There’s a great scene in the new biopic of Stephen Hawking in which he woos his soon-to-be wife with an analysis of why the men’s white shirts luminesce in the ultraviolet lights of a cocktail party. “Optical whiteners,” he says—and she swoons. I didn’t know that that meant, so I did a little research. Optical whiteners are synthetic chemicals that make laundry look brighter; they absorb ultraviolet light and re-emit it as blue light. (It’s basically the same reason why white-haired ladies use blue shampoo.) So since optical brighteners have nothing to do with getting your laundry actually clean, I wondered if there was a downside to these chemicals—and if you…