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A Brief History of Timekeeping


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A guest lecture delivered to a sophomore seminar class on time.

Published in: Technology, Education
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A Brief History of Timekeeping

  1. 1. From Stonehenge to NIST F1<br />Chad Orzel, Department of Physics and Astronomy<br />A Brief History of Timekeeping<br />
  2. 2. Time According to Physicists<br />Theories of Space-Time:<br />Clocks:<br />
  3. 3. A clock is something that “ticks”<br />Regular, repeated action used to measure time<br />What Is a Clock?<br />
  4. 4. Earth orbiting Sun:<br />Earth rotating on axis:<br />Astronomy<br />
  5. 5. ~ 1 day<br />~ 1 hour<br />Astronomical Clocks<br />
  6. 6. Drips and Drops<br />Chinese water clock:<br />Hourglass:<br />Mark time by emptying vessel<br />~1 min<br />
  7. 7. Pendulum Clocks<br />Pendulum oscillation depends only on length<br />Keep time to within seconds<br />
  8. 8. Longitude<br />John Harrison (1693-1776)<br />Clocks to keep time at sea<br />Lose ~10 sec/month<br />
  9. 9. Quartz Oscillators<br />Quartz crystals vibrate when voltage applied (32,768 vib/s)<br />Use as reference for watches<br />Accurate to ~10s/year<br />
  10. 10. Light as a Clock<br />Light: Electromagnetic wave<br />Extremely regular oscillation<br />No moving parts<br />Use atoms as a reference:<br />
  11. 11. Atomic Clock<br />oven<br />RF<br />1 second = 9,192,631,770 cycles<br />of light associated with a transition in cesium<br />Procedure:<br />1) Synchronize clock with atom<br />2) Wait some time<br />3) Check against atom<br />4) Adjust as needed<br />NIST-7: lose 1s in 3,000,000 years<br />
  12. 12. Fountain Clock<br />Improve by going to “fountain” clock<br />1) Launch atoms upward<br />2) Synchronize on way up<br />3) Fly up, fall back (T~1s)<br />4) Check on way down<br />5) Adjust as needed<br />Better performance for two reasons:<br /> Only one interaction cavity<br /> Longer time between checks<br />Performance: Lose 1s in 20,000,000 years<br />
  13. 13. Who Cares?<br />Global Positioning System (GPS): 24 Atomic Clocks in Space<br />
  14. 14.
  15. 15. Global Positioning System<br />1) Satellites broadcast time<br />2) Compare signals from 4 satellites<br />3) Get distance from delay time<br /> Gives position on Earth to within a few meters.<br />
  16. 16. Future Clocks<br />Ion Clocks<br />Higher frequency, better stability<br />Lose 1s in ~400 million years<br />Frequency Comb<br />Connect different frequencies<br />2005 Nobel Prize in Physics<br />Astro-Comb<br />