File:Hairy ball.gif
Summary
| Description |
English: A simplified (but easy to understand) version of the hairy ball theorem tells us that, if you try to lie flat the hair on a sphere, you will get some defect somewhere, no matter what. |
| Date | |
| Source | https://twitter.com/j_bertolotti/status/1293561744107986944 |
| Author | Jacopo Bertolotti |
| Permission (Reusing this file) |
https://twitter.com/j_bertolotti/status/1030470604418428929 |
Mathematica 12.1 code
sinstep[t_] := Sin[\[Pi]/2 t]^2
R = 5; n = 800;
points = R*SpherePoints[n];
field[{x_, y_, z_}] := (1 - sinstep[t]) {x, y, z}/Norm[{x, y, z}] +
sinstep[t] {y, -x, 0}/Norm[{y, -x, 0}]
p0 = Table[
Graphics3D[{
Orange, Opacity[0.8], Sphere[{0, 0, 0}, R], Opacity[1], Black, Point[points], Thick, Purple,
Table[
Line[{points[[j]], points[[j]] + field[points[[j]] ]}]
, {j, 1, n}]
}, Boxed -> False, Lighting -> "Neutral", Background -> Black,
PlotRange -> 1.2*R*{{-1, 1}, {-1, 1}, {-1, 1}}]
, {t, 0, 1, 0.025}];
t = 1;
defect1 = Graphics3D[{
Orange, Opacity[0.8], Sphere[{0, 0, 0}, R], Opacity[1], Black, Point[points], Thick, Purple,
Table[
Line[{points[[j]], points[[j]] + field[points[[j]] ]}]
, {j, 1, n}]
, Red, Sphere[{0, 0, R}, 0.5], , Sphere[{0, 0, -R}, 0.5]
}, Boxed -> False, Lighting -> "Neutral", Background -> Black,
PlotRange -> 1.2*R*{{-1, 1}, {-1, 1}, {-1, 1}}]
t =.;
field[{x_, y_, z_}] := {0, 1, 0}/Norm[{0, 1, 0}];
norm[{x_, y_, z_}] := {x, y, z}/Norm[{x, y, z}];
fnorm[{x_, y_, z_}] := (field[{x, y, z}].norm[{x, y, z}]) norm[{x, y, z}];
ftang[{x_, y_, z_}] := (field[{x, y, z}] - fnorm[{x, y, z}])/Norm[field[{x, y, z}] - fnorm[{x, y, z}]];
field2[{x_, y_, z_}] := (1 - sinstep[t]) {x, y, z}/Norm[{x, y, z}] + sinstep[t] ftang[{x, y, z}];
p1 = Table[
Graphics3D[{
Orange, Opacity[0.8], Sphere[{0, 0, 0}, R], Opacity[1], Black, Point[points], Thick, Purple,
Table[
Line[{points[[j]], points[[j]] + field2[points[[j]] ]}]
, {j, 1, n}]
}, Boxed -> False, Lighting -> "Neutral", Background -> Black,
PlotRange -> 1.2*R*{{-1, 1}, {-1, 1}, {-1, 1}}]
, {t, 0, 1, 0.025}];
t = 1;
defect2 = Graphics3D[{
Orange, Opacity[0.8], Sphere[{0, 0, 0}, R], Opacity[1], Black, Point[points], Thick, Purple,
Table[
Line[{points[[j]], points[[j]] + field2[points[[j]] ]}]
, {j, 1, n}]
, Red, Sphere[{0, R, 0}, 0.5], , Sphere[{0, -R, 0}, 0.5]
}, Boxed -> False, Lighting -> "Neutral", Background -> Black,
PlotRange -> 1.2*R*{{-1, 1}, {-1, 1}, {-1, 1}}]
t =.;
ListAnimate[
Join[p0, Table[defect1, {5}], Table[p0[[-1]], {5}], Table[defect1, {5}], Table[p0[[-1]], {5}], Table[defect1, {5}], Table[p0[[-1]], {5}], Reverse[p0],
p1, Table[defect2, {5}], Table[p1[[-1]], {5}], Table[defect2, {5}], Table[p1[[-1]], {5}], Table[defect2, {5}], Table[p1[[-1]], {5}], Reverse[p1] ]
]
Licensing
I, the copyright holder of this work, hereby publish it under the following license:
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This file is made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication. |
| The person who associated a work with this deed has dedicated the work to the public domain by waiving all of their rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law. You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.
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