Expectation Value Question with Unknown Operator

[mkosmos2]

Homework Statement

Consider an observable A associated to an operator A with eigenvalues a_n.
Using the formula <A> = ∫ψ*Aψ compute the expectation value of A for the following wave function:

\Psi=\frac{1}{\sqrt{3}}\phi_{1}+\frac{1}{\sqrt{6}}\phi_{2}+\frac{1}{\sqrt{2}}\phi_{3}

where \phi_{1,2,3} are normalized and orthogonal.

Homework Equations

The only other equation I can think of is the eigenvalue equation A\phi_{n}=a_{n}\phi_{n} but it really just puts the first part of the question into an equation, which doesn't help. I really can't think of any other relevant equations.

The Attempt at a Solution

I understand that I need to determine the operator A in order to compute the integral, I'm just having trouble determining A from the given wave equation. I get the feeling this question should be straight forward, yet I'm stuck right off the bat...

Also, I'm sorry if this is in the wrong part of the forum. I tried reading about what constitutes intro vs. upper level physics, but I'm not familiar with the divisions for junior and senior years in U.S. institutions. Anyways, this question is from my third year Elements of QM course, if that changes anything.
Thanks in advance for any help you can offer!

-Mike

 

[praharmitra]

Since nothing else is mentioned in the question, I think it is safe to assume the eigenvalue equation you have written, i.e. \hat{A} \phi_n = a_n \phi_n. In this case, what is \hat{A}\psi?

Then explicitly find the integral. Remember that \phi_{1,2,3} are all normalized. (What does that mean?)

 

[mkosmos2]

So the \phi_{n} functions being normalized means that when I integrate \Psi, they will each become 1, correct?

I'm still confused as to what happens with the operator. Based on the given information,

A\Psi=a_{n}[\frac{1}{\sqrt{3}}\phi_{1}+\frac{1}{\sqrt{6}}\phi_{2}+\frac{1}{\sqrt{2}}\phi_{3}]

does it not?

Do I not need to know what A represents in order to calculate a_{n}, to explicitly calculate the integral?

Sorry, this class is the first time I've been exposed to concepts of QM. They're still pretty confusing for me.

 

[praharmitra]

Hi,

It's ok if these are confusing you. Let me explain.

When the question says A\phi_n = a_n \phi_n it means

A\phi_1 = a_1 \phi_1,~ A\phi_2 = a_2 \phi_2,~A\phi_3 = a_3 \phi_3

The question is asking you to find the expectation of A in terms of a_1,a_2,a_3. You do NOT need to calculate what a_1,a_2,a_3 are.

 

[mkosmos2]

Oh okay, so the values of a_1, a_2, and a_3 are given, and I just need to compute &lt;A&gt; as the sum of three separate integrals?

 

[praharmitra]

Oh okay, so the values of a_1, a_2, and a_3 are given, and I just need to compute &lt;A&gt; as the sum of three separate integrals?

yes.

 

[mkosmos2]

Thank you so much! I understand the general idea much better now.