8 ms·

The best part about this batch of changes is they push the mole and Avogadro's constant out on their own where the belong, not linked to any other units. Now we

The best part about this batch of changes is they push the mole and Avogadro's constant out on their own where the belong, not linked to any other units. Now we'll have only a single mass unit (kg) instead of two (kg and unified atomic mass unit) that we do now. This will knock carbon 12 of its perch as the definition of the "other" mass unit that's been essential to use SI's mole but was not actually SI itself.

tomp 11y ago
But wouldn't the most straightforward definition of a base mass unit (kg) be linked to the mass of an atom?
- dalke 11y ago
  Yes. I believe Asbostos is commenting that they currently are not linked. ("Yes", with the proviso that the proposed silicon atom definition is a more obviously direct link than the watt balance.)
  - tomp 11y ago
    What I mean is, the proposed new definition defines kilogramm using a relationship to Planck constant. Instead, I think that a more intuitive definition would be something like "the mass of {huge number} of C or Si atoms".
    - marcosdumay 11y ago
      The proposal is to define all fundamental physics constants, and derive all the units from them. The Avogadro number isn't a fundamental physics constant, it's just a unit conversion constant, and thus, didn't get to define any basic unit.
      - tomp 11y ago
        > The Avogadro number isn't a fundamental physics constant That's a pretty arbitrary assertion. Who defines what "fundamental" is? The number of periods of <something> of Caesium atoms used to define the duration of one second doesn't sound very fundamental either.
        jsprogrammer 11y ago
        You seem to be assuming that the atom is fundamental. It's not. Why not base things off the mass of an electron instead? What you want, is to go down to the smallest unit possible, which is apparently Planck's Constant.
        marcosdumay 11y ago
        In fact, the second and candela aren't choices aren't that fundamental, mostly reflecting how those are measured. All the others come from the speed of light, fundamental charge, Plank's constant, and gases constant; those are pretty fundamental to physics. Avogadro's number is used to define the atomic units, what makes a lot more sense than pushing it into metric calculations. The wikipedia page is great: https://en.wikipedia.org/wiki/Metric_system#Future_developments https://en.wikipedia.org/wiki/Metric_system#Future_developme...
        duaneb 11y ago
        It's an arbitrary physics constant.
    - Retra 11y ago
      The mass of a chunk of atoms depends on their temperature and their purity, which are things you can't measure very accurately the more atoms you have. Essentially, that's what they were doing with Le Grande K. It's unstable because molecular properties are unstable in aggregate.
      - tomp 11y ago
        The second depends on a specific isotope at a specific temperature, 0K, so it's all theoretical anyways. How you define something doesn't need to coincide with how you measure it.
        Retra 11y ago
        It's far easier to measure a single isotope at a specific temperature limit than it is a whole rod of them. Just because they're both theoretical doesn't mean there isn't a huge practicality component to the decision.
      - sklogic 11y ago
        How exactly mass depends on a temperature?
        Retra 11y ago
        Measured mass is a function of total energy.
        sklogic 11y ago
        It is not a mass (or a "rest" mass, if you want to use this incorrect and unnecessary notion) that you measure.
        IvyMike 11y ago
        E=mc^2 You may be thinking "Oh, you fool--E=mc^2 is for fission and fusion, and certainly doesn't apply to things like the energy in the heat of an object". But weirdly, it does. https://physics.stackexchange.com/questions/87259/does-decrease-in-temperature-affect-mass-e-mc2 https://physics.stackexchange.com/questions/87259/does-decre...
- masklinn 11y ago
  That's what the new definition does, using Si(28) as the "reference atom". In the current definition the kilogram is not related to atomic mass.
  - tomp 11y ago
    No, the new definition defines kg using Planck constant (according to Wikipedia).

tomp 11y ago

But wouldn't the most straightforward definition of a base mass unit (kg) be linked to the mass of an atom?

dalke 11y ago
Yes. I believe Asbostos is commenting that they currently are not linked. ("Yes", with the proviso that the proposed silicon atom definition is a more obviously direct link than the watt balance.)
- tomp 11y ago
  What I mean is, the proposed new definition defines kilogramm using a relationship to Planck constant. Instead, I think that a more intuitive definition would be something like "the mass of {huge number} of C or Si atoms".
  - marcosdumay 11y ago
    The proposal is to define all fundamental physics constants, and derive all the units from them. The Avogadro number isn't a fundamental physics constant, it's just a unit conversion constant, and thus, didn't get to define any basic unit.
    - tomp 11y ago
      > The Avogadro number isn't a fundamental physics constant That's a pretty arbitrary assertion. Who defines what "fundamental" is? The number of periods of <something> of Caesium atoms used to define the duration of one second doesn't sound very fundamental either.
      - jsprogrammer 11y ago
        You seem to be assuming that the atom is fundamental. It's not. Why not base things off the mass of an electron instead? What you want, is to go down to the smallest unit possible, which is apparently Planck's Constant.
      - marcosdumay 11y ago
        In fact, the second and candela aren't choices aren't that fundamental, mostly reflecting how those are measured. All the others come from the speed of light, fundamental charge, Plank's constant, and gases constant; those are pretty fundamental to physics. Avogadro's number is used to define the atomic units, what makes a lot more sense than pushing it into metric calculations. The wikipedia page is great: https://en.wikipedia.org/wiki/Metric_system#Future_developments https://en.wikipedia.org/wiki/Metric_system#Future_developme...
      - duaneb 11y ago
        It's an arbitrary physics constant.
  - Retra 11y ago
    The mass of a chunk of atoms depends on their temperature and their purity, which are things you can't measure very accurately the more atoms you have. Essentially, that's what they were doing with Le Grande K. It's unstable because molecular properties are unstable in aggregate.
    - tomp 11y ago
      The second depends on a specific isotope at a specific temperature, 0K, so it's all theoretical anyways. How you define something doesn't need to coincide with how you measure it.
      - Retra 11y ago
        It's far easier to measure a single isotope at a specific temperature limit than it is a whole rod of them. Just because they're both theoretical doesn't mean there isn't a huge practicality component to the decision.
    - sklogic 11y ago
      How exactly mass depends on a temperature?
      - Retra 11y ago
        Measured mass is a function of total energy.
        sklogic 11y ago
        It is not a mass (or a "rest" mass, if you want to use this incorrect and unnecessary notion) that you measure.
      - IvyMike 11y ago
        E=mc^2 You may be thinking "Oh, you fool--E=mc^2 is for fission and fusion, and certainly doesn't apply to things like the energy in the heat of an object". But weirdly, it does. https://physics.stackexchange.com/questions/87259/does-decrease-in-temperature-affect-mass-e-mc2 https://physics.stackexchange.com/questions/87259/does-decre...
masklinn 11y ago
That's what the new definition does, using Si(28) as the "reference atom". In the current definition the kilogram is not related to atomic mass.
- tomp 11y ago
  No, the new definition defines kg using Planck constant (according to Wikipedia).