Ligo, a gravitational wave observatory, detected ripples in space-time. They sent out an alert, and astronomers in Caltech were able to detect the merging of the two black holes that caused the ripples. Normally, it's not possible to see a collision of two black holes, but they were in the vicinity of a massive black hole, which illuminated the collision and made it possible for the astronomers to observe it. This was the first time colliding black holes have been observed optically.

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