Saturday, 1 February 2020

Discrete Values for Observables vs Average Values (Quantum Mechanics)


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 

Now we have an arbitrary state |ψ and expand in the basis {|ai}


|ψ =ici|ai

ψ|=iciai|
Now normalising ψ would require the condition i|ci|2=1
With that now we can what ci would mean physically, |ci|2 is probability of finding |ψ  in the eigen state |ai . Hence the sum of probabilities is one(from the above equation).


When you do a measurement on |ψ of the observable ˆA, i.e. ˆA|ψ =ˆAici|ai=iciˆA|ai=iciai|ai

and ψ|ˆA|ψ=i|ci|2ai
with the interpretation that |ci|2 as the probability this would become the average value of the observable.


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.


No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...