Positive and Negative Feedback Loops

If output makes more of the same,
Then “positive feedback” ‘s the name.
But if something corrects
And reverses effects,
Then “negative” feedback’s the game.

An important concept from complex systems theory that people often misunderstand are positive and negative feedback loops.

Positive feedback loops occur when a system’s output serves as an input that drives the system in the same direction. This can lead to exponential growth or exponential decline.

Negative feedback loops happen when an output of a system causes counteractive effects, serving as an input that pushes the system in the opposite direction. This often leads to stability or equilibrium.

Abstract, I know. Let’s give an example of each.

Example 1

Imagine a spacecraft in orbit around the Earth. Despite what people often think, when orbiting the Earth there is still gravity acting on the spacecraft. The spacecraft is moving forward at a speed that allows it to continuously “fall” around the Earth without crashing into it.

Also despite what people often think, a spacecraft in orbit around the Earth (especially low Earth orbit) has not fully escaped the Earth’s atmosphere. The Earth’s atmosphere thins out, but keeps going for thousands of miles.

There are fewer particle for spacecraft to collide with up there, but anything in orbit is still colliding with particles. These collisions cause drag, which slows the spacecraft down. This is why to stay in orbit spacecraft have to fire their thrusters periodically to counteract this drag and maintain their speed. If they do not, they will slow down and eventually fall back to Earth.

Okay, we have enough background info to discuss feedback loops.

Say the spacecraft runs out of fuel and can no longer fire its thrusters to couteract the drag. As it slows down, it will fall closer to the Earth. That also means that it’s falling farther into the Earth’s atmosphere, where there are more particles to collide with. This increase of particle collisions causes more drag, which causes it to slow down even more, which causes it to fall faster. And so on, and so on.

Now, here’s the question: what kind of feedback loop is this?

Take a minute to think about it before reading on.

Got your answer?

You might be surprised to hear that this is a positive feedback loop.
The output (the spacecraft slowing down) serves as an input that drives the system in the same direction (slowing down even more).

Many people would mistakenly think this is a negative feedback loop. The spacecraft is slowing down, it’s in a decaying orbit, it’s in a state of decline—there’s plenty of negative imagery going on. But the nature of a feedback loop doesn’t have anything to do with value judgements about the desirability of an outcome.

But from a systems standpoint, what we are trying to assess is whether the feedback loop makes the effect stronger or weaker.

The only thing that matters is whether the output continually makes itself stronger (positive feedback loop) or whether the output serves as an input that pushes the system in the opposite direction (negative feedback loop).

The slowing down is actually driving the system in the same direction (slowing down even more), which is why it’s a positive feedback loop.

The more it slows down, the more it slows down.

I’m trying to think of a way to turn this into a negative feedback loop. This will be a bit of a stretch, but stick with me. Say that the spacecraft is orbiting a planet that has an atmosphere that is rotating around the planet extremely fast in a perpetual storm and it only goes in one direction. This also happens to be the same direction the spacecraft is orbiting.

Now instead of colliding with particles that make it slow down, the spacecraft is getting hit by particles from behind that cause it to speed up.

If the spacecraft tries to slow down, it will fall farther into the atmosphere and get hit from behind by more of these fast moving particles, that push the spacecraft slightly causing it to speed up. So in this aggressively hypothetical case, the output (the spacecraft slowing down) serves as an input that drives the system in the opposite direction (speeding up again). This would be a negative feedback loop.

Example 2

Okay, now let’s do a real example of a negative feedback loop.

Do you know how the wheels of a train stay on those narrow metal tracks?
This is a weird one. We generally think of wheels as being…y’know…wheel shaped. But that’s not how train wheels look. The wheels of trains are actually shaped like cones, with a narrow end and a wider end. The narrow end of the cone is on the inside, and the wider end is on the outside.

When a train goes around a curve, the wheels on the outside of the curve have to travel farther than the wheels on the inside of the curve. If the wheels were perfectly round, they would both have to travel the same distance, which would cause problems. But because of their conical shape, when a train goes around a curve, the wheels on the outside of the curve will naturally move up onto their wider end, while the wheels on the inside of the curve will move down onto their narrower end. This allows both sets of wheels to travel different distances while still staying on the tracks.

Likewise, if the train starts to drift to one side, the conical shape of the wheels will cause it to naturally correct itself and stay on the tracks. If the train drifts to the left, that action of drifting to the left moves the train onto the wider end of their left wheels and the narrower end of their right wheels. This creates a restoring force that pushes the train back towards the center of the tracks.

This is a negative feedback loop. The output (the train drifting to the left) serves as an input that drives the system in the opposite direction (pushing it back towards the center of the tracks).

So you can think of a negative feedback loop as a self-correcting mechanism that helps maintain stability, while a positive feedback loop is a self-reinforcing mechanism that can lead to exponential growth or decline.