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Math Science

Perfect Silicon Sphere to Redefine the Kilogram 453

MrCreosote writes "The Age reports optical specialists at CSIRO are helping create a new standard for the kilogram, based on a precise number of atoms in a perfect sphere of silicon. This will replace the International Prototype, a lump of metal alloy in a vault in Paris."
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Perfect Silicon Sphere to Redefine the Kilogram

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  • alternate theories (Score:5, Informative)

    by arun_s ( 877518 ) on Friday June 15, 2007 @06:33AM (#19517089) Homepage Journal

    I found some alternate theories that are also attempting to precisely measure the kilogram at everything2 []. They look pretty interesting, here's a small excerpt:

    Superconducting levitation

    This method works along essentially the same principles as the Watt Balance. In it, a superconductor of a known mass is placed within a superconducting coil. By running current through the coil, a magnetic field is generated that causes the superconducting mass to levitate. By levitating it at different positions and measuring the current required to do so, the magnetic flux can be calculated. Magnetic flux relates directly to Planck's constant, and because the force generated by the magnetically-induced levitation and the downward force of gravity must be equal, Planck's constant can thus be precisely related to the kilogram.

    Hey wait, TFA skims over what they're going to do with the Silicon ball once its made. Again, from everything1:

    X-ray interferometry is used to determine the distance between lattice planes in the silicon crystal, permitting physicists to determine, as closely as possible, the number of atoms in these spheres. Currently, a measurement accuracy of one part in 10^7 is possible, after considering all of the various sorts of error introduced in the process, but it is hoped that ten times this accuracy will be possible within five years.
    • by JanneM ( 7445 ) on Friday June 15, 2007 @06:48AM (#19517193) Homepage
      This method works along essentially the same principles as the Watt Balance. In it, a superconductor of a known mass is placed within a superconducting coil.

      If you have a lump of anything of a known mass, why bother with the rest?

      • Re: (Score:3, Insightful)

        maybe because you are using the lump of known mass to measure something else.

        Duck Measurer: "I put a duck on one side of the scale, and use weights (lumps of known mass) on the other side to determine the mass of the duck."
        Some Guy: "Umm, but you already know the mass of the weights, why are you bothering?"
      • by Delirium Tremens ( 214596 ) on Friday June 15, 2007 @07:18AM (#19517423) Journal
        You are confusing mass and weight.
        Mass = how much matter there is in an object.
        Weight = how much pull does a particular gravity (like Earth's g) has on that quantity of matter.

        That's why you could be floating (weightless) in a space ship without having lost any of your fingers or other parts of your body (mass) ;-)
        • Re: (Score:3, Insightful)

          by Software ( 179033 )
          At the risk of being pedantic, people in spaceships orbiting earth are not weightless; the Earth's gravity has an effect on them (namely, keeping them in orbit). Their sensation of weight [] will be zero, because they are essentially in free fall, but they still have weight.

          To be fair, though, you didn't specify that the person was in orbit. Maybe you were thinking of in interstellar space, where a person would be weightless.

          • Re: (Score:3, Insightful)

            by thegnu ( 557446 )

            Maybe you were thinking of in interstellar space, where a person would be weightless.

            Not to be pedantic here, either, but maybe you'll just be in an imperceptibly slow freefall in an imperceptibly large orbit.
          • Re: (Score:3, Informative)

            by mobby_6kl ( 668092 )
            At the risk of sounding ignorant due to the lack of a PhD in physics, I thought people in spaceships (in orbit at least) are weightless. The gravity is still present so it's not zero gravity, but since there's no force acting on them other than gravitational acceleration, they are indeed in free fall and are thus weightless.

            Walter Lewin [] seems to agree with this:

            "If I jump from a tower which is 100 (?) meters high, I will be weightless for about 4 seconds, ignoring the air drag."

            From this [] video lecture, wic

    • by jmv ( 93421 ) on Friday June 15, 2007 @06:58AM (#19517257) Homepage
      I personally wouldn't put too much trust into a measurement that depends on gravitational acceleration for several reasons.
      1) It means you can't move the setup somewhere else easily because gravity is location-dependent
      2) Events like the 2004 tsunami has a slight (but measurable) effect on the Earth's rotation and hence on the acceleration (because of centrifugal force) ... and most importantly
      3) Your measurement will (*literally*) depend on the phase of the moon (just like tides)
      • by _Eric ( 25017 ) on Friday June 15, 2007 @07:38AM (#19517595)
        You're mixing up mass (an amount of matter) and weight (the gravitational force felt by matter). The kilogram can be used anywhere. Only using a device based on absolute measurement of weight (spring based scale) will render the device dependent on the the local gravity field. Yet, true enough, this is how most of modern electronic scales work (they could still weight a known internal mass for calibration to work that around, but I don't know if or how this is actually done).
  • First of all (Score:2, Insightful)

    by alx5000 ( 896642 )
    ... no sphere made of atoms will ever be a perfect one.
    Second, if that rusty lump in Paris defines what a kilogram is, in no way is this sphere gonna change that.
    • Re:First of all (Score:4, Informative)

      by Corporate Troll ( 537873 ) on Friday June 15, 2007 @06:45AM (#19517179) Homepage Journal

      Second, if that rusty lump in Paris defines what a kilogram is, in no way is this sphere gonna change that.

      That's wrong. The lump is not rusty, because the lump is platinum-iridium which is quite unreactive so that corrosion ("rust") won't affect the material. Corrosion alters the weight, you know.

      Second, it can change the definition. The metre used to be a platinum rod in Paris, now it is defined in how much distance light does in a certain (very short) time. Here it will be that the kilogram will be defined as N silicium atoms. (Where N is a very large number) Scientists do not like definitions based on objects, they prefer definitions based on universal constants. All this could of course be read in the article....

    • Second, if that rusty lump in Paris defines what a kilogram is, in no way is this sphere gonna change that.

      Wow, what a great straight line. There's just so many opportunities, I feel like Quagmire in an adult entertainment store. Giggity-giggity!

      "Rusty lump? Oh, I'm sure she can afford to go with actual silicone."

      "Wow, did that come out in the cavity search?"

      "Biggest ben wa ball ever."

      "That's not a rusty lump, that's my watch, you insensitive clod!"

  • by antifoidulus ( 807088 ) on Friday June 15, 2007 @06:39AM (#19517131) Homepage Journal
    is because they are embarrassed of the fact that a T-rex managed to steal [] the original one and now they need a replacement.
  • Ah yes... (Score:5, Funny)

    by Nerdposeur ( 910128 ) on Friday June 15, 2007 @06:40AM (#19517137) Journal
    ..but how can they make sure the new kilogram weighs a kilogram? :)
    • by Zaatxe ( 939368 )
      ..but how can they make sure the new kilogram weighs a kilogram? :)

      They put it on a scale, silly!
    • by Zaatxe ( 939368 )
      ..but how can they make sure the new kilogram weighs a kilogram? :)

      Actually the kilogram is a unit of mass, not weight. We say "it weights X kilograms", but we should really say "it weights under Earth's surface gravity the same as X kilograms". But of course we don't say that. We don't even say kilogram, we usually say "kilo", which means only "a thousand"!!
    • by Jamu ( 852752 )
      Weigh it against the old kilogram.
    • Re:Ah yes... (Score:4, Informative)

      by at0mjack ( 953726 ) on Friday June 15, 2007 @07:19AM (#19517435)

      They don't. The idea isn't to make the new sphere weigh a kilogram. The idea is to redefine the kilogram in terms of the weight of an atom of silicon (i.e. 602383623523895723945743 atoms of Si-14 weigh exactly 14 grams). The idea of the ultrapure and ultraround Si sphere is that (a) you can measure the lattice spacing of the Si atoms in it using x-ray crystallography, so you know how far apart the Si atoms are, and (b) you can measure the diameter of your ultraround sphere very accurately, so you can calculate its volume very accurately. Given these two, you can calculate with very small error bars how many atoms of Si there are in the sphere, and given the definition of the kg in terms of how many atoms of Si make up a kg you can calculate exactly how much the sphere weighs.

      You can then stick it on your balance that needs calibrating, and twiddle the dials until the balance thinks that the sphere weighs the same as the calculated weight.

  • by oliverthered ( 187439 ) <oliverthered@hotmail . c om> on Friday June 15, 2007 @06:43AM (#19517161) Journal
    A kilogram was equal to 1000 millilitres of water and that 1000 millilitres of water would fit into a space 10cm cubed.

    If they've already defined the metre using constants, isn't something like this the best way of defining a kilogram.
    • Shhh it's a make work project. We were taught [in Canada] that 1g = 1ml = 1cm^3.

    • One problem with that is the density of water varies w/ the temperature (it's this characteristic which makes life on earth possible --- water gets more dense as it approaches the freezing point, then less dense when it freezes), so the definition has to include a temperature &c.


      • by Nimey ( 114278 )
        I'm pretty sure that STP (standard temperature & pressure) is implied.

        ...except according to Wackypedia, various standards bodies don't agree [] on STP:

    • by at0mjack ( 953726 ) on Friday June 15, 2007 @07:12AM (#19517377)

      The main problem with this as a definition is that water expands and contracts with temperature. So, if you wanted to define the kilogram in terms of a volume of water, you need to specify the temperature at which you are making the measurement. Temperature isn't something you can measure with very high precision (parts per million or parts per billion), so you end up with unavoidably large errors. As a result this is useless as a basic standard, the essence of which is that you should be able to repeat the standard measurement and get the same answer to N decimal places.

      • Re: (Score:3, Informative)

        by gkhan1 ( 886823 )

        That's wrong, actually. I mean, practicality doesn't really figure into defining the other units, how practical is it to define a meter as the length light travels in 1/299,792,458ths of a second? The point is that it has to be absolute. 1 dm^3 of water at 4 degrees Celsius (whatever that is in Kelvin) is exact and absolute. Plus, we already have a similar definition, the Kelvin is defined as exactly 1/273.16 of the temperature of the triple point of water. If the hokey-ness of measuring temperature figures

  • by Max Romantschuk ( 132276 ) <> on Friday June 15, 2007 @06:44AM (#19517175) Homepage
    Except for the challenges of making one, what's it useful for? You can't use it to calibrate anything, the wear and tear caused by the friction of handling would eventually change it's mass and defy it's purpose. Is the actual "finished product" good for anything else than sitting in another vault somewhere?
    • Re: (Score:3, Interesting)

      by meringuoid ( 568297 )
      Except for the challenges of making one, what's it useful for? You can't use it to calibrate anything, the wear and tear caused by the friction of handling would eventually change it's mass and defy it's purpose.

      It's hierarchical. You use the standard kilogram to calibrate other, slightly less exalted standard kilograms. So the one kept in London and the one in New York and the one in Tokyo get calibrated against the one in Paris. Then you calibrate actual working weights against those.

    • I guess it would be used to calibrate a (limited by wear and tear, yes) number of "second generation" reference weights. Which would obviously not be quite as accurate, but still good enough to serve as reference to calibrate commercially weights and weighing machines.

      The great advantage of this approach is that you can reproduce the original reference weight if necessary, while the loss of the current prototype would mean a much bigger problem.
    • by dargaud ( 518470 ) <slashdot2 AT gdargaud DOT net> on Friday June 15, 2007 @07:10AM (#19517357) Homepage

      You can't use it to calibrate anything, the wear and tear caused by the friction of handling would eventually change it's mass and defy it's purpose.
      Yes you can. The problem with the current reference weight is that it cannot be reproduced. Here you have a definition: this volume (4/3.Pi.R^3) contains such an amount of Si atoms. We define their individual mass and we define the whole sphere to be one kilo, ergo we can build another one. Just like defining the meter as a distance covered by light, here it's the weight of a given number of atoms.
      • Re: (Score:3, Interesting)

        by jabuzz ( 182671 )
        It's extremely unlikely that the spehere has a mass of exactly one kilogram. What you do is assign a mass to an individual Si atom, and count the number of atoms in the sphere. You then have a mass for that sphere. The more accurately you can count the atoms the more accurate the mass assigned to the sphere.

        The idea behind this is that rather than having a absolute reference mass in a vault in Paris, we can create new reference masses at will, so we can have additional ones in London, one in New York, one i
  • by Dan East ( 318230 ) on Friday June 15, 2007 @06:51AM (#19517219) Journal
    A perfect sphere, down to the atom, of 1 kg silicon would require pi to what precision?

    Dan East
    • By definition, a "perfect" sphere would require pi to infinite precision. Let me know when you've got that figured out.
      • an atom might have an diameter of about 0.1 nm, they say they have a perfect sphere which is just 35nm from being perfect, what ever that means. So you don't need that many positions of PI at all..
    • Re: (Score:3, Funny)

      640 digits of PI ought to be good enough for anyone.

      -- Should you believe authority without question?
  • Didn't David Bowie have a few of these the institute could borrow instead of making new ones? Just keep them away from baby sitters.
  • I've lived all my life in countries that use the metric system, so I have to ask... does the pound have an "official reference" like the kilogram?
    • I've lived all my life in countries that use the metric system, so I have to ask... does the pound have an "official reference" like the kilogram? Yes, it is one of those Kilogram weights, cut into a piece about 45.359237% smaller
    • The pound is a weight (mass times acceleration due to gravity). There is no standard because weight is not a fundamental dimension of measurement. I have no idea if there is an "official stone".
      • Re: (Score:3, Informative)

        by Rostin ( 691447 )
        Actually, a pound is sometimes both a mass and a weight. It's one of the stupider quirks of the customary system. When the distinction is important and not obvious from the context, pounds mass (lbm) or pounds force (lbf) is specified. The "conversion" between the two involves a constant, usually written as g-sub-c.

        lbf = lbm * (accel due to gravity) / g-sub-c

        constant = 32.174 lbm*ft*s^-2*lbf^-1

        That way, one lbm weighs very close to 1 lbf. []
    • Re: (Score:3, Informative)

      by simong ( 32944 )
      Yes, there are standard Imperial weights and measures. I seem to recall that one set are on display at the Tower of London. However, the pound now seems to be legally defined as 0.453 592 37 kilogram.
  • That's a lotta light!
  • How do they account for different isotopes? Or do they just get a sphere that weighs the same as N many atoms of pure silicon 28 would weigh?
    • Re: (Score:3, Interesting)

      by Tom Womack ( 8005 )
      The 'special crystal which took three years to grow in Russia' that the article writes about is made of pure silicon-28 precisely to get around this objection.

      I think the concern is that samples of silicon from different sources (consider, for example, 'depleted silicon' from the scrapyard of the Russian isotope-enrichment facility) might have different isotope distributions at the 10^-7 level, whilst good laser enrichment can ensure a really very constant isotope distribution.
  • by femto ( 459605 ) on Friday June 15, 2007 @07:09AM (#19517353) Homepage
    The CSIRO project is about determining how many silicon atoms are equivalent in mass to the current standard kilogram. Once that number is established the actual kilogram in Paris is redundant. If it gets lost or destroyed we can reconstruct the kilogram by counting out 'n' silicon atoms. It also means anyone can construct their own kilogram by counting out 'n' silicon atoms, without having to go to Paris to do a comparison.

    It is a separate (but related) project to figure out the second part of the project: how to easily count out 'n' silicon atoms, so creating a universally available standard. One way might be to make a silicon sphere, like the CSIRO, but most people don't have the ability to do that.
  • by siddesu ( 698447 ) on Friday June 15, 2007 @07:14AM (#19517387)
    silicon spheres will define the standard ... will they be coming in pairs by any chance?
  • Jenna Jameson do your part for science.
  • by Bromskloss ( 750445 ) <auxiliary.addres ... l.DEBIANcom minu> on Friday June 15, 2007 @07:23AM (#19517449)

    One horsepower is the power of the reference horse in an archive in Paris.

    • I mean, come on, would you Australians to define scientific standards? I won't!

      For starters, those guys believe the South is on the top and North is at the bottom of the maps! I feel upside down just thinking of it. And on which side of the road are they driving already? North or south? See: you cannot trust those guys!

      Second, the issue with "the" current "reference" in Paris (there are three cylinders in fact) is that is loses atoms sometimes, so its mass diminishes. I mean it is still The Kilogramme but t
  • single isotope (Score:3, Informative)

    by Tzinger ( 550448 ) on Friday June 15, 2007 @07:29AM (#19517511) Homepage
    Standards for weights, mass, distance or any other measure, are critical in the calibration of instruments. This calibration provide the means that to compare product specifications and research results.

    This particular effort is a very interesting set of challenges. It requires the use of single isotope of silicon; calibrations for distance and roundness, and a sophisticated means to to count the atoms. This last step requires the silicon to be perfectly crystalline.

    Measurement is itself a very interesting study bordering on metaphysics and philosophy. The desire to measure things has been at the heart of a lot of scientific investigation, economics and other areas of study. Ref "Abstract Measurement Theory" by Louis Narens ttype=2&tid=6345/ []
  • Exam (Score:5, Funny)

    by matt me ( 850665 ) on Friday June 15, 2007 @07:37AM (#19517593)
    Weird. I read about this in an exam I took last week. It stated that the present standard kilogram is a mass of platinum and iridium kept at STP underground, and asked what factors might affect the mass of the standard kilogram when it is measured. I answered if any isotopes of platinum or iridium decay, or if the standard kilogram had a velocity close to the speed of light.
    • by Migraineman ( 632203 ) on Friday June 15, 2007 @08:50AM (#19518277)
      Worse - you can't touch the Sphere of One-ness with anything.

      Q: May I put my greasy paws on it?
      A: No. Fingerprints will alter the mass in a measureable way.

      Q: White gloves?
      A: Abrasive.

      Q: Use a special cradle that's machined to exactly the same radius profile such that you won't scratch or deform the Sphere of One-ness?
      A: Nope. That'll result in a molecular interference fit. You'll never get the two pieces apart.

      So ultimately, they're building a very precise bauble that no one will ever be allowed to touch. I suspect that bouncing photons off the surface may displace an atom or two, so they'll keep it in a dark room ... in a vacuum chamber ... at the bottom of a flight of stairs, in a disused lavatory with a sign on the door reading "prendre garde du léopard."
      • Re: (Score:3, Insightful)

        by gardyloo ( 512791 )
        Haha. Love the Douglas Adams reference. Such a lead-in, too!

            Seriously, though, photons of anywhere near visible frequencies won't displace the atoms; light bouncing is almost always a purely electronic transition thing. And if this thing is ultrapure silicon, atoms are NOT going to want to displace. No worries there.
  • by frostilicus2 ( 889524 ) on Friday June 15, 2007 @07:52AM (#19517719)
    I'd prefer A non-perfect sphere of Silicone [].
  • by ribuck ( 943217 ) on Friday June 15, 2007 @08:16AM (#19517907) Homepage
    A new standard is needed because the "standard kilogram" held in France has been slowly losing mass, about 50 micrograms in the last 100 years, compared to other reference masses. It's not known how this has occurred.

    Wikipedia - Kilogram []

    Slashdot: The Changing Definition of 'Kilogram' 7/023252 []
  • by clickety6 ( 141178 ) on Friday June 15, 2007 @08:19AM (#19517941)
    ...that way they'll be able to find it again after they put it down somewhere.

    "Zut alors! Pierre, le sphere parfait - ou est-ce que tu le placer?

    "C'etait sur le table, Jean-Claude"

    "Merde, il avait roller sous le sofa encore!"

  • Too complicated! (Score:3, Interesting)

    by Muad'Dave ( 255648 ) on Friday June 15, 2007 @08:47AM (#19518231) Homepage
    We've got a precise definition of the 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.
    and the meter:

    the distance traveled by light in absolute vacuum in 1/299,792,458 of a second
    Why not tie the kg to the second and meter via the Newton? 1 Newton is defined as 1 kg * m * s^-2. Using centripetal force in a plane normal to the earth's gravitational field eliminates gravity from the equation.
  • by TropicalCoder ( 898500 ) on Friday June 15, 2007 @12:43PM (#19521643) Homepage Journal

    The picture shows a beautiful shot of the perfect silicon sphere. Out of curiosity, I looked very closly at the scene reflected by it's surface, thinking perhaps I might get a glimpse of the photographer. However, he was nowhere to be seen.

    Then I got to thinking - it should be easy to reconstruct the scene that is portrayed in the reflection from the surface of the sphere. All that is needed is to cut out the image of the silicon sphere and paste onto the surface of a three dimensional sphere. Then we could rotate it this way and that and look around the scientist's lab. So I did this - using a software simulation. I cut out the silicon sphere from the article's photo, and used it as a texture on a spherical 3D mesh, and added a little code to rotate it back and forth so that I could look around the scientist's lab. Guess what - there is no sign of the photographer! What we see is a very messy lab, with a closed door on the right. There are florescent fixtures on the ceiling that are currently turned off. There is a large window at the end of the room. I do believe that the ceiling, though it meets the left wall at the usual 90 degree angle, curves down to the wall at the right - a very unusual space, as if it was crammed into to an attic. At the extreme right of the room I believe we see a curtain hastily thrown over whatever would have been on the right side of the view. If the photographer is in the room, as he must be, I think he must be kneeling to the left of the window about three-quarters of the way back, and using a telephoto lens.

    I have made available the exe that I created on my web site so that you may take a look for yourself. The code is a hasty adaptation of Microsoft's DX3D mesh tutorial "Tut_06Meshes" from the DxSDK 9.0, which is also included. You can get the zip package here []. Perhaps you could modify the code to produce an even better view, but unfortunately, the resolution of the original image is really too low to get much out of it. It was a lot of fun doing this, and if you come up with a better result than me I would like to hear from you.

Economists state their GNP growth projections to the nearest tenth of a percentage point to prove they have a sense of humor. -- Edgar R. Fiedler