Assistant
Professor of Physics David Hall (left) works on his cooling device with a student,
Ted Reber ’03. The cooling chambers are at the far end
of the machine; the elements in the foreground tune and direct laser beams.
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Redefining cold
It’s cold at the end of the cosmos, in the blackness beyond the last
star. But on most days David Hall’s laboratory in the Merrill Science Center
is colder than that—several billion times colder. It is, in fact, the coldest
place in the universe, at least for 30 seconds at a time.
The device that creates these low
temperatures looks like a Rube Goldberg fantasy, and was hand-built by Hall ’91
(now an assistant professor of physics at Amherst) and his students entirely
from scratch, using various electronic, optical and mechanical parts, carefully
aligned and optimized to create and handle delicate atomic samples. Despite its
homemade
appearance, it is a supremely sophisticated piece of equipment, one that accomplishes
something that scientists have been trying to do for 70 years. There are only
about 50 such machines in the world, and the one at Amherst is the only one at
an undergraduate institution.
Its purpose is to create a new form of matter called a Bose-Einstein condensate.
The condensate, predicted by Albert Einstein in 1924, allows physicists for the
first time to see atomic processes. Einstein calculated that if one could cool
a dilute gas to a temperature a few tens of billionths
of a degree above absolute zero (the point where all molecular motion stops—zero
on the Kelvin scale; minus 459.67 degrees Fahrenheit), most of the atoms in that
gas would drop to the lowest possible energy level. Moreover, they would all
be identical, and, Einstein predicted, they would condense to a single point.
In doing so,
the atoms would reinforce each other,
acting like a single, gigantic “superatom,” which would allow scientists
to directly observe its peculiar quantum-mechanical properties.
This image shows a Bose-Einstein condensate created by David Hall’s machine.
As a condensate emerges from confinement in the magnetic trap, it rebounds into
this signature ovoid shape. The false colors indicate the density of atoms — white
being most dense and blue the least dense. |
That’s just what Hall is doing with his machine. The first time anyone
managed to make a Bose-Einstein condensate (Bose is the physicist whose research
prompted Einstein’s predictions) was at the University of Colorado in 1995.
Hall worked with the Colorado team as a post-doctoral researcher in 1997, and
when he came to Amherst in 1999, he decided to build his own machine. Hall’s
device, which has
no official name, uses two different techniques to cool gases. First, six laser
beams capture, confine and cool a dilute rubidium gas (among the most obliging
of
the few elements that will condense this way—“God’s atom,” in
Hall’s words). A photon of light from a precisely tuned laser hits a rubidium
atom, which absorbs the photon and then releases it again, losing some of its
own energy. As the process is repeated with each succeeding photon, the atom
moves more and more slowly and becomes increasingly cold. The laser assembly
can cool the rubidium gas to about 50 millionths of a degree Kelvin: extremely
cold, but not enough to condense.
To further cool the gas, it is transferred to a chamber where
it is exposed to the second technique, forced evaporative cooling. This is the
same thing that happens when you blow on a bowl of hot soup: the most energetic
molecules jump up above the surface of the soup, where your breath carries them
away before they can fall back and reintroduce their energy. In the case of Hall’s
machine, the bowl is a magnetic field, and rather than blowing on it, the computers
controlling the process gradually lower the sides of the magnetic bowl, letting
the most energetic atoms
escape. The ones that remain are about 0.00000005 degrees Kelvin—cold enough
to condense in the Bose-Einstein pattern. (For comparison, the coldest spot in
outer space is estimated to be 2.7 degrees Kelvin.)
The resulting condensate is about one-fifth the thickness of
a sheet of paper, which is enormous by atomic standards—large enough to be photographed in
images like the one shown here. The matter exists for only about 30 seconds,
so Hall and his students (currently Mark Wheeler ’04) use the images to
study its properties.
So far, Hall says, they are just exploring the terrain of this new world; no
one is
yet dealing with practical applications for Bose-Einstein condensates. But the
basic science is valuable on its own terms, helping scientists flesh out what
had been only theoretical assumptions. “The system is richer than the theories,” he
says. “You can come up with some theory that seems to be perfect and explains
every
aspect, but nature isn’t exactly like that. When you throw in reality,
you find quite a number of phenomena that you might never have suspected would
exist from the simplest models.”
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Photo: Frank Ward
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