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.
> 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)
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.
The Avogadro project isn't the only horse in this race. The electromechanical approach, known as a Watt balance, is underway in several national standards labs around the world.
For dieters, you're entirely correct. A lighter kilo makes the dieter have a greater mass when measured in kilograms.
> Besides, can't they recreate a "canonical kilo" with the required measurements?
This is the main point of the drive to redefine the kilogram. Right now, the IPK (the French kilogram) is the SI standard for the kilogram. Groups are working hard to redefine the kilo in terms of either Planck's constant or the Avogadro number. In order to do so, the experiments that measure one of those fundamental constants must be more reliable than the IPK.
They're very close, but absolute mass measurements of the required precision at the kilogram scale have never been done before.
Because the SI specifies the kilogram as the base unit of mass.
If you want to generate a kilogram to a certain precision (say, a part in 10^8) as a sum of a thousand well-measured grams, the grams need to be measured sqrt(1000) better (about a factor of 30). If you could multiply, you're fine, but since you're required to make (or measure) an actual kilo, you have to sum.
If we were to accept the multiplication method, it would be equivalent to restandardizing the SI upon the gram. That would be an okay thing to do, but to maintain continuity with past measurements (referenced to the IPK), we'd still have to make or measure an actual kilo, requiring precision sufficient to do the sum.
I don't think the problem is that we can't get anything that big to be precise, but rather that we just don't have any mass experiments that are precise enough period. If you look at the definition of a second, you can see that scientists aren't squeamish at all about multiplying things.
Valid point, but your starting mass also increases.
Then there's also that if the measurement changes, the results can become pretty bizarre. Say, you are on a diet for one month. At the start of the month, the kilo prototype weights 1 skg and at the end 1 ekg = 0.9 skg. You measure yourself at the start of the diet, and get that you weight 100 skg. You use some very precise diet-fu to draft a program that should see you losing 5 skg during the month. And lo and behold, you manage to stick to the program and loose exactly 5 skg.
But at the end of the month, when you measure yourself against the now-lighter prototype, you get that you now weight 106 ekg. So, you actually achieved your target and lost 5 kilos, but meanwhile ended up with 6 kilos more body mass than before the diet!
(Disclaimer: real changes in the prototype mass are of course way, way smaller.)
Ha, love it. I wonder if that's like receiving dividends on shares in a foreign currency. Your share value (weight in kg) changes over time due to currency fluctuations (lowering value of 1kg), but the dividends paid out (variable weight loss for a given time period) are also affected by the same currency fluctuations (lowering value of 1kg).
If the kilo is getting lighter, it means that your mass in kilograms is going up! Better hit that treadmill before the meter starts shrinking and you need to run farther.
You're some random anonymous guy on the internet asking a public and very personal question about somebody's mental health with no obvious context, no sense that some people might consider that a sensitive topic, and apparently no awareness that doing so looks like you're calling them clueless. You do all this while claiming to be asking a serious question. (Rather than, say, emailing me.) And you're wondering if I have have problems functioning in society?
The headline writer for the original article put in a joke. Xentronium, correctly, pointed out he appeared to get the joke backwards. Somebody else said, "it's just a joke", which is a thoughtless thing to say when you're trying to analyze why a joke does or doesn't work. If there's anywhere it's reasonable to nerd out on a nerdy joke, it's here.
So I pointed out a great source, written by experts in funny, explaining why getting to truth really matters in comedy. I am unable to fathom why you think that's a sign of a social disorder, let alone sufficient license to act like a dick.
Have you? Semi-serious question, genuinely not intended to offend.
It's perfectly reasonable to have the opinion that wpietri advanced, and it's perfectly reasonable to raise it in this context. One could believe that for aesthetic reasons, or for political reasons, or other reasons, or a combination thereof. And I suggest that it's a higher caliber of comment than the mean HN comment, given that it suggests a source for deeper study of the commenter's opinion.
On the other hand, attempting to (tentatively) diagnose psychiatric disorders based on a single sentence on HN is completely implausible, totally insensitive (to actually ASD sufferers, I mean.. I'm sure wpietri can take it), totally oblivious to social context, and suggests that you might have a "stereotyped pattern of behaviour or interest".
Unless, of course, you have professional qualifications for such a (tentative) diagnosis, in which case I take it back.
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.
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 (and more besides: there was a really interesting paper I saw recently which irritatingly I can't find now)
... 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.
I completely agree, It's time to start counting atoms/molecules of something (being careful with isotopes).
Integer numbers are very good for "perfect" (well, very good) abstract definitions.
And it would also really prolong the French Revolution spirit in the sense that this unit would become so universal that you don't need a physical reference anymore, like what we could achieve with the other base units (like second then meter).
"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.
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.
Exactly. They'd COMPARE it to the existing physical object used as the reference for the kg. Assuming water allover the world hasnt changed its properties over the last few centuries they'd know whether it became heavier or lighter.
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.
"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."
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.
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.
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.
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?
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.
Check out this veritasium video https://www.youtube.com/watch?v=ZMByI4s-D-Y - It goes into a little detail on how the kilogram got it's name, it is also very interesting in it's own right.
Other SI units can and obviously are used with it. Micrograms, milligrams, and grams are really commonly used. I'm not sure why they chose to do it this way, but it definitely doesn't really have any bearing on how it's used.
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).
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.
And, specifically, you only have "extra" mass from the viewpoint of someone traveling at a different speed or direction. From your own frame of reference, your mass never changes.
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?
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.
Sure, you will only need two approximately equal masses, a high precision scale and a round-trip airplane ticket. Seems easy enough for me. Strictly speaking this seems like the most obvious way to test your theory. Of course it would be mighty odd if you would find any effect, but who knows?
I'm not sure you're right, but I totally agree with the idea behind your idea, which seems to be that it's better to explain the difference rather than work around it.
From a coder's perspective, I'm looking at this like you can either identify the root cause and fix the system itself, or you can start putting in spaghetti code to fix it.
Now, if the system itself isn't behaving like you expected, it makes sense to me that there's something more fundamental going on that needs to be looked into. Why work around the problem when the problem itself could be telling you something very interesting and useful?
Since you use equations of relativity to calculate the mass increase, you could use the same equations to calculate the mass at rest and show that it doesn't increase.
well, you seems to be on to something. These other kilos are kept at different places that have different gravitation (no mentioning that they control for that) - thus different time. So 1 year for IPK in Paris isn't a precise 1 year for its copy in another place. So whatever changes happen to the kilos they happen a bit quicker for some and a bit slower for the others.
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?
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.
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?
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.
Perhaps. But at least one complexity there is that there are a couple of isotopes (of different mass) for both oxygen and hydrogen.
There's the Vienna Standard Mean Ocean Water[1][2] which accounts for differing isotopic compositions in different parts of the world.
There's been some competing attempts to the Standard Kilogram, including a pure silicon sphere of extremely precise physical dimensions (which we're good at), and using the lattice spacing to determine the number of atoms.
I used to live next door to one of the guys working on the KG. Very smart guy! Basically from what I gather, weight is the last unstandardized unit of measurement at the moment and the problem derives from finding something that has constant mass. Apparently it's pretty near to being solved though, I believe their project was sold to Canada last year though.
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!
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.
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
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.
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.
In order to make practical use of that (i.e., calibrate a weight-measurement-tool) you need to be able to somehow make an artifact with that certain number of carbon 12 atoms, or measure the count of atoms with a sufficient accuracy.
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.
grecy, the meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
A 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.
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..
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...
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... . 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.
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).
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.
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?
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.youtube.com/watch?v=ZMByI4s-D-Y