Basic introduction to the mathematics of curved spacetime

Curved spacetime is a strange way of thinking about the world that suggests that the three dimensions that we are used to (height, width and length) can be combined in a fourth, time. It is a special type of math that helps us to understand how the Universe works, and is an important part of Albert Einstein's Theory of Relativity. In curved spacetime, objects such as stars, planets and light can take curved paths, like a river running around rocks in a riverbed. Instead of everything being straight and flat, curved spacetime suggests that the Universe is curved and not flat. To understand this strange concept, think of the Earth: if you wanted to walk from New York to Los Angeles, it will take much longer by going North and around the Earth, than if you flew straight across the curvature of the planet. In the same way, curved spacetime helps us understand how objects like planets and stars can take curved paths in the Universe.