Finding My Way into the Sandbox

Bulbul Chakraborty

My current scientific obsession is with understanding systems that do not fit neatly into known paradigms.  We all know that walking on a sand dune is much more exhausting than walking on a paved path. The sand grains respond to the pressures our feet exert on them in ways that surprise us: they flow around our feet while also supporting our weight. I ask why. This question leads me down paths I think I’ve never explored before, but then I find that one starts to look familiar, reminds me of other paths I’ve walked in search of answers to other questions. I feel elated! Sometimes these familiar paths lead to dead ends, but they offer clues that help me discover other paths, and the quest continues.  I gravitate towards physics questions that push me to go out on a limb.  I tend to take lesser known paths.  I have often wondered why.  Is the way I practice physics a reflection of the experiences that shaped me as a person?

Prof. Bulbul Chakraborty

I grew up in India in a large, close-knit family. My siblings, five of them, were much older than me. From my young perspective, they were five close friends who were constantly engaged in exciting new adventures, exploring both intellectual and physical boundaries. I was too young to participate, but I could sense their excitement as they pushed limits, coming up with ingenious ways of escaping our parents’ watchful eyes.  My siblings and parents taught me to love books and to love the outdoors.  I was constantly immersed in stories: stories I read in books, stories they told me, and stories that unfolded in front of me as we explored the outside world together.  And then there was that most fascinating and mystifying story: the myth of the Goddess—the Goddess that emerged from the coalescence of all the energy in the universe to defeat the demons and establish peace in heaven and on earth.  But the Goddess was also a wife and a mother and a daughter who came to visit her family on earth every fall, and we celebrated her homecoming!  This myth still fascinates me, and this portrait of a woman has been my lifelong companion.

What drew me into this field was not these phenomena. I was not consciously aware that these were interesting physics questions. What drew me in was a challenge.

Growing up, I was in awe of my siblings. They did things I thought I could never do. They were singers, dancers, budding scientists and engineers. I could not compete with them on their turf, so I found my own. Once, I went on a solitary exploration to collect all of the different species of wild grass that I could find. Another time, someone gave me a box of 3D puzzles, and I fell in love with those.  By the time I was in middle school, I was the only child left at home.  That is when my father and I started bonding deeply.  He was a civil engineer with an infectious enthusiasm for mathematics.  It is at this time, I think, that I realized I liked mathematics.  

Looking back, I think I was always attracted to what challenged me.  It could be a mathematical puzzle, a song I was told wasn’t easy to sing, a book I was told I shouldn’t attempt to read because I was too young. What drew me in was the challenge—the thrill of uncovering something new.  A new way of singing a set of notes.  A pattern in a puzzle. A subtle message in a book.

My path through physics has also been driven by perceived challenges rather than a deep desire to understand a particular physical phenomenon. My most recent obsession, which has lasted more than a decade now, is to understand the laws that govern the behavior of “sand.” The quotes imply a class of materials with sand grains as a paradigm. If we pause to think, it seems amazing that we, physicists, do not have a theory that can predict the angle of repose of a sandpile, or explain how a grain silo jams or unjams, or why, in these YouTube videos, people can be seen running on vats of “oobleck” into which they sink as soon as they stop running.   

What drew me into this field was not these phenomena.  I was not consciously aware that these were interesting physics questions. What drew me in was a challenge. During a visit to a university to give a talk, a young colleague showed me a paper addressing the question of how stresses get transmitted in granular materials. My colleague couldn’t see the connection between what seemed to him an esoteric theoretical framework and the very real granular problem. As I read the paper, I realized that the esoteric theory connected closely to one that I had explored in connection with a very different problem in quantum spin systems. That hooked me in.  Could it really be that these problems in granular materials might be understood using a theoretical formalism with its roots in quantum spin systems? 

As I dug deeper, I discovered the fascinating world of sand. The elusive, beguiling behavior of these materials slowly opened up in front of me. I had “seen” them before, but not really!  I had amazing mentors who guided me through this world. The theoretical challenge got connected to real physical phenomena.  Today, I am as enamored by the beauty of these physical phenomena as I am by the beauty of the mathematical framework, but it did not start that way. It started with a challenge.  I am always pulled toward a light at the end of a tunnel, but I know that as I get closer, other tunnels will pull me in, and who knows maybe I will enter one that will open my eyes to wonders of nature that I’d never paid any attention to.

Am I still on my solitary exploration collecting different species of wild grass? Maybe!