Fluctuation dissipation theorem

The fluctuation dissipation theorem (FDT) helps us understand how things behave when they're not in equilibrium, like when they're being heated or cooled. It says that if we know how much something fluctuates (like its temperature), we can use that to figure out how much energy it will dissipate (like when a warm object cools down).

It's kind of like when you're playing with a toy car and you push it really fast. As it slows down, it gets cooler, right? That's because it's dissipating its energy. The FDT helps us understand how much energy the toy car will dissipate based on how much it's fluctuating.

The FDT is really useful because it helps us understand how systems work even when they're not in perfect equilibrium. We can use it to make predictions about how things will behave even when they're changing over time, which is really helpful in lots of different fields.