The keywords related to this are "spacelike" and "timelike". https://en.wikipedia.org/wiki/Causal_structure https://en.wikipedia.org/wiki/Causal_structure For

The keywords related to this are "spacelike" and "timelike". https://en.wikipedia.org/wiki/Causal_structure https://en.wikipedia.org/wiki/Causal_structure For example, the distance between Earth and Moon is about 1.3 light-seconds. That means that is A happens on Earth now, and B happens on Moon 1 second later (from our perspective), it is possible that from someone else's perspective B happened before A. (Note that "someone else" in this example does not refer to the Moon, because its perspective is very similar to the perspective of the Earth. It refers to a hypothetical alien spaceship flying at an extremely high speed in a line parallel to the line between the Earth and the Moon.) However, if C happens on Earth now, and D happens on Moon 2 seconds later (from our perspective), then C happened before D from everyone's perspective. Individual aliens would disagree on the exact time interval between C and D, where 0.7 would be the smallest possible value, but it could be any larger value. (You get the smallest possible value when you are not moving relatively to the Earth and the Moon.) This is because a hypothetical light particle emitted at A couldn't reach B in time, but a hypothetical light particle emitted at C could reach D in time. The speed of light is the speed of causality, kind of. Events A and B couldn't possibly have caused each other (they could have a common cause X, but that's different), but C hypothetically could have caused D, which is why C happened before D from everyone's perspective.