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----------------------------- 2682 - ATOMIC CLOCKS - keep accurate time?
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- Today atomic clocks are on the ground helping satellites and deep space probes navigate throughout the solar system. Soon they will be on board these space probes providing built-in transmission accuracy. This will allow one way communications instead of the clock being on the ground and the signal traveling to the space probe and traveling back.
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- That accurate clock signal allows astronomers to accurately measure the speed and position of the space craft.
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- Most of today’s atomic clocks are too large and power hungry to be carried on the spacecraft. But, NASA did launch a 35 pound toaster size atomic clock to prove the concept of their improved accuracy of navigation just to see how it worked.
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- When spacecraft can routinely carry their own atomic clocks it will cut the time in half of the time needed to determine speed and position. These clocks will be accurate enough to measure one second in a period of 10,000,000 years. Read this again. Too amazing. This is 50 times more accurate than today’s Global Positioning System that is using atomic clocks.
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- Most of today’s atomic clocks us the silver-white metal cesium. This element melts at 83.1 degrees Fahrenheit. It is not radioactive and is the only stable isotope of Cesium , Cs-133.
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- The technology being investigated today has aspirations of designing a clock the size of a grain of rice. Wow, what a difference that will make! We will have to wait a few years for this one.
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- Today the 31 satellites in the GPS system use “rubidium atomic clocks“. Every new satellite has four of these on board. In addition there are 400 atomic clocks around the world keeping track of “Coordinated Universal Time“.
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- Because the Earth’s rotation is not 100% consistent, every year the clocks need to add a “leap second”, to our standard time every year or so. 23.59.59 goes to 23.59.60 in order to sneak in that extra second to account for the Earth’s rotation slowing down. This slower rotation is primarily caused by the friction of the tides as the Moon is pulling them back and forth across the Earth’s oceans.
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- The atomic clock works by precisely controlling the oscillations of the cesium atom. The lock itself is constructed of 8 parts. It starts with s vacuum tube of cesium. This is surrounded by a shield to protect stray magnetic fields from entering the tube.
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- Next there is an oven to keep the cesium atoms at a constant temperature. The Cs-133 isotope is heated to evaporate the metal in to a gaseous state. The gas atoms then can escape the oven in a hole that sends the atoms in to a tube.
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- The free atoms are traveling at 820 feet per second into this chamber where a magnet separates those atoms that can still absorb energy. The others are deflected into a graphite “getter“. One gram of cesium is this oven will last about one year.
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- The beam then enters a microwave cavity, called a “Ramsey cavity“, where microwave energy resonates at a frequency near to the natural frequency of the cesium atom’s vibration. When the frequencies exactly match the oscillations absorb the microwave energy
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- A second magnet follows to separate those atoms that have absorbed the energy and discard those that haven’t into another getter.
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- Next a detector captures the higher energy atoms. These atoms resonant at the same frequency as the microwaves in the Ramsey cavity.
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- The last step after the detector is the frequency divider that matches the atom frequency of the cesium atom, 9,192,631,770 vibrations per second. This is the frequency that accurately measures the time durations.
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- In 1989 Norman Ramsey , (1915 - 2011), was awarded the Nobel prize for his work on this atomic clock.
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- But, he stood on the shoulders of many other scientists. Starting in 1879, Lord Kelvin proposed using atoms to measure time. In 1945 scientists used his suggestion to design a clock using atomic beam in magnetic resonance. And, in 1949, the first atomic clock was built using ammonium molecules , but , it was less accurate than the currently available quartz clock.
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- In 1955, the first atomic clock based on cesium atoms was finally built. In 1967, the International Standards set the definition of one second as 9,192,631,770 vibrations of
C-133. Previously the second was defined as 1 / 86,400 of a “mean’ solar day.
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- In 1972, Greenwich Mean time was based on the atomic clock. In 1999, an atomic clock was invented using laser cooled cesium atoms. This clock was accurate to one second in 20,000,000 years.
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- In 2008, NIST even improved on that accuracy using strontium ions instead of cesium. In 2011, the National Lab claimed to have accuracies of one second in 130,000,000 years.
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- It doesn’t get any better than that. I will never be late to class again.
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- March 28, 2020 2682
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--------------------- Sunday, March 29, 2020 -------------------------
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