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Perfect Silicon Sphere to Redefine the Kilogram
MrCreosote writes "The Age reports optical specialists at CSIRO are helping create a new standard for the kilogram, based on a precise number of atoms in a perfect sphere of silicon. This will replace the International Prototype, a lump of metal alloy in a vault in Paris."
I found some alternate theories that are also attempting to precisely measure the kilogram at everything2. They look pretty interesting, here's a small excerpt:
Hey wait, TFA skims over what they're going to do with the Silicon ball once its made. Again, from everything1:
I can explain it for you, but I can't understand it for you.
Second, if that rusty lump in Paris defines what a kilogram is, in no way is this sphere gonna change that.
That's wrong. The lump is not rusty, because the lump is platinum-iridium which is quite unreactive so that corrosion ("rust") won't affect the material. Corrosion alters the weight, you know.
Second, it can change the definition. The metre used to be a platinum rod in Paris, now it is defined in how much distance light does in a certain (very short) time. Here it will be that the kilogram will be defined as N silicium atoms. (Where N is a very large number) Scientists do not like definitions based on objects, they prefer definitions based on universal constants. All this could of course be read in the article....
It's important enough for laboratories in Germany, Italy, Belgium, Japan, Australia and USA to invest a great deal of time and effort.
The spheres are being made by CSIRO's Centre for Precision Optics. They've been making precision spheres for research since the late '80s, and have all the recognition they need from anyone who has a clue.
Have a look here; http://www.tip.csiro.au/IMP/Optical/spheres.htm. It might help you understand the project better.
"I've got more toys than Teruhisa Kitahara."
Mass is constant (assuming it is at rest), and has nothing to do with the force of gravity on an object. The mass times gravitational acceleration is the weight, often reported in lbs or Newtons. In other words, a kilogram here is a kilogram everywhere.
BTW, in the English measurement system, mass is measured in stones.
A lot of units can be defined using physical properties : a second is 9,192,631,770 periods of a precise physical reaction (transition between the two hyperfine levels of the ground state of the caesium-133 atom according to the wikipedia), a meter is the distance travelled by light in a 1/299,792,458 of a second and so one, Volts, Joules, etc... are defined this way. Mass, however, was not yet related to physics constants. So there is a "yardstick" for kilograms. A platinium cylinder was made a century ago, the closest we could get to what was considered a kilogram at this time and it was proclaimed "the exact measurement of a kilogram is the mass of this particular object". It is stored somewhere in Paris. I am sure that modern scientists will manage to conceive an experiment with a great precision to transform the kilogram unit into the abstraction it is supposed to be.
The Wise adapts himself to the world. The Fool adapts the world to himself. Therefore, all progress depends on the Fool.
The CSIRO project is about determining how many silicon atoms are equivalent in mass to the current standard kilogram. Once that number is established the actual kilogram in Paris is redundant. If it gets lost or destroyed we can reconstruct the kilogram by counting out 'n' silicon atoms. It also means anyone can construct their own kilogram by counting out 'n' silicon atoms, without having to go to Paris to do a comparison.
It is a separate (but related) project to figure out the second part of the project: how to easily count out 'n' silicon atoms, so creating a universally available standard. One way might be to make a silicon sphere, like the CSIRO, but most people don't have the ability to do that.
Non-Linux Penguins ?
The main problem with this as a definition is that water expands and contracts with temperature. So, if you wanted to define the kilogram in terms of a volume of water, you need to specify the temperature at which you are making the measurement. Temperature isn't something you can measure with very high precision (parts per million or parts per billion), so you end up with unavoidably large errors. As a result this is useless as a basic standard, the essence of which is that you should be able to repeat the standard measurement and get the same answer to N decimal places.
OK, someone's going to have to explain this for me. Why do we have to have an actual object to define a weight?
:) It's also mentioned that a similar approach was taken to defining the meter, based on an absolute definition of the speed of light.
You don't. That's just the way we've done it in the past. I read a really interesting article a couple months ago in American Scientist magazine called An Exact Value for Avogadro's Number that addresses exactly this question. In the past, Avogadro's Number (6.02andchange x 10^23) was defined experimentally, based on the reference kilogram. These scientists propose reversing that -- defining the number absolutely, based on the number of atoms of a particular element that fit within a sphere of a certain size. It's sort of similar to what they're doing with the silicon sphere, but it's all done on paper, rather than by actually manufacturing an artifact.
The advantage of this, they say, is that the number will remain constant and not be affected over time as refinements in building and measuring such "reference kilograms" change the accepted mass of a kilogram. They make several other arguments, as well, but it's much better if you just read the article.
They don't. The idea isn't to make the new sphere weigh a kilogram. The idea is to redefine the kilogram in terms of the weight of an atom of silicon (i.e. 602383623523895723945743 atoms of Si-14 weigh exactly 14 grams). The idea of the ultrapure and ultraround Si sphere is that (a) you can measure the lattice spacing of the Si atoms in it using x-ray crystallography, so you know how far apart the Si atoms are, and (b) you can measure the diameter of your ultraround sphere very accurately, so you can calculate its volume very accurately. Given these two, you can calculate with very small error bars how many atoms of Si there are in the sphere, and given the definition of the kg in terms of how many atoms of Si make up a kg you can calculate exactly how much the sphere weighs.
You can then stick it on your balance that needs calibrating, and twiddle the dials until the balance thinks that the sphere weighs the same as the calculated weight.
Yes, there are standard Imperial weights and measures. I seem to recall that one set are on display at the Tower of London. However, the pound now seems to be legally defined as 0.453 592 37 kilogram.
Standards for weights, mass, distance or any other measure, are critical in the calibration of instruments. This calibration provide the means that to compare product specifications and research results.
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This particular effort is a very interesting set of challenges. It requires the use of single isotope of silicon; calibrations for distance and roundness, and a sophisticated means to to count the atoms. This last step requires the silicon to be perfectly crystalline.
Measurement is itself a very interesting study bordering on metaphysics and philosophy. The desire to measure things has been at the heart of a lot of scientific investigation, economics and other areas of study. Ref "Abstract Measurement Theory" by Louis Narens https://mitpress.mit.edu/catalog/item/default.asp
"If all the American people want is security, let them live in prisons." Eisenhower
The kg is a unit of mass, not weight.
I am not a crackpot.
You are being very dense here.
TODAY: 1kg = the mass of the "rusty lump" in Paris. We don't know how many atoms of Ir and Pt the "rusty lump" have. So, if the "rusty lump" changes mass (and it changes with time because of being rustier all the time) AND because the "rusty lump" is used to calibrate scales all over the world, the kilogram is effectively changing with time. This is BAD.
WHAT THE GUYS ARE DOING: they are trying to make the most perfect silicon sphere possible that weights the same as the "rusty lump". Once they get to do that, they will count the atoms of silicon on the sphere, using interferometry. Suppose the # of atoms of the shpere is M.
WHAT WILL WE GOT THEN: 1kg = M atoms of silicon. This definition will never change, and if the silicon spheres rust or break or change weight by any circunstances, we make new ones with M atoms and we have a forever-constant definition of a kilogram. This is GOOD.
Got it? They did the a similar thing with the meter -- the original was a rod roughly 1m in size, then they did some measurements and said (*) "oh, one meter is the length that the light takes 1/299,792,458 of a second to go through in vacuum." and now they can do as many calibrating rods as needed, provided they make them the length that the light takes 1/299,792,458 of a second to go thru.
(*) actually the meter had an intermediate definition of "1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum", but the new definition has the advantage of setting the light speed at exactly 299,792,458 m/s.
It's better to be the foot on the boot than the face on the pavement. ~~ tkx Kadin2048
I have moved from metric countries to the US, and have actually researched that issue. Turns out, that the US is implicitly metric!!! The "Customary US Units of Measurement" are defined by relating them to metric base units, e.g. 1 yd = 3 ft = 36 in = 36 in * 2.54 cm/in = 91.44 cm. Weights are similarly defined. The "implicit conversion" is based on bills passed in the early 70's
-- Cave quid dicis, quando, et cui
Actually, a pound is sometimes both a mass and a weight. It's one of the stupider quirks of the customary system. When the distinction is important and not obvious from the context, pounds mass (lbm) or pounds force (lbf) is specified. The "conversion" between the two involves a constant, usually written as g-sub-c.
lbf = lbm * (accel due to gravity) / g-sub-c
constant = 32.174 lbm*ft*s^-2*lbf^-1
That way, one lbm weighs very close to 1 lbf.
http://en.wikipedia.org/wiki/Pound-force
No, it's widely accepted as a necessary step towards being able to define the unit of mass in terms of a specific number of carbon 12 atoms. Look, it would be a lot better for this discussion if you made the effort to learn what the project was for.
Just because you personally don't understand it doesn't make it "media spin" or otherwise redundant. There's more information here http://www.npl.co.uk/mass/avogadro.html, including an FAQ which might clear up some of your misconceptions.
"I've got more toys than Teruhisa Kitahara."
Wrong what we are trying to do is count the number of Si atoms in a sphere so we can say sphere X has a mass of Y, and then use the sphere to calibrate other masses. We already have a number for how many silicon atoms in a kilogram, and once we can count atoms precisely enough that preexisting number will be fixed.
A new standard is needed because the "standard kilogram" held in France has been slowly losing mass, about 50 micrograms in the last 100 years, compared to other reference masses. It's not known how this has occurred.
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Wikipedia - Kilogram
http://en.wikipedia.org/wiki/Kilogram
Slashdot: The Changing Definition of 'Kilogram'
http://science.slashdot.org/article.pl?sid=03/05/
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The sphere itself, no; you would use it in some experiments (carefully, to reduce error) for whatever purpose, and then you would put it back in the vault. However, once this Si sphere is complete they will know to some level of accuracy X how many Si atoms make up the sphere. That way this company can just create more spheres, count the number of Si atoms in it and as long as the interferometry matches the number of Si atoms they set as being 1kg to within their limits of measurement, then these other Si spheres can also be used to do the same experiments with the same level of accuracy and precision (two different things).
The key difference between these Si spheres and the hung of metal alloy in paris is that these spheres are reproduceable. You would have an incredibly hard time creating a hunk of metal alloy the exact same mass as that in Paris; furthermore, I presume these spheres are less susceptible to degradation due to environmental effects.
In essence, we're re-defining the kilogram so that instead of the formula 1kg=1(Hunk of Metal) we have 1kg=X(Si atoms), where X (my understanding, anyway) is what they are trying to determine as closely as possible. Once this standard is actually set, they will select an exact number of Si atoms in the middle of their error distribution and say "this is the definition for 1kg!", even if that means there's a tiny change in what 1kg really is, since the difference is so small that we wouldn't be able to tell there was one until years down the road when we have more accurate sensors to measure both mass and number of atoms. But even then, the number they chose wouldn't change.
Aikon-
That's wrong, actually. I mean, practicality doesn't really figure into defining the other units, how practical is it to define a meter as the length light travels in 1/299,792,458ths of a second? The point is that it has to be absolute. 1 dm^3 of water at 4 degrees Celsius (whatever that is in Kelvin) is exact and absolute. Plus, we already have a similar definition, the Kelvin is defined as exactly 1/273.16 of the temperature of the triple point of water. If the hokey-ness of measuring temperature figures into the kg, it should figure into the definition of the Kelvin.
No, the REAL reason this is an impossible way to define mass is pressure. Water density depends ever so slightly on the pressure it is in. Pressure is measured in Pascals, and as we all know that 1 Pa = 1 N / 1 m^2 is the definition of the Pascal, and that Newton's second law is 1 N = 1 kg * 1 m/s^2. Which means that if you used this method, you would use the kilogram to define the kilogram. Not dice soldier. That's why we have a rusty lump.