The Einstein[mu, nu], displayed as , is a computational representation for the Einstein tensor, defined in terms of the Ricci tensor as

From this definition, and because of the symmetry of the Ricci tensor with respect to interchanging the position of its two indices, the Einstein tensor is also symmetric.

When the indices of Einstein assume integer values they are expected to be between 0 and the spacetime dimension, prefixed by ~ when they are contravariant, and the corresponding value of Einstein is returned. When the indices have symbolic values Einstein returns unevaluated after normalizing its indices taking into account their symmetry property.

Computations performed with the Physics package commands take into account Einstein's sum rule for repeated indices - see `.` and Simplify. The distinction between covariant and contravariant indices in the input of tensors is done by prefixing contravariant ones with ~, say as in ~mu; in the output, contravariant indices are displayed as superscripts. For contracted indices, you can enter them one covariant and one contravariant. Note however that - provided that the spacetime metric is galilean (Euclidean or Minkowski), or the object is a tensor also in curvilinear coordinates - this distinction in the input is not relevant, and so contracted indices can be entered as both covariant or both contravariant, in which case they will be automatically rewritten as one covariant and one contravariant. Tensors can have spacetime and space indices at the same time. To change the type of letter used to represent spacetime or space indices see Setup.

When Physics is loaded, the default spacetime is galilean, of Minkowski type. During a Maple session, the value of any component of  is automatically determined by the value of the spacetime metric and so all the elements of Einstein are automatically zero. To set the spacetime metric to something different, use Setup. Also, at least one coordinate system must be set in order to compute the derivatives entering the definition of the Christoffel symbols, used to construct the Ricci tensor and so Einstein. For that purpose see Coordinates or Setup.

Besides being indexed with two indices, Einstein accepts two keywords:

matrix: (synonyms: Matrix, array, Array, or no indices whatsoever, as in Einstein[]) returns a Matrix that when indexed with numerical values from 1 to the dimension of spacetime returns the value of each of the components of Einstein. If this keyword is passed together with indices, that can be covariant or contravariant, the resulting array takes into account the character of the indices.

nonzero: returns a set of equations, with the left-hand-side as a sequence of two positive numbers identifying the element of  and the corresponding value in the right-hand-side. Note that this set is actually the output of the ArrayElems command when passing to it the Array obtained with the keyword array.

Some automatic checking and normalization are carried out each time you enter Einstein[...]. The checking is concerned with possible syntax errors. The automatic normalization takes into account the symmetry of Einstein[mu,nu] with respect to interchanging the positions of the indices mu and nu.

The %Einstein command is the inert form of Einstein, so it represents the same mathematical operation but without performing it. To perform the operation, use value.

The Physics[Einstein] command was introduced in Maple 16.

For more information on Maple 16 changes, see Updates in Maple 16.

Landau, L.D., and Lifshitz, E.M. The Classical Theory of Fields, Course of Theoretical Physics Volume 2, fourth revised English edition. Elsevier, 1975.