In 1925 Heisenberg introduces a new mechanics that no longer attributes specific orbits to the electrons in the atom. In 1926 Schrödinger expands de Broglie's idea of matter waves and formulates a wave equation for electrons. It soon becomes clear that the two theories are practically equivalent, and quantum mechanics develops from them.

In June 1926 Born interprets the Schrödinger waves as probabilities. In the following year Heisenberg formulates the uncertainty principle in his theory: it states that certain properties of particles – e.g. position and velocity – cannot be determined simultaneously with arbitrary accuracy.

Einstein is convinced that a theory involving such uncertainties, which only makes probability statements, cannot give a complete description of physical reality. He defends his position with thought experiments aimed at proving that quantum mechanics is incomplete. The most famous is the Einstein-Podolsky-Rosen thought experiment, which does not disprove quantum mechanics but at least shows how counterintuitive it is.