From Stonehenge to NIST F1 provides a brief history of timekeeping. It discusses early astronomical clocks based on the earth's orbit and rotation. It then describes the development of pendulum clocks in the 17th century and John Harrison's sea clocks in the 18th century. Quartz oscillators were developed in the 1920s providing more accurate timekeeping. Atomic clocks were developed in the 1950s using the vibration of cesium atoms as the most accurate clock yet, losing only 1 second every 20-30 million years. Precise timekeeping from atomic clocks is necessary for applications like global positioning systems.

1. From Stonehenge to NIST F1Chad Orzel, Department of Physics and AstronomyA Brief History of Timekeeping

2. Time According to PhysicistsTheories of Space-Time:Clocks:

3. A clock is something that “ticks”Regular, repeated action used to measure timeWhat Is a Clock?

4. Earth orbiting Sun:Earth rotating on axis:Astronomy

5. ~ 1 day~ 1 hourAstronomical Clocks

6. Drips and DropsChinese water clock:Hourglass:Mark time by emptying vessel~1 min

7. Pendulum ClocksPendulum oscillation depends only on lengthKeep time to within seconds

8. LongitudeJohn Harrison (1693-1776)Clocks to keep time at seaLose ~10 sec/month

9. Quartz OscillatorsQuartz crystals vibrate when voltage applied (32,768 vib/s)Use as reference for watchesAccurate to ~10s/year

10. Light as a ClockLight: Electromagnetic waveExtremely regular oscillationNo moving partsUse atoms as a reference:

11. Atomic ClockovenRF1 second = 9,192,631,770 cyclesof light associated with a transition in cesiumProcedure:1) Synchronize clock with atom2) Wait some time3) Check against atom4) Adjust as neededNIST-7: lose 1s in 3,000,000 years

12. Fountain ClockImprove by going to “fountain” clock1) Launch atoms upward2) Synchronize on way up3) Fly up, fall back (T~1s)4) Check on way down5) Adjust as neededBetter performance for two reasons: Only one interaction cavity Longer time between checksPerformance: Lose 1s in 20,000,000 years

13. Who Cares?Global Positioning System (GPS): 24 Atomic Clocks in Space

15. Global Positioning System1) Satellites broadcast time2) Compare signals from 4 satellites3) Get distance from delay time Gives position on Earth to within a few meters.

16. Future ClocksIon ClocksHigher frequency, better stabilityLose 1s in ~400 million yearsFrequency CombConnect different frequencies2005 Nobel Prize in PhysicsAstro-Comb