Understanding Change of Basis & Superpositioning of States

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Discussion Overview

The discussion revolves around the concepts of change of basis and superpositioning of states in quantum mechanics, particularly focusing on the implications of applying rotation operators to observable operators and the relationship between eigenvectors in different bases. Participants explore theoretical aspects, mathematical representations, and the implications for measurement probabilities.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the role of change of basis in superpositioning states and proposes a scenario involving a rotation operator applied to an observable operator.
  • Another participant clarifies that when applying a unitary transformation like rotation, both the operator and the state must be transformed, leading to a different representation of the observable.
  • Concerns are raised about the measurement of probabilities in different bases, with one participant asserting that the expectation value remains unchanged when the matrix is represented in different bases.
  • Some participants argue that observables remain observable regardless of the basis used, while others suggest that orthogonality of eigenvectors may change with different bases.
  • A later reply emphasizes that all physical outcomes are independent of the chosen basis, drawing an analogy to vectors in Euclidean space.
  • One participant reflects on the implications of changing basis on the length of state vectors and the invariance of probabilities across different representations.

Areas of Agreement / Disagreement

Participants express differing views on the implications of changing basis for observables and measurements. While some agree that observables remain observable and that probabilities do not change with basis, others raise concerns about the orthogonality of eigenvectors and the proper application of transformations. The discussion remains unresolved with multiple competing views.

Contextual Notes

Participants highlight the importance of Hermitian matrices and unitary transformations in quantum mechanics, as well as the potential misconceptions regarding the measurement of observables in different states and bases. There is an ongoing exploration of the mathematical relationships between operators and their eigenvalues/eigenvectors.

  • #31
Well, in general the to an eigenvalue ##a## there are several linearly independent eigenvectors (in QT one talks about "degeneracy"), and then I label them as ##|a,\beta\rangle##. You can always choose them as orthonormal sets. For a pure state represented by a normalized ##|\psi \rangle## the probability to measure the value ##a## of the observable ##A## then is
$$P(a)=\sum_{\beta} |\langle a,\beta|\psi \rangle|^2.$$
That's of course the same as
$$P(a)=\sum_{\beta} \langle a,\beta|\hat{\rho}|a,\beta \rangle$$
with
$$\hat{\rho}=|\psi \rangle \langle \psi|.$$
 
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