# Wick's theorem

## Theorem for reducing high-order derivatives / From Wikipedia, the free encyclopedia

#### Dear Wikiwand AI, let's keep it short by simply answering these key questions:

Can you list the top facts and stats about Wick contraction?

Summarize this article for a 10 year old

**Wick's theorem** is a method of reducing high-order derivatives to a combinatorics problem.^{[1]} It is named after Italian physicist Gian-Carlo Wick.^{[2]} It is used extensively in quantum field theory to reduce arbitrary products of creation and annihilation operators to sums of products of pairs of these operators. This allows for the use of Green's function methods, and consequently the use of Feynman diagrams in the field under study. A more general idea in probability theory is Isserlis' theorem.

In perturbative quantum field theory, Wick's theorem is used to quickly rewrite each time ordered summand in the Dyson series as a sum of normal ordered terms. In the limit of asymptotically free ingoing and outgoing states, these terms correspond to Feynman diagrams.