When considering observables and their corresponding operators, would it be correct to believe that discerning discrete values for an observable is possible ONLY when ψ is an eigenfunction of the operator? Alternatively, would it also be correct to believe that the average value of an observable is ALWAYS obtainable regardless if ψ is an eigenfunction of the operator?
Thanks for your help.
Answer
I will try to answer from what I have understood so far.
Every Hermitian operator has a set of Linearly independent eigenvectors and hence we can use it construct a basis(provided it spans the space). Lets say say operator is ˆA and their eigenvector set {|ai⟩} with the eigenvalue equation,
ˆA|ai⟩=ai|ai⟩
|ψ⟩ =∑ici|ai⟩
When you do a measurement on |ψ⟩ of the observable ˆA, i.e. ˆA|ψ⟩ =ˆA∑ici|ai⟩=∑iciˆA|ai⟩=∑iciai|ai⟩
It is important to realise, when you do a single measurement the outcome is such that you will obtain a value ai with a probability |ci|2.
Remember your measurement will yield only a single value, this value is obtained by a number of measurements and averaging over them.
Now if the wavefunction is in an eigenstate of the observable, say |ψ⟩=|ak⟩, if you do a measurement of the operator ˆA you will always obtain the value ak.
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