To Save Drowning People, Ask Yourself “What Would Light Do?”

Imagine you’re a lifeguard and you see someone struggling to stay afloat. Being a responsible lifeguard, you want to get to them as quickly as possible. You’re pretty fast when swimming, but even faster running on sand. So what’s the quickest route to get to the swimmer? It may not sound like it, but this puzzle, which was laid out by famed physicist Richard Feynman, is actually an analogy for the behavior of light. Although I first read it over 10 years ago, its lesson about how light travels has stuck with me.

At first thought you might consider whether a straight line (path A) is the fastest path. This is indeed the shortest one, but it isn’t the quickest. You can do better, because if you run further along the beach, you’ll cover more distance on land than in water. And since you’re faster on land, you get there in less time.

So maybe option B is quickest? Of all the choices, this path involves the least swimming. But that’s not right either. Although you’re moving faster now, this route is too long, and it slows you down.

As you can see, there’s a trade-off here. As Feynman” The quickest route is C—a very specific path that lies somewhere between A and B.