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 2.15×10^25 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 2.15×10^25 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.