Why Define Inner Products for Complex Vector Spaces Using Complex Conjugation?

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Discussion Overview

The discussion revolves around the definition of inner products for complex vector spaces, specifically the use of complex conjugation in the formulation of the inner product. Participants explore the implications of this definition on properties such as length and the ordering of numbers, as well as its utility in mathematical contexts.

Discussion Character

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

Main Points Raised

  • Some participants question the necessity of using complex conjugation in the inner product definition, suggesting that both forms yield a scalar.
  • Others argue that the use of complex conjugation is useful, particularly in maintaining properties like the triangle inequality and ensuring that yields a real number.
  • One participant notes that without complex conjugation, the inner product could lose desirable properties, such as the ability to define length in a meaningful way.
  • Another point raised is that the complex inner product aligns with the real dot product when viewed through the lens of real vector spaces.
  • Some participants highlight the importance of having as a real number for the structure of prehilbert spaces, which is essential for generalizing Euclidean space to complex fields.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and implications of defining the inner product with complex conjugation. There is no consensus on whether the definition is essential or merely a matter of convenience.

Contextual Notes

Participants acknowledge that using complex conjugation affects properties such as length and the ordering of numbers, but the discussion does not resolve the implications of these differences.

Who May Find This Useful

This discussion may be of interest to those studying complex vector spaces, inner product spaces, or mathematical structures that involve complex numbers.

qbslug
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What is the motivation behind defining the inner product for a vector space over a complex field as
<v,u> = v1*u1 + v2*u2 + v3*u3
where * means complex conjugate
as opposed to just
<v,u> = v1u1 + v2u2 + v3u3
They both give you back a scalar. The only reason I can see is the special case for <v,v> in which you get a real number but what does that matter.
 
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qbslug said:
What is the motivation behind defining the inner product for a vector space over a complex field as
<v,u> = v1*u1 + v2*u2 + v3*u3
where * means complex conjugate
Because it's useful. :smile:

One thing to note is that if you "forget" the complex structure and view such a vector space as a vector space of the reals, the complex inner product gives you the same value as the real dot product
 
qbslug said:
The only reason I can see is the special case for <v,v> in which you get a real number but what does that matter.

It matters a lot in terms of "usefulness". For example, real numbers can be ordered with the relations "greater than" and "less than", but complex numbers can not. <v,v> represents the "length" of v, so if <v,v> was a complex quantity, general results like the triangle inequality would not apply to it.
 
Ok thanks. This is the only axiom of inner products that bothers me. So we could define the inner product of a complex vector space as
<v,u> = v1u1 + v2u2 + v3u3
with no complex conjugates but we would lose some nice properties that are convenient such as length?
 
One reason that comes to mind is that by defining the inner product as <v,u> = v1*u1 + v2*u2 + v3*u3 you get a real number for <v,v> as you said and that is needed for a prehilbert space (where <v,v>=0 <=> v=0 and otherwise <v,v> > 0), which is basically the generalization of the Euclidean space for a complex field.
 

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