GW 190412 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 12 April 2019.^[1][2] In April 2020, it was announced as the first time a collision of a pair of very differently sized black holes has been detected.^[3] As a result of this asymmetry, the signal included two measurable harmonics with frequencies approximately a factor 1.5 (a perfect fifth) apart.^[2][4]

The collision took place 2.4 billion light-years away.^[3] The heavier of the black holes had a mass of 29.7 solar masses, and the lighter one around 8.4 solar masses.^[3] The difference in mass meant that the secondary harmonic in the signal was strong enough to be detected, allowing researchers to perform a test of general relativity and determine that the larger black hole was spinning.^[3]