General Relativity

In Einstein's Special Relativity, it is established that information could not be transmitted faster than the speed of light in vacuum, the so called "cosmic speed limit". The diagram above shows the trajectory of an object described by a "world line", the sequence of spacetime points (or "events") corresponding to the history of the object. The limit imposed by the speed of light on information travel divides the surrounding spacetime into causally distinct regions. For a given object, A, the present is its locus, a single point in spacetime at the vertex of a light cone. A light cone is the conic consisting of all events that can be connected to A via light rays. Events falling within the past light cone can reach or exert causal influence on A. Events falling within the future light cone can in turn be influenced by the A. Events outside of A's lightcone cannot be causally linked to A, because in order to do so, information has to be transmitted faster than the cosmic speed limit. When we look at the Universe we are actually seeing just our past light cone in all of spacetime.