a brief history of timekeeping
DESCRIPTION
A guest lecture delivered to a sophomore seminar class on time.TRANSCRIPT
From Stonehenge to NIST F1
Chad Orzel, Department of Physics and Astronomy
A Brief History of Timekeeping
Theories of Space-Time: Clocks:
Time According to Physicists
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 DropsChinese 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 OscillatorsQuartz 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
1 second = 9,192,631,770 cyclesof light associated with a transition in cesium
Procedure:
1) Synchronize clock with atom
2) Wait some time
3) Check against atom
4) Adjust as needed
oven
RF
NIST-7: lose 1s in 3,000,000 years
Fountain ClockImprove 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
5) Adjust as needed
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
1) Satellites broadcast time
2) Compare signals from 4 satellites
3) Get distance from delay time
Gives position on Earth to within a few meters.
Future ClocksIon Clocks
Higher frequency, better stability
Lose 1s in ~400 million years
Frequency Comb
Connect different frequencies
2005 Nobel Prize in Physics
Astro-Comb