Quantum theory is the foundation for our current understanding of nature. When people first begin to learn quantum theory, they tend to question its necessity, wondering if the same phenomena could be explained using something that feels more like common sense. Such questions are healthy as long as we pay close attention to the experimental facts.
Part of what we might have called common sense leads to an inequality called the CHSH bound. This article explains how this inequality is derived and then describes a class of experiments in which the CHSH bound is violated. For people whose goal is to understand how nature works, this is one of the most important phenomena ever observed, because it shows in a relatively direct way that part of what we might have called common sense cannot be correct.
