Causality refers to how one event can directly lead to another event in a cause-and-effect relationship. Einstein took this idea further when he realized that nothing can travel faster than light. This led to the concept of "light cones," which represent all the possible paths that light — or any signal moving at the speed of light — can take from a given event in space and time. Anything inside the light cone of an event could potentially be influenced by that event, while anything outside the light cone cannot be affected by it due to the limitation imposed by the speed of light. The outer boundary of this cone is called the "event horizon."