In 1905 Henri Poincaré first suggested that accelerated masses in a relativistic field theory of gravity should produce gravitational waves in analogous fashion as an accelerating electrical charge produces electromagnetic waves. When Einstein published his theory of general relativity in 1915 he was skeptical of Poincaré's idea since the theory implied there were no "gravitational dipoles". Nonetheless he still pursued the idea and based on various approximations came to the conclusion that there must, in fact, be three types of gravitational waves.
This text is adapted from a Wikipedia article Gravitational wave. This text is licensed under the Creative Commons Attribution-Share-Alike License 3.0.