---
author: Veritasium
date: '2026-11-04'
guest: ''
layout: post.njk
source: https://www.youtube.com/watch?v=KfODG5REyss
speaker: Veritasium
tags:
- superfluidity
- liquid-helium
- bose-einstein-condensate
- zero-point-energy
title: 超流氦的发现历程与物理机制
summary: 文章回顾了从气体液化到卡末林·昂内斯液化氦气的探索历程，展现了液氦在2.17开尔文下转变为超流体的奇异特性，并探讨了艾伦、迈斯纳、卡皮查与朗道等科学家通过双流体模型与量子激发对其零粘度、爬壁、热反流及常压下无法冻结等物理现象的理论解释。
insight: ''
draft: true
series: ''
category: science
area: knowledge-meta
project: []
people:
- 海克·卡末林·昂内斯
- 迈克尔·法拉第
- 詹姆斯·杜瓦
- 彼得·卡皮查
- 列夫·朗道
- 杰克·艾伦
- 唐·迈斯纳
- 阿尔伯特·爱因斯坦
companies_orgs:
- 诺丁汉大学
- 欧洲核子研究中心
products_models:
- 大型强子对撞机
media_books:
- 《科学革命的结构》
- 《自然》
status: evergreen
---
### 超流氦的奇特现象
- 这是液氦。
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- This is liquid helium.
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It's currently sitting at
negative 269 degrees Celsius.
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That's just four degrees
above absolute zero.
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But watch what happens if
you start cooling it down.
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It starts to boil violently.
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- That's aggressive.
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- There are bubbles everywhere.
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That is until 2.17 Kelvin.
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At that point, something strange happens.
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The helium has transformed
into a superfluid.
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- Ooh. It's completely still again.
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- What we've got is a beaker
with a leak tight aerogel base.
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- This beaker holds
the liquid helium well,
but the moment it turns superfluid, it starts to leak
through.
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- Oh, the transition below is
happening. Yeah, it's dripping.
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- Yeah, it's dripping on the right.
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- And the colder it gets,
the faster it pours.
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- Oh wow. That's gorgeous. So fast.
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- That's spectacular.
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Superfluid helium can do things that seem impossible.
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If you put it in a cup,
a thin film climbs up
and over the walls, so it empties itself.
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You can also use superfluid
helium to create a fountain
with no moving parts.
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It is powered solely by light.
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Scientists spent 30 years
looking at this liquid without
realizing what they had created.
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- This is really one
of the great phenomenon
in 20th century physics,
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- And it makes you look at
matter in a different way.
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It allows us to explore
the boundary of quantum
and regular matter.
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So we teamed up with the
gravity laboratory at Nottingham
University to recreate the experiments
and to find out what really
happens when you just
keep cooling down.
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The obsession with cooling
things down as much
as possible began in the 1800s.
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People noticed that when you
let a gas expand against a
piston, its temperature drops.
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That's because as the
particles hit the piston,
they give it a little push
and transfer some of their
energy to the piston.
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So the particles slow down and
their temperature decreases.
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Lord Kelvin learned of this
and realized it offered a
tantalizing possibility, that
of an absolute temperature scale.
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Imagine that you let the
gas expand an extreme amount
to the point where all the
particles effectively stop.
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The particles no longer
exert any force on the piston
and they have no more energy left to give.
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This would be the absolute
coldest you could get,
absolute zero.
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Kelvin quickly computed what
temperature it corresponded to
and found it was around
negative 273 degrees Celsius.
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People speculated
what strange things might
happen down there, a place
with no thermal motion.
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Kelvin himself suggested that metals,
which are normally great
conductors of electricity,
might turn into perfect insulators
and not conduct any electricity at all.
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But there was only one way to find out
and that was you had to get there.
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Fortunately, the race
to absolute zero was
already well underway.
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It had started by
accident 25 years earlier.
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In 1823, Michael Faraday
was playing around
with chlorine hydrate
and his scientific mentor suggested
that heating it in a
sealed tube might lead
to some interesting results.
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So Faraday sealed it
up, warmed it in water,
and watched as a yellow
atmosphere emerged.
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Then as the pressure increased
to around four atmospheres,
he noticed that a bright
yellow fluid formed.
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He had just turned chlorine
gas into chlorine liquid.
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Faraday then went on a
bit of a liquefying spree,
liquefying six more
gases within five weeks.
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But just as he was hitting
his stride, he hit a wall.
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Some gases like ethylene
refused to liquefy no matter
how much he squeezed them.
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You see why you need to zoom into a gas.
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There you'd find a bunch of
particles whizzing around
as fast as tiny bullets.
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And while it might not seem like it,
the particles also attract each other.
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It's just that they're so spread out
that they meet relatively little,
and when they do, they're going so fast
that they mostly just
bounce off each other.
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In some cases though, if you
squeeze the particles together
by increasing the pressure,
you can get those particles
to stick together and
slide past each other.
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In other words, you get a liquid.
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This is what happened
with Faraday's chlorine,
but that doesn't work for all gases.
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If the attraction between
particles isn't strong enough
and the particles are
too hot, then no amount
of squeezing can get them to liquefy.
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And this is what happened with ethylene.
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Luckily, there is another way.
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If you cool the gas down,
the particles will slow
and eventually they
start sticking together
and sliding past each other
and you get your liquid.
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So to liquefy more tricky gases,
Faraday needed colder temperatures
and he got a lucky break.
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A French inventor had
discovered how to make dry ice,
which was about negative
79 degrees Celsius.
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So 20 years after his initial chlorine
success, he tried again.
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Faraday put ethylene gas in
a glass tube with a bend,
compressed it using pumps,
and then lowered the bend into
a bath of dry ice and ether.
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As the dry ice cooled, the ethylene,
a clear colorless liquid formed.
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Feraday used the same technique
and liquefied five more
gases pursuing ever colder
temperatures in the process
and getting all the way down
to negative 110 degrees Celsius
or 163 Kelvin.
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But six gases still resisted
his attempts, including oxygen,
hydrogen and nitrogen.
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He just couldn't get cold enough.
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Over the following decades,
scientists all over the world picked up
where Faraday had left off.
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Liquefying a new gas became
a matter of national pride
and the race to absolute zero was on.
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In 1883, two Polish scientists
liquefied oxygen and nitrogen
and the record low
temperature fell to 77 Kelvin.
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15 years later, James Dewar
liquified hydrogen at 20 Kelvin
before freezing it at 14.
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By the turn of the 20th century,
there was just one gas
left standing, helium,
but some thought helium might
be impossible to liquefy.
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And there was good reason to think that
because to turn a gas into
a liquid, its particles have
to attract each other.
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But helium is a very simple element.
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Its most common isotope
consists of two protons,
two neutrons and two electrons.
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Electrons form shells around the nucleus
and helium, like all noble
gases, has a full outer shell.
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So it isn't really seeking to
gain or lose any electrons,
and as a result, it doesn't really
bind together with anything.
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That made people wonder
whether helium atoms
attract each other at all.
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And if they didn't, you could
cool down helium as much
as you wanted, but you
would never get a liquid.
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And even if the atoms did
attract each other, well,
the attraction might be so small
that its liquefaction temperature
could be much colder than
anyone could practically reach.
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So when Haiku Kamerlingh Onnes
set his sights on liquefying
helium, he knew he was in for a challenge.
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Onnes had joined the race
to absolute zero in 1882,
and since then he'd been
building an industrial scale
laboratory in Leiden, complete
with chemists, technicians,
glassblowers, and massive liquefiers.
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By 1906, he too had liquefied hydrogen,
and from then on, he
dedicated himself fully
to liquefying helium.
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But working with helium came
with two major problems.
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For one thing, just
getting it was difficult.
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Remember, helium doesn't
react with anything,
so there are no compounds
you can get helium from,
and the air only contains
around five parts per million.
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So on earth, there's almost no helium,
which is at least a little surprising
because in the universe,
helium is everywhere.
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It's the second most abundant
element after hydrogen.
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The issue is just how light helium is.
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Release any helium and
it mixes into the air.
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Over hundreds of thousands
of years, it slowly drifts up
to the top of the atmosphere
where it gets heated up
by the sun and it speeds up.
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Now, a small fraction ends
up going faster than 11
kilometers per second, which
is Earth's escape velocity,
the velocity required to escape
Earth's gravitational well.
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And so it escapes into space.
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A much larger fraction gets
ionized by solar radiation
and then gets carried away
by the polar winds along
the Earth's magnetic field
before ultimately merging
with the solar wind.
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And either way, the outcome is the same.
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The helium is lost to space forever.
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So if I pop this balloon, that
helium is effectively gone.
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As a result, pure helium at
the time was hard to come by
and the world's total
supply was likely less than
50 party balloons worth.
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So Onnes had to get his helium
from a different source
and he turned to sand,
specifically two large bags
of monazite sand.
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That's because monazite is a
mineral that contains around 5%
thorium by weight and
thorium is radioactive.
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When it decays, it spits
out an alpha particle,
which is two protons and two neutrons.
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In other words, the nucleus of helium.
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That nucleus then gets slowed
down by the surrounding rock
where it steals two electrons
and becomes a helium atom.
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So over millions of years,
you get this buildup
of helium atoms trapped inside the rock.
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In fact, basically all the
helium we have on earth today
comes from radioactive decay.
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It's essentially a radioactive fossil,
although now we mostly get
it from natural gas wells
where it accumulates over the eons.
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But Onnes didn't have that luxury.
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He had to liberate it from the sand.
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So he recruited four chemists.
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They heated up the sand to
release the gas, exploded it
with oxygen, cooled it with liquid air
to freeze out impurities,
burned over copper oxide
to get rid of the hydrogen,
and last of all,
compressed it over charcoal
to absorb any remaining impurities.
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After several months, Onnes
had about 25 party balloons
of pure helium, an estimated
80% of the world supply.
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Onnes quickly conducted
careful experiments to try
and tease out whether helium
atoms attract each other at all.
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- Kamerlingh Onnes did a lot of work
and I think he was becoming more
and more convinced that helium
can in fact be liquefied.
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- It seemed like the helium
atoms did attract each other
just ever so slightly.
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So how do two neutral
atoms that don't react
with anything attract each other?
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Well, a helium atom is just
a nucleus with two electrons
around it, but in reality
you never know exactly
where those electrons are.
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So a more accurate picture
looks like this, a nucleus
with an electron cloud around it.
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But that cloud isn't always
perfectly evenly distributed.
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At any particular instant,
there's slightly more negative charge
on one side than the other.
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And now if there's another
helium atom close enough, well,
this charge imballence can
induce a charge imballence
on that atom.
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And now the two exert a force
of attraction on each other.
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This is known as a van der Waals force,
and it's why every atom
attracts every other if
you get it close enough.
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But helium holds its two
electrons very tightly,
so the cloud hardly budges
and the van der Waals force
is tiny, the smallest
of any element, but
perhaps it's just enough.
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The second problem with helium
is that no one knew exactly
how cold it would have to get
to liquefy, only that it had
to be colder than liquid
hydrogen, possibly much colder.
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And that's the issue
because the rate at which
heat transfers in from the
environment depends on the
temperature difference.
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The bigger the gap, the more heat flows in
and cooling something down
to possibly a few degrees
above absolute zero while the
room it's in is 290 degrees
above it, well, that's a humongous gap.
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Fortunately, Dewar had
already invented one
key piece of apparatus.
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Essentially, one glass
flask nested inside another
with a vacuum in between.
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This limited heat conduction
to a small connection point at the top.
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Convection got ruled out
entirely due to the vacuum
and optionally by covering the insides
with the silver coating, you
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could also reflect radiation.
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This reduced heat losses
by a factor of 30,
and it's the basic design behind
every thermos flask today.
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But for Onnes, it still wasn't good enough.
So he nested several Dewar
flasks inside each other.
The helium would go in
the inner one, a bath
of liquid hydrogen would sit around that,
and a bath of liquid air around that.
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So each layer acted as a
heat shield for the next,
and the helium would see a wall
of 20 Kelvin instead of 300,
cutting the radiated heat
coming in by a factor of 50,000.
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To measure the temperature
inside the inner vessel,
Onnes inserted a small glass bulb filled
with dilute helium gas
held at a constant volume.
A thin capillary connected this to
what was essentially a pressure
meter outside the liquefier,
because for a gas at a constant volume,
the temperature is directly
proportional to the pressure.
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So if you first measure
the baseline pressure at a
temperature you know, like
the freezing point of water
around 273 Kelvin, you
could set that to 100%.
Then if you later measure a
pressure that's only 10% of
that amount, then the
temperature must also be 10%
of the original or around 27 Kelvin.
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By using this trick,
HOnnes could measure the
temperature even at a few degrees
above absolute zero.
Now in their case, they
read off a level of mercury,
but for simplicity, we'll animate
it as a temperature scale.
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Onnes finally had everything ready.
At Professor Weinfurtner's
gravity laboratory, we got
to see the setup we'd use.
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- Is this also what it would
have looked like back in the
day when Onnes was doing it?
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- Yeah, so this is actually
a really similar setup
to Kamerlingh Onnes.
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- The biggest difference
is that we only have two
Dewars nested inside each other.
The outer one will be
filled with liquid nitrogen
and the inner one is
where the helium will go.
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By the 10th of July 1908,
Onnes decided to go for it.
He arrived at his lab around 5:30 AM
where his technicians
were already hard at work.
They prepared 75 liters of
liquid air the day before
and Onnes now ordered them to
start making liquid hydrogen.
By 1:30 PM they had 20
liters, which was enough
to launch their attack on helium.
They started by filling the outer vessels.
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- First step, we're
going to fill the outer
Dewar with liquid nitrogen.
That's going to precool
the whole system down
to about 77 Kelvin.
Once we're full, we'll
start filling with helium.
So we've used about 25
liters of nitrogen to get to that point.
Now consider how big the
Dewar is, that probably holds,
you know, less than 10 liters.
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- With the liquid air and
hydrogen in place, they started
to cool the liquefier.
After 30 minutes, it
had cooled to 93 Kelvin.
Now it was time to start
cycling the helium.
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A pump compressed it to 100 atmospheres
before it went to the liquefier,
where it got precooled to
around 15 Kelvin.
The gas was then rapidly
expanded into the inner vessel,
greatly increasing the
distance between atoms.
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But remember, helium atoms do
slightly attract each other,
so moving them apart costs energy
and they pay for this with
some of their kinetic energy,
causing the particles to slow down
and their temperature to drop.
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The gas then got pumped
back to the compressor,
but the smart thing was that
they let this colder gas cool
the incoming helium,
slightly lowering its
temperature before it expanded.
And by doing this over
and over, the incoming
helium got colder and colder.
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- It's so faint.
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- Yeah.
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- It feels like a ghostly
fluid or something.
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- You know, it's not
liquid that's escaping.
It's more of kind of a super dense vapor.
It's only 10 degrees inside,
but it's dropping fast.
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- A little after 6:30 PM, Onnes
and his team had gotten
down to six degrees
above absolute zero.
That is closer than anyone had ever gotten
before, and they kept going.
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But just below five Kelvin, the
temperature stopped falling,
so they started to worry that
perhaps something was wrong.
But then a chemist in the room
remarked, "It's so stable,
it's almost as if the thermometer
is submerged in a bath
of liquid." So they
rushed to the apparatus,
but still saw nothing
until Onnes grabbed a torch
and shot it from a low angle,
and they saw a reflection.
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After the surface had once been seen,
it was no more lost sight of, Onnes wrote,
"It stood out sharply
defined like the edge
of a knife against a glass wall.
They had just made liquid
helium."
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- That's really cool.
It's just hitting the light
on it while it's yeah.
So beautiful. It's a,
it's a helium kettle.
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- Four degrees above absolute zero.
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- You can see that really
faint liquid level on the
inside. That's the liquid helium.
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- Oh wow. Okay. So the,
the refractive index
of liquid helium is very
close to air, is it?
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- 1.025. Okay.
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- So yeah, really, really
difficult to see by eye. Wow.
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- Isn't it crazy to think
that that is about four
degrees above absolute
zero? It doesn't look like.
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- It doesn't, there's nothing
that would tip you off.
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- Yeah. Back in Leiden,
everyone was elated.
For the next hour, they
ran basic experiments until
by 8:30 PM only 10 millilitres were left.
But then Onnes thought, "Well,
we've made liquid helium.
Why not make solid helium?" So
he was going to cool it down,
and the way you can cool down any liquid
is by pumping on it.
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- When we pump on it, we're removing all
of the hottest atoms from the system,
which takes about a
third of the fluid away.
So we initially take the level
way higher than we need it
so it can drop down.
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- So Onnes started his pumps and so did we.
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- Are we ready?
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- Three, two, one, go.
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- Oh,
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- Wow. Okay.
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- Oh wow. It's gone down so much.
It's gone down like,
what, five centimeters?
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- Yeah, immediately.
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- Immediately.
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- Because we're removing all the
hottest atoms from the system.
So we're just shy of 3.5K.
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- This rapidly evaporating
helium is quickly cooling down.
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- So we're about 2.6K. We're below 2.3K.
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- Okay.
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- Oh,
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- It's going super fast, super
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- Fast.
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- It keeps boiling until
right at 2.17 Kelvin,
it abruptly stops
and the liquid goes
eerily quiet. There it is.
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- Ooh. It's completely still again.
Man, you can hardly see it. It
feels like a mirror surface.
It's crazy how smooth that is.
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- Onnes pumped his helium
down to 1.7 Kelvin,
colder than outer space,
but it didn't freeze.
Instead, it just sat
there, remarkably still.
Onnes concluded it must
just freeze at some lower
temperature, and since
everyone was exhausted,
they called it a day.
Little did he know what sat
there evaporating in his flask.
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Onnes still wanted to freeze helium,
so over the next two years,
he tried several more times,
getting all the way down to one Kelvin,
but all his attempts failed.
This led him to wonder
whether helium will not remain
perhaps liquid even if it is
cooled to the absolute zero.
And indeed, scientists later
discovered this is true.
If you take any substance
and keep cooling it down,
eventually it always freezes,
except for helium.
Somehow it's the only
substance that never freezes
by just cooling it down.
Original English
So Onnes was wise when he gave up on
trying to freeze helium.
Instead, he set out to finally
test Kelvin's prediction
that the electrical conductivity
of metals should stop near absolute zero.
In other words, that their
electrical resistance should
become effectively infinite.
To do this, he submerged
different metals in liquid helium
and measured their resistance.
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He started with gold and platinum,
but their resistance just
kept slowly decreasing
until finally settling on a steady value.
Then on the 8th of April,
1911, he measured mercury
and wrote down just three words
mercury practically zero.
As you cooled down, its
resistance slowly decreased until
around 4.2 Kelvin, where
it suddenly plummets to
what appeared to be zero.
Electricity that flowed
with no apparent losses,
the exact opposite of what
Kelvin had hypothesized,
Onnes had just discovered
superconductivity.
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- So that's the story most people know.
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- Okay.
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- What most people
don't know is that later
that same day, he also
cooled down the helium more
and he wrote, "Right
before the helium reached
the coldest temperature,
which was 1.8 Kelvin, the
boiling suddenly stopped."
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- He noticed.
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- He noticed.
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- So what did Onnes do
after seeing this sudden change?
Almost nothing. He guessed at an ordinary
explanation and moved on.
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But helium kept dropping clues.
When they measured its density,
they discovered it had a
curious peak around 2.17 Kelvin.
And when they measured the
latent heat, they found that
around the same temperature,
something happened
to the helium, which was
perhaps even discontinuous.
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Yet the biggest smoking gun
of all happened when they
measured a property known
as the specific heat, which
is how much energy it takes
to heat up one gram by one kelvin.
They found that as the
temperature dropped,
the specific heat declined
until they approached 2.17 Kelvin,
where it suddenly shot up.
This was as clear a sign as any
that something was
happening at this point.
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- But they knew what this was.
This was condensation happening, you know,
experimental error.
So they had these points,
but they, they wrote like,
"We felt justified in disregarding
these irregular points,
and they only published
the part above 2.61 Kelvin,
which neatly fit the
formula they had for it."
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- Onnes brushed off hint
after hint, even while seeing
it happen with his own eyes.
One day, Onnes put one
container of liquid helium
inside another, and he noticed
that below 2.17 Kelvin,
if one level started higher,
it always quickly evened out.
He even remarked how
strikingly fast it happened.
He thought it was too
fast for distillation,
but since he had no better
explanation, he stuck with that.
If only he had raised his container,
he might have discovered the truth.
Here, we've got a polymer
cup. This is our container.
Original English
- So we got the bowl loaded up.
It's way at the bottom, and I
think what we're going to try
and do is lift it up.
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- Yeah. The colder we get, the
better this effect is going
to be, and it's a super subtle
effect at the best of times.
We want to get down to about 1.5, 1.55 K.
I'm going to try and raise it
just above the fluid surface.
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- Whoa.
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- Can you see that?
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- Yeah, yeah,
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- Yeah. Yeah.
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- Yeah, no, you see like waves almost.
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- Yeah.
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- Yeah. Ah, you can
really see a pulse down there.
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- If you look at the bottom,
you can see it's dripping.
And if we speed it up, you
can see it keeps dripping.
More and more liquid helium
is going to the bottom.
What's happening is that somehow
the helium is climbing up
and over the walls, literally
emptying itself out.
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This was not distillation.
Something far stranger was happening.
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Onnes's successors grew suspicious
of the specific heat
data, so they remeasured
and they became convinced
it wasn't an error.
This was real. Something was
happening at 2.17 Kelvin.
Since the curve resembled
the Greek letter Lambda,
they called this the Lambda point
and they suggested that a phase
transition must be happening
here, but not an ordinary
phase transition.
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This would be a phase
transition to a more exotic kind
of matter, from a normal
liquid to something else.
But what?
Original English
Figuring out what's really happening when there are so many different
clues can be confusing.
If not, outright impossible.
Original English
The same is still true today.
Recently, I came across this
story where scientists used AI
to design viruses not found in nature,
but depending on where you read the story,
you might have a completely
different reaction.
If you read IFLS first,
you might think this is an
exciting scientific breakthrough,
AI designing a virus
that can kill bacteria.
Original English
If you'd seen this first,
you'd get a more complete,
albeit complex picture,
16 viruses from scratch,
but with possible downsides.
Original English
And if you saw this New
York Post article first,
you'd most likely focus
on what could go wrong.
Original English
So three headlines,
but three completely
different conclusions.
Original English
Most people just get their
news from one source,
and as a result, their takeaway
is a little bit like the
luck of the draw, which outlet
did you happen to see first?
Original English
That's why we use today's
sponsor, Ground News.
They're a website, an app
that brings a data-driven
and objective approach
to reading the news.
Original English
They pull thousands of stories daily,
and for each, they break
down the political leaning,
how factual the reporting is,
and who owns the reporting.
Original English
For example, with the AI virus story,
it immediately tells you
that over 200 news outlets reported on it
and that it got covered fairly evenly
across the political spectrum.
Original English
Below that, you can also see
the factuality, ownership,
and even who broke the news
first, Forbes in this case.
Original English
The bias comparison feature
highlights specific differences
in the contents of the articles,
but my favorite feature might
be their blind spot feed.
It shows you stories that are
under-reported by one side
of the political spectrum or the other.
Original English
Trying to figure all these
differences out by yourself
for every story would be a full-time job.
That's what makes Ground News so valuable.
Their data-driven approach
helps make these stories
digestible in seconds.
Original English
So if you, like us, care
about getting to the truth, go
to ground.news/ve or scan this QR code.
Our link gets you 40%
off their vantage plan.
Original English
I'd like to thank Ground
News for sponsoring this part
of the video, and now back to
what was happening when you
keep cooling down helium.
The answer started to take
shape throughout the 1930s.
Original English
- So in 1945,
there was this graduate
student called Don Misener,
and he needed to do some experiments
for his coursework to get enough credit.
And he was aware of everything
that was happening in helium,
but he realized that no one
had measured its viscosity.
So he's like, "Oh, that's
weird. I will measure it."
Original English
- So how do you measure the viscosity?
Original English
- There are two ways. You have a disc
and you try to see what the
resistance of the liquid is.
Another way is to have a small
capillary, and then you push
the liquid through that,
and the viscosity you measure
between the two methods
should be exactly the same.
Original English
- Misener chose a variation
of the first method.
He submerged a cylinder that was connected
through a torsion fiber in liquid helium.
He then rotated the cylinder
and let go, causing the
cylinder to swing back
and forth to oscillate.
Original English
Now, by watching how long it oscillated,
he could determine the viscosity.
The longer it oscillated,
the less friction it must
experience, so the lower the viscosity.
Original English
And what he found was that at 4.2 Kelvin,
the viscosity was about 14%.
That of air, already tiny.
Then, as he decreased the temperature,
the viscosity slowly increased
until at the transition point,
it suddenly plummeted, not
to zero, but to about 5% that of air.
Original English
This caught the attention of Jack Allen, a
27-year-old physicist who'd
just come to Cambridge.
Allen had inherited a helium
liquefier from someone
who could no longer use it,
and now he resolved to use it
to unravel the mysteries of helium.
Original English
Of course, he could use some help,
and Misener seemed like
the perfect PhD student,
so Allen convinced him,
and together, they continued
studying helium's viscosity.
Only now they turn to the
second method of studying it,
which works something like this.
Original English
I've got a tube filled with
two layers of glycerol.
Now, if I let it flow,
what's going to happen?
Well, you'd expect the layers
to drop, which they do,
but not in a straightforward way.
It doesn't maintain its shape.
Instead, it stretches out in the middle.
Original English
What's happening is that
if you zoom in, you'd find
that the molecules near
the wall get stuck,
they experience friction,
and then the molecules
sliding past them experience
friction with those molecules.
The internal friction is
basically what viscosity is.
Original English
In general, if you make
the channel smaller,
the flow rate should decrease,
and if you push on the
liquid, it should increase.
Now, by carefully keeping track of
how things change when you
alter these parameters,
you can determine the viscosity.
Original English
In November of 1937, Allen
and Misener did a version
of this experiment
with liquid helium at around one kelvin.
They let it flow through
one wide capillary
and one narrow one, and they
found that the flow speed
through both was almost the same,
despite the narrow one
having a surface area a
thousand times as small.
Original English
Even weirder was
that the speed hardly changed
when the pressure increased.
They concluded that any known
formula cannot, from our data,
give a value of the viscosity,
which would have much meaning.
As far as they could tell, the
viscosity was non-existent.
Original English
- They know they've got gold
on their hands here, right?
They know this is a, like
a bombshell of a result.
Original English
- So they spent the next three
weeks making sure they hadn't
made any mistakes.
They repeated all the experiments
and did all the other measurements they
needed, ready to write it up.
Original English
- But then on the last day, one
of Kapitsa's old students walks in
with a letter from Kapitsa.
Original English
- "Who's Kapitsa? Well,
remember that allen had inherited
that liquefier from someone
who couldn't use it anymore?
Well, that was Soviet
physicist Pyotr Kapitsa.
He built the liquefier and
ran the lab before allen.
Original English
But then during a trip back
home in the summer of 1934,
Stalin had his passport confiscated.
He was to work and stay
in the Soviet Union, where
for the past few years, Kapitsa
had been rebuilding his lab,
telling almost no one exactly
what he was working on.
Original English
But Allen and Misener
were about to find out.
Original English
- He had done the same sort of experiment,
found the viscosity was vanishingly small,
and send a note to the editor of Nature
that he would be very grateful
if they could publish it
as soon as possible.
Don't bother to send the
proofs to me here to Moscow.
It takes too much time. He had
just scooped this old laugh.
Original English
- He knew what he had
and he raced to publish.
Allen and Misener were freaking out,
but over the next four days,
they wrote up all their results
and also sent it to nature.
Original English
On the 8th of January, 1938,
the two papers appeared side
by side, reporting the
vanishingly small viscosity,
perhaps even zero.
And then in analogy to superconductors,
Kapitsa called it a superfluid.
Original English
This was the breakthrough
everyone had been waiting for
because it finally began to explain some
of the strange things they'd been seeing.
Above 2.17 Kelvin, helium
is just a normal fluid
and it has internal friction
or viscosity, so when it flows
through tiny cracks, part
of it gets stuck to the walls.
Original English
Now, in the case of the
bucket, the bottom is made
of a polymer aerogel, which is filled
with microscopic channels up
to tens of nanometers across.
That's so small
that the helium gets fully
clamped to the walls.
It can't flow through.
Original English
But as you cool down below 2.17 Kelvin,
the helium transforms,
and now it can suddenly
pass through tiny cracks
with no friction, so it leaks
straight through the bottom.
Original English
It also explained what was
happening with the little cup.
How? Well, remember
those van der Waals forces
that cause every atom
to attract every other?
Well, between helium
atoms, they're very weak,
but the atoms in the walls
of the cup have many more
electrons in their outer shell,
so the charge imballences become larger.
Original English
And as a result, the helium
atoms are much more strongly
drawn to the wall than to each other.
So they climb up the wall
and within seconds wet the
entire surface with a layer
that's around 30 nanometers thick.
Original English
And because the bottom
of this thin sheet is
below the liquid layer,
it now acts as a siphon.
Normal fluid can't flow through
because it experiences friction,
but superfluid can, so it flows up
and over the walls until
it drips out the bottom.
Original English
But for all the idea of
a superfluid explained,
it also raised a much
more fundamental question.
How do you get a superfluid?
Original English
- Like if it is actually zero,
I want you to think about
how crazy that is, right?
Because if we think back to,
okay, what is this viscosity?
It's internal friction
between the molecules.
So if you have actually
zero viscosity, it means
that you have like a billion,
billion particles all
traveling through this pipe
and not a single one is
bouncing off of anything
and losing some energy or bouncing into
each other and losing energy.
They all must flow perfectly
and somehow not lose a
single bit of energy.
It seems preposterous.
Original English
- Within two months of the nature papers,
Fritz London proposed
a possible mechanism.
There was this forgotten
paper from 13 years earlier.
It predicted that if you just
kept cooling down a certain
type of particle, then
the atoms would fall
into an exotic state.
And the author of that
forgotten paper was none other
than Albert Einstein.
Original English
To understand what Einstein had predicted,
consider this plane of particles.
They're not interacting
and they're free to move
around in every direction
with different velocities and
different kinetic energies.
Original English
Now at room temperature,
you can just think of them
as point particles,
but if you start cooling them down,
what Einstein realized is that, well,
this is no longer the case
because now quantum theory
starts to play a role.
And according to that
theory, particles aren't just
particles, they also have
a wave-like character.
Original English
- So-called wave particle
duality, the quantum
wave function tells you the likelihood of
where it would be found if you were
to try to measure its location.
Original English
- The important thing here
is that the wavelength
of these waves or how
spread out they are is equal
to planks constant over their momentum
or mass times velocity.
Original English
- So if you're cooling
these particles down,
they on average have less
and less momentum per particle,
their quantum wavelength
or the de Broglie wavelength gets longer.
And so eventually the sort
of quantum state associated
with each of those seemingly
independent particles starts
to smoosh out and overlap.
This distinguishability in
space starts to melt away.
Original English
- What happens next depends on which kind
of particle you're cooling down.
Every particle in the
universe is one of two kinds.
It's either a boson or a fermion.
Fermions are most of the
particles you're familiar with,
like protons, neutrons and electrons,
and bosons are particles like photons.
Original English
You can see the difference between bosons
and fermions by looking at two
indistinguishable particles.
Now, one particle just
has its own wave function,
but if you take two of them together,
something special happens.
They get a combined wave function
that describes both of them together.
Original English
In the case of bosons, if you
could see that wave function,
you'd see it's the same on both sides,
that is it's symmetric.
But if those particles are fermions,
then their combined wave function is the
opposite on both sides.
It's anti-symmetric.
Original English
Now I want you to think about
what this means if you try
to move those particles together.
In the case of the bosons, the two sides
of the wave function add up
and you just get two bosons
in the same place or state.
Original English
But now try doing the same for fermions.
You get one peak to
overlap with one valley
and they'd cancel out.
But that would suggest that all
of a sudden there's a 0% probability
of finding the particles anywhere,
which doesn't make any sense.
They have to be somewhere. So
Original English
what this is really telling us is
that two indistinguishable
fermions can never occupy
the same quantum state.
This is known as Pauli's
exclusion principle,
and it's why particles like
electrons can never be in
exactly the same state.
If the particles you're
cooling down are fermions,
then they start filling up the
energy states like a ladder
from lowest to the higher states,
with only one fermion per quantum state.
Original English
But bosons don't have this restriction.
They're very happy to share
the same quantum state.
When you keep cooling them down, more
and more of them start to occupy lower
and lower energy levels.
And Einstein realized there comes a point
where a macroscopic portion
suddenly condenses into the
lowest energy state
or ground state, which
in this case is just when
they're completely still.
Original English
- These little particles
could actually act in a way
where they would sort of
lose, so to speak, their,
their individuality, that they could act
as a single coherent state
of matter, a whole new state
of matter that had no analog.
Original English
- This became known as a
bose Einstein condensate
after Bose whose work
Einstein was building on.
But Einstein doubted this
could happen in real life.
He wrote, "The theory is pretty,
but is there also some truth
to it?" 13 years later,
London believed there might be
and that perhaps superfluid helium was a
bose Einstein condensate.
Original English
At first, that might seem impossible
because helium is made
up of protons, neutrons,
and electrons, and every
one of those is a fermion.
So you'd expect helium
to be a fermion too.
And remember, fermions
can't share the same state,
but helium is not a fermion.
bosons and fermions differ
in one other fundamental way,
their intrinsic angular momentum or spin.
For bosons, this is always
an integer number, zero, one,
two, three, and so on.
But fermions always have a
half integer spin, like a half.
Original English
So what happens if you
have multiple fermions
together, say in one atom?
Well, now you have to
add their spins together
to get an overall spin for the atom,
and then use that to see whether
it's a bozon or a fermion.
In the case of helium, which
has six half spin particles,
then no matter how you add them up,
you always end up with an integer number.
Original English
As a result, helium
atoms behave as bosons.
And as you cool them
down below 2.17 Kelvin,
a macroscopic fraction
of the atoms condense into
the lowest energy state.
They lose their individuality
and about a trillion trillion
atoms all behave as one.
Normally, you think of wave functions
as these teeny tiny things,
but here it's literally
the size of the container.
It's as if after the transition,
the helium has turned into
one big atom with all these
atoms behaving as one.
Original English
- This is quantum mechanics
with your own eyes.
Original English
- And it makes you look at
matter in a different way.
It allows us to explore
the boundary of quantum
and regular matter.
Original English
So it seems like we've solved it.
The atoms are in the lowest energy state.
They've lost their individuality
and act as one, so they can't scatter
and they flow with zero internal friction.
That's how I first understood superfluids
and how I'd explain it to
anyone who wanted to know.
I mean, it made sense and we're all happy.
The only problem is that it's wrong.
Original English
I mean, the first issue is that in a BEC,
all the atoms occupy the state
where the particles are at rest,
but in a superfluid, well, they flow.
Maybe that issue could be excused
because you could imagine moving
along with the superfluid
and finding a frame in which
it effectively stands still,
but there's an even
bigger problem, which is
that in a non-interacting
Bows Einstein condensate,
even the tiniest push can
excite any of the atoms.
So if you'd try to let it flow,
the smallest disturbance would knock
atoms out of the ground state.
They would scatter and the
flow would lose energy.
So clearly a BEC alone isn't enough.
You need something else, but what?
Original English
Fortunately, Kapitsa knew
exactly who could solve the problem.
Unfortunately, he had been locked
inside Stalin's prison for the last year.
Original English
- This was right around the
time of Stalin's great terror
where he was executing
or imprisoning all the enemies of state,
whether they were real or imaginary.
Original English
- But in the case of Lev Landau,
the physicist Kapitsa was thinking about,
it seems like there was some truth to it.
Landau was known as a genius theorist
who cared deeply about
elevating Soviet physics.
He was the head of the physics department
and personally trained some
of the best physicists of the century.
But around the mid 1930s,
he became disillusioned
with Stalin's regime.
He reportedly told one colleague
that Soviet officials were
"born subhuman." And according
to KGB Files, he was accused
of heading a counter-revolutionary
organization.
Original English
Landau was arrested on
the 28th of April 1938
and taken to the Lubyanka Prison.
The first day he was forced
to stand for seven hours,
followed by six days of
12-hour persuasion sessions.
After two months, Landau broke
and wrote a six-page confession.
It reads, "We came to the conclusion
that the party had degenerated
and that the Soviet government
no longer acted in the
interests of workers, but in the interests
of a small ruling group.
The interests of the country
demanded the overthrow
of the existing government."
Original English
- That's a pretty,
pretty strong confession.
Original English
- This confession at a time
where people were being executed
for no reason should have
meant the end for Landau.
But fortunately for him, he
had someone defending him.
The morning Landau got arrested,
Kapitsa immediately wrote to Stalin.
His character, to put it simply, is nasty.
He is a bully and a brawler,
but I find it very hard to
believe that Landau was capable
of anything disOnnes.
Landau is young and he still has much
to accomplish in science.
No one else can write about all of this,
which is why I'm writing to you.
Original English
Now, Stalin never replied.
So a year later, Kapitsa wrote to one
of Stalin's closest allies.
He told him how he had
discovered superfluidity
and that no theorist other
than Landau could explain it.
Three weeks later, Kapitsa
was summoned to the prison.
As he recalled, "I was
summoned at 1:00 AM.
They escorted me to a big
office occupied by two men.
Do you understand for
whom you are pleading?
He's a most dangerous
criminal, a spy who confessed
to everything." We talked
till four in the morning.
Towards the end, one of them
said, "All right, Kapitsa,
if you pledge your word for Landau,
then give us a written guarantee.
If anything happens, you
will be held responsible."
Original English
Kapitsa wrote the letter
and two days later, Landau
turned up at the institute, ready
to get back to what he
loved doing most, physics.
Kapitsa had put his neck on the line.
Now it was up to Landau to
prove he was up to the task,
and superfluid helium had
only gotten more interesting.
Original English
Back in January 1938, just days
after the nature papers, allen was studying
how superfluid helium conducts heat.
To do this, he packed some Emery powder,
basically fine sand, in a
glass bulb with an open bottom.
To see what was happening, he
shone his pocket torch at it.
And according to him, as the
Emery powder tube came into
view, helium spurted
out the top of the tube.
It continued to spurt as long
as the light shone on the powder
and ceased when a card cut off the light.
allen later showed this
fountain with no moving parts
to the Royal Society, during
which famous writer H.G.
Wells crawled under the table looking
for a hidden pump, but found nothing.
Original English
- Yeah, so we had this
kind of specialist made.
It's a single piece of glass.
There are no seams in it whatsoever.
The filtered disc is fused in,
and then it's just filled
with carbon stones,
and we just hit it with a bright light
that heats it up, drives
the fountain flow.
Original English
- Amazing. Fingers
crossed. You guys ready?
Three, two, one. Torch it.
Woah! Yeah.
Let's go. What?
That's absurd.
Original English
- That's awesome. Woo. Let's go.
Original English
- So what was happening?
How could light cause
the helium to shoot out?
Original English
- Any theory now had to take
into consideration all these phenomena.
So for theoreticians
was a really heavy task.
Original English
- Landau spent the next
eight years working on it.
He started by considering a
superfluid flowing at absolute
zero, and he realized
what London had left out,
the forces between atoms.
In a Bose Einstein condensate,
there are no forces
between particles, and as a result,
a push can knock a single atom out
of the lowest energy state,
resulting in easy friction.
But in liquid helium,
this is not the case.
The atoms interact, so if you push on one,
you're pushing on a bunch of them.
Original English
Landau realized that this rigidity meant
that the only way you can
create excitations in a superfluid
and lose energy in the flow is
through the collective
motion of the atoms.
He argued that these excitations
come in two main flavors.
The first is that if you push
hard enough, you get the atoms
to bump into each other
and create ripples throughout the fluid,
basically sound waves,
just a quantized version
of them called phonons.
Similar to how photons
are particles of light,
these are particles of sound.
Original English
The second kind of excitation
is something known as a roton,
because originally people
thought of them as tiny whirls,
but now they're seeing
more like a patch of atoms
that suddenly orders themselves
into a crystal lattice just
for a split second.
You can think of them almost
as tiny ghosts of a solid.
And just like a phonon
creating a roton costs both
energy and momentum.
So Landau argued that these phonons
and rotons were the only way
that the flow could lose energy.
Original English
- And you're like, okay,
well, it's quite easy,
you know, to make a sound wave.
Like all I have to do
is knock on it, right?
That in terms of like
how much energy it takes
to create a phonon, it
can be very little energy.
It can be as little energy as you want,
but you don't just have
to conserve energy.
You also have to conserve momentum.
So when you push on it
or when the liquid bumps
into something, it both needs
to satisfy conservation of energy
and conservation of momentum.
Original English
And there is a very nice
relationship that tells you exactly
how the momentum of a phonon
corresponds to the energy
of a phonon, and it's
connected with the speed
of sound in a material.
And so the only way you can
satisfy both constraints is if
the flow is going at
least the speed of sound,
which in helium two is very fast.
It's about, I think, 220
or 240 meters per second.
Rotons, he realizes, can get
created a little bit easier.
I think the modern value is about 60
meters per second, but it's still a lot.
Original English
- So in Landau's theory, below
about 60 meters per second,
the flow can't create a
single phonon or roton.
The atoms all stay in the ground state
and no energy is lost.
So it's not that the
friction is vanishingly
small in a superfluid.
Landau proved that it's strictly zero.
Friction is forbidden.
In practice, other excitations
are created at lower speeds
and therefore the actual flow
rate at which superfluidity
breaks down is lower.
Original English
But if the friction is actually zero,
that implies something else.
You should be able to get a
superfluid flowing in a loop,
and once it's going, it
should just keep going like a
perpetual motion machine.
In 1964, two physicists
at Yale tested this.
They got superfluid
helium circling in a stack
of thin rings and waited.
And what they found was
that no matter whether
they waited five minutes
or more than 12 hours, the
flow was just as strong.
Original English
- But then, you know,
if the friction is zero,
how is it possible that Misener, you know,
when he did his like torsion
experiment, he didn't measure zero.
Original English
- The liquid wasn't just
passing over liquid,
like you had like a cylinder
of something in there?
Original English
- Yep. - Yep. I don't know.
Is it coming from the wire?
Is it coming from the cylinder?
Like the different materials?
What is it coming from?
Original English
- Your instinct is
that Misener must have
done something wrong.
He didn't. Both experiments
are true. How can both be true?
Original English
- You're measuring different things.
I don't know, but how are you
measuring different things?
Original English
This is where the second part
of Landau's work comes in.
So far, Landau had only looked
at a fluid at absolute zero,
but every real experiment
happened at higher temperatures.
So next he considered what if it flows at
some finite temperature?
To do this, he built on the
work of Tisza, who suggested
that helium below 2.17 Kelvin acts
as a mixture of two fluids.
One is the superfluid. It's
effectively at absolute zero.
It carries no heat and
flows without friction.
You can think of this
as the background fluid.
And the second is the normal
fluid, which is made up
of the phonons and rotons.
It carries all the heat and has viscosity.
At absolute zero, the
whole fluid is superfluid,
but as it heats up, part
of the superfluid
turns into normal fluid.
The superfluid fraction decreases
and the normal fraction
increases until at 2.17 Kelvin,
the superfluid component is zero.
Original English
- Now, if you were to look
at one atom in the fluid
and say, "Is that superfluid
or normal fluid?" Well, it's hard to tell
because the superfluid atoms
are constantly exchanging
energy with the normal fluid atoms.
Original English
- Sure. - So any one atom
is constantly bouncing up
and down between these two modes.
Original English
- Now, only the superfluid
can flow through tiny channels.
So when Allen and Misener pushed helium
through their capillaries,
they only measured the super
fluid part and they
found a zero viscosity.
But Misener's torsion experiment sat in a
mixture of both fluids.
So the cylinder felt the
viscosity from the normal fluid.
Misener hadn't made a mistake,
they were just measuring different things.
And once you know what you're looking for,
you can also physically see the effect.
Just below the transition
temperature, say at 2.15 Kelvin,
only around 13% of the
helium is superfluid.
So only that 13% can leak
through the bottom and you get a drip.
But if you cool it down
to 1.5 Kelvin, well now
around 89% is superfluid.
That's almost seven times
as much helium that can flow
through, so it just pours out the bottom.
Original English
It even explains how light
can power a fountain.
When the light hits the
rocks, the rocks absorb some
of the photons and heat up a little
Original English
- Bit.
Original English
- This causes part of the superfluid
to turn into normal fluid,
but now the superfluid density is lower
inside the fountain, so more
superfluid immediately rushes
in through the bottom to
restore the proportions.
That bottom, however, is just
like the bottom of the bucket.
It only allows superfluid through.
So the normal fluid can't go out that way.
The only way the excess
helium can go is up and out.
And so you end up with this,
a fountain powered solely by light.
The heat from the light gets
turned into the mechanical
energy of the spouting helium.
Original English
But the two fluids do
more than make fountains.
They change how helium transfers heat.
Take a bath of liquid helium
below the lambda point
with a heater on one side.
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- The helium atoms that are in
contact with that hot face -
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- Yeah.
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- Are going to gain energy. They're
going to jump up this ladder.
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- Yeah. - So they're going to
be part of the normal fluid.
Original English
- Right.
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- So my normal fluid is going
to flow away from the heater,
carrying all of the heat
and the entropy that's
imparted into the fluid away.
Original English
- Yeah. - But as the normal
fluid flows away - Yeah.
The superfluid flows
back to replace it. Right.
And the overall density of
the fluid remains constant.
Original English
- Sure.
Original English
- So this is a two-fluid
thermomechanical counterflow.
They basically flow through each other
like they weren't even there.
Original English
- But you can take this one step further.
Imagine changing the temperature
of the heat source periodically.
What is going to happen?
Well, the amount of
normal fluid created will
also change periodically.
So you get these fluctuations
in the normal fluid
and superfluid densities.
They spread out and travel away,
almost like a sort of sound.
Original English
- When we think of first
sound, regular sound - Yeah.
That's density waves in the air driven
by tiny pressure gradients.
Original English
- But with a superfluid, you
can get density waves in the
normal and superfluid components.
And since the normal fluid
carries all the heat,
that means you get temperature waves.
Original English
- And they can spread
out like sound waves.
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- Yeah.
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- So we call temperature waves
in helium second sound.
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- The crazy thing about it
is it's a completely new mode
of heat transport, unlike
anything else in the uni - right?
Yeah. When I heard that, I was like,
"That's crazy." So you
always think of conduction,
convection, and radiation.
And now helium gets a fourth.
Original English
- This is a combination of second sound
and thermal counterflow,
and it is effective,
around a thousand times
more conductive than copper,
which is one of the best
thermal conductors we know of.
It finally helps explain one
last thing you've been seeing
this whole time, that is why
does the boiling suddenly stop?
Well, above 2.17 Kelvin,
helium boils just like a normal liquid.
Original English
- Say, for example, I've
got a bucket of water
and I take a red hot
poker and I dump it in.
The bubbles aren't going to
form anywhere in the bucket.
They're going to form around that poker
because you've got this
high temperature region
where you've dumped in
enough energy locally
to vaporize the water
and then that's where the
bubbles are going to form.
The amount of energy it takes
to do this is the latent
heat of vaporization.
In water, that is about
2,200 joules per gram.
In helium, it's about 23 joules per gram.
So if you've got helium
in its liquid state,
a tiny thermal fluctuation is enough
to locally vaporize a small pocket,
which then forms a bubble.
And that's why boiling is so constant,
because tiny thermal
fluctuations will form
bubbles very, very easily.
Original English
- But as you cool helium
down below 2.17 kelvin,
thermal counterflows take over.
They transfer heat so effectively
that the entire liquid almost immediately
reaches the same temperature.
So there are no hotspots
and no places for bubbles to form,
the boiling suddenly stops.
This lack of boiling paired
with its unparalleled heat
transfer also makes superfluid
helium the ideal coolant
at one or two kelvin,
and that's why CERN uses
around 700,000 liters of it
to cool the LHC's 9,000
superconducting magnets.
Original English
- There is still one thing
that bothers me though, Chris,
which is that, you know,
if you take any substance
and you keep cooling it
down, it always freezes.
But helium never does, yet
it does liquefy, which
is the annoying part.
So because it liquefies, it means
that there is some
attraction between the atoms.
And so you'd expect, you know,
I can just keep cooling
it down more and more
and more until eventually
the kinetic energy becomes
arbitrarily small
and it must, like the attractive
energy must be much larger
and so it must form a solid.
Original English
- Yes. So this is the kind of
intuitive picture that you would expect.
Well, you can't answer
that in an intuitive way.
You have to bring in quantum mechanics.
Original English
- Because according to quantum mechanics,
the more precisely you
know a particle's position,
the less you know about its
momentum and vice versa.
This is known as Heisenberg's
uncertainty principle,
and mathematically it tells you
that the uncertainty in
position delta x times the
uncertainty in momentum
delta p is greater than
or equal to h-bar over two.
For simplicity, let's just
say it's around h-bar.
Original English
- Let's say I've got an atom in a liquid
and it has some freedom.
It can move around,
but you know, it's very closely packs in
with all of its neighboring atoms.
Let's say that it occupies
some spherical volume
with a radius r.
Well, we can say that that r
is The uncertainty in its position.
So if the uncertainty in the
momentum is going to look like
h-bar, planks constant, over R.
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- But if it's got an
uncertainty in momentum,
it means it must be jiggling
around inside that sphere.
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- We know if it's jiggling,
we can say it's got an energy.
Well, we know what it's momentum is.
It's planks constant over, over R.
So how do we get the
energy? Well, it's easy.
We say it's the momentum
squared over twice the mass.
Now, helium being such a light atom,
that mass turn is very small.
So that means that the energy that it has,
because it's confined,
is very, very large.
Now, this is what we call
the zero point energy.
This is the energy it cannot get rid of.
It's nothing to do with thermal energy.
It's not because there's a
temperature in the liquid.
In fact, when you're in the liquid state,
that zero point energy
accounts for more than 98%
of the total energy in the fluid.
Original English
- Wait, what?
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- It is, it is enormous.
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- That is crazy. I had no clue.
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- This zero point energy is
too large for the atoms ever
to lock down in place.
So helium never freezes
just by cooling it down.
Original English
- So when you're thinking of
this kind of ladder of states,
this kind of BEC picture, like, you know,
everything's macroscopically
occupying this bottom state,
we think we, the, the
intuition is to think of it
as like ladder where each rung
is nice and evenly spaced.
But the way to think of it in terms
of this zero point energy is that
that first rung is 10
stories off the floor.
And then the thermal states
stack up on top of that.
Original English
- Oh, that's so good.
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- But there is one way in which you
can freeze helium, which was
discovered by Onnes's successor.
That is by cooling helium
down to about one Kelvin
and by increasing the
pressure to 25 atmospheres.
If you do that, helium finally freezes.
As for Onnes, he died in
1926 at the age of 72.
He had been making super
fluid helium for 18 years,
looking at it, seeing hint after hint,
and yet he never realized
what he'd been staring at the entire time.
Now, you might look at this
and think HOnnes was a little
stupid or ignorant to miss it
and to not put the pieces together,
but I don't think that's the case.
Instead, I think it reveals a fundamental
feature of being human.
I think it goes to show a lot
of people think you got
to see it to believe it.
And I think often you have to
believe it in order to see it.
It makes me think a lot
about Kuhns, the structure
of scientific revolutions,
because it focuses a lot
on, you know, you have the
existing science, you
have the existing paradigms,
you have the existing formulas
that these things are supposed to follow.
And how often would the
data point going off the
line just be some error,
someone messed up, condensation,
something happening?
It's easy to write those things off.
So I have a lot of sympathy
and I don't think, "Oh, how
could they miss it?" I kind
of think like, "No, that's actually
how we operate." Especially, I
don't know if scientists are supposed
to be a little bit more observant
or a little bit more skeptical,
but at the same time, you're up against
so much chaos in the
world that you do kind
of sometimes just see,
"Yeah, yeah, that fits that.
Yeah, that fits the data."
And they sort of ignore things
that don't fit because you're looking
for something. I don't know.
Original English
- Yeah. But I think probably
especially if you are a
scientist, it's these unexpected things
that can end up having
the biggest breakthrough
because those are the
things that disconfirm
your view of reality.
Original English
- And the thing is, often all the signs
Original English
for the breakthrough are there.
It's just that we're
not ready to see them.
Original English
So it makes you wonder what
signs are we missing right now?
Original English
I used to think that the
reason someone didn't make the
breakthrough was because maybe
they weren't smart enough
or they didn't work hard enough
or didn't get lucky enough.
Original English
But now I feel like I've
always missed something.
Just look at Onnes. He
had all of those things.
He was an experimental genius,
had an immense work ethic
and even discovered
super connectivity, one
of the biggest breakthroughs
of the 20th century.
He got three out of three.
Original English
So perhaps sometimes making the
breakthrough doesn't require
any extra intelligence,
hard work or even luck.
Maybe all it takes is for us to let go
of the way we believe the world to be
and instead, just for a moment, try
to see it as it really is.
What if that is all it takes?
Original English
And Onnes, well, he may
have missed superfluidity,
but he changed cold
temperature physics forever.
And in 1913, he received
the Nobel Prize in physics
for his achievements at finding out
what happens when you
just keep cooling down.
Original English
Hey, two quick final things.
First, this video would not
have been possible without the
help of Professor Weinfurtner's
Gravity Laboratory at
Nottingham University and the funding
provided to them to run their
superfluid experiments.
So I want to give a big
thank you to all of them
for helping us recreate these experiments.
Original English
The second thing is
that we didn't even get
to show you their research
because while it might seem
like we've solved all the
mysteries of superfluids,
it turns out we're just getting started
because really weird things happen when
you start rotating them.
Original English
And this rotation
or the physics of it can play
a role in everything from
neutron stars to simulating
rotating black holes
to a black hole bomb.
And because if you look
closely enough, you can kind
of see the entire universe
in a drop of superfluid.
So let us know if you want us
to make a part two on that.