Energy vs. position eigenstates

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Discussion Overview

The discussion revolves around the relationship between energy and position eigenstates in quantum mechanics, specifically focusing on the implications of commuting operators. Participants explore whether commuting operators necessarily share identical eigenstates and the nature of these eigenstates in terms of their stationarity.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the energy and position operators, which seem to commute, share identical eigenstates and whether position eigenstates are stationary like energy eigenstates.
  • Another participant asserts that the Hamiltonian (energy) and position do not commute in general, clarifying that while commuting operators share a mutual complete set of eigenkets, not all eigenkets of one operator are necessarily eigenkets of the other.
  • A further contribution discusses the conditions under which operators commute, emphasizing the need for certain assumptions like self-adjointness and detailing the implications for operators in quantum mechanics.
  • In a specific case, it is noted that the free-particle Hamiltonian commutes with the momentum operator but not with the coordinate operator due to the kinetic term and fundamental commutation relations.

Areas of Agreement / Disagreement

Participants express differing views on the commutation of energy and position operators, with some asserting that they do not commute in general, while others explore the implications of their apparent commutation in specific contexts. The discussion remains unresolved regarding the nature of the eigenstates.

Contextual Notes

Limitations include the need for specific assumptions about the operators involved, such as self-adjointness, and the complexity of the relationships between different operators in quantum mechanics.

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This might be a silly question, but I'm not sure about the answer...

Commuting operators have identical eigenstates. For example: energy and position operators seem to commute:
[E,x] = Ex - xE = 0

Does it mean that position and energy operators share identical eigenstates? Because eigenstates of energy are stationary, are position eigenstates stationary as well?
 
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mpv_plate said:
This might be a silly question, but I'm not sure about the answer...

Commuting operators have identical eigenstates. For example: energy and position operators seem to commute:
[E,x] = Ex - xE = 0

Does it mean that position and energy operators share identical eigenstates? Because eigenstates of energy are stationary, are position eigenstates stationary as well?
1) The hamiltonian (energy) and position do not commute in general.

2) If two operators, say A and B, commute they share a mutual complete set of eigenkets. This is not to say all eigenkets of A are necessarily eigenkets of B.
 
Defining commutative operators in the general case is impossible, one needs certain simplifying assumptions, such as (essential) self-adjointness. Two (essentially) self-adjoint operators commute iff the (generalized) projectors from their spectral decomposition commute. This mathematical reasoning applies thoroughly to all operators in quantum mechanics which describe observables.

In 1D, the free-particle Hamiltonian commutes with the momentum operator, but not with the coordinate operator, due to the kinetic term (proportional to p^2) and the fundamental commutation relations of Born and Jordan (1925).
 
Thank you for the answers.
 

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