Feedback

Feedback.

If you grew up during the 80s, you might hear this word and think about interference on a TV that uses rabbit ears. Those rabbit ears (or aluminum foil, in a pinch) would amplify not only the television signal (helpful), but also all kinds of background electromagnetic radiation (not helpful). The noise was fed back in, so to speak.

You might think about feedback loops, like the thermostat in your house when you were growing up. You might know that there’s a little bit of mercury in there, and when the temperature in your house rises, the mercury—a very sensitive metal—expands in proportion to the rising temperature. That mercury, in turn, flips a switch that ultimately turns the heat off when it reaches a certain temp.

Maybe your mind turns instead to feedback on a project you’re working on. This metaphor for the physical sort of feedback has become the dominant way the word feedback is used today.

All of these ways we use feedback are unique, but they all have something important in common too. They describe a system where one of the inputs goes back in to the system itself. This feedback can form a sort of self-regulation, like that thermostat I mentioned, or it can cause a runaway effect, like how a nuclear reactor creates a critical mass.

Either way, something created by the system itself goes back in and affects that same system. This is the essence of feedback.

The word is a 20th century invention, but the idea is very old. 2500 years ago, Heraclitus talked about the interplay of opposites in a dynamic state of balance in nature, like a constant tug of war between forces. Similarly, the Tao Te Ching introduces abstract opposites called yin and yang—dark and light in a not-quite-equilibrium.

By the time of Heron of Alexandria, Roman engineers were beginning to create machines that involved feedback. One of these was a fountain that kept itself full and running all the time:

Heron's Fountain was a self-replenishing fountain that seemed like magic. It used the principles of air pressure and displacement to create a continuous display of water flowing upwards.

This wasn’t a true feedback loop like a thermostat, but it was impressive for a closed system. Heron even built an early version of a steam engine:

The Aeolipile consisted of a hollow sphere mounted on a pair of hollow tubes. Water was poured into the sphere, and then it was heated, turning the water into steam. The steam escaped from the sphere through bent tubes projecting from its equator, creating a strong force that could make the sphere spin around.

It was another innovation in feedback that allowed James Watt to make improvements to steam engines just as the fallout from the American and French Revolutions was beginning to be felt.

Watt’s governor adjusted the speed of the engine automatically based on the output. He devised a really clever mechanism that let less steam in as the speed went up, and he did this by using the centrifugal force. As the engine spun faster, balls attached to a rod in the middle would move outward, striking a lever and causing a valve to close a bit, letting less steam in. The opposite happened whenever it slowed down, creating a nice dynamic balance.

The material world wasn’t the only place where feedback has come to prominence during the modern era. Writing around the same time as Watts was working, Adam Smith spoke of an invisible hand that would lead to better outcomes than heavy regulation, and it was due to feedback and dynamic equilibrium that he thought it might work:

Smith also noted that if prices are allowed to fluctuate as demand rises or falls, then resources are allocated more efficiently. When the government dictated what could be produced, how it could be produced, and how much people should be paid for their labor, people were unable to respond to market signals—the needs and wants of other people.

One additional example really ushers in the modern era: Darwin’s theory of natural selection. Today, we owe our oxygen-rich atmosphere to early cyanobacteria:

About 2.4 billion years ago, there was already lots of life on our planet, but the atmosphere was mostly nitrogen, carbon dioxide, and methane. Blue-green algae by now had begun to turn the Sun’s light into useful energy, using the process we call photosynthesis to convert carbon dioxide and water into sugars and oxygen.

These algae (also called cyanobacteria) were producing oxygen, but they were fragile. By way of contrast, the early eukaryotes—single-celled organisms with a nucleus—couldn’t photosynthesize, but they were much tougher, with an exterior membrane that kept potentially harmful molecules out of the cell.

One day, an early eukaryote engulfed a cyanobacteria through a process biologists call phagocytosis. Instead of digesting the cyanobacteria, a symbiotic relationship developed where the eukaryote was able to harness the photosynthesis of the cyanobacteria while also getting the benefit of a sturdier home that could absorb nutrients through something like eating.

Over time, any eukaryotes who had eaten chloroplasts were able to outcompete the others in particular environments. The little cyanobacteria living inside the eukaryotes evolved to become little organelles, which is how today’s plants and algae produce oxygen.

This isn’t an exhaustive study of feedback, but I’m hopeful that the different examples are more than enough to get your brain cooking today. Of course, there’s one additional request I need from you in order to continue to make this place better: feedback.