# On the interpretation of a correlator with different definite states

1. Jul 25, 2012

### Israel.cma

Hello everyone, I was reading Ashos Das book on field theory, chapter 4.3, and I had this question.

This expression:

$$\left< \psi_f | \psi_i \right>$$

is the transition amplitude of two states.

This expression:

$$\frac{ \left< \psi|T(U_1...U_n)|\psi \right>}{\left< \psi | \psi \right>} =\left< T(U_1...U_n) \right>$$

is the expectation value of the operators U when used in that state (for example, the ground state).

But what does it would mean?

$$\frac{ \left< \psi_f|T(U_1...U_n)|\psi_i \right>}{\left< \psi_f | \psi_i \right>}$$

I though that maybe is the expectation value of the operators U IF after the measure the state changes from the state 1 to the state 2.

Does anyone have seen that formula in another books or knows that does it means?