Newscripts

March 17, 2008

Volume 86, Number 11

p. 248

Science Friction with Bob Wolke

Will all those who are wearing wristwatches please raise their left hands. (Southpaws, raise your right hands.)

Now, will all those who think of their wristwatches primarily as jewelry or status symbols please lower their hands and leave the room.

May I assume that those who remain think of their watches primarily as instruments for telling time?

Good. You're the ones I want to talk to. But if all you want from your watch is to determine lunchtime and quitting time, you can stop reading now, because my message is that if you use your watch to tell time, you should have one that tells it as accurately as possible. You're a scientist, aren't you? You make laboratory measurements as accurately as your instruments will allow. Why should you have to settle for a rough approximation of the time? Time is precious. It deserves all the accuracy we can muster.

In the International System of Units (SI), the second is one of only seven base units from which all others can be derived: the meter, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. Of these, only the kilogram remains arbitrarily defined as the mass of a certain chunk of Pt-Ir alloy in S??vres, France. (But not for long; see page 48.) Even the meter, until recently taken as the length of another chunk of French metal, is now defined in terms of the second: the length of a path traveled by light in vacuum during a time interval of 1/299,792,458 of a second-the reciprocal of the speed of light in meters per second.

But how should we define a second? Is it 1/86,400 of a revolution of our Mother Earth? No, because after some 1.6 trillion pirouettes, the ol' lady has been slowing down. But nothing is more unvarying over the lifetime of the universe than the object of our affections as chemists: a stable (nonradioactive) atom.

But how should we define a second? Is it 1/86,400 of a revolution of our Mother Earth? No, because after some 1.6 trillion pirouettes, the ol' lady has been slowing down. But nothing is more unvarying over the lifetime of the universe than the object of our affections as chemists: a stable (nonradioactive) atom.

Atoms of the lighter elements were created by fusion reactions in stars. But the formation of heavier elements requires the temperatures of supernovae, which have been spewing into space these elements, some of which ultimately formed our planet. The atoms we find today are exactly the same as when they were made and will presumably remain so forever. That includes their atomic spectra. So let's pick an atom and base our standard of time on one of its energy transitions.

I have on my wrist a watch that not only gives me an up-to-the-second digital readout, but also sets itself automatically every midnight to Coordinated Universal Time (UTC, the official world time) with a precision of two parts in 10¹⁵. It does that by receiving radio signals broadcast by the National Institute of Standards &; Technology from Fort Collins, Colo. There, NIST maintains an "atomic clock" based on the transition between the two hyperfine levels of the ground state of ¹³³Cs at absolute zero. Why ¹³³Cs? It is the sole stable isotope of an easily vaporized element.

The cesium "atomic clock," designated NIST-F1, corrals a ball of ¹³³Cs atoms in a vacuum and cools it to near 0 K by bombarding it from six directions with photons from finely tuned lasers. (Photons, as Albert Einstein taught us, have momentum.) The ball is then tossed upward into a microwave cavity, where the ground state splits into its two hyperfine levels. Another laser then excites resonance between the two states at their resonance frequency of 9,192,631,770 Hz. That number of cycles is taken to define the second.

Why am I telling you all this? (Oh, sorry. You can put your hands down now.) Because I believe that a watch should be not only accurate but also easy to read. In the 21st century, when we do almost everything digitally except flossing our teeth (well, we do use our fingers), we still tell time by reading an analog display. We must guesstimate the positions of two pointers or "hands" (Mickey Mouse's included) moving around a numbered dial: the ubiquitous analog clock face. (The hands move around "clockwise" to mimic the path of the sun as viewed from the Northern Hemisphere.) But isn't it silly to be able to know the exact time to the femtosecond and then have to settle for the nearest minute? We might as well be reading the shadow on a sundial.

Arise, then, thou seekers of Truth! Discard your medieval dials and avail yourselves of what science hath wrought: a digital "atomic" watch. Others may not care what time it is, but you and I will always know.

Chemical & Engineering News

ISSN 0009-2347

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