Normal bundle

In differential geometry, a field of mathematics, a normal bundle is a particular kind of vector bundle, complementary to the tangent bundle, and coming from an embedding (or immersion).

Definition

Riemannian manifold

Let $(M,g)$ be a Riemannian manifold, and $S\subset M$ a Riemannian submanifold. Define, for a given $p\in S$, a vector $n\in \mathrm {T} _{p}M$ to be normal to $S$ whenever $g(n,v)=0$ for all $v\in \mathrm {T} _{p}S$ (so that $n$ is orthogonal to $\mathrm {T} _{p}S$). The set $\mathrm {N} _{p}S$ of all such $n$ is then called the normal space to $S$ at $p$.

Just as the total space of the tangent bundle to a manifold is constructed from all tangent spaces to the manifold, the total space of the normal bundle $\mathrm {N} S$ to $S$ is defined as

$\mathrm {N} S:=\coprod _{p\in S}\mathrm {N} _{p}S$.

The conormal bundle is defined as the dual bundle to the normal bundle. It can be realised naturally as a sub-bundle of the cotangent bundle.

General definition

More abstractly, given an immersion $i\colon N\to M$ (for instance an embedding), one can define a normal bundle of N in M, by at each point of N, taking the quotient space of the tangent space on M by the tangent space on N. For a Riemannian manifold one can identify this quotient with the orthogonal complement, but in general one cannot (such a choice is equivalent to a section of the projection $V\to V/W$).

Thus the normal bundle is in general a quotient of the tangent bundle of the ambient space restricted to the subspace.

Formally, the normal bundle to N in M is a quotient bundle of the tangent bundle on M: one has the short exact sequence of vector bundles on N:

$0\to TN\to TM\vert _{i(N)}\to T_{M/N}:=TM\vert _{i(N)}/TN\to 0$

where $TM\vert _{i(N)}$ is the restriction of the tangent bundle on M to N (properly, the pullback $i^{*}TM$ of the tangent bundle on M to a vector bundle on N via the map $i$).

Stable normal bundle

Abstract manifolds have a canonical tangent bundle, but do not have a normal bundle: only an embedding (or immersion) of a manifold in another yields a normal bundle. However, since every compact manifold can be embedded in $\mathbf {R} ^{N}$, by the Whitney embedding theorem, every manifold admits a normal bundle, given such an embedding.

There is in general no natural choice of embedding, but for a given M, any two embeddings in $\mathbf {R} ^{N}$ for sufficiently large N are regular homotopic, and hence induce the same normal bundle. The resulting class of normal bundles (it is a class of bundles and not a specific bundle because N could vary) is called the stable normal bundle.

Dual to tangent bundle

The normal bundle is dual to the tangent bundle in the sense of K-theory: by the above short exact sequence,

$[TN]+[T_{M/N}]=[TM]$

in the Grothendieck group. In case of an immersion in $\mathbf {R} ^{N}$, the tangent bundle of the ambient space is trivial (since $\mathbf {R} ^{N}$ is contractible, hence parallelizable), so $[TN]+[T_{M/N}]=0$, and thus $[T_{M/N}]=-[TN]$.

This is useful in the computation of characteristic classes, and allows one to prove lower bounds on immersability and embeddability of manifolds in Euclidean space.