Final answer:
In Einstein's theory of general relativity, gravity is described as the effect of mass curving spacetime.
Step-by-step explanation:
In Einstein's theory of general relativity, gravity is described by considering how mass affect(s) spacetime. This revolutionary theory proposed by Albert Einstein in 1915 diverged from Newton's view of gravity as a force pulling between masses. Instead, Einstein envisioned that the presence of mass warps the very fabric of spacetime, and this curvature of spacetime is what we perceive as gravity. Objects, including light, moving through this curved spacetime have their paths altered, which to an observer gives the appearance of the gravitational pull. American physicist John Wheeler famously summarized this as: "Matter tells spacetime how to curve; spacetime tells matter how to move." The implications of Einstein's theory have been vast, leading to predictions like black holes and gravitational waves—both of which have been experimentally confirmed.