Why Can't Scalars and Matrices Be Added in Pauli Matrix Calculations?

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Homework Help Overview

The discussion revolves around the calculation involving Pauli matrices and vector operations, specifically the expression (\sigma \cdot a)(\sigma \cdot b) = (a \cdot b) + i \sigma \cdot (a \times b). The original poster is attempting to understand the addition of scalars and matrices in this context.

Discussion Character

  • Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • The original poster expresses uncertainty about their calculations, particularly regarding the addition of scalar and matrix terms. They seek clarification on the structure of the result and the necessity of including an identity matrix with the dot product. Other participants suggest that the identity matrix is required to maintain consistency with the matrix structure of the other terms.

Discussion Status

Participants are exploring the implications of matrix operations and the structure of the expression. Some guidance has been offered regarding the inclusion of the identity matrix, and the original poster acknowledges this input as helpful. However, there is still an ongoing inquiry into the underlying rules governing these operations.

Contextual Notes

The discussion includes references to fundamental properties of Pauli matrices, such as their commutation and anti-commutation relations, which may be relevant to the problem but are not fully resolved in the conversation.

frerk
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Homework Statement



Hey :-)
I just need some help for a short calculation.
I have to show, that
(\sigma \cdot a)(\sigma \cdot b) = (a \cdot b) + i \sigma \cdot (a \times b)

The Attempt at a Solution



I am quiet sure, that my mistake is on the right side, so I will show you my calculation for this one:
a_xb_x + a_yb_y+a_zb_z + i\sigma_x (a_yb_z - a_3b_2) + i\sigma_y (a_zb_x-a_xb_z) + i\sigma_z (a_xb_y -a_yb_x)

The last 3 terms are a 2x2 matrix and the first 3 terms are just a scalar...
So i can`t add them.

would be happy fora small hint what is wrong :-)
Thank you
 
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There is actually an identity matrix to be multiplied wit ##a\cdot b##.
 
blue_leaf77 said:
There is actually an identity matrix to be multiplied wit ##a\cdot b##.
hey. thank you for your answer.
Yes, right. that brings to to the result I want.
Is there a rule, why I have to multiply the result of the dot product with the idendity matrix?
Because the other terms include a Pauli Matrix and the result
of the dot produkt must adapt to that structure?
 
frerk said:
hey. thank you for your answer.
Yes, right. that brings to to the result I want.
Is there a rule, why I have to multiply the result of the dot product with the idendity matrix?
Because the other terms include a Pauli Matrix and the result
of the dot produkt must adapt to that structure?
Of course it can be proven using the more fundamental properties of Pauli matrices, especially their commutation and anti-commutation. An easy prove can be found here.
 

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