Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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

    Thanks alot !!

    : )
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook