We have already seen that, at a fundamental level, the gravitational field is nothing more than the shape of spacetime itself, a revelation that was also due to a unifying idea. By combining space with time, Einstein revealed a profound truth: that only in four-dimensional spacetime can all observers (however fast they are moving relative to each other) agree on the separation between two events. A decade later, his general theory of relativity gave the world a new and more accurate picture of how mass and energy cause this spacetime to curve.
