I put my desk fan on top of a book, and noticed it was Neuenschwander's book about Noether's theorem. That got me thinking about the symmetries of the fan: while there is a periodic symmetry for each $1/3$rd turn of the fan, there is a simple continuous screw symmetry along the time axis affecting the $x$-$y$ plane. So shouldn't there be a conserved Noether charge in this system associated with this symmetry?

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    $\begingroup$ A charge associated with screw symmetry of pitch $a$, ie a translation of distance $d$ along $z$ is accompanied by a rotation of angle $ad$ about $z$, is $a p_z+L_z$ $\endgroup$
    – LPZ
    Jul 25, 2022 at 12:51

1 Answer 1


Any screw transformation can be decomposed into a translation and a rotation (either along the screw axis or by decomposing it into a translation and a couple). Thus any conserved quantity related to such a symmetry would be decomposable into a combination of momentum and angular momentum.

In your particular case, with the translation being only along the time axis, I do believe the conserved quantity would be entirely angular momentum.


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