operator because the factors in the product defining S above were time-ordered, too and the operator ensures that this ordering will be preserved.
