Quantum mechanics is arguably the most accurate and successful theory in the history of science. But unlike the case for special relativity, for which two physical principles suffice to derive the whole theory, physicists are still seeking the entire set of underlying principles for quantum mechanics. Recently1, 2, they have been trying to understand one of the most intriguing predictions of quantum mechanics: that quantum correlations violate mathematical relationships known as Bell inequalities, which are valid for any local realistic (classical) theory, but that they do so only up to a certain value, whereas more general theories allow violations up to greater values. On page 490 of this issue, Lapkiewicz et al.3 describe an experiment suggesting that a wider perspective, beyond Bell inequalities, is needed to understand why quantum correlations can attain only certain values.
In Bell-inequality experiments (Fig. 1a), tests are performed on two widely separated parts of a composite system. The experimenters then extract the correlations between the outcomes of each of several pairs of tests. In any theory in which the outcomes of these tests are pre-established, the sum of these correlations cannot take a value beyond a certain upper limit. However, quantum mechanics predicts greater values.