A Brief History of Timekeeping

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
4) Adjust as needed
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
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 Clocks
Ion Clocks
Higher frequency, better stability
Lose 1s in ~400 million years
Frequency Comb
Connect different frequencies
2005 Nobel Prize in Physics
Astro-Comb