## 2014-11-14

### oscillations and the metric

In class, I was solving the normal-mode problem for a solid object near equilibrium, using generalized coordinates, in the usual manner. This starts by orthogonalizing the coordinates to make (what I call) the "mass tensor" (the tensor that comes in to the quadratic kinetic-energy term) proportional to (or identical to) the identity. This operation was annoying me: Why do we have to get explicit about the coordinates? The whole point is that the coordinates are general and we don't have to get specific about their form!

In my anger, I solved the problem without this orthogonalization. It turns out that this solution is easier! Of course it is: I can do everything with pure matrix operations.

I had two other in-class epiphanies about the problem. The first is that the solution you get when you don't do the orthogonalization is more analogous to the simple one-dimensional problem in every way. The second is that, in a D-dimensional problem with D generalized coordinates, the tensor that goes in to the kinetic energy term is some kind of spatial metric for a D-dimensional dynamical problem. (Or proportional to it, anyway.) That is simultaneously obvious and deep.