Forever Changes

5:30 a.m., Brianna Pelletier gets ready for her daily pounding. As she lies on the couch, her dad beats her chest, then her back, coaxing the mucus out of her lungs. The pounding doesn't take care of everything. Brianna's held out for a long time, but a body with cystic fibrosis doesn't last forever. It doesn't matter that Brianna has a brilliant mathematical mind or that she's a shoo-in for MIT. Or even that her two best friends are beautiful, popular, and loyal. In the grand scheme of things, none of that stuff matters at all. The standard life, lasting maybe seventy-five years, is no more than a speck in the sum total of the universe. At eighteen, and doubting she'll make nineteen, Brianna is practically a nonentity. Of course she's done the math. But in her senior year of high school, Brianna learns of another kind of math, in which an infinitely small, near-zero quantity can have profound effects on an entire system. If these tiny quantities didn't exist, things wouldn't make the same sense.