= Werner Heisenberg
{c}
{tag=Physicist}
{title2=1901–1976}
= W. Heisenberg
{c}
{synonym}
<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>.<On the quantum reinterpretation of kinematical and mechanical relationships by Werner Heisenberg>[1925 paper, transition quantities and their multiplication rule]{ref}
A <physicist>.https://www.mpp.mpg.de/en/about-us/history[Independent professional profile]{ref}
\Image[https://upload.wikimedia.org/wikipedia/commons/1/12/Werner_Heisenberg_Portrait.jpg]
{title=Portrait of <Werner Heisenberg>}
{width=240}
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