Vector Spaces – Is Cross Product Defined on Vector Space?

inner-productsvector-spaces

In Wikipedia, a cross product between two "vectors" is defined in terms of the angle between the vectors and their magnitudes.

  1. As I learned cross product in linear
    algebra, which I understand to be a
    topic about vector space, I now
    wonder if cross product is not
    defined on vector space, but instead
    can only be defined on an inner
    product space so that the angle
    between the vectors and their
    magnitudes can make sense?
  2. Or is there other definition of
    cross product on vector space?

Thanks and regards!

Best Answer

One point of view is that the cross product is the composition of the Hodge dual and the exterior product $V \times V \to \Lambda^2 V$ in three dimensions. The Hodge dual requires additional structure to define: you need not only an inner product, but an orientation. This reflects the fact that there is a choice of handedness in the definition of the cross product.

Another point of view is that the cross product is not an operation on spatial vectors in $\mathbb{R}^3$, but the Lie bracket on the Lie algebra of $\text{SO}(3)$. This is the definition, for example, which is relevant to the physics of angular momentum. Of course, if you insist on taking the cross product of spatial vectors you need a way to identify spatial vectors with elements of the Lie algebra of $\text{SO}(3)$. Writing $\text{SO}(3)$ as $\text{Aut}(V)$ where $V$ is a real oriented 3-dimensional inner product space, the Lie algebra of $\text{SO}(3)$ is naturally isomorphic to $\Lambda^2 V$, so this identification is again the Hodge dual.

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