 From antiquity, it was believed that the idea of empty space is a conceptual impossibility. Space is nothing but an abstraction we use to compare different arrangements of the objects. Concerning time, it was believed that there can be no lapse of time without change occurring somewhere. Time is merely a measure of cycles of change within the world. Then in 1686, Isaac Newton founded classical mechanics on the view that space is real and distinct from objects and that time is real and passes uniformly without regard to whether anything moves in the world. He spoke of absolute space and absolute time as a stage within which matter existed and moved as time flowed at a constant rate. It was understood that space and time tell matter how to move, but matter has no effect on space and time. We have seen that special relativity broke the paradigm of absolute space and absolute time because the constancy of the speed of light required a trade-off between space and time across different inertial frames. In addition, and more to our point, the idea that space and time act on matter, but that matter does not act on space and time, troubled Einstein. With these considerations in mind, and noting that light curved in a gravitational field, Einstein proposed that the mass of an object does indeed act on the space and time it exists in. Specifically, he proposed that the presence of matter curves spacetime. To see how this can be, we need to examine the geometry of space a little deeper. We'll start with the Euclidean geometry of flat surfaces and generalize it to curved spacetime.