<html>#2759: TwoPunctures swap_xz results in left-handed coordinate system
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<tr><td style='text-align:right'> Reporter:</td><td>Zach Etienne</td></tr>
<tr><td style='text-align:right'> Status:</td><td>new</td></tr>
<tr><td style='text-align:right'>Milestone:</td><td></td></tr>
<tr><td style='text-align:right'> Version:</td><td></td></tr>
<tr><td style='text-align:right'> Type:</td><td>bug</td></tr>
<tr><td style='text-align:right'> Priority:</td><td>minor</td></tr>
<tr><td style='text-align:right'>Component:</td><td></td></tr>
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<p>A word of caution:</p>
<p>Swapping x<->z in TwoPunctures (parameter swap_xz = true) results in a <strong>non-right-handed coordinate system</strong> (coordinate permutation zyx). This flips the sign of the y-component of the BH spin vectors away from what one might expect.</p>
<p>In BH@H (<a data-is-external-link="true" href="https://github.com/nrpy/nrpy/blob/main/nrpy/infrastructures/BHaH/general_relativity/TwoPunctures/TP_interp.py" rel="nofollow">https://github.com/nrpy/nrpy/blob/main/nrpy/infrastructures/BHaH/general_relativity/TwoPunctures/TP_interp.py</a>) 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.</p>
<p>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.</p>
<p>--<br/>
Ticket URL: <a href='https://bitbucket.org/einsteintoolkit/tickets/issues/2759/twopunctures-swap_xz-results-in-left'>https://bitbucket.org/einsteintoolkit/tickets/issues/2759/twopunctures-swap_xz-results-in-left</a></p>
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