Dear Einstein Toolkit community,
I am a retired researcher (background in mathematics and computer science) working independently from China. Over the past ten years I have been building a test-particle geodesic tracker for the Einstein Toolkit, and I would like to share it with the community.
The Geodesic thorn integrates geodesics for large numbers of test particles in Kerr–Schild Cartesian coordinates. It supports (i) the exact Kerr–Schild metric, independent of the grid (verified symbolically to be the vacuum solution to machine precision), and (ii) 27-point local Lagrange interpolation of the grid metric, so it also works for arbitrary spacetimes carried on the grid (e.g., GRHydro evolutions). The per-particle integration is fully independent and parallelized with OpenMP (validated with 10^4 particles).
The code (GPLv2+), the configuration files used in the validation runs, and a paper draft describing the method and verification are all available in this public repository: https://github.com/wylyhzh/Geodesic
I plan to submit the paper to arXiv (gr-qc, cross-list astro-ph.HE) once it has benefited from the community's feedback.
I would appreciate: (1) feedback on whether the thorn meets ET standards, and what would be needed to have it listed with the ET thorns; (2) any comments on the paper. I am most comfortable discussing in writing, so I would prefer to continue the conversation here on the mailing list or by email, whichever is more convenient.
I am happy to provide any further information.
Best regards,
Yuhua Li
Dear Yuhua,
Thank you for writing and for putting the code and the paper draft in a public repository under GPLv2+. Independent contributions of this kind are welcome, and a test-particle geodesic tracker is something we don't have: the ET currently ships fluid tracers (particle_tracerET) and puncture tracking (PunctureTracker), but not a timelike geodesic integrator on the 3+1 metric.
Were you hoping to have it included in the ET, or were you just looking to share the code with the community? If so, we can help you with the process. Please take a look at https://einsteintoolkit.org/contribute.html if you haven't already.
If you would like to do something less formal, we can share your link on our list of "thorns we know of": https://docs.einsteintoolkit.org/et-docs/Thorns_we_know_of
Best regards,
Steven R. Brandt
On 9/8/2026 12:28 AM, Yuhua Li via Users wrote:
Dear Einstein Toolkit community,
I am a retired researcher (background in mathematics and computer science) working independently from China. Over the past ten years I have been building a test-particle geodesic tracker for the Einstein Toolkit, and I would like to share it with the community.
The Geodesic thorn integrates geodesics for large numbers of test particles in Kerr–Schild Cartesian coordinates. It supports (i) the exact Kerr–Schild metric, independent of the grid (verified symbolically to be the vacuum solution to machine precision), and (ii) 27-point local Lagrange interpolation of the grid metric, so it also works for arbitrary spacetimes carried on the grid (e.g., GRHydro evolutions). The per-particle integration is fully independent and parallelized with OpenMP (validated with 10^4 particles).
The code (GPLv2+), the configuration files used in the validation runs, and a paper draft describing the method and verification are all available in this public repository: https://github.com/wylyhzh/Geodesic
I plan to submit the paper to arXiv (gr-qc, cross-list astro-ph.HE) once it has benefited from the community's feedback.
I would appreciate: (1) feedback on whether the thorn meets ET standards, and what would be needed to have it listed with the ET thorns; (2) any comments on the paper. I am most comfortable discussing in writing, so I would prefer to continue the conversation here on the mailing list or by email, whichever is more convenient.
I am happy to provide any further information.
Best regards,
Yuhua Li
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Yuhua Li