This artist's animation shows the merger of two black holes and the gravitational waves that ripple outward during the event. The black holes—which represent those detected by LIGO on Dec. 26, 2015—were 14 and 8 times the mass of the sun, until they merged, forming a single black hole 21 times the sun's mass. One solar mass was converted to gravitational waves. In reality, the area near the black holes would appear highly warped, and the gravitational waves would be difficult to see directly.