The Roundest Object:
I’m about to touch a one kilogram sphere of silicon 28 atoms they’re about 2.15 times 10 to the 25 of them. It feels absolutely incredible. 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 it has been so precisely sculpted. It’s priceless, this you’re looking at now is the roundest object in the world. How can you say for sure that it’s the roundest object? I mean the earth is pretty grand isn’t it. That is shockingly round.
Reason behind everything :
Why would you invest 1 million euros and thousands of man hours in perfecting a pure polished silicon sphere? Well, the answer is grey or rather grav, as it would have been pronounced in the original french. You see the graph was the original name for the base unit of mass in the metric system. Which became the systeme international dunite or SI units. In 1793 a commission which included notable scientists 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 liter of ice water. The name grav came from the latin gravitas meaning weight but it wasn’t to last, it sounded too similar to the aristocratic title graph which is the equivalent of an earl or account and with the french revolution. In full swing with a 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 grav would be too big for the things. They wanted to measure anyway and so they settled on a gram which was just a thousandth of a grav but soon they realized that a gram was too small and so they returned to the grav. But since they couldn’t call it that they invented the kilogram, a thousand grams and that is why out of the seven base six 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 liter of water at four degrees celsius. 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. 90 years later in 1889 the kilogram was upgraded to a platinum iridium alloy cylinder. It was much harder than the original but was otherwise basically identical and to this day it remains the definition of the kilogram.
The Importance of it:
It is officially called the international prototype kilogram though it’s affectionately known as legrand k or big k and it’s about this big k. It is the only thing in the entire universe with a mass of exactly one kilogram because it is the kilogram. 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 actually 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.
In 1948 the kilograms were reunited for a weigh-in and this is when the problem 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. 40 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 thing 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 the seven base si units four of them depend on the mass of the kilogram. Not to mention all the derived units like newtons, jewels, volts and watts. At this point those of you in countries that have not adopted the metric system like liberia burma and the u.s. The avoid pound is no longer defined by a physical object, instead it is defined as precisely 90.45359237 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, but you can calculate how many are in there because this material is silicon. There’s no voids or dislocations. That’s right, not only is it pure silicon, it contains only one isotope of 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 characterize the entire dimension of the object. That explains why the sphere has to be the roundest object ever created but how do you actually make something that round? Actually we start with an oversized sphere, it is about two millimeters larger in diameter and then we just grind it progressively finer and finer using an 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 we need to accurately measure its diameter. The diameter is actually measured via a laser so you’re actually measuring. Halving the sphere in the center 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 of silicon is known to high precision you can then 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 12 grams of carbon-12. Which would then define the kilogram.
Without the mass:
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. If you would like to see the official definition of the kilogram, a kilogram is the mass of 2.15 times 10 to the 25 silicon 28 atoms. Yes it is, 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 complementary. Each one provides a check. On the other hand, if they show good agreement and are able to bring their uncertainties down to about 20 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 liter of water at its densest temperature, then how well did we do? Well, if you look at a liter of water at nearly 4 degrees celsius 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, it does seem to be the frontrunner in terms of redefining the kilogram.
One last thing I should point out is 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.