
An old artifact kept in a vault outside Paris is no longer the standard kilogram - WalterSobchak
https://news.mit.edu/2019/kilo-standard-change-0516
======
inopinatus
And with that, I mourn the obsolescence of my last remaining "absolute unit"
joke and will have to develop something equally satisfying based on dressed
lumber to replace it.

~~~
StavrosK
What about the imperial units? Are those defined against metric as well?

~~~
Etheryte
A quick glance at Wikipedia seems to indicate so, but I didn't go over every
base unit.

~~~
not2b
Yes, for example an inch is now defined as precisely 2.54 cm, so a foot
exactly 30.48 cm, a mile is exactly 1609.344 m.

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lelf
Definition of mole changes too.

 _One mole contains exactly 6.022 140 76×10²³ elementary entities_ (as opposed
to the number of atoms in 12 grams of ¹²C). Effective 20 May 2019.

~~~
Izkata
Eh? Straight 6.022*10^23 is what I learned in highschool chemistry in 2003..

~~~
hexane360
The difference is more epistemological than practical. Before, 6.022x10^23 was
the _measured_ value of the conversion factor between a gram (exactly 1/100 of
the Prototype Kilogram) and a dalton (exactly 1/12 the mass of a carbon-12
atom). After the redefinition, a dalton is _defined_ as being 1/6.022x10^23th
of a gram, and _measured_ to be approximately equal to 1/12 the mass of a
carbon-12 atom. In essence, the dependency chain was reversed, so now it all
flows from the kilogram outwards.

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pferde
Defining a unit of weight in terms of a unit of time and a unit of length.
Explain _that_ to your grandma! :)

~~~
groestl
Your grandma might be a physicist, and she'll explain it to you ;)

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anvandare
For the next revision: remove the mole from the SI. It's completely pointless.
A number is not a unit! Instead replace it by the bit, the fundamental unit of
information.

~~~
GuB-42
The reason the mole is in the SI puzzled me too.

It seems that the reason is that chemists don't care how many atoms there are
in their reactions. It would be inconvenient: fractions of atoms don't make
sense, and the numbers are far from human scale. They just want an agreement,
so that even if they don't know the exact number of atoms in their test tube,
they know they all have the same number. That's what units are for.

The definition of units are done in the most convenient and precise way
possible. And they are updated as science progresses. For the kg, the artifact
used to be the best we had, alternative methods weren't precise enough, they
changed that to the Kibble balance because now, it is better. It may change
again at a later time if we find something better.

Back to the mole, it used to be defined as the ratio between the kilogram and
the mass of a 12C atom because it is the best we had. Now that we can count
atoms with more precision, we decided to change it to just the Avogadro
constant, which becomes fixed. Again it might change. What may happen (or may
have happened, see Avogadro project) is that we can define the kilogram using
the mole, making it fundamental.

~~~
garmaine
That is what has happened, essentially. The mole is atomically precise, and
will never change.

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Causality1
I've always been mildly irritated that the name of the SI mass base unit has a
prefix. Second, liter, meter, Newton, kilogram. Were I emperor of the world I
would slide the name scale up so the mass equal to 2.205lbs was called a gram.

~~~
shpx
If you were emperor of the world, I would implore you to finance a
multibillion dollar project to measure the gravitational constant to higher
percision so that we could move to units that actually make sense.

[https://en.wikipedia.org/wiki/Planck_units](https://en.wikipedia.org/wiki/Planck_units)

~~~
kragen
Shpx, gung'f n shpxvat terng vqrn.

~~~
stilley2
V fcrag na rzoneenffvatyl ybat gvzr penpxvat guvf pbqr...

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aivisol
Reading about standard kilogram always reminds me 2014 movie "1001 grams" [0].
Not sure how accurate it reflects how the things are actually run there, but
was interesting to watch it nevertheless.

[0]
[https://www.imdb.com/title/tt3346824/](https://www.imdb.com/title/tt3346824/)

~~~
fatbob
Directed by Bent Hammer

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mehrdadn
Confused, so how do they measure 1 kilo of something now when they need to
calibrate the most precise instruments? They count 10^40 photons? How? And how
does that translate into a physical mass they want to carry around?

Edit: Seems I completely missed a paragraph somehow. Thanks!

~~~
sanxiyn
The term of art is "realisation". The article mentions two realisations but
does not elaborate: "In practice, there are currently two known methods for
measuring such masses with great precision. These are known as the Kibble
balance and the single-crystal silicon sphere". You should be able to google
from that.

~~~
crankylinuxuser
I never understood why a sphere was even in the discussion of a standardized
atomic count.

Any rational fractions using pi = pi atoms or molecules. And that's absurd to
have r=(3V/4pi)^1/3 . No matter how you mess with it, it's irrational to the
extreme!

Now.. Face-centered or ody-centered cubic follows a nice quadratic expansion
for integer based counting. We don't need no steenkin pi's!

~~~
protonfish
It's not difficult to round to the nearest atom.

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dwighttk
>Despite the greatest of precautions, every time the standard kilo was handled
— for example, to compare it to another unit that could then be used to
calibrate instruments — it would shed some atoms and its mass would be
slightly changed. Over its lifetime, that standard kilo is estimated to have
lost about 50 micrograms.

Why did the standard kilo ever need to be handled? Just store it on a balance
and only handle the comparison unit.

~~~
rough-sea
The balance still would need to be balanced.

~~~
dwighttk
right...

~~~
thenewwazoo
and then left...

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misnome
This article sounds like it got written by a scientist but got butchered
through a PR department.

It talks about the "Mass of a photon" and weighing them directly, rather than
the energy-equivalent mass, which is presumably what they are intending to
talk about?

~~~
fsh
As the article correctly states, photons have a finite mass that could in
principle be measured by putting an optical cavity on a scale.

~~~
renoz
I know that zero is a finite number, but since the photon is a massless
particle, wouldn't it be impossible to get a kilogram worth of photons
bouncing around in an optical cavity?

~~~
amluto
No, it’s entirely possible in principle. If you build a perfect optical
cavity, weigh it, pump a bunch of light in, and weigh it again, you’ll find
that the weight went up by ghf/c^2 summed over each photon added to the
cavity. Divide by g and you get the sum of the masses of the photons. 1 kg of
photons would be enough to destroy your lab and the rest of your city, so only
a very advanced civilization would ever do the experiment on this scale.

Photons indeed have no _rest_ mass, but it’s impossible to ever find a photon
at rest unless you yourself have no rest mass, in which case you also have no
ability to do experiments.

~~~
phaemon
Actually, as a physicist with no rest mass, I'm perfectly capable of
performing experiments. I just never seem to find the time...

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another-dave
Physicists finally moving from SVN to Git.

~~~
vincnetas
More like from JPG to SVG

~~~
anoncake
Not JPG, a physical picture that cannot be copied perfectly.

~~~
bpye
If you consider re-encoding a JPG perhaps the initial comparison holds.

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huehuemon
Veritasium has a video back in 2017 explaining kibble balance
[https://youtu.be/Oo0jm1PPRuo](https://youtu.be/Oo0jm1PPRuo)

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inflatableDodo
I wonder if there is a process that has to be gone through to now update the
pound avoirdupois. It is defined as 453.59237 grams but given the formal
definition of the gram has changed could there be a need for a formal
acknowledgement, or does the law encompass changes in methods so it just
happens automagically?

~~~
sanxiyn
I don't know about US, but this does need formal amendment to Weights and
Measures Act 1985 (current UK standard in force) Schedule 1 Part V, see
[http://www.legislation.gov.uk/ukpga/1985/72/schedule/1/part/...](http://www.legislation.gov.uk/ukpga/1985/72/schedule/1/part/V).

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donalhunt
Change is scheduled for 20 May 2019.

 _checks date_

fake news... the artifact is still the official reference for the standard
kilogram for another 72 hours... ;)

~~~
orblivion
Yeah, jokes aside I think _technically_ the post title should be changed.
(Even the article's title is wrong. Though, it wouldn't be the first time NH's
title is more accurate than that of the article it points to.)

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6006135
Is there a reason why this took so long?

~~~
6006135
Don't know why this was downvoted.

But here is the answer [1]: "The new definition only became possible when
instruments were devised to measure the Planck constant with sufficient
accuracy based on the IPK definition of the kilogram."

[https://en.wikipedia.org/wiki/Kilogram](https://en.wikipedia.org/wiki/Kilogram)

------
wnissen
Le Grand K is dead! Long live Le Grand K!

------
jelder
The new reference kilogram is an engineering marvel, but I’ve always wondered
this: why didn’t they just build a reference gram instead? Seems like that
would be at least a thousand times easier to pull off.

~~~
saalweachter
Basically, it's part of the lie of the mythology of the metric system.

The metric system did some great things (eliminated the use of measures that
depended on the substance being measured, dry pints vs wet pints, bushel of
wheat versus bushel of oats; simple relationship between units of length, area
and volume), some things that were already common at the time (eliminated
regional definitions of units; related the volume and mass of water), and some
stupid things (metric prefixes).

But at the end of the day, the size of the meter and kilogram were chosen to
be very nearly 3 Parisian feet and 2 Parisian pounds, because that made it
easier to adopt.

~~~
0xffff2
This reply doesn't seem to address the comment it's replying to at all...

~~~
saalweachter
The answer to the question "why is the unit weight the kilogram instead of the
gram" is "because the kilogram is about 2 Parisian pounds, which is what
everyone at the time actually cared about".

