# Where does the mass of a kilogram come from? **URL:** \ **Category:** Factual Questions\ **Created:** [November 11, 2010, 8:12pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350 "2010-11-11T20:12:35Z")\ **Posts on this page:** 19\ **Page:** 2
**Author:** ![wheresmymind](https://avatars.discourse-cdn.com/v4/letter/w/91b2a8/32.png) [@wheresmymind](https://boards.straightdope.com/u/wheresmymind)\ **Post date:** [November 12, 2010, 12:55am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/21 "2010-11-12T00:55:25Z")
> [@beowulff](#): > > The new standard will be a really cool Single-isotope Silicon [sphere](http://www.wired.com/wiredscience/2010/10/platinum-silicon-kilogram-standard/). I remember reading (possibly in Wired, actually) that there are other teams trying to define a new standard using ultra-sensitive watt balances. ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [November 14, 2010, 11:43pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/22 "2010-11-14T23:43:39Z")
> [@JoelUpchurch](#): > > At this level, I think you also have to compensate by time compression. IIRC it you put an atomic clock on a plane flying east and another plane flying west you will get a slightly different answer when the return to their starting location. > > I just goggled it and here is a description. > > [http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html#c2](http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html#c2) IIRC, the referenced paper was included in the Proceedings of the Frequency Control Symposium possibly one year after the original publication or it might even have been the original publication. Again, IIRC, one couldn’t move an inch without running into another discussion of that paper; it generated more interest than any I could ever recall. My copy was lost somewhere over the years and I’m delighted to have access to it again. Thanks for the link. ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [November 14, 2010, 11:47pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/23 "2010-11-14T23:47:59Z")
> [@Spatial\_Rift\_47](#): > > In order: you don’t, you use the equations of atomic structure and energy from quantum mechanics, and you use a spectrometer to measure the radiation emitted by a large assembly of cesium-133 atoms as they make that particular transition when you induce it with a magnetic field. > > The choice of that particular transition from that particular atom is largely arbitrary, since it serves only to identify photons of a particular frequency. Any quantum mechanically specified transition would serve the same purpose. We could just as solidly define the second as 2.4967x10[sup]15[/sup] periods of the radiation corresponding to the Lyman-alpha transition of neutral hydrogen atoms (in the absence of any fine or hyperfine splitting). Thank you; my ignorance has been fought but, regrettably, not vanquished. All I want to know is everything; that shouldn’t be too much to ask. ---
**Author:** ![Rhythmdvl](https://avatars.discourse-cdn.com/v4/letter/r/85f322/32.png) [@Rhythmdvl](https://boards.straightdope.com/u/Rhythmdvl)\ **Post date:** [November 14, 2010, 11:52pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/24 "2010-11-14T23:52:58Z")
> [@glowacks](#): > > Yes, but you know your clock to be accurate because you’ve used the definition of a second, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, to build it. How does the counting work? A team of dedicated actuarial physicists with high-tech versions of those clicky-counter things umpires use? Some gadget that’s capable of not only counting to 9 billion in a second, but hitting “start” on the Doomsday Device within the margin for error? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 15, 2010, 12:13am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/25 "2010-11-15T00:13:41Z")
> [@Rhythmdvl](#): > > How does the counting work? A team of dedicated actuarial physicists with high-tech versions of those clicky-counter things umpires use? Some gadget that’s capable of not only counting to 9 billion in a second, but hitting “start” on the Doomsday Device within the margin for error? You just buy one of [these](http://www.axiomtest.com/Agilent___HP_5340A_18GHZ_COUNTER6360-details.aspx). ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [November 15, 2010, 2:32am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/26 "2010-11-15T02:32:43Z")
> [@Dewey\_Finn](#): > > This part (“A team of scientists based in Germany measured the [Avogadro] constant by counting the atoms in a painstakingly crafted one-kilogram sphere of silicon-28.”) of that article is hilarious. So the standard kilogram is something for which the _number of atoms_ have been counted? Well, not yet, but that’s the general idea behind the new standard. One mole of carbon-12 is defined to be that number of atoms of carbon-12 such that their total mass is precisely 12 grams. We can use precision measurements to find out what the ratio of the mass of an atom of silicon to an atom of carbon is, so we know how many grams a mole of silicon should weigh. What we don’t know is precisely how many atoms are in a mole. So if we can manufacture some physical object for which we know the number (and type) of atoms extremely precisely, and we weigh that object relative to our “old kilogram” standard, then that allows use to precisely measure how many atoms (of silicon, say) make up an “old kilogram”. Then we define the “new kilogram” to be the weight of that number of atoms of silicon, and the iridium bar is no longer needed. ---
**Author:** ![ChrisBooth12](https://avatars.discourse-cdn.com/v4/letter/c/e95f7d/32.png) [@ChrisBooth12](https://boards.straightdope.com/u/ChrisBooth12)\ **Post date:** [November 15, 2010, 3:02am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/27 "2010-11-15T03:02:14Z")
they should just say take 1 carbon-12 atom and multiply till you get 1 kilo. I don’t get the standard block I mean are scales really calibrated using that? I doubt it so whats the point? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 15, 2010, 5:25am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/28 "2010-11-15T05:25:38Z")
> [@](#): > > they should just say take 1 carbon-12 atom and multiply till you get 1 kilo. I don’t get the standard block I mean are scales really calibrated using that? I doubt it so whats the point? In practice, what’s usually done is that you use the primary standard to make a bunch of secondary standards for major national labs around the world, and then you use those secondary standards to make tertiary standards that are used by all the other labs that need precise standards. And yes, the process of making those secondary and tertiary standards does involve putting the primary and secondary standards on scales of some sort. ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [November 15, 2010, 6:34am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/29 "2010-11-15T06:34:21Z")
> [@LouisB](#): > > Thank you; my ignorance has been fought but, regrettably, not vanquished. All I want to know is everything; that shouldn’t be too much to ask. [Start here,](http://www.internetlastpage.com/) and work your way back. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [November 15, 2010, 12:59pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/30 "2010-11-15T12:59:38Z")
> [@ChrisBooth12](#): > > they should just say take 1 carbon-12 atom and multiply till you get 1 kilo. Well, yes, but multiply by what? That’s what this precision silicon sphere project is supposed to do, in the end: tell us precisely how many atoms are in a mole (and, by extension, in a kilogram as currently defined.) ---
**Author:** ![Scuba\_Ben](https://avatars.discourse-cdn.com/v4/letter/s/278dde/32.png) [@Scuba\_Ben](https://boards.straightdope.com/u/Scuba_Ben)\ **Post date:** [November 15, 2010, 2:17pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/31 "2010-11-15T14:17:19Z")
> [@MikeS](#): > > Well, yes, but multiply by what? That’s what this precision silicon sphere project is supposed to do, in the end: tell us precisely how many atoms are in a mole (and, by extension, in a kilogram as currently defined.) And that, right there, is the engineering problem. The SI meter is defined exactly, based on the SI second. SI mass would need to be defined exactly. No “6.02245 E23 +/- 1E18”, but exactly. ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [November 15, 2010, 9:06pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/32 "2010-11-15T21:06:29Z")
> [@Cheshire\_Human](#): > > [Start here,](http://www.internetlastpage.com/) and work your way back. Oh, thank you, thank you; that greatly simplifies my task. I had hoped for a link to the first page, but one must start somewhere. ---
**Author:** ![LouisB](https://avatars.discourse-cdn.com/v4/letter/l/dfb087/32.png) [@LouisB](https://boards.straightdope.com/u/LouisB)\ **Post date:** [November 15, 2010, 9:08pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/33 "2010-11-15T21:08:41Z")
> [@beowulff](#): > > You just buy one of [these](http://www.axiomtest.com/Agilent___HP_5340A_18GHZ_COUNTER6360-details.aspx). Just thought I’d mention------the first frequency counter I ever used was an HP with nixie tubes. I get headaches just remembering those things. ---
**Author:** ![RadicalPi](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/radicalpi/32/9806_2.png) [@RadicalPi](https://boards.straightdope.com/u/RadicalPi)\ **Post date:** [November 16, 2010, 4:16am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/34 "2010-11-16T04:16:41Z")
> [@Simplicio](#): > > The chunk of metal used to formally define the Kilogram was machined so that it would be close to the weight of 1,000 cubic centimeters of water at 4 degrees. I’m not really sure why they haven’t taken the obvious step of just using that as the definition. Perhaps water doesn’t have as fixed a density as I’d think at a set temperature? I believe that part of the reason that they didn’t do this was that the volume of water depends a bit on the pressure of the air above it. So, any definition that doesn’t take pressure into account is going to be less accurate than the platinum-iridium cylinder. And any definition that _does_ take it into account is going to be circular since pressure is force per unit of area, and force is mass times acceleration. So you’d need to have mass defined before you can define mass. Someone please correct me if I’m wrong. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 16, 2010, 5:55am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/35 "2010-11-16T05:55:44Z")
Well, you could specify that it be vacuum over the water (or rather, nothing but water vapor at its equilibrium vapor pressure), but you’d still have to specify a temperature. And the shape of the container and its orientation, since the water will be pressurized slightly by its own weight, and that’ll depend on how tall the water is (and on the local gravity, for that matter). And then there’s the question of how precisely you could measure the mass of that water, and how it compares to the precision of a metal proof mass. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [November 16, 2010, 2:40pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/36 "2010-11-16T14:40:11Z")
> [@Squink](#): > > How do you think _Iridium_ feels about the change? > That [old Kg standard](http://en.wikipedia.org/wiki/Kilogram) made up 90% of its reason for being. Iridium is still used to make flares IIRC ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 16, 2010, 6:23pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/37 "2010-11-16T18:23:12Z")
> [@](#): > > Iridium is still used to make flares IIRC An Iridium flare has almost nothing to do with iridium, and very little to do with flares. It’s a specular reflection of sunlight off of an Iridium communications satellite, visible from the ground. And the Iridium satellites were named that because originally, there were 77 of them, and the structure of 77 satellites orbiting the Earth reminded the folks who founded the company of 77 electrons “orbiting” a nucleus. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [November 16, 2010, 6:46pm UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/38 "2010-11-16T18:46:46Z")
> [@LouisB](#): > > Oh, thank you, thank you; that greatly simplifies my task. I had hoped for a link to the first page, but one must start somewhere. [Here](http://www.internetfirstpage.com/) you go! ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [December 5, 2010, 3:58am UTC](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350/39 "2010-12-05T03:58:26Z")
> [@](#): > > the International Bureau of Weights and Measures (BIPM) outside Paris > … > in the basement of the BIPM’s main lab building, which houses a vault secured with three locks. Three antique keys, held by citizens of different countries, open these locks. > … > the three key holders produce the keys, open the vault, unlock a safe inside the vault and inspect its principal contents: the platinum–iridium cylinder that defines the kilogram. [Au revoir, kilogram](http://physicsworld.com/cws/article/indepth/44445) (with a nice picture of the kilogram inside its three nested bell jars). So In Paris:In a Basement:In a Vault:In a Safe:Inside 3 bell jars. [Previous page](https://boards.straightdope.com/t/where-does-the-mass-of-a-kilogram-come-from/560350.md?page=1)