Angular momentum commutation relations

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The discussion centers on proving that the commutator \([l_i^2, l_j^2]\) vanishes for the state \(|l, m\rangle\) when \(l=1\). Participants clarify that this means demonstrating \(\langle m, l| [l_i^2, l_j^2] |l, m \rangle = 0\). Initial attempts involve manipulating the commutators and utilizing properties of angular momentum operators, including their Hermitian nature and eigenvalue equations. The conversation highlights that the proof can be simplified by recognizing that any component of angular momentum commutes with the total angular momentum. Ultimately, the conclusion suggests that this result holds not only for \(l=1\) but generally for all \(|l, m\rangle\) states.
McLaren Rulez
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Homework Statement


Show that ##|l, m\rangle## for ##l=1## vanishes for the commutator ##[l_i^2, l_j^2]##.

Homework Equations


##L^2 = l_1^2 + l_2^2 + l_3^2## and ##[l_i^2,L^2]=0##

The Attempt at a Solution


I managed to so far prove that ##[l_1^2, l_2^2] = [l_2^2, l_3^2] = [l_3^2, l_1^2]##. I know that for ##l=1##, I have ##m=-1,0,1## but I'm not really sure how to proceed from here though. Any tips?
 
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McLaren Rulez said:

Homework Statement


Show that ##|l, m\rangle## for ##l=1## vanishes for the commutator ##[l_i^2, l_j^2]##.
What is the complete problem statement ? Or: what do you mean when you write ##|l, m\rangle## vanishes ??
 
BvU said:
What is the complete problem statement ? Or: what do you mean when you write ##|l, m\rangle## vanishes ??
That is the full question. I believe it means that the commutator acting on the state ##|l,m\rangle## i.e. ##\langle m, l| [l_i^2, l_j^2] |l, m \rangle = 0##. Of course ##l## and ##m## are the usual quantum numbers corresponding to ##L^2## and ##l_3##
 
Ah, clear. Sorry to have pulled you off the unanswred threads list :oops:

So the thing to do is work out ##\left ( l_i^2 l_j^2 - l_j^2 l_i^2 \right ) \left | 1,m \right \rangle ## and conclude that it's zero :rolleyes:|
But you knew that already.

I tried working them out but had a hard time keeping it from getting longer and longer. Still looking for a shorter path through.
 
BvU said:
Ah, clear. Sorry to have pulled you off the unanswred threads list :oops:

That's okay, thank you for helping. I guess the idea is to somehow write ##l_1## and ##l_2## in terms of the ladder operators but some basic manipulations haven't really led me anywhere.
 
Same idea here. I'm looking at $$ \begin{align*} L_- L_+ & = L_x^2 + L_y^2 +i \left ( L_x L_y - L_y L_x \right ) \\ & = L^2 -L_z^2 -\hbar L_z \end{align*}$$which is from Ballentine (7.9) but haven't worked it towards our commutator yet.
 
You can prove that ##\langle l, m| [l_i^2, l_j^2] |l, m \rangle = 0## when i or j is equal to 3 in about two lines by calculating explicitly the commutator and using the fact that ##|l, m \rangle## is an eigenfunction of ##l_3##.

Combining that with
McLaren Rulez said:
I managed to so far prove that ##[l_1^2, l_2^2] = [l_2^2, l_3^2] = [l_3^2, l_1^2]##
should complete the proof.
 
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I feel silly for not trying the obvious. Thank you Dr. Claude.
 
For the admiring spectators (including me :rolleyes:): can you show it ?
 
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  • #10
Of course. Let's start with the ##[l_2, l_3]## commutator

##
\begin{align*}
\langle m, l | [l_2^2, l_3^2]| l,m \rangle &= \langle m, l | l_2^2l_3^2 - l_3^2l_2^2| l,m \rangle \\
&= \langle m, l | l_2^2l_3^2| l,m \rangle - \langle m, l | l_3^2l_2^2| l,m \rangle \\
\end{align*}
##

Utilize the fact that ##l_i## are Hermitian and the eigenvalue equation ##l_3| l,m \rangle = m| l,m \rangle##, we get
##
\begin{align*}
\langle m, l | l_2^2l_3^2| l,m \rangle - \langle m, l | l_3^2l_2^2| l,m \rangle &= m^2(\langle m, l | l_2^2| l,m \rangle - \langle m, l | l_2^2| l,m \rangle) \\
& = 0
\end{align*}
##

As for the commutator result, we have
##
\begin{align*}
[l_1^2, l_2^2] &= [l_1^2, L^2 - l_1^2 - l_3^2] \\
&= -[l_1^2, l_3^2] \\
& = [l_3^2, l_1^2],
\end{align*}
##
where we use that any component of angular momentum commutes with the total angular momentum. A similar proof can be done for the ##[l_2^2, l_3^2]## commutator.

Also, correct me if I am wrong but it appears that this is true for all ##|l,m\rangle##, not just the specific case of ##l=1##.
 
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  • #11
McLaren Rulez said:
Also, correct me if I am wrong but it appears that this is true for all ##|l,m\rangle##, not just the specific case of ##l=1##.
You indeed never need to make use of that. I wonder if this is a red herring or if the the person who wrote the question had another proof in mind, involving ladder operators (where square ladder operators only need to involve two states, ##|l, l\rangle## and ##|l, -l\rangle##).
 
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  • #12
McLaren Rulez said:

Homework Statement


Show that ##|l, m\rangle## for ##l=1## vanishes for the commutator ##[l_i^2, l_j^2]##.
Maybe this means to show that ##[l_i^2, l_j^2] \, |l, m\rangle = 0## for ## l = 1##.

You might consider using the matrix representation of the angular momentum operators for ##l = 1##.
http://quantummechanics.ucsd.edu/ph130a/130_notes/node247.html
 

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