Feedback loops

A thermostat is the clearest case. The room gets cold. The heater comes on. The room gets warm. The heater goes off. The output of the system, warmth, is fed back in as the thing that decides what happens next. The loop holds the room near one number without anyone watching.

That kind of loop fights change. Push it and it pushes back. Your body does it with temperature and sugar. A market does it with prices: a shortage lifts the price, the high price brings in sellers, and the shortage eases. Loops like this are why some things are stubbornly hard to shift.

The other kind feeds change instead. More people on a phone network makes the network worth more, which brings more people. A bank in trouble frightens savers, who take their money out, which puts the bank in more trouble. Nothing in the loop says stop. It runs until it hits something outside itself.

Most real systems have both kinds, and which one is in charge can switch. A fire grows by feeding itself until it runs out of wood. Then the loop that was driving it has nothing to work on, and the fire dies fast.

Delay is what makes loops hard to see and hard to live with. Turn the shower tap and the water takes a few seconds to change. If you keep turning while you wait, you overshoot, then you overshoot the other way. Public policy, hiring and stock control all work like that shower, with delays of months.

So when something will not shift, look for the loop holding it. When something runs away, look for the loop feeding it. And when something swings back and forth with no one steering, look at the delay. The loop usually explains more than the parts do.