
International System of Units overhauled in historic vote - daegloe
http://www.npl.co.uk/news/international-system-of-units-overhauled-in-historic-vote
======
kibwen
See, the US wasn't being stubborn in not adopting metric, it was just waiting
patiently for it to progress from beta to 1.0. :)

(Fun fact: all the US customary units have been officially defined in terms of
their metric counterparts since 1893:
[https://www.nist.gov/sites/default/files/documents/pml/wmd/m...](https://www.nist.gov/sites/default/files/documents/pml/wmd/metric/1136a.pdf))

~~~
blhack
I'm one of the weird people that actually _really_ likes imperial units. For
instance:

1 foot -- base 12. This is a superior base to 10. It can be easily divided
into 4ths, 3rd, and 2nds. Base 10 can only easily be divided into 2nds.

1 inch -- an easily identifiable unit of measure for smallish things. About
the width of my thumb. A pretty good unit.

For low precision, inches become 1/2, 1/4, 1/8, 1/6, 1/64, etc. Each one is
half the size of the previous one. I actually like this one a lot. An eight
being half of a quarter is a really easy way to work with things when you're
building stuff. Think about drilling a bolt hole in the center of a piece
(half the width), or drilling two bolt holes with something in the center
(divide those halves in half again) etc. Fractional is really good for
building stuff.

For precision: thousands of an inch. Harder to visualize, but precise (has the
same problems as mm imho). Millionths of an inch when you get into serious
metrology.

Okay temperature: In imperial units:

0 = REALLY cold. 100 = REALLY hot. 50 = somewhere in the middle. Put on a
sweater, but not dangerous.

100 = about the temperature of a human body.

Water boils at 212F and freezes at 32F. There are 180 degrees (degrees!)
between freezing and boiling. 180 is, again, base 12. It's the 15th order of
12.

I actually _love_ imperial units. I greatly prefer them to metric (even though
I do use metric very frequently, and can see the appeal). I think I just
actually prefer base 12 to base 10.

I also think that F is seriously superior to C.

~~~
naikrovek
You are definitely (and thankfully) not the norm. Imperial units are awful.
They're more ... human, maybe, in that a foot is about one human foot long,
and that an inch is about one human thumb width, and how 0F feels "pretty
cold" and 100F feels "pretty hot", and to me the charm of imperial units
begins and ends right there.

Those are literally the only redeemable qualities about imperial units, and
they have nothing at all to do with their utility as a tool for measurement.

I am glad that you like them. Use them all you like. I won't.

~~~
blhack
>They're more ... human, maybe, in that a foot is about one human foot long,
and that an inch is about one human thumb width, and how 0F feels "pretty
cold" and 100F feels "pretty hot", and to me the charm of imperial units
begins and ends right there.

As far as we know, humans are the only thing in the universe that care about
any of this stuff. A computer couldn't care less if you are using 1 foot or
0.3048 meters.

Why not optimize for humans, the things actually using these things?
Optimizing for the computer just seems...silly. It seems like something
somebody from the 1970s would have thought was a futuristic idea.

~~~
naikrovek
Because adding 7/16ths of an inch to 7/8ths of a mile and getting an answer in
inches isn't easy for a human. Adding 1cm to 982m and getting an answer in
centimeters is dead easy for a human.

THAT'S why human intuition about measurement doesn't mean a damn thing.

Using metric means using the system optimized for humans. Period.

~~~
Just_Smith
Your absolutism is a bit silly. I don't think the Imperial system is good for
anything beyond construction or cooking, but there's a reason it excels those
areas. And it's exactly because it's more usable by humans in certain contexts
(like construction or cooking). 1/3 of a meter should not be an irrational
number - how is that "optimized for humans"?

A scalable system that shares the divisibility of the foot would certainly
trump both systems.

~~~
tylerhou
Nit: 1/3 of a meter is rational. You're looking for "whole number."

~~~
Just_Smith
Correct - I've admittedly had a few beers.

Which might be obvious due to me debating the merits of the number 12 on the
internet.

~~~
kiriakasis
Well 12 is amazing, but arguably there are too many other negative aspects in
the imperial system as a whole.

------
yholio
The actual text of the resolution, for those with low tolerance for dumbed-
down PR releases:

[https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-
EN...](https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf)

~~~
FiveSquared
I have low tolerance for long ass papers ;)

~~~
ZainRiz
Isn't that just toilet paper? (long ass-papers)

~~~
nathancahill
One of the OG xkcd's: [https://www.xkcd.com/37/](https://www.xkcd.com/37/)

------
pfbtgom
How would you use the new definition to calibrate instruments? My current
understanding is that there is a lineage of artifacts calibrated against
another artifact until one of those was calibrated against the one true
artifact. So how would NIST or another certifying source say that my 1kg
standard is 1kg +/\- tolerance?

Is it anyone with a kibble balance can now certify calibrations? How do you
know your kibble balance is as accurate as the next guy's kibble balance?

[https://www.nist.gov/si-redefinition/kilogram-kibble-
balance](https://www.nist.gov/si-redefinition/kilogram-kibble-balance)

Edit: Found some information explaining this from NIST:
[https://www.nist.gov/si-redefinition/kilogram-
disseminating-...](https://www.nist.gov/si-redefinition/kilogram-
disseminating-new-kilogram)

~~~
simcop2387
Essentially this brings it one more step away from using another artifact for
calibrating things. Instead this lets us define the kilogram against what we
believe are universal constants, in this case properties about the
electromagnetic force.

It's much simpler to understand looking at it with a watt balance, even though
it's not going to be as precise or accurate as a kibble balance [1]. Basically
now anyone with access to a kibble balance and the right set of
numbers/information can make an exact 1kg object.

[1]
[https://www.youtube.com/watch?v=ewQkE8t0xgQ](https://www.youtube.com/watch?v=ewQkE8t0xgQ)

~~~
mjevans
Or potentially more usefully, ascertain an accurate measurement of how closely
a given measuring device is to the target to calibrate it's use in actual
measurements (outside of the expensive validation mechanisms).

------
3pt14159
This is probably the dumbest thing I'll type on HN.

In university I just gave up trying to understand why we even needed the
Avogadro constant / mole as a fundamental constant. It still confuses me. Why
have a difference between molar mass and mass? Why couldn't it just be "1" and
everything else change around it?

~~~
jcranmer
Understanding mass in molar terms is necessary to do a lot of chemistry
correctly. The mole is effectively a count of the number of molecules kicking
about, although the count is large enough that terms like "quadrillion" don't
cut it. (One mole is about 600 sextillion molecules). Knowing how many
molecules are around lets you actually compute how much stuff can react in a
given chemical environment, and other aspects of chemistry end up being pretty
related to molecule counts. Vanilla mass doesn't cut it since an atom of
iodine weighs about 6.6 times that of fluorine but can still only react with
one other molecule.

The flip side is that the molecular count is less useful to us in the everyday
world. We can gauge the weight of a kilogram much more than we can gauge a
septillion molecules. And if we're trying to figure out how much stuff a shelf
can hold before it collapses, it's the weight that matters, not the actual
molecular count. (Note for pedants: in the familiar environment of Earth's
surface, mass and weight can be treated as the same quantity in most cases.)

So mass and molecular count are both very important quantities that have
importance in different fields of science, and they don't have a trivial
relationship to each other. Avogadro's constant and molar mass is a way to
express their relationship.

~~~
jackpirate
I think the GP is making the claim that Avogadro's constant should be 1
(effectively eliminating it), not that there's no need to count molecules.

~~~
maccam94
An Avogadro's constant number of molecules is one mole. One mole of hydrogen
has much less mass than one mole of iron. You have to choose an arbitrary mass
of a single element as the base quantity. Hydrogen might be the best
theoretically, it's just a proton and an electron, but it is tricky to work
with because it's a gas at room temperature. So instead the mole has been
defined as the number of molecules in a specific mass of carbon-12.

~~~
mbreese
I think this is exactly the rationale. But, from a more practical
perspective...

If instead of g/mol, we referred to molecules/g -- we would end up populating
tables and charts with really big numbers. This would make lookup tables hard
to read, difficult to publish, and hard to work with. Imagine if you had to do
math with a bunch of 10^23 exponents all of the time.

Instead, it was agreed to effectively pull out a constant value from each of
those to make the math significantly easier. Now, instead of dealing with a
lot of big numbers, all of the lookup tables could now list smaller g/mol
values. And we would be left with just the one single large (Avogadro's)
number in the equations.

Honestly, we don't _need_ a set mole constant, but it makes chemistry
significantly easier to do so. Unlike the other constants mentioned in the OP,
Avogadro's number is completely arbitrary. It could be '1' as the parent
suggested, except then it makes the rest of the math more difficult.

Even for this SI overhaul, we didn't really even need to redefine the mole,
except for the fact that it was previously defined in terms of the old kg.
This was just "fixing a glitch".

~~~
thaumasiotes
> Even for this SI overhaul, we didn't really even need to redefine the mole,
> except for the fact that it was previously defined in terms of the old kg.
> This was just "fixing a glitch".

I came to complain about the article calling the mole a "base unit of the SI",
and this seems like an appropriate thread.

Why is the mole a defined unit at all? As far as I understand things, "one
mole" is the same thing as Avogadro's number -- neither can be a unit, because
they're both dimensionless constants (well, they're both one and the same
dimensionless constant). Applying actual units, "one mole _of water molecules_
" is the same thing as "Avogadro's number of water molecules". Avogadro's
number, and therefore the mole, is the conversion factor between atomic mass
units and grams. Similarly, 3 is the conversion factor between feet and yards,
but nobody thinks 3 is a fundamental base unit of the imperial system. The
foot is a base unit of the imperial system, measuring length, the yard is a
non-base unit also measuring length, and 3 is a number with no special
relationship to the system at all. It would be total nonsense to say that
yards are defined by reference to 3. How is Avogadro's number different?

Wouldn't "fixing the glitch" be abandoning the idea of calling the mole a unit
in the first place?

~~~
int_19h
The new definition of mole is exactly that - it's just 6.02214076×10^23 items
of whatever, and no longer depends on kilogram.

~~~
thaumasiotes
That's not a base unit, or a unit of any kind. What's it doing in a list of SI
base units?

~~~
kiriakasis
Or you can say it is the conversion between a gram and a dalton, which is the
same but more complex as it is now intertwined with the gram

~~~
thaumasiotes
I already said exactly that:

>>> Avogadro's number, and therefore the mole, is the conversion factor
between atomic mass units and grams.

But that doesn't intertwine anything with grams. I went on to say

>>> Similarly, 3 is the conversion factor between feet and yards, but nobody
thinks 3 is a fundamental base unit of the imperial system.

>>> It would be total nonsense to say that yards are defined by reference to
3.

------
bostonpete
I get that this is a significant milestone but this seems extremely
overstated:

 _" the new changes will have wide-reaching impact in science, technology,
trade, health and the environment, among many other sectors."_

Am I missing something?

~~~
lmcarreiro
With the old definitions, the values of the known constants were just
approximations, but now that the unit is defined based on the constants, we
can consider the constant as being the exact value, no matter how imprecise it
was when it was measured using old definitions of the units.

~~~
jobigoud
So it's the equivalent of removing a magic number from your code and putting
it into a const variable.

------
JohnJamesRambo
I read the uk address and was afraid they had voted to leave the metric
system, as some sort of Brexit associated madness.

------
nudgeee
Veritasium did a great video [0] on this leading up to the vote.

[0] [https://youtu.be/c_e1wITe_ig](https://youtu.be/c_e1wITe_ig)

~~~
ThetaOneOne
I wish he would spend a bit more time exsplining were some of the constants
come from, although that would probably make the video a bit weaker overall.
Wasting time on such information.

~~~
delecti
He's probably done videos on many of them already.

------
gorn
Recently I have been thinking about if any of our units makes sense in cosmic
perspective. Let's take speed of light for example. It's approximately 300000
km/s. But then what is a second? It's 1/60 of a minute which is 1/60 of an
hour which is 1/24 of a day(and so it goes) and all those numbers are
arbitrary. A day doesn't make any sense outside our planet anyway, I doubt
that there is another celestial body in the universe that takes the same time
to complete a rotation. Period of some natural phenomena (like atomic electron
transition) sounds better as a unit but it's a really tiny period of time so
we have to scale it to make it practical for us. We will use decimal numeral
system to do that, another arbitrary choice. What if we had 12 fingers or 8?
This can be extended to all kinds of measurements so I wonder if any of this
would make sense to another civilization. What would a cosmic system of units
would like? Any reading about this would be greatly appreciated.

~~~
carljoseph
The definition of a second isn't based on the Earth's motion, but some natural
phenomena like you recommended. "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." [1]

[1] [https://www.scientificamerican.com/article/how-does-one-
arri...](https://www.scientificamerican.com/article/how-does-one-arrive-at-
th/)

~~~
saagarjha
Of course, _that_ definition was made to make our current definition of the
second continue to work.

------
adamnemecek
The Wikipedia entry is pretty informative
[https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...](https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units)

------
tehaugmenter
Very cool explanation of the math as well as the concept behind the standard:

[https://www.youtube.com/watch?v=Oo0jm1PPRuo](https://www.youtube.com/watch?v=Oo0jm1PPRuo)

------
ballenf
Has a "metric" version of time ever been proposed? Seems odd to me that we're
"ok" with divisions of 12 for time but not length.

I guess for science _seconds_ are basically the only measure of time that
usually matters?

~~~
eutropia
Heinlein's "Starship Troopers" has the space marines using "kiloseconds" and
"megaseconds" conversationally, and it was always tricky for me to convert
those to hours and minutes in my head accurately.

~~~
TylerE
Odd choices... since 1000 seconds is ~17 minutes and 1M seconds is 4.6 hours.

~~~
majewsky

      $ units --verbose
      Currency exchange rates from FloatRates (USD base) on 2018-10-25 
      3070 units, 109 prefixes, 109 nonlinear units
    
      You have: 1 megasecond
      You want: 1 day
              1 megasecond = 11.574074 * 1 day
              1 megasecond = (1 / 0.0864) * 1 day

~~~
yesenadam
Gee, I never knew about _units_ , thank you. Very cool. It can convert
meters/s to furlongs/fortnight, cm^3 to gallons.. Write fractions like '1|2'.
And you can add your own units to the units file.

------
sandov
>The definition of the kilogram for more than 130 years, the International
Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the
BIPM in France, will now be retired. It will be replaced by the Planck
constant

Maybe I interpreted it wrong (non-native English speaker), but does it say
that the Kilogram will be equal to the Planck constant? Shouldn't it be that
the definition of the Kg will be _based_ on the Planck constant?

>Although the size of these units will not change (a kilogram will still be a
kilogram)

Are our current measurements of the IPK _that_ exact so the old kg is
_exactly_ equal to the new Kg? How can they measeure it with 0 error?. It
doesn't make any sense to me.

~~~
m1el
> but does it say that the Kilogram will be equal to the Planck constant?

No, this statement means that Kilogram will give up its place as a fundamental
unit to Planck constant. Earlier, Kilogram was used as a fundamental unit,
defined by a physical object. Not anymore.

The kilogram will be defined by Planck constant. Namely, m = E/c^2 = hf/c^2.

> How can they measeure it with 0 error?

We can't, all measurements have errors. But we _can_ measure things better
than a physical object, which _has_ changed with time. So we only have to
measure Plank constant with precision higher than variation of physical
object's mass for the purpose of replacing the definition of mass.

------
ggm
Never understood why the reference weight was one and not a maintained average
over several or tens. This could have included aperiodic less used reference
weights stored differently, different washing cycles &c.

I know several bipm participant nations had clones but they were always
understood to be daughter weights.

Level surfaces (for instance) used to be made as triplets and never pairs:
pairs can form lenses.

Not that a kibble balance definition isn't better: I just don't understand why
the kg definition was a Singleton and not statistically satisfied.

~~~
kderbyma
Decay would be exponentially greater with more weights. Causing a larger
decrease over time.

~~~
ggm
Except some kg reference weights gain weight. The washing regime ablates, but
the metal matrix is porus at the atomic level (I am told) and so they accrete.

Therefore there would be gains and loses. Not just losses

------
rubenhak
Finally there is order in units for science community. Hopefully such
transformation will come to US one day.

~~~
clord
The US units are defined in terms of SI since the Mendenhall Order[0], so they
also benefit from this change.

[0]:
[https://en.wikipedia.org/wiki/Mendenhall_Order](https://en.wikipedia.org/wiki/Mendenhall_Order)

~~~
kevin_thibedeau
It wasn't until 1959 that the yard was metricized. This still affects
precision machinery manufactured before the changeover.

~~~
clord
It was metricized with all the rest before that. What happened in 59 was
harmonization with other countries including Canada, who had a different
agreement on the metric yard. The older american definition was 1 yard =
​3600/3937 meter. The international metric yard is exactly 0.9144m. (A
difference of 0.002mm)

------
24gttghh
So...How many Planck's is one Kilogram then?

>The mass of the international prototype of the kilogram m(K) is equal to 1 kg
within a relative standard uncertainty equal to that of the recommended value
of h at the time this Resolution was adopted, namely 1.0 × 10-8 and that in
the future its value will be determined experimentally,

Is the Planck simply the most standard deviation the kilogram can stray from
now?

~~~
zamadatix
This wikipedia section breaks down the new definition nicely
[https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units#...](https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units#Kilogram)

~~~
24gttghh
It is still not apparent to me. Is a kilogram of something now it's joules x
seconds or something? Does the kilogram now vary depending on what the
kilogram is of? Like, is a kilogram of Iron a different mass from a kilogram
of Aluminum?

>The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the
fixed numerical value of the Planck constant h to be 6.62607015×10−34 when
expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and
the second are defined in terms of c and ΔνCs.

So mass is now some sort of length times a given elapsed time?

~~~
conbandit
I believe it means something like:

Given the definition of metre and the definition of second, the kilogram is
whatever value that makes the Planck constant h precisely 6.62607015×10−34 kg
m^2 s^(-1).

~~~
24gttghh
But isn't that a bit of a recursive definition? What might one use as the
value of kg in that example?

~~~
zamadatix
There is only one value a kg could be in that example, everything else is
constant. The question I think you're trying to ask is "why 6.62607015×10−34
instead of some other simpler value" and the answer is a bit longer.

Original 1 kg was the mass of a cubic decimeter of water at 4 C at 1 ATM. Why
a cubic decimeter at 4 C? Water is densest at 4 C and a cubic decimeter of it
is a weight that people can work with on a day-to-day scale. Unfortunately
this was a bit hard to measure so they made the IPK (international prototype
kilogram) which was a lump of metal. Fast forward 100 years and the lump of
metal proved to be too unreliable for modern standards as it kept losing very
small amounts of mass, also it Earth's gravity isn't even so it requires you
average it out and then calculate the local offset and a whole bunch of other
weird things that can mean a microgram or two. This is inconvenient but we
still needed a way to say "1 usefull measurement of mass" but unfortunately in
the universe 1 Planck's constant is far too small to ever use daily.
Thankfully it has become easier to accurately measure Planck's constant
against the IPK so now we have solidified 1 kg to be exactly what we measured.

The nice thing is 1 kg will forever be the same thing now and is easy to
measure to extraordinary accuracy. The downside I think you're asking about is
1 kg by itself isn't some significant relation of the physical world, it's
just a useful-in-daily-life multiple of mass as defined by Planck's constant.

------
gwbas1c
I can imagine that in 100-200 years we'll have to adjust some unit because our
understanding of some constant was just slightly wrong.

~~~
Koshkin
According to the new definitions, some of the fundamental constants can no
longer be wrong. We could even have defined them as 1 (which is what
physicists have been doing for a long time already.)

~~~
anticensor
If you do that, magnitudes of resulting units will either be huge or very
tiny, depending on the composition of fundamental constants in said measure.
The constants are _very carefully_ chosen to match old units.

------
turdnagel
So there was a definition of a kilogram based on an actual physical thing with
an arbitrary amount of mass.

Does this new definition essentially try to approximate that mass as close as
possible? What is the margin of error there?

I would love to know this for a second as well, although I don't know what we
used to measure a second before we locked it to the vibrations of a cesium
atom.

~~~
bcoates
The Earth?

~~~
tialaramex
Yes. In two ways.

First, the assumption was that the Earth's rotational period was 86400
seconds. Mechanical clocks had given us no reason to doubt this was so.

By the mid-20th century quartz timers were able to discern that the Earth has
longer and shorter days, as numerous factors cause the spin to speed up or
slow down. Instead of a single day the second was taken to be one 86400th part
of an average day over the whole year.

A few decades later the availability of atomic clocks made this seem silly and
we uncoupled the second from the variably spinning Earth. The completely
arbitrary seeming atomic clock definition of a second was basically chosen to
be indistinguishable from our last guess at the "averaged over a year" second.

------
ajb1
Something I've considered recently is the case of estimating heighth (or any
length) in customary vs. metric.

In the US, if I'm describing someone, perhaps to a police officer, I can
fairly easily conceptualize the difference between 5ft, 5ft 6in, 6ft, or even
smaller degrees of difference.

Though I'm certain this is likely just a result of me having grown up with the
customary system, it seems like it would be more difficult (or at least more
tedious) to estimate the heighth of a person using meters, given that most
humans are in the 1.5-1.8m range (by my estimation). Especially when looking
only at one gender, the range of possible heights is quite small in meters,
requiring more precision to describe.

For example, I can reliably understand and visualize the difference between
5'0" and 5'4".

I'm curious for those in the rest of the world - can you meaningfully
visualize the difference between 1.5m and 1.6m? Or perhaps 1.55m and 1.58m?

~~~
llukas
This is _only_ due to you being used to certain units.

I was raised in EU using Celsius scale and can relate to full scale. In US I
can relate only to 70-80 range as this is what I consciously experienced (ie.
adjusting air conditioning). I still need to convert Fahrenheit to Celsius for
anything outside of that range.

Also imperial sucks utterly as metric allows almost seamless exchange of mass
measurements with volume for of pretty much anything in the kitchen (1g of
water is roughly 1cm^3, almost everything we eat is very close in density to
water).

------
imhoguy
I curiously wonder on what ground the future overhauls will happen. ToE?
[https://en.wikipedia.org/wiki/Theory_of_everything](https://en.wikipedia.org/wiki/Theory_of_everything)

------
Millennium
The main problem with imperial units is that it isn't a system. It's a
hodgepodge of pieces of different historical systems, many of which made a lot
of sense in their original context, but the context has been stripped from
them and they've been mashed together with completely unrelated systems. A
base-12 system would actually have some advantages, but very few of the
imperial units are actually defined in terms of base-12, so it doesn't
qualify.

------
xaduha
> a cylinder of a platinum alloy stored at the BIPM in France, will now be
> retired. It will be replaced by the Planck constant – the fundamental
> constant of quantum physics.

I thought they did that long time ago.

~~~
keketi
I wonder how much the cylinder will auction for.

~~~
adrianmonk
At least there will be no disagreement about the shipping weight on the
auction listing.

~~~
Devagamster
The real question is how many boxes will the fulfilment center place the kg
in. If Amazon is anything to go by my money is on 3

~~~
reaperducer
OT, but _this_!

I recently ordered a picture frame from Amazon. It came in its packaging box,
which was placed in a box by the manufacturer. That box was put in another box
by Amazon and shrink wrapped to a large piece of cardboard, which was then put
inside yet another box.

On the surface, it seems crazy, but Amazon did manage to get it to me
undamaged.

~~~
toast0
My wife recently ordered five storage boxes. They arrived in two packages on
the same day. One package neatly fit four of the boxes. The other package was
large enough for five, and contained one storage box and a excessive amount of
packing material. Not an Amazon purchase, so it must be an industry issue.

------
edoo
The US didn't adopt the metric system because it was considered blasphemous
since the French used it. I'm not joking. We got screwed by a bunch of
demagogues.

~~~
gnopgnip
The US officially adopted the metric system in 1975 with the Metric Conversion
Act.

~~~
edoo
The act does not require the US to use metric in any capacity. It is optional.
If it did kids would not be taught using the imperial system.

~~~
slededit
The US uses customary measures. Imperial is the old UK system. It has some
important differences like the size of a gallon - and perhaps most importantly
if you like beer: the size of a pint.

------
iharhajster
For an engineer it's quite an anventure throughout your career discovering how
many "languages" people speak: imperial, metric, IEC, ANSI, metric thread, NPT
thread, all the units and industrial standards. And still foreign ISS parts
mate in orbit on their first date. Or ITER parts fit toghether in plant.
That's electrifying and inspiring.

------
zzo38computer
The old definition of kilogram (with IPK) is no good, so it is very good that
they are correcting it.

Of course you can still use imperial and other units in whatever context they
are useful, and so SI units can also be used in the context where they are
useful. But still I am glad they change it, because the old definition of
kilogram is no good.

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SethTro
The Doubleclicks write a song about the IPK, the international prototype
kilogram, and how it must feel about this.

[https://youtu.be/VHd4HPmz5aE](https://youtu.be/VHd4HPmz5aE)

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Digit-Al
Can anyone tell this poor ignoramus whether the kg is getting heavier or
lighter - and how much by (approximately, I know it's not going to be an exact
figure given the reason they are changing it)?

~~~
shittyadmin
This doesn't impact the actual weight of a kg. What this impacts is how the kg
is defined, no longer does it rely on a physical kg.

It used to literally be defined as the mass of "IPK" \- the international
prototype kilogram.
[https://en.wikipedia.org/wiki/Kilogram#International_prototy...](https://en.wikipedia.org/wiki/Kilogram#International_prototype_kilogram)

This switch now defines it in terms of the force equivalent of the energy
exerted by a single photon - if my understanding is correct.

Theoretically a more scientifically sound way of keeping 1 kg a constant
permanently.

~~~
cco
That doesn't make sense to me, the definition has changed.

The IPK was 1 kg and the copies of it were all slightly heavier or lighter,
for argument let's say IPK1 was 1.1 kg and IPK2 was 0.9 kg. Now that we have a
constant for the mass of 1 kg, all of those IPK's will now have a new mass
relative to the new definition.

Unless the new definition was based on the current IPK and set its mass as of
today as the standard, and that standard now will be unchanging?

~~~
ekimekim
> Unless the new definition was based on the current IPK and set its mass as
> of today as the standard, and that standard now will be unchanging?

Correct. The current IPK will still be 1kg to within margin of error at the
time the new definition comes into effect, but any further changes to it will
make it, for the first time, drift away from that value.

~~~
cco
Ah I see, that makes sense, thanks!

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yeukhon
So now packages relying on older values would have to be updated, right? In
that case, wow, all those ancient - but usable FORTRAN/C code. Correct me if I
am wrong.

~~~
delecti
Ideally the weight of a kilogram defined by the new definition should be
identical to the weight of the old physical " _the_ kilogram" object. The goal
of this isn't to change how much a kilogram weighs, it's just to change how
it's defined.

For a comparison, it used to be that we measured the speed of light as being
299,792,458 meters/second. Since then, we have _defined_ the meter to be
1/299,792,458 of the distance that light travels in one second. An object that
was 1 meter long before is still 1 meter long, but the way we communicate how
long a meter is has gone from "it's how long this particular stick in France
is" to a precise definition based on fundamental properties of the universe.

------
tempodox
Welcome news.

Of course the U.S. will continue to be be the leader of the 3rd world by
measuring in Royal Hangnails, Princely Bladders and Regal Farts.

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pulverizer
About metric vs imperial. We all adopted 10 based numbering. That puts any
system not 10 based at disadvantage.

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orblivion
I guess the next redefinition will happen if and when there's detectable drift
in "fundamental" constants?

~~~
reaperducer
_I guess the next redefinition will happen if and when there 's detectable
drift in "fundamental" constants?_

And until then, all the metric fanbois will constantly tell you that metric is
superior to imperial because its measurements are immutable. Until suddenly
they are.

~~~
nafey
As opposed imperial constants? How is a foot standardized anyway? I suspect
most manufactures rely on SI metrics for that.

~~~
tialaramex
There are no other standards. Metrology is expensive and doing it over again
would offer no benefit whatsoever. All prior standards in widespread use were
abandoned during the 20th century in favour of defining the relevant units in
terms of SI units.

Specifically the international yard equals 0.9144 meters and the international
pound equals 0.45359237 kilograms

There literally isn't another definition, in the US when you say "100 yards"
the only legal meaning that has is 91.44 metres, which is whatever CGPM / BIPM
says it is.

------
LTL_FTC
My poor textbooks! Welp, I guess there will be some great bargains next year.

------
garmaine
This is massive news.

~~~
redial
The went to great lengths working on this announcement.

~~~
garmaine
No they didn’t, the lengths stayed the same!

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onetimemanytime
if civilization fails....how we are going to measure these things? Or then,
we'll have bigger problems so who cares

------
stickfigure
On the rare chance that anyone here missed today's XKCD:

[https://xkcd.com/2073/](https://xkcd.com/2073/)

~~~
mbigras
Thank you for linking this, just made my day!

------
xvilka
Too bad US doesn't use them.

~~~
lb1lf
Fun (?) anecdote - metric apparently has been used in US engineering for quite
a while; a petrolhead buddy of mine was puzzled as to all the fractions used
on the drawings of his (I think) Chrysler Hemi V8 from the fifties - on a
hunch, we converted a few to metric. Bingo.

(Just making up a number here) 3.937" stroke? Why on Earth... Ah. 100.0mm.
That's why.

~~~
ggcdn
Another anecdote - In structural engineering, US units are way easier to work
with, and even in Canada, about 90% the buildings I work on use kips, ft, ksi,
etc.

~~~
jbay808
They aren't easier to work with for any reason other than that sizes of
standard components (pipes, beams, etc) are sized conveniently in US units and
have US units printed on the spec sheets.

If you go overseas outside of North America, you find that the sizes of
everyday objects are conveniently sized in metric, and building codes and
standards and material strengths and densities are specified in SI units, and
suddenly working with the metric system in those industries is easy and
convenient.

Do you deal with building insulation? How would you interpret a material spec
of "1 BTU ft/(in^2 hr °F)"? I had to work with these units in the US and it
was not at all convenient.

~~~
ggcdn
I don't deal with building envelope. Its not surprising or unexpected that
there are differing opinions among various professions on which set of units
are more convenient.

In Canada, all our building code, material design codes, etc are in metric,
and yet we are still using US units for day to day design.

~~~
jbay808
What lengths and widths of wood can you buy from the lumberyard?

What is the common width and thickness of drywall sheet you can buy and the
hardware store? What door sizes are available? What size of screws are cheaply
available in Canada?

I think these factors are far more likely to affect what unit of measure is
commonly used in construction, rather than any intrinsic merit of the
measurement system. If you were shopping at a Japanese or German hardware
store, you'd probably suddenly find all of those 12 foot dimensions quite
frustrating and not be at all surprised to find the hard-hat-wearing locals
happily using the metric system for the same tasks.

