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.)
It is transparent to any other color.
Third, you would have to either use very dim light, or you would have
to look very quickly, because shining light on the condensate quickly heats
it up so much that it evaporates 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 density of the different parts of the cloud,
they could see a big peak form in the center as they got the cloud colder.
You can see this in an animation 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
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 called "the Heisenberg uncertainty principle" that says you cannot
simultaneously know the exact location and the exact velocity of anything, including atoms.
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That sounds just like my hyperactive little brother!
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Huh?
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Yeah, the more you try to get him to stay in one room, the faster he runs
around bouncing off all the walls. It is impossible to keep track of how fast he is moving!
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That is a very good analogy. Atoms in the BEC are just the same.
Einstein says they are supposed to go to the lowest possible energy in the
trap, but if that was the very bottom it would mean that we know exactly where
they are (the location of the bottom of the trap), and exactly how fast they are
going (standing still). That would violate the uncertainty principle, so they compromise.
They go into the lowest allowed energy level, but that is not quite the bottom.
They still have a little bit of energy (called "zero point energy") that makes them
jiggle around so that we do not know exactly where they are or exactly how fast they
are moving. This little bit of jiggling energy is what makes the condensate slowly
spread out. The atoms also spread out more like a wave does than a bunch of individual atoms.
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Is there anything else special about the way the condensate behaves?
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Yes, several things. If you take two lumps of the same kind of
condensate and put them on top of each other, you don't just see one
lump that looks the same but is twice as dense. Scientists at MIT showed
that the two condensates interfere like waves so that the density varies
wildly up and down in the lump. Wieman and Cornell have also pushed two
different kinds of condensates together and found that they act very funny.
You know how when you put two normal gases together in a container they just
mix up and hardly notice each other?
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Sure. In the air nitrogen and oxygen are always all mixed up.
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But these two condensates of Wieman and Cornell, when pushed against
each other, would not mix at all. They stayed apart like two water balloons,
even though they are 10,000 times less dense than the air we breath!
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Have they done anything else with BEC?
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Yes, there are lots people studying various other things about BEC.
A favorite experiment is to poke it and look at how it quivers, sort of
like a bowl of jello. From how it quivers they can learn a lot about it.
They have also looked at many other properties.
Researchers at Colorado,
Rice, and
MIT all have
web sites presenting their research into Bose-Einstein condensation
in gases.
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Ok, you convinced me that it is kind of weird stuff, but is it good for anything?
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