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When certain substances are cooled to very low temperatures, their properties undergo radical changes. At temperatures a couple of degrees above absolute zero, helium becomes superfluid and the liquid flows without friction. One of Lev Landau’s many contributions within theoretical physics came in 1941, when he applied quantum theory to the movement of superfluid liquid helium. Among other things, he introduced the concept of quasiparticles as the equivalent of sound vibrations and vortexes. This allowed him to develop his theoretical explanation for superfluidity.