>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.
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].
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.
And for that matter, isn't it a bit frivol to call it a constant, if it's based on something mutable? Perhaps "the Avogadro number" would be a more appropriate name.
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.
> 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
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!
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.
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.
Down to eight significant figures it is an issue (effect of the moon's gravity on the surface of the earth peaks at 1.1 × 10−7 g), but you can always point the normal of the axial plane of the balance in the earth relative right ascension direction of the moon to avoid this effect (regardless of the moon's relative declension, δ).
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.
> 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.
Turns out they know that :-)
The problem isn't definition, its the accuracy of reproducibility. You need a mechanism that can be built from scratch and can be believed to produce identical results. You can pull that off counting wavelengths of an unknown number of a known pure atom. Counting actual numbers of atoms is quite difficult. Typically we do it statistically...via mass! We could get it precisely by using a small number, but then actually measuring the mass (rather than calculating it) would be difficult.
A second (time) is already defined on cesium transition, a meter (length) is defined as a fraction of the distance light travels in a second. Both things which we can accurately measure for some time now, and which universal and stable over eons.
What is the problem that would be solved by switching to a single element?
I don't recall. I just read some article recently about trying to define time and length with sodium; trying to define everything with a single element that is very common. Of course it could also have been a 50 year old Asimov book.
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.
Wait no this doesn't make sense. It's not an 8th decimal of a pound, it's an 8th decimal of what you're measuring. If you're measuring out a pound of Carfentanil you might be off by 0.05mg, but if you're measuring a dose you'll actually take, you won't.
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.
Right now the mass of the kilogram prototype is defined to be exactly 1 kg. If someone adds or removes matter from the prototype, then the numerical value we assign to the mass of everything else in the world would change, but the prototype would remain 1 kg. On the other hand, currently, the value of Planck's constant is known only to a certain level of accuracy.
Under the new system, Planck's constant would be defined to be exactly 6.626070040e−34 kg.m^2.s^-1, with no error bars, and the prototype would no longer be exactly 1 kg. If we refine our estimate for the physical value of Planck's constant, its numerical value of 6.626070040e−34 kg.m^2.s^-1 would not change, but the numerical value for everything's mass would.
This definition of 1 kg requires we first define 1 m and 1 s, but there are already good definitions for these quantities based on fundamental physical properties (namely, the speed of light and the frequency of the transition between the two hyperfine levels of ground state of the caesium-133 atom).
Because in order to measure something in terms of a constant, you cancel out all the other units except the one you're measuring.
I.e., if you know how to measure a second, you could define the meter based on the speed of light by creating light in a vacuum and timing it.
Meanwhile, you can't define a meter or a second in terms of pi, because it has no units. It's just a ratio of the surface area of certain classes of object or shape to thheir dimensions.
The same way a measurement in the units of m/s (i.e. The speed of light) can be used to define the meter, given the second is defined via another method.
The constant is measured via experiments and because mass appears in the expression of the constant given above, a measure of the kg is obtained. I.e. the closer we approximate the constant (6.62...) the closer we get to determine what a kilogram actually weights (in relation to meters and seconds).
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:
Because of how SI units evolved historically; basically, there was the "cgs" version of the metric system, and the "mks" version, and the latter won out. (Note that neither version has all three "base" units pure in terms of metric prefixes--the first has centimeters instead of meters, the second has, as you note, kilograms instead of grams.)
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
How to define fundamental units has always been a matter of precision measurements, not theoretical purity. If the standards body decides to base the definition on Planck's constant, instead of a certain amount of electrons, the rationale will be higher accuracy in measurements of Planck's constant than of the mass of electrons.
It's OK though, because the metre and second are already based on physical constants: the second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom", and the metre is defined from the second by fixing the speed of light at 299 792 458 m/s.
>For electrons or electron holes in a solid, the effective mass is usually stated in units of the rest mass of an electron, me (9.11×10−31 kg). In these units it is usually in the range 0.01 to 10, but can also be lower or higher—for example, reaching 1,000 in exotic heavy fermion materials, or anywhere from zero to infinity (depending on definition) in graphene. <
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?
The article explains, albeit it's maybe not very obvious if you aren't acquanited with metrology.
> So in 2014, at the quadrennial General Conference on Weights and Measures (yep, that's a thing), the scientific community resolved to redefine the kilogram based on Planck's constant, a value from quantum mechanics that describes the packets energy comes in. If physicists could get a good enough measure of Planck's constant, the committee would calculate a kilogram from that value.
> “But it's a very difficult constant to measure,” Pratt said. He would know: He and his colleagues at NIST have spent much of the past few years trying to come up with a number accurate and precise enough to please the finicky physics community.
Basically, it was hard to measure Planck's constant precisely enough to be as precise as the old standard. For compatibility reasons, the way this usually works is that they will measure Planck's constant and then define the kilogram so that `(k * Planck's constant) = (old mass of the kilogram)`, where `k` is whatever constant that makes this work out. To do this properly, you need to be able to measure Planck's constant with the same level of precision (and accuracy) as the old mass of the kilogram was known. Apparently this wasn't easy, presumably because Planck's constant is very small.
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?
Defining your fundamental unit of mass in terms of a single arbitrary physical object costs a lot of theoretical purity, though it might not cause a lot of problems in practice. You mention a potential loss of precision. But we should gain accuracy by defining it in terms of fundamental constants. When the object itself changes, do we simply have a standard that drifts more than the physical constants of the universe? Do textbook publishers need to update all examples and problems using micrograms because the unit drift at that scale became significant? Should anyone without access to a reference object not get a real, accurate value for the kilogram? Do we keep using the mass value we know the reference object had, or does our kilogram really change with the object? Do we need to update any equations with a constant in them involving mass in any unit or derived unit in the equation once a year for high precision applications?
The problem with a physical standard like that is that you can't (easily) ship it to labs all over the world so that they can calibrate to it. At least when you use fundamental constants, each lab can set up their own equipment to produce the correct measurement.
Also, anything physical will shed atoms, which will affect the mass.
> Cumpson suspects that because the kilos living in national labs have been retrieved and handled more frequently than the international kilo, more carbon-containing contaminants have built up on them over time.
The simple platinum cylinder's margins of error are pretty much unbounded over time. If you come up with a good physical definition, our knowledge of the value will only get more precise over time as the means for measurement are improved.
It's the opposite: it makes sense practically, because it means a setup that can be reproduced "anywhere" based on pure knowledge, without depending on being able to ensure the continued integrity of a physical object that has been shown to change over time.
Most people can continue to depend on prototypes for calibration, so it does not cost us any complexity in that respect. What we're gaining is to be able to independently re-calibrate multiple prototypes.
I wouldn't say politically, but I'm sure that scientific aesthetics play a role. There are people who have devoted their careers to the development of fundamental standards. We search for the next digit of h, for the same reason why we search for the Higgs particle -- because we're curious.
A better kilogram might not have a practical use right away, but it might in the future, if for instance we want to do things like look for time dependent variations in the physical constants, tiny departures from existing theories of mechanics, etc.
> 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_seven...). Also all the US conventional units have been based on the SI metre and kilogramme since 1959. And of course the metric system is familiar to any American in the military and/or who uses illegal drugs.
Although its not part of the BIPM, my favorite such standards organization is the International Earth Rotation Service (justified paying my taxes -- what if they stopped???). Sadly they recently renamed themselves "International Earth Rotation and Reference Systems Service"
Apropos of little: I used to live quite close (a couple of hundred metres) to an official metre, as there is one on the wall across the street from the French Senate. When the system was originally promulgated, markers were erected around France; you could bring something (piece of string or whatnot) and make your "own" metre to bring home and measure things. There are two or three of them still extant.
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.
Just in case folks are unaware, this is the parapsychology guy that believes the following:
> Sheldrake's morphic resonance hypothesis posits that "memory is inherent in nature" and that "natural systems, such as termite colonies, or pigeons, or orchid plants, or insulin molecules, inherit a collective memory from all previous things of their kind" ... Sheldrake proposes that it is also responsible for "telepathy-type interconnections between organisms". His advocacy of the idea encompasses paranormal subjects such as precognition, telepathy and the psychic staring effect as well as unconventional explanations of standard subjects in biology such as development, inheritance, and memory.
The reason his TED talk was pulled is because he's a crank, and the entire talk is a confused defense of pure BS.
Yes, but did you watch the section on fundamental constants? Even if he's a crank, his questions are valid, and metrologists who define away problems with fundamental constant measurements are to be questioned.
>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.