Sign up ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

For two particles, $\langle {\mathcal T} a(t_1) a^\dagger (t_2) \rangle = \langle a(t_1) a^\dagger (t_2)\rangle \theta (t_1-t_2) + \xi \langle a^\dagger (t_2)a(t_1) \rangle \theta (t_2-t_1)$ with $\xi$ is a plus sign for bosons and a minus sign for fermions.

How would I write, for example, $\langle {\mathcal T} a(t_1) a^\dagger (t_2) a(t_3) a^\dagger (t_4) \rangle$ ?

share|cite|improve this question

1 Answer 1

up vote 4 down vote accepted

Just sum over each permutation of [1,2,3,4], for each permutation $[I_1,I_2,I_3,I_4]$you would have a factor of $\theta(t_{I_1}-t_{I_2})\theta(t_{I_2}-t_{I_3})\theta(t_{I_3}-t_{I_4})$ times the corresponding operator product, etc.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.