Measuring Angular Momentum for Free Particles: L-square =l(l+1)?

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Discussion Overview

The discussion revolves around the measurement of angular momentum for free particles in the context of quantum mechanics, particularly focusing on the eigenvalue relation L-square = l(l+1) and the implications of measuring angular momentum without a potential. Participants explore the nature of angular momentum, its quantization, and the relationship between angular momentum and particle classification as bosons or fermions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether measuring L-square for a free particle will yield the eigenvalue L-square = l(l+1) and if angular momentum can be measured directly.
  • Another participant notes that angular momentum is measured with respect to a reference point.
  • A concern is raised about the quantization of angular momentum in the absence of a potential and whether this has been experimentally verified.
  • It is mentioned that to measure angular momentum, interaction with a potential is necessary, and scattering against potentials involves all angular momentum components.
  • One participant suggests that measuring angular momentum could yield various L-square values with different probabilities.
  • There is a discussion about the distinction between bosons and fermions, with one participant asserting that this classification depends on intrinsic angular momentum (spin), while another relates it to orbital angular momentum.
  • A participant expresses confusion about energy quantization for free particles and asks for clarification.
  • Responses indicate that the energy of a free particle is its initial energy, and there is a suggestion to study scattering theory for further understanding.

Areas of Agreement / Disagreement

Participants express differing views on the quantization of angular momentum for free particles and the relationship between angular momentum and particle classification. The discussion remains unresolved regarding the implications of measuring angular momentum without a potential and the quantization of energy.

Contextual Notes

There are limitations in the discussion regarding assumptions about the measurement of angular momentum in free particles, the dependence on reference points, and the lack of clarity on the quantization of energy.

sanjibghosh
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In case of 3d free particle Schrödinger equation solution, the angular momentum eigenvalue L-square =l(l+1) and a free particle has a wavefunction as the superposition of all 'l'(angular momentum) states.Now the difficulty is that when I will measure the L-square, is it true that I will endup with a result L-squrae=l(l+1) even for a free particle? Can anybody measure the anguler momentum directly?
 
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you measure the angular momentum with respect to some reference point.
 
ok...but without potential how can I expect the angular momentum quantization? Is it experimentally verified that the angular momentum of a free particle is quantized?
 
you write the plane wave as series expansion of angular momenta.

in order to measure the angular momentum of a particle you need something to make it interact with, a potential ..

but yes scattering of particles against potentials include terms for all angular momentum components of the incoming particle.
 
So when I will measure the angular momentum, I can get all the L-square with different probability..Is it true?
 
yes.
 
So, how can I know whether this is Boson or Fermion?
 
what has that anything to do with angular momentum?
 
but Boson or Fermion depends on it's angular momentum.
 
  • #10
no, they depend on their INTRINSIC angular momentum; The Spin.

What you and me have discussed now is a particles orbital angular momentum.
 
  • #11
OK thanks and sorry for the stupidity
 
  • #12
But for the same reason, why is not energy quantized?
 
Last edited:
  • #13
the energy of a free particle is the initial energy...

have you learned about scattering theory yet? If not, you might want to study it.
 
  • #14
just studying...
 
  • #15
what is scattering theory?
 
  • #17
It is not so clear to me but I only know that this is the quantum mechanical description of scattering of some incoming particles by some scatterer (potential).
 
  • #18
and I only know the partial wave method.
 
  • #19
energy is conserved in inelastic scattering, i don't know where your "why is not energy quantized?" comes from - what makes you ask such question??

use "edit" feature.
 
  • #20
ok thanks..
actually my mother tongue is Bengali that's why I have some problem in English.
 

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