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pnictogen-horses
@pnictogen-horses

I may be the Pnictogen Wing's "science officer" so to speak but I am really just an amateur, and an experimentalist at heart. "Science" to me is hands-on laboratory or field work, and I am somewhat weak on theory. I have had some graduate-level coursework in such subjects as quantum chemistry and fluid dynamics and other heavy-duty topics, but I am not great with that stuff. I am badly in need of refresher courses.

Hence I feel like I am on uncertain ground here when I say: doesn't the 2018 redefinition of the kilogram by the General Conference on Weights and Measures... smell funny? I don't like what they did, and I can't say exactly why. I am not @qualia or anyone else good with physical fundamentals. My objection is chiefly on experimental grounds: faced with an inexplicable physical phenomenon, the international boffins of scientific definition simply gave up and redefined the kilogram in terms of a quantity which is simply asserted by fiat—the value of Planck's constant h, which is now merely defined by international law. It's not an experimentally determined value any more. h is fixed by arbitrary human decision.

Doesn't that seem strange to anyone else? The second, the fundamental SI unit of time, is now, so far as I know, the only SI unit that's defined in terms of experimental observation, in terms of the frequency of photons emitted by the "hyperfine" transition between the two ground states of the valence electron of cesium-133, whose energy level is split by the nuclear spin of the 133Cs atom. Everything else is dependent on some constant that's been fixed by an arbitrary decision of an international standards body. h (Planck's constant), c (the speed of light), e (the elementary charge), k (Boltzmann's constant), and A (Avogadro's number) are all simply...defined. They're not based in experiment; they're defined solely by human authority.

I don't like this. I don't like this at all, especially because of why this happened, specifically to the kilogram.

The kilo used to be defined in terms of a physical object, a standard kilogram fashioned out of iridium-platinum alloy in 1889. The "International Prototype of the Kilogram was (still is, I guess) kept in Paris, and a number of standard duplicates made and kept in different places—you can see the whole list in that Wikipedia article. Elaborate cleaning and weighing procedures were defined when comparing these standard kilograms against each other and for standardizing other masses. Iridium-platinum alloy is one of the most chemically resistant metals known, scarcely subject to surface oxidation, and therefore it was thought these Ir-Pt cylinders would make excellent mass standards.

Until...they didn't. A very strange thing happened, which you can see in the chart I've excerpted from the Wikipedia article: the masses of the standard replicas began diverging from each other by many micrograms over time. And nobody could explain why.

If you search around you'll find papers about this matter, conjecturing all sorts of things. Was the Ir-Pt alloy actually subject to more oxidation than once thought? Were they acquiring surface contaminants? There were many hypotheses and tests, all of which came to nothing. The international standards boffins simply gave up, blamed the standard itself, and then fudged up their Planck's-constant definition to replace it, which was basically equivalent to saying: OK we'll just define the kilogram in terms of an arbitrary number. Yes, there's an extremely elaborate device called a "watt balance" or Kibble balance that produces a force proportional to the product of a known voltage and current (hence "watt balance"), and this is now used to standardize masses. But because all the quantities involved in deriving mass from this information are arbitrarily defined anyway (except for the second) I'm not sure how this is an adequate replacement for the standard kilogram. It seems, to my naïve horse sense, like a cheat.

There was a competing project to replace the Ir-Pt standard kilogram with another physical object, a perfect sphere of single-crystal silicon. One could then calculate exactly how many atoms of silicon ought to be present in a sphere of a given size, correct for the superficial oxidation of the silicon, and obtain a mass standard that way. I liked this idea a lot; it was in keeping with the nature of the original standard, and suited my temperament as an experimental scientist. But the international boffins didn't like it, I guess. In my opinion they were wowed by the idea of having everything nice and "theoretical" (i.e. defined solely in terms of arbitrary constants, and cut loose from experimental verification.)

The experimentalist in me is hollering in protest at this whole business. An experimentally verified phenomenon was taking place—the divergence of the standard kilograms from various places—and nobody could explain it, so what did the scientific authorities do? NOTHING! They shrugged it off, as if to say, "lol I guess that's what happens when you rely on physical objects, you know how wacky and unpredictable they are!" I tell you, it's bullshit. Something really stinks about this whole business. It's like they wanted to sweep an awkward physical finding under the rug because it got in the way of an image they wanted to project—an illusion, not a reality, that the international scientific authorities had everything sewn up and neatly defined and fixed.

Yeah, it's fixed all right.

~Alyx Woodward


pnictogen-wing
@pnictogen-wing

I'd really appreciate it, @qualia, if you could look over what Alyx has just written here, and maybe correct her misconceptions (if any). ~Chara


qualia
@qualia
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FoxBall
@FoxBall

im no scientist, coming from more of the engineering side of things, but isnt every unit arbitrary? on the engineering side, measuring things, or more fancily, metrology, is very important to get right.

the kg was a weight, that a committee decided 'hey, this is equal to 1 now', and all things were measured in comparison to it. but as the graph you posted shows, there is inherent problems with this, as since nearly all the top quality derived weights gained mass over time, who's to say it wasnt actually the IPK losing some mass? but you cant say that because it is defined to be 1. and therein lies the problem. by messing with the IPK, even slightly, you throw off the calibration of every single scale in the world.

the metre had the same problem. it too was a physical bar of metal, and suffered from differences in length over the time it was used. think of the kinds of chaos that could cause in the machine shops responsible for making the measuring equipment, and then propagating out to the ones that have to use these new, oh so slightly different tools that never quite agree with their old ones.

while its extremely cool to have something so precise and so important, the problems behind that are exactly what push us to not do that wherever possible.

instead we look for physical constants or reliable natural phenomena (atomic clocks, etc). in the case of previous measurements like the metre, we can turn them on their head and use another base unit that does have a basis in constants or reliable natural phenomena to derive it. so long as we know what a measurement is supposed to be, we can calibrate our measuring device using that known quantity.

there is a sort of beauty to be had here. without a basis on physical things: if say, the building containing the IPK burns down or something, you can find out how big the si units should be, just by doing some experiments to calibrate your instruments.

call it a math cheat all you want, but defining stuff this way prevents a lot of drift induced headache, and allows a measurement recorded today, to be the same length as an equal measurement recorded 200 years from now. that was not something that was possible previously.


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in reply to @pnictogen-wing's post:

I think everyone comes to it with a different perspective, but while I'm a little sad that there aren't standard reference objects anymore I think defining by fiat more closely resembles what we're actually trying to do. The kilogram isn't a real thing that we're trying to divine in greater and greater detail, it's just an arbitrary quantity that it is useful for us to all* agree on. That's not very romantic but it is functional.

*Obvious exceptions apply; it may help that in my personal context metric have always been the default units and I only remember alternatives enough to communicate with elderly people and americans

(boy I pissed off some people with this one, I'm a bit surprised) yeah I admit that ultimately, this fades off into philosophical problems I'm ill-equipped to think about. maybe I'm just biased here, as an experimentalist at heart, but—science is supposed to be an empirical thing, so anything that pushes it a bit further away from the need for experimental verification seems...odd, to me. and there's still the physical phenomenon left unexplained. people are saying weird things like "physical objects change" and it's like...did they miss the bit where people tried to find out WHY but couldn't? doesn't that bug anyone but me? ~Alyx

oh for sure. and maybe that's all that matters: you've got your new machine for generating a standard force, you compare masses to it, and that presumably cascades all the way down through a million corporate interconnections to, like, postal scales and stuff. I still feel irritated by a mystery phenomenon that got ignored, though. it's a significant drift in something that ought to have been constant, or at least should have changed in roughly the same way everywhere. but even the greatest change over a century wasn't enough to, like, affect how they measure loads of coal or whatever. ~Alyx

in reply to @qualia's post:

not sure if my last comment posted (cohost hiccuped on me?) so apologies if this is a double notification but

if you're still looking for an answer, I studied physics and metrology (albeit am now non-practicing), and I'd be happy to try and write up a good explanation for this one. it's an interesting topic and there are a few ways to approach it from, incl the philosophy of metrology (why we went from "measure the standard" to "define a standard and attempt to measure it better")