Musical isomorphism

In mathematics—more specifically, in differential geometry—the musical isomorphism is an isomorphism between the tangent bundle and the cotangent bundle of a Riemannian or pseudo-Riemannian manifold induced by its metric tensor. There are similar isomorphisms on symplectic manifolds. These isomorphisms are global versions of the canonical isomorphism between an inner product space and its dual. The term musical refers to the use of the musical notation symbols and .
In the notation of Ricci calculus and mathematical physics, the idea is expressed as the raising and lowering of indices. Raising and lowering indices are a form of index manipulation in tensor expressions.
In certain specialized applications, such as on Poisson manifolds, the relationship may fail to be an isomorphism at singular points, and so, for these cases, is technically only a homomorphism.

Motivation

In linear algebra, a finite-dimensional vector space is isomorphic to its dual space, but not canonically isomorphic to it. This is to say that given a fixed basis for the vector space, there is a natural way to go back and forth between vectors and linear functionals: vectors are represented in the basis by column vectors, and linear functionals are represented in the basis by row vectors, and one can go back and forth by transposing. However, without a fixed basis, there is no way to go back and forth between vectors and linear functionals. This is what is meant by that there is no canonical isomorphism.
On the other hand, a finite-dimensional vector space endowed with a non-degenerate bilinear form is canonically isomorphic to its dual. The canonical isomorphism is given by
The non-degeneracy of means exactly that the above map is an isomorphism. An example is where and is the dot product.
In a basis, the canonical isomorphism above is represented as follows. Let be the components of the non-degenerate bilinear form and let be the components of the inverse matrix to. Let be the dual basis of. A vector is written in the basis as using Einstein summation notation, i.e., has components in the basis. The canonical isomorphism applied to gives an element of the dual, which is called a covector. The covector has components in the dual basis given by contracting with :
This is what is meant by lowering the index. Conversely, contracting a covector with the inverse of gives a vector with components
in the basis. This process is called raising the index.
Raising and then lowering the same index are inverse operations, which is reflected in and being inverses:
where is the Kronecker delta or identity matrix.
The musical isomorphisms are the global version of the canonical isomorphism and its inverse for the tangent bundle and cotangent bundle of a Riemannian manifold. They are canonical isomorphisms of vector bundles which are at any point the canonical isomorphism applied to the tangent space of at endowed with the inner product.
Because every smooth manifold can be endowed with a Riemannian metric, the musical isomorphisms show that a vector bundle on a smooth manifold is isomorphic to its dual.

Discussion

Let be a Riemannian manifold. At each point, the map is a non-degenerate bilinear form on the tangent space. If is a vector in, its flat is the covector
in. Since this is a smooth map that preserves the point, it defines a morphism of smooth vector bundles. By non-degeneracy of the metric, has an inverse at each point, characterized by
for in and in. The vector is called the sharp of. The sharp map is a smooth bundle map.
Flat and sharp are mutually inverse isomorphisms of smooth vector bundles, hence, for each in, there are mutually inverse vector space isomorphisms between and.
The flat and sharp maps can be applied to vector fields and covector fields by applying them to each point. Hence, if is a vector field and is a covector field,
and

In a moving frame

Suppose is a moving tangent frame for the tangent bundle with, as dual frame, the moving coframe . Then the pseudo-Riemannian metric, which is a 2-covariant tensor field, can be written locally in this coframe as using Einstein summation notation.
Given a vector field and denoting, its flat is
This is referred to as lowering an index, because the components of are written with an upper index, whereas the components of are written with a lower index.
In the same way, given a covector field and denoting, its sharp is
where are the components of the inverse metric tensor. Taking the sharp of a covector field is referred to as raising an index.

Extension to tensor products

The musical isomorphisms may also be extended, for each, to an isomorphism between the bundle
of tensors and the bundle of tensors. Here can be positive or negative, so long as and.
Lowering an index of an tensor gives a tensor, while raising an index gives a. Which index is to be raised or lowered must be indicated.
For instance, consider the tensor. Raising the second index, we get the tensor
In other words, the components of are given by
Similar formulas are available for tensors of other orders. For example, for a tensor, all indices are raised by:
For a tensor, all indices are lowered by:
For a mixed tensor of order, all lower indices are raised and all upper indices are lowered by
Well-formulated expressions are constrained by the rules of Einstein summation notation: any index may appear at most twice and furthermore a raised index must contract with a lowered index. With these rules we can immediately see that an expression such as
is well formulated while is not.

Extension to ''k''-vectors and ''k''-forms

In the context of exterior algebra, an extension of the musical operators may be defined on and its dual, and are again mutual inverses:
defined by
In this extension, in which maps k-vectors to k-covectors and maps k-covectors to k-vectors, all the indices of a totally antisymmetric tensor are simultaneously raised or lowered, and so no index need be indicated:
This works not just for k-vectors in the context of linear algebra but also for k-forms in the context of a Riemannian manifold:

Vector bundles with bundle metrics

More generally, musical isomorphisms always exist between a vector bundle endowed with a bundle metric and its dual.

Trace of a tensor

Given a tensor, we define the trace of through the metric tensor by
Observe that the definition of trace is independent of the choice of index to raise, since the metric tensor is symmetric.
The trace of an tensor can be taken in a similar way, so long as one specifies which two distinct indices are to be traced. This process is also called contracting the two indices. For example, if is an tensor with, then the indices and can be contracted to give an tensor with components

Example computations

In Minkowski spacetime

The covariant 4-position is given by
with components:
and the Minkowski metric tensor with metric signature is defined as
in components:
To raise the index, multiply by the tensor and contract:
then for :
and for :
So the index-raised contravariant 4-position is:
This operation is equivalent to the matrix multiplication
Given two vectors, and, we can write down their inner product in two ways:
By lowering indices, we can write this expression as
In matrix notation, the first expression can be written as
while the second is, after lowering the indices of,

In electromagnetism

For a tensor, twice contracting with the inverse metric tensor and contracting in different indices raises each index:
Similarly, twice contracting with the metric tensor and contracting in different indices lowers each index:
Let's apply this to the theory of electromagnetism.
The contravariant electromagnetic tensor in the signature is given by
In components,
To obtain the covariant tensor, contract with the inverse metric tensor:
and since and, this reduces to
Now for, :
and by antisymmetry, for, :
then finally for, ;
The lower indexed tensor is then:
This operation is equivalent to the matrix multiplication