Calculating expectation value

1. Aug 16, 2010

dudy

Let A be an observable (opeator), and we're assuming that for a given state psi(x), the value of A is given by A acting on psi(x), namely - A|psi>.
Also we assume that - P(x) = |psi(x)|^2
So, I'de expect the Expectation value of A to be defined like so:
<A> = Integral[-Inf:+Inf]{ P(x) A psi(x) dx} = Integral[-Inf:+Inf]{ |psi(x)|^2 A psi(x) dx} , which is not <psi|A|psi>, and that's clearly not right. where did i go wrong here?

2. Aug 16, 2010

Staff: Mentor

No. $A|\psi\rangle$ is not a value. It's another state, in general.

If $|\psi\rangle$ happens to be an eigenstate of the operator A, then $A|\psi\rangle = a|\psi\rangle$, where a is an eigenvalue of the operator A.

3. Aug 16, 2010

dudy

got it, thank you!