This slide shows a spacetime diagram focusing on the event consisting of my talk. In this picture, I'm only showing 2 dimensions of space and time. I'm omitting the z-direction so that I can draw the picture on the 2-dimensional screen. In spacetime diagrams, distances are usually divided by the speed of light so that we're measuring both distance and time in units of time. This makes the slopes of the paths of light in the diagram equal to 1.

We expand the definition of the future of an event to include all events that it could send information to and be a cause of. Since nothing travels faster than c, the paths of light traveling from the event bound the future of an event. The region of spacetime in the future of an event is called the future light cone of the event. In this case, my banishment and the exercise of your better judgment are in the future of my talk and are contained within the future light cone of my talk. We define the past light cone in an analogous way and my decision to give this talk and the time when you read about it are contained within the past light cone for the talk.

The collection of all possible causal relationships in the universe make up the causal structure of the universe. General relativity describes how the causal structure of the universe changes in response to events in the universe. For example, it describes how light cones tilt toward matter (or any other form of energy), which we experience as the gravitational force. Another example is how moving matter can cause the light cones to oscillate thereby producing gravitational waves.