Comment on recent papers regarding next-to-leading order spin-spin effects in gravitational interaction

Understand

It is argued that the tetrad in a recent paper by Porto and Rothstein on gravitational spin-spin coupling should not have the given form.

• The fixation of that tetrad was suggested by Steinhoff, Hergt, and Schaefer as a possible source for the disagreement found in the spin-squared dynamics.
• However, this inconsistency will only show up in the next-to-leading order spin-orbit dynamics and not in the spin-squared dynamics.
• Instead, the disagreement found at the next-to-leading order spin-squared level is due to a sign typo in the spin-squared paper by Porto and Rothstein.