Is energy conserved when nobody looks?

Abstract

Conservation principles are essential to describe and quantify classical and quantum mechanical processes. Classically, the conservation holds objectively because the description of reality can be considered independent of observation. In quantum mechanics, however, invasive observations change quantities drastically, even if they are expected to be conserved classically. One may hope to overcome this problem by considering weak, non-invasive quantum measurements. Interestingly, we find that the non-conservation is manifest even in weakly measured correlations if some of the observables don't commute with the conserved quantity. Our observation casts some doubt on the fundamental compatibility of conservation laws and quantum objectivity.