German <a href="https://en.wikipedia.org/wiki/Theoretical_physics" id="mwug" rel="mw:WikiLink" title="Theoretical physics">theoretical physicist</a> <a href="https://en.wikipedia.org/wiki/Werner_Heisenberg" id="mwuw" rel="mw:WikiLink" title="Werner Heisenberg">Werner Heisenberg</a> writes a letter to fellow physicist <a href="https://en.wikipedia.org/wiki/Wolfgang_Pauli" id="mwvA" rel="mw:WikiLink" title="Wolfgang Pauli">Wolfgang Pauli</a>, in which he describes his <a href="https://en.wikipedia.org/wiki/Uncertainty_principle" id="mwvQ" rel="mw:WikiLink" title="Uncertainty principle">uncertainty principle</a> for the first time.