「物理討論四」 Why is gravity described by the curvature of spacetime? Consider a 2D creature (denoted as $) on a sphere; $ indicates its current position, and it starts walking along the sphere. After a while, $ goes back to the original position he indicated. From the point of view of $, he thinks he is walking along a straight line. But based on the above observation, he realizes that he is moving in a curved space embedded in higher dimensions.