 Christoffel symbols

In mathematics and physics, the Christoffel symbols, named for Elwin Bruno Christoffel (1829–1900), are numerical arrays of real numbers that describe, in coordinates, the effects of parallel transport in curved surfaces and, more generally, manifolds. As such, they are coordinatespace expressions for the LeviCivita connection derived from the metric tensor. In a broader sense, the connection coefficients of an arbitrary (not necessarily metric) affine connection in a coordinate basis are also called Christoffel symbols.^{[1]} The Christoffel symbols may be used for performing practical calculations in differential geometry. For example, the Riemann curvature tensor can be expressed entirely in terms of the Christoffel symbols and their first partial derivatives.
At each point of the underlying ndimensional manifold, for any local coordinate system, the Christoffel symbol is an array with three dimensions: n × n × n. Each of the n^{3} components is a real number.
Under linear coordinate transformations on the manifold, it behaves like a tensor, but under general coordinate transformations, it does not. In many practical problems, most components of the Christoffel symbols are equal to zero, provided the coordinate system and the metric tensor possess some common symmetries.
In general relativity, the Christoffel symbol plays the role of the gravitational force field with the corresponding gravitational potential being the metric tensor.
Contents
Preliminaries
The definitions given below are valid for both Riemannian manifolds and pseudoRiemannian manifolds, such as those of general relativity, with careful distinction being made between upper and lower indices (contravariant and covariant indices). The formulas hold for either sign convention, unless otherwise noted. Einstein summation convention is used in this article.
Definition
If x^{i}, i = 1,2,...,n, is a local coordinate system on a manifold M, then the tangent vectors
define a basis of the tangent space of M at each point.
Christoffel symbols of the first kind
The Christoffel symbols of the first kind can be derived from the Christoffel symbols of the second kind and the metric,
Christoffel symbols of the second kind (symmetric definition)
The Christoffel symbols of the second kind, using the definition symmetric in i and j,^{[2]} (sometimes Γ^{k}_{ij} ) are defined as the unique coefficients such that the equation
holds, where is the LeviCivita connection on M taken in the coordinate direction e_{i}.
The Christoffel symbols can be derived from the vanishing of the covariant derivative of the metric tensor :
As a shorthand notation, the nabla symbol and the partial derivative symbols are frequently dropped, and instead a semicolon and a comma are used to set off the index that is being used for the derivative. Thus, the above is sometimes written as
By permuting the indices, and resumming, one can solve explicitly for the Christoffel symbols as a function of the metric tensor:
where the matrix is an inverse of the matrix , defined as (using the Kronecker delta, and Einstein notation for summation) . Although the Christoffel symbols are written in the same notation as tensors with index notation, they are not tensors,^{[3]} since they do not transform like tensors under a change of coordinates; see below.
The Christoffel symbols are most typically defined in a coordinate basis, which is the convention followed here. However, the Christoffel symbols can also be defined in an arbitrary basis of tangent vectors e_{i} by
Explicitly, in terms of the metric tensor, this is^{[2]}
where are the commutation coefficients of the basis; that is,
where e_{k} are the basis vectors and is the Lie bracket. The standard unit vectors in spherical and cylindrical coordinates furnish an example of a basis with nonvanishing commutation coefficients.
The expressions below are valid only in a coordinate basis, unless otherwise noted.
Christoffel symbols of the second kind (asymmetric definition)
A different definition of Christoffel symbols of the second kind is Misner et al.'s 1973 definition, which is asymmetric in i and j:^{[2]}
Relationship to indexfree notation
Let X and Y be vector fields with components and . Then the kth component of the covariant derivative of Y with respect to X is given by
Here, the Einstein notation is used, so repeated indices indicate summation over indices and contraction with the metric tensor serves to raise and lower indices:
Keep in mind that and that , the Kronecker delta. The convention is that the metric tensor is the one with the lower indices; the correct way to obtain from is to solve the linear equations .
The statement that the connection is torsionfree, namely that
is equivalent to the statement that the Christoffel symbol is symmetric in the lower two indices:
The indexless transformation properties of a tensor are given by pullbacks for covariant indices, and pushforwards for contravariant indices. The article on covariant derivatives provides additional discussion of the correspondence between indexfree and indexed notation.
Covariant derivatives of tensors
The covariant derivative of a vector field is
The covariant derivative of a scalar field is just
and the covariant derivative of a covector field is
The symmetry of the Christoffel symbol now implies
for any scalar field, but in general the covariant derivatives of higher order tensor fields do not commute (see curvature tensor).
The covariant derivative of a type (2,0) tensor field is
that is,
If the tensor field is mixed then its covariant derivative is
and if the tensor field is of type (0,2) then its covariant derivative is
Change of variable
Under a change of variable from to , vectors transform as
and so
where the overline denotes the Christoffel symbols in the y coordinate system. Note that the Christoffel symbol does not transform as a tensor, but rather as an object in the jet bundle.
In fact, at each point, there exist coordinate systems in which the Christoffel symbols vanish at the point.^{[4]} These are called (geodesic) normal coordinates, and are often used in Riemannian geometry.
Applications to general relativity
The Christoffel symbols find frequent use in Einstein's theory of general relativity, where spacetime is represented by a curved 4dimensional Lorentz manifold with a LeviCivita connection. The Einstein field equations—which determine the geometry of spacetime in the presence of matter—contain the Ricci tensor, and so calculating the Christoffel symbols is essential. Once the geometry is determined, the paths of particles and light beams are calculated by solving the geodesic equations in which the Christoffel symbols explicitly appear.
See also
 Basic introduction to the mathematics of curved spacetime
 Proofs involving Christoffel symbols
 Differentiable manifold
 List of formulas in Riemannian geometry
 Riemann–Christoffel tensor
 Gauss–Codazzi equations
Notes
 ^ See, for instance, (Spivak 1999) and (ChoquetBruhat & DeWittMorette 1977)
 ^ ^{a} ^{b} ^{c} http://mathworld.wolfram.com/ChristoffelSymboloftheSecondKind.html.
 ^ See, for example, (Kreyszig 1991), page 141
 ^ This is assuming that the connection is symmetric (e.g., the LeviCivita connection). If the connection has torsion, then only the symmetric part of the Christoffel symbol can be made to vanish.
References
 Abraham, Ralph; Marsden, Jerrold E. (1978), Foundations of Mechanics, Benjamin/Cummings Publishing, pp. See chapter 2, paragraph 2.7.1, ISBN 080530102X
 ChoquetBruhat, Yvonne; DeWittMorette, Cécile (1977), Analysis, Manifolds and Physics, Amsterdam: Elsevier, ISBN 9780720404944
 Landau, Lev Davidovich; Lifshitz, Evgeny Mikhailovich (1951), The Classical Theory of Fields, Course of Theoretical Physics, Volume 2 (Fourth Revised English ed.), Pergamon Press, pp. See chapter 10, paragraphs 85, 86 and 87, ISBN 0080250726
 Kreyszig, Erwin (1991), Differential Geometry, Dover Publications, ISBN 9780486667218
 Misner, Charles W.; Thorne, Kip S.; Wheeler, John Archibald (1970), Gravitation, W.H. Freeman, pp. See chapter 8, paragraph 8.5, ISBN 0716703440
 Spivak, Michael (1999), A Comprehensive introduction to differential geometry, Volume 2, Publish or Perish, ISBN 0914098713
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