One of the greatest challenges in all of science is to understand life. How does it arise? How does it evolve and change over time? How does it work?

The answers are proving to be complex in the extreme, because life does nothing in a simple way. But then, that may be why more and more physicists are being drawn to this challenge: they can bring with them decades of experience in understanding complex systems generally.

Even leaving life aside, after all, the world is rife with complex systems. Examples range from turbulence in fluids to the interaction of particles within atoms, and from economic markets to the origins of the large-scale structure of the universe. Much of physicists’ insight into this kind of complexity has come—of course—through rapid advances in computer simulation. But just as important has been their deeper understanding of the nonlinear mathematics that describe complex systems. The hallmark of nonlinear equations is that they are (usually) simple to write down but can have surprising and unexpected results.­ For example, a nonlinear system may suddenly go from apparent stability to completely chaotic behavior—one reason why nonlinear mathematics is often referred to in the popular media as chaos theory.

The physicists’ work, in turn, is part of a much broader effort to understand complexity that cuts across a wide variety of disciplines, from mathematics and computer science to economics and anthropology. NSF has been a major supporter of this effort over the years—perhaps most famously for its early backing of the independent Santa Fe Institute in New Mexico, but also through its many grants to individual academic researchers, as well as through its creation of centers for complexity research at Northwestern University, the University of Illinois and elsewhere.

To see how these techniques might apply to life, consider the human genome—or for that matter, any of the dozens of other genomes that are now being sequenced, from chimp to mouse to mustard weed. The problem is that a genome by itself is just a blueprint, a parts list, a set of instructions for building the zillions of protein molecules that actually do most of the work in the cell. What’s missing is a deep understanding of how these components go together to create a working system. And the reason it’s missing is complexity. Even the simplest chemical process in the cell may involve dozens of proteins made by dozens of genes, each of which is regulated many other genes.

Thus the opening for physicists. The hope is that by applying a combination of computer modeling and complexity analysis to these multiple interactions, they can help shed light on some key questions. For example, what cellular conditions cause viruses to replicate and infest their host or instead, lie dormant to wait for conditions to improve?

Complexity analysis may also explain how organisms manage to survive even though the precise genetic blueprint varies from one individual to the next. Rather than the tightly controlled chemistry of a test-tube experiment, organisms from bacteria to humans may have evolved complex networks of reactions that get the job done, despite a wide range of individual variation.

Similar nonlinear interactions exist in ecology, and the physics of complex systems may help unravel the principles that govern how populations vary over time. Species thrive or decline depending on complex interactions among individuals, competition from other species and environmental conditions beyond their control, ranging from the phase of the moon to the harshness of the weather.

At the farthest frontier of complexity research, physicists are also contemplating the physical laws from which human consciousness emerges. Competing theories abound. Perhaps consciousness emerges from the biochemical pathways of intertwined neurons once brain-size passes a certain threshold. Or perhaps the brain uses microscopic structures within neurons to create a highly advanced quantum computer, able to perform complex calculations, such as those needed to talk, catch a ball or ponder the meaning of life.

Pushing physics advances in modeling complex systems toward better understanding of how life works will be a major research challenge for the 21 st century.

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