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# Re: [Phys-l] tiny bubbles

Is a super fluid, at non zero T, a mixture of super and normal, or is this just a convenient model? If so (former) then as I was thinking of condensate at zero -- would the "zero point" motion still make homogeneous fluid impossible?

bc, who assisted a student doing fourth sound

John Denker wrote:

On 09/15/2006 12:57 AM, Bernard Cleyet wrote:

no liquid has uniform density (except a Bose condensate?)

That points to a misconception about superfluids, and reminds me of some
other misconceptions.

1) Any solid (even a "perfect" crystal) has nonuniform density at any
nonzero temperature, because of thermal phonons.

By the same token, any fluid (even a superfluid) has nonuniform density
at any nonzero temperature.

2) A crystal is ordered with respect to /position/. That is: If you
tell me the position of a few atoms in the crystal, I can calculate
the positions of all the others. (This does not mean they all have
the same position, merely that they have predictable positions.)

In contrast, a superfluid is ordered with respect to /momentum/.
That is: If you tell me the momenta of a few atoms in the
superfluid, I can calculate the momenta of all the others. (This
does not mean they all have the same momentum, merely that they
have predictable momenta.)

This has a nice interpretation in phase space: A crystal "crystallizes
out" along the position axis, while a superfluid "crystallizes out"
along the momentum axis.

You can have superfluidity in porous media, where there is no hope
of long-range positional order.

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