Paradoxical comfort

In trying to comfort Penny, who was crying over her mixed feelings toward her former boyfriend, Leonard talks about a paradox, recalling Sheldon's comments from a short while before (at the beginning of the show) about the double-slit experiment, where light is shown to behave both as particles and as a wave. Here he also mentions two famous physicists, Christiaan Huygens, who first described light behaving as a wave back in the 1600s, and Albert Einstein.

Leonard's attempt at comfort falls flat. Penny stares at him, uncomprehending, and he goes back to Sheldon rather desperately and says "Well, I didn't make it worse ..."