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Are particle masses commonly normalized against a natural standard?

  1. Jul 26, 2013 #1

    rbj

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    in HEP research and lit, is it common to think and write about the masses of the various leptons and quarks as relative to the invariant mass of some chosen standard particle? such as the electron? or something else?
     
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  3. Jul 26, 2013 #2
    I think the normal way is to express the mass in units of eV/c^2.
     
  4. Jul 27, 2013 #3

    mfb

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    eV is the common unit. The speed of light is usually set to 1, so there are no factors of c hanging around. meV, keV, MeV and GeV give the prefactors needed for particle masses.

    In theory, you could express all masses as fraction of the Planck mass, but that would give really unhandy numbers.
     
  5. Jul 27, 2013 #4

    clem

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    It usually depends on what you are calculating.
    In pion interactions, it is convenient to take the pion mass =1, etc.
    The MeV is widely used, but is an arbitrary unit that depends on how the volt is defined.
    It would be more appropriate to set the proton mass=1, and that may eventually be used (if I am elected).
    Then we could compare our masses to those of alien civilizations.
     
  6. Jul 28, 2013 #5

    mfb

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    Can you show where this is done? I don't remember seeing that.
    Right
    ?
    We can compare ratios, that's fine.
     
  7. Jul 28, 2013 #6

    OmCheeto

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    I thought Carbon-12 was the "natural" standard for mass?
     
  8. Jul 28, 2013 #7

    mfb

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    Carbon-12 is used for the definition of the atomic mass unit. This is used in chemistry and maybe nuclear physics, but not in particle physics.
     
  9. Jul 29, 2013 #8

    QuantumPion

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    Mass has no fundamental way of being defined, unlike time and distance which are defined by the speed of light. The kilogram is defined as the weight of the international prototype kilogram (a platinum bar made to be about the same weight of 1 liter of water at room temperature).
     
  10. Jul 29, 2013 #9

    mfb

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    The second has a completely arbitrary definition as multiples of a period of electromagnetic waves emitted in a transition in Cs-atoms. Meters are derived from that and the speed of light, indeed.
    I expect this to change in the next 2-3 years.
     
  11. Jul 29, 2013 #10

    OmCheeto

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    Cool....

    Do you think it will involve Carbon-12?

    :tongue2:
     
  12. Jul 29, 2013 #11

    QuantumPion

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    I think it has something to do with precise measurements of the mass of silicon crystal atoms, I saw a video about it somewhere recently.
     
  13. Jul 30, 2013 #12

    Bill_K

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    Carbon-12 atoms could be used, but we are still looking for a volunteer to count them.
     
  14. Jul 30, 2013 #13

    OmCheeto

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    :biggrin:

    Here's an interesting article from about 6 years back:

    Obviously brilliant scientists.....

    Oh! And there's the silicon sphere:

    And if I knew the following, I'd forgotten it:

    And you are apparently correct about finding someone to count out these carbon atoms:

    Where's mfb? I suspect he knows about some secret government project.

    hmm...... google google google

    ah ha!

    Just as I suspected.

    So one is the carbon-12 cube, and the other is:

    A quantum kilogram? hmmm...... several lame jokes just popped into my head......
     
  15. Jul 30, 2013 #14

    mfb

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    That sounds like a weird description of the Watt balance project.
     
  16. Aug 12, 2013 #15
    Let's see what's happened to the other metric-system units.

    Length is now defined from time by fixing c, the speed of light in a vacuum. Strictly speaking, that's the speed of something that travels on a null geodesic. That's general-relativity-speak for a straight-line-like path with zero space-time distance along it in space-time.

    Time is now defined in terms of the frequency of the hyperfine transition of cesium-133. It's to be improved by making it the ground-state one at 0 K temperature.

    Proposed redefinition of SI base units - Wikipedia

    Planck's constant, the elementary charge, Avogadro's number, and Boltzmann's constant are to be fixed, as c has been.

    Planck's constant will give mass in terms of length and time.

    The fixing of the elementary charge will make the electric permittivity of the vacuum a measured quantity instead of a defined one, as it currently is. The magnetic permeability of the vacuum and the impedance of free space will follow along with it.

    The fixing of Avogadro's number will force one of two possibilities. Currently, a carbon-12 atom is defined as 12 daltons / atomic mass units, and a mole of carbon atoms is defined as 12 grams. So either (1) carbon-12 atoms will continue to weigh exactly 12 daltons and the mass of a mole of carbon-12 atoms will become a measured quantity, or (2) a mole of carbon-12 atoms will continue to weigh exactly 12 grams, and the mass of a carbon-12 atom will become a measured quantity.

    The fixing of Boltzmann's constant will make the temperature of the triple point of water a measured quantity instead of a defined one. That's where solid, liquid, and gas phases coexist.


    What does this mean for the electron volt? It's defined as
    ((elementary charge)/(1 coulomb)) * (1 joule).

    It'll change from a measured quantity to a defined quantity, being given as an exact though non-integer multiple of the energy of a Cs-133 hyperfine-line photon. That energy is about 3.80*10^(-5) eV, and the multiplication factor is about 26300.
     
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