One of the most well-known counterintuitive discoveries was arguably Heisenberg's greatest act of quantum conjuring. The uncertainty principle forbids, at any given moment, the precise determination of both the position and the momentum of a particle. It is possible to measure exactly either where a particle is or how fast it is moving, but not both simultaneously. In a quantum dance of give-and-take, the more accurately one is measured the less precisely the other can be known or predicted. Heisenberg's uncertainty principle is not due to any technological shortcomings of the equipment, but a deep underlying truth about the nature of things.

According to some, including Heisenberg, there is no quantum reality beyond what is revealed by an experiment, by an act of observation. Take Erwin Schrödinger's famous mythical cat trapped in a box with a vial of poison. It is argued that the cat is neither dead nor alive but in a ghostly mixture, or superposition, of states that range from being totally dead to completely alive and every conceivable combination in between until the box is opened.