The Double-Slit Experiment

Quantum physics informs us that a system exists in superposition — that is, in all possible states — until we observe that it is only in one specific state.

According to a 2002 poll of Physics World readers, the most beautiful experiment in physics is one that simply and elegantly demonstrates how observation affects quantum systems: The double slit experiment. The double slit sets aside causality, determinism, and the notion that reality is out there as it blurs the line between the observer and the system being observed.