# Have you lost weight?November 2, 2018 6:44 AM   Subscribe

Max Fagin gets a little excited on Twitter about the potential shift in how we define a kilogram.

The main link goes to the Threadreader version of his tweets, but here's a link to the original on Twitter, as well, so you can read the replies.
posted by jacquilynne (21 comments total) 21 users marked this as a favorite

The thread doesn't say it, but I thought the gram was the definition, and it's defined as the weight of 1 cubic centimeter (1 ml) of water at 4deg C.

(Though that makes it sound like a derived property, but the chart shows how inter-related the SI units are)
posted by k5.user at 6:53 AM on November 2, 2018 [2 favorites]

Of course if the IPK loses weight, we'll all gain kilos by definition.
posted by chavenet at 6:54 AM on November 2, 2018 [2 favorites]

Oh, that's so cool. I've always been fascinated that we have this one super important unit that is just a metal bar kept in a vault somewhere, I didn't know that it was on its way out too.
posted by jeather at 7:00 AM on November 2, 2018 [3 favorites]

That’s great, but I wanted to know more about the complex setup that now defines a kilogram (the tweets were sorta hand wavy there), so here’s this from NIST:
Named after its inventor, Bryan Kibble at the U.K.’s National Physical Laboratory (NPL), who conceptualized it in 1975, the Kibble balance is an exquisitely accurate weighing machine. Like any balance, it is designed to equalize one force with another: In this case, the weight of a test mass is exactly offset by a force produced when an electrical current is run through a coil of wire immersed in a surrounding magnetic field.
Now, or soon, the force produced by the current in the coil will be the SI for the kilogram rather than the lump of mass on the other side of the balance.
posted by notyou at 7:03 AM on November 2, 2018 [7 favorites]

The thread doesn't say it, but I thought the gram was the definition, and it's defined as the weight of 1 cubic centimeter (1 ml) of water at 4deg C.

The older water standard comes up in some of the replies -- using water turns out to be more difficult than is practical, because so much can affect how water measures -- purity, temperature, pressure, evaporation, etc. It was originally defined that way, but they switched the reference standard in the past to what it is now, the reference weight in the pictures.
posted by jacquilynne at 7:07 AM on November 2, 2018 [8 favorites]

I get tangentially involved in metrology and calibrations occasionally, as we have instruments that need traceable pedigrees. The standardization of the world is really fascinating, and I think it's pretty cool that (for a time, anyway) you could take an instrument out of one of our labs and trace its calibration pedigree all the way back to a lump of material or a rod or whatever stored in a vault somewhere.
posted by backseatpilot at 7:20 AM on November 2, 2018 [2 favorites]

And this is frustrating since, over the last 100 years, there is evidence that the IPK has actually lost some of its material (even though, paradoxically, the IPK is the one object in the entire universe that *cannot* loose mass.

I love little geeky facts like this. Even though I'm not a Big Bang Theory fan, I can totally hear Sheldon saying this.
posted by TedW at 7:29 AM on November 2, 2018

Lord knows some years really do feel ten million pi seconds long.
posted by Glomar response at 7:34 AM on November 2, 2018 [10 favorites]

So you're saying time is running out on the ultimate heist?
posted by ckape at 7:45 AM on November 2, 2018 [3 favorites]

Wikipedia: Proposed redefinition of SI base units.
posted by 1970s Antihero at 7:52 AM on November 2, 2018

I remember reading about the Electric Kilogram when I was in uni just after the turn of the century. They've taken the time to get it right, and I truly respect that. I would love to see the old SI kilogram in a museum sometime.

Does this really mean that even if all artifacts were destroyed, we could reconstruct our measurements from scratch? That would be a really cool project: construct independently and confirm consistency.
posted by Wrinkled Stumpskin at 8:05 AM on November 2, 2018 [1 favorite]

Loose? Loose!!???
[...]
I weep for our current educational system.
"The term includes errors due to mechanical failure or slips of the hand or finger, but excludes errors of ignorance [...]"
posted by Horkus at 9:02 AM on November 2, 2018 [1 favorite]

Honestly, the reason I'm most excited about this redefinition is that it means that a mole will no longer be a base unit, and we physicists can finally ignore the chemists completely.
posted by Johnny Assay at 9:19 AM on November 2, 2018 [6 favorites]

And yet we still can't agree on how to measure our own feet, so we can order shoes online.
posted by Cobalt at 9:35 AM on November 2, 2018

Wikipedia has a good article on the Kibble balance too.

One of the fun things I learned when travelling around Scotland is that every old town had a market square, and in that market square was the standard ell. A long metal rod or marked piece of stone of a standard length, used to compare measures of cloth, etc. An ell is about 18 inches, defined the same way as the cubit: the length of a man's forearm. Except not really, the Scottish ell was 37 inches, the English ell was 45, the German ell was 23 inches. How these societies all produced a standard unit of length with the same name and yet radically different sizes is confusing.
posted by Nelson at 10:17 AM on November 2, 2018 [3 favorites]

Actually, come to think of it, is there some kind of general relativity effect where it'll alter the defined length of a meter between a vacuum chamber on Earth versus completely flat intergalactic space for example, by an amount of time that's some fraction of an oscillation of a cesium atom's electron? Or is it one of those things where the general relativity equivalents of length contraction and time dilation cancel each other out?
posted by XMLicious at 11:47 AM on November 2, 2018 [1 favorite]

Absolute unit!
posted by L.P. Hatecraft at 3:15 PM on November 2, 2018 [10 favorites]

So you're saying time is running out on the ultimate heist?

Come one step closer and you're all going to weigh twice as much!

*menacingly waves hacksaw at stolen reference kilogram*
posted by sebastienbailard at 6:14 AM on November 4, 2018 [3 favorites]

An article in Physics World about the SI redefinitions. In a sort of reversal of the definition of mass via water volume, the mole will at the same time be redefined as a fixed number, with the help of measurements of 1 kg ultra-pure silicon-28 spheres (discussed in a bit more depth in this 2011 article).
posted by mubba at 12:40 PM on November 4, 2018 [2 favorites]

At a mass gathering in Versailles, the world's scientists have weighed in, and have decided to redefine the kilogram in terms of the Planck constant.

The definitions of the Ampere and, Kelvin, and Mole have also been changed, and the definition of the second has been refined to improve precision.
posted by schmod at 7:10 AM on November 16, 2018 [1 favorite]

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