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I know that Bose-Einstein condensate is not always a superfluid. So what can cause viscosity in a BEC gas?

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  • $\begingroup$ This might help - Does every superfluid have a normal and a superfluid component? $\endgroup$
    – mmesser314
    Dec 15, 2023 at 14:55
  • $\begingroup$ A Galilean BEC (by which I mean a BEC of massive bosons whose kinetic energy $\sim p^2$) has quadratically dispersing low energy excitations, whereas a superfluid has linearly dispersing excitations. The linear dispersion in superfluids is crucial to zero viscosity -- see Landau's ingenious argument, discussed in most references on the topic . $\endgroup$ Dec 17, 2023 at 4:19

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