moving frame
Let be a smooth manifold. A moving frame
(sometimes just a frame) on is a choice, for every , of a basis for the tangent space to at . More formally (and abstractly), a frame is a (smooth) section
of the principal bundle for over .
Examples and Remarks
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If , then any basis of trivially gives a frame as well.
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A more interesting example (and perhaps a source for the definition) is when and we take the vectors and at a point . Note that this frame cannot be extended to a smooth frame on all of .
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Similar to the previous example, one can show that the 2-sphere admits no frames. A manifold which admits a (global) frame is called parallelizable.
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A key example of a frame is the Frenet frame.
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One places adjective in front of ”moving frame” if that adjective pertains to each basis, e.g. an orthogonal frame is a frame for which each basis is orthogonal (with respect to a given inner product). Given any frame, one can always ”orthonormalize” it in a smooth manner to provide an orthonormal frame.
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Frames arise in general relativity as a formalization of the observation that there is no “preferred” observer standpoint.
References
- 1 Wikipedia’s http://en.wikipedia.org/wiki/Moving_frameentry on moving frame