Atomic Clocks
What Time is it, Really?
Carolyn Lauer EE ‘03
You are running across campus wearing a heavy backpack and your shoes are untied. The trek to DeBartolo seems to be miles farther than yesterday and the watch on your wrist is passing time an outrageous pace. A stream of sweat rolls down your cheek as you are imagining the test that your classmates are probably taking without you. You’re late. Or are you? Do you know what time it is, really?
Society today is becoming increasingly fast paced and ruled by schedules, time frames and the clock. Our obsession with time is growing each day and with this obsession comes a desire to make the most of the time we have, and also to measure it as accurately as possible. Atomic clocks are the most precise time measuring devices that exist today and are becoming progressively more important with the evolution and sophistication of technology today.
What is an Atomic Clock?
An atomic clock is not radioactive as the name may connote. These clocks do not keep time according to atomic decay, but rather by the oscillation of the nucleus of an atom and its surrounding electrons. The mass of the nucleus and the gravity and electrostatic “spring” between the positively charged nucleus and the electrons set the oscillation frequencies. This oscillation is not directly analogous to the time keeping of normal clocks, but both types of clocks use oscillations to keep track of time. Atomic clocks are more reliable time keepers because their oscillations are not subject to change according to environmental factors such as humidity that would decrease the accuracy of normal clocks.Evolution of Atomic Clocks
For many years scientists have realized that atoms have resonant frequencies that can be attributed to the ability of each to absorb and emit electromagnetic radiation. In the 1930s and 1940s the appropriate high frequency communication and radar equipment was developed to interact with the frequencies at which these atoms and molecules resonated and the idea of the atomic clock was conceived.
The first atomic clock was built in 1949 by the National Institute of Standards and Technology (NIST) and utilized ammonia as the source of the clock setting vibrations. However, this clock was not much more accurate that the existing time standard and the next generation of atomic clocks was built with cesium.
After the implementation of cesium in the atomic clocks the time keeping became much more accurate than the current standard. There was such a dramatic change in the accuracy of the measurement of time that in 1967 the General Conference on Weights and Measures defined the SI second as 9,192,631,770 oscillations of the cesium atom at its resonant frequency. This meant that the world’s time keeping no longer had correlation with the earth’s motion. The world’s most stable atomic clock was completed in 1968 and was used as part of the NIST time keeping system well into the 1990s.
An example of an atomic clock during the first years this technology was being developed.
One of the most recent advancements in atomic clocks today is the addition of a laser cooling system. This system improves the signal-to-noise ratio and also decreases the uncertainty in the clock signal. To accommodate this cooling system and all the other equipment used to better the time measuring cesium clocks used today are about the size of a railroad car, although commercial clocks can be as small as a suitcase. One such lab atomic clock keeps the official time for the United States. This clock is located in Boulder, Colorado and is the most accurate clock on earth. To give an idea of this clock’s precision, it only loses two nanoseconds a day or one second in 1,400,000 years.
How do Atomic Clocks Work?
As can be imagined, the time keeping method of cesium clocks is very complicated. First, the liquid cesium is put into an oven and heated until it changes into a gas. These cesium atoms escape through a small hole in the oven at very high speeds. After passing through the hole these fast-moving atoms electromagnets force the atoms to split into separate beams according to their energy. The beam of the appropriate energy is sent through a U shaped hole and the atoms are exposed to radiant energy by specific wavelengths of microwaves. The wavelengths used in this procedure are in a very small range that includes 9,192,631,770 Hz. These microwaves give the atoms more energy and when they receive the energy at exactly the right frequency, the atom is excited to another energy state. All the atoms are sent through another magnetic field, which filters out those that have been excited to another energy state by the microwaves. A detector at the end of the tube gives an output according to the number of cesium atoms striking it and peaks when the frequency is absolutely correct. This peak is then used to make slight corrections to the crystal oscillator that controls the clocking mechanism, locking in the frequency. This locked frequency is then divided by 9,192,631,770 which results in the familiar one pulse per second.
This early atomic clock retains its accuracy within 1 sec for every 300 years and was among one of the first clocks to better the accuracy of conventional timekeeping methods.
Although the most common and widely used atomic clocks utilize the properties of cesium, there are other types of atomic clocks. The difference between these clocks is the element used and also the means of detecting when the energy level changes. Other materials used in these clocks are hydrogen and rubidium. Hydrogen atomic clocks function in mostly the same way as described above, but they require a container with walls of a special material so that the atoms do not lose the high-energy state too rapidly. Rubidium clocks are the simplest and most compact of all atomic clocks and use a glass cell of rubidium gas that changes its light absorption when exposed to the proper microwave frequency.
Clocks used in laboratory applications, such as the ones shown here, are much bigger and more powerful than their commercial counterparts.
This is an example of an early atomic clock which utilized cesium for its timekeeping.
Why is Accurate Time Necessary?
Today time can be kept with extreme precision, but why is this important? The accuracy with which time can be measured and controlled is imperative to systems such as mobile telephone, land line telephones, the internet, GPS, aviation programs, and digital television. At first it the accuracy with which we can measure time does not seem to correlate, but consider the average phone line. Thousands of phone calls are being transferred on a small amount of phone lines. This is possible only because the phone call is not transferred in it entirety. The phone company separates the conversation into tiny packets and even omits some of the information. It sends these packets across the phone line along with the packets from other conversations and at the other end your conversation is recovered without interference from the conversations of all those other people. A clocking system on the phone line is able to designate which packets belong to each conversation because of the precise time that the information was sent. Another implementation of precise time is Global Positioning System (GPS.) This system consists of 24 satellites, which broadcast their positions and times accurately. Any GPS receiver can connect with these satellites and compare the times broadcasted by each. The difference in these times allows for the user to pinpoint his location. If these clocks were not highly accurate the GPS system would be impractical and unreliable.
Atomic clocks have many uses and one of the more familiar ones is the Global Positioning System. This is an example of a receiver that utilizes such a system.
Even with advancing technology and atomic clocks there is still imprecision in the universe. The earth wobbles and moves on its axis causing the random fluctuations in the length of years and days. In the past, these variations would have gone unnoticed, as the tools for measurement were too imprecise to detect them. However, much to the dismay of researchers and scientists, the precise time keeping with atomic clocks must be adjusted to compensate for anomalies in the real world. Atomic clocks are amazing tools for the advancing technology of today, but their usefulness can only extend as far as the limits of nature will allow.
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