That is where the evaporative cooling comes in. It is the same physics that
cools your cup of hot coffee. In your coffee, the most energetic coffee
molecules escape from the cup and come off as steam. When they do this, they
take away more than their share of heat, and the atoms left behind in the cup
are colder because they have lost energy. To make BEC, the most energetic
atoms are allowed to escape from the magnetic trap/bowl.
Hey this goes pretty slowly, but I can get the atoms cold a lot faster if I
lower the sides of the bowl.
Yes, that is exactly the same as in the BEC experiments. But if you lower the
sides really quickly, notice that you end up with very few cold atoms. It
turns out that you not only have to get the atoms very cold to cause
Bose-Einstein condensate, but you also have to have enough of them left in the
trap. Try to lower the edge of the bowl at the rate that gives you the most
cold atoms in a given amount of time. That is exactly what they do in the BEC
experiments. Just as in those experiments, if you do it in the right way to
give you enough cold atoms, you will see BEC.
of atoms as it is cooled!
But does this demonstration really act like the real experiment?
Well in the real experiments the atoms are smaller but there are more of them
so they bump into each other just about as often and they are going just about
as fast as in this demonstration.
No way. I heard that atoms move about 1000 miles per hour, but these balls
are only going about an inch or two a second.
Ah, but remember that the colder atoms get, the slower they go. One thousand
miles per hour is the speed that atoms in the air move when they are at room
temperature. When you get down to less than a millionth of a degree above
Absolute Zero, the atoms are just crawling along at about the speed of these
balls. Another thing that is different is that the Bose condensate, or "super
atom", does not look like this picture.
What does it look like?