This is a theory of gravity proposed by Albert Einstein in the early 20th century. According to general relativity, gravity is not a force that allows objects to attract each other, but rather a curvature of space and time that is caused by the presence of mass and energy. This curvature of space and time is known as spacetime, and it describes how objects move and interact with each other in the universe. General relativity also predicts the existence of black holes and gravitational waves, which have been observed by modern astrophysical instruments.