14 ms·
An old artifact kept in a vault outside Paris is no longer the standard kilogram
- inflatableDodo 7y agoI wonder if there is a process that has to be gone through to now update the pound avoirdupois. It is defined as 453.59237 grams but given the formal definition of the gram has changed could there be a need for a formal acknowledgement, or does the law encompass changes in methods so it just happens automagically?
- sanxiyn 7y agoI don't know about US, but this does need formal amendment to Weights and Measures Act 1985 (current UK standard in force) Schedule 1 Part V, see http://www.legislation.gov.uk/ukpga/1985/72/schedule/1/part/V http://www.legislation.gov.uk/ukpga/1985/72/schedule/1/part/....
- lifthrasiir 7y agoI think the US is not affected by this change, as the NIST didn't define what kilogram actually is back in 1959 [1]. [1] https://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf https://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.p...
- pferde 7y agoDefining a unit of weight in terms of a unit of time and a unit of length. Explain that to your grandma! :)
- groestl 7y agoYour grandma might be a physicist, and she'll explain it to you ;)
- hnuser123456 7y agoshe's not
- tim333 7y agoOr to me. So I guess a weight = some number of square meters per second. I'm trying to think of a intuitive idea why it should be thus.
- zamadatix 7y agoIt's actually the unit of mass in terms of action (time and energy). You can think of it as "how much of something is required to balance a scale where I'm pulling this much" https://ichef.bbci.co.uk/news/624/cpsprodpb/AE6A/production/_104305644_kg_v4_640-nc.png https://ichef.bbci.co.uk/news/624/cpsprodpb/AE6A/production/...
- epicide 7y agoUnit of mass.
- zamadatix 7y agoUnit of weight in terms of time and length? It's the unit of mass in terms of action (time and energy). It's very easy to explain https://ichef.bbci.co.uk/news/624/cpsprodpb/AE6A/production/_104305644_kg_v4_640-nc.png https://ichef.bbci.co.uk/news/624/cpsprodpb/AE6A/production/...
- hiccuphippo 7y agoThe unit of length is also defined in terms of the unit of time (distance travelled by light in a specific interval of time) ;)
- Maken 7y agoWeight is mass times acceleration. Now explain that.
- misnome 7y agoThis article sounds like it got written by a scientist but got butchered through a PR department. It talks about the "Mass of a photon" and weighing them directly, rather than the energy-equivalent mass, which is presumably what they are intending to talk about?
- fsh 7y agoAs the article correctly states, photons have a finite mass that could in principle be measured by putting an optical cavity on a scale.
- renoz 7y agoI know that zero is a finite number, but since the photon is a massless particle, wouldn't it be impossible to get a kilogram worth of photons bouncing around in an optical cavity?
- bonoboTP 7y agoFor some reason, zero is often considered not finite. I personally find this terminology quite unintuitive, but it is well established.
- amluto 7y agoNo, it’s entirely possible in principle. If you build a perfect optical cavity, weigh it, pump a bunch of light in, and weigh it again, you’ll find that the weight went up by ghf/c^2 summed over each photon added to the cavity. Divide by g and you get the sum of the masses of the photons. 1 kg of photons would be enough to destroy your lab and the rest of your city, so only a very advanced civilization would ever do the experiment on this scale. Photons indeed have no rest mass, but it’s impossible to ever find a photon at rest unless you yourself have no rest mass, in which case you also have no ability to do experiments.
- phaemon 7y agoActually, as a physicist with no rest mass, I'm perfectly capable of performing experiments. I just never seem to find the time...
- inopinatus 7y agoAnd with that, I mourn the obsolescence of my last remaining "absolute unit" joke and will have to develop something equally satisfying based on dressed lumber to replace it.
- StavrosK 7y agoWhat about the imperial units? Are those defined against metric as well?
- mehrdadn 7y agoConfused, so how do they measure 1 kilo of something now when they need to calibrate the most precise instruments? They count 10^40 photons? How? And how does that translate into a physical mass they want to carry around? Edit: Seems I completely missed a paragraph somehow. Thanks!
- sanxiyn 7y agoThe term of art is "realisation". The article mentions two realisations but does not elaborate: "In practice, there are currently two known methods for measuring such masses with great precision. These are known as the Kibble balance and the single-crystal silicon sphere". You should be able to google from that.
- deleted 7y ago[deleted]
- crankylinuxuser 7y agoI never understood why a sphere was even in the discussion of a standardized atomic count. Any rational fractions using pi = pi atoms or molecules. And that's absurd to have r=(3V/4pi)^1/3 . No matter how you mess with it, it's irrational to the extreme! Now.. Face-centered or ody-centered cubic follows a nice quadratic expansion for integer based counting. We don't need no steenkin pi's!
- protonfish 7y agoIt's not difficult to round to the nearest atom.
- fl0wenol 7y agoBecause it's easiest to polish perfectly and verify the diameter of if there's no preferrer orientation. Edges of any kind are liable to wear.
- tty2300 7y agoFor most intents and purposes its OK if you are a bit inaccurate. The important thing is that we will never drift further and further away. In 1000 years time we will still have exactly the same kg as we have now.
- Causality1 7y agoI've always been mildly irritated that the name of the SI mass base unit has a prefix. Second, liter, meter, Newton, kilogram. Were I emperor of the world I would slide the name scale up so the mass equal to 2.205lbs was called a gram.
- function_seven 7y agoHow about we keep the current name and use it as a basis for prefixing? gram -> millikilogram milligram -> microkilogram metric ton -> kilokilogram ...never mind.
- bdowling 7y agoIt's almost a crime that a metric ton isn't called a "megagram". Aside, are the mega and giga prefixes ever used for anything except bits and bytes? (Edit: OK, OK. I forgot about megaohms, megahertz, megawatts. Also megapixels and Megadeth.)
- another-dave 7y agoPhysicists finally moving from SVN to Git.
- mehrdadn 7y agoGit is fundamental now?
- another-dave 7y agoWas just thinking centralised repository to distributed, where everyone can have their own copy :)
- vincnetas 7y agoMore like from JPG to SVG
- walrus01 7y agomore like from cvs/cvsup to git.
- highhedgehog 7y agolaughs in imperial units
- deleted 7y ago[deleted]
- huehuemon 7y agoVeritasium has a video back in 2017 explaining kibble balance https://youtu.be/Oo0jm1PPRuo https://youtu.be/Oo0jm1PPRuo
- anvandare 7y agoFor the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.
- rimliu 7y agoHow do you imagine that? And what has the bit to do with it?
- snops 7y agoThe SI units are supposed to be used to define all other units. The mole is just a unitless number which isn't used for many definitions, while the bit (or preferably, Shannon) has a fundamental use in information theory and the wider field of physics. The mole is a human specific number, the bit is a property of the universe. Information is a physical quantity, just like mass or length, and deserves to have a unit in the SI system. You could use other bases, like the nebit (natural log) or Hart (base 10), but bit is most elegant as it uses the smallest integer base possible, 2.
- no_identd 7y ago"The smallest=the most elegant=the most desirable" seems like terrible reasoning, and similar to the reasoning behind the infamous 'Uno': https://en.wikipedia.org/wiki/Parts-per_notation#Uno https://en.wikipedia.org/wiki/Parts-per_notation#Uno However, I don't entirely disagree with where you go with this, but in my opinion, one ought to: 1. Use the nat an normalize everything else (which might seem like an odd idea at first, but think about it): https://en.wikipedia.org/wiki/Nat_%28unit%29 https://en.wikipedia.org/wiki/Nat_%28unit%29 But, if we assume you want to avoid the normalization, then it would seem more sensible, due to numerical efficiency, to use balanced ternary, which actually has, unlike what people who rather cluelessly apply the literal textbook (and terrible) metric of 'Radix efficiency'(an entirely artificial measure originally invented to account for engineering concerns related to vacuum tube spacing.) would presume, higher numeric efficiency than unsigned ternary, as shown here by a then Indian computer science student, Abhijit Bhattacharjee, whose work on this unfortunately only remains accessible via the Internet Archive Wayback Machine: https://web.archive.org/web/20090312094241/http://abhijit.info/tristate/tristate.html#Base3 https://web.archive.org/web/20090312094241/http://abhijit.in... He actually got a reply by Marvin Minsky(to an email he had sent him) at some point (also available via archive.org, but only if you do a lot of trickery which I can't do from my mobile phone, I've however previously verified the quote he provided on the bottom of the page and it does, by all things I could account for, indeed seem genuine), which I'll quote here: "Yes it is a fascinating subject, and your explorations and descriptions are extremely readable. I hope it gets the interest of more mathematicians."
- deleted 7y ago[deleted]
- donalhunt 7y agoChange is scheduled for 20 May 2019. checks date fake news... the artifact is still the official reference for the standard kilogram for another 72 hours... ;)
- orblivion 7y agoYeah, jokes aside I think _technically_ the post title should be changed. (Even the article's title is wrong. Though, it wouldn't be the first time NH's title is more accurate than that of the article it points to.)
- lelf 7y agoDefinition of mole changes too. One mole contains exactly 6.022 140 76×10²³ elementary entities (as opposed to the number of atoms in 12 grams of ¹²C). Effective 20 May 2019.
- Izkata 7y agoEh? Straight 6.022*10^23 is what I learned in highschool chemistry in 2003..
- kfichter 7y agoIt's being slightly tweaked: https://chemistry.stackexchange.com/questions/91692/avogadros-number-6-023-x-1023-or-6-022-x-1023 https://chemistry.stackexchange.com/questions/91692/avogadro...
- hexane360 7y agoThe difference is more epistemological than practical. Before, 6.022x10^23 was the measured value of the conversion factor between a gram (exactly 1/100 of the Prototype Kilogram) and a dalton (exactly 1/12 the mass of a carbon-12 atom). After the redefinition, a dalton is defined as being 1/6.022x10^23th of a gram, and measured to be approximately equal to 1/12 the mass of a carbon-12 atom. In essence, the dependency chain was reversed, so now it all flows from the kilogram outwards.
- gubbrora 7y agoThe real wtf is that we had a unitless unit, ie a number whose value changes over time with some reference object.
- aivisol 7y agoReading about standard kilogram always reminds me 2014 movie "1001 grams" [0]. Not sure how accurate it reflects how the things are actually run there, but was interesting to watch it nevertheless. [0] https://www.imdb.com/title/tt3346824/ https://www.imdb.com/title/tt3346824/
- fatbob 7y agoDirected by Bent Hammer
- dwighttk 7y ago>Despite the greatest of precautions, every time the standard kilo was handled — for example, to compare it to another unit that could then be used to calibrate instruments — it would shed some atoms and its mass would be slightly changed. Over its lifetime, that standard kilo is estimated to have lost about 50 micrograms. Why did the standard kilo ever need to be handled? Just store it on a balance and only handle the comparison unit.
- hiccuphippo 7y agoI think they needed to take the replicas from other countries like the US to the master one in Paris for comparison every year. Edit: see this Veritasium episode https://youtu.be/SmSJXC6_qQ8 https://youtu.be/SmSJXC6_qQ8
- forgot-my-pw 7y agoAlso relevant: https://www.youtube.com/watch?v=ZMByI4s-D-Y https://www.youtube.com/watch?v=ZMByI4s-D-Y
- rough-sea 7y agoThe balance still would need to be balanced.
- dwighttk 7y agoright...
- thenewwazoo 7y agoand then left...
- petschge 7y agoAt this precision you weigh things twice. Once with the reference on the left arm of the balance and the test object on the right and another time with objects swapped.
- jelder 7y agoThe new reference kilogram is an engineering marvel, but I’ve always wondered this: why didn’t they just build a reference gram instead? Seems like that would be at least a thousand times easier to pull off.
- gppk 7y agoLikely a lot harder to get the tolerances down on something much smaller.
- Etheryte 7y agoAs covered in the article, the reference object loses some of it's atoms every time it is handled. The smaller the reference object, the larger the relative error induced this way.
- swiley 7y agoNot just that, it's probably just the surface (or near surface) that sublimes. The surface only increases with the square of the radius while the volume increases with the cube. so multiplying the volume by 27 only multiplies the surface by 9.
- ISL 7y agoIt was easier and more reliable to manufacture the reference kilo in the past. With the new redefinition, you can make your own reference gram, nanogram, megagram, etc. Your intuition is correct -- it is possible to build more-precise intercomparison tools at the gram scale than the kilogram scale, but until next Monday, our units are referenced to Le Grand K, and any gram-reference must subdivide the absolute standard. After Monday, you're free to build your own gram-scale Kibble balance and call it an absolute standard.
- saalweachter 7y agoBasically, it's part of the lie of the mythology of the metric system. The metric system did some great things (eliminated the use of measures that depended on the substance being measured, dry pints vs wet pints, bushel of wheat versus bushel of oats; simple relationship between units of length, area and volume), some things that were already common at the time (eliminated regional definitions of units; related the volume and mass of water), and some stupid things (metric prefixes). But at the end of the day, the size of the meter and kilogram were chosen to be very nearly 3 Parisian feet and 2 Parisian pounds, because that made it easier to adopt.
- deleted 7y ago[deleted]
- wnissen 7y agoLe Grand K is dead! Long live Le Grand K!
- 6006135 7y agoIs there a reason why this took so long?
- 6006135 7y agoDon't know why this was downvoted. But here is the answer [1]: "The new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram." https://en.wikipedia.org/wiki/Kilogram https://en.wikipedia.org/wiki/Kilogram