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Thread: De-artifacting the kilogram

  1. #1
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    De-artifacting the kilogram

    International System of Units revised in historic vote On November 16, delegates from the member nations of the International Bureau of Weights and Measures (Bureau Internaltional des Poids et Mesures: BIPM) voted to revise the definitions of some of the seven fundamental units of the SI, the International System of Units (Système International d'Unites).

    The biggest one was its revision of the kilogram. It is no longer given by a platinum-iridium cylinder in a suburb of Paris, but by Planck's constant being officially fixed.

    It thus goes the way of the meter, once officially the length of a platinum-iridium bar in that same suburb of Paris, but for the last quarter-century by the speed of light in a vacuum officially being fixed.

    The elementary charge will also be fixed, for making precision measurements of voltage and electrical resistance with the Josephson effect and the quantum Hall effect.

    Also fixed are Boltzmann's constant, giving temperature in terms of energy, and Avogadro's number or constant, giving the atomic mass unit or dalton in terms of the kilogram.

    A consequence is that all of the SI's units now depend on one physical reference, the one in the definition of the second, with all the rest being handled by very successful theories: relativity, quantum mechanics, thermodynamics, and electromagnetism. Something like how energy is handled, as a result of recognition that different kinds of energy are interconvertible.

  2. #2
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    I think that the International Prototype Kilogram, as it is called, will now become a secondary mass standard, like similar platinum-iridium cylinders in several national laboratories all over the world. BTW, it is nicknamed Le Grand K, The Big K.

    History of the seven SI units.

    Candela - standard of visible-light intensity.
    • (Original standards) Lamps with specified construction.
    • 1946: the blackbody luminosity of melting-point platinum
    • 1979: a certain energy flux of a certain wavelength of visible light


    Mole - the number of grams of something equal to the number of atomic mass units or daltons of its component parts. Thus, a mole of hydrogen weighs 1.008 grams, because the atomic weight of hydrogen is 1.008 amu. The number of amu's in a gram is Avogadro's number or Avogadro's constant.
    • 1803 (John Dalton) 1 amu = mass of hydrogen atom
    • 1903: 16 amu = mass of oxygen atom
    • 1961: 12 amu = mass of carbon-12 atom
    • Recently: Avogadro's number fixed, defining the amu in terms of the kilogram


    Kelvin - thermodynamic temperature, an offshoot of the Celsius scale
    • 1742 (Anders Celsius): Freezing point of water = 100 C, boiling point of water at sea level = 0 C
    • 1744 (Carl Linnaeus): Freezing point of water = 0 C, boiling point of water at sea level = 100 C
    • 1848 (William Thomson, Lord Kelvin): Absolute zero = 0 K, freezing point of water = 273 K
    • 1954: Triple point of water = 273.16 K
    • 2005: The water's composition specified: Vienna Standard Mean Ocean Water
    • Recently: Boltzmann's constant fixed, defining the kelvin in terms of the joule, defined in turn in terms of length, time, and mass


    Kilogram - mass
    • 1795: 1 gram = mass of one cubic centimeter of water at 0 C
    • 1799: At 4 C instead (maximum density of water)
    • 1799: A platinum cylinder, the Kilogramme des Archives (Kilogram of the Archives)
    • 1889: A platinum-iridium cylinder, the International Prototype Kilogram
    • Recently: Planck's constant fixed, defining the kilogram in terms of length and time


    Meter - length
    • 1795: 1 meter = 1/10,000,000 of equator-pole distance
    • 1799: A platinum bar, the Mètre des Archives (Meter of the Archives)
    • 1889: A platinum-iridium bar, the International Prototype meter
    • 1960: In terms of an electronic transition of krypton-86
    • 1975: The speed of light in a vacuum fixed, defining the meter in terms of time


    Second - time
    • Prehistoric: observation of the day, the month, and the year
    • Antiquity: division of daytime and nighttime into 12 hours each
    • 14th cy: division of the day into 24 hours
    • 16th cy: 1 hour = 60 of pars minuta prima (first tiny part: the minute), 1 minute = 60 of pars minuta secunda (second tiny part: the second). No pars minuta tertia defined.
    • 19th cy(?): 1 second = 1/86,400 of a mean solar day
    • 1956: a fraction of a certain year
    • 1967: in terms of the cesium-133 ground-state hyperfine transition's frequency


    Ampere - electric current, electric charge per unit time. Long defined from mechanical effects of interacting electric charges and currents, the elementary charge is now fixed for convenience in using the Josephson effect and the quantum Hall effect for precision measurements of voltage and resistance, and from them, current.
    Last edited by lpetrich; 2018-Nov-30 at 02:41 AM. Reason: Fixed typo in C and O amu definitions

  3. #3
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    This is so that if all the meter sticks, graduated cylinders. clocks and such are destroyed in some cataclysm, we'll still be able to get those values back.

    All we have to do is fire up the spectrometer, gas chromatography, cyclotron, computers, and ... D'oh!


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    Quote Originally Posted by DonM435 View Post
    This is so that if all the meter sticks, graduated cylinders. clocks and such are destroyed in some cataclysm, we'll still be able to get those values back.

    All we have to do is fire up the spectrometer, gas chromatography, cyclotron, computers, and ... D'oh!

    Yes indeed.

    Aside from such a scenario, having universally-available measurement standards means that it is not necessary to go to some central location to calibrate one's measurement systems. If we colonize other celestial bodies and/or build free-flying space colonies, then it would not make much sense to have to send one's mass standards back to the Earth to be compared to The Big K in Paris. Or even send copies of them for doing so.

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    Apparently the way this was done was by declaring a value for one of the physical constants of the universe: "it is the number we say it is". It has not been made clear to me how that's a good thing.

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    Quote Originally Posted by Delvo View Post
    Apparently the way this was done was by declaring a value for one of the physical constants of the universe: "it is the number we say it is". It has not been made clear to me how that's a good thing.
    Because the numbers picked are completely arbitrary scaling units used to relate our whacky system of units to each other. They are not like pi, e or some other meaningful constant - they are simply scaling factors to take into account our penchant for using units based on the duration of our orbit or a segment of the Earth's circumfrence.

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    Quote Originally Posted by Delvo View Post
    Apparently the way this was done was by declaring a value for one of the physical constants of the universe: "it is the number we say it is". It has not been made clear to me how that's a good thing.
    To measure something, you need some reference. That is easy for counting, but for doing lengths and times and masses and the like, one needs to have reference lengths, reference times, reference masses, etc. to compare to. These need not be physical objects, and they can instead be measurement algorithms. But they have to be present in some form or another.

    If you and I use feet as measurement units, and I use my foot length as a unit and you use your foot length as a unit, then our measurements will not be comparable until we compare the lengths of your and my feet. This is what led to standardized measurement units.

    The nice thing about natural phenomena is that they are universally available. But even with natural phenomena like the motions of the celestial bodies, one has to use conventions. The day has variously been begun at sunrise, noon, sunset, and midnight, for instance, and there have been numerous conventions for when to begin years. Even at the present, many organizations have their own fiscal years. For synodic or Sun-relative months, it is much easier. The universal convention there is new moon.

    As to all the oddball numerical values, that is from making new standards as close to old ones as is reasonably feasible.

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    It must be noted that we recognize a variety of kinds of energy: kinetic energy, gravitational potential energy, mechanical potential energy, thermal energy, phase-change energy, chemical energy, electromagnetic energy, nuclear energy, and elementary-particle energy. The primary units of energy are kinetic-energy units, defined in terms of length, time, and mass: erg (CGS) and joule (MKS/SI). All other units of energy, like the calorie and the electron volt, are now defined in terms of them.

    Likewise, various physical theories are used to relate various units:
    • Meter (length) -- relativity -- time
    • Kilogram (mass) -- quantum mechanics -- length, time
    • Kelvin (temperature) -- thermodynamics -- energy (length, time, mass)
    • Candela (intensity of light) -- electromagnetism -- energy (length, time, mass) / time

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    it is nice to know that it all depends on the second when we know there is no absolute time, there is spacetime. two observers watching the same atomic clock might record two different times, especially if they are adjacent to two different amounts of mass! Just an aside , I am sure it all makes perfect sense.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Could this definition of a kg still be viable even if it was found that Planck's Constant was not constant?

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    Quote Originally Posted by profloater View Post
    it is nice to know that it all depends on the second when we know there is no absolute time, there is spacetime. two observers watching the same atomic clock might record two different times, especially if they are adjacent to two different amounts of mass! Just an aside , I am sure it all makes perfect sense.
    Yes and no. Two observers each using their own clock might disagree on the time elapsed between two events. And observers might disagree on what time on a given clock aligns with the occurrence of events that are far from that clock. But if there is a single atomic clock that is right next to both the starting event and the ending event you're concerned about, then all observers will agree on the amount of time that specific clock reads between those events. That's just a measurement of the proper time along whatever path that clock follows between those two events, and it's invariant. It won't be affected by the movement or gravitational effects on any observers.
    Conserve energy. Commute with the Hamiltonian.

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    Quote Originally Posted by wd40 View Post
    Could this definition of a kg still be viable even if it was found that Planck's Constant was not constant?
    It would probably run into some problems (just like our other measurement standards could run into problems if the speed of light were not constant). But just what those problems might be and how serious they were would depend on just how Planck's Constant changed. It has to be a pretty small variation, or we'd have noticed it already, especially in the process of verifying that it's realistic to define the standard this way.
    Conserve energy. Commute with the Hamiltonian.

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