Skating on Thin Ice

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