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I am trying to make a 3D plot of arrows showing the directions of my magnetic field on different points on a sphere. I am constantly facing errors about the scaling which I do not know how to fix. I would be thankful if somebody can help.

d = {Sin[2 θ] Cos[ϕ], Sin[2 θ] Sin[ϕ], Cos[ 2 θ]}
σ = {PauliMatrix[1], PauliMatrix[2], PauliMatrix[3]}
B = d.σ
dat = N[Table[B, {θ, .15, 3, .1}, {ϕ, 0, 6, .2}]]
spherearrow1 = Graphics3D[Table[{Arrowheads[.01], Arrow[dat[[i]]]}, {i, 1, Length[dat]}]
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  • $\begingroup$ It looks like there's some code missing. What do you define dvec1 and \[Sigma]vec to be? Graphics3D is expecting that each coordinate is given as {x, y, z}. So make sure that dat[[1]] for example, is something like {{x1, y1, z1}, {x2, y2, z2}} so that it knows where to start each arrow and where to end each arrow. $\endgroup$ – MassDefect Jan 27 '19 at 23:22
  • $\begingroup$ I fixed that, thank you. the problem is that my B is 2 by 2 matrix, but I still want to plot the arrows. So Isn't it possible to show the direction of B on each point of a 3D configuration (like a sphere)? $\endgroup$ – Delaram Nematollahi Jan 27 '19 at 23:27
  • $\begingroup$ Well, a 2 x 2 matrix doesn't seem to specify a direction in 3D Cartesian or spherical coordinates. For an example of theta = 0, phi = 0, we get d = {0, 0, 1} and B = {{1, 0}, {0, -1}}. It's not clear to me which direction such an arrow should be pointing. Should it be a vector that goes from the point {1, 0, 0} to {0, -1, 0} or something? That would be fairly easy to do, but I'm guessing that's not it since the Pauli matrices show up. $\endgroup$ – MassDefect Jan 27 '19 at 23:40
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I figured out my problem. Here I wanted to plot the direction of magnetic field. The direction of B is defined by vector "d". So I should not use the Pauli matrices to plot the external magnetic field direction (which was also causing the error because of the B dimensions).

  d = {Sin[2 \[Theta]] Cos[\[Phi]], Sin[2 \[Theta]] Sin[\[Phi]], 
  Cos[ 2 \[Theta]]}
  dat = N[Table[{{Sin[2 \[Theta]] Cos[\[Phi]], 
  Sin[2 \[Theta]] Sin[\[Phi]], 
  Cos[ 2 \[Theta]]}, {Sin[2 \[Theta]] Cos[\[Phi]], 
  Sin[2 \[Theta]] Sin[\[Phi]], Cos[ 2 \[Theta]]} + 
  d}, {\[Theta], .15, 3, .1}, {\[Phi], 0, 6, .2}]]
  datflat = Flatten[dat, 1];
  spherearrow1 = 
  Graphics3D[
  Table[{Arrowheads[.01], Arrow[dat[[i]]]}, {i, 1, Length[dat]}]]
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