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What is the definition of 1 meter?

  1. May 17, 2003 #1
    What is the defination of 1 meter? My physics teacher told me it's related to the frequency of a certain EM wave, but he couldn't remember the whole story. Perhaps it also explains why the speed of light is 299792458 m/s, which is an integer.
    Last edited by a moderator: Feb 4, 2013
  2. jcsd
  3. May 17, 2003 #2


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    Re: defination

    The original physical definition of a meter
    was not quite precise. Then it became the
    steel metal bar that is currently kept
    in a vacuum safe in the International
    Standards Institute (I think that's its name)
    in Paris. I don't know if it was converted
    into an EM frequency definition (if we have
    an exact metric definition of c), but of
    course you can find the ("almost" at least)
    accurate value by [nu] = c / [lamb] <Hz> with
    [lamb] = 1 m .

    Live long and prosper.
  4. May 17, 2003 #3
    Re: definition

    Hi KL,
    The "meter" is the length of a sterling silver bar (at some given temperature) on display somewhere in England (I think). Supposedly the geodic reference is 10^-7 of the distance from the north pole to the equator of the Earth.
    BTW: If an integral value of "c" were really desired why not alter the length of the meter such that the velocity of light would be exactly 3. E +8 neo-meters/second? Cheers, Jim
    Last edited: May 17, 2003
  5. May 17, 2003 #4


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    Re: Re: definition

    England ?! I'm pretty certain it's Paris and
    the kilogram is there too, I think (the
    English mesauring units are feet and inches.)
    I don't remember about the material and
    temp. parts (I still believe it's vaccum).
    What are you talking about ?!
    Hmm... tradition maybe ? :wink:

    Live long and prosper.
  6. May 17, 2003 #5
    I think, old meter standard was x-shaped (in cross section) Pt-Ir alloy bar with two scratches on it, kept somewhere in Paris (not in a vacuum, because Pt and Ir are quite stable against elements, and because from time to time people come with their meter stcks to check against prime one).

    Currently I believe a standard of length is dead.

    New standard is instead a standard of time (to be exact, of frequency), and a meter is simply calculated using speed of light: l =ct=c/f.
    Last edited by a moderator: May 17, 2003
  7. May 17, 2003 #6


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    Yes, the current standard makes use of the constancy of speed of light in vacuum for all observers, plus a standard for time (which makes sense given the achievable accuracy for time measurements). It defines the meter as the distance covered by a light beam in 1/299792458 of a second. That is why the value of c is now exactly 299792458 m/s.
  8. May 17, 2003 #7
    As Drag suggested, meter shall be simplified to have speed of light exactly 3E8 m/s Very good idea.

    Shall we block NIST entrances, or just strike at our business locations?

    It would also be nice to reduce all constants to 1E1 (and get rid of units).
  9. May 17, 2003 #8


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    I suggested that ?
  10. May 17, 2003 #9
    Sorry, my mistake, Neo suggested that indeed.

    But I really think it is a cool idea - to simplify those hard to memorise trains of numbers.

    I think in QED and GR books they are already close to that (they routinely use h=G=c=1 system).

    I wonder if phys 101-201 texts will look easier (and attracts more females) with such system?
  11. May 17, 2003 #10
    Hi Drag,
    Here's what Feynman - Leighton - Sands Vol I page 5-10 says:

    "It might be thought that it would be a good idea to use some natural length as our unit of length -- say the radius of the earth or some fraction of it. The meter was originally intended to be such a unit and was defined to be ([pi]/2) X 10^-7 times the earths radius. - -
    For a long time it has been agreed internationally that the meter would be defined as the distamce between two scratches on a bar kept in a special laboratory in France. It is currently being considered that a new definition be adopted, an agreed upon (arbitrary) number of wave lengths of a chosen spectral line."
    My original post had a mistake (recently edited) that had stated that there were 10,000 kilometers between the north pole and the equator A quick calc. using Feynman's formula and assuming that the earth's circumference is 25,000 miles and each mile contains 1.6 kilometers, gives that same number as the distance between the north pole and the equator.
    I didn't know Leighton but I knew Dick Feynman in Feb, 1945 and Matt Sands was my "Differential Equations" Prof in 1946 at Univ of Los Alamos, NM. Cheers, Jim
  12. May 17, 2003 #11


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    NIST has some information about this topic.
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