[ET Trac] #2759: TwoPunctures swap_xz results in left-handed coordinate system

[Zach Etienne]

#2759: TwoPunctures swap_xz results in left-handed coordinate system

 Reporter: Zach Etienne
   Status: new
Milestone: 
  Version: 
     Type: bug
 Priority: minor
Component: 

A word of caution:

Swapping x<->z in TwoPunctures \(parameter swap\_xz = true\) results in a **non-right-handed coordinate system** \(coordinate permutation zyx\). This flips the sign of the y-component of the BH spin vectors away from what one might expect.

In BH at H \([https://github.com/nrpy/nrpy/blob/main/nrpy/infrastructures/BHaH/general\_relativity/TwoPunctures/TP\_interp.py](https://github.com/nrpy/nrpy/blob/main/nrpy/infrastructures/BHaH/general_relativity/TwoPunctures/TP_interp.py)\) I implemented a permutation xyz → zxy in TwoPunctures that maintains right-handedness, so the BHs will orbit on the zx plane instead of the xy plane. The inputs, including y-components of spin vectors, are consistent with the BHs initially orbiting on the xy plane, which is generally assumed in PN theory.

I would recommend implementing a warning in TwoPunctures when swap\_xz is enabled, to make sure the user is aware of the implications of swapping x and z.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2759/twopunctures-swap_xz-results-in-left
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