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.
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.
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.
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?
