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# A Brief History of Timekeeping

## by Chad Orzel, Physics Professor at Union College on Oct 15, 2009

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

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

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## A Brief History of TimekeepingPresentation Transcript

• From Stonehenge to NIST F1
Chad Orzel, Department of Physics and Astronomy
A Brief History of Timekeeping
• Time According to Physicists
Theories of Space-Time:
Clocks:
• A clock is something that “ticks”
Regular, repeated action used to measure time
What Is a Clock?
• Earth orbiting Sun:
Earth rotating on axis:
Astronomy
• ~ 1 day
~ 1 hour
Astronomical Clocks
• Drips and Drops
Chinese water clock:
Hourglass:
Mark time by emptying vessel
~1 min
• Pendulum Clocks
Pendulum oscillation depends only on length
Keep time to within seconds
• Longitude
John Harrison (1693-1776)
Clocks to keep time at sea
Lose ~10 sec/month
• Quartz Oscillators
Quartz crystals vibrate when voltage applied (32,768 vib/s)
Use as reference for watches
Accurate to ~10s/year
• Light as a Clock
Light: Electromagnetic wave
Extremely regular oscillation
No moving parts
Use atoms as a reference:
• Atomic Clock
oven
RF
1 second = 9,192,631,770 cycles
of light associated with a transition in cesium
Procedure:
1) Synchronize clock with atom
2) Wait some time
3) Check against atom
NIST-7: lose 1s in 3,000,000 years
• Fountain Clock
Improve by going to “fountain” clock
1) Launch atoms upward
2) Synchronize on way up
3) Fly up, fall back (T~1s)
4) Check on way down
Better performance for two reasons:
 Only one interaction cavity
 Longer time between checks
Performance: Lose 1s in 20,000,000 years
• Who Cares?
Global Positioning System (GPS): 24 Atomic Clocks in Space
• Global Positioning System