Editorialised title - actual title is "The Search for a More Perfect Kilogram". Wired should use your title though ("Everything is Getting Heavier") - it's a catchy title for an interesting article. Also the picture half-way down is totally a TARDIS.
The article did state that the 'secondary' versions of the kilogram were getting lighter with respect to the Grand K and it was up to 500 mG which is quite significant.
It would be interesting to know what is the source of that error (I didn't find the speculation of out gassing to be particularly compelling, but I didn't recall them mentioning how the companion standards were built).
Can somebody (preferrably, a physicist or a chemist) explain to me two things.
1. Why do we need both kilogram and mole as fundamental units when they measure essentially the same thing (at least in case when the thing being measured is pure)?
2. Why not reverse the definition: postulate that 1 mole is Avogadro's number of elementary particles; 1 mole of C-12 has 12 grams and derive (kilo)gram from that.
1. They are not both fundamental, a mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 0.012kg of the isotope carbon-12. So the mole is defined in terms of the kg.
2. This is essentially Arnold Nicolaus' project (described in the article). He wants to define the kg as the mass of a certain number of atoms. But we don't yet have an accurate way of counting atoms, so if we pick a number (i.e. define Avogadro's number by fiat) today, then later when we count more carefully the mass of everything (denoted in kg) will fluctuate.
1) The two values don't measure exactly the same thing. The mole is a unit for the amount of of material, while the kilogram is the unit for mass. From a chemistry perspective, the difference probably doesn't matter. If you have 1 mol of carbon 12, you know that it's exactly 12g. On the other hand, what if you had 1 mol of electrons? The conversion between the two quantities only works because we can use the isotope number as a conversion factor.
2) We don't exactly know Avogadro's constant. We've measured it fairly accurately, but, for your proposal to work, we'd be forced to define all 23 digits. We can then declare that 12g of carbon 12 is exactly 12g, but how do you know that you have exactly 12g of carbon twelve? Just from basic counting statistics, you'll probably be off by a few hundred billion atoms, and that's assuming that you have someone is and count all 6.02 * 10^23 atoms individually. The result is that the mass of the kilogram is now a derived and empirical value, adding uncertainty to every measurement of mass in every experiment. It's far preferable to leave the kilogram as an exactly defined unit and put the uncertainty into Avogadro's number, where it will only crop up when someone needs to make the conversion.
Interesting article. I was under the impression that the kilogram was defined as the mass of one liter of water, but that appears to just be an approximation now.
"In fact, of the seven fundamental metric units — the kilogram, meter, second, ampere, kelvin, mole, and candela — only the kilogram is still dependent on a physical artifact."
Followed by:
"And the luminosity of light, or candela, is measured in terms of power, designated in watts, or joules per second."
I'm not an SI expert, but Wikipedia suggests the candela is in fact an SI base unit with a definition based on physical phenomenon of the universe, so the second sentence seems erroneous.
The candela is based on Watts, but weighted to adjust for human perception. While the "standard human eye" could be considered a physical artifact, we use experimentally determined photopic (in bright light) and scotopic (in low light) response curves, which are supposedly fixed.
It isn't quite as fixed as some measurements since the average human's vision could continue to evolve and change the curve, or because we could find out that we measured it wrong, but it's not dependent on a single physical sample like the kilogram.
What I don't get is why ampere is a fundamental unit while coulomb isn't. I mean, it seems more natural to think of amperes and coulombs per second rather than to define coulombs as the amount of charged carried by a 1 amp current in 1 second.
Now, in the abstract, yours is the more natural definition. But experimentally, it turns out to not give the best accuracy.
I believe one starts with (1) the meter (which is based on a second defined using a atomic transition, plus light) and (2) the Newton (which is the force required to accelerate the all-important kilogram at 1 meter per second squared). Then an Ampere is the amount of current flowing through two parallel wires required to produce a force of one Newton per meter between the wires.
Would it be possible to define the KG as the amount of copper wire, when arranged in a configuration (a coil?), that causes a magnetic field of a certain size when a specified current is applied?
The problem lies in how will you define the other parameters over here.
First up is the question of defining that 1 KG itself and the configuration; how will we define the parameters of the copper wire? (edit: to clarify after reemrevnivek's excellent comment the configuration of the solenoid is important because the strength of the magnetic field generated is directly related to the configuration of the coil) We can't use the radius, length of the copper wire accurately because as it gets heated up it will expand not only longitudinally but radially as well.
Second, how will the magnetic field be measured to the precision needed? If you use induction of current in another coil then according to lenz law that will create a downward force on our coil leading to inaccurate measurements (I'm assuming that it will be constantly measured) , this means that the coil needs to be strictly constrained, by how do you constrain it without adding/rubbing material on to the surface of the copper wire?
The magnetic field in a coil is dependent on the number of turns and current in a coil (as well as, to a lesser degree) the position of the wires in the coil. It is almost completely unrelated to the number of copper atoms in the coil.
Read up on the Biot-Savart law if you want to know more about calculating magnetic fields.
The article makes it quite clear why it isn't possible to relate weight to fundamental physical constants yet: the constants that can be used for this haven't been measured precisely enough yet. Until they are, using a block of metal in a vault that has lost about half a speck of dust worth of matter in two hundred years is more exact.
"Aside from a yearly ceremonial peek inside its vault, which can be unlocked only with three keys held by three different officials, the prototype goes unmolested for decades. Yet every 40 years or so, protocol requires that it be washed with alcohol, dried with a chamois cloth, given a steam bath, allowed to air dry, and then weighed against the freshly scrubbed national standards, all transported to France."
Sounds sensible and reliable. No issue with corrosion, infiltration of gasses or lint I guess. And what about the other standards? Who knows which is changing?
"Steiner argues, the watt balance, with its Planck constant, is “a better realization,” because his system is self-contained and replicable, whereas the Avogadro project spans several continents and relies on a single artifact. "
A non-reproducible result is not science. It doesn't matter how many zeroes are involved; its not actually a verifiable fact if there's only one experiment, right?
