
NIST to redefine the kilogram based on a fundamental universal constant - nixme
https://www.washingtonpost.com/news/speaking-of-science/wp/2017/07/05/scientists-are-about-to-change-what-a-kilogram-is-thats-massive
======
anonymfus
_> 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.

------
kiernanmcgowan
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...](https://en.wikipedia.org/wiki/SI_base_unit#Seven_SI_base_units)

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

~~~
grondilu
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.com/watch?v=60OkanvToFI](https://www.youtube.com/watch?v=60OkanvToFI)

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

~~~
analog31
The half-marathon is a race that commemorates the Greek soldier who ran
halfway from Marathon to Athens after the Battle of Marathon.

~~~
amptorn
Your username is "analog31".

------
sanxiyn
> 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](https://phys.org/news/2013-01-kilogram-weight.html)

~~~
DonbunEf7
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](https://www.youtube.com/watch?v=ZMByI4s-D-Y)

~~~
cmurf
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.

~~~
mattashii
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.

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

[https://www.nist.gov/physical-measurement-
laboratory/silicon...](https://www.nist.gov/physical-measurement-
laboratory/silicon-spheres-and-international-avogadro-project)

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.

~~~
dtech
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?

~~~
madengr
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.

------
ZeljkoS
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](https://en.wikipedia.org/wiki/Pound_\(mass\)#Current_use))

~~~
zorked
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.

~~~
GCA10
I was about to argue that the illegal drug trade might be a counter-example,
but haven't they gone metric, too?

~~~
randlet
Even if they did use the imperial system, no drug dealer is going to care
about a change in the 8th decimal place of the definition of the pound.

~~~
undersuit
Carfentanil is LD50 at 0.05mg. Don't be Bill Gates, 640KB is not enough for
everyone and the 8th decimal of the pound is a lethal dose of Carfentanil.

~~~
MBlume
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.

~~~
undersuit
It still stands that a LD50 of Carfentanil is able to be estimated away
because of the change in the precision of a pound.

------
shawncampbell
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._

~~~
JoeAltmaier
Right - can somebody explain how this unit of kg-m2/s (weight diffusion? work-
seconds?) can be used to define weight? Seem circular.

~~~
plus
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).

------
msimpson
I always get a kick out of this:

[https://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Pr...](https://upload.wikimedia.org/wikipedia/commons/thumb/c/c9/Prototype_mass_drifts.jpg/399px-
Prototype_mass_drifts.jpg)

~~~
hsod
context?

~~~
msimpson
It's how the mass of the International Prototype Kilogram and its copies have
drifted over time:

[http://www.bipm.org/en/bipm/mass/ipk/](http://www.bipm.org/en/bipm/mass/ipk/)
(verifications tab)

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

~~~
FabHK
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...](https://physics.stackexchange.com/questions/64562/why-metric-
system-uses-kilogram-as-a-basic-si-unit)

------
Aardwolf
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

~~~
roywiggins
One way has been by counting silicon atoms in a nearly-perfect spherical
crystal.

[http://aip.scitation.org/doi/full/10.1063/1.4921240](http://aip.scitation.org/doi/full/10.1063/1.4921240)

~~~
marcosdumay
And they are using this as a definition for the unity of atomic mass.

------
kronos29296
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?

~~~
gh02t
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.

~~~
kronos29296
I used to wonder (basically my physics class where they took an entire class
about dimensions and verification based on that. Now I know better.)

------
leeoniya
[https://www.wired.com/2011/09/ff_kilogram/](https://www.wired.com/2011/09/ff_kilogram/)

------
slim
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

~~~
oh_sigh
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.

~~~
zimpenfish
Or, indeed, get coated in atmospheric gunk.

[https://www.wired.com/2013/01/keeping-kilogram-
constant/](https://www.wired.com/2013/01/keeping-kilogram-constant/)

> 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.

------
kazinator
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.

~~~
gumby
> 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...](http://www.bipm.org/en/about-us/member-
states/original_seventeen.html)). 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.

~~~
kazinator
Submission title says "NIST".

> _and /or who uses illegal drugs._

:)

~~~
gumby
Yes, but the article is clear. The headline is from the Washington Post. The
apparatus itself was indeed developed by NIST.

------
moonbug22
That's also a dreadful pun.

~~~
tartuffe78
For those who missed it, the original submission title (from the article) was
"Scientists are about to change what a kilogram is. That’s massive."

~~~
mabbo
Thank you, that _is_ awful. I love it.

~~~
scott_s
The last line of the article is also an awful groaner.

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

[https://youtu.be/JKHUaNAxsTg?t=591](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.

~~~
jasonwatkinspdx
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.

~~~
cpr
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.

