World's Roundest Object!

Key Takeaways

[Applause] can I hold it I need you promise to be really really careful I promise I will be so incredibly careful I will be incredibly careful with it okay I promise all right so it's slip free be careful all right are you ready I'm about to touch a 1 kg sphere of silicon 28 atoms they're about 2.15 * 10 to the 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 but now that it has been so precisely sculpted how much is that worth it's Priceless this you're looking at now is the the roundest object in the world how can you say for sure that 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 and the lowest Valley would be about 14 M apart that is shocking that is shockingly round but why would you invest 1 million EUR and thousands of manh hours perfecting a pure polished silicon sphere well the answer is grave or rather grav as it would have been pronounced in the original French you see the grav was the original name for the base unit of mass in the metric system which became the system international dun or SI units in 1793 a commission which included notable scientist and Aristocrat Antoine lavier defined the base unit of mass as the weight of a cubic decimeter of water at the melting temperature of ice 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 a count 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 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 gra but since they couldn't call it that they invented the kilogram a th000 g and that is why out of the seven base 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 4° C 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 not 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 idium 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 L Grand C or Big K oh and it's about this big it is the only thing in the entire universe with a mass of exactly 1 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 kilg 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 are 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 rep 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 weighin 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 microgram 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 idium 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 Newton's Jewels 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 base unit of mass the avad dupa pound is no longer defined by a physical object now instead it is defined as precisely 0.45359237 kilg sucked in so clearly something needs to be done to eliminate the kilograms 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 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 characterize 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 2 mm larger in diameter and then we just grind it progressively finer and finer using um abrasive it's actually massaging atoms you're down at that level of 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 measured via a laser so you're actually measuring um having 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 avagadro's constant at the moment avagadro's constant is defined based on the kilogram it is equal to the number of atoms in 12 G of carbon 12 but using this approach the number of silicon atoms in the sphere would be used to fix avagadro 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 2.15 * 10 25 silicon 28 atoms yes is not there's a there's a likelihood a high likelihood that is going to happen but there is another approach to redefining the kilogram which involves fixing Plank'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 20 microG 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° C it has a mass of 9999975 G 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 W balance let me know in the comments and I will see what I can do it does seem to be the front runner 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 a 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 grab