>>42746790
>quantum marechanics
Pinkie has A LOT of energy levels (which would imply heavier nucleus, but we already know she's kinda pudgy).
Every time she goes down an energy level, she emits a quantum of FUN. It can be absorbed by nearby mares to raise their own energy level, i.e. get excited, but when they're already residing at a high energy level, a stimulated emission of FUN happens, triggering a party event.