[Users] New thorn proposal: Geodesic — test-particle geodesic tracking for the Einstein Toolkit

[Steven R. Brandt]

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
>
> _______________________________________________
> Users mailing list
> Users at einsteintoolkit.org
> http://lists.einsteintoolkit.org/mailman/listinfo/users
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.einsteintoolkit.org/pipermail/users/attachments/20260908/202033a5/attachment.htm>