Summary edit

Mathematica 11.0 code edit

ClearAll["Global`*"]
\[Mu]0 = 1.;
dt = 0.1; (*time interval*)

B[pos_, m_] := \[Mu]0/(4 \[Pi]) (3 ( pos (m.pos))/Norm[pos]^5 - m/Norm[pos]^3);
mm = 1.; (*magnitue of the magnetic dipoles. Same for all of them*)

mI = 1; (*moment of inertia of each magnetic dipole. Same for all of them*)
ndip = 36; (*number of dipoles. As they are arranged on a square it MUST be a perfect square*)

ell = 1; (*distance between a dipole and its nearest neighbour*)

dipoles = 
  RandomReal[{0, 2 \[Pi]}, {ndip}] ;(*angle with respect to the vertical of each dipole*)
vd = Table[0, {ndip}];
positions = Flatten[Table[ell {i, j, 0}, {i, 1, Sqrt[ndip]}, {j, 1, Sqrt[ndip]}], 1];

nsteps = 3000;
evo = Reap[Do[
      torques = 
       Table[If[i == j, {0, 0, 0}, 
         Cross[mm {Cos[dipoles[[j]]], Sin[dipoles[[j]]], 0}, 
          B[positions[[j]] - positions[[i]], 
           mm {Cos[dipoles[[i]]], Sin[dipoles[[i]]], 0}] ] ], {i, 1, 
         ndip}, {j, 1, 
         ndip}]; (*torque generated by the ith dipole on the jth dipole (assuming I=1)*)
      
      ttorques = 
       Table[Sum[torques[[i, j]], {i, 1, ndip}], {j, 1, ndip}][[All, 
        3]];
      vdold = vd;
      vd = vdold + (ttorques - 0.03 vdold)/mI dt;
      dold = dipoles;
      dipoles = dipoles + vd dt;
      Sow[dipoles];
      , nsteps];][[2, 1]];

p1 = Table[Graphics[{Thick,
     Table[Disk[positions[[i]][[1 ;; 2]], 0.05], {i, 1, ndip}], 
     Table[Arrow[{positions[[i]][[1 ;; 2]] - 
         ell/3 {Cos[evo[[k, i]]], Sin[evo[[k, i]]]}, 
        positions[[i]][[1 ;; 2]] + 
         ell/3 {Cos[evo[[k, i]]], Sin[evo[[k, i]]]}}], {i, 1, ndip}]
     }, PlotRange -> {{0, ell (Sqrt[ndip] + 1)}, {0, 
       ell (Sqrt[ndip] + 1)}}, AspectRatio -> Automatic], {k, 1, 
    nsteps, 5}];
ListAnimate[p1]

Licensing edit

I, the copyright holder of this work, hereby publish it under the following license:

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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. http://creativecommons.org/publicdomain/zero/1.0/deed.enCC0Creative Commons Zero, Public Domain Dedicationfalsefalse

This file, which was originally posted to https://twitter.com/j_bertolotti/status/1030470604418428929, was reviewed on 5 February 2019 by reviewer Ronhjones, who confirmed that it was available there under the stated license on that date.