“Fragments in Time and Space,” a new exhibition at the Hirshhorn, can be experienced as a kind of summer sorbet, a refreshing immersion in cool art. Or it can take you deeper, into some of the thorniest problems of representation, most of them raised by the advent of photography and its frustrating power to capture a moment from the flow of existence — and isolate it and drain it of something essential. The background to this show, organized by Hirshhorn chief curator Kerry Brougher, is laid out in the first pages of a book published more than two centuries ago: “The Critique of Pure Reason” by Immanuel Kant.