Precise time measurement has practical uses

Ever since the ancients discovered that the endless cyclical parade of celestial events, such as the motion of planets and phases of the moon, are tied to changes of season here on Earth, humans have been interested in measuring the passage of time.

Early attempts, such as 4,000-year-old Stonehenge on the Salisbury Plain of England, simply kept track of the seasons. Centuries later the Egyptians, Romans and Chinese invented water clocks, hourglasses, hour candles and sundials to mark the passage of the day.

But, until well into the modern era, time was reckoned by celestial motion, with the second being defined as 1/86,400 of a mean solar day. The latter is defined as being exactly 24 hours. James Jespersen and Jane Fitz-Randolph have chronicled the early methods for measuring time in their book “From Sundials to Atomic Clocks.”

The first mechanical clock was built sometime in the 14th century and was powered by a dropping weight that engaged a notched wheel which, in turn, caused a small weighted arm to swing back and forth. It was accurate to about 15 minutes a day. In 1656, Christian Huygens constructed a pendulum clock that was accurate to 10 seconds per day. Robert Drullinger in the July-August 1994 issue of Compressed Air says that the Shortt double-pendulum clock was the most accurate mechanical clock ever built. Constructed in the 1920s, it had an error of only a few seconds per year.

Despite their precision, pendulums are awkward to use and timepieces with balance wheels and hairsprings soon became the norm. In 1713, the British Admiralty offered a large cash prize to anyone who could devise a means of determining longitude to within a half-degree. This required the construction of a timepiece that could withstand the rigors of months at sea without gaining or losing time. John Harrison, a clockmaker, spent 40 years testing one model after another until, in 1761 when he was 68 years old, his chronometer made the three-month trip to Jamaica while losing only 54 seconds and giving him the prize.

The next major advance in measuring time came when clockmakers switched to counting the number of oscillations in a vibrating material. In 1959, the balance wheel was replaced by a tiny tuning fork that vibrated at 360 cycles per second and was accurate to about one minute per month.

These soon gave way to quartz crystals that would vibrate in tune with an oscillating electric charge placed upon it. This is known as the piezoelectric effect and is used today in ultrasound imaging. F.G. Major, in his book “The Quantum Beat,” writes that vibrating quartz watches became commonplace in the early 1970s and were accurate to less than five seconds per month. In order to make further refinements, clockmakers had to find something that would oscillate far faster than any crystal known. The answer lies with the oscillation of electrons between energy levels in an atom.

An atom of the element cesium has a single electron in its outermost valence shell that oscillates rapidly between two energy states when stimulated by an outside energy source. Metallic cesium is vaporized and beamed through a chamber, where the atoms are irradiated with microwaves at a frequency exactly equal to the electron transition energy. This frequency equaled 9,192,631,770 hertz (cycles per second). The new definition of a second became the time needed for exactly this number of cycles to occur.

The first atomic clock, developed by the National Bureau of Standards in the 1950s, had an accuracy of 0.00001 second per day. The last, in 1993, had an accuracy of 3 billionths of a second per day. At this point, uncertainties associated with the rapidly moving electrons made it seem that the limits of measurement had been reached.

But now two papers, one by Pierre Lemonde in the January 2001 issue of Physics World and the other by James Bergquist et al. in the March 2001 issue of Physics Today, say that this barrier has been surpassed by cooling the cesium vapor down to a few millionths of a degree above absolute zero. This has the effect of slowing the atoms down to the point where the accuracy of frequency measurement has been increased twenty fold and, according to Peter Weiss in the Aug. 7, 1999, issue of Science News, future atomic clocks with an accuracy of one second in 300 million years are envisioned.

Why are clocks with such a fantastic accuracy needed? Physicists have used them to verify time dilation, a basic tenet of Einstein’s theory of relativity while NASA used atomic clocks to help track the Voyager spacecraft. But the most practical use, from an everyday viewpoint, is in the Global Positioning System, where the accuracy makes it possible to triangulate to within 30 meters any point on the planet’s surface.

Clair Wood taught chemistry and physics for more than 10 years at Eastern Maine Technical College.