There are over 7,000,000,000 people in the world, over 320,000,000 of whom live in the United States alone. When I try to picture populations this big, I go “okay, a thousand people would be how many fit in a really big room, and then a million is… a lot more than that, and a billion is… even more?”
So here’s a way I’ve been recently trying to think about big populations like these, namely: How many rare events are happening to someone right now? Here are some back-of-the-envelope, order-of-magnitude calculations. No promises on accuracy, but the gist is about right.
Say a person’s life contains 30,000 days on average (a little over eighty years). That means that for any roughly once-in-a-lifetime event you can think of—having a baby, losing a parent, your house catching on fire, etc., on any given day about 10,000 people in the U.S. (320 million divided by 30 thousand) are experiencing it. That’s about 500 people every hour—one every few seconds! And if you pick any two once-in-a-lifetime events (as long as they’re reasonably independent of each other), 10 or so people in the world are experiencing them both today. Someone just got lost their job and had their first baby. Someone just picked up their future favorite book on the same day that they met their future spouse.
Sometimes the scope of the world is better expressed not by how large our population is, but by how commonplace the improbable is.