Macrorealism

Macrorealism is the view that macroscopic objects possess definite properties at all times, independent of observation, and that these properties can be measured without disturbing the system. In this view, a macroscopic system is always in one of a set of distinct states, even if no measurement is made. A closely related idea is noninvasive measurability, the claim that it is possible to determine the state of the system without affecting its future evolution. When combined with a principle of induction, these assumptions define macrorealism per se.

In 1985, Anthony J. Leggett and Anupam Garg formulated a test for macrorealism by deriving temporal inequalities, now known as the Leggett–Garg inequalities, that constrain the correlations of a single system’s variable measured at different times if macrorealism holds. Violations of these inequalities would rule out macrorealism under the stated assumptions.

Experiments have sought to observe Leggett–Garg violations in systems ranging from superconducting circuits to nanomechanical resonators, spin ensembles, and photonic setups. Some experiments report results consistent with quantum mechanics and in tension with macrorealism. However, interpretations are complicated by experimental loopholes, notably the potential invasiveness of measurements (the clumsiness loophole) and the precise modeling of system dynamics.

The macrorealism program illuminates the quantum–classical boundary and complements studies of locality (Bell inequalities) by focusing on temporal correlations. Ongoing work aims to close loopholes, refine definitions of noninvasive measurements, and explore whether macrorealism can be reformulated or is fundamentally incompatible with quantum theory.