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Big Test Coming Up For Kilogram Redefinition (ieee.org)
szotz writes: Electromechanical balances have got to be better than an aged lump of platinum and iridium right? Teams are working to get kilograms measured and shipped to Paris in time for a test to see whether the technology (along with another that uses ultrapure silicon spheres) is now ready to redefine the kilogram. Why is this redefinition interesting? Because it's about using physics to overcome one problem with weight standards based on tightly held exemplars in standards bodies' inner sanctums: the mass of those exemplars can change, however subtly, introducing uncertainty and confusion. From the article: The world's metrologists aim to change this state of affairs in 2018 by fixing the kilogram to the Planck constant, a fundamental physical constant. That shift would, at least in principle, allow any laboratory to "realize" the kilogram from scratch with a series of experiments and specialized equipment. But for that scheme to work, the kilogram derived by one laboratory must be the same as those derived by others.
To bolster the argument, look at the fine-structure constant. When Arnold Sommerfeld introduced the constant in 1916, Arthur Eddington argued that you could get to it by pure math and found that for completely logical reasons, the constant should be exactly 1/136. When later measurements put the value closer to 1/137, he discovered an error in his deduction and published a new paper that the constant should be for even more logical reasons exactly 1/137. Currently measurements put the value of the fine-structure constant at about 1/137.036, and no numerological explanation so far has been accepted.
Well, this certainly qualifies as news for nerds! It's news, technical and amazingly esoteric.
Why the planck constant then? Why not e, or (pi), or any other constant, for that matter?
Neither e nor pi are physical constants. They are unitless mathematical constants, so you'd have to specify e or pi *somethings*. It's the somethings that are important at which point neither e nor pi would come into it all that much.
By way of example:
The second is defined in terms of a physical constant: "the duration of 9192631770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." The important thing about this is that this was not the original definition of a second. The second was a bit more vague, so at some point a bunch of metrologists got together and declared that from now on this SHALL be the definition of a second and shall supercede all previous definitions.
The kit to measure a second is withing reach of well funded science labs and can be reproduced independently. You need the high frequency counter (capable of 10GHz operation), some pure caesium, and an assload of expensive support equipment and liquid helium and you can measure a second.
Once you have the second, you can move on. The meter is defined in terms of the second and the speed of light: as the distance travelled by light in a specific fraction (1/299 792 458) of a second. Much like before, this is a declaration by fiat, and is very very close to and supersedes the old platinum iriduim rod in Paris.
Now, here's where it gets interesting!
First, an aside:
The reason for Planck's constant comes down to what is colloquially known as E=mc^2, or more generally E= h v where v is momentum and h is plank's constant. In other words, Planck's constant connects energy, mass and time.
Here's a nice link:
www.bipm.org/utils/common/pdf/RoySoc/Michael_Stock.pdf
It's a bit more detailed, but essentially it relates the Kg, Planck's constant and a few others which are known. So, if you know what the Kg is exactly then you can measure Planck's constant with a Watt balance very accurately.
So what you do is calibrate the Watt balance with the prototype Kg, and measure Planck's constant. You then declare (by fiat) that Planck's constant is EXACTLY what you've written down and so now the Kg is defined in terms of that number, not the other way around.
In principal, now someone can build their own Watt balance, plug in the numbers which are now just numbers and measure their own chunk of metal to find out how much it weighs in Kg.
So that is a nutshell is why h is appropriate and pi and e aren't.
The other option is to build a very very pure, very very precise silicon sphere, in which case the Kg will be essentially determined by a single number which is the number of silicon atoms in a Kg. That will be measures in terms of the meter (for both the bond spacing of silicon and the radius of the sphere). In that case, Planck's constant will still be defined in terms of the Kg, not the reverse. In this case, pi would make it into the definition, via the volume of a sphere, of course, but in a somewhat peripheral role.
The question is whether we (collectively) can make silicon spheres more accurately than we can make Watt balances, or the reverse, right now.
SJW n. One who posts facts.
The US approach is silly. The apparatus must be isolated from the environment to such a degree that it is impractical. Oner must monitor and dissuade wildlife a quarter mile away from the apparatus to get useful measurements. In essence the US approach is not to make a standard but a very impractical scale. The German approach is not so touchy. There is nonsense about only one Australian guy being able to form the spherical reference but that is ridiculous cult of personality. The German approach is both defined by a physical description and produces an actual physical kilogram reference. The US approach has the wow factor of a physical constant used to define the kilo but who cares? The US approach would result in inconsistent kilos if adopted. Anything that touchy is not suitable as a definitive reference.