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  • Can I hold it?

  • Only if you promise to be really, really careful.

  • I promise I will be so incredibly careful. I will be incredibly careful with it. I promise.

  • So, it's slippery, be careful. Alright, are we ready?

  • I'm about to touch a 1kg sphere of silicon-28 atoms. There are about 2.15x10^25 of them.

  • It feels absolutely incredible. Wow, that is amazing.

  • Besides its creators, I am one of only a handful of people ever to hold this sphere.

  • The raw material used to make it was worth 1 million Euros but now that it has been so

  • precisely sculpted --

  • How much is that worth?

  • It's priceless. ... This you are looking at now is the roundest object in the world.

  • How can you say for sure it's the roundest object? I mean the Earth is pretty round,

  • isn't it? If this was the Earth...

  • If this were the Earth then the highest mountain to the lowest valley would be... about 14m

  • apart.

  • That is shocking. That is shockingly round.

  • But why would you invest one million Euros and thousands of man-hours perfecting a pure,

  • polished silicon sphere?

  • Well the answer is grave. Or rather 'grave' as it would have been pronounced in the original

  • French.

  • You see the grave was the original name for the base unit of mass in the metric system,

  • which became the Systeme International d'unites or SI units. In 1793, a commision which included

  • notable scientist and aristocrat Antoine Lavoisier, defined the base unit of mass as the weight

  • of a cubic decimeter of water at the melting temperature of ice -- essentially just a litre

  • of ice water. The name grave came from the Latin gravitas, meaning weight.

  • But it wasn't to last. It sounded too similar to the aristocratic title 'graf' -- which

  • is the equivalent of an earl or a count. And with the French revolution in full swing with

  • the rallying cry of equality for all, you couldn't exactly have one unit nobler than

  • the others. At this Lavoisier lost his head, literally, not because he helped devise one

  • of the greatest systems of measurement of all time, but because he was collecting taxes

  • as a nobleman.

  • So things really were grave.

  • The new republican government believed a grave would be too big for the things they wanted

  • to measure anyway and and so they settled on the gramme, which was just a thousandth

  • of the grave.

  • But soon they realized that a gram was too small and so they returned to the grave, but

  • since they couldn't call it that, they invented the kilogram -- a thousand grams. And that

  • is why out of the seven base SI units, the kilogram is the only one to have a prefix

  • in its name.

  • In 1799 the kilogram definition was refined to be the mass of a litre of water at 4 degrees

  • Celcius -- the temperature at which it is densest. But water itself is obviously not

  • the most sensible thing to use as a mass standard. So a pure platinum cylinder was created to

  • have the same mass as the water definition and it was declared Kilogram of the Archives.

  • Now it's important to note at this point the kilogram is no longer tied to the mass of

  • a volume of water -- the kilogram of the archives is by definition THE kilogram.

  • 90 years later, in 1889 the kilogram was upgraded to a platinum-iridium alloy cylinder. Now

  • it was much harder than the original but was otherwise basically identical. And to this

  • day, it remains the definition of the kilogram. It is officially called the International

  • Prototype Kilogram, though it's affectionately known as Le Grand K -- or Big K. Oh, and it's

  • about this big...

  • It is the only thing in the entire universe with a mass of exactly one kilogram because

  • it IS the kilogram. It is also the only SI unit that is still defined by a physical object.

  • It sits under three bell jars, next to six sister kilograms, in a climate-controlled

  • vault locked by three independently controlled keys, in the basement of the International

  • Bureau of Weights and Measures on the outskirts of Paris.

  • Now if you were able to break into the vault and tamper with Big K, you would be changing

  • the definition of the kilogram, a definition on which many of our measurements rely, and

  • so you would throw the world into chaos! Well no, not actually-- but how would anyone ever

  • know if the mass of Big K changed?

  • Well when it was first created, 40 identical replicas were also made. Well they weren't

  • quite identical - they had a mass which was slightly different to Big K but those offsets

  • were recorded. Now these replicas were sent out to countries around the world to serve

  • as their national standards.

  • In 1948 the kilograms were reunited for a weigh-in. And this is when the problems started.

  • Because even though all the cylinders were made of the same alloy and stored under virtually

  • the same conditions, their masses had diverged over time. The mass of Big K wasn't even the

  • same as the six sister cylinders stored with it. And to make matters worse when they were

  • brought together again forty years later, their masses had further diverged, up to about

  • 50 micrograms - that's about the weight of a fingerprint. But fingerprints were not the

  • culprits since the kilograms were carefully washed before their weigh-ins.

  • So some physical process must have actually changed the mass of the cylinders, but how

  • that exactly works remains a matter of speculation. One this is for certain, the mass of a platinum-iridium

  • cylinder is not stable over time. And this is a big problem. You can't have a unit which

  • changes its value. And the fallout isn't limited to measurements

  • of mass since of the seven base SI units, four of them depend on the mass of the kilogram,

  • not to mention all the derived units like Newtons, Joules, Volts and Watts.

  • At this point those of you in countries that have not adopted the metric system--yes I'm

  • speaking to you Liberia, Burma, and the US--you may be feeling rather smug that your unit

  • of mass, the avoirdupois pound, is no longer defined by a physical object. No, instead

  • it is defined as precisely 0.45359237

  • kilograms. Sucked in.

  • So clearly something needs to be done to eliminate the kilogram's dependence on a physical object

  • and this is where the silicon sphere comes in, but how exactly does that help?

  • Here you have a physical object and it's beautiful but you know it's still a physical object.

  • You're trying to get away from that. We're trying to get away from the physical

  • object but what we're doing with this particular object is counting how many atoms are in there.

  • You can't actually count how many are in there can you?

  • You can't count how many are in there but you can calculate how many are in there because

  • this material is silicon, there's no voids or dislocations.

  • So this is like a perfect crystal of silicon. That's right.

  • Not only is it pure silicon, it contains only one isotope of silicon, silicon-28, and that

  • explains why the original material was so expensive.

  • And why a sphere? Well, a sphere is a pretty simple object.

  • If you know the diameter of the sphere you can characterise the entire dimension of the

  • object. Well that explains why the sphere has to be

  • the roundest object ever created, but how do you actually make something that round?

  • We actually start with an oversized sphere. So it was about two millimetres larger in

  • diameter and then we just grind it progressively finer and finer using abrasive. It's actually

  • massaging atoms. You're down at that level of trying to control the shape of an object

  • down at the atomic level. But making the sphere is only half the battle,

  • then you need to accurately measure its diameter. The diameter is actually measured via a laser.

  • So you're actually measuring having the sphere in the centre of a cavity and a laser is hitting

  • both sides and you're actually measuring the gap.

  • By knowing the diameter you can determine its volume. And since the atom spacing in

  • silicon is known to high precision, you can the calculate how many atoms make up the sphere.

  • This allows you to redefine Avogadro's constant. At the moment, Avogadro's constant is defined

  • based on the kilogram. It is equal to the number of atoms in twelve grams of carbon

  • 12. But using this approach, the number of silicon atoms in the sphere would be used

  • to fix Avogadro's constant, which would then define the kilogram.

  • So even if the silicon spheres were lost or damaged, it would have no effect on the definition

  • of the kilogram because it would be defined not by a physical object but by a concept.

  • You would like to see the official definition of the kilogram say "a kilogram is the mass

  • of 2.15x10^25 silicon-28 atoms" Yes.

  • Is it - is it going to happen? There's a likelihood, a high likelihood that

  • it's going to happen. But there is another approach to redefining

  • the kilogram which involves fixing Planck's constant and it's done using something called

  • a Watt Balance. These two approaches are complimentary. Each one provides a check on the other, and

  • if they show good agreement and are able to bring their uncertainties down to about twenty

  • micrograms they may redefine the kilogram as early as 2014. And then the kilogram finally

  • will be an unchanging unit, no longer defined by a physical object in the basement vault

  • of some place in Paris. Now if the kilogram was originally intended

  • to be the mass of a litre of water at its densest temperature then how well did we do?

  • Well if you look at a litre of water at nearly four degrees Celcius it has a mass of 999.975

  • grams. So I guess you could look at this two ways. On the one hand you could say the kilogram

  • is slightly heavier than it should be, but on the other hand 214 years ago, scientists

  • were able to create an artifact that was correct within the margin of error of a grain of rice.

  • Now that is truly remarkable. Now if you want to hear more about the Watt Balance, let me

  • know in the comments and I will see what I can do. It does seem to be the frontrunner

  • in terms of redefining the kilogram, so we will have to wait and see what happens. One

  • last thing, I should point out that it took an international collaboration of scientists

  • to create the silicon sphere but don't you think that the scientist who originally conceived

  • of silicon as an element should receive some of the credit. Well in 1787, that was none

  • other than Antoine Lavoisier. So he's been involved in the definition of a kilogram from

  • start to finish or from cradle to grave.

Can I hold it?

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B1 US kilogram sphere silicon object physical object grave

World's Roundest Object!

  • 149 6
    Tony posted on 2014/06/23
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