Building Invariants from Vector, Levi-Civita Symbol

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Discussion Overview

The discussion revolves around constructing invariants from a contravariant vector using various mathematical approaches, including the use of covariant vectors and axial vectors. Participants explore the implications of these constructions, particularly in relation to complex metrics and the behavior of certain quantities under coordinate inversion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that a contravariant vector can be used to create invariants through direct products with covariant vectors, resulting in scalars, or with axial vectors, resulting in pseudoscalars.
  • There is a suggestion that a vector could be acted upon by an object that yields a result neither scalar nor pseudoscalar, acquiring a factor of e^{\imath \phi} upon coordinate inversion, raising questions about the necessity of a complex metric.
  • One participant notes that if a quantity acquires a factor of e^{\imath \pi} after inversion, it implies that under double inversion, the quantity should return to its original state, suggesting a relationship to pseudoscalars.
  • Another participant challenges the existence of objects that change under inversion in a non-standard way, referencing the classification of representations of the O(3) group.
  • There is a discussion about the potential for new irreducible representations depending on the nature of the objects representing physical states, indicating that the exploration is not limited to vectors.
  • A later reply introduces the concept of group actions on sets as a generalization of group representations, emphasizing the behavior of objects under transformations.

Areas of Agreement / Disagreement

Participants express differing views on the possibility of constructing new invariants and representations, with some asserting limitations based on established mathematical frameworks while others remain open to exploration of new concepts.

Contextual Notes

The discussion includes assumptions about the nature of physical states and the mathematical structures involved, which may not be universally accepted or defined. The implications of complex metrics and the classification of representations are also points of contention.

tannin
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Suppose we have a vector (contravariant) and we want to build an invariant.
a) we may take the direct product of the vector with some covariant vector (1-form obtained through metric tensor) and contract. The result is scalar.
b) we may take it's product with an axial vector (built with Levi-Civita symbol from antisymmetric tensor). The result is pseudoscalar.

I wonder if our vector may be acted on with some object giving result, that is neither scalar nor pseudoscalar, but having the following property:
with inversion of coordinates (in 3D) it acquires factor of e^{\imath \phi} ?
Does it imply necessarily that the metric should be complex?
 
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I guess you might be working toward the notion of a "covariant" as that term is used in invariant theory. See for example Peter J. Olver, Equivalence, Invariants, and Symmetry, 2nd ed., Springer, 1995.
 
tannin said:
Suppose we have a vector (contravariant) and we want to build an invariant.
a) we may take the direct product of the vector with some covariant vector (1-form obtained through metric tensor) and contract. The result is scalar.
b) we may take it's product with an axial vector (built with Levi-Civita symbol from antisymmetric tensor). The result is pseudoscalar.

I wonder if our vector may be acted on with some object giving result, that is neither scalar nor pseudoscalar, but having the following property:
with inversion of coordinates (in 3D) it acquires factor of e^{\imath \phi} ?
Does it imply necessarily that the metric should be complex?
If something acquires a factor e^{\imath \pi} after inversion then after a double inversion, this something should not change, so it seems we must have \phi=\pi. So something just changes sign. So it seems that you are asking for a pseudoscalar (a quantity that changes sign under inversion of coordinates).
 
Last edited:
Thanks for your answers!
The particular case of \phi=\pi indeed gives pseudoscalar. But I've stated the question in the full generality. Double inversion ought not to be a "return to innocence" and some phase factor may be acquired (as in the case of two-dimensional spinors). The question is whether this behaviour can be expanded for any real numbers (and not necessary integer or rational multiples of \pi).
BTW what is the tag for LaTeX?
Thanks again
 
The LaTeX is entered as [ tex ] \ alpha [ / tex ] (remove spaces).

I think you are trying to invent a generalization of spinors. A spinor changes sign when you rotate by 2\pi. But a spinor is not multiplied by \exp(\imath \pi/2) under rotation by \pi. And also inversion just leaves a spinor invariant (as far as I remember) because inversion is not equvalent to a rotation (in three dimensions).

I don't think there can exist objects that are multiplied by some weird number under inversion. The reason is that you want a group of transformations, say O(3), to act on your objects. In other words, you want a representation of O(3). Representations of O(3) are known and classified, and there are no such beasts among them.
 
Now you understand me :-)
But, I am not sure, that no new irreducible representation can be proposed.
Everything depends on what kind of object is assumed to represent a physical state. If it is not a vector...
Let me think awhile and thanks for your remark
Thanks for the tag \imath
 
tannin said:
Now you understand me :-)
But, I am not sure, that no new irreducible representation can be proposed.
Everything depends on what kind of object is assumed to represent a physical state. If it is not a vector...
Let me think awhile and thanks for your remark
Thanks for the tag \imath
If you don't want to get into category theory, then a group action on a set is the most general generalization of a group representation. (we call such a thing a G-set)

Let 1 be the identity element of your group G. Then, 1(x) = x, for any element x of any G-set.

If you have an element t of G satisfying tt = 1 (such as the element of O(3) that swaps the x and y axes of the canonical representation on R^3), then t(t(x)) = x, because t(t(x)) = (tt)(x) = 1(x) = x.

So, for any sort of group action of O(3), if you have an object and perform an inversion twice to it, you must get the object back.
 

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