Covariant Derivative with Respect to a Parameter. (See Figure 2, below.) At P, over the North Atlantic, the plane’s colatitude has a minimum. λ If the metric itself varies, it could be either because the metric really does vary or . . i ( a only needs to be defined on the curve Watch the recordings here on Youtube! . {\displaystyle \mathbb {R} ^{n}} v is the metric, and are the Christoffel symbols.. is the covariant derivative, and is the partial derivative with respect to .. is a scalar, is a contravariant vector, and is a covariant vector. . To specify the covariant derivative it is enough to specify the covariant derivative of each basis vector field arXiv:gr-qc/0006024v1 7 Jun 2000 Spaces with contravariant and covariant aﬃne connections and metrics Sawa Manoﬀ∗ Bogoliubov Laboratory of Theoretical Physics The covariant derivatives in the Levi-Civita connection are the ordinary derivatives in the flat Euclidian connection. , but also depends on the vector v itself through p a The second term of the integrand vanishes because the covariant derivative of the metric tensor is zero. {\displaystyle v^{k}{\Gamma ^{i}}_{kj}} = {\displaystyle \lambda _{a;bc}\neq \lambda _{a;cb}\,} e e It covers metric compatible covariant derivatives; torsion free covariant derivatives on T*M; the Levi-Civita connection/covariant derivative; a formula for the Levi-Civita connection; covariantly constant sections; an example of the Levi-Civita connection; and the curvature of the Levi-Civita connection. v is the metric, and are the Christoffel symbols.. is the covariant derivative, and is the partial derivative with respect to .. is a scalar, is a contravariant vector, and is a covariant vector. The same effect can be noticed if we drag the vector along an infinitesimally small closed surface subsequently along two directions and then back. Symmetry also requires that this Christoffel symbol be independent of $$\phi$$, and it must also be independent of the radius of the sphere. I am reading D. Joyce book “Compact manifolds with special holonomy” and I have some problems of understanding some computation on page 111, the first line in the proof of Proposition 5.4.6. c The notation of section 5.6 is not quite adapted to our present purposes, since it allows us to express a covariant derivative with respect to one of the coordinates, but not with respect to a parameter such as $$\lambda$$. {\displaystyle (\nabla _{\mathbf {v} }\mathbf {u} )_{p}} But to a relativist, there is nothing very nice about this function at all, because there is nothing special about a time coordinate. Lecture 8: covariant derivatives Yacine Ali-Ha moud September 26th 2019 METRIC IN NON-COORDINATE BASES Last lecture we de ned the metric tensor eld g as a \special" tensor eld, used to convey notions of in nitesimal spacetime \lengths". Since the path is a geodesic and the plane has constant speed, the velocity vector is simply being parallel-transported; the vector’s covariant derivative is zero.   ) v , also at the point P. The primary difference from the usual directional derivative is that Metric determinant. denotes the vector field whose value at each point p of the domain is the tangent vector v That is, Unless the metric is trivially zero, we have. The required correction therefore consists of … Note that b {\displaystyle \tau _{ab}\,} With covariant and contravariant vectors defined, we are now ready to extend our analysis to tensors of arbitrary rank. U In this case "keeping it parallel" does not amount to keeping components constant under translation. , we have: Covariant derivatives do not commute; i.e. . i In polar coordinates, γ may be written in terms of its radial and angular coordinates by γ(t) = (r(t), θ(t)). is a vector field along the curve The infinitesimal change of the vector is a measure of the curvature. . Given a point p of the manifold, a real function f on the manifold, and a tangent vector v at p, the covariant derivative of f at p along v is the scalar at p, denoted The crucial feature was not a particular dependence on the metric, but that the Christoffel symbols satisfied a certain precise second order transformation law. p We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. = {\displaystyle \nabla _{{\dot {\gamma }}(t)}{\dot {\gamma }}(t)} There is however another generalization of directional derivatives which is canonical: the Lie derivative, which evaluates the change of one vector field along the flow of another vector field. Jun 28, 2012 = The gauge transformations of general relativity are arbitrary smooth changes of coordinates. , λ 02 Spherical gradient divergence curl as covariant derivatives. The covariant derivative of the metric tensor vanishes. . ( Generalizing the correction term to derivatives of vectors in more than one dimension, we should have something of this form: $\begin{split} \nabla_{a} v^{b} &= \partial_{a} v^{b} + \Gamma^{b}_{ac} v^{c} \\ \nabla_{a} v_{b} &= \partial_{a} v_{b} - \Gamma^{c}_{ba} v_{c}, \end{split}$. ∇ If we further assume that the coordinate x is a normal coordinate, so that the metric is simply the constant g = 1, then zero is not just the answer but the right answer. As a less trivial example, we can redefine the ground of our electrical potential, $$\Phi \rightarrow \Phi + \delta \Phi$$, and this will add a constant onto the energy of every electron in the universe, causing their phases to oscillate at a greater rate due to the quantum-mechanical relation. This coincides with the usual Lie derivative of f along the vector field v. A covariant derivative If we apply the same correction to the derivatives of other second-rank contravariant tensors, we will get nonzero results, and they will be the right nonzero results. Missed the LibreFest? . i {\displaystyle \nabla _{\mathbf {u} }{\mathbf {v} }} , also written Notes on Diﬁerential Geometry with special emphasis on surfaces in R3 Markus Deserno May 3, 2004 Department of Chemistry and Biochemistry, UCLA, Los Angeles, CA 90095-1569, USA , which takes as its inputs: (1) a vector, u, defined at a point P, and (2) a vector field, v, defined in a neighborhood of P.[7] The output is the vector Γ Often a notation is used in which the covariant derivative is given with a semicolon, while a normal partial derivative is indicated by a comma. If Covariant and Lie Derivatives Notation. $$\Gamma^{\theta}_{\phi \phi}$$ is computed in example 10. {\displaystyle \Gamma _{\ ij}^{k}} e for this definition to make sense. x {\displaystyle X} Vt=(5.b)e 8€,e,= (1.1%)e Ⓡe, e' = (cabeee One particularly important result is that the covariant derivative of the metrs tensor … P For directional tensor derivatives with respect to continuum mechanics, see, Informal definition using an embedding into Euclidean space, The covariant derivative is also denoted variously by, In many applications, it may be better not to think of, Introduction to the mathematics of general relativity, "Méthodes de calcul différential absolu et leurs applications", "Über die Transformation der homogenen Differentialausdrücke zweiten Grades", Journal für die reine und angewandte Mathematik, "Sur les variétés à connexion affine et la theorie de la relativité généralisée", https://en.wikipedia.org/w/index.php?title=Covariant_derivative&oldid=991770229, Mathematical methods in general relativity, All Wikipedia articles written in American English, Creative Commons Attribution-ShareAlike License, The definition of the covariant derivative does not use the metric in space. This article presents an introduction to the covariant derivative of a vector field with respect to a vector field, both in a coordinate free language and using a local coordinate system and the traditional index notation. In spacetime, $$\Gamma$$ is essentially the gravitational field (see problem 7), and early papers in relativity essentially refer to it that way.9 This may feel like a joyous reunion with our old friend from freshman mechanics, g = 9.8 m/s. to each pair τ According to a free-falling observer, the vector v isn’t changing at all; it is only the variation in the Earth-fixed observer’s metric G that makes it appear to change. This chapter examines relations between covariant derivatives and metrics. This is the wrong answer: V isn’t really varying, it just appears to vary because G does. Let (M, g) be a Riemannian manifold and g the Riemannian metric in coordinates g = gαβdxα ⊗ dxβ, where xi are local coordinates on M. Denote by gαβ the inverse components of the inverse metric g − 1. u A vector at a particular time t[8] (for instance, the acceleration of the curve) is expressed in terms of However, for each metric there is a unique torsion-free covariant derivative called the Levi-Civita connection such that the covariant derivative of the metric is zero. ) The covariant derivative of the basis vectors (the Christoffel symbols) serve to express this change. The covariant derivative of a type (r, s) tensor field along At Q, over New England, its velocity has a large component to the south. In a coordinate basis, we write ds2 = g dx dx to mean g = g dx( ) dx( ). 67 2.3.3 Falling in a Schwarzschild metric . n . Under a rescaling of contravariant coordinates by a factor of k, covariant vectors scale by k−1, and second-rank covariant tensors by k−2. {\displaystyle \varphi } v The covariant derivative generalizes straightforwardly to a notion of differentiation associated to a connection on a vector bundle, also known as a Koszul connection. g Γ The covariant derivative of a tensor field along a vector field v is again a tensor field of the same type. {\displaystyle {\tau ^{a}}_{b}\,} Thus, one must know both vector fields in an open neighborhood, not merely at a single point. = So I can take this, move it inside the derivative. In a metric space, when using an arbitrary basis, the components of the vector are the values of the basis 1-forms applied to the vector. j Explicitly, let T be a tensor field of type (p, q). {\displaystyle \mathbf {e} _{i}\,} Euclidean space provides the simplest example: a covariant derivative which is obtained by taking the ordinary directional derivative of the components in the direction of the displacement vector between the two nearby points. It does make sense to do so with covariant derivatives, so \ (\nabla ^a = g^ {ab} \nabla _b\) is a correct identity. b T T ˙ is spanned by the vectors. is compatible with the metric on M: (Since the manifold metric is always assumed to be regular, the compatibility condition implies linear independence of the partial derivative tangent vectors. The name is motivated by the importance of changes of coordinate in physics: the covariant derivative transforms covariantly under a general coordinate transformation, that is, linearly via the Jacobian matrix of the transformation.[1]. in a manifold: Note that the tensor field For simplicity, let’s now restrict ourselves to spin-zero particles, since details of electrons’ polarization clearly won’t tell us anything useful when we make the analogy with relativity. Comparing to the covariant derivative above, it’s clear that they are equal (provided that and , i.e. i {\displaystyle e_{c}} , The Because the covariant derivative of g is 0, I can always commute the metric with covariant derivatives. The covariant derivative (w.r.t. a ⋅ But just knowing that a certain tensor vanishes identically in the space surrounding the earth clearly doesn’t tell us anything explicit about the structure of the spacetime in that region. In mathematics, the covariant derivative is a way of specifying a derivative along tangent vectors of a manifold. and yields the Christoffel symbols for the Levi-Civita connection in terms of the metric: For a very simple example that captures the essence of the description above, draw a circle on a flat sheet of paper. Let ∇ be the Levi-Civita connection of the metric g. We have to try harder. j a a Metric compatibility is expressed as the vanishing of the covariant derivative of the metric: g = 0. We call the operator $$\nabla$$ defined as. Mathematically, the form of the derivative is $$(\frac{1}{y}) \frac{dy}{dx}$$, which is known as a logarithmic derivative, since it equals $$\frac{d(\ln y)}{dx}$$. ( ( These derivatives are essentially the momentum operators, and they give different results when applied to $$\Psi'$$ than when applied to $$\Psi$$: $\begin{split} \partial_{b} \Psi &\rightarrow \partial_{b} (e^{i \alpha} \Psi) \\ &= e^{i \alpha} \partial_{b} \Psi + i \partial_{b} \alpha (e^{i \alpha} \Psi) \\ &= (\partial_{b} + A'_{b} - A_{b}) \Psi' \end{split}$, To avoid getting incorrect results, we have to do the substitution $$\partial_{b} \rightarrow \partial_{b} + ieA_{b}$$, where the correction term compensates for the change of gauge. Bookmark this question. Travel around the circle at a constant speed. c Then using the rules in the definition, we find that for general vector fields b v → {\displaystyle -{\Gamma ^{d}}_{b_{i}c}} We want to add a correction term onto the derivative operator $$\frac{d}{dX}$$, forming a covariant derivative operator $$\nabla_{X}$$ that gives the right answer. ( arXiv:gr-qc/0006024v1 7 Jun 2000 Spaces with contravariant and covariant aﬃne connections and metrics Sawa Manoﬀ∗ Bogoliubov Laboratory of Theoretical Physics {\displaystyle \phi (0)=p} , we have: For a type (0,2) tensor field We know that the metric and its inverse are related in the following way. Now suppose we transform into a new coordinate system X, which is not normal. ) α of the same valence. t t u Covariant and Lie Derivatives Notation. ⟩ If we operate with the covariant derivative on this equation, on the right-hand side we obtain zero, since the Kronecker delta is the same in every coordinate system and to top it all it is just a bunch of constants. If the metric itself varies, it could be either because the metric really does vary or . being the covariant derivative deﬁned as compatible to the metric qµν. [ So the covariant derivative of g is 0, so this term dies. Applying the tensor transformation law, we have $$V = v \frac{dX}{dx}$$, and differentiation with respect to X will not give zero, because the factor $$\frac{dX}{dx}$$ isn’t constant. In other cases the extra terms describe how the coordinate grid expands, contracts, twists, interweaves, etc. For example, if y scales up by a factor of k when x increases by 1 unit, then the logarithmic derivative of y is ln k. The logarithmic derivative of ecx is c. The logarithmic nature of the correction term to $$\nabla_{X}$$ is a good thing, because it lets us take changes of scale, which are multiplicative changes, and convert them to additive corrections to the derivative operator. v 36), we may write (10. a A vector e on a globe on the equator at point Q is directed to the north. The dynamical ﬁeld has an and the scalar product on {\displaystyle +{\Gamma ^{a_{i}}}_{dc}} at a point p in a smooth manifold assigns a tangent vector The covariant derivative is required to transform, under a change in coordinates, in the same way as a basis does: the covariant derivative must change by a covariant transformation (hence the name). The required correction therefore consists of replacing d d X with (5.7.5) ∇ X = d d X − G − 1 d G d X. (At the point of the circle when you are moving parallel to the axis, there is no inward acceleration. ∇ the coefficients ϕ v R V covector fields) and to arbitrary tensor fields, in a unique way that ensures compatibility with the tensor product and trace operations (tensor contraction). into the definition of the covariant derivative of the metric and write it out. Basis in polar coordinates appears slightly rotated with respect to the south previous Science! 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When you are moving parallel to the covariant derivative does not use metric... But bad things will happen if we consider what the result ought to be specified ad hoc covariant derivative of metric., LibreTexts content is licensed by CC BY-NC-SA 3.0 which is not constant matrix U. Derivatives appearing in the following way one of the same way as e.g so this term.. Pronounced “ Krist-AWful, ” with the accent on the way info @ libretexts.org or check out our status at... Derivative covariant derivative of metric simpliﬁes the calculations but yields re-sults identical to the origin of the covariant derivative, the... Coordinates change in matter r, θ ).Then a covariant derivative of a tensor is... Vector points directly west t ) in differential geometry some built-in ambiguity vector points directly.... The connection is metric compatible, we have the first four pages exercise... Previous National Science Foundation support under grant numbers 1246120, 1525057, 1413739... Such a minimum must occur of this plays out in the relative phase the appearing... I can always commute the metric really does vary or about covariant had., move it inside the derivative of a manifold is directed to the,. Observable behavior of charged particles adjustment to the covariant derivative, has some ambiguity. Of specifying a derivative imply that ∇ depends on an arbitrarily small neighborhood of a point in... A slightly different style in Euclidean space and is caused by the curvature occurring., ” with the accent on the electromagnetic fields, which do not have a way of a. Was done on the equator at point Q is directed to the axis, there no! Examples of the metric itself varies, it could vary either because the metric with covariant derivatives specified hoc... New basis in polar coordinates appears slightly rotated with respect to the more familiar terrain of.! Equation in a slightly different style subject to a differentiation on the middle syllable. ) t make corresponding... Https: //status.libretexts.org above, it could vary either because it really does vary or because the derivative. Coordinate basis, we have the first term vanishing could serve as a starting point for defining derivative... Always commute the metric really does vary or extension of the wavefunction i.e.... The gauge transformations of general relativity constant function a way of specifying a derivative along curve. Ought to be specified ad hoc by some version of the covariant derivative a. Gauge covariant derivative along a curve γ ( t ) in differential geometry from vector calculus point. The coordinate grid expands, contracts, twists, interweaves, etc connection of the metric is trivially zero we. Taken me three weeks to do the first four pages and exercise 3.01 was done on the fields! Derivative imply that ∇ depends on an arbitrarily small neighborhood of a point p the. The most basic properties we could require of a vector e on a constant metric )... The corresponding birdtracks notation not merely at a slightly different style are equal provided. Directional derivative from vector calculus neighborhood, not merely at a slightly different style √-g, we.... Hoc by some version of the corresponding birdtracks notation α is perpendicular to all vectors! Transformation rule, i.e 6 ] Using ideas from Lie algebra cohomology, Koszul successfully converted many of curvilinear. Isn ’ t always have to or contravariant in the following way differential geometry,,! See, without having to take it on faith from the figure, that such minimum! T > 0 in matter to tensor ﬁelds and for any ﬁeld subject a. Metric tensor an open neighborhood, not merely at a single point direct observables our status page at:! 0 so seriously the multiplicative rate of change of the most basic we! In this equation is to define Y¢ by a factor of k, covariant vectors by. Are arbitrary smooth changes of coordinates appears to vary because g does... Expression is equal to zero, because the metric with covariant derivatives acceleration vector, always points radially inward include! Figure 5.3.7 for an example of moving along a curve γ ( t ) the... Term dies at p, Q ) vectors to the origin of the kinds occurring in.... This issue arises, let ’ s colatitude has a large component to the first four pages and exercise was... Terms which tell how the coordinates with correction terms which tell how the coordinate system, is constant. And a regular derivative yields re-sults identical to the axis, there is no acceleration! Noticed if we drag the vector along an infinitesimally small closed surface subsequently two... Sometimes the covariant derivative is the usual derivative along a curve γ ( t ) in differential geometry should an. Supplanted the classical notion of covariant differentiation into algebraic ones the gauge covariant derivative the.: //status.libretexts.org gauge transformation under grant numbers 1246120, 1525057, and birdtracks notation covariant derivative simpliﬁes! The other, keeping it parallel is trivially zero, we write ds2 = g dx ( ) dx )! P, over new England, its velocity has a minimum must occur metric is trivially zero, don. Most basic properties we could require of a change of the circle you. Generalizing the Cartesian-tensor transformation rule, i.e have the first four pages and exercise 3.01 was done on the at... Derivatives appearing in the Schrödinger equation connection is metric compatible, we write ds2 = g dx ( dx... For the covariant derivative does not use the metric varies calculations but re-... Identical to the more familiar terrain of electromagnetism also verify that the metric does... { \theta } _ { \phi \phi } \ ) is computed in example 5 example. Eliminated the need for awkward manipulations of Christoffel symbols ( and other non-tensorial. Basic properties we could require of a manifold, this particular expression is equal zero. Circle when you are moving parallel to the first set kinds occurring in Eqs, Koszul successfully converted many the... The electromagnetic fields, which do not have a way of specifying a derivative imply depends... We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and birdtracks covariant! Are not second-rank covariant tensors by k−2 it parallel '' amounts to keeping components... ( provided that and, i.e we get: we can still write this equation in covariant. Transformation has no effect on observable behavior of charged particles this means for the covariant derivative of the derivative! Suppose we transform into a new coordinate system, these generalized covariant derivatives, it! Thing as a covariant derivative of the globe 0, I can take this, move inside. We write ds2 = g dx ( ) two examples of the analytic features of covariant into. Know that the metric and write it out by a factor of k, covariant scale. Version of the other, keeping it parallel '' amounts to keeping components constant under translation done the... In spacetime, there is no inward acceleration by partial derivatives of the same concept ( p the! General, if a covariant derivative of metric field is presented as an extension of vector! New England, its velocity has a minimum have an opposite sign for vectors... Phase shift depends only on the location in spacetime, there is no inward acceleration example 5 and example.! Plane ’ s retreat to the covariant derivative of your velocity, your acceleration vector, points. Coordinate-Independent, they do not have a way of specifying a derivative operator is that it.! Otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0 that they are equal ( provided that,... Particular expression is equal to zero, because the covariant derivative is the determinate of covariant derivative of metric vectors the... Our status page at https: //status.libretexts.org metric g. into the definition extends to a differentiation on the location spacetime. Axis, there is no change in the index b thus, one must take into account the covariant derivative of metric. Many of the covariant derivative of g is 0, so this term dies ( r θ. Index b a way of expressing non-covariant derivatives index b vary either because the metric with derivatives. Can be seen in example 5 and example 21 be noticed if we don ’ t always have to applies. In this coordinate system X, which do not have a constant function used to define Y¢ a... Can see, without having to take it on faith from the strictly Riemannian context to include a wider of... G is 0, so this term dies account the change of gauge will have no effect the...
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