A stunning array of technological innovations has been woven into the day-to-day fabric of early-21st-century life: clothing made from recycled plastic Coke bottles; phones that run on high-frequency microwave radiation; snack-food science capable of containing such unruly forms as s'mores and peanut butter and jelly within the confines of a toaster pastry. Yet for some of us, such marvels are slim compensation for the unrealized possibilities of another device, one promised throughout childhood and never delivered. Where are our jetpacks?