Oppenheimer made important contributions to the theory of cosmic ray showers. He also worked on the problem of field electron emission. This work contributed to the development of the concept of quantum tunneling. In 1931, he co-wrote a paper, "Relativistic Theory of the Photoelectric Effect," with his student Harvey Hall, in which, based on empirical evidence, he correctly disputed Paul Dirac's assertion that two of the energy levels of the hydrogen atom have the same energy. Subsequently, one of his doctoral students, Willis Lamb, determined that this was a consequence of what became known as the Lamb shift, for which Lamb was awarded the Nobel Prize in physics in 1955.