Imagine you are a photon, a packet of light. You are a tiny blip of energy, hurtling through the universe on your own. But you have a twin, another photon to whom you have been intimately connected since the day you were born. No matter what distance separates you, be it the width of a lab bench or the breadth of the universe, you mirror each other. Whatever happens to your twin instantaneously affects you, and vice versa. You are like the mouse siblings in "An American Tail," wrenched apart by fate but feeling the same feelings and singing the same song beneath the same glowing moon.