
Kilogram conflict resolved at last - ColinWright
http://www.nature.com/news/kilogram-conflict-resolved-at-last-1.18550
======
Asbostos
The best part about this batch of changes is they push the mole and Avogadro's
constant out on their own where the belong, not linked to any other units. Now
we'll have only a single mass unit (kg) instead of two (kg and unified atomic
mass unit) that we do now. This will knock carbon 12 of its perch as the
definition of the "other" mass unit that's been essential to use SI's mole but
was not actually SI itself.

~~~
tomp
But wouldn't the most straightforward definition of a base mass unit (kg) be
linked to the mass of an atom?

~~~
dalke
Yes. I believe Asbostos is commenting that they currently are not linked.

("Yes", with the proviso that the proposed silicon atom definition is a more
obviously direct link than the watt balance.)

~~~
tomp
What I mean is, the proposed new definition defines kilogramm using a
relationship to Planck constant. Instead, I think that a more intuitive
definition would be something like "the mass of {huge number} of C or Si
atoms".

~~~
Retra
The mass of a chunk of atoms depends on their temperature and their purity,
which are things you can't measure very accurately the more atoms you have.

Essentially, that's what they were doing with Le Grande K. It's unstable
because molecular properties are unstable in aggregate.

~~~
sklogic
How exactly mass depends on a temperature?

~~~
IvyMike
E=mc^2

You may be thinking "Oh, you fool--E=mc^2 is for fission and fusion, and
certainly doesn't apply to things like the energy in the heat of an object".
But weirdly, it does. [https://physics.stackexchange.com/questions/87259/does-
decre...](https://physics.stackexchange.com/questions/87259/does-decrease-in-
temperature-affect-mass-e-mc2)

~~~
Pyxl101
It also applies to things like gravitational potential energy, or velocity.
Climb a flight of stairs, and you gain energy. Gaining potential energy really
means that you've gained mass, and so you're heavier at the top of the stairs
than at the bottom. The _amount_ that you are heavier depends on the amount of
energy you gained, converted into mass. Similarly, as you gain velocity, you
gain kinetic energy, which also makes you heavier. If you wave your hand in
front of your face, your hand's motion causes it to gain mass. Velocity also
dilates time, so time passes ever so slightly slower than for the rest of your
body.

This typically only matters in a relativistic context, and doesn't impact day
to day life. The difference in passage of time can be measured by an atomic
clock if you put it on a rocket into space, but is otherwise insignificant.
It's unlikely any scale could measure your weight-gain from climbing stairs,
since the amount of energy you gain is insignificant when converted into mass.
You can compute the mass gain by solving for "m" in the formula E=m*c^2, so
m=E/(c^2).

The speed of light c is a really big number, and so to compute the mass you
gain, you're dividing the energy by c^2, which is a much bigger number. Thus a
gain to kinetic or potential energy does not noticeably affect your mass in
day to day situations. Conversely, if you can convert any meaningful part of
your mass into energy, then it's an absolutely tremendous amount of energy:
atomic weapons.

~~~
tomp
> Gaining potential energy really means that you've gained mass

I've never heard this before. Can you link to some further explanation?
Intuitively, if anything, you'd lose mass, because you're in a place now where
space is less curved than where you were before.

~~~
marcosdumay
The system composed of you + planet gains mass. You can not measure a
different rest mass for yourself at either situation, so you'll probably
attribute the extra mass to the planet (and the planet to you).

And yes, there's also some change due to changes in gravity. I'd expect that
to be much smaller.

But IANAP, and not that good with relativity.

------
jakeogh
silicon-28 sphere(s?):
[https://www.youtube.com/watch?v=ZMByI4s-D-Y](https://www.youtube.com/watch?v=ZMByI4s-D-Y)
(yep, they let him palm it)

watt balance:
[https://www.youtube.com/watch?v=VlJSwb4i_uQ](https://www.youtube.com/watch?v=VlJSwb4i_uQ)

~~~
jahnu
Surprised that the cotton gloves or particles that might float onto them or be
coughed or breathed onto them aren't abrasive.

------
zb
I was amused to read this:

"They never found the cause for the disagreement, but in late 2014 the NIST
team achieved a match with the other two"

at a time when this story, also from Nature, is also on the front page:
[https://news.ycombinator.com/item?id=10383984](https://news.ycombinator.com/item?id=10383984)

~~~
rubidium
I was a bit more concerned then amused.

Having worked with some folks who do these high precision measurements, it's
concerning that they never found the cause for disagreement. Pinning down
systemic error is really, really, really hard.

As Feynmann pointed out in reference to the oil drop
experiment([https://en.wikipedia.org/wiki/Oil_drop_experiment](https://en.wikipedia.org/wiki/Oil_drop_experiment)):

"We have learned a lot from experience about how to handle some of the ways we
fool ourselves. One example: Millikan measured the charge on an electron by an
experiment with falling oil drops, and got an answer which we now know not to
be quite right. It's a little bit off because he had the incorrect value for
the viscosity of air. It's interesting to look at the history of measurements
of the charge of an electron, after Millikan. If you plot them as a function
of time, you find that one is a little bit bigger than Millikan's, and the
next one's a little bit bigger than that, and the next one's a little bit
bigger than that, until finally they settle down to a number which is higher.

Why didn't they discover the new number was higher right away? It's a thing
that scientists are ashamed of—this history—because it's apparent that people
did things like this: When they got a number that was too high above
Millikan's, they thought something must be wrong—and they would look for and
find a reason why something might be wrong. When they got a number close to
Millikan's value they didn't look so hard. And so they eliminated the numbers
that were too far off, and did other things like that..."

~~~
dtparr
Regarding the Feynmann quote, I suspect that behavior itself is intrinsic to
humans. Few people want to be the guy coming up with a wildly different answer
from a consensus/authoritative answer. We see the same thing in election
polling results exhibiting 'herding'. If a pollster has a result wildly off
from the polling average (especially, near the election when the variance is
expected to be lower), they'll sometimes 'put a thumb on the scale' as
fivethirtyeight[0] put it. This makes some sense, from a CYA perspective even
if not from a scientific one. If they publish an outlier and are wrong, they
look bad. If they publish close to the average and are wrong, well, at least
everyone else missed it too.

[0] - [http://fivethirtyeight.com/features/heres-proof-some-
pollste...](http://fivethirtyeight.com/features/heres-proof-some-pollsters-
are-putting-a-thumb-on-the-scale/)

~~~
danharaj
I don't think it's a human thing; I think it's a problem whenever you rely on
previous knowledge or sharing knowledge, which science is exactly about. There
is no practical experimental set-up that systematically reestablishes all
prior knowledge from scratch, so there has to be trust in other scientists and
reevaluating that trust must be grinded out the same way we grind out new
scientific results: methodically, reproducibly.

What is problematic when it comes to humans is how our social structures are
organized for doing science. They are hierarchical, resources are controlled
centrally, and scientists are forced to compete with each other instead of
cooperating with each other. Science is a career. We injure science and
scientists by tying up their economic prosperity with their ability to
convince the rest of the world that their work is worth anything. This creates
a huge incentive to push forward and a huge disincentive to reevaluate past
results: Your reputation can be damaged because you might be undermining the
legacy of a high status scientist, and if you confirm the past result then you
haven't done anything new and that reflects poorly on your 'performance'.

Science succeeds in spite of status, institutional monopolies, and
hierarchical social organization. It would flourish in a more egalitarian
society.

Is it intrinsic to humans to be hierarchical and status based? I want to say
no. I don't think so. It is in the interests of the prevailing powers of the
world to convince people that it is the case though, because they'd rather we
not imagine a world where there isn't power to accumulate and hold on to.

~~~
kosievdmerwe
Another thing is is if you do work in fields considered "fringe". No matter
how diligently you follow the scientific method, you'll be ridiculed if you
find the "wrong" results.

I've come to the opinion that scientists are ideologues, but the ideology is
based in the current understanding of physics rather than the results of
experiments and the scientific method. A famous example is Arago's dot (aka
Poisson's dot), where Freshnel was ridiculed for his wave-based theory of
light by Poisson despite the latter not even bothering to do an experiment.

------
LoSboccacc
so the proposed definition was set by fixing the numerical value of the Planck
constant to 6.62606X×10−34 s−1·m2·kg

and the conundrum was that they still needed to have a precise enough
measurement of that constant because it's an experimental measurement.

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

[https://www.quora.com/How-is-Plancks-constant-
derived](https://www.quora.com/How-is-Plancks-constant-derived)

------
MarcusP
Are metric measurements all derived from the value 1kg? If so, does this mean
that the entire metric weight range can now be officially based on
mathematics?

~~~
markild
There are seven so called "base units" defined in the SI; meter, second, mol,
ampere, kelvin, candela and kilogram [1].

Of these, mole and kilogram are dependent of the kilogram.

edit: this is a good, if maybe a bit misleading, illustration:
[http://www.nist.gov/pml/wmd/metric/upload/SI_Diagram_Color_A...](http://www.nist.gov/pml/wmd/metric/upload/SI_Diagram_Color_April_2008-2.pdf)

[1]: [http://www.nist.gov/pml/wmd/metric/si-
units.cfm](http://www.nist.gov/pml/wmd/metric/si-units.cfm)

~~~
delecti
> Of these, mole and kilogram are dependent of the kilogram.

Did you make a typo? While it's technically true that the kilogram is defined
by the definition of the kilogram, I just wanted to make sure.

~~~
markild
Not a typo, but maybe a bit vague. I meant to say that the SI units both are
dependant on the real world kilogram object.

------
tinkerdol
Reminds me of this movie:
[https://www.youtube.com/watch?v=5dPnFO_JCdc](https://www.youtube.com/watch?v=5dPnFO_JCdc)
(haven't seen it, but it looks interesting)

------
dfc
The kilogram is still the only base unit that contains an SI prefix in the
base unit's name.

~~~
dkbrk
Yeah, that's still a bit wierd. There's also the [centimetre-gram-
second]([https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%...](https://en.wikipedia.org/wiki/Centimetre%E2%80%93gram%E2%80%93second_system_of_units))
system that's used in some areas that has the prefix on the length unit.

On a related note, tonne (i.e metric tonne) is widely used, though not
generally in the scientific arena. This is partly for historical reasons, but
I think there's an element of it being a little awkward to apply prefixes to
the kilogram, as one must multiply the "kilo". Nonetheless, we should exploit
the full power of SI prefixes and use "megagram" instead. With two short
syllables rather than one long syllable it takes about the same amount of time
to say and is far less ambiguous.

This works well elsewhere; Fat Man wasn't 21 kilotonnes of TNT, it was 21
gigagrams.

~~~
fluxquanta
As a student in physics, I always used to joke about how we should say "3
megadollars" and "5 gigadollars" instead of "3 million dollars" and "5 billion
dollars".

~~~
SamReidHughes
There's a backup service that lists its pricing in picodollars per byte (or
byte-month).

~~~
mmastrac
[http://www.tarsnap.com/picoUSD-why.html](http://www.tarsnap.com/picoUSD-
why.html)

    
    
      There are three major reasons why Tarsnap pricing is defined in terms of picodollars per byte rather than dollars per gigabyte:
    
        Tarsnap's author is a geek. Applying SI prefixes to non-SI units is a geeky thing to do.
        If prices were listed in dollars per GB instead of picodollars per byte, it would be harder to avoid the what-is-a-GB confusion (a GB is 10^9 bytes, but some people don't understand SI prefixes). Picodollars are perfectly clear — nobody is going to think that a picodollar is 2^(-40) dollars.
        Specifying prices in picodollars reinforces the point that if you have very small backups, you can pay very small amounts. Unlike some people, I don't believe in rounding up to $0.01 — the Tarsnap accounting code keeps track of everything in attodollars and when it internally converts storage prices from picodollars per month to attodollars per day it rounds the prices down.

------
pervycreeper
What is the level of accuracy they are aiming for? If it entails have some
uncertainty over the precise number of atoms in the silicon sphere, then how
did they choose this level of accuracy?

~~~
jessriedel
I don't know what fundamentally sets the accuracy, but it's definitely a lot
coarser than the number of atoms in the silicon sphere. G is known to 4 or 5
significant digits, while atomic accuracy would be 26 significant digits.

~~~
sanxiyn
G doesn't matter if you use dual-pan balances, and they do.

------
Animats
Ah, they went with the "electric kilogram". The other plan was to build up a
regular structure with a known number of silicon atoms. That idea was to make
a perfect crystal and count the number of atoms on each face. Apparently
that's almost possible, although hard to do.

~~~
hugh4
Silicon will oxidise the moment you put it in air, were they going to keep it
in ultra-high vacuum or just deal with the oxidisation?

~~~
copsarebastards
Ultra-high vacuum is way harder to maintain than simply keeping it in
nitrogen.

~~~
hugh4
Was that the plan then? You'd need to have one hundred percent purity nitrogen
with no oxygen and no water, is it really possible to keep it at the level of
purity you need?

~~~
copsarebastards
I don't know, I was just presenting an alternative solution to the problem you
were talking about. I imagine that the sides of the encasement would have to
be pretty impermeable to keep oxygen out, but it's probably harder if the
oxygen-containing air is pushing in and nothing is pushing out.

Playing around with the chemistry more, it might be possible to use a gas that
oxidizes more readily than silicon, so that if any oxygen _does_ get in it
will be neutralized before it gets anywhere near the silicon. I'm not sure if
that's possible to do with a gas, though.

------
Aoyagi
Here I thought a kilogram was defined by water... oh well, looks like that
definition is slightly outdated.

[https://en.wikipedia.org/wiki/Litre#Rough_conversions](https://en.wikipedia.org/wiki/Litre#Rough_conversions)

~~~
fredley
The used to be defined as one kg of water, but it is not an official SI unit
(it is part of the SI accepted metric system, which does not require such
rigorous definitions).

~~~
jacquesm
> The used to be defined as one kg of water

I think you meant 'one litre'

~~~
rangibaby
One kg and one litre of water are the same thing.

~~~
kqr
Inofficially, it has been. Technically, it's not quite true. Water has a
density of 999.97 kg/m³.

~~~
seunosewa
That's unfortunate. How did it come to be that way?

~~~
lvh
Water is too hard to keep constant, and too hard to reproduce. Keeping it
exactly in 1dm3 is tricky, for example. Not for "regular" use, with a few
significant digits of course; but when you're defining fundamental physical
constants, atoms count.

------
spydum
i was hoping this was going to explain the kg differences between the original
and the copies. instead it just resolves it by changing the standard. good for
science i guess, sad for my curiosity

~~~
sanxiyn
We actually know why. Check the paper series "Stability of reference masses"
published in Metrologia. Summary: mercury contamination.

~~~
michaelhoffman
Before I was disturbed by the change in apparent mass without explanation.

Now I'm disturbed that the people working in these laboratories were exposed
to mercury themselves. Nasty stuff.

------
unwind
Duplicate, very close in time:
[https://news.ycombinator.com/item?id=10385743](https://news.ycombinator.com/item?id=10385743).

------
novaleaf
If you are interested in hearing more expert commentary, NPR Science Friday
did a piece on this in July:

[http://www.sciencefriday.com/segment/07/17/2015/redefining-t...](http://www.sciencefriday.com/segment/07/17/2015/redefining-
the-kilogram.html)

------
thieving_magpie
A planet money podcast from a few years ago on the kilogram:
[http://www.npr.org/templates/story/story.php?storyId=1120033...](http://www.npr.org/templates/story/story.php?storyId=112003322)

------
bartvbl
I wonder: the article states that the SI unit for Kg up to this point was
defined using a single object. Doesn't this definition also involve the fact
that it's placed on earth, thus requiring two objects for its definition?

~~~
tomp
Nope. Mass != weight. The weight of 1kg on Earth is about 10N (and varies by
location). The _weight_ of the same 1kg object on the moon would be much less,
but the _mass_ remains the same.

------
acqq
I didn't understand what then will be used: the Si sphere or the Watt balance?

~~~
marcosdumay
The 2011 proposal is to define the kg by the Watt balance, and use the Si
sphere to define Avogadro's number.

~~~
acqq
And what was then actually the conflict now being resolved I also couldn't
figure out from the fine article if the experiments anyway had the different
goals since 2011. Anybody knows?

~~~
marcosdumay
Not everybody agreed with the decision, because nobody knew if the experiments
would agree.

Now, both experiments have agreed to a precision high enough that current
metrology best practices will not have to change.

------
TomGullen
Lived next door to a PHD NPL physist who was working on this a few years ago.
I think they ended up handing the project over to Canada or somewhere like
that IIRC. Fascinating project and guy.

------
justhw
There's a good Radiolab episode related to this .

[http://www.radiolab.org/story/kg/](http://www.radiolab.org/story/kg/)

~~~
tantalor
That site is terribly broken until Flash is enabled. There's no obvious way to
play the episode, just a tiny "stream" button that doesn't stream. Why does it
have to use Flash anyway?

------
lifeformed
Previously, why didn't they have a reference gram instead of a kilogram? Seems
like it'd be easier to create and maintain, and transport.

~~~
Aloha
The base measure for the KG was originally the gram, wiki has a good
explanation why it was changed.

------
mtgx
Now even the U.S. can adopt it.

~~~
dtech
Unfortunately, this is not the reason the US isn't adopting SI.

~~~
k__
What is?

~~~
anonymfus
Ronald Reagan

~~~
anonymfus
That's from Wikipedia:

[https://en.wikipedia.org/wiki/Metrication_in_the_United_Stat...](https://en.wikipedia.org/wiki/Metrication_in_the_United_States#20th_century)

>In 1981, the USMB reported to Congress that it lacked the clear Congressional
mandate necessary to bring about national conversion. Because of this
ineffectiveness and an effort of the Reagan administration — particularly from
Lyn Nofziger's efforts ([http://www.washingtonpost.com/wp-
dyn/content/article/2006/03...](http://www.washingtonpost.com/wp-
dyn/content/article/2006/03/28/AR2006032802142.html) ) as a White House
advisor to the Reagan administration, to reduce federal spending — the USMB
was disbanded in the autumn of 1982.

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

>The metrification assessment board existed from 1975 to 1982, ending when
President Ronald Reagan abolished it, largely on the recommendation of Frank
Mankiewicz and Lyn Nofziger. Overall, it made little impact on implementing
the metric system in the United States.

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

>According to Mankiewicz, he prompted Lyn Nofziger's efforts to halt the 1970s
U.S. metrication effort, who convinced President Ronald Reagan to shut down
the United States Metric Board

[http://www.washingtonpost.com/wp-
dyn/content/article/2006/03...](http://www.washingtonpost.com/wp-
dyn/content/article/2006/03/28/AR2006032802142.html)

> So, during that first year of Reagan's presidency, I sent Lyn another copy
> of a column I had written a few years before, attacking and satirizing the
> attempt by some organized do-gooders to inflict the metric system on
> Americans, a view of mine Lyn had enthusiastically endorsed. So, in 1981,
> when I reminded him that a commission actually existed to further the
> adoption of the metric system and the damage we both felt this could wreak
> on our country, Lyn went to work with material provided by each of us. He
> was able, he told me, to prevail on the president to dissolve the commission
> and make sure that, at least in the Reagan presidency, there would be no
> further effort to sell metric.

>It was a signal victory, but one which we recognized would have to be shared
only between the two of us, lest public opinion once again began to head
toward metrification.

------
castratikron
Strange to see Planck's constant used that way, defining a kilogram. Planck's
constant usually only shows up when you're doing quantum mechanics and the
things you're working with are really small.

~~~
castratikron
Really? A downvote?

