Quantum Mechanics is the science behind nuclear energy, smart phones, and particle collisions. Yet, almost a century after its discovery, there is still controversy over what the theory actually means. The problem is that its key element, the quantum-mechanical wave function describing atoms and subatomic particles, isn’t observable. As physics is an experimental science, physicists continue to argue over whether the wave function can be taken as real, or whether it is just a tool to make predictions about what can be measured—typically large, “classical” everyday objects. (What Really Is Everything? Quantum Reality Space, Time, and Entanglement)