"... if a space is curved, it is impossible to compare two distant vectors without some method of parallel transport of vectors throughout the curved space. The amount of curvature is a measure of the mismatch of a vector with a copy of itself which has undergone a complete circuit. ... The parallel transport is provided by a structure which is added to the manifold and is called the connection. In the theory of general relativity, the connection is provided by an object calledthe Christoffel symbol G_ij^k. This is a very compact notation for a set of 40 (= 64 -24) functions on the 4-d spacetime. If the symbol carried two asymmetric lower indices, there would be 64 (= 4 x 4 x 4) functions; but the symmetry of the lower indices reduces the independent functions to 40. The standard Christoffel symbol of general relativity is symmetric in the two lower indicies i,j, and generates a connection called the Levi-Civita connection. However, there are geometries for which an asymmetic Christoffel symbol is employed in addition to the the symmetric Christoffel symbol. The asymmetry is carried by a tensor T called the torsion. We can write:
G_ij^k - G_ji^k = T_ij^k
Thus although the Christoffel symbols are not tensors, their difference is a tensor. In physics, we expect tensors to correspond to measurable quantities. If T is 0, then the torsion is zero, and the symbol must be symmetric. A very special case of parallel transport is called absolute parallelism. While ordinary parallel transport guarantees that the vectors will be rotated only by the curvature along the particular path in the circuit, an absolute parallelism connection guarantees that the vectors will remain unrotated by travel along any circuit that follows vector field flow lines. This implies that there is no curvature for this absolute parallelism connection. However [there] will, in general, be a gap in this circuit caused by a "vertical" motion of the ... moving vector. After making the ciruit, the moving vector and its stay-at-home twin will, end up parallel to each other but separated by this "vertical" gap. This gap is called the torsion. ... The connection structure which provides curvature, is based on the symmetric Christoffel symbol. Thus this connection (called the Levi-Civita connection) has zero torsion. By contrast, the absolute parallelism connection which provides torsion has zero curvature. ... there are good examples of spaces carrying both these connections. These spaces are Lie group manifolds. In fact, later work by Joseph Wolf proved that the only spaces that carry an absolute parallelism (Cartan) connection are Lie groups--with one exception: the seven-sphere S7. ... the only spheres that carry an absolute parallelism are spheres of dimension 1, 3, and 7. And the only spheres that are Lie groups are spheres of dimensions 1 & 3. The Lie group structures of these spheres are called U(1) and SU(2). Moreover, S1 (= U(1)) is the set of all unit complex numbers, while S3 (= SU(2)) is the set of all unit quaternions, and S7 is the set of all unit octonions (or Cayley numbers); it is because octonions are not an associative algebra that S7 fails to be a Lie group; but the octonion structure provides an absolute parallelism on S7. ... it is the left-invariance (or right invariance) of the Lie algebra vector fields the provides absolute parallelism. As Cartan discovered, there are three canonical connections on a Lie group manifold. These three connections are generated by three different actions of the Lie group on itself:
(1) Left action: g --> h g (where g and h are group elements of Lie group G)
(2) Right action: g --> g h [(where g and h are group elements of Lie group G)]
(3) Adjoint action: g --> h^(-1) gh (where h^(-1) is the inverse element of h )
... The set of all ... tangent planes together form a vector bundle called the tangent bundle of the Lie group. For the Lie group G, the symbol for the tangent bundle is TG, and it is simply the direct product of the Lie group G and the Lie algebra g. ... In contrast to the case of an ordinary manifold, which is not a Lie group, we say that TG is a trivial bundle because it is direct product of the base space G with the the fiber g, this implies a global trivialization of the bundle structure; moreover, this global trivialization corresponds to the absolute parallelism afforded by the group action on the group manifold and thus on the parallel transport of vectors of the Lie algebra, as described above. The intimate relationship between the Lie group G and the Lie algebra g, has the consequence that the torsion of G ... is simply the Lie product, [x,y], of g ... for the torsion T of a Lie group manifold we can write:
[ X_i ,Y_j ] = T_ij^k Z^k
... where the componets of T are the structure constants of the Lie algebra; and X, Y, and Zare Lie algebra elements, i.e., left-invariant vector fields on G. For right invariant vector fields the torsion tensor is the would be -T_ij^k. ... In general, the curvature tensor describing the curvature of the Lie group manifold is the Riemann curvature tensor which can be written in terms of the Lie algebra structure constants:
R_i,kl^k = (1/4) C_hi^j C_kl^ h
The Riemann curvature tensor is the tensor generated by the Levi-Civita connection, for which there is zero torsion. Thus on the Lie group manifolds we have two radically different connections: the Cartan asymmetric connection which has torsion but no curvature, and the symmetric Levi-Civita connection which has curvature but no torsion. Given two connections on the same manifold, the difference between the Christoffel symbols is a tensor, called the difference tensor. In the case of these two connections on a Lie group manifold the difference tensor is the contorsion tensor K.
We can write:
K_ij^k = G_ij^k - G_ij^k
where the first Christoffel symbol is the Cartan connection of absolute parallelism (with 64 independent functions); the second ... Christoffel symbol is the Levi-Civita connection (with 40 independent functions ...); and K is the contorsion tensor (with 24 independent functions). The contorsion tensor is also the tensor of Ricci rotation coefficients. ...".