Editing Pseudotensor

{{Short description|Type of physical quantity}}
{{Use American English|date=March 2019}}

In [[physics]] and [[mathematics]], a '''pseudotensor''' is usually a quantity that transforms like a [[tensor]] under an orientation-preserving [[coordinate transformation]] (e.g. a [[proper rotation]]) but additionally changes sign under an orientation-reversing coordinate transformation (e.g., an [[improper rotation]]), which is a transformation that can be expressed as a proper rotation followed by [[Reflection (mathematics)|reflection]]. This is a generalization of a ''[[pseudovector]]''. To evaluate a tensor or pseudotensor sign, it has to be [[Tensor contraction|contracted]] with some vectors, as many as its [[Tensor (intrinsic definition)#Tensor rank|rank]] is, belonging to the space where the rotation is made while keeping the tensor coordinates unaffected (differently from what one does in the case of a base change). Under improper rotation a pseudotensor and a proper tensor of the same rank will have different sign which depends on the rank being [[Parity (mathematics)|even or odd]]. Sometimes inversion of the axes is used as an example of an improper rotation to see the behaviour of a pseudotensor, but it works only if vector space dimensions is odd otherwise inversion is a proper rotation without an additional reflection.

There is a second meaning for '''pseudotensor''' (and likewise for ''pseudovector''), restricted to [[general relativity]]. Tensors obey strict transformation laws, but pseudotensors in this sense are not so constrained. Consequently, the form of a pseudotensor will, in general, change as the [[frame of reference]] is altered. An equation containing pseudotensors which holds in one frame will not necessarily hold in a different frame. This makes pseudotensors of limited relevance because equations in which they appear are not [[Covariance and contravariance of vectors|invariant]] in form.

Mathematical developments in the 1980s have allowed pseudotensors to be understood as [[Section (fiber bundle)|sections]] of [[jet bundle]]s.

==Definition==
Two quite different mathematical objects are called a pseudotensor in different contexts.

The first context is essentially a tensor multiplied by an extra sign factor, such that the pseudotensor changes sign under reflections when a normal tensor does not. According to one definition, a pseudotensor '''P''' of the type <math>(p, q)</math> is a geometric object whose components in an arbitrary basis are enumerated by <math>(p + q)</math>indices and obey the transformation rule
<math display=block>\hat{P}^{i_1\ldots i_q}_{\,j_1\ldots j_p} =
(-1)^A A^{i_1} {}_{k_1}\cdots A^{i_q} {}_{k_q}
B^{l_1} {}_{j_1}\cdots B^{l_p} {}_{j_p}
P^{k_1\ldots k_q}_{l_1\ldots l_p}</math> 
under a change of basis.<ref>Sharipov, R.A. (1996). Course of Differential Geometry, Ufa:Bashkir State University, Russia, p. 34, eq. 6.15. {{ISBN|5-7477-0129-0}}, {{arxiv|math/0412421v1}}</ref><ref>Lawden, Derek F. (1982). An Introduction to Tensor Calculus, Relativity and Cosmology. Chichester:John Wiley & Sons Ltd., p. 29, eq. 13.1. {{ISBN|0-471-10082-X}}</ref><ref>Borisenko, A. I. and Tarapov, I. E. (1968). Vector and Tensor Analysis with Applications, New York:Dover Publications, Inc., p. 124, eq. 3.34. {{ISBN|0-486-63833-2}}</ref>

Here <math>\hat{P}^{i_1 \ldots i_q}_{\,j_1 \ldots j_p}, P^{k_1 \ldots k_q}_{l_1 \ldots l_p}</math> are the components of the pseudotensor in the new and old bases, respectively, <math>A^{i_q} {}_{k_q}</math> is the transition matrix for the [[Covariance and contravariance of vectors|contravariant]] indices, <math>B^{l_p} {}_{j_p}</math> is the transition matrix for the [[Covariance|covariant]] indices, and <math>(-1)^A = \mathrm{sign}\left(\det\left(A^{i_q} {}_{k_q}\right)\right) = \pm{1}.</math> 
This transformation rule differs from the rule for an ordinary tensor only by the presence of the factor <math>(-1)^A.</math>

The second context where the word "pseudotensor" is used is [[general relativity]]. In that theory, one cannot describe the energy and momentum of the gravitational field by an energy–momentum tensor. Instead, one introduces objects that behave as tensors only with respect to restricted coordinate transformations. Strictly speaking, such objects are not tensors at all. A famous example of such a pseudotensor is the [[Landau–Lifshitz pseudotensor]].

==Examples==
On [[Orientable manifold|non-orientable manifolds]], one cannot define a [[volume form]] globally due to the non-orientability, but one can define a [[volume element]], which is formally a [[Density on a manifold|density]], and may also be called a ''pseudo-volume form'', due to the additional sign twist (tensoring with the sign bundle).  The volume element is a pseudotensor density according to the first definition.

A [[Integration by substitution|change of variable]]s in multi-dimensional integration may be achieved through the incorporation of a factor of the absolute value of the [[determinant]] of the [[Jacobian matrix and determinant|Jacobian matrix]].  The use of the absolute value introduces a sign change for improper coordinate transformations to compensate for the convention of keeping integration (volume) element positive; as such, an [[integrand]] is an example of a pseudotensor density according to the first definition.

The [[Christoffel symbols]] of an [[affine connection]] on a manifold can be thought of as the correction terms to the partial derivatives of a coordinate expression of a vector field with respect to the coordinates to render it the vector field's covariant derivative.  While the affine connection itself doesn't depend on the choice of coordinates, its Christoffel symbols do, making them a pseudotensor quantity according to the second definition.

==See also==

* {{annotated link|Action (physics)}}
* {{annotated link|Conservation law (physics)|Conservation law}}
* {{annotated link|General relativity}}
* {{annotated link|Tensor}}
* {{annotated link|Tensor density}}
* {{annotated link|Tensor field}}
* {{annotated link|Noether's theorem}}
* {{annotated link|Pseudovector}}
* {{annotated link|Variational principle}}

==References==

{{reflist}}

==External links==
* [http://mathworld.wolfram.com/Pseudotensor.html Mathworld description for pseudotensor].

{{Manifolds}}
{{Tensors}}

[[Category:Differential geometry]]
[[Category:Tensors]]
[[Category:Tensors in general relativity]]