In Newtonian gravity the angular momentum of each component of a
point-particle binary system is conserved: the orbital angular momentum of the
binary, and the individual angular momenta of the two objects in orbit. In
general relativity this is no longer true; there are spin-orbit and spin-spin
couplings between the individual angular momenta of the binary components, and
as a result the orbital plane precesses around the direction of the total
angular momentum. General relativistic precession has previously been measured
in binary pulsars, where the precession frequency was several degrees per year.
The effect can be far stronger in binaries consisting of black holes in close
orbit. It has long been anticipated that strong-field precession will be
measured in gravitational-wave observations of the late inspiral and merger of
two black holes. While there is compelling evidence that the binary-black-hole
population includes precessing binaries, precession has not been unambiguously
measured in any one of the $\sim$90 LIGO-Virgo-Kagra (LVK) gravitational-wave
detections to date. Here we report strong evidence for the measurement of
strong-field precession, which we find in the LVK gravitational-wave signal
GW200129. The binary’s orbit precesses at a rate ten orders of magnitude larger
than previously measured from binary pulsars. We also report that the primary
black hole is likely highly spinning.

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