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{{for|the book|Schild's Ladder}}
[[Image:Schild's ladder step 4.svg|thumb|right|Two rungs of Schild's ladder.  The segments ''A''<sub>1</sub>''X''<sub>1</sub> and ''A''<sub>2</sub>''X''<sub>2</sub> are an approximation to first order of the [[parallel transport]] of ''A''<sub>0</sub>''X''<sub>0</sub> along the curve.]]
In the theory of [[general relativity]], and [[differential geometry]] more generally, '''Schild's ladder''' is a [[First-order approximation|first-order]] method for ''approximating'' [[parallel transport]] of a vector along a curve using only [[Torsion of connection#Geodesics and the absorption of torsion|affinely parametrized]] [[geodesic]]s.  The method is named for [[Alfred Schild]], who introduced the method during lectures at [[Princeton University]].


== Construction ==


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The idea is to identify a tangent vector ''x'' at a point <math>A_0</math> with a geodesic segment of unit length <math>A_0X_0</math>, and to construct an approximate parallelogram with approximately parallel sides <math>A_0X_0</math> and <math>A_1X_1</math> as an approximation of the [[Levi-Civita parallelogramoid]]; the new segment <math>A_1X_1</math> thus corresponds to an approximately parallel translated tangent vector at <math>A_1.</math>
 
{|
|[[Image:Schild's ladder step 1.svg|thumb|250px|right|A curve in ''M'' with a "vector" ''X''<sub>0</sub> at ''A''<sub>0</sub>, identified here as a geodesic segment.]]
|[[Image:Schild's ladder step 2.svg|thumb|250px|right|Select ''A''<sub>1</sub> on the original curve. The point ''P''<sub>1</sub> is the midpoint of the geodesic segment ''X''<sub>0</sub>''A''<sub>1</sub>.]]
|[[Image:Schild's ladder step 3.svg|thumb|250px|right|The point ''X''<sub>1</sub> is obtained by following the geodesic ''A''<sub>0</sub>''P''<sub>1</sub> for twice its parameter length.]]
|}
 
Formally, consider a curve &gamma; through a point ''A''<sub>0</sub> in a [[Riemannian manifold]] ''M'', and let ''x'' be a [[tangent vector]] at ''A''<sub>0</sub>. Then ''x'' can be identified with a geodesic segment ''A''<sub>0</sub>''X''<sub>0</sub> via the [[exponential map]]. This geodesic &sigma; satisfies
 
:<math>\sigma(0)=A_0\,</math>
:<math>\sigma'(0) = x.\,</math>
 
The steps of the Schild's ladder construction are:
* Let ''X''<sub>0</sub> = &sigma;(1), so the geodesic segment <math>A_0X_0</math> has unit length.
* Now let ''A''<sub>1</sub> be a point on &gamma; close to ''A''<sub>0</sub>, and construct the geodesic ''X''<sub>0</sub>''A''<sub>1</sub>.
* Let ''P''<sub>1</sub> be the midpoint  of ''X''<sub>0</sub>''A''<sub>1</sub> in the sense that the segments ''X''<sub>0</sub>''P''<sub>1</sub> and ''P''<sub>1</sub>''A''<sub>1</sub> take an equal affine parameter to traverse.  
* Construct the geodesic ''A''<sub>0</sub>''P''<sub>1</sub>, and extend it to a point ''X''<sub>1</sub> so that the parameter length of ''A''<sub>0</sub>''X''<sub>1</sub> is double that of ''A''<sub>0</sub>''P''<sub>1</sub>.
* Finally construct the geodesic ''A''<sub>1</sub>''X''<sub>1</sub>.  The tangent to this geodesic ''x''<sub>1</sub> is then the parallel transport of ''X''<sub>0</sub> to ''A''<sub>1</sub>, at least to first order.
 
== Approximation ==
This is a discrete approximation of the continuous process of parallel transport. If the ambient space is flat, this is exactly parallel transport, and the steps define [[parallelograms]], which agree with the [[Levi-Civita parallelogramoid]].
 
In a curved space, the error is given by [[holonomy]] around the triangle <math>A_1A_0X_0,</math> which is equal to the integral of the [[curvature]] over the interior of the triangle, by the [[Ambrose-Singer theorem]]; this is a form of [[Green's theorem]] (integral around a curve related to integral over interior).
 
== Notes ==
# Schild's ladder requires not only geodesics but also relative distance along geodesics. Relative distance may be provided by affine parametrization of geodesics, from which the required midpoints may be determined.
# The parallel transport which is constructed by Schild's ladder is necessarily [[Torsion of connection|torsion]]-free.
# A Riemannian metric is not required to generate the geodesics. But if the geodesics are generated from a Riemannian metric, the parallel transport which is constructed in the limit by Schild's ladder is the same as the Levi-Civita connection because this connection is defined to be torsion-free.
 
==References==
*{{citation|first1=Arkady|last1=Kheyfets|first2=Warner A.|last2=Miller|first3=Gregory A.|last3=Newton|title=Schild's ladder parallel transport procedure for an arbitrary connection. International Journal of Theoretical Physics
20001200, Volume 39, Issue 12, pp 2891-2898 }}.
* {{Citation
| first1=Charles W.
| last1=Misner
  | authorlink1=Charles W. Misner
| first2=Kip S.
| last2=Thorne
| authorlink2=Kip S. Thorne
| first3=John A.
| last3=Wheeler
| authorlink3=John Archibald Wheeler
| title=Gravitation
| publisher= W. H. Freeman
| year=1973
| isbn=0-7167-0344-0
}}
 
[[Category:Connection (mathematics)]]
[[Category:First order methods]]

Revision as of 15:18, 12 January 2014

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Two rungs of Schild's ladder. The segments A1X1 and A2X2 are an approximation to first order of the parallel transport of A0X0 along the curve.

In the theory of general relativity, and differential geometry more generally, Schild's ladder is a first-order method for approximating parallel transport of a vector along a curve using only affinely parametrized geodesics. The method is named for Alfred Schild, who introduced the method during lectures at Princeton University.

Construction

The idea is to identify a tangent vector x at a point A0 with a geodesic segment of unit length A0X0, and to construct an approximate parallelogram with approximately parallel sides A0X0 and A1X1 as an approximation of the Levi-Civita parallelogramoid; the new segment A1X1 thus corresponds to an approximately parallel translated tangent vector at A1.

A curve in M with a "vector" X0 at A0, identified here as a geodesic segment.
Select A1 on the original curve. The point P1 is the midpoint of the geodesic segment X0A1.
The point X1 is obtained by following the geodesic A0P1 for twice its parameter length.

Formally, consider a curve γ through a point A0 in a Riemannian manifold M, and let x be a tangent vector at A0. Then x can be identified with a geodesic segment A0X0 via the exponential map. This geodesic σ satisfies

σ(0)=A0
σ(0)=x.

The steps of the Schild's ladder construction are:

  • Let X0 = σ(1), so the geodesic segment A0X0 has unit length.
  • Now let A1 be a point on γ close to A0, and construct the geodesic X0A1.
  • Let P1 be the midpoint of X0A1 in the sense that the segments X0P1 and P1A1 take an equal affine parameter to traverse.
  • Construct the geodesic A0P1, and extend it to a point X1 so that the parameter length of A0X1 is double that of A0P1.
  • Finally construct the geodesic A1X1. The tangent to this geodesic x1 is then the parallel transport of X0 to A1, at least to first order.

Approximation

This is a discrete approximation of the continuous process of parallel transport. If the ambient space is flat, this is exactly parallel transport, and the steps define parallelograms, which agree with the Levi-Civita parallelogramoid.

In a curved space, the error is given by holonomy around the triangle A1A0X0, which is equal to the integral of the curvature over the interior of the triangle, by the Ambrose-Singer theorem; this is a form of Green's theorem (integral around a curve related to integral over interior).

Notes

  1. Schild's ladder requires not only geodesics but also relative distance along geodesics. Relative distance may be provided by affine parametrization of geodesics, from which the required midpoints may be determined.
  2. The parallel transport which is constructed by Schild's ladder is necessarily torsion-free.
  3. A Riemannian metric is not required to generate the geodesics. But if the geodesics are generated from a Riemannian metric, the parallel transport which is constructed in the limit by Schild's ladder is the same as the Levi-Civita connection because this connection is defined to be torsion-free.

References

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