Relativity and Cosmology

   

A Comparison of Light Path Equations in General Relativity Using a Taylor Series Approach vs Jacobian Elliptic Functions

Authors: Gerald Malczewski

Using a method to calculate the deflection of light near the Sun based on General Relativity (GR), a formula is derived which expresses the light path as a function of the radial distance r from a gravitating central body. This method, based on the GR geodesic equation applied to the Schwarzschild metric, uses an infinite Taylor series expansion. This requires a ‘finite cutoff’ of the series to compute the polar angle of a light path coordinate for a given radial distance. We take the light path to be a trajectory of a light ray originating at radial infinity and then departing to radial infinity after reaching its point of closest approach to the central body. Constraints are found that limit the discussion to central bodies of mass M which obey M/R< 1/3 where R is the turning point, the point of closet approach. A comparison is then made with a different approach previously published using Jacobian elliptic functions which yields a closed expression for the light path equation. It is shown that the two approaches are equivalent if a finite cutoff is not taken for the Taylor series. If the cutoff is taken, then computationally the two methods yield approximately the same result for light path locations near the point of closest approach to the central body. The elliptic function method has the advantage that the method can be applied to the case where M/R ≥ 1/3, including calculating light paths inside a black hole horizon. This case is outside the scope of this paper.

Comments: 33 Pages. Mathematical corrections have been made but with no material effect on the conclusions. A Predictive Strength measure has been added.

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Submission history

[v1] 2024-11-04 18:16:21
[v2] 2024-12-05 21:03:41
[v3] 2025-08-28 14:49:23

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