Summary: The one that took four days. A detector that worked finally arrived for the experiment, so work on EM II has been somewhat slower. Also, the example here uses a lot of material from prior examples and requires being on your toes. This example is all about showing that a rather abstruse looking rotation matrix is in fact a rotation matrix. It involves recognizing dot and cross products when they're written in tensor index notation and having rock solid index skills. At the end of the day though, it's pretty cool, but it still seems like there should be an even simpler way to do this than the one shown here. The game is to show that the following is a rotation matrix in that when multiplied by its transpose, the result is the identity matrix: $M_{ij} = \delta_{ij}cos \alpha + n_i n_j \left(1 - cos \alpha\right) + \epsilon_{ijk}n_k sin \alpha$ Keep in mind that $n_i$ is defined to be a unit vector. The transpose relation that we're su...