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Cesius-133 atom ?

  1. Nov 21, 2008 #1
    Hi all,

    I am a new member in your forum and I am very pleased to join it. This my first theard in this great forum.

    My Q.:

    The unit of time was originally defined as 1/86400 of the mean solar day. Because of irregularities in the earth roatation, the definition was changed to the '' emphemeris second '', i.e., 1/31556925.9747 of the tropical year 1900. In 1967 , this definition was replaced.

    the second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine of the ground state of the cesius-133 atom.

    The defintion is based on the atomic clock. One of the best atomic clocks ( NIST-F1) is precise to within about 1 second in 60 million years, or 5 parts in 10^16. Commercially available atomic clocks are precise to within 3 parts in 10^12.

    I didn't understand this paragraph .. it's from my textbook !

    Concepts in Engineering
    Second Edition
    Mark Holtzapple and W. Reece
    McGraw-Hill
    2008


    Would you mind explaining it, please?
     
    Last edited by a moderator: Nov 21, 2008
  2. jcsd
  3. Nov 21, 2008 #2
    no reply ?!!

    :(
     
  4. Nov 21, 2008 #3

    Redbelly98

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    It means that if you set the clock "exactly", and then wait 60 million years, it could be reading the wrong time. But, it will only be off by about 1 second or so, after all that time.

    "5 parts in 10^16" means the time could be off by 5 seconds after having run for 10^16 seconds (that's about 300 million years).

    Or it could be off by 5 nanoseconds after having run for 10^16 nanoseconds (that's 16.5 weeks). A nanosecond is a billionth of a second or 10^-9 seconds.

    Many of us (in USA) were sleeping when you first posted.
     
  5. Nov 22, 2008 #4
    Redbelly98

    Thanks alot !!

    : )
     
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