Geometric and Physical Interpretation of Diagonalization

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Discussion Overview

The discussion centers around the significance and interpretation of simultaneous diagonalization of matrices in quantum mechanics, exploring both geometric and physical implications. Participants examine the relevance of this concept in the context of observable quantities and measurement in quantum systems.

Discussion Character

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

Main Points Raised

  • Some participants propose that simultaneous diagonalization of matrices indicates compatibility of the corresponding operators, allowing for simultaneous measurement of observables without uncertainty relations.
  • Others argue that while a common eigenvector basis implies definite values for both observables, it does not necessarily mean one observable can be expressed as a function of the other, as they may be independent.
  • A participant notes the importance of eigenvectors in studying matrices and suggests that having a basis of eigenvectors can simplify analysis.
  • There is a question raised about the necessity of forming the matrix S from normalized eigenvectors for diagonalization, indicating uncertainty about the requirements for unitary matrices in this context.

Areas of Agreement / Disagreement

Participants express varying views on the implications of simultaneous diagonalization, particularly regarding the relationship between observables and the necessity of unitary matrices. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Some statements rely on assumptions about the nature of the matrices (e.g., being Hermitian) and the definitions of compatibility and independence of observables, which are not fully explored in the discussion.

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I am a second year student in quantum mechanics. I heard in lecture that simultaneous diagonalization of matrices is important in quantum mechanics. I would like to know why is it significant when two matrices can be simultaneous diagonalized, and what is the geometric and physical interpretation of simultaneous diagonalization of matrices in quantum mechanics and in physics in general.

Thank you for everyone's help.
 
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The physical interpretation of this result is that if you can simultaneously diagonalise two matrices then the corresponding operators (if the matrices in question are Hermitian) are compatiable. This means that they commute since it is possible to find a commom eigenvector basis for both of the matrices. Ultimately, this means that one can simultaneously measure both of those observables on the quantum system of interest. Thus, in this case there should be no uncertainty relation between the operators and one can be expressed as a function of the other.
 
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To put it a little more physically, (hermitian) matrices represent observable physical quantities. If you have a state which, when acted upon by a matrix, returns itself multiplied by a constant, that state has a definite value of the observable associated with the matrix. A compete set of such states will diagonalize the matrix.

If a common set of states can diagonalize two matrices simultaneously, it means that those states have definite values of both observables. So, the statement that there is no uncertainty relation between the observables is quite correct. However, it is not the case that one can be expressed as a function of the other. In general, the observables are independent (like the energy and angular momentum of the electron in a hydrogen atom).
 
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I would like to know why is it significant when two matrices can be simultaneous diagonalized, and what is the geometric and physical interpretation of simultaneous diagonalization of matrices in quantum mechanics and in physics in general.
It's often useful, when studying a matrix, to look at the eigenvectors. Even better if you can choose a basis whose vectors are all eigenvectors.

So, if two matrices can be simultaneously diagonalized...
 
while diagonalizing an operator A with a matrix S (formed from eigenvectors of A), does S need to be unitary (i.e., whether I have to form it with the NORMALIZED eigenvectors)? Even if S is not unitary, A gets diagonalized.
 

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