What does a Bose-Einstein condensate look like?
The simple answer is that it looks like a dense little lump in the bottom
of the magnetic trap/bowl. This lump of condensate is like a drop of water
condensing out of damp air when it is cooled. When it first forms,
the condensate is still surrounded by the normal trapped gas atoms, so it
looks a bit like a pit inside a cherry.
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Could I just look into the experiment and see it?
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In principle yes, but there are several reasons it is difficult.
First, it is quite a small lump, so you would need a microscope.
Second, you would have to illuminate it with the special deep red color
to which the rubidium atom responds.
(Click here to
learn more about atoms and the colors to which they respond.)
Third, you would have to look very quickly, because shining light on the
condensate quickly heats it up so much that it goes back into
being normal gas atoms. The way Wieman and Cornell first looked at it
was to turn off the trap, and then, after a little while, take a
snapshot picture of the cloud. When they looked at the darkness of
the different parts of the cloud, they could see a very dark blob form
in the center as they got the cloud colder. You can see this in the
pictures of their actual data as they cool the atoms from 400 billionths
of a degree above absolute zero down to 50 billionths.
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Click to view larger image. Grey scale has been converted to color, with
white indicating darkest densest part of the cloud.
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So the atoms on the edges of the picture are spreading out, but the Bose-Einstein
condensate peak in the middle doesn't?
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You are right about the atoms on the side, and almost right about the condensate.
It actually does spread out, but the way it spreads out shows some of the reasons
it is quite a special little lump.
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I was beginning to wonder what the big deal is over just a little lump.
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This lump spreads out as slowly as any atoms possibly can that are not
stuck together, as they are in any solid material. There is a basic law of
physics requiring them to spread called "the Heisenberg uncertainty principle"
that says you cannot simultaneously know the exact location and the exact
speed of anything, including atoms. Since we can see about where they are
located, then we can't know exactly how fast they are moving. If they were
stationary, we would know they were moving with zero speed. So that's why
they spread out. But to really understand the uncertainty principle is
another story.
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