Complex Inner Products: Skew Symmetry & Linearity

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Discussion Overview

The discussion revolves around the properties of complex inner products, specifically focusing on skew symmetry and linearity. Participants explore differing conventions between mathematics and physics regarding the treatment of linearity in inner products, particularly in the context of quantum mechanics.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants note that the complex inner product is traditionally viewed as satisfying skew symmetry and linearity in the first component, as expressed in the equation - = a +b.
  • Others argue that in quantum mechanics, as presented by Shankar, linearity is applied to the ket vector, leading to the expression = b +c, which raises questions about the placement of complex conjugates.
  • One participant emphasizes that the differing definitions of the inner product reflect a matter of convention between mathematicians and physicists, suggesting that consistency is key.
  • Another participant points out that the alternate definition of the complex inner product results in different placements of complex conjugates, indicating that the two definitions may not be equivalent.
  • Participants reference the definition of "bar" in the context of integrating the complex conjugate of one function multiplied by another, as used in Griffiths's "Introduction to Quantum Mechanics," which is cited to support their points.
  • Concerns are raised about Griffiths's approach, which is seen as making general statements based on specific examples, potentially leading to confusion in understanding the broader context.

Areas of Agreement / Disagreement

Participants express disagreement regarding the conventions of linearity in complex inner products, with no consensus reached on which definition is superior or more applicable. The discussion remains unresolved as differing perspectives are presented.

Contextual Notes

Participants highlight limitations in the definitions and assumptions used, particularly regarding the contexts in which different inner product conventions apply, such as function spaces versus discrete spaces.

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Previously i learned from maths that the complex inner product satisfied skew symmetry and linearity in the first component, ie - <aA+bB,C> = a<A,C> +b<B,C>

But after studying Shankar in quantum mechanics, he claims the linearity is in the ket vector, ie - <A,bB+cC> = b<A,B> +c<A,C> which would mean that the complex conjugate of the constants b,c appear in the wrong spots?
 
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This is a matter of convention. Mathematicians do it one way, but physicists do it another. It doesn't actually change anything, as long as you remain consistent.
 
But with this alternate complex inner product you actually get the complex conjugate, so both definitions are not equivalent as with one you get the conjugate of the other.
 
The definition of "bar" is
integrating{(complex conjugate of[f(x)]) *g(x)} over x,
when <f(x)| hit |g(x)>.

So <aA| equal the complex conjugate of a multiplying <A|.

The definition is used in J.Griffiths's 《Introduction to Quantum Mechanics》.
 
Yes, it's just convention, and amounts to a change in order of the vectors, i.e., <A,B> is for a physicist what <B,A> is for a mathematician.
 
quanjia said:
The definition of "bar" is
integrating{(complex conjugate of[f(x)]) *g(x)} over x,
when <f(x)| hit |g(x)>.

So <aA| equal the complex conjugate of a multiplying <A|.

The definition is used in J.Griffiths's ?Introduction to Quantum Mechanics?.

Griffiths's book presumes that we're working in a function space in the position representation. I could be working in a discrete space working in the coherent state representation. This is one of the many problems I have with that book: it tries to make general statements using very specific examples, and that leads to confusion.
 

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