Gravitational waves are ripples in the fabric of space-time, caused by energetic processes in the Universe, including catastrophic events such as colliding black holes, supernovae collapses, and others. Though their existence has been known for a long time, gravitational waves were physically detected for the first time on September 14, 2015 by LIGO, when distortions in space-time made by gravitational waves created by two colliding black holes were detected.