Monday, January 21, 2013

Inner Life of Stuff

The Up-Goer Five tool challenges you:

Can you explain a hard idea using only the ten hundred most used words? It's not very easy. Type in the box to try it out.

More about the tool here from its creator, Theo Sanderson. I saw it in action first at Suvrat Kher's post describing his research. Here's my attempt, a possible abstract for an upcoming talk on "computational modeling of microstructures and their evolution."

Inner Life of Stuff

We study pieces of matter. Each such piece, called stuff, could be water-like or rock-like, and we are especially interested in what goes on inside rock-like stuff. From the outside, such stuff may look as if it is the same at every place inside, but it is not! It turns out that, much as a house is made up of rooms, stuff is made up of lots and lots of small things, called parts. But, while rooms in a house can't grow bigger or smaller, parts inside stuff can! Some parts may grow bigger while others grow smaller. All this happens, quite easily even, when stuff gets hot.

So, here is the Big Idea: though stuff has no life to speak of, it enjoys some seriously beautiful inner life!

Why is this inner life of stuff important? Because it decides how well stuff works -- some kinds of packing are better than the others. So we need to understand how the parts come to be packed the way they are, and to figure out other ways of packing that make stuff work better. With this understanding, we can do some pretty cool things. We can take soft stuff and make it hard (or go the other way). Or, we can decide what, and how much, can move through it. Many things we use in our lives today -- lights, cars, flying air buses, computers -- are possible because we know how to control and change the inner life of stuff.

So, how do we get to understand the inner life of stuff? By building ideas about how parts change their form. Actually, we take a slightly different line: instead of focusing on parts, we focus on the walls between the parts. When a wall moves, the part on one side grows bigger, while that on the other side grows smaller. By watching walls move, we figure out how parts change their form.

While this is all easy to say, it is actually very, very hard to pull off. Instead of trying to crack the problem by writing on paper (and feeling let down!), we build make-believe stuff inside a computer. Just like real stuff has real parts and walls, the make-believe stuff has its own parts and walls. We then ask the computer to let its not-so-real walls move around just like real walls do. By watching changes in the make-believe stuff brought about by its moving walls, we get a good idea about changes in real stuff.

This talk is about how we study the inner life of stuff, and about what we get out of our studies. We start with walls, and why and how they move. We then show how we build make-believe stuff in a computer to watch moving walls and form-changing parts. We then talk about two very different kinds of stuff to show how our ideas work. Near the end of the talk, we have some things to say about looking at and studying stuff using computers, and about how it is not very different from studying stuff using things (such as focusing glasses) other than computers or paper.


  1. gaddeswarup said...

    "All this happens, quite easily even, when stuff gets hot."
    Which parts get smaller in this case?

  2. Abi said...

    Swarup: Think of a collection of bubbles (like those at the head of a glass of beer). With time, some bubbles grow at the expense of their neighbors (which shrink).

    This sort of stuff happens in many everyday materials ("stuff" in the post): they contain grains ("parts" in the post) of many different sizes packed quite tightly inside them, and at high temperatures, some grains grow at the expense of the others (which shrink). While modeling this process, it's (sometimes) useful to focus only on the walls between the grains, and compute their migration. This is what we do -- I hope this came through okay in the "ten hundred word" version.