## Evaporative Cooling

Okay, so now that we have a great thermos, how do we get the atoms in it cold enough for BEC?

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.

See the page for a movie of an actual cloud
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?

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