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Computational physicist Sharon Glotzer is uncovering the rules by which complex collective phenomena emerge from simple building blocks.
By Natalie Wolchover
March 8, 2017
Sharon Glotzer has made a number of career-shifting discoveries, each one the kind “that completely changes the way you look at the world,” she said, “and causes you to say, ‘Wow, I need to follow this.’”
A theoretical soft condensed matter physicist by training who now heads a thriving 33-person research group spanning three departments at the University of Michigan in Ann Arbor, Glotzer uses computer simulations to study emergence — the phenomenon whereby simple objects give rise to surprising collective behaviors. “When flocks of starlings make these incredible patterns in the sky that look like they’re not even real, the way they’re changing constantly — people have been seeing those patterns since people were on the planet,” she said. “But only recently have scientists started to ask the question, how do they do that? How are the birds communicating so that it seems like they’re all following a blueprint?”
Glotzer is searching for the fundamental principles that govern how macroscopic properties emerge from microscopic interactions and arrangements. One big breakthrough came in the late 1990s when she was a young researcher at the National Institute of Standards and Technology in Gaithersburg, Maryland. She and her team developed some of the earliest and best computer simulations of liquids approaching the transition into glass, a common yet mysterious phase of matter in which atoms are stuck in place, but not crystallized. The simulations revealed strings of fast-moving atoms that glide through the otherwise frustrated material like a conga line. Similar flow patterns were later also observed in granular systems, crowds and traffic jams. The findings demonstrated the ability of simulations to illuminate emergent phenomena.
A more recent “wow” moment occurred in 2009 when Glotzer and her group at Michigan discovered that entropy, a concept commonly conflated with disorder, can actually organize things. Their simulations showed that entropy drives simple pyramidal shapes called tetrahedra to spontaneously assemble into a quasicrystal — a spatial pattern so complex that it never exactly repeats. The discovery was the first indication of the powerful, paradoxical role that entropy plays in the emergence of complexity and order.
Lately, Glotzer and company have been engaged in what she calls “digital alchemy.” Let’s say a materials scientist wants to create a specific structure or material. Glotzer’s team can reverse-engineer the shape of the microscopic building blocks that will assemble themselves into the desired form. It’s like whipping up gold from scratch — only in modern times, the coveted substance might be a colloidal crystal or macromolecular assembly.
Glotzer ultimately seeks the rules that govern emergence in general: a single framework for describing self-assembling quasicrystals, crystallizing proteins, or living cells that spontaneously arise from simple precursors. She discussed her eureka-studded path with Quanta Magazine in February; a condensed and edited version of the interview follows.
Reprinted by permission from Macmillan Publishers Ltd: Nature 462, 773-777, copyright (2009)
QUANTA MAGAZINE: Tell me about your famous 2009 Nature paper that linked self-assembly with entropy….