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04/10/2001
Absolute Zero - It's Cold!
A couple weeks ago, my wife and I went down to Washington,
DC to attend a meeting of The Electrochemical Society (ECS).
Sadly, she had a bad cold and, outside, the wind chill was in the
teens or even single digits. The cherry blossoms, which were
supposed to be in full bloom, wisely chose to remain tightly
huddled in their buds. Like the blossoms, my wife remained
huddled in our hotel with its modicum of warmth while I
attended the meeting. Neither of us got out to a single museum
or any of Washington''s other great attractions.
Cold as it was, it didn''t compare with the frigidity of the topic of
the plenary speaker at the meeting. He was Nobel Prize winner
William Phillips and his lecture was titled "Almost Absolute
Zero: The Story of Laser Cooling and Trapping". If you''re
unclear about the concept of Absolute Zero, simply put, it''s as
cold as you can get. Phillips had a more sophisticated take on
the subject. At normal room temperature, the atoms or
molecules in a gas such as our air are on average moving at over
300 meters a second, the speed of sound in a gas at that
temperature. Since it''s baseball season, a molecule moving at
that speed would take about half a second to travel from home
plate over the centerfield wall. That''s assuming it didn''t bump
into anther molecule. As you might expect, half a second is
about the time it takes for the sound of the crack of the bat to
reach you if you''re sitting in the bleachers at Yankee Stadium. (I
sat there for a couple World Series games many years ago.)
What''s this got to do with temperature? Well, one measure of
temperature is how fast the atoms in a gas or vapor are moving.
In fact, if you can measure how fast the atoms or molecules in
gas are moving, you can calculate the temperature. What is
Absolute Zero? It''s the temperature at which the atoms or
molecules don''t move at all. Here''s where Phillips shows why he
won the Nobel - "What can be slower than stopped?" As I said,
you can''t get any colder.
How cold is Absolute Zero in terms of our temperature scale? It
turns out we have three temperature scales to choose from, one
of them pretty weird. The weird one is, of course, the Fahrenheit
scale we use all the time in the United States. It''s based on the
scale proposed by a German fellow named Fahrenheit. The
freezing point of water is 32 degrees F, the boiling point 212 F
(let''s drop the degree bit). A Swedish guy named Celsius had a
more sensible idea - make water''s freezing point 0 C and the
boiling point 100 C. When I first studied science, the "C" stood
for Centigrade. However, in 1948 it officially and, logically,
became Celsius. I still say Centigrade!
The third and most useful scale scientifically is the Kelvin scale,
named after its father, William Thomson. Thomson was a famed
British mathematician and physicist who published some 600
papers. He was knighted in 1866 and in 1892 elevated to the
peerage, becoming Baron Kelvin of Largs, or Lord Kelvin to his
buddies. His reason for suggesting the new scale was based on
studies of gases that showed an interesting behavior. If you have
a gas in a container and measure the pressure with a gage of
some sort, the pressure rises or falls if you raise or lower the
temperature. This doesn''t surprise you, hopefully. You know
you shouldn''t put those sealed frozen dinners in the microwave
without puncturing the plastic - otherwise the pressure increase
on heating might blow the steam or hot contents of the dinner
onto your face or hands as you take it out of the oven.
What was surprising back in the 1800s was the following. If you
take the data on various gases and plot the pressure against the
temperature, you get a straight line. If you put a ruler down on
the line and continue it to very low temperatures it comes to a
point where the pressure is zero - zilch! And that temperature is
the same, -273 C, for every gas. Lord Kelvin suggested that this
temperature be designated as the new zero. Today, Absolute Zero
is -273.15 C (-459.67 F). On the Kelvin scale then, the freezing
point of water is 273.15 K. Virtually all the fundamental
equations in physics and chemistry that involve temperature
require that the temperature be in degrees Kelvin.
All this business of different temperature scales and the
confusion that results from having to convert from one to another
reminds me that yesterday NASA launched another mission to
Mars. After two failed missions in a row, NASA is on the
griddle to produce. Dr. Phillips works for the National Institute
of Science and Technology (NIST). Why mention this? A few
years ago NIST launched an unsuccessful campaign in the U.S.
to convert to the metric system of units. The campaign failed
miserably with the American public and I plead equally guilty,
even as a scientist, to not feeling comfortable with having
weather reports in Celsius and yardages on golf courses in
meters. Had NIST succeeded, one of those failed Mars missions
would have been ok. That was the one that failed because the
contractor reported data in one set of units and the Mars
spacecraft was programmed in another.
NIST, born as the National Bureau of Standards is celebrating its
centennial this year - it''s birth date was March 3, 1901. At the
meeting in Washington, I picked up a very nice softbound
publication detailing the highlights of NIST''s history. It was the
first federal research agency devoted to the physical sciences.
Its mission was to establish a national set of standards to replace
a hodgepodge of standards then prevailing throughout the nation.
NIST''s responsibilities included such things as defining and
maintaining standards for units of length, time and weight.
When you buy a pound of coffee, you want to be sure that you
don''t get half a pound.
Temperature has always been an item in NIST''s repertoire. Back
when I was a graduate student and early in my Bell Labs career, I
used thermocouples to measure the temperatures in my furnaces
or other apparatus. By measuring thermocouple voltages I could
get the temperature from a table of voltages and temperatures for
that particular type of couple. But no two thermocouples are the
same and I needed to calibrate them to see how far they departed
from the tables. To do this, I bought from the then Bureau of
Standards certified bars of such metals as tin, lead or silver. I
would melt one of them in a pot, cool the melted metal down
until it froze and measure the voltage of my thermocouple. The
NBS certified the freezing point for that standard sample, so I
then had a calibration point to tell me how far my couple differed
from the one in the table.
You might think that, with the mission of setting and maintaining
standards, NIST would be a pretty humdrum place to work. Just
three years after its founding, in 1904, there was a huge fire in
Baltimore. Firefighters and their apparatus from Washington and
even New York were rushed to the scene. However, their
couplings to the fire hydrants for the most part didn''t fit the
hoses. Over 1,500 buildings burned in that conflagration. NBS
had already on hand over 600 sizes and configurations from an
earlier investigation and joined in setting a national standard.
Today you can feel assured that a neighboring town can join with
your town''s firefighters in fighting a blaze.
In 1915, NBS published what proves to be a precursor to
Consumer Reports, "Measurements for the Household". The
journal Nature lauded the publication as being a "treatise on
domestic science". Among the subjects covered were the
operations of thermometers and clocks. Again, you might be
surprised that a Nobel Prize would be given to a fellow working
in an establishment devoted to such mundane pursuits as
measuring temperature and time. But it turns out that Dr. Philips
won the prize in large measure doing just that - achieving and
measuring very low temperatures. And, as a consequence of his
work, improving the accuracy of clocks!
Perhaps we shouldn''t be surprised that measuring temperature
can be a key to winning a Nobel. After all, Bell Labs'' Arno
Penzias and Robert Wilson got the prize for measuring a pretty
low temperature, 2.73 K. Ok, they were measuring the
temperature of the universe, or at least the "empty" space in the
universe. And it did provide strong confirmation of the big Bang
theory. The temperature they measured was that of the infrared
radiation left over from the Bang.
I find that I have rambled on and now do not have enough space
left to properly describe Dr. Phillips'' work or his lecture. To be
perfectly honest, I was hoping this would happen. I''ve been so
engrossed in TurboTax this past week that I haven''t had time to
collect my thoughts on laser cooling and trapping. Actually, I
did want to lay the groundwork for Absolute Zero. I probably
should have mentioned that there''s a law of thermodynamics that
says you can never reach Absolute Zero. But next week we''ll see
that Dr. Phillips and others have come awfully close! We''ll also
talk about his lecture, the most entertaining lecture I''ve ever
attended, and his flat, frozen Frisbee balloons.
Allen F. Bortrum
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