The world around us is full of patterns that repeat themselves over periods of seconds, hours, days and months; The sun rises and sets, seasons come and go and the moon waxes and wanes. This is true for living animals, plants and humans as well as inanimate objects. People wake and sleep, hearts beat in a steady rhythm, and flowers open and close again. 

 

Humans, animals and even plants and bacteria possess a Circadian rhythm. This is a 24 hour rhythm that regulates and keeps various bodily functions in check. Most obviously, you get sleepy in the evening and wake up in the morning. In essence, we have an internal clock that sets off different alarms at regularly times every day. As jetlag illustrates, we don’t automatically get sleepy when it gets dark outside, but have to wait until our Circadian clock tells us it’s bedtime.

 

Tamarind tree flower and frond.

Tamarind tree flower and frond.

 

In the 4th century B.C., Androsthenes, an officer under Alexander The Great, noticed that tamarind trees would flower in the morning and close at night. This was clearly periodic behaviour, and there were two possible explanations for how the trees followed this 24 hour cycle. Either they simply detected sunrise & sunset, or the trees possessed an internal alarm clock, which triggered a new cycle of opening and closing every 24 hours. An experiment could prove which mechanism was at work: Place a plant in a sealed, dark room, where the temperature & light levels don’t change over, and see what happens. If the plant still flowers & closes on a daily rhythm, it has an internal clock. This experiment was performed almost 1500 years after Androsthenes’ observation, and proved that trees possessed an internal clock.

 

In scientific terms, each of these clocks is an ‘oscillator,’ and they are present on every biological level, from single cells to organs, animals and whole populations. Bacteria divide and grow every few hours, the pancreas releases insulin on a regular schedule each day, and menstrual cycles repeat every 4 weeks. In the wild, population levels of prey and predator are seldom stable, and these oscillate too. Wolves eat too many rabbits, causing a drop in rabbit population, followed by a drop in wolf population as wolves starve, which then allows rabbits to reproduce more, and the rabbit population grows. Now able to feed again, the wolf population begins to grow, and the cycle starts again.

 

 

Graph showing two sinusoidal lines out of phase, reflecting oscillations in predator and prey populations

Oscillations in predator and prey populations.

 

Each of these processes are controlled and implemented by a different system, but they exhibit common features that are present in any oscillator. The most important such feature is feedback. Feedback can take many different forms, but basically, it enables a machine, system or organism to monitor itself and change what it’s doing. For a thermostat, the thermostat measures the temperature of a room, and whenever the temperature deviates from the set point, the heating gets turned up or down a notch. In this way, the room’s heating is constantly changing in response to changes in temperature. Similarly, your body can monitor and maintain its temperature, so if you get a bit too hot, you will start sweating and cool down, and if you get a bit too cold, your heart will start pumping more blood around your body to heat it up.

 

The feedback present in thermostats and body temperature regulation is negative feedback. When the temperature changes from the set point, the feedback brings the temperature back towards the set point. If you get too hot, you start cooling down, and vice versa. It’s like a ball stuck at the bottom of a valley. Whichever way you kick the ball, it will roll back down to the middle. That’s gravity providing negative feedback, keeping the ball in the same place! In contrast, imagine a ball on top of a hill. If the ball gets nudged out of place, it will roll away, and keep rolling. This is called positive feedback. In both cases, gravity is the source of the feedback, along with the particulars of the system: Is the ball on top of a hill, or at the bottom of a valley?

 

Graph showing sinusoidal change in core body temperature over the course of the day. Average: ~37.0 degrees celsius.

Change in core body temperature over the course of a day.

 

Another example of positive feedback is population count. If population increases, more people reproduce, which in turn leads to a faster growing population. A small increase in population can eventually lead to a much larger population as more people are born, and more go on to reproduce. Positive feedback occurs in chain reactions in nuclear reactors too, where each reaction causes further reactions to occur.

 

A nuclear chain reaction starting with one neutron and nucleus, doubling in two steps to end with 4 fissile nuclei.

Nuclear reactions can grow rapidly, but their progress is controlled by absorption and moderation of neutrons.

 

So why doesn’t the human population grow indefinitely? Why doesn’t every nuclear reaction get bigger and bigger until it leads to a Chernobyl-like disaster? Well, systems can have multiple types of feedback happening at the same time, and at different strengths. Positive feedback in the form of more couples reproducing can cause population to increase, while scarce food, disease and violence act as negative feedback, causing populations to reduce. In fact, for millenia, human population grew slowly. The positive and negative mechanisms that caused people to be born and die, respectively, were quite balanced. Scientific advances in the form of medicine, and agriculture enabled more people to be fed, and caused less people to die of disease. This reduced the amount of negative feedback in our Earth-wide system, and with less people dying, population started growing rapidly, and continues to do so. That said, human population may still stabilise, even if people are not starving or dying of disease; as people get wealthier, they tend to have less children, which slows down population growth, and is a form of negative feedback.

 

Graph showing world population growth, 1700 - 2100, including total population and annual population growth. Includes projections from the present to 2100.Source - Our World In Data

World population growth, 1700 – 2100, including projections. Source: Our World In Data

 

Coming back to the topic of oscillators, the interplay of positive and negative feedback is what makes oscillators possible. The positive feedback pushes the system away from the neutral, normal or zero position, while the negative feedback tries to bring it back towards neutral. The system might start with strong positive feedback, and the negative feedback grows in strength over time, which eventually brings the system back to the neutral point. A good example is a pendulum, where the pendulum might start hanging straight down, and then get pushed, beginning the oscillatory process. The pendulum will keep moving until gravity slows it down and turns it back towards the central point. But the pendulum will keep moving when it reaches the centrepoint and the process will repeat many times – it is an oscillator! 

 

Instead of pendulums, nature’s oscillators are made up of many different types of interacting objects. In the case of bacteria, different genes and chemicals interact with each other to produce periodic changes in the concentration of particular chemicals. In the animal world, as with the wolf and rabbit populations mentioned above, predators hunt prey, sometimes overhunting, leading to oscillations in the population of both species. We also see oscillations in the atmosphere and in tidal patterns, such as El Niño. Most of these oscillations are more complicated than a simple pendulum. They might involve a number of different chemicals, animals or tides interacting with each other in complicated ways and result in more complicated oscillations than we see with a pendulum, but in each case, you will find both positive and negative feedback driving the phenomenon.



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