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

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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