Physics Progress "To those who do not know mathematics it is difficult to get across a real feeling as to the beauty, the deepest beauty, of nature ... If you want to learn about nature, to appreciate nature, it is necessary to understand the language that she speaks in."  The Character of Physical Law Once the project was complete Feynman accepted a teaching position at Cornell University. However, Arline's death had quite an impact on him and he seemed quite stagnant intellectually. In fact, Richard was worried that he was at the end of his career (Feynman Online). However, the rest of the world did not seem to think so. He found that he a variety of offers from several other prestigious institutions. In the end, Feynman chose to accept a professorship from California Institute of Technology (Caltech). At Caltech he would find a perfect balance between interaction with his students and research opportunities. Feynman teaching: http://ysfine.com/feynman/ At Caltech Feynman did his most important theoretical work. He worked on  applying the sum of histories approach directly to quantumelectrodynamics (QED). This approach calculates the path a particle can take as a sum of all possible paths, a theory which makes very accurate predictions of particle behavior (Wikipedia). The theory won him a Nobel Prize in 1965, along with Julian Schwinger and Sin-Itiro Tomanaga for the same work. Feynman also worked on the issue of superfluidity of cold liquid helium, a liquid that shows no frictional resistance while flowing (Feynman Online). Feynman used the solution to Schrodinger's equation to show that the liquid was showing quantum behavior at a macroscopic level (Feynman Online). Feynman also collaborated with Murray Gell-Mann on the principle of weak decay ( the decay of a neutron into an electron, a proton, and finally into an anti-neutrino is a general instance of weak decay). This was ground-breaking work in which Feynman was directly involved in the defining of a newly discovered law of nature. Feynman and Gell-mann: http://photos.aip.org/quickSearch.jsp?qsearch=feynman&group=30