Black holes merging. Image 1 of 2. Supercomputer simulation of two non-spinning black holes (grey discs right and left of centre) merging. According to Einstein's theory of general relativity, the merger of two massive objects causes ripples (yellow lines) in space-time. These ripples, known as gravitational waves travel at the speed of light but have not been observed directly. A black hole is formed when the core of a star collapses under its own weight, increasing its gravitational field to the point where nothing can escape, not even light. Black hole mergers are the most powerful events in the universe, releasing more energy than all of the stars in the universe combined. This simulation was created in 2006 on the NASA Columbia supercomputer in California, USA.