from Hacker News

NIST to redefine the kilogram based on a fundamental universal constant

by nixme on 7/6/17, 10:19 AM with 84 comments

  • by anonymfus on 7/6/17, 1:26 PM

    >When the International Committee for Weights and Measures announced that it would reconsider the kilogram definition, it said it would require three measurements with uncertainties below 50 parts per billion, and one below 20 ppb. But with the new NIST measurement, the world now has at least three experiments below 20 ppb — another was conducted by a Canadian team using a Kibble balance, the third by an international group that calculates the Planck constant based on the number of atoms in a sphere of pure silicon.

    >The weights and measures committee will meet this month to establish a global value for Planck's constant by averaging the values calculated at NIST and other labs. And in 2018, at the next General Conference on Weights and Measures, the scientific community will draft a resolution to redefine kilogram based on this constant.

    Looks like the current title "NIST to redefine the kilogram based on a fundamental universal constant" is confusing because it implies that NIST defines kilogram but it's International Committee's for Weights and Measures job.

  • by kiernanmcgowan on 7/6/17, 2:38 PM

    The kilogram is not the only unit that will be redefined based on universal constants. The seven base units[0] will transition to being based on elementary charge and the Planck, Boltzmann, and Avogadro constants[1].

    [0] https://en.wikipedia.org/wiki/SI_base_unit#Seven_SI_base_uni...

    [1] https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...

  • by sanxiyn on 7/6/17, 1:07 PM

    > Scientists don't know whether the BIPM prototype is losing mass, perhaps because of loss of impurities in the metals, or if the witnesses are gaining mass by accumulating contaminants.

    Can we stop this nonsense? It would be a big problem if it were true, but it isn't. It's the later (contamination weight gain) and we have fairly good understanding of what's going on. For example, see https://phys.org/news/2013-01-kilogram-weight.html

  • by madengr on 7/6/17, 4:35 PM

    There is an alternate definition using a sphere of silicon with N atoms:

    https://www.nist.gov/physical-measurement-laboratory/silicon...

    What's really need though is a universal, stable over eons, single standard for time, length, and mass. I believe time is N cycles of an excited sodium (light) emission. Length is N wavelengths of that same emission in a vacuum. Mass would be N atoms.

    So why are they not using a single element to define everything? Is it a matter of finding the proper element that is easy to excite and stable enough (chemically and atomically) over the long term? Sodium is very reactive and easy to excite. Silicon is probably the opposite.

  • by ZeljkoS on 7/6/17, 2:44 PM

    Interesting fact: this is important for US too, because pound is defined as exactly 0.45359237 kg (https://en.wikipedia.org/wiki/Pound_(mass)#Current_use)

  • by shawncampbell on 7/6/17, 2:56 PM

    It's a little ironic that the article expressed the value of Planck's constant using an SI Unit with kilograms.

    >Based on 16 months' worth of measurements, it calculated Planck's constant to be 6.626069934 x 10−34 kg∙m2/s.

  • by msimpson on 7/6/17, 3:38 PM

  • by nerdponx on 7/6/17, 5:05 PM

    Why is the kilogram the base unit and not the gram?

  • by Aardwolf on 7/6/17, 1:32 PM

    They define it based on Planck's constant, so the results also depends on the definition of meter and seconds if I understand it correctly.

    Would it have been possible to define it as the weight of N amount of electrons (assuming all electrons have the exact same weight under all circumstances) or another fundamental particle?

    EDIT: it would be the weight of 9.10938356e31 electrons at rest

  • by kronos29296 on 7/6/17, 5:28 PM

    Always thought why didn't we have some super complex standard for weight when we had one for length and time. Now my thoughts have become reality. Though since I am non sciencey, It makes me ask Why so long?

  • by leeoniya on 7/6/17, 4:15 PM

  • by slim on 7/6/17, 4:08 PM

    It does not make sense, practically. So they'll be using a balance with multiple moving parts made of multiple minerals that have to be precisely calibrated with margins of error adding up, instead of a simple platinum cylinder?

    Although, it makes sense politically

  • by kazinator on 7/6/17, 4:23 PM

    Ha, the irony! The USA's NIST defines a SI unit to the rest of the world; meanwhile, most of citizens don't know what it is.

  • by moonbug22 on 7/6/17, 11:08 AM

    That's also a dreadful pun.

  • by cpr on 7/6/17, 4:59 PM

    An interesting (banned) TEDx talk by Rupert Sheldrake, one part on the changing "fundamental constants", starting here:

    https://youtu.be/JKHUaNAxsTg?t=591

    The other parts are a bit "woo" and I'm sure would be laughed at by the HN crowd. But his points about fundamental "constants" changing, and the metrologists' dogmatic (really, anti-scientific) response, are worth pondering.