Are quadratic quasinormal modes present in observed ringdowns?
In plain words
General relativity is nonlinear, so two ringing tones of a black hole should combine into weaker extra tones at the sum of their frequencies. Detecting them would test the nonlinear part of Einstein's equations.
Precise statement
Second-order perturbation theory predicts a mode at frequency $2\omega_{220}$ in the $(4,4)$ harmonic with amplitude ratio $A_{220x220}/(A_{220})^2$ fixed by the remnant spin (measured in simulations, 2023). Determine whether such quadratic modes are detected in gravitational-wave data at $5\sigma$ or more, confirmed by independent pipelines, and whether the amplitude ratio matches the general-relativity value. An answer is a detection with a measured ratio, or an upper limit inconsistent with the prediction.
What would settle it
A detection of the quadratic $(4,4)$ mode at $5\sigma$ in one or more loud events with the amplitude ratio measured.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Non-collaboration analyses of GW250114 report quadratic-mode evidence at $3.4\,\sigma$ (Wang et al., arXiv:2601.05734, 2026) with the coupling consistent with general relativity within $1.3\,\sigma$ (Yang, Shi, Hu, arXiv:2609.28953, 2026); there is no collaboration or $5\,\sigma$ detection as of October 2026.
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