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03/28/2000
Skating on Thin Ice
This is the last of my columns originating in Marco Island,
Florida. I had not planned any more follow-ups of subject matter
discussed in earlier columns but several articles Brian Trumbore
sent down here demand prompt attention. Also the size of the
moon entered into my pre-dawn walks this past week. For the
past few years, I''ve looked forward to morning of the full moon
setting in the west as the sun rises in the east. This happened last
week and I started my walk under a crystal clear sky with my
pronounced "moonshadow" following me on the beach. The
moon appeared larger as it neared the horizon. Then a dense fog
rolled in, deleting the stream of moonshine on the water and
washing out other visual cues while still allowing the moon itself
to be visible. Am I skating on thin ice or, without those visual
cues, didn''t that moon seem significantly smaller than before the
fog appeared? Wasn''t it just what we talked about earlier
(2/1/00); that is, the closer the moon seems, the smaller it
appears?
One of the articles Brian sent was from the February 18 Wall
Street Journal and it dealt with dust mites. You may recall the
recent column (2/15/00) on fabrics formulated to kill the fungi
that make our shed skin a suitable repast for these tiny
critters. I must admit to being utterly astounded by and skeptical
of a statistic quoted in the Journal article. The statistic: "the
average mattress will double in weight in 10 years as a result of
being filled with dead dust mites and their detritus". Now, I
don''t know about you, but a chore I really hate is flipping and
turning our queen-size mattress, an activity I only undertake
when prodded by my spouse. Over the years, I have noticed that
the job gets harder and harder but I''ve attributed this to my
advancing age. It never even occurred to me that the mattress
could be getting heavier due to dust mites! Has there actually
been experimental data to support this and, if so, where do I find
a scale big enough to weigh my mattress? Or is the author of the
article skating on thin ice when she quotes this statistic for the
"average" mattress? I welcome any input on this subject.
Another follow-up is on the subject of dark matter. One of my
first columns (5/25/99) reported on a lecture I had by Tony
Tyson of Bell Labs. He showed how a map of this dark matter,
which comprises almost all of the matter in the universe, could
be inferred from its effect on the bending of light from distant
celestial objects. We mentioned that one candidate for this dark
matter was the WIMP (weakly interacting massive particle).
This particle was strictly a theoretical postulate and its existence
highly speculative - possibly until now?
A group of researchers at the University of Rome in Italy have
claimed to have detected this WIMP by conducting three years''
worth of experiments deep underground. By going underground,
they hoped to eliminate all but the most penetrating particles,
specifically the WIMPs, if they exist. In their work they used the
compound sodium iodide, which emits flashes of light when
struck by particles. What the researchers have done is to count
the number of flashes and in doing so, they find a seasonal effect.
They claim that such an effect is expected based on the
inclination of the earth and its orbit around the sun, combined
with the distribution of dark matter in our galaxy. If their
interpretation is correct, the WIMP they have detected weighs
about 60 times as much as a proton, about as much as an atom of
nickel.
It''s hard for me to understand how such a heavy particle can go
traveling about the universe without banging into all kinds of
things. However, if they do exist, it seems that about a billion
may be passing through our bodies every second or so. In this
regard the WIMP is like the much lighter neutrino, only recently
shown to have any mass at all! The Rome group claims to have
found those rare events when a WIMP does bang into and
interact with something. There is a considerable amount of
skepticism in the physics community over the Rome results,
partly due to the possibility that the seasonal effect could have
some other explanation. Let''s hope they are not skating on thin
ice.
Speaking of ice skating, I have only strapped on ice skates two or
three times in my life. More my speed as a kid was playing
hockey on roller skates on the sidewalks and streets of
Mechanicsburg, Pennsylvania. I can still feel the impact of my
head hitting a tree while concentrating on the puck instead of my
surroundings. Although my experience at ice skating is marginal
at best, I thought I understood why skaters glide so smoothly and
why ice is so slippery. Specifically, the melting point of ice is
lowered under the pressure of the skater''s weight concentrated in
the blades of the skates. That is, the skater is gliding along on a
film of water formed under the skates.
Or so I thought until I stumbled on an article by two physicists
from the University of Washington in the February 2000 issue of
Scientific American. The article, "Melting Below Zero"
concerns a phenomenon known as "surface melting". To
understand surface melting let''s go down inside an ice crystal.
Here, each water molecule is bonded in some manner to its
neighbors and there is a nice regular arrangement of these
molecules. Very close to the surface, however, things get a bit
dicey. At the surface there aren''t any molecules above the
surface molecule and in a sense there''s nothing to hang on to or
to keep them anchored. Accordingly these surface and near-
surface molecules may move around, even tens of degrees below
the melting point of water ice. This surface layer thus has
properties like water itself and the film becomes more like water
as we approach the melting point. This water-like film is a
contributing factor to such things as the upheaval of soil and
rocks in cold weather and also participates in the transport of
charges that build up on ice crystals and water droplets in clouds
that lead to thunderstorms. In the clouds the small ice crystals in
an updraft collide with the falling hailstones. The collisions
result in the ice crystal apparently losing electrons to the
hailstones, which thus acquire a negative while the up-flowing
ice crystals now have a positive charge. The ice crystal also pick
up a layer of water in the process. A consequence of these
collisions is that the cloud develops a negative charge at the
bottom and a positive charge at the top and the stage is set for
lightning.
Quite aside from the surface melting, another factor which leads
to a liquid or liquid-like layer on the is the pickup of impurities
from the air. These atmospheric impurities can be salts or other
compounds that lower the melting point of water at the surface.
So, we see that surface melting and impurities can contribute to
the slipperiness of ice. The authors of the article point out that
pressure only lowers the melting point of water a degree or so
and imply that it''s not as important factor as one might think,
especially at temperatures much below the melting point.
However, I must say that I personally find walking on ice much
less hazardous when it''s really cold compared to when the
temperature is at or just a tad below freezing.
So much for skating on thin ice. I''m off to a walk on the beach!
Allen F. Bortrum
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