Active and passive transformation

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In the active transformation (left), a point moves from position P to P' by rotating clockwise by an angle θ about the origin of the coordinate system. In the passive transformation (right), point P does not move, while the coordinate system rotates counterclockwise by an angle θ about its origin. The coordinates of P' in the active case (that is, relative to the original coordinate system) are the same as the coordinates of P relative to the rotated coordinate system.

In analytic geometry, spatial transformations in the 3-dimensional Euclidean space are distinguished into active or alibi transformations, and passive or alias transformations. An active transformation[1] is a transformation which actually changes the physical position (alibi, elsewhere) of a point, or rigid body, which can be defined in the absence of a coordinate system; whereas a passive transformation[2] is merely a change in the coordinate system in which the object is described (alias, other name) (change of coordinate map, or change of basis). By transformation, mathematicians usually refer to active transformations, while physicists and engineers could mean either. Both types of transformation can be represented by a combination of a translation and a linear transformation.

Put differently, a passive transformation refers to description of the same object in two different coordinate systems.[3] On the other hand, an active transformation is a transformation of one or more objects with respect to the same coordinate system. For instance, active transformations are useful to describe successive positions of a rigid body. On the other hand, passive transformations may be useful in human motion analysis to observe the motion of the tibia relative to the femur, that is, its motion relative to a (local) coordinate system which moves together with the femur, rather than a (global) coordinate system which is fixed to the floor.[3]

Example[edit]

Rotation considered as a passive (alias) or active (alibi) transformation
Translation and rotation as passive (alias) or active (alibi) transformations

As an example, let the vector , be a vector in the plane. A rotation of the vector through an angle θ in counterclockwise direction is given by the rotation matrix:

which can be viewed either as an active transformation or a passive transformation (where the above matrix will be inverted), as described below.

Spatial transformations in the Euclidean space R3[edit]

In general a spatial transformation may consist of a translation and a linear transformation. In the following, the translation will be omitted, and the linear transformation will be represented by a 3×3 matrix .

Active transformation[edit]

As an active transformation, transforms the initial vector into a new vector .

If one views as a new basis, then the coordinates of the new vector in the new basis are the same as those of in the original basis. Note that active transformations make sense even as a linear transformation into a different vector space. It makes sense to write the new vector in the unprimed basis (as above) only when the transformation is from the space into itself.

Passive transformation[edit]

On the other hand, when one views as a passive transformation, the initial vector is left unchanged, while the coordinate system and its basis vectors are transformed in the opposite direction, that is, with the inverse transformation .[4] This gives a new coordinate system XYZ with basis vectors:

The new coordinates of with respect to the new coordinate system XYZ are given by:

From this equation one sees that the new coordinates are given by

As a passive transformation transforms the old coordinates into the new ones.

Note the equivalence between the two kinds of transformations: the coordinates of the new point in the active transformation and the new coordinates of the point in the passive transformation are the same, namely

In abstract vector spaces[edit]

The distinction between active and passive transformations can be seen mathematically by considering abstract vector spaces.

Fix a finite-dimensional vector space over a field (thought of as or ), and a basis of . This basis provides an isomorphism via the component map .

An active transformation is then an endomorphism on , that is, a linear map from to itself. Taking such a transformation , a vector transforms as . The components of with respect to the basis are defined via the equation . Then, the components of transform as .

A passive transformation is instead an endomorphism on . This is applied to the components: . The new basis is determined by asking that , from which the expression can be derived.

Although the spaces and are isomorphic, they are not canonically isomorphic. Nevertheless a choice of basis allows construction of an isomorphism.

As left- and right-actions[edit]

Often one restricts to the case where the maps are invertible, so that active transformations are the general linear group of transformations while passive transformations are the group .

The transformations can then be understood as acting on the space of bases for . An active transformation sends the basis . Meanwhile a passive transformation sends the basis .

The inverse in the passive transformation ensures the components transform identically under and . This then gives a sharp distinction between active and passive transformations: active transformations act from the left on bases, while the passive transformations act from the right, due to the inverse.

This observation is made more natural by viewing bases as a choice of isomorphism . The space of bases is equivalently the space of such isomorphisms, denoted . Active transformations, identified with , act on from the left by composition, while passive transformations, identified with acts on from the right by pre-composition.

This turns the space of bases into a left -torsor and a right -torsor.

From a physical perspective, active transformations can be characterized as transformations of physical space, while passive transformations are characterized as redundancies in the description of physical space. This plays an important role in mathematical gauge theory, where gauge transformations are described mathematically by transition maps which act from the right on fibers.

See also[edit]

References[edit]

  1. ^ Weisstein, Eric W. "Alibi Transformation." From MathWorld--A Wolfram Web Resource.
  2. ^ Weisstein, Eric W. "Alias Transformation." From MathWorld--A Wolfram Web Resource.
  3. ^ a b Joseph K. Davidson, Kenneth Henderson Hunt (2004). "§4.4.1 The active interpretation and the active transformation". Robots and screw theory: applications of kinematics and statics to robotics. Oxford University Press. p. 74 ff. ISBN 0-19-856245-4.
  4. ^ Amidror, Isaac (2007). "Appendix D: Remark D.12". The theory of the Moiré phenomenon: Aperiodic layers. Springer. p. 346. ISBN 978-1-4020-5457-0.

External links[edit]