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NIST to redefine the kilogram based on a fundamental universal constant

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Re: NIST to redefine the kilogram based on a fundamental universal constant

#61

Post by astrodust »

sanxiyn wrote: post #3

> 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-k…

Do they account for potential variations in gravity where they've been doing the measurements? Given how ridiculously sensitive the equipment is the position of the moon probably has an impact on the readings they're getting.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#62

Post by astrodust »

GCA10 wrote: post #48
zorked wrote: post #31

Earlier quoted context omitted.

It's kind of not important because nobody measures the kinds of things that require the level of precision applied to the kilogram definition using pounds.

I was about to argue that the illegal drug trade might be a counter-example, but haven't they gone metric, too?

It's ounces, grams and kilos in that business, but the ounces are an old-school unit that's dying out. You might buy an "ounce" of weed, but an ounce of cocaine is ridiculous unless you're Charlie Sheen.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#63

Post by kazinator »

gumby wrote: post #44
kazinator wrote: post #36

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.

> 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. ...except actually it's the International Committee for Weights and Measures. However I will help you retain your justifiable sense of ironic superiority: the US is one of the 17 original signatories to the metre convention (in May 1875: http://www.bipm.org/en/about-us/member-states/original_…

Submission title says "NIST".

> and/or who uses illegal drugs.

:)

Re: NIST to redefine the kilogram based on a fundamental universal constant

#64

Post by FabHK »

nerdponx wrote: post #45

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

Good question. Apparently, the unit (mass of 1 litre of water at the ice point) was supposed to be called "grave", but then it was considered that most measurements would be for much smaller amounts, and so it was switched to the gramme, but then for the definition they stuck to the original idea, now re-christened kg.

I don't quite understand this, as they could've defined it to be the mass of one cubic centimetre of water, rather than a cubic decimetre of water. Or they could've said that the unit is gram, and 1 g is 1/1000 of the mass of this artefact.

I've also read stories that revolutionaries objected to the name "grave", as it is close to Grave (French), Graf (German), that is, the title of nobility (anathema for the Republicans of the French Revolution). Thus, instead of 1 grave we have 1 kilogram.

At any rate, it was all rather messy and political, as the delightful book Whatever Happened to the Metric System?: How America Kept Its Feet by John Bemelmans Marciano recounts.

See also precisely this question at Physics StackExchange:

https://physics.stackexchange.com/questions/64562/why-metric...

Re: NIST to redefine the kilogram based on a fundamental universal constant

#66

Post by mattashii »

cmurf wrote: post #59
DonbunEf7 wrote: post #52

Earlier quoted context omitted.

As Veritasium explains [0], all of the reference kilograms have drifted, and some appear to weigh less than they used to. So even if we know the precise mechanism of action of the drift, it doesn't help with the fact that our measurements are less reliable than they used to be. Of course, what's probably happened is that our measuring tools have gotten more accurate! [0] https://www.youtube.com/watch?v=ZMByI4s-D-Y

The kilogram is a measure of mass, not weight which is a measure of force. In order to really figure out mass, I'd think the localized gravity has to be accounted for. e.g. GRACE maps gravity variation around the earth by measuring the acceleration difference between a pair of satellites.

The measurements were taken at the same place at around the same time, so fluctuations in local gravity should have been negligible compared to actual mass fluctuations.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#67

Post by gph »

astrodust wrote: post #62
GCA10 wrote: post #48

Earlier quoted context omitted.

I was about to argue that the illegal drug trade might be a counter-example, but haven't they gone metric, too?

It's ounces, grams and kilos in that business, but the ounces are an old-school unit that's dying out. You might buy an "ounce" of weed, but an ounce of cocaine is ridiculous unless you're Charlie Sheen.

An Eight-ball is an eighth of an ounce, and it's still one of the most popular ways of asking for coke. It's not really dying out as much as you say.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#68

Post by [deleted] »

Jyaif wrote: post #49
ramshorns wrote: post #34

Earlier quoted context omitted.

Four of the seven physical constants used to define the seven base SI units will change, so a bunch of physical constants that currently have exactly defined values will start being subject to measurement uncertainty. So μ_0 will no longer be exactly 4π × 10^-7 H/m, k_C = 1/4πε_0 will no longer be exactly 8,987,551,787.3681764 N m^2/C^2, and 1 mol of carbon-12 will no longer have a mass of exactly 12 g.

Why can't they update the Avogadro constant and keep having 1 mole weight 12g of carbon 12?

[deleted]

Re: NIST to redefine the kilogram based on a fundamental universal constant

#69

Post by amptorn »

grondilu wrote: post #23
kiernanmcgowan wrote: post #11

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...

Slightly off-topic, but looking at the temperature section in your second link reminded me of the latest video from Linus Tech Tips[1] where he gets bashed for using the expression "degrees Kelvin". Personally, I don't see the problem. Since a kelvin is a hundredth of the difference between boiling and freezing temperatures of water, there is a notion of scale so the term "degrees" makes sense. 1. https://www.youtube…

Yes, in the same way that a half-marathon is 13 degrees mile long.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#70

Post by astrodust »

gph wrote: post #67
astrodust wrote: post #62

Earlier quoted context omitted.

It's ounces, grams and kilos in that business, but the ounces are an old-school unit that's dying out. You might buy an "ounce" of weed, but an ounce of cocaine is ridiculous unless you're Charlie Sheen.

An Eight-ball is an eighth of an ounce, and it's still one of the most popular ways of asking for coke. It's not really dying out as much as you say.

Okay grandpa.
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