Kind: captions
Language: en
thermometry is kind of a key safety
diagnostic to make sure that we're well
controlled thermometry thermometry what
if it gets too high here in trouble or
sodium expands when it eats the vessel
has a certain volume there's a
temperature above which it's over full
and it's better not to be over fault
what temperature is that uh I think it's
around a hundred and thirty degrees have
you ever been slows no no because of our
mama trees have we have mana tree and
backup from our tree and backup to the
backup the wrong tree let's back up and
explain what is this thing this is a
three meter sphere of molten sodium that
is 12.5 tons of hot liquid metal
designed to span it up to four
revolutions per second and at that rate
the outer surface of the sphere is
moving at over a hundred and thirty
kilometers an hour
that's over 80 miles per hour basically
it's representing the earth what they're
trying to do is find out how the Earth's
magnetic field is generated and some of
the first scientists to study this
starting with William Gilbert in 1600
proposed that the earth was a huge
permanent magnet now today we know that
the inner core of the earth is solid and
it's mostly made up of iron and nickel
which are ferromagnetic elements however
the temperature of the inner core is
nearly 6,000 Kelvin which is way above
their Curie temperatures and so there is
no way that they could maintain a
permanent magnetic field the same is
true of the solid mantle of the earth
much of it is way too hot for permanent
magnets which leaves only the liquid
metal outer core as the place where
Earth's magnetic fields could be
generated and this is what is modeled by
the 3-meter spinning sphere of molten
sodium it needs to be as large as
practicable it needs to be as high-speed
as practicable and we use liquid sodium
because it's the best electric conductor
of any liquid so it has the downsides of
being hazardous and flammable but the
failure would be modestly Cass trophic
it's gonna weird twist a phrase but it's
modestly catice most buildings have
sprinkler systems in case of fire but
not this lab to see why take a look at
a single drop of water falling on a
single drop of molten sodium now
multiply that by 12.5 tonnes and you
have modestly catastrophic but the
researchers have thought about this and
they have a plan there is a liquid
nitrogen Dewar that sits up top so one
of these mostly full you can just hit a
switch and dump the entire contents
liquid nitrogen
top of the experiment so this is like a
physicist super fire extinguisher then
you know it's back-up plan the plan is
no leak no fire the back-up plan
put the fire out with cryogenics it
actually works great this is three
hundred degrees colder than the sodium
and so sodium cannot burn nitrogen gas
atmosphere and the cold freezes it and
leak gets frozen and everything just
chills but when everything's working
normally how is a spinning sphere of
liquid metal meant to generate a
magnetic field well for the earth the
thinking goes like this first as the
earth radiates heat into space its core
cools this means the solid inner core is
growing as iron and nickel from the
outer core solidify at the boundary now
this leaves some lighter elements like
sulfur for example behind in the outer
core and so patches with those lighter
elements experience a buoyant force and
rise away from the court in turbulent
convection currents now add to this the
fact that the earth is rotating the
rotation causes those convection
currents to spiral around and access
roughly parallel with the Earth's axis
of rotation
these spiraling turbulent flows of
liquid metal can trap magnetic field
lines and as they move and stretch the
field they create electric currents that
produce more magnetic fields if the
magnetic fields are generated faster
than they dissipate due to the
resistance of the metal well then you
have a self-sustaining electric
generator or a dynamo we talk about the
dynamo generation of magnetic fields to
mean when you have a turbulent conductor
let's say convecting like the core
that's spontaneously generates a
magnetic field if you have a little bit
of current that happens it causes a
little bit of magnetic field that
couples into the motion
that causes more currents cause more
magnetic fields couples into the motion
and then you're just converting motion
to currents but it goes it goes unstable
it's kind of counterintuitive though
because you started out with no magnetic
field so how do you get the current in
the first place you don't I mean like no
no oh you know all you need is a little
infinitesimal amount a thermal variation
a random current will just grow to full
size so one Sun is unstable actually you
don't really care where the the tickle
came from the states we have now don't
generate their own magnetic field so
we've not reached the dynamo threshold
although the hope is to do so in a
modified version of the experiment so we
apply magnetic fields from the outside
and then look at the amplification in
those magnetic fields by the rotating
turbulence and we get a copious amounts
of amplification depending on where we
are in the operating conditions for
instance we apply these external
magnetic fields sort of go through the
experiment like that they get twisted up
by the flow and the magnetic fields in
the angular direction can be up to ten
times larger than the fields we apply so
we actually have lots of gain even
though we've not yet have a closed loop
dynamo so why is it important to
understand the Earth's magnetic field we
know the Earth's magnetic field is part
of what makes Earth a habitable planet
one of the ways is that it forms a
shield the magnetosphere which deflects
the worst parts of the solar storms
around but lately that shield appears to
be weakening so the Earth's magnetic
field was dropped 10% in the last 170
years we don't know why there's a very
weak spot of the Earth's magnetic field
in the South Atlantic the South Atlantic
anomaly that's growing weaker we do know
the Earth's magnetic field has reversed
many hundreds of times in its history we
don't know where it's going now we
actually don't currently have a
predictive science of those magnetic
field why is that important so the
Earth's magnetic field has whether it
has changes you might want to have a
forecast for planning purposes will this
spinning sphere make that forecasting
possible it's an experiment so no one
has run a device like this ever it's
world unique so you don't entirely know
what's gonna happen we run it but that's
why we do experiments so in fact we
don't know whether it's possible or not
to predict the Earth's future
magnetically
it's an open question Hey
so in about a week I am taking the 13
hour flight to Sydney
I am touring the country and launching a
new feature length documentary there's
more info in the description but on that
flight will be my wife and my two kids
who are 2 and under and I kind of have
these visions who her by if I do get a
moment to myself I am going to sit back
and relax with the sponsor of today's
video which is audible the leading
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Trevor Noah it's about his life growing
up in South Africa it opens with him
getting pushed out of a car by his
mother and it's actually read by Trevor
Noah and he is South African my family
comes from South Africa and I would love
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not to mention the fact that it is a
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you can return it and get that credit
back and if you don't use a credit in a
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month and if you ever cancel your
membership well all your books are still
yours to keep and you can listen to them
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stop because you've stopped your
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like about audible if you are out
traveling and you want to get some more
stories into your life I highly
recommend you give them a shot and just
go to audible.com slash veritasium or
text veritasium to 500 500 so I want to
thank audible for supporting me and I
want to thank you for watching
fingers crossed I will get some time to
listen to this book on the plane