  The general-relativistic phenomenon of spin-induced orbital precession has
not yet been observed in strong-field gravity. Gravitational-wave observations
of binary black holes (BBHs) are prime candidates, since we expect the
astrophysical binary population to contain precessing binaries. Imprints of
precession have been investigated in several signals, but no definitive
identification of orbital precession has been reported in any one of the 84 BBH
observations to date by the Advanced LIGO and Virgo detectors. Here we report
the measurement of strong-field precession in the LIGO-Virgo-Kagra (LVK)
gravitational-wave signal GW200129. The binary's orbit precesses at a rate ten
orders of magnitude faster than previous weak-field measurements from binary
pulsars. We also find that the primary black hole is likely highly spinning.
According to current binary population estimates a GW200129-like signal is
extremely unlikely, and therefore presents a direct challenge to many current
binary formation models.
