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Re: [Phys-l] BEC



Well Jack, I guess my answer is take it or leave it. I am responding
to this thread in a spirit of public service, if you do not accept the
accuracy of this information it is of no concern to me.

The only other suggestion I can make is to read the literature, follow
the research and make up your own mind i.e. go and create it yourself.

Terry




JU> So how do we know whether any part of what your friend told you is
JU> correct? What should the teachers on this net tell their students.
JU> "I got a message on the net that ..."?

JU> When I teach physics or math I keep a sign on the wall with Feynman's
JU> words on it:
JU> "That which I cannot create, I do not understand."
JU> Regards,
JU> Jack


JU> On Thu, 18 May 2006, Terry Scott wrote:

Ludwik said:

LK> It is not my cup of tee but I have a comment. You are saying that atoms
LK> are bosons, not naked nuclei. Consider a molecule, in which electrons
LK> are said to be "shared" between atoms. Which electrons should be
LK> counted? If atoms are treated as single bosons then why not molecules?
LK> And if molecules then why not groups of molecules, for example a
LK> nonocrystal, or a much larger object?

I have spoken to my friend the BEC theoretician again, and I am
summarising his response to this question.

It is all a question of energy scales. If the interaction of the atoms
occurs at a low energy, for example in the formation of a molecule or
in the cooling and trapping of a collection of molecules, then the
internal structure of the atomic nuclei is not involved. At this
energy scale, only atomic scale factors are important i.e. total spin,
including nuclear spin, electronic configuration etc. This means that
two fermionic atoms are able to combine to form a bosonic molecule,
and these molecules may be condensed into a molecular BEC. This has in
fact been done in experiments by Deborah Jin at NIST in Colorado and
Rudy Grimm at Innsbruck in Austria.

On the question of the bosonic or fermionic character of crytals, the
interactions between two large crystals (due to electrostatic forces
etc) would far exceed any subtle intrinsic symmetry of the crystal as
a composite entity. So while it may be tempting to say that a crystal
with an even number of fermions is a composite boson, there may be no
practical way to explore the implications of this. Indeed we know that
for the case of He-4, a composite boson, it does Bose condense when it
is in the its superfluid phase, but the presence of the condensate is
strongly masked by the interaction between the atoms (and relatively
speaking He- atoms interact with each other only weakly).

Hopefully this answers your question Ludwik.

Terry Scott

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