Macroscopic measuring apparatuses and the role of global symmetries

The idea that any statement made by a scientific theory - be that an equation, a postulate, a principle - either descends from or is confirmed by some experimental evidence lies at the heart of the so-called scientific method. In fact, the Oxford English Dictionary defines it as "a method or procedure that has characterised natural science since the 17th century, consisting in systematic observation, measurement, and experiment, and the formulation, testing, and modification of hypotheses". The experimental procedure adopted for testing any statement thus acquires a foundational role in the theory itself: its relevance, however, is not always recognised. The above procedure is often taken for granted, with attention rather focused upon improving the properties of the instrumentation (such as resolution or sensitivity). This picture is challenged by quantum mechanics, a theory where the production of experimental results complies with rules which need in fact to be considered as an integral part of the theoretical system. This is typically done in the form of the "measurement postulate", or "Born's rule". As a result, the amount of research that has been devoted to the subject since the first formulations of the theory is huge, and yet physicists continue to argue that our understanding of the process is unsatisfactory. This feeling has grown even stronger in recent decades, due to the recently acquired capability of controlling individual quantum objects with ever greater precision. This pushes fundamental research in at least two areas: (1) in the design and realisation of new tests of quantum mechanics itself, including of rival successor theories and of various interpretational viewpoints and (2) the use of such objects as highly sensitive probes which can enable measurements of unprecedented fidelity, for example of gravitational waves and other tiny effects.