A Brief History of Timekeeping

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