Abmact-In this correspondence, we show that images that could not have arisen from shading on a smooth surface with uniform reflecting properties and lighting exist. Much work has been done on recovering surface shape from images, and there has been some attention paid to the question of uniqueness. It has been shown, for example, that singular points curtail ambiguity. However, little has been...

A well-recognized open conceptual problem in relativistic quantum field theory concerns the relation between measurement and causality. Naive generalizations of rules can allow for superluminal signaling (``impossible measurements''). This raises delineating physically allowed measurements operations. We analyze this issue a recently proposed framework which local (in possibly curved spacetime)...