Bose Einstein Condensates?

Question

What are the physical changes, on an atomic level, that something such as caesium undergoes when it is supercooled to the point of becoming a BEC. I know that it becomes a 'squiggle' but can anyone explain it in a little more detail. Thanks.

Answer 1

The cesium does not undergo any changes. Bose-Einstein condensates are a very basic property of quantum mechanical systems formed by bosons, i.e. particles with integer spin.

In classical mechanics there is a minimum distance between atoms which is more or less given by the interactions between them (you can add external pressure and push them together, but only to a point). QM says that, under the right circumstances, wave functions can overlap and "condense" in such a way that they seem to overlap completely. It is a little bit like as if a solid body could go through another solid body. Well, not quite. It just happens that these atoms "stop seeing each other" because they all become practically the same (they are not differentiated any more by the directions of their random thermal motions) and so they can overlap and form this quantum mechanical state that is different from an ordinary solid or liquid or gas.

These people have some very nice interactive animations:

http://www.colorado.edu/physics/2000/bec/

I am not sure anyone can explain the meaning of all of this very well without the math... this is very much about quantum statistics of atoms in different energy states and not so much about something one can demonstrate with drawings of classical systems.