When scientists speak seriously about something like a space-time cloak, they're either a) extremely smart or b) a tiny bit crazy. Prof. Martin McCall of Imperial College in London is not crazy. The British physicist published a paper in the journal Optics describing the theoretical possibility of something he calls "metamaterials," fabric or other forms of matter that could be molecularly engineered to scramble the usual flow of electromagnetic energy. Light passing through it would emerge unevenly, creating gaps in time and space. O.K., that's hard to follow, but his only half-joking description of a safecracker entering a room, robbing a safe and leaving while a surveillance camera reveals nothing amiss isn't. One tiny glitch in the plan: given the speed at which light travels, invisibility for even a few minutes would require a cloak about 100 million meters (320 million ft) long.