Abstract

Active time reversal in the sense of “object reversal” and passive time reversal in the sense of a frame reversal of time are discussed separately and then together so as to bring out their dual nature. An understanding of that duality makes it unavoidable to contrast symmetry properties of matter with symmetry properties to be assigned to antimatter. Only frame reversal of time can “see” all conceivable active time reversals relevant to physical objects. Only frame reversal of time can be used for a meaningful extension of the Neumann principle to the time domain