Briefly, a tangent is a derivative of a curve. Translated into topology this means that you can effectively remove one dimension from a picture. Typically, when working with space time we can can perform one of two operations: Either remove time as a way to see frozen 3D images or we can remove one spatial dimension and therefore represent space time as a curved surface (the net like drawings so typically used to represent topological surfaces in pictures or such phenomena as black holes which are often drawn as shrinking cones).
Tangent spaces are therefore only a representation of what we understand to be one dimension simpler than the problems of topology. It is a useful tool for visualising space time arguments and positions.
So far we have defined smooth maps on smooth manifolds by requiring the corresponding maps on euclidean space to be smooth. In this section we will generalize the notion of derivative on euclidean space to a notion of the derivative of functions between manifolds.
Recall our definition of the derivative on euclidean space:
Definition 1: Let
. Then the derivative of
at
, if it exists, is a linear map
such that

Remark 2:
is unique if it exists, and can be identified with the jacobian matrix
. This is left as an exercise to the reader. This way of defining the derivative does nt, unfortunately, lend itself to generalization to the manifold level. Instead, we will construct another definition of the derivative on euclidean space.
Definition 3: A smooth curve on
is a smooth function
. Let
be smooth curves on
such that
. Define the equivalence relation
. Define the tangent space of
at
as the space
of all equivalence classes
of smooth curves
on
such that
.
Remark 4: Note that we only need smooth curves to be defined on an open subset of
containing
.
Lemma 5:
is isomorphic to
as a vector space for any
.
Proof: Since for any smooth curve
on
,
is a vector in
, there is a natural bijection
. Let
be this bijection, and give
the vector space structure
, and
becomes an isomorphism of vector spaces. ∎
Remark 6: Unlike
,
does not have a natural basis.
Lemma 7: Let
be a smooth curve on
with
and
. Then
where
.
Proof: First off, note that
, so it makes sense to compare them. Secondly,
, so
. ∎
Definition 8: Let
be a smooth function. Then the differential of
at
is the map
given by
.
Lemma 9:
is well defined.
Proof: Let
where
. Then
by the chain rule and using the usual derivative, therefore
and so
is well defined. ∎
Lemma 10: Let
. Then if
, then
.
Proof: Let
be any curve at
. Then if
we have
. ∎
Thus the differential encodes the information about the derivative. However, it also encodes information about
. Unlike the previous definition of the derivative, the differential can, with some slight modifications, be generalized to work on manifolds. That is the topic of the next subsection.
Definition 11: A smooth curve on a manifold
at
is a function
such that
. If
are smooth curves on
at
, we define the equivalence relation
if and only if there exists a chart
with
such that
.
Remark 12: We can differentiate
since it is a function between euclidean spaces, for which we already have a developed theory of differentiation. Also, the equivalence relation is well defined since if it holds for one chart, it holds for all compatible charts as well.
Definition 13: The tangent space of
at
is the space of all equivalence classes of curves on
at
.