Quantum-ness Put on the Scale

How much weirder would it be to see Schrödinger’s living/dead cat than to observe a tiny cluster of atoms interfering with itself in a double-slit experiment? That judgment might be in the eye of the observer, but physicists are interested in quantifying the effective size of macroscopic quantum objects in order to explore the border between quantum and classical physics. A new definition for this quantum size, or so-called macroscopicity, is based on how much leeway a given experiment leaves for quantum mechanics to be modified at a fundamental level. In a paper in Physical Review Letters, Stefan Nimmrichter of the University of Vienna, Austria, and Klaus Hornberger of the University of Duisburg-Essen, Germany, apply this formalism to a wide range of experiments, both completed and hypothetical.