Protecting Us With Physics
I spent several decades as a research scientist studying the behavior of what are known as complex systems. A complex system is one in which the parts that make up the system interact in such a way that the whole can respond to changes in its surroundings by adapting to these changes. Examples include ecological systems or even the complex adaptive systems that make up the molecular machinery inside a biological cell.
Since my training is in Chemistry and Physics, my research often focused on very small things, such as the complex molecular systems that make up a single cell or the metabolic and other pathways that connect cells to one another. Over time, my interests broadened and I soon found myself working with medical doctors and neuroscientists, trying to apply the insights I had arrived at about complex molecular systems to networks of neurons in the brain, a most complex system indeed.
The complex systems approach was very successful in understanding even something as complex as the brain. We found that the methods and concepts we had developed to describe the complex behavior of molecular systems translated well to the cellular level and beyond. I was not terribly surprised at this outcome, since I knew that the principles that govern the behavior of complex systems are found at all levels of organization, from the molecular to the cellular, tissue and organ level, even to the whole organism. Nevertheless, I was surprised when, after decades of doing research and working as a university professor, I moved into work in science policy, and found that these same ideas were also helping policy-makers understand people and our behavior.
Social groups are another example of a complex system, and have, for years, been fruitfully applying the ideas developed by physicists and mathematicians for describing complex systems to communities, organizations—even entire societies. I learned about one surprising example from Prof. Dirk Helbing of ETH in Switzerland who has applied ideas about self-organizing fluids, a well-studied example of a complex system, to groups of pedestrians. He has found, for example, that crowds of people can move gently at times, like a smoothly flowing stream, but at other times the flow of people can become “turbulent.” This can be a very scary situation, since a “turbulent” crowd is essentially a stampede. The annual Hajj ritual that attracts millions of Muslim pilgrims to Mecca is one area where Helbing’s ideas have had surprising and profound impact. In recent years, tragic stampedes have occurred during the Hajj, leading to the deaths of hundreds—251 people died in 2004 and 362 more in 2006, all trampled under the feet of panicking pilgrims.
Helbing’s work attracted the attention of Saudi authorities, who hoped to prevent more such tragedies. After studying video footage of the crowd at Mecca, Helbing noticed that the movement near a narrow entryway onto the Jamarat bridge was smooth like slow-moving water until just before the stampede broke out. The videos show that, as the crowding near the bottleneck becomes heavier, the flow goes from smooth to a stop-and-go forward motion Helbing calls “shock waves.” Suddenly, without warning, a different kind of movement begins– rapid back and forth motions in a direction perpendicular to the flow of the crowd.
This lateral motion, explains Helbing, is a signature that the flow is about to go turbulent, like water flowing through a hose when the faucet is suddenly turned on full-blast. Indeed, shortly after lateral movements appear in the crowd, panic breaks out. The video is difficult to watch. Even though the footage shows only dots on the screen, it is hard to escape the knowledge that the dots are people, some of whom are falling under the feet of others and dying. More details of the study can be viewed here.
Helbing recommended a redesign of the entryway to the bridge, including some one-way lanes. These changes in design take into account the self-organization of the crowd that Helbing observed in the video data. It is interesting to note that since Helbing’s ideas have been implemented, no further stampedes have occurred.
I was able to experience some of the benefits of Helbing’s work myself when, in January, 2009, I attended the inauguration of the new US President—along with over a million and a half other people. Although I didn’t know it at the time, Prof. Helbing and his colleague, Anders Fredrick Johansson, also of ETH in Switzerland, worked with officials who were organizing the inaugural festivities in Washington DC. The group made changes in the planned layout of barriers, as explained in an article in Popular Mechanics, that incorporated the same ideas as those used to redesign the crowd-flow systems in Mecca.
I’ve included a photo I took of the crowds that day, one of the few in which I could get a good look at a large number of people. Most of the day I spent looking at somebody’s back, unable to see much at all, and I occasionally thought about those tragic events that occur when a panic breaks out in a crowd of people. I certainly didn’t want that to happen to us, and I was very relieved when we made it out, safe, and were able to move into a more open area.
If I had known at the time that ideas from the physics of self-organizing fluids had been used to design the protective barriers, I would have felt safer, since I’d seen how well this had worked in Mecca. I, for one, am very pleased that policy makers, in my country as well as in others, are taking ideas from science, solid ideas that describe how people behave, and applying them to their decision making processes. Protecting us with Physics seems, to me, to be a most excellent strategy.
Raima Larter holds a PhD in Physical Chemistry and served, for over twenty years, as faculty member, chairperson and associate dean at Indiana University-Purdue University at Indianapolis (IUPUI) in the United States. She currently works as a freelance writer and publishes a blog, Complexity Simplified.