Kind: captions
Language: en
what do I have to push sub
basement I'm at the National Institute
of Standards and Technology near
Washington DC and I'm going to the sub
basement it's getting dark down here
we're going to find out how they're
going to redefine a
kilogram the kilogram is in trouble
since 1799 it's been defined as the mass
of a metal cylinder in a locked vault in
a basement in Paris but over the last
century careful measurements of this
International prototype kilogram and in
theory identical national standards from
around the world have shown that their
masses are diverging the spread has
grown to around 50 micrograms or 50
parts per billion and having a standard
of mass that changes is unacceptable
plus the kilogram is the last of the
base SI units to still be defined by a
physical object the meter for example
used to be defined as the length of a
platinum bar in Paris but in 1983 it was
redefined as the distance light travels
in 1 over
299792458 of a second this definition
means that the speed of light is set to
exactly
Etc
m/s note how this works first you take
the existing definition say the length
of that meter bar and you measure as
carefully as you can how it relates to a
physical constant of the universe the
speed of light then you set the exact
value of that constant and use it to
redefine how long a meter is I know this
might seem circular but importantly it
moves the point of Truth off of the
physical object and onto the unchanging
constant of the universe so naturally
the thought is to do the same thing with
the kilogram but using which constant
and
how well there were a number of
different strategies that were attempted
but the two that achieved the greatest
success were number one using a silicon
sphere to determine and set avagadro's
number and number two to use a watt
balance to determine and set planks
constant how you doing Derek pretty good
nice to meet you where is the W balance
the W balance is Behind These closed
doors and it's in there it's correct and
right now the problem is that um we are
in a crunch to get a number by the end
of May what's the number the Plank's
constant this is what we measure with
the w balance in 2011 the general
conference on weights and measures
decided that the kilogram should be
redefined based on Plank's constant but
that doesn't mean that the avagadro
approach was futile I mean you can use
avagadro's number to calculate Plank's
constant and vice versa so ultimately
both approaches are going to be used to
redefine Plank's constant and avator
number simultaneously one good thing
about having a zic sphere is that you
only want to redefine if you have
agreement between different numbers
right and the Silicon sphere method is a
method in my mind that comes out of
chemistry but the measure of Godus
constant just a constant that comes
comes out of chemistry this method comes
out of physics you measure pl's constant
so if they both agree is a pretty strong
sign right because you know chemistry
and physics agree now since I've already
discussed the avagadro approach in a
previous video here I want to focus on
the watt balance it's actually now
called a kibble balance in honor of its
inventor Brian kibble who actually
passed away in 2016 you know traditional
balances work by equating the
gravitational forces on objects in two
pans the kibble balance looks kind of
similar but all of the balancing happens
on the left hand side where a mass pan
is attached to a coil of wire in a
magnetic field on the right hand side is
a motor the whole apparatus is sealed
and operated in vacuum the balance
operates in two modes weighing mode and
velocity mode and both are required to
determine Plank's constant in weighing
mode a kilogram Mass standard is placed
on the mass pan and then current is
passed through the coil in the magnetic
field and adjust it until the weight of
the kilogram is equal and opposite to
the electromagnetic force on the the
coil the equation for this is mass time
the local gravitational acceleration is
equal to the magnetic field time the
length of wire in the coil times the
current flowing through it in this
equation the variables that are
difficult to measure exactly are the
magnetic field strength and the length
of wire in the coil but luckily the
kibble balance allows us to get around
this problem using velocity mode in
velocity mode the kilogram mass is
lifted off the mass pan and now the
motor on the other side of the balance
is used to move the coil back and forth
at constant velocity through the
magnetic field this motion induces a
voltage in the coil which is equal to
the magnetic field times the length of
wire in the coil times its velocity now
we have two equations which we can solve
for b * L and so we can set them equal
to each other and eliminate these
variables without having to know
precisely what their values are and if
we rearrange a little bit you get
voltage time current equals mass time
gravity time velocity on the left hand
side there is electrical power and on
the right hand side mechanical Power and
that's why this was called the watt the
unit of Power Balance but how do you go
from this to Plank's constant the number
that relates a photon's frequency to its
energy well it turns out there's
actually a way of measuring voltage
accurately using a macroscopic Quantum
effect that involves Joseph's
injunctions so a Joseph's injunction
consists of two superconductors
separated by a thin piece of insulator
now if you apply microwave radiation to
that Junction you create a voltage
across the device and its value is
precisely known to be HF over 2E where H
is planks constant f is the frequency of
the radiation and E is the charge on an
electron Now by tuning that frequency
and stacking as many of these Joseph
injunctions as you want in series you
can create virtually any voltage you
like very very precisely the way this is
used in the kibble balance is a stack of
hundreds of thousands of Joseph's
injunctions are put into the circuit
with the coil as it is moved through the
field and so you exactly balance the
voltage which is induced in the coil
using those Josephs injunctions so you
can measure that voltage very very
accurately but how do we measure current
well it turns out this voltage measuring
method is so good that instead of trying
to measure current directly we instead
measure V on R which is the same thing
so this current is passed through a
resistor and we measure that voltage
again using Joseph's injunctions and
then to measure resistance we use
another macroscopic quantum Effect
called the quantum Hall effect which is
beyond the scope of this video but
suffice it to say that the resistance
measurement will be an integer fraction
1 over P times Plank's constant divided
by the charge on the electron squared so
if we sub all of this into our equation
and solve for H we have that Plank's
constant is equal to 4 over pn^ 2 those
are all constant numbers that we know
times the local acceleration due to
gravity time velocity divid frequency
squar time the mass which is 1 kg so
here we have a very precise equation for
Plank's constant in terms of the mass of
1 kilogram now to get an answer that's
good to say 10 parts per billion you
need to know all of these values very
accurately so to measure V for example
the velocity of the coil as it moves
through the magnetic field we use a
laser interferometer as the distance to
the coil changes the interference
fringes pass over a detector and
essentially by counting how many fringes
go past in a certain period of time you
can determine the speed of the core oil
very accurately to measure g a device
called a gravimeter was used to map out
the local acceleration due to gravity in
the balance room before it was built in
there the gravimeter actually drops a
corner reflector down a vacuum tube and
measures its acceleration again through
interferometry counting the fringes as
they pass this is a 3D printed map of
the acceleration due to gravity in the
kibble balance room the bump is due to
the mass of the powerful and very heavy
permanent magnet that's in the balance
the acceleration gravity must
continually be measured because it can
be affected at this level of precision
by the positions of the Sun and Moon and
even the water table underneath the
building in 2018 the kilogram will no
longer be defined by an object in Paris
instead it will be defined based on the
fixed value of planks constant which is
being finalized right now as a result of
all these measurements from the kibble
balances and silicon spheres so right
now what we do is we put the mass in and
we get age out and 20 of the
redefinition age will be fixed and we
use that to realize you the
M easy yeah just that just that easy
yeah just that simple
simple hey this episode of veritasium
was supported in part by viewers Like
You on patreon and by audible who as you
probably know because they're longtime
supporters of the channel are leading
providers of spoken audio information
including Audi books original
programming news comedy and more and for
viewers of this channel they offer a
free 30-day trial just go to
audible.com/veritasium you know recently
I've been traveling around the world to
Israel London Mexico City and tomorrow
I'm off to New Orleans because I'm
shooting uh stuff for Netflix and what
I'll be listening to on the plane is
Steven Pinker the better angels of our
nature this is an awesome book that
takes a scientific and statistical
approach to the question of when is the
best time to be alive as a human and his
answer is now whether it seems like it
or not violence and all the terrible
things that humans have to deal with has
been on the decline for centuries and if
you want the statistical proof for that
you should check out this book it is
fantastic um and if you want you can
download it for free by going to
audible.com/veritasium or you can pick
any other book of your choosing for a
one-month free trial so I want to thank
Audible for supporting me and I want to
thank you for watching