A couple of days ago I wrote a post discussing chemists who did significant work after receiving a Nobel Prize. The examples are few but noteworthy; accomplishing one significant piece of scientific work is hard enough, so if you manage more than one you should definitely be recognized. A commenter pointed out that there's likely more such cases in physics so I decided to find out. I looked at the list of all Nobel Prizes in physics starting from 1900 and found interesting examples and trends.
Let's start with the two physicists who are considered the most important ones of the twentieth century in terms of their scientific accomplishments and philosophical influence - Albert Einstein and Niels Bohr. Einstein got a Nobel Prize in 1921 after he had already done work for which he would go down in history; this included the five groundbreaking papers published in the "annus mirabilis" of 1905, his collaboration on Bose-Einstein statistics with Satyendranath Bose and his work on the foundations of the laser. After 1921 Einstein did not accomplish anything of similar stature but he became famous for one enduring controversy, his battle with Niels Bohr about the interpretation of quantum theory that started at the Solvay conference in 1927 and continued until the end of his life. This led to the famous paper on the EPR paradox in 1935 that set the stage for all further discussions of the weird phenomenon known as quantum entanglement.
Bohr himself was on the cusp of greatness when he received his prize in 1922. He was already famous for his atomic model of 1913, but he was not yet known as the great teacher of physics - perhaps the greatest of the century - who was to guide not just the philosophical development of quantum theory but the careers of some of the century's foremost theoretical physicists, including Heisenberg, Gamow, Pauli and Wheeler. Apart from the rejoinders to Einstein's objections to quantum mechanics that Bohr published in the 30s, he contributed one other idea of overwhelming importance, both for physics and for world affairs. In 1939, while tramping across the snow from Princeton University to the Institute for Advanced Study, Bohr realized that it was uranium-235 which was responsible for nuclear fission. This paved the path toward the separation of U-235 from its heavier brother U-238 and led directly to the atomic bomb. Along the same lines, Bohr collaborated with his young protege John Wheeler to formulate the so-called liquid drop model of fission that likened the nucleus to a drop of water; shoot an appropriately energetic neutron into this assembly and it wobbles and finally breaks apart. Otto Hahn who was the chief discoverer of nuclear fission later won the Nobel Prize and it seems to me that along with Fritz Strassman, Lisa Meitner and Otto Frisch, Bohr also deserved a share of this award.
Since we are talking about Nobel Prizes, what better second act than one that results in another Nobel Prize. As everyone knows, this singular achievement belongs to John Bardeen who remains the only person to win two physics Nobels, one for the invention of the transistor and another for the theory of superconductivity. And like his chemistry counterpart Fred Sanger who also won two prizes in the same discipline, Bardeen may be the most unassuming physicist of the twentieth century. Also along similar lines, Marie Curie won another prize in chemistry after her pathbreaking work on radioactivity with Pierre Curie.
Let's consider other noteworthy second acts. When Hans Bethe won the prize for his explanation of the fusion reactions that fuel the sun, the Nobel committee told him that they had trouble deciding which one of his accomplishments they should reward. Perhaps no other physicist of the twentieth century contributed to physics so persistently over such a long time. The sheer magnitude of Bethe's body of work is staggering and he kept on working productively well into his nineties. After making several important contributions to nuclear, quantum and solid-state physics in the 1930s and serving as the head of the theoretical division at Los Alamos during the war, Bethe opened the door to the crowning jewel of quantum electrodynamics by making the first decisive calculation of the so-called Lamb shift that was challenging the minds of the best physicists. This work culminated in the Nobel Prize being awarded to Feynman, Schwinger and Tomonaga in 1965. Later at an age when most physicists are just lucky to be alive, Bethe provided an important solution to the solar neutrino puzzle in which neutrinos change from one type to another as they travel to the earth from the sun. There's no doubt that Bethe was a supreme example of a second act.
Another outstanding example is Enrico Fermi, perhaps the most versatile physicist of the twentieth century, equally accomplished in both theory and experiment. After winning a prize in 1938 for his research on neutron-induced reactions, Fermi was the key force behind the construction of the world's first nuclear reactor. That the same man who designed the first nuclear reactor also formulated Fermi-Dirac statistics and the theory of beta decay is a fact that still beggars belief. The sheer number of concepts, laws and theories (not to mention schools, buildings and labs) named after him is a testament to his mind. And he achieved all this before his life was cut short at the young age of 53.
Speaking of diversity, no discussion of second acts can ignore Philip Anderson. Anderson spent his entire career at Bell Labs before moving to Princeton. The extent of Anderson's influence on physics becomes clear when we realize that most people today talk about his non-Nobel Prize winning ideas. These include one of the first descriptions of the Higgs mechanism (Anderson is still regarded by some as a possible contender for a Higgs Nobel) and his firing of the first salvo into the "reductionism wars"; this came in the form of a 1972 Science article called "More is Different" which has since turned into a classic critique of reductionism. Now in his eighties, Anderson continues to write papers and has written a book that nicely showcases his wide-ranging interests and his incisive, acerbic and humorous style.
There's other interesting candidates who show up in the list. Luis Alvarez was an outstanding experimental physicist who made important contributions to particle and nuclear physics. But after his Nobel Prize in 1968 he re-invented himself and contributed to a very different set of fields; planetary science and evolutionary biology. In 1980, along with his son Walter, Alvarez wrote a seminal paper proposing a giant asteroid as the cause for the extinction of the dinosaurs. This discovery about the "K-Pg boundary" really changed our understanding of the earth's history and is also one of the most exemplary examples of a father-son collaboration.
There's a few more scientists to consider including Murray Gell-Mann, Steven Weinberg, Werner Heisenberg, Charles Townes and Patrick Blackett who continued to make important contributions. It's worth noting that this list focuses on achievements after winning the prize; a "lifetime achievement" list would include many more scientists like Lev Landau, Subrahmanyan Chandrasekhar and Max Born. It's also important to focus on non-research activities that are still science-related. A list of these achievements would include teaching (Feynman, Fermi, Bohr, Born), writing (Blackett, Feynman, Bridgman, Weinberg), science and government policy (Bethe, Compton, Millikan, Rabi) and administration (Bragg, Thomson, de Gennes, Rubia). Bonafide research is not the only thing at which great scientists excel.