Meet the world's most accurate clock. It's the standard instrument used for Universal Coordinated Time (UCT) and it lives in England. It uses a very sophisticated design called "Continuous Cold Cesium Fountain", where Cesium 133 atoms are cooled to near absolute zero and then probed in atomic fountains in a microwave-filled cavity to measure their atomic state transitions. It is mind-blowing and brilliant technology:
"To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately. This is normally done by locking a crystal oscillator to the principal resonance of the cesium atom. This signal is in the microwave range of the radio spectrum...cesium is heated so that atoms boil off and pass down a tube maintained at a high vacuum. First they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hz*. The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.
At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state when the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9,192,631,770 to give the familiar one pulse per second required by the real world."
This clock's accuracy is unsurpassed, it loses about a billionth of a second every two months.
Creating ultra-accurate clocks like this might seem like an unimportant or frivolous exercise, like calculating Pi to billions of decimal places, but it's not. Our modern communication and navigation systems require incredibly accurate clocks to function correctly. For example, the time-delays involved in calculating position with signals from the positioning satellites for GPS are very very tiny.
The Cesium Clock...another awesome creation of the ever-flexible human mind.
"To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately. This is normally done by locking a crystal oscillator to the principal resonance of the cesium atom. This signal is in the microwave range of the radio spectrum...cesium is heated so that atoms boil off and pass down a tube maintained at a high vacuum. First they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hz*. The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.
At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state when the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9,192,631,770 to give the familiar one pulse per second required by the real world."
This clock's accuracy is unsurpassed, it loses about a billionth of a second every two months.
Creating ultra-accurate clocks like this might seem like an unimportant or frivolous exercise, like calculating Pi to billions of decimal places, but it's not. Our modern communication and navigation systems require incredibly accurate clocks to function correctly. For example, the time-delays involved in calculating position with signals from the positioning satellites for GPS are very very tiny.
The Cesium Clock...another awesome creation of the ever-flexible human mind.
*In 1967, the International System of Units defined the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the cesium-133 atom. That means that a second is no longer linked to the Earth day as it was historically. Due to variations in the rotation of the earth, that old-fashioned second was far too inaccurate for modern use.
Off subject, but in a similar way, the definition of a meter is no longer tied to the physical standard platinum/iridium bar cast in 1889. A meter is now defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
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