 Werner Heisenberg

Werner Heisenberg's 1925 reformulation of quantum mechanics replaced unobservable classical electron orbits with quantities attached to transitions between stationary quantum states. Transition frequencies combine as ${\nu }_{nm}+{\nu }_{mk}={\nu }_{nk}$, suggesting the indexed product rule that became matrix multiplication. The resulting physical quantities need not commute. This approach made measured spectral lines and transition strengths the starting data for a new dynamics rather than requiring a miniature classical planetary model of the atom.^[ref]

A physicist.^[ref]

Werner Heisenberg (1901–1976) was a German physicist who made significant contributions to quantum mechanics and is best known for formulating the Heisenberg Uncertainty Principle, a fundamental concept in quantum theory. This principle states that certain pairs of physical properties, like position and momentum, cannot both be precisely measured simultaneously.

Participated in the German nuclear weapons program, ouch.