When we teach about S-parameters and modeling, someone never fails to ask “What is causality?” or “What happens if a model isn’t causal?” Our go-to explanation is that it's a cause and effect thing: a system is causal if its output doesn’t respond before its input changes. In a causal universe, the light shouldn't turn on before you flip the switch. This seems straightforward. After all, if you're modeling or measuring a physical system — say, interconnects between a drive
