Finding the Eigen Value of A.σ Vector Product

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Discussion Overview

The discussion revolves around finding the eigenvalues of the scalar product of an arbitrary vector A with the Pauli matrices, denoted as A.σ. Participants explore the implications of this product in the context of quantum mechanics and spinors.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks for the eigenvalue of the product A.σ, where A is an arbitrary vector.
  • Another participant expresses confusion about how A.σ, being a vector, can have eigenvalues.
  • A clarification is provided that A.σ represents a scalar product resulting in a 3x3 matrix that can indeed have eigenvalues.
  • There is a suggestion that the eigenvalues may correspond to eigenspinors aligned with the direction of ±A, with an eigenvalue of |A|.
  • One participant proposes that the eigenvalues should be ±|A|.
  • A further elaboration discusses the nature of eigenspinors in relation to the z-direction and their corresponding eigenvalues.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the eigenvalues and eigenspinors, with some proposing specific values while others clarify the conceptual framework. No consensus is reached on the exact eigenvalues or their interpretations.

Contextual Notes

The discussion includes assumptions about the properties of Pauli matrices and the nature of eigenvalues in quantum mechanics, which may not be fully articulated by all participants.

rupesh57272
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Can anyone tell me what is eigen value of product of a vector with pauli matrices i.e
A.σ where A is an arbitrary vector ?
 
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welcome to pf!

hi rupesh57272! welcome to pf! :smile:

i don't follow you :redface:

A.σ is a vector, so how does it have eigenvalues? :confused:
 
He means a sort of 'scalar' product, which would be (after performing the sum) a 3x3 matrix which can have eigenvalues.

\vec{A}\cdot\vec{\sigma} = A_{x}\sigma_x + A_{y}\sigma_y + A_{z}\sigma_z.
 
ohh!

then won't they be eigenspinors rather than eigenvectors, in the directions of ±A, and with eigenvalue |A| ?
 


Sorry I forgot to mention that it is scalar product of a Vector and Pauli Spin matrices. What is the Eigen Value of it ?
 
I think it should be ±|A|
 
sorry, yes, ±|A| :smile:

eg for Sz, or for S-z, the two eigenspinors are the same …

spinor in the z direction (which we call spin-up, with positive eigenvector, for Sz and spin-down, with negative eigenvector, for S-z)

spinor in the minus-z direction (which we call spin-down, with negative eigenvector, for Sz and spin-up, with positive eigenvector, for S-z) :wink:
 

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