Kind: captions
Language: en
[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
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