This leads us to talk about causal relationships. An event A is a cause of an event B if A was necessary for B to occur. Two events A and B have only 3 possible causal relationships, either A is a cause of B, B is a cause of A, or A is not a cause of B and B is not a cause of A. If A is a cause of B, then we say that A is in the past of B. If B is a cause of A, then we say that A is in the future of B.

Time and causality are synonymous and the idea of a synchronous moment of time doesn't make sense. A causal universe evolves as information flows between causally connected events. This is how general relativity describes our universe, subject to the restriction that nothing can travel faster than c.

Here's an example of a causal web. The arrows indicate causal relationships. (Tell story of causal web on slide. The Andromeda Galaxy (M31) is approximately 2.5 million light years away.) The events at the bottom are in the past of my talk while the events at the top are in the future of my talk.