# How chaos theory helps explain the weirdness of the Covid-19 pandemic

The pandemic, chaos, and coming to terms with uncertainty.

As we’ve learned, it takes just one person infected with Covid-19 to unleash chaos.

In Washington state, a person with the virus attended a choir practice, and more than half of the other singers subsequently got sick. In South Korea, a 29-year-old man went out to nightclubs; he was Covid-19 positive, and he has since been linked to at least 54 new cases. In China, nine people sitting in the path of an air conditioning vent in a restaurant all got sick, most likely from one person, as the duct blew viral particles across their faces.

Small things could have changed these outcomes. The clubber could have decided to watch TV instead of going out dancing. If the choir practice was rescheduled for the next day, maybe the person would have felt sick and stayed home. The air conditioner in the restaurant could have been turned off.

As Stephen Kissler, an infectious disease modeler, puts it, “Little shifts can have really disproportionately sized impacts” in a pandemic. And scientists have a name for systems that operate like this: chaos.

What I’ve found, talking to scientists like Kissler, is that knowing this pandemic is chaotic is one of the keys to understanding why the future is uncertain. But it can also help us appreciate why this uncertainty need not be so debilitating.

### The double pendulum teaches us about the nature of chaos

There’s a simple mechanism that is helping me understand the many possible futures we face with the Covid-19 pandemic.

It’s the double pendulum, and as a physical object, it’s very simple: A pendulum (a string and a weight) is attached to the bottom of another. Its movement is explained by the laws of motion written by Isaac Newton hundreds of years ago.

But slight changes in the initial condition of the pendulum — say it starts its swing from a little higher up, or if the weight of the pendulum balls is a little heavier, or one of the pendulum arms is a bit longer than the other — lead to wildly different outcomes that are very hard to predict.

The double pendulum is chaotic because the motion of the first pendulum influences the motion of the second, which then influences the entire apparatus. There isn’t a simple scale or ratio to describe how the inputs relate to the outputs. A one-gram change to the weight of a pendulum ball can result in a very different swing pattern than a two-gram change.

It teaches us to understand the mechanics of a system — the science of how it works — without being able to precisely predict its future. It helps us visualize how something that seems like it should be linear and predictable just isn’t.

The double pendulum shows us that simple systems aren’t simple at all. Complicated ones, then? God knows.

### The pandemic is chaos. We can still understand it.

An outbreak isn’t a double pendulum; it’s much more convoluted. Myriad chains of events, operating in overlapping networks, conspire to chart its course.

That’s why, when pressed, epidemiologists have to say they don’t know what’s going to happen.

Still, they know the mechanics of outbreaks. The chaos “doesn’t necessarily mean we know nothing,” Kissler says. They understand the conditions that make an outbreak worse and the conditions that make it better.

There is a tough tension of the current moment that we all need to work through: The future is clouded in chaos, but we know the mechanics of this system. We know what’s possible. Just as we know that, when pushed, pendulums swing, we can sense the direction the future of the Covid-19 pandemic is heading.

Here are the mechanics. Scientists know that if we let up on social distancing, without an alternative plan in place, the virus can infect more people. They know this virus is likely to persist for at least a few years without a vaccine. They know it’s very contagious. That it’s very deadly. They also know that its pandemic potential is hardly spent, and that most of the population of the United States and the world is still vulnerable to it.

And so, scientists fear big resurgences of the virus over the next months and years, and their fears are grounded in history and scientific analysis. But yet, they say, their view of the future is more occluded than ever, as the response to the pandemic grows more varied.

“I think we have more uncertainty now than almost any other time,” epidemiologist Eleanor Murray, at Boston University, recently told me. “Now at this moment, some places are thinking about continuing lockdown. Some places are thinking about opening. Opening means different things in different places. There’s such a range of possible actions that different areas are taking, that it’s really hard to predict.” Our actions influence the results, which then, in turn, influence our actions.

Will residents keep up with mask-wearing and social distancing, even when their leaders relax regulations? Plus, there are scientific questions about the virus still not understood: Will it diminish transmission in a seasonal pattern? Do children contribute greatly to its spread? How long does immunity last after an infection? Why do some people breathe out more of the virus than others? The answers to these questions will influence the future, and we do not know the answers.

Scientists are still unraveling what makes the difference between a sprawling outbreak in one city and a more manageable one in another. Some of it is the result of policy, some is the result of demographics, some is about structural inequality and racism, and some comes down to individual behavior. Some of it is just luck. That’s chaos for you.

### Chaos rules our world. But it doesn’t mean we’re powerless.

“I don’t see uncertainty as a lack of knowledge,” says Philipp Lorenz-Spreen, a physicist who studies the chaos of a different sort of viral dynamics. “I think it’s a fundamental part of how our world works. It’s not our fault we do not know where this all will go.”

Newton clearly told us what happens when an object drops from the sky. But follow his laws, and find that the path of a double pendulum is very, very difficult to predict. Climate scientists clearly tell us adding CO2 to the air will increase global temperatures. Yet they argue about when the worst effects of climate change will be felt and how bad it will be. Epidemiologists are clearly telling us what happens when you bring masses of people together during a pandemic. But they can’t tell us the exact shape this outbreak will take.

Thinking about the future of the pandemic means wrestling with uncertainty, both personally and as a wider community. It also means dealing with what’s not likely to happen: The virus disappears in the next several months. If it does, it will do so for reasons scientists do not understand or can currently explain.

There’s a lot in this chaotic pandemic system we can’t control. Let’s be serious about the ones we can.

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