A Brief History of Time

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A Brief History of Time

• Patrick Hofmann
• 11/15/02

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Ancient Calendars

• Celestial Bodies The sun, moon, stars, and
  planets provided the only means for ancient
  civilizations to measure the passage of time.
• Civilizations from South America, China, Africa,
  and the Middle East all determined measurements
  of time using celestial bodies for thousands of
  years

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Ancient Civilizations

• Early Egyptian calendars based off the cycles of
  the moon, then also the star Sirius.
• Ancient Babylonians used 12 alternating 29/30 day
  lunar months, giving a 354 year.
• Mayans and Aztecs relied on the planet Venus, to
  establish 260 day and 365 day calendars.

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Early Clocks

• Ancient civilizations also devised primitive
  clocks.
• Egyptians used Obelisks and Shadow Clocks, as
  well as Water Clocks.
• Water Clocks also used in Greek and Roman lands,
  as well as the Far East.
• Rate of water flow was difficult to control
  accurately, thus necessitating new types of
  clocks.

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Advancements in Timekeeping

• From ancient times until the end of the 13th
  century, sundials and the early clocks were the
  only type used, as innovation was severely
  depressed during the medieval period
• By the first half of the 14th century, large
  mechanical clocks began to appear in Italy,
  however they were still inaccurate due to
  variable amounts of friction and driving forces.
• Then spring-powered clocks came into existence
  between 1500 and 1510 by Peter Henlein. These
  were pocket size, but lost their accuracy as the
  main spring unwound.
• In 1656, the first pendulum clock was invented,
  very accurate for the time. Other developments
  based on the pendulum clock came along during the
  next 150 years, such as the balancing wheel
  clock.
• By the 1920s, the development of the quartz
  cystal oscillator mechanisms antiquated the
  pendulum clocks.

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The Atomic Age of Clocks

• First atomic clock built by NIST in 1949, based
  on the ammonia molecule. Not very successful, so
  attention turned towards atomic-beam devices
  based on cesium.
• First practical cesium atomic frequency standard
  built in England in 1955.
• By 1960, cesium standards had been refined enough
  to become the official timekeeping system of NIST.

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Measuring Distances

• Ancient civilizations used units that were based
  on bodily proportions, such as the inch, foot,
  yard and fathom.
• These measurements based on human proportions
  were used for thousands of years.
• King Edward I of England (1300) ordered the
  first permanent, consistent measuring stick made
  of iron to serve as a master standard.
• First national move to a standard was under
  Napoleon in France (1793).
• In 1960, the CGPM adopted the International
  System of Units as the official standard for
  measurements.
• Meter now measured according to distance light
  travels in a set amount of time.

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NIST Photograph

• Picture of the latest and most accurate cesium
  atomic clock NIST-F1 Fountain Clock
• How does it work?
• Two separate kinds The Cesium Beam Clock and
  The Cesium Fountain Clock

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Cesium Fountain Clock

• Trapping Cesium atoms are trapped and cooled in
  a  magneto-optical trap.
• Launch The atoms are launched upwards.
• Preparation The atoms are pumped into the upper
  level of the clock transition .
• Interrogation The atoms follow a fountain-like
  course, passing through the microwave cavity
  twice.
• Detection The atoms are detected.
• The fountain cycle is repeated.

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Cesium Beam Clock

• Hot atoms, heated in oven to 350K
• Horizontal launch at 250m/s (velocity of gaseous
  cesium atoms at 350K)
• Using magnets, atoms in two atomic ground states
  are separated.  Atoms in the desired state enter
  the microwave cavity the other atoms are
  discarded.
• The atoms undergo Ramsey interrogation as they
  pass through the two parts of the double cavity. 
  Interrogation time 0.005 s
• Source provides a continuous flow of atoms

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Differences

• Fountain clock much more accurate due to
• Interrogation time of atoms in the fountain is
  longer than it is for atoms in the beam clock.
• Better Microwave cavity design in Fountain clock
• No atoms are discarded in Fountain clock
  greater stability
• However, continuous pulse from beam clock better
  than pulsed operation of Fountain clock

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References

• National Institute of Standards
• http//physics.nist.gov/GenInt/Time/ancient.html
• http//physics.nist.gov/cuu/Units/history.html
• Other helpful Websites
• http//www.unc.edu/rowlett/units/custom.html
• http//whatis.techtarget.com/definition/0,,sid9_gc
  i523639,00.html
• http//www.cftech.com/BrainBank/OTHERREFERENCE/WEI
  GHTSandMEASURES/MetricHistory.html
• http//www.nrc.ca/inms/images/time/fcs1/en/