Can Angular Momentum Eigenstates Defy the Uncertainty Principle?

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Discussion Overview

The discussion revolves around the relationship between angular momentum eigenstates and the Heisenberg Uncertainty Principle (HUP), specifically whether simultaneous eigenstates of angular momentum components can exist under certain conditions. The scope includes theoretical considerations and interpretations of quantum mechanics.

Discussion Character

  • Debate/contested, Conceptual clarification, Technical explanation

Main Points Raised

  • Some participants note that the operators for angular momentum components do not commute, leading to the uncertainty relation: \(\sigma_{L_x}\sigma_{L_y} \geq \tfrac{\hslash}{2}|\langle L_z \rangle|\).
  • Others propose that if \(\langle L_z \rangle = 0\), it may allow for simultaneous eigenstates of \(L_x\) and \(L_y\), questioning the implications of the uncertainty principle in this scenario.
  • A participant asks for examples of situations where simultaneous eigenstates of \(L_x\) and \(L_y\) can exist, referencing the hydrogen atom's ground state as a potential case.
  • Some participants express uncertainty about the commonality of such cases, with one suggesting that the hydrogen atom is prevalent in the universe, yet questioning the rarity of simultaneous eigenstates.
  • Another participant emphasizes that having \(\langle L_z \rangle = 0\) does not necessarily imply that \(L_x\) and \(L_y\) can have simultaneous eigenstates, reiterating the nature of the uncertainty relation as an inequality.
  • One participant suggests computing the action of angular momentum operators on the hydrogen atom's ground state to verify the uncertainty relation.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the uncertainty principle in relation to angular momentum eigenstates, with no consensus reached on whether simultaneous eigenstates can exist when \(\langle L_z \rangle = 0\).

Contextual Notes

The discussion highlights potential limitations in the understanding of the uncertainty principle and its application to angular momentum, particularly regarding the conditions under which simultaneous eigenstates may or may not exist.

broegger
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Hi,

The operators representing the components of angular momentum are incompatible, since


[tex][L_x,L_y] = i\hslash L_z[/tex].

If you apply the uncertainty principle to this you get:


[tex]\sigma_{L_x}\sigma_{L_y} \geq \tfrac{\hslash}2|\langle L_z \rangle|[/tex].

But what if [tex]\langle L_z \rangle = 0[/tex]? Then the HUT does not prevent simultanoeus eigenstates of Lx and Ly or what?
 
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Yes,it's true.U can,under certain conditions,measure eigenvalues of [itex]\hat{L}_{x}[/itex] and [itex]\hat{L}_{y}[/itex] exactly...

I think you meant

[tex]\left[\hat{L}_{x},\hat{L}_{y}\right]_{-}=i\hbar\hat{L}_{z}[/tex]

Daniel.
 
dextercioby said:
Yes,it's true.U can,under certain conditions,measure eigenvalues of [itex]\hat{L}_{x}[/itex] and [itex]\hat{L}_{y}[/itex] exactly...

Oh, could you give a quick example? My book doesn't mention this: it just states that it is impossible to have simultaneous eigenstates of Lx and Ly because they don't commute. Everytime <Lz>=0 (like the ground state of hydrogen, right?) the HUT allows simultaneous eigenstates of Lx and Ly, but is it very uncommon (since my book doesn't mention it)?

dextercioby said:
I think you meant

[tex]\left[\hat{L}_{x},\hat{L}_{y}\right]_{-}=i\hbar\hat{L}_{z}[/tex]

Daniel.

Yes, edited.
 
broegger said:
Oh, could you give a quick example?

I think you found it yourself.H atom in the fundamental state (1s) has

[tex]\langle\hat{L}_{z}\rangle_{|1,0,0\rangle}=0[/tex]



broegger said:
My book doesn't mention this: it just states that it is impossible to have simultaneous eigenstates of Lx and Ly because they don't commute.

This means that they don't analyze the cases of the general uncertainty relations...That's bad...


broegger said:
Everytime <Lz>=0 (like the ground state of hydrogen, right?) the HUT allows simultaneous eigenstates of Lx and Ly, but is it very uncommon (since my book doesn't mention it)?

Why is it very uncommon.I'd say the H atom is very common...~75% of solar mass is hydrogen...


Daniel.
 
Yeah, the H-atom ground state has <Lz>=0 but that doesn't necessarily imply that it is a simultaneous eigenstate of Lx and Ly: the HUT is an inequality, [tex]\sigma_{L_x}\sigma_{L_y} \geq 0[/tex], so I don't know.
 
Compute

[tex]\hat{L}_{x}|1,0,0\rangle[/tex]

[tex]\hat{L}_{y}|1,0,0\rangle[/tex]

[tex]\hat{L}_{z}|1,0,0\rangle[/tex]

And then the uncertainties...And verify the uncertainty relation...

Daniel.
 

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