Matter Tensors and Coordinate Transformations
Vectors, the subject of the previous two chapters, may be classified as members of a class of mathematical entities called tensors, insofar as they can be expressed in the form of ordered arrays, or matrices, and insofar as they further conform to conditions to be explored in the present chapter. Tensors can have various ranks, and vectors are tensors of the first rank, which in three-dimensional space have 31 or three components. Much of this, and later, chapters deals with tensors of the second rank which in the same space have 32 or nine components. Tensors of higher (nth) rank do exist and have 3n components, and so do, at least nominally, tensors of zero rank with a single, or 30, component, which makes them scalars. Tensors of the second rank for three dimensions are written as three-by-three matrices with each component marked by two subscripts, which may be either letters or numbers.