Autorotation: Invited Comment

Jakob Bohr, Steen Markvorsen

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A continuous autorotation vector field along a framed space curve is defined, which describes the rotational progression of the frame. We obtain an exact integral for the length of the autorotation vector. This invokes the infinitesimal rotation vector of the frame progression and the unit vector field for the corresponding autorotation vector field. For closed curves we define an autorotation number whose integer value depends on the starting point of the curve. Upon curve deformations, the autorotation number is either constant, or can make a jump of (multiples of) plus-minus two, which corresponds to a change in rotation of multiples of 4π. The autorotation number is therefore not topologically conserved under all transformations. We discuss this within the context of generalised inflection points and of frame revisit points. The results may be applicable to physical systems such as polymers, proteins, and DNA. Finally, turbulence is discussed in the light of autorotation, as is the Philippine wine dance, the Dirac belt trick, and the 4π cycle of the flying snake.
Original languageEnglish
Article number023005
JournalPhysica Scripta
Issue number2
Number of pages9
Publication statusPublished - 2016


  • Closed space curves
  • Darboux vector
  • DNA
  • Dirac belt trick
  • Integration of generalised Frenet Serret equations
  • Ribbon
  • Signed 3D curvature


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