
The kilo is losing weight, changing all of science - eaxitect
http://www.extremetech.com/extreme/169448-good-news-dieters-the-kilo-is-losing-weight-changing-all-of-science-but-unfortunately-we-dont-know-why
======
velodrome
The scientific community is working on the replacement:

 _Known as the Avogadro Project, the plan is to bring together enough atoms of
one substance – silicon – to make a kilo.

Attention has focused on silicon because:

\- its characteristics are very well understood

\- a single crystal of the right size can be grown

\- its atomic structure is extremely uniform

\- its widespread use in the computer industry means it can be obtained with
relative ease at high purity and resonable cost.

A spherical shape was chosen because a sphere has no edges that might get
damaged and only one dimension has to be measured in order to calculate its
volume._

\---

[http://www.csiro.au/content/ps35k](http://www.csiro.au/content/ps35k)

[http://www.youtube.com/watch?v=ZMByI4s-D-Y](http://www.youtube.com/watch?v=ZMByI4s-D-Y)

~~~
Shish2k
> A spherical shape was chosen because a sphere has no edges that might get
> damaged and only one dimension has to be measured in order to calculate its
> volume.

Surely _every_ dimension needs to be checked, to ensure that it's a perfect
sphere? (Which I assume is harder than checking a cuboid has six flat sides
joined at right angles)

~~~
ISL
If you build an apparatus for measuring a sphere's diameter, it's possible to
measure it in any orientation. If their test mass were cylindrical, they'd
need the ability to measure both the diameter and the length to the requisite
precision. I believe it is in this sense that 'dimension' was used.

------
xentronium
Kinda off topic:

How is this good news for dieters? New kilo is lighter than old one, thus
making all new measurements bigger in absolute numbers.

Besides, can't they recreate a "canonical kilo" with the required
measurements?

~~~
martin-adams
It's because dieters measure their progress in kilos lost. So if it's lighter,
they can lose more kilos for the same effort.

~~~
ewzimm
Those dieters who measure fifty millionths of a gram in weight loss have
bigger problems.

~~~
solistice
For example the incredibly devastating fluctuations due to shedding of skin
and subsequent gains due to snowflakes.

~~~
ewzimm
I refuse to go outside because I'll get fat.

------
speeder
There is another competing proposal for solution that the article does not
mention. It is define kilo as a specific number of Silicon-28 atoms, that have
a well known mass. The laboratory with that proposal made the most perfect
sphere ever, in attempt to allow measurements.

~~~
Osmium
A side-benefit of this project (The Avogadro Project) is that there are a load
of interesting things you can do with almost-pure Silicon-28, e.g. see
[http://www.nist.gov/pml/div684/enriched-silicon-
project.cfm](http://www.nist.gov/pml/div684/enriched-silicon-project.cfm) (and
more besides: there was a really interesting paper I saw recently which
irritatingly I can't find now)

~~~
philsnow
That is really cool

... is that _aluminum foil_ wrapped around a hose in one of the pictures ? I
wrapped an unshielded 12 foot vga cable in aluminum foil once and it greatly
(subjectively) improved the video quality.

------
EGreg
_" Weirdly, it’s not even known if the IPK is getting lighter, or if the
national prototypes are getting heavier — but either way, something is causing
these kilos to change weight, by around 50 micrograms every 100 years."_

Well they COULD always do that thing with the water where they heat it to 4
degrees celcius and measure its volume and weight. Then they'd know which it
is.

That said, there's a nice video on this:
[http://youtube.com/watch?v=ZMByI4s-D-Y](http://youtube.com/watch?v=ZMByI4s-D-Y)

~~~
fragsworth
> measure its volume and weight

Except they'd have to measure its weight (mass), which is in... Kg...

~~~
emhs
Well, they're using the static, known density at 4 degrees Celsius to produce
water with a known mass, and then test what the measurement of that mass comes
to. Basically they're using a known mass of water as a calibration aid. But I
don't know that this level of precision could compete with the efforts
underway.

------
Zenst
I often wonder why they don't make a set of scales with a electro magnet at
one end that attracts one end of a balance and you place the weight on the
other end and adjust the strength of the magnet until you get a balance and
from there can measure out that same weight. Now would need very well pression
made electro magnet and balance.

Though idealy the ability to measure out a fixed amout of atoms of element and
wheigh that and work out the relationship of how many atoms of element X is
needed for a kilo. Well until then it is one of the last area's of measurment
that history still firmly has its teeth into.

Was nice TV show in the UK not long ago that covered the whole area of weights
and measures from the science and history of them comming about. One of the
better science shows.

[http://www.bbc.co.uk/programmes/b02xgf5d](http://www.bbc.co.uk/programmes/b02xgf5d)

    
    
      "Deep underground in a vault beneath Paris lives the most important lump of metal in the world - Le Grand K. Created in the 19th century, it's the world's master kilogramme, the weight on which every other weight is based. But there is a problem with Le Grand K - it is losing weight. Professor Marcus du Sautoy explores the history of this strange object and the astonishing modern day race to replace it."

~~~
ISL
You've described a Watt balance and the Avogadro project.

------
rmrfrmrf
I'm not a physicist, but it's irritating to me that this article is using
weight and mass interchangeably. I doubt they'd ever allow this, but the
"simple" solution is to bring the weight aboard the ISS and capture its mass
on an inertial balance.

~~~
ISL
Nobody can build a kilogram-scale inertial balance of sufficient absolute
precision. If we knew how, we'd be doing it.

Also, the ISS is a tricky place to work for a precision measurement. It's
electrically, seismically, and gravitationally noisy (and huge gradients).
Precision gravitational measurements are generally carried out in dedicated
spacecraft with careful attention to those concerns, if they can't be done on
the ground.

~~~
rmrfrmrf
Serves me right for acting like a know-it-all. Kinda glad I did, though,
because you gave me a lot of good information. Thanks!

~~~
CamperBob2
Most people don't realize that the gravity on ISS is almost as strong as it is
on the surface of the Earth. They're continuously falling toward Earth, going
just fast enough to miss the ground.

~~~
ronaldx
Gravitational acceleration is approx 10% less? Based on the radius of the
earth and the radial distance of the ISS.

Some nice discussion:
[http://physics.stackexchange.com/questions/29929/gravity-
on-...](http://physics.stackexchange.com/questions/29929/gravity-on-the-
international-space-station)

------
muuh-gnu
Can somebody explain to me why an SI unit has the "kilo" prefix already in it,
making other SI prefixes unusable with it?

1 kJ = 1000 standard units of energy.

1 kg = 1 unit of mass.

Was this just an unfortunate historical accident? But if the too that much
care to make a unit system that makes intuitive sense, why would they let in
such an annoying exception? Why didnt they just make "gram" the standard unit
or just made up another name?

~~~
knappador
The order of magnitude chosen to calibrate the metric on doesn't change the
prefix magnitude.

1 kJ = 1000J 1 kg = 1000g

Adjusting prefixes puts the units on the order of magnitude where they are
easy to write in typical domain problems. For a heat-capacity and heat-
transfer problem, I might end up with J/kg or kJ/kg being convenient, while
for expressing the specific energy of jet fuel, I'll use MJ/kg because it's
39.

In the case of the kg, I'll bet that 1g was simply too small for the tools of
the time to make an accurate model that was reproducible to the desired
accuracy. For all I know, the weights were die-cast from the same melt, and
the effect of scratches gets reduced as the surface-area/volume ratio goes
down.

------
cantrevealname
OK, here's a wacky theory about why this is happening: The article says that
the kilogram copies are brought to Paris to be compared. Therefore the copies
are undergoing significant acceleration (e.g., transported on airplanes or
trains) while the original in Paris remains stationary.

From the stationary kilogram's point of view, all of the other kilograms had
undergone relativistic mass increases during the time of their travel. Suppose
a tiny amount of this mass increase is somehow actually retained when all the
transported kilograms are brought to the same frame of reference (i.e., when
the airplanes land in Paris).

What's a simple way to disprove this idea?

~~~
sp332
The "mass" gained from motion is really just the extra energy. When the energy
is removed from the point of view of the stationary kilogram, all the extra
"mass" is gone too.

~~~
cantrevealname
Yes I know that. That's why I said if the mass were "somehow actually
retained". I'm positing that the some of the extra energy somehow becomes
_real_ mass. Yes, this is not how special relativity works. That's why I said
"wacky theory". But is there a simple and obvious way to falsify this?

~~~
mtdewcmu
If all of the kilograms were undergoing radioactive decay, then the ones that
traveled back and forth would be younger, and thus heavier, because of the
twin paradox. You'd probably need a new undetectable form of radioactive decay
and relativistic trains to make this plausible.

------
legierski
I always thought 1 litre of water equals 1 kilogram. And as we know how long
1m is from nature, why don't they use 10cm x 10cm x 10cm of water at a
specified temperature as a prototype of kilogram?

~~~
stan_rogers
That was the original definition. Things change over the course of a couple of
hundred years or so, though, and the imprecision of that definition was
noticed early on.

~~~
ajasmin
Where does the imprecision come from exactly? Can't we derive the kilogram
from the number of hydrogen and oxygen atoms in a litre of water at an uniform
temperature?

~~~
jjoonathan
But then your definition depends on the definition of temperature
(energy/entropy) which in turn depends on the definition of mass. You might
say "make all measurements at the triple point" but then you are still stuck
with the problem of dealing with thermal fluctuations and structural variation
(liquids have uniform structure only in a weak statistical sense) in addition
to the practical problem of ensuring uniform composition at the triple point.
This idea doesn't work for benchtop measurements (How do you determine that
you have 1 mole of H2O? By measuring it's mass!) and it doesn't make sense for
high-precision measurements where you need to provide a mechanism to allow the
experimenter to make the error arbitrarily small.

One of the leading proposals for the 2014 redefinition is very similar in
spirit, though: defining Avagadro's number to be an actual number rather than
a derived quantity (effectively this uses Carbon 12 as the mass standard).

Another proposal is to define Planck's constant. I can't comment on the
measurability of this one, but it would probably make people studying atomic
physics happy because they like to use units that set most common constants to
one, and the redefinition would make more of these implicit factors of 1
defined, rather than measured quantities.

------
ronaldx
The IPK being 2nd lightest out of 12 on the given graph is not statistically
significant: even before accounting for post-hoc analysis.

The even distribution suggests (to my eye) that this is as likely random as by
some systematic effect.

~~~
blahedo
It's third lightest---there's an outlier that's way lower. But, again just
from eyeballing, it looks like there's two clusters, one that has tracked with
the IPK and one that is now heavier than the IPK. Based only on that, it
strikes me as more likely that some of the national copies have gained
mass(/weight), due to storage conditions or whatever, while others have kept
their same weight.

Of course this "doesn't matter" in that whatever the mechanism, it's still a
problem for the science!

~~~
ronaldx
> It's third lightest---there's an outlier that's way lower.

wow, good spot, thanks :)

Although it could be a problem for science, somehow it's nice that we can get
a sense of margin of error by this method.

------
KaiserPro
_90% platinum and 10% iridium for its virtual immunity to oxidization, and
because it’s extremely hard-wearing_

well thats bollocks. Platinum is hilariously soft.

There are a few issues, one is radioactivity. There are radioactive impurities
that as they decay loose weight. Second, its postulated that there is a build
up of trace amounts of mercury on the IPK due to environmental factors.

thirdly, they are not cleaned anymore. they used to be cleaned with shammy
leather.

~~~
tlb
Pure platinum is soft and malleable, but the heat-treated alloy can be as hard
as stainless steel.

[https://en.wikipedia.org/wiki/Platinum-
iridium_alloy](https://en.wikipedia.org/wiki/Platinum-iridium_alloy)

------
CoryG89
Wow, is it surprising to anyone else that a 1.54 inch tall/wide cylinder of
90% Platinum / 10% Iridium would weigh a kilo (2.2 pounds)?

~~~
smcl
Yes, you're not the only one. I've never owned or held anything platinum but I
still didn't expect such a small amount to weigh so much

------
NAFV_P
Originally the meter was defined as the distance around the Earth,
perpendicular to the equator, divided by 40 million. The French spent a while
triangulating the French countryside from North to South to calculate the
Earth's curvature. Unfortunately it was assumed that the earth was a prolate
spheroid, when actually it is oblate. This is one of the reasons it was
reformulated. The East India Company triangulated most of the length of India,
but it was tremendously expensive.
[http://en.wikipedia.org/wiki/Great_Trigonometric_Survey](http://en.wikipedia.org/wiki/Great_Trigonometric_Survey)

------
mistercow
It has always bugged me that scientists consider this obviously terrible
kludge of a standardization method acceptable.

A civilizaton looking back 20,000 years from now and translating our
scientific literature would be able to figure out what we meant by "second" by
measuring the decay of a cesium 133 atom. But a kilogram (or any unit derived
therefrom)? Sorry, the prototype is at the bottom of a crater. You can't miss
it - it's the size of a whole golf ball, after all.

~~~
ISL
Your concern is at the heart of the effort to redefine the kilo. Everyone
wants to link it to fundamental constants. The trouble is that the IPK is
still more reliable than our measurements of Planck's constant and Avogadro's
number.

The meter was an artifact until measurements of the speed of light surpassed
the prototype. The situation here is the same.

If the world ended and all of the distributed artifacts were lost, you could
use existing measurements of fundamental constants to redefine the kilo at
slightly worse precision.

------
alan
Could the fact that the other kilogram weights are being moved around maybe
affect their weight?

Like they're more exposed to chemical disturbance? Some form of tiny
relativistic effect?

------
scotchmi_st
I'm not sure I understand: surely we can just say that one kilo is a certain
number of carbon 12 atoms, and have done? Would seem sensible to me.

~~~
logicallee
OK - how many?

~~~
Leszek
Presumably 6.0221413 * 10^23 * 1000 / 12 = 5.0184511 * 10^25

------
kunai
For anyone interested in the Planck and Avogadro projects to redefine the
kilo, this video is an imperative watch:
[https://www.youtube.com/watch?v=ZMByI4s-D-Y](https://www.youtube.com/watch?v=ZMByI4s-D-Y)

------
leeoniya
fascinating article, circa 2011.

[http://www.wired.com/magazine/2011/09/ff_kilogram/all/1](http://www.wired.com/magazine/2011/09/ff_kilogram/all/1)

------
harigov
If they made 40 copies of the prototype, they know exactly how to make it -
don't they? Why can't they make one more?

~~~
PeterisP
They can make a copy by adjusting its weight against an etalon weight (cutting
to the "same size" wouldn't be nearly as accurate as the current differences)
- so they can make one more kilo-as-it-is-right-now, but they can't magically
make one more of kilo-as-it-was-in-1879 unless they get a time machine.

------
pontifier
We just need more mass surveillance on those kilograms... Kidding aside, can
we please define the kilogram already?

------
mmanfrin
Doesn't 1cc of water (at 4C) == 1 gram? Why not use 1000cc of water as the
measurement for the kilo?

~~~
heydenberk
1 cc of water is very nearly 1 gram, but not exactly. A few centuries ago,
they were definitionally the same; now they're merely very similar.

------
skj
> This is a problem, because the weight of the IPK is the kilo.

A kilogram is not a unit of weight, but a unit of mass.

------
D9u
Since gold is allegedly the most inert element, why wasn't the the prototype
made of gold?

~~~
maxerickson
Platinum is less reactive than gold.

------
Aloha
I've always wondered why the KG was not defined as a certain volume of Water
at Sea Level.

~~~
grecy
How do you define the unit used to measure the volume?

~~~
jevinskie
By using the well defined meter.

~~~
grecy
Remind me again what that's defined as?

~~~
nknighthb
Per
[http://www.bipm.org/en/CGPM/db/17/1/](http://www.bipm.org/en/CGPM/db/17/1/)

"The metre is the length of the path travelled by light in vacuum during a
time interval of 1/299 792 458 of a second."

------
omegant
Interesting, anybody knows something tath weights something similar to the
variation registered at the standar kilo? Like a virus or so.. On a side note
it must be a slow day to get this news at the middle of the front page with 3
votes in one hour..

~~~
gus_massa
The difference is 25 or 50 micrograms. It’s muuuch heavier than a virus.

The milligram weigh sets (1 milligram = 1000 microgram) are/were standard
equipment in a chemistry lab (Now, if you are lucky, you get an electronic
balance.) The smaller weights are only a small tin of 1 milligram. See the
images in the middle of this page: [http://www.ebay.com/itm/STAINLESS-STEEL-
MILLIGRAM-WEIGHT-SET...](http://www.ebay.com/itm/STAINLESS-STEEL-MILLIGRAM-
WEIGHT-SET-100-g-TO-1-mg-/330539476607)

I found a microgram weight set in Internet. (But I haven’t seen any of them in
real life)
[http://cn.mt.com/cn/zh/home/products/Laboratory_Weighing_Sol...](http://cn.mt.com/cn/zh/home/products/Laboratory_Weighing_Solutions/Weights/microgram_weights.html)
. These weights are only a small piece of thin wire, and you _must_ handle
them with a special tool because the fingers are too dirty. The smaller weight
is 50 micrograms, that is approximately the variation between the kilo
prototypes.

------
jsonmez
no matter how much we think we know about the universe, we still don't know
sh#*!

------
ye
The correct way to define a kilogram would be N number of molecules of Carbon
(pick your favorite isotope) at relative speed = 0.

~~~
berntb
Which _type_ of C molecules? I think you want to say "X mol of Y. Weight
measured in a vacuum, which necessitates an Y with very low evaporation."?

Since it isn't done like that, I assume it isn't easy to find an Y which is
both easy to measure exactly and to keep pure. (I have a smattering of organic
chemistry but no inorganic, so any more opinions from me would be without
value.)

Edit: Other people here talk about Si. We can keep that clean, at least (see
electronics).

~~~
yk
They try it, the problem is that you can compare mass better than you can
count molecules. So one of the approaches is, to manufacture a Si sphere as
perfect as possible and calculate the mass from the radius and the properties
of Si crystals. As far as I know, this had no practical results, except cool
photos of strangely rendered looking spheres.

[1]
[http://www.theepochtimes.com/n2/images/stories/large/2011/03...](http://www.theepochtimes.com/n2/images/stories/large/2011/03/27/10301_big_2.jpg)
[2]
[http://www.ptb.de/cms/en/themenrundgaenge/hueterindereinheit...](http://www.ptb.de/cms/en/themenrundgaenge/hueterindereinheiten/das-
si/avogadro.html)

------
n0mad01
maybe it's because the original kilo is a curiosity and therefore was passed
through thousands of hands (maybe someone even dropped it?) the last 100
years?

this surely can have a "losing weight" effect.

~~~
KaiserPro
As they cost billions to produce, they've held securely since they've been
made.

