The term "atomic clock" may conjure up scary, 1950s-horror movie mental images: A Doomsday device, constructed by a lab coat-wearing maniac in a mountain fortress, is ticking away the seconds before it wipes out our entire planet. In reality, though, atomic clocks are one of the more benign inventions to emerge from the explosion -- oops, maybe not the best word choice -- of knowledge about the workings of the atom and its parts. That knowledge came in the wake of the World War II Manhattan Project to develop the atomic bomb.
Unlike the bomb, though, atomic clocks don't split atoms and they don't blow up. Instead, they use oscillation -- that is, the change in the flow of electrical charge -- in between an atom's nucleus and its surrounding electrons, the same way an old-fashioned grandfather clock might use a pendulum. Because an atom's oscillation involves incredibly small units of time -- a cesium atom, for example, has a frequency of 9,192,631,770 cycles per second -- and is extraordinarily consistent, a clock set to that oscillation can keep time much better time than that old grandfather clock [source: Britannica].
That's why, since their invention in the late 1940s, atomic clocks have become a critical tool in a modern world dependent on technology. Atomic clocks make it possible to synchronize time across complex systems, ranging from the Internet to the system of Global Positioning Satellites.
But for something that's become such a normal and helpful part of our lives, atomic clocks still remain a bit arcane and mysterious. Here are some of the myriad strange and surprising facts about these helpful devices.
Atomic clocks have changed how we calculate common units of time.
Back when humans began to track the passage of time thousands of years ago, they did it by watching the apparent movement of the sun across the sky -- which actually was due to the Earth's rotation -- and basing units of time on that journey. Traditionally, for example, a second was defined as 1/86,400 of the average length of a solar day.
But with the advent of atomic clocks, which were far more reliable than the motion of the Earth itself, it became necessary to change that standard. In 1967, the second was redefined as the time that it took for an atom of the isotope cesium 133 to oscillate 9,192,631,770 cycles [source: Sciencemuseum.org.uk].
An atomic clock uses a single electron from each cesium atom to keep time.
As we explained previously, electrons orbit the center of an atom, which is called the nucleus. Imagine an extremely tiny version of our solar system, with planets revolving around the sun, and you'll get the general idea. Physicists have discovered that electrons are amazingly regular in their movements -- they tend to remain within a narrow range of orbits, with the distance from the nucleus determined by how much radiation they're emitting at a given moment. The distance between the lowest orbit and the highest orbit that an electron moves in is the frequency.
In the case of cesium, which is used in atomic clocks, scientists focus on just one of the element's 55 electrons -- the outermost one, which occupies an orbit that's conspicuously higher than the rest. The difference in energy between the outermost electron's closest orbit to the nucleus and its farthest orbit corresponds to a radio frequency of 9,192,631,770 cycles. That's the part that scientists actually use to calculate time and break it into incredibly brief units of less than a billionth of a second [source: Sciencemuseum.org.uk].
Atomic clocks are incredibly reliable now, but the first ones weren't.
In 1948, the U.S. National Bureau of Standards built the world's first atomic clock. Instead of cesium, the first clock used ammonia atoms, which were heated and shot out of a copper pipe. While the first clock proved that the concept of atomic clocks worked, it was never actually used for time keeping. The first atomic clock was off by about one second every four months. That made it less reliable than an existing technology, the quartz clock, which measured the oscillation of a piece of quartz when an electrical charge was applied to it.
Eventually, the scientists switched to using cesium, which had shorter oscillations, and improved the design in various ways. A 1959 model managed to keep time with an error of one second per 2,000 years, and by 1964, the clocks had become so precise that it took 6,000 years for them to lose or gain a second. Today a state-of-the-art atomic clock would be off by just one second after 6 million years of use [source: Sciencemuseum.org.uk].
Cesium, the stuff used in atomic clocks, is kind of an oddball element.
For starters, it's sometimes spelled "caesium." Cesium was discovered in 1860 by Robert Bunsen, better known to high school chemistry students as the inventor of the Bunsen burner. And it's such strangely fascinating stuff that in the early 1990s it inspired the creation of an Internet newsgroup, Alt.cesium, which was devoted to "discussion, praise, veneration, and adoration, the posting of songs, poetry, stories, and parables of and about that most sublime of elements" [source: Nelson]. Commonly known as "the other golden metal," it's one of only three metals that aren't gray or shiny silver in color (the other two are gold and copper) [source: Scientific American].
The type of cesium found in nature, cesium 133, is pretty difficult to locate. The natural source that yields the greatest quantity of it is a rare mineral called pollucite, which in the U.S. is found in ore from Maine and South Dakota. Though it's a metal, cesium melts at a really low temperature -- 82 degrees Fahrenheit (22.7 degrees Celsius) -- and explodes when it comes in contact with cold water [source: Argonne National Laboratory]. In air, it sometimes catches fire spontaneously, burning with a brilliant sky-blue flame [source: Nelson].
An atomic clock actually uses a hunk of quartz, not a cesium atom, to tell time.
You're probably a bit puzzled by this, given that we've spent all this time telling you how much more accurate an atomic clock is due to its use of the oscillation of cesium. But the part of the clock that actually keeps time is a standard quartz crystal oscillator, which subjects a piece of the crystal to electrical current to make it vibrate. The difference is that in most ordinary quartz clocks, the oscillator is tuned accurately when the clock is built, but its frequency is never checked or adjusted after that, which means that over time, slight variations develop that make the clock a little fast or a little slow. In an atomic clock, however, the oscillation of the cesium is used to check the frequency of the quartz device, which is what gives the clock such amazing accuracy [source: Sciencemuseum.org.uk].
In 2008, a second was added simultaneously to every atomic clock on Earth.
Just before 7 p.m. on Dec. 31, 2008, scientists wound the atomic clocks around the world ahead exactly one second, in order to synchronize Coordinated Universal Time (UTC), the international standard for atomic clocks, with the Earth's rotation. It wasn't the clocks that were off, but rather the planet, whose rotational speed is slowed down about two milliseconds each day by a variety of brakes: space dust, magnetic storms, solar winds, resistance from its own atmosphere, and most importantly, the tug of the moon's gravity on Earth, which not only causes ocean tides, but also makes the entire planet bulge.
The effect of all that is to lengthen the solar day, and throw it ever so slightly off in comparison to our super-accurate atomic clocks. It would take hundreds of years for the discrepancy to become noticeable, so that the position of the sun in the sky would be different from the time on a house clock, (which you've probably set according to the correct time phone number, which is based on the UTC). To prevent that from ever happening, in 1972, an international agreement decreed that atomic clocks periodically would be adjusted in unison [source: Dowling].
An atomic clock was used to prove that the higher you live above sea level, the faster you age.
The idea that someone who lives on a mountain ages faster than a person who lives on the beach may seem a little preposterous, but it's actually the truth. This concept was first advanced about a century ago by physicist Albert Einstein, whose theory of special relativity postulated that time is not constant, but relative. (That's why they call it relativity.) In 2010, James Chin-Wen Chou and colleagues from the National Institute of Standards and Technology (NIST) conducted an experiment to test Einstein's reasoning. They positioned two atomic clocks about 30 centimeters apart above sea level, and found that the higher of the two clocks ran slightly faster. In real terms, though, the difference wouldn't be noticeable; the mountain dweller would age about 90 billionths of a second faster over a 79-year lifetime, according to Chou [source: Connor].
Scientists make atomic clocks more accurate by using laser beams.
If you've ever seen that scene in the movie "Goldfinger" during which the villain threatens to slice James Bond in half with a laser, you're probably wondering why a laser wouldn't burn a hole through an atomic clock, instead of making it run more precisely. But it actually can do the latter. Bear with us because this gets pretty complicated.
Atomic clocks essentially bombard cesium atoms with microwaves to stir up some action, which scientists can then measure. The limitation of conventional atomic clocks has been that they can only catch a small portion of the cesium atoms with the microwave. By subjecting the atoms to a laser beam -- a process called laser optical pumping -- you can slow down the atoms' speed, which gives the microwaves more chance to hit them. That, in turn, makes for a more precise signal, which enables scientists to use the cesium oscillation to mark off time even more accurately. Oddly, the process also cools down the cesium atoms, right down to a few millionths of a degree above absolute zero on the Kelvin Scale [source: Buell and Jaduszliwer].
Atomic clocks make your phone conversations comprehensible.
These days, telecom companies transmit phone calls in bits and pieces called packets, which enables them to pump a vast number of conversations through their wires at the same time. When you call someone in another city, your words are broken up and transmitted between computers at each end, which flick back and forth between one conversation and another, thousands of times every second. For that to work, however, the two computers have to stay in perfect sync, like a pair of incredibly nimble ping pong players who can hit the equivalent of a truckload of little balls at each other at blazing speed and never miss even one. If they do miss, the calls will get jumbled and sound like gibberish. That's why telecom companies these days have their own atomic clocks to prevent this from happening, by keeping the computers almost perfectly in step with one another at all times [source: Sciencemuseum.org.uk].
A next-generation atomic clock may be able to keep perfect time with the universe.
Scientists keep dreaming up ways to make atomic clocks more and more accurate, but researchers at the Georgia Institute of Technology and the University of Nevada recently proposed a truly mind-blowing advance. In vastly oversimplified terms, here's the deal: They want to use lasers to rearrange the pieces of an atom, so that they can use an orbiting neutron, rather than an electron, as the equivalent of a pendulum. The result might be a clock that would be 100 times more accurate than any now in existence, so precise that it would only lose or gain less than one-twentieth of a second in 14 billion years. Consider this: The universe itself is roughly 14 billion years old, so if this clock somehow could be sent back in a time machine to the moment of the big bang that started everything, it would still be ticking along today, in virtually perfect step with every moment that has ever occurred [source: Science Daily].
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I've always been fascinated with time keeping and clocks, and as an elementary school student in the 1960s, I first learned about the wonder of atomic clocks that could measure tiny units of time with incredible precision. But until I started doing research for this piece, it had never occurred to me that it was possible to construct a clock that is more precise at keeping time than the Earth's rotation. But since atomic clocks have been used to prove Einstein's theory that time is relative to one's position and velocity rather than constant, I have to wonder whether there really is such a thing as correct time at all. I think I'm going to use that as an excuse the next time that I'm late for a deadline!
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- "Proposed Nuclear Clock May Keep Time With The Universe." Science Daily. March 8, 2012. (April 17, 2012) http://www.sciencedaily.com/releases/2012/03/120308101331.htm
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