Expectation Values of Angular Momentum Operators

In summary, the discussion is about proving that the commutator of Lx and Ly is equal to zero. There are various methods that can be used, such as substituting different commutation values and using ladder operators. However, it is also possible to use a shortcut by realizing that the eigenstate of Lz should not distinguish between the x and y directions, which can be proven by a 90-degree rotation about the z-axis.
  • #1
KiwiBlack
2
0

Homework Statement


Show that

< l,m | Lx2 - Ly2 | l,m > = 0

Homework Equations



L2 = Lx2 + Ly2 + Lz2

[ Lx, Ly ] = i [STRIKE]h[/STRIKE] Lz

[ L, Lz ] = i [STRIKE]h[/STRIKE] Lx

[ Lz, Lx ] = i [STRIKE]h[/STRIKE] Ly



The Attempt at a Solution



I tried substituting different commutation values in place of Lx and Ly, but I'm not reducing it any further. I also tried ladder operations, but my professor said they're not needed to solve the problem.
 
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  • #2
Well I think ladder operators are fine, but there are some "shortcut" ways too:

You could probably get away with just saying that it's "obvious" that
[tex]\langle l,m \lvert L_x^2 \lvert l,m \rangle = \langle l,m \lvert L_y^2 \lvert l,m \rangle[/tex]
because an Lz eigenstate shouldn't know the difference between the x and y directions. If you wanted to make that idea precise, you could find out what happens to the eigenstate and to the angular momentum operators when you do a 90-degree rotation about the z-axis.
 

What are expectation values of angular momentum operators?

Expectation values of angular momentum operators are mathematical quantities used in quantum mechanics to describe the average value of a physical quantity, specifically the angular momentum, in a given state. They are represented by the symbol ⟨L⟩ and are calculated by taking the inner product of the state vector with the operator representing the observable.

How are expectation values of angular momentum operators calculated?

The expectation value of an angular momentum operator is calculated by taking the inner product of the state vector with the operator and then normalizing it by dividing by the norm of the state vector. This gives the average value of the angular momentum in that state.

What are the physical implications of expectation values of angular momentum operators?

The expectation values of angular momentum operators have physical implications as they represent the average value of the angular momentum in a given state. This means they can be used to predict the outcome of measurements of the angular momentum in that state.

Why are expectation values of angular momentum operators important in quantum mechanics?

Expectation values of angular momentum operators are important in quantum mechanics as they provide a way to describe and predict the behavior of quantum systems. They are used to calculate measurable quantities and are a fundamental concept in understanding the quantum world.

How do expectation values of angular momentum operators relate to the uncertainty principle?

Expectation values of angular momentum operators are related to the uncertainty principle as they are subject to the same restrictions. This means that the product of the uncertainty in the measurements of angular momentum and the uncertainty in the position of a particle cannot be smaller than a certain value, known as the uncertainty relation.

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