Superfluidity and Superconductivity

I AM sorry that Professor Landau was unable to come for two personal reasons. The first is that I have worked on this problem of helium upon which he has also done so much, and I would have liked to have spoken with him about it. The other reason is that I wouldn't have had to give this lecture. Quantum mechanics was developed in 1926, and in the following decade it was rapidly applied to all kinds of phenomena with an enormous qualitative success. The theories of metals, other solids, liquids, chemistry, etc. came out very well. But as we continued to advance the frontiers of knowledge, we left behind two cities under siege which were completely surrounded by knowledge although they themselves remained isolated and unassailable. The first of the two very similar phenomena for which we still lacked a qualitative explanation is the superfluidity of helium. I may remind you that the discovery that helium flows without resistance through very thin tubes was made as far back as 1911. The second phenomenon is the superconductivity of metals. The fact that electricity flows through some metals without any resistance at low temperatures was discovered, I believe, in 1905. Of these two phenomena, I think we now understand qualitatively the superfluidity of helium, but we do not yet understand qualitatively the superconductivity. I propose to give a brief summary of views on liquid helium insofar as they may give some clue as to the kind of thing that is involved in superconductivity. It is clear, however, that the solution of the superfluidity problem does not give us a very good clue because we have solved the first problem but not the second. First, I make some semiphilosophical remarks about the kind of problem facing us, and the kind of view that I take toward it. I do not want to discuss all of the aspects of superconductors and of superfluids. I want to discuss only the interesting qualitative features, i.e., the curious problem of how does it work more or less. In other words, we would like to make an analogy between the problem of, say, superconductivity and the problem of friction. After all, how is friction explained on the basis of the Schrodinger equation? No one has ever computed the coefficient of friction of two blocks of copper, but qualitatively we feel that