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Encyclopedia > Axial vector

In physics and mathematics, a pseudovector (or axial vector) is a quantity that transforms like a vector under a proper rotation, but gains an additional sign flip under an improper rotation (a transformation that can be expressed as an inversion followed by a proper rotation). The opposite of a pseudovector is a (true) vector or a polar vector.

A common way of constructing a pseudovector p is by taking the cross product of two vectors a and b:

A simple example of an improper rotation is a coordinate inversion: x goes to _x. Under this transformation, a and b go to _a and _b (by the definition of a vector), but p clearly does not change. It follows that any improper rotation multiplies p by _1 compared to the rotation's effect on a true vector.

This concept can be further generalized to pseudoscalars and pseudotensors, both of which gain an extra sign flip under improper rotations compared to a true scalar or tensor.

Many occurrences of pseudovectors in mathematics and physics are more naturally analyzed as bivectors, following the calculus of differential forms; the double negation is natural for a bivector. However, bivectors are "less intuitive" in some senses than ordinary vectors, and since in every bivector has a unique dual vector , it is this dual which is more often used.

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Physical examples of pseudovectors include the magnetic field and the angular momentum.

Often, the distinction between vectors and pseudovectors is overlooked, but it becomes important in understanding and exploiting the effect of symmetry on the solution to physical systems. For example, consider the case of an electrical current loop in the z=0 plane: this system is symmetric (invariant) under mirror reflections through the plane (an improper rotation), but the magnetic field is anti-symmetric (flips sign) under that mirror plane—this contradiction is resolved by realizing that the mirror reflection of the field induces an extra sign flip because of its pseudovector nature.

To the extent that physical laws are the same for right-handed and left-handed coordinate systems (i.e. invariant under inversion), the sum of a vector and a pseudovector is not meaningful. However, the weak nuclear force that governs beta decay does depend on the chirality of the universe, and in this case pseudovectors and vectors are added. Results from FactBites:

 PlanetMath: axial vector (114 words) Axial vector behaves like a vector (polar vector) except that it is invariant under the inversion of its coordinate axes. The cross product of two axial vectors stays as an axial vector, whereas the croos product of an axial vector and a polar vector turns to be a polar vector. This is version 3 of axial vector, born on 2005-05-09, modified 2005-05-12.
 Axial vector - definition of Axial vector in Encyclopedia (319 words) In physics and mathematics, a pseudovector (or axial vector) is a quantity that transforms like a vector under a proper rotation, but gains an additional sign flip under an improper rotation (a transformation that can be expressed as an inversion followed by a proper rotation). The opposite of a pseudovector is a (true) vector or a polar vector. invariant under inversion), the sum of a vector and a pseudovector is not meaningful.
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