Momentum operator in quantum mechanics

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Discussion Overview

The discussion revolves around the nature of the momentum operator in quantum mechanics, specifically whether it should be classified as a vector or scalar operator in one and three spatial dimensions. The conversation touches on theoretical implications and representations in quantum mechanics.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant states that the momentum operator in one spatial dimension is -iħd/dx, while in three dimensions it is -iħ∇, questioning its classification as a vector or scalar operator.
  • Another participant argues that in three dimensions, momentum is a polar vector and thus must be represented by a vector operator in quantum mechanics, supporting this with the transformation properties of the operator.
  • A third participant comments that the distinction between scalar and vector operators is not unique to quantum mechanics, suggesting that the objection raised is more general.
  • One participant introduces the idea of the momentum operator being a covector, adding a technical nuance to the discussion.
  • Another participant notes that the concept of momenta as co-vectors is also applicable in classical physics, indicating a broader context for the discussion.
  • A later reply acknowledges a pedantic tone in the conversation, suggesting a light-hearted approach to the technical details.

Areas of Agreement / Disagreement

Participants express differing views on the classification of the momentum operator, with no consensus reached on whether it is a vector or scalar operator, and the discussion remains unresolved.

Contextual Notes

The discussion includes assumptions about the nature of operators in different dimensions and their representations, which may depend on the definitions used in quantum mechanics and classical physics.

adosar
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The momentum operator for one spation dimension is -iħd/dx (which isn't a vector operator) but for 3 spatial dimensions is -iħ∇ which is a vector operator. So is it a vector or a scalar operator ?
 
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In 3D the momentum is a (polar) vector and thus must be represented by a vector operator in quantum mechanics, and that's indeed the case as you correctly wrote: In the position representation,
$$\hat{\vec{p}}=-\mathrm{i} \hbar \vec{\nabla},$$
and this transforms indeed as a vector when operating on scalar fields (and the Schrödinger wave function is a scalar field under rotations!).
 
Your objection, that 3-d operators in 3-d can appear as scalar operators in 1-d, isn't really specific to QM.
 
Well, technically, as a covector :P
 
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That's no quantum specific. Also in classical physics (canonical) momenta are co-vectors.
 
I know. I'm in a pedantic mood ;)
 
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