|MadSci Network: Physics|
Your question brings up in a new way one of the main difficulties we have in dealing with the world of quantum mechanics. We have a very real lack of good descriptive terms that make any sense in an every day sort of way or allow us to make analogies that aren't more wrong than they are right for what goes on at the quantum level. Many people have noted this over the years and probably most agree that it is in large part because we just don't see behavior for the common physical objects about us that in terms of energy, momentum or position obeys rules much at all like those for things the size of atoms and smaller. Lacking this everyday experience we not only don't have good unambiguous words for it but have trouble even imaging it even though we can describe it in exquisite detail mathematically. That being said, here we go: What do we mean by rigid? In our everyday world nothing is perfectly rigid. There is always a delay between pushing on one side of an object and the vibration caused making it to the other side. We usually interpret this to be something like the speed of sound in that material. It happens because molecules take time to either bump into one another or for the distortion of an atomic or molecular bond to be noticed by a neighboring atom or molecule, much liked a wave in a spring. In fact if something were perfectly rigid any disturbance of one part would cause instantaneous motion of all parts. It's the word "instantaneous" that gets us in trouble here since that would imply knowledge of what was going on in one place to travel infinitely fast (much more rapidly than even the speed of light!) to some other place. If you check what we know of BEC you find that it is a group of "indistinguishable" (you can't tell which is which) atoms all in the same energy state in the same volume of space. Saying it's now all "one big atom" is perhaps a bit misleading but even if true would make it no more rigid than any other ordinary atom. It's probably not a good idea to talk of "all the atoms moving in unison" either since this implies in our ordinary view of the world that they all go the same way at the same time instantly. It certainly sounds that way and as you correctly point out if that's what happened it would be perfectly rigid. Rather what happens is that the energy of the group is disturbed as a whole but in fact we don't really know where any one atom of the group is any more precisely than any other and for none of them any better than Heisenberg's Uncertainty Principle allows. This is why we say, (and experimentally see) all the atoms "condensed" into the same volume of space with the same energy or quantum state. If you haven�t already been there check out the entire site at http://www.colorado.edu/physics/2000/bec/ Especially their interactive energy level diagram. I hope this is helpful Chris. Keep on asking good questions.
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