Physics and electrical engineering! Complex numbers are an elegant way to model reactive power, the key insight into developing power factor correction circuits which are a critical piece of electrical infrastructure without which our electrical grid would fail!
Physics and electrical engineering! Complex numbers are an elegant way to model reactive power, the key insight into developing power factor correction circuits which are a critical piece of electrical infrastructure without which our electrical grid would fail!
Quantum mechanics too, basically any process that has a periodic component with amplitude and phase