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04/17/2001
Coldness -Part 2
Last week''s topic was Absolute Zero, 0 degrees Kelvin or just
plain 0 K. You can''t get any colder than 0 K and you cannot ever
absolutely reach Absolute Zero. Coldness is big business. Where
would we be without ice cubes for our gin and tonic, that ice
cream cone or air conditioning on a hot summer day? Most of
our everyday coldness revolves around the freezing point of
water at 273 K.
A big increase in coldness is in the domain of liquid nitrogen at
77 K, its boiling point. It is widely used by dermatologists and in
various kinds of cryosurgery. Liquid nitrogen is also used to
preserve sperm or, as I read this week in the paper, is even
(controversially) being used to freeze eggs of young women for
conception in later years. Liquid nitrogen is plentiful and cheap.
To get a lot colder, go for the more expensive liquid helium and
you''re down to 4 K, its boiling point. Pump on liquid helium,
and we''re less than a degree above Absolute Zero.
Why would anybody care about going down so low in
temperature? For one thing, it''s a great way to win Nobel Prizes!
A Russian physicist, Peter Kapitza, won the 1978 prize for just
watching the behavior of one form of liquid helium at a couple
degrees above 0 K. At that temperature, liquid helium is not
your standard, everyday liquid. You might think that because the
temperature was so low it would be pretty viscous, sort of like
molasses. Just the opposite. This weird form of liquid helium
has no viscosity at all; Kapitza had discovered "superfluidity".
Just as a superconductor has no resistance to the flow of
electrons, there''s no resistance to flow in a superfluid. In fact,
the liquid helium wants to flow like crazy, even climbing up and
overflowing the walls of its container.
If you''re still a skeptic, you might say "Ok, you''ve found
something interesting but surely there''s no point to exerting
yourself to try to get even closer to Absolute Zero?" On the
contrary, if you''re interested in clocks there''s a very good reason.
You say you''re not particularly interested in clocks? Suppose
you''ve run out of gas and are lost on a side road in Death Valley.
Wisely, you have one of these GPS (Global Positioning System)
units you''ve seen on commercials for some upscale autos. You
sound the alarm and help arrives soon enough to save your life.
The reason rescuers could pinpoint your location depended on
having a clock that could precisely compare the times it took for
your signal to bounce off orbiting satellites and arrive at the
central station monitoring your unit''s signal. The more accurate
and precise the clock, the closer they know your position. Hey,
now you''re interested in clocks!
Nobelist William Phillips, whose lecture "Absolute Zero - Laser
Cooling and Trapping" spurred these two columns on coldness,
works at the National Institute of Science and Technology
(NIST). One of NIST''s main functions is to precisely measure
time. Without going into detail here, the methods used to
measure time with ultimate precision depend on how long it
takes atoms to travel from one place to another. The slower the
atoms move, the more precisely you can measure the time.
When you slow atoms down, as we discussed last week, you
lower their temperature.
Phillips gave a most entertaining lecture. After tossing the
contents of a jug of liquid nitrogen on the floor near the front
rows to get our attention, Phillips blew up a few balloons. He
them jammed them down into the nitrogen in another jug. I
expected they would shrivel up as the gas inside them cooled.
Actually, when Phillips retrieved them they were thin and flat,
like Frisbees. He sailed these frozen Frisbees out at the
audience. Amazingly, before they landed they heated up enough
that they were back to being balloons. A couple blew up with a
loud pop for extra effect.
Phillips then got down to serious coldness - laser cooling and
trapping. As we said, if atoms in a gas are moving slowly that
means that the temperature of the gas is lower than if they are
moving rapidly. The object of laser cooling is, therefore, simply
to slow down the atoms as much as possible. If you stop them
completely, that''s Absolute Zero. How do you contain the
atoms? Phillips pulled out a very large salad bowl and tossed in
his atom, a ping-pong ball. As long as the ping-pong ball wasn''t
moving too fast, it stayed in the bowl.
Actually, a salad bowl isn''t a great container for atoms in a gas
at low temperatures. You need a "magnetic bottle". Phillips had
a dramatic demonstration of such a bottle. His props were a top
and what looked like a regular hotplate. The top was fashioned
around a magnet, with its north and south poles. The ''hotplate''
wasn''t a heater but instead contained an arrangement of magnets.
Phillips showed how, if you held the top normally, it was
repelled by the stationary magnets and rose into the air above the
plate. However, it quickly flipped and was drawn to the magnets
in the ''hotplate''. How to keep the top suspended in midair?
How did you keep a top standing up straight as a kid? Answer -
spin it.
So, Phillips got out his handy little spinner and set the spinning
top/magnet on a plate of glass sitting on the ''hotplate''. He then
slowly lifted the glass plate and, sure enough, the top took off on
its own in midair, spinning merrily. Phillips removed the glass
plate and passed his hand between the spinning top and the
magnetic plate - neat! We have our magnetic bottle. If we
accept the fact that atoms, with their circulating electrons, can
behave like little magnets, it''s not a stretch to think that a bunch
of atoms could be held in a magnetic bottle.
Now Phillips really gets down to business and is ready to use
lasers to cool sodium atoms in his magnetic bottle. But I''ve had
laser eye surgery and know that it''s the concentration of the laser
energy that burns a small area in the eye. High-power lasers are
even used industrially to drill holes in metal alloy or ceramic
objects. Since heating is the hallmark of laser applications, how
can we possibly use lasers to cool our bunch of sodium atoms?
Phillips explained that sodium atoms absorb or emit photons of
light at certain definite colors (frequencies). This absorption or
emission of photons gives the atom a jolt, and may slow it down
or speed it up depending on its direction and speed. Let''s line up
three pairs of lasers shining their photons into our magnetic
bottle to give us 3-D coverage of our atoms so that no matter
which way they move they''re hitting streams of photons. We''re
also smart and tune these photons to just the right colors. It gets
a bit complicated here and involves the Doppler effect. We
know the Doppler effect as the change in frequency (pitch) of a
train whistle or auto horn as they move towards or away from
you.
As our atom in the magnetic bottle moves toward or away from
one of the streams of laser photons, it "sees" an increase or
decrease in frequency (a change in color). Since we''ve tuned
our lasers just right, the moving atom sees just the right
frequencies for the photons to be absorbed or emitted. In the
process, the atom gets jostled and, with lasers hitting it in three
dimensions, slows down. Not one to stand still, Philips animates
this by pretending to be an atom dancing back and forth across
the stage with appropriate little jolts on emitting or absorbing a
photon. The faster atoms escape from the bottle and we''re left
with slow atoms.
If you would like to see and participate in a demonstration of
laser cooling, I found a neat University of Colorado Web site -
http://www.colorado.edu/physics/2000/bec/lascool1.html
On this site there are several interactive demos. In one, you
simply aim photons at the moving atoms, the analogy being that
if you shoot enough ping-pong balls at a bowling ball, you can
slow it down or stop it. Another demo brings in the color bit.
You not only shoot the photons at the atoms but you change their
color. When you hit the right color, the atom absorbs the
photons and slows down. It''s worth a shot to make you feel more
comfortable with the concept and it''s also fun. At least I thought
so.
The net result of all this is that Phillips and other workers in the
field of laser cooling really slowed those atoms down. The
prevailing view was that a temperature of just 240 millionths of a
degree above Absolute Zero had been achieved. That''s not only
really cold but it''s also as low as it was possible to go in that type
of experiment. At least that''s what the theorists said and it''s
always comforting to have a result that agrees with theory.
But wait! Remember that Phillips and his NIST colleagues are in
the business of measuring things very precisely. They devised a
fancy method for measuring the temperature and, what do you
know? They found the temperature was only 40 millionths of a
degree above Absolute Zero, six times lower than the 240 figure!
Theory said that''s impossible and the NIST workers were
shocked. So, they tried other approaches and came up with the
same answer. When they published their results some snickered,
but soon others had repeated the work and, sure enough, 40 is the
correct answer. Last week we noted that at room temperature
atoms in a gas zip along at over 300 meters (over a thousand
feet) a second. At 40 millionths of a degree Kelvin, those
sodium atoms are moving at only 7 millimeters or less than half
an inch a second - not stopped, but pretty darn close!
In an earlier column (Tick, Tock August 2000) I talked about
clocks that, if they could run that long, would gain or lose only a
second every 20 million years. If I heard him correctly, Phillips
said that with the slower atoms a clock could run for 30 million
years and only be off one second! Who am I to doubt him?
Allen F. Bortrum
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