Conservative physical quantities

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Discussion Overview

The discussion revolves around the concept of conservative physical quantities, specifically focusing on the angular momentum operator and its components in the context of Hamiltonian mechanics. Participants explore the conditions under which these quantities are conserved and the implications of commutation relations.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether the conservation of the angular momentum operator vector L implies that its three components (Lx, Ly, Lz) are also conserved.
  • Another participant states that the conservation of motion is dependent on the Hamiltonian, noting that for a symmetric potential like that of the hydrogen atom, angular momentum and its components are conserved.
  • A participant expresses concern about whether finding that [H, L] = 0 guarantees that the individual components [H, Lx], [H, Ly], and [H, Lz] are also zero, or if they need to be checked separately.
  • One participant asserts that if the vector L commutes with the Hamiltonian, then its components will also commute with the Hamiltonian.

Areas of Agreement / Disagreement

Participants generally agree that the Hamiltonian plays a crucial role in determining whether an operator is conserved. However, there is uncertainty regarding whether the commutation of the vector L with the Hamiltonian directly implies the commutation of its components without further verification.

Contextual Notes

The discussion does not resolve the question of whether the commutation relations for the components must be verified independently, leaving some assumptions and dependencies on definitions unaddressed.

KostasV
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Hello ppl !
If i find that a physical quantity (lets say angular momentum operator vector L) is conservative (this means [H,L]=0 - H=hamiltonian ) then its 3 components Lx , Ly and Lz are being conserved too ?
That happens with every conservative vector operator ? Like spin vector S and his components?
I am confused ... :S
 
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The constants of the motion is dependent on the Hamiltonian, for simple Hamiltonian such as hydrogen atom whose potential is symmetric, angular momentum L and its components are indeed conserved in time.
KostasV said:
That happens with every conservative vector operator ?
Its the Hamiltonian which decides whether an operator is conserved in time or not.
 
blue_leaf77 said:
The constants of the motion is dependent on the Hamiltonian, for simple Hamiltonian such as hydrogen atom whose potential is symmetric, angular momentum L and its components are indeed conserved in time.

Its the Hamiltonian which decides whether an operator is conserved in time or not.
i am concerned about the components !
Ok, let's say that i find that [H,L]=0 (so L -angular momentum vector- is being conserved) ! Do i have to find the commutators [H,Lx] , [H,Ly] , [H,Lz] or i am sure that they will all be zero due to the fact that [H,L]=0 ?
 
In that case, where the one which commutes the Hamiltonian is the vector L, the components all commute with the Hamiltonian as well.
 
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blue_leaf77 said:
In that case, where the one which commutes the Hamiltonian is the vector L, the components all commute with the Hamiltonian as well.
Thank you very much for the help ;)
 

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