Re: [Phys-l] tiny bubbles

[Bernard Cleyet <bernardcleyet@redshift.com>]

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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