The scene opened on a room with a red sofa, a potted plant and the kind of bland modern art you’d see on a therapist’s wall. In the room was Michelle Obama, or someone who looked exactly like her. Wearing a low-cut top with a black bra visible underneath, she writhed lustily for the camera and flashed her unmistakable smile. Then, the former first lady’s doppelgänger began to strip. The video, which appeared on the online forum Reddit, was what’s known as a “deepfake” — an ultrarealistic fake video made with artificial intelligence software. It was created using a program...