as y approaches x but the difference between them has a well-defined limit. This allows us to define :φ(x)χ(x):. Wick's theorem Wick's theorem states the relationship between the time ordered product of fields and a sum of normal ordered products. This may be expressed for even as where the summation is over all the distinct ways in which one may pair up fields. The result for odd looks the same except for the last line which reads This theorem provides a simple method for computing vacuum expectation values of time ordered products of operators and was the motivation behind the introduction of normal ordering. Alternative definitions The most general definition of normal ordering involves splitting all quantum fields into two parts (for example see Evans and Steer 1996) . In a product of fields, the fields are split into the two parts and the parts are moved so as to be always to the left of all the parts. In the usual case considered in the rest of the article, the contains only creation operators, while the contains only annihilation operators. As this is a mathematical identity, one can split fields in any way one likes. However, for this to be a useful procedure one demands that the normal ordered product of any combination of fields has zero expectation value It is also important for practical calculations that all the commutators (anti-commutator for fermionic fields) of all and are all c-numbers. These two properties means that we can apply Wick's theorem in the usual way, turning expectation values of time-ordered products of fields into products of c-number pairs, the contractions. In this generalised setting, the contraction is defined to be the difference between the time-ordered product and the normal ordered product of a pair of fields. The simplest example is found in the context of Thermal quantum field theory (Evans and Steer 1996). In this case the expectation values of interest are statistical ensembles, traces over all states weighted by . For instance, for a single bosonic quantum harmonic oscillator we have that the thermal expectation value of the number operator is simply the Bose–Einstein distribution So here the number operator is normal ordered in the usual sense used in the rest of the article yet its thermal expectation values are non-zero. Applying Wick's theorem and doing calculation with the usual normal ordering in this thermal context is possible but