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Sunday, 22 July 2012

The Lie algebra su(2)

Previously, I noted that the three generator matrices
tx=(000001010)ty=(001000100)tz=(010100000) obey the commutation relations
[tx,ty]=tz[ty,tz]=tx[tz,tx]=ty It turns out that the matrices tx, ty and tz may appear in a very much different form and still follow the same set of rules (1). E.g. the three complex-valued matrices
tx=i2(0110)ty=i2(0ii0)tz=i2(1001) also fulfil (1). These matrices generate three "sort-of-rotations" in a two-dimensional space with complex coordinates. Visualizing (in three dimensions) points moving in two-dimensional complex space (four dimensional space with real coordinates) is too difficult for me. Here is my best-effort result:
The red crosses and blue circles mark two unit squares [(0,0), (1,0), (1,i), (0,i)], one in each of the two complex planes which constitute the two-dimensional complex space. The animations show the motion of the two squares when rotated by 1+αtx (bottom left), 1+αty (bottom right) and 1+αtz (top left); α denotes the (infinitesimal) rotation angle. Note that it requires an accumulated rotation of 4π for the squares to return to their original position.

The Lie algebra described by the relations (1) is known as su(2) - by physicists, mathematicians call it A1.

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