Keeping Time to the Nanosecond: Atomic Clocks

A New Standard for a New Age

Since the first creation of the sundial, thousands of years ago, man has struggled to keep accurate track of the time. Until the 20th century, all developments with clocks still depended ultimately on one thing, the rotation of the earth in the creation of a full day. With origin of the atomic age in the first half of the 20th century, a new standard was created to keep track of the time: the atomic clock.

The first atomic clock was created by the National Institute of Standards and Technology (NIST). Unlike modern atomic clocks that use the cesium ion cesium-133, this original atomic clock used an ammonia molecule standard. In 1952, NIST made an accurate measurement of a cesium resonance, which would become the base of the NBS-1 clock. However, the first cesium beam clock was built in 1955 at the National Physics Laboratory in England by physicist Louis Essen.

As these clocks continued to develop, they became increasingly accurate with telling time. In 1967, the 13th General Conference on Weights and Measures redefined the second as being 9,192,631,770 vibrations of the cesium atom. This was the first time in the history of mankind that time was not defined on an astronomic basis.

This new method led to the creation of two international time standards: International Atomic Time (TAI) and the Coordinated Universal Time (UTC). International Atomic Time is based purely on the atomic clock, while UTC, used to disseminate civic time, is based on the TAI readings but also adjusted to synchronize more precisely with the natural transitions between day and night.

The current time standard in the United States is the NIST-F1 atomic clock located in Boulder, Colorado. This is the official clock of the United States, that also plays a role in the calculation of both TAI and UTC. It was first built in 1999, and is the continuation of a long line of atomic clocks built by the National Institute of Standards and Technology, including a long line of NBS clocks and the NIST-7.

The NIST-F1 is so accurate that it will not gain or lose more than a second of time in 60 million years. To put that into perspective, it was about 60 million years ago that the dinosaurs went extinct. Had the NIST-F1 gone into operation then and continually ran until now, it would not be off by more than a second!

How an Atomic Clock Works

An atomic clock works on the same principles as an ordinary pendulum clock. As the traditional pendulum clock counts off seconds based on the oscillation of the swinging pendulum, so does the atomic clock base its seconds on the oscillation frequency of the cesium-133 ion.

The NIST-F1 is referred to as a fountain clock. First, a gas of the cesium-133 ions is introduced into a special vacuum chamber. 6 lasers positioned at right angles with each other are shot at the gas, forcing the ions into a ball-like mass. These lasers also cool the atoms until they reach a temperature of almost absolute zero.

At this point the lasers are shut off, and two lasers are used to lift the ball upwards like a fountain (hence the name) through a special microwave chamber. The microwaves excite the atoms, and then the ball returns to the vacuum chamber. Some of these ions are charged at a particular frequency: 9,192,631,770 Hz, the standard used to measure the second. At this frequency the ions emit light. Special devices within the chamber take readings of the ions to see which are at the proper state. The process is then repeated until the proper frequency is found to keep the ions moving at 9,192,631,770 Hz, and thus define the second.

Although cesium-133 is used for the international standard, other types of atomic clocks do exist. In Russia, hydrogen masers are often used to create atomic clocks. For commercial enterprises, rubidium is a popular choice. Both of these elements offer cheaper methods of developing atomic clocks, and both offer excellent short-term stability, although they cannot provide the same long-term accuracy that a cesium clock has.

Atomic Clocks in Your Home

While most people aren’t going to be building an atomic clock in their own home in order to tell the time, there are ways to make sure that your clocks are set to the official standard. Often referred to as atomic clocks (although more properly called radio clocks), these clocks never need to be set and always give the most accurate, up to the second time.

A number of different companies offer these atomic radio clocks, in all shapes and sizes, and even atomic radio watches. NIST-F1 in Boulder, Colorado, sends out constant radio signals informing of the current time. Radio watches are tuned into this frequency, and check themselves at least once a day against the standard being kept by that clock.

Because it is based on these radio signals, atomic clocks and watches are not able to work everywhere in the world. However, for the contiguous US and even some parts beyond, you are covered by the area of the radio signal and thus your clock or watch will always be kept updated. These clocks are not available in Alaska or Hawaii.

If you are looking for another way to keep the clocks in your house up to date, the official US time can also be found on the official US time web site (link given below). The site offers two options for finding out the official time, one is a special Java clock and the other is a static clock that must be constantly refreshed to keep giving the accurate time, but works if your computer does not support Java.

Unfortunately, these clocks are set to the official standard, and are not corrected based upon the speed of your internet connection. That means that if your internet connection is slow, the time shown on the site might actually be several seconds off from the actual official time.

There are also many software programs that work on your computer that update themselves on the official time, such as Atomic Clock Sync (I personally use this program, as it is free). These programs connect to the official time server and ping the server to calculate the speed of your connection. The program then makes adjustments to the signal from the server so as to set your computer clock accurately the official server.

If you want to know that your clock is always correct, there is no better way to go than to keep your clocks set to the official US standard. Never before in the history of the world has a more accurate clock existed, and it is possible for you to have this incredible accuracy in the privacy of your own home.

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