Why not formulate QM in terms of |ψ| squared?

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Discussion Overview

The discussion revolves around the formulation of quantum mechanics (QM) in terms of the absolute square of the wave function, |ψ| squared, and the implications of such a formulation. Participants explore the theoretical foundations, applications, and limitations of using |ψ| squared directly, as opposed to the traditional wave function approach.

Discussion Character

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

Main Points Raised

  • Some participants suggest that formulating QM directly in terms of |ψ| squared is indirect, as it only applies to pure states and does not account for other observables like momentum.
  • Others argue that the density matrix formulation, which incorporates |ψ| squared, can provide a more comprehensive understanding of quantum systems, particularly in quantum statistical mechanics.
  • A participant mentions that the use of complex numbers in wave functions simplifies solving the Schrödinger equation, which may not be the case if only |ψ| squared is used.
  • Some participants highlight that using |ψ| squared loses phase information, which is crucial for analyzing phenomena like diffraction and interference.
  • There is a discussion about the implications of the two-slit experiment, suggesting that classical probability rules do not adequately describe quantum behavior, necessitating a more complex formulation.
  • A later reply questions whether the density matrix formulation allows for the determination of momentum solely from the density matrix.
  • Another participant proposes that it is possible to reformulate the Schrödinger equation using |ψ| squared, but this requires introducing additional variables, such as phase.
  • Some participants express curiosity about the relationship between charge density and momentum in the context of second quantization.

Areas of Agreement / Disagreement

Participants generally do not reach a consensus on the advantages or disadvantages of formulating QM in terms of |ψ| squared. Multiple competing views remain regarding the implications of such a formulation and its applicability to various quantum phenomena.

Contextual Notes

Limitations include the dependence on the definitions of pure states and density matrices, as well as unresolved questions about the role of phase information in quantum mechanics.

  • #31
DrDu said:
You are obviously right, here. Nevertheless my question remains: can we quantize charge density, viewed as a field, directly?

Together with charge density, you need st least the associated charge current operaots, which complement the density to a conservation law. Quantizing this is called current algebra. There have been varous attempts and there is significant literature, but apart from the 1+1D case (where current algebra essentially turns into Kac-Moody algebra) no real breakthrough.
 

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