Normally I understand Wick theorem as used in particle physics, but I guess I have a problem with using it in condensed matter physics. Or at least, I have a problem with a use of Wick theorem in the book(adsbygoogle = window.adsbygoogle || []).push({});

T. Lancaster, S.J. Blundell, Quantum Field Theory for the Gifted Amateur

at page 381, Eq. (43.4)

This equation is of the form

##a^{\dagger}a^{\dagger}aa=N[a^{\dagger}a^{\dagger}aa]##

+ terms with contractions,

where ##N[...]## denotes normal ordering. However, the expression on the left ##a^{\dagger}a^{\dagger}aa## is already normal ordered, so there should be no terms with contractions on the right. The equation would make sense if different ordering (e.g. time ordering) was understood on the left, but I don't see where such different ordering comes from, given that ##a^{\dagger}a^{\dagger}aa## is essentially a Hamiltonian in Eq. (43.2).

What am I missing?

**Physics Forums - The Fusion of Science and Community**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Wick theorem in "QFT for the Gifted Amateur"

Loading...

Similar Threads - Wick theorem Gifted | Date |
---|---|

I Wick's theorem and Nucleon scattering | Aug 25, 2017 |

A Expert on Wick's Theorem needed | Jan 27, 2017 |

Vacuum diagrams vs. tree diagrams vs. loop diagrams | Nov 24, 2015 |

Wick's Theorem for free fields only | Jan 16, 2015 |

A question about Wick theorem? | Feb 10, 2014 |

**Physics Forums - The Fusion of Science and Community**