YT YouTube2Text

YouTube2Text → Transcripts → I finally saw quantum mechanics with my own eyes... (It's spookier than I expected)

I finally saw quantum mechanics with my own eyes... (It's spookier than I expected) — Transcript

by Veritasium · 10,726 words · 1,407 segments · language en · Watch on YouTube

Open in transcriber Summarize with AI Download SRT/VTT

Full transcript

0:00- This is liquid helium.
0:01It's currently sitting at negative 269 degrees Celsius.
0:05That's just four degrees above absolute zero.
0:08But watch what happens if you start cooling it down.
0:13It starts to boil violently.
0:15- That's aggressive. - There are bubbles everywhere.
0:20That is until 2.17 Kelvin.
0:23At that point, something strange happens.
0:29The helium has transformed into a superfluid.
0:33- Ooh. It's completely still again.
0:36- What we've got is a beaker with a leak tight aerogel base.
0:42- This beaker holds the liquid helium well,
0:45but the moment it turns superfluid, it starts to leak
0:49through.
0:50- Oh, the transition below is
0:52happening. Yeah, it's dripping.
0:53- Yeah, it's dripping on the right.
0:56- And the colder it gets, the faster it pours.
1:00- Oh wow. That's gorgeous. So fast.
1:05- That's spectacular.
1:08Superfluid helium can do things that seem impossible.
1:11If you put it in a cup, a thin film climbs up
1:14and over the walls, so it empties itself.
1:18You can also use superfluid helium to create a fountain
1:21with no moving parts.
1:23It is powered solely by light.
1:26Scientists spent 30 years looking at this liquid without
1:28realizing what they had created.
1:30- This is really one of the great phenomenon
1:32in 20th century physics,
1:34- And it makes you look at matter in a different way.
1:36It allows us to explore the boundary of quantum
1:40and regular matter.
1:42So we teamed up with the gravity laboratory at Nottingham
1:44University to recreate the experiments
1:47and to find out what really happens when you just
1:50keep cooling down.
1:59The obsession with cooling things down as much
2:01as possible began in the 1800s.
2:04People noticed that when you let a gas expand against a
2:07piston, its temperature drops.
2:10That's because as the particles hit the piston,
2:12they give it a little push
2:13and transfer some of their energy to the piston.
2:16So the particles slow down and their temperature decreases.
2:21Lord Kelvin learned of this
2:22and realized it offered a tantalizing possibility, that
2:26of an absolute temperature scale.
2:30Imagine that you let the gas expand an extreme amount
2:33to the point where all the particles effectively stop.
2:37The particles no longer exert any force on the piston
2:40and they have no more energy left to give.
2:43This would be the absolute coldest you could get,
2:47absolute zero.
2:51Kelvin quickly computed what temperature it corresponded to
2:54and found it was around negative 273 degrees Celsius.
2:59People speculated
3:00what strange things might happen down there, a place
3:03with no thermal motion.
3:05Kelvin himself suggested that metals,
3:07which are normally great conductors of electricity,
3:10might turn into perfect insulators
3:12and not conduct any electricity at all.
3:15But there was only one way to find out
3:17and that was you had to get there.
3:20Fortunately, the race
3:21to absolute zero was already well underway.
3:25It had started by accident 25 years earlier.
3:28In 1823, Michael Faraday was playing around
3:31with chlorine hydrate
3:32and his scientific mentor suggested
3:34that heating it in a sealed tube might lead
3:37to some interesting results.
3:39So Faraday sealed it up, warmed it in water,
3:44and watched as a yellow atmosphere emerged.
3:48Then as the pressure increased to around four atmospheres,
3:51he noticed that a bright yellow fluid formed.
3:54He had just turned chlorine gas into chlorine liquid.
3:59Faraday then went on a bit of a liquefying spree,
4:02liquefying six more gases within five weeks.
4:05But just as he was hitting his stride, he hit a wall.
4:09Some gases like ethylene refused to liquefy no matter
4:13how much he squeezed them.
4:15To see why you need to zoom into a gas.
4:19There you'd find a bunch of particles whizzing around
4:21as fast as tiny bullets.
4:24And while it might not seem like it,
4:25the particles also attract each other.
4:29It's just that they're so spread out
4:30that they meet relatively little,
4:32and when they do, they're going so fast
4:34that they mostly just bounce off each other.
4:37In some cases though, if you squeeze the particles together
4:40by increasing the pressure, you can get those particles
4:43to stick together and slide past each other.
4:45In other words, you get a liquid.
4:47This is what happened with Faraday's chlorine,
4:50but that doesn't work for all gases.
4:52If the attraction between particles isn't strong enough
4:54and the particles are too hot, then no amount
4:57of squeezing can get them to liquefy.
5:00And this is what happened with ethylene.
5:03Luckily, there is another way.
5:06If you cool the gas down, the particles will slow
5:09and eventually they start sticking together
5:11and sliding past each other and you get your liquid.
5:16So to liquefy more tricky gases,
5:19Faraday needed colder temperatures and he got a lucky break.
5:23A French inventor had discovered how to make dry ice,
5:25which was about negative 79 degrees Celsius.
5:28So 20 years after his initial chlorine
5:31success, he tried again.
5:33Faraday put ethylene gas in a glass tube with a bend,
5:36compressed it using pumps,
5:38and then lowered the bend into a bath of dry ice and ether.
5:41As the dry ice cooled, the ethylene,
5:44a clear colorless liquid formed.
5:47Faraday used the same technique
5:48and liquefied five more gases pursuing ever colder
5:51temperatures in the process
5:53and getting all the way down to negative 110 degrees Celsius
5:56or 163 Kelvin.
5:58But six gases still resisted his attempts, including oxygen,
6:02hydrogen and nitrogen.
6:04He just couldn't get cold enough.
6:06Over the following decades,
6:08scientists all over the world picked up
6:10where Faraday had left off.
6:11Liquefying a new gas became a matter of national pride
6:16and the race to absolute zero was on.
6:20In 1883, two Polish scientists liquefied oxygen and nitrogen
6:24and the record low temperature fell to 77 Kelvin.
6:2715 years later, James Dewar liquified hydrogen at 20 Kelvin
6:31before freezing it at 14.
6:34By the turn of the 20th century,
6:36there was just one gas left standing, helium,
6:39but some thought helium might be impossible to liquefy.
6:43And there was good reason to think that
6:45because to turn a gas into a liquid, its particles have
6:48to attract each other.
6:50But helium is a very simple element.
6:53Its most common isotope consists of two protons,
6:56two neutrons and two electrons.
6:58Electrons form shells around the nucleus
7:00and helium, like all noble gases, has a full outer shell.
7:05So it isn't really seeking to gain or lose any electrons,
7:08and as a result, it doesn't really
7:10bind together with anything.
7:12That made people wonder whether helium atoms
7:15attract each other at all.
7:16And if they didn't, you could cool down helium as much
7:19as you wanted, but you would never get a liquid.
7:22And even if the atoms did attract each other, well,
7:25the attraction might be so small
7:27that its liquefaction temperature could be much colder than
7:30anyone could practically reach.
7:34So when Heike Kamerlingh Onnes set his sights on liquefying
7:38helium, he knew he was in for a challenge.
7:41Onnes had joined the race to absolute zero in 1882,
7:44and since then he'd been building an industrial scale
7:47laboratory in Leiden, complete with chemists, technicians,
7:51glassblowers, and massive liquefiers.
7:53By 1906, he too had liquefied hydrogen,
7:56and from then on, he dedicated himself fully
7:59to liquefying helium.
8:02But working with helium came with two major problems.
8:06For one thing, just getting it was difficult.
8:09Remember, helium doesn't react with anything,
8:12so there are no compounds you can get helium from,
8:15and the air only contains around five parts per million.
8:19So on earth, there's almost no helium,
8:22which is at least a little surprising
8:24because in the universe, helium is everywhere.
8:26It's the second most abundant element after hydrogen.
8:30The issue is just how light helium is.
8:33Release any helium and it mixes into the air.
8:36Over hundreds of thousands of years, it slowly drifts up
8:39to the top of the atmosphere where it gets heated up
8:42by the sun and it speeds up.
8:44Now, a small fraction ends up going faster than 11
8:47kilometers per second, which is Earth's escape velocity,
8:51the velocity required to escape Earth's gravitational well.
8:55And so it escapes into space.
8:57A much larger fraction gets ionized by solar radiation
9:00and then gets carried away
9:02by the polar winds along the Earth's magnetic field
9:04before ultimately merging with the solar wind.
9:07And either way, the outcome is the same.
9:10The helium is lost to space forever.
9:13So if I pop this balloon, that helium is effectively gone.
9:18As a result, pure helium at the time was hard to come by
9:22and the world's total supply was likely less than
9:2550 party balloons worth.
9:27So Onnes had to get his helium from a different source
9:31and he turned to sand, specifically two large bags
9:35of monazite sand.
9:37That's because monazite is a mineral that contains around 5%
9:41thorium by weight and thorium is radioactive.
9:45When it decays, it spits out an alpha particle,
9:47which is two protons and two neutrons.
9:50In other words, the nucleus of helium.
9:53That nucleus then gets slowed down by the surrounding rock
9:56where it steals two electrons and becomes a helium atom.
10:00So over millions of years, you get this buildup
10:02of helium atoms trapped inside the rock.
10:06In fact, basically all the helium we have on earth today
10:09comes from radioactive decay.
10:11It's essentially a radioactive fossil,
10:13although now we mostly get it from natural gas wells
10:16where it accumulates over the eons.
10:19But Onnes didn't have that luxury.
10:21He had to liberate it from the sand.
10:24So he recruited four chemists.
10:26They heated up the sand to release the gas, exploded it
10:30with oxygen, cooled it with liquid air
10:32to freeze out impurities, burned over copper oxide
10:36to get rid of the hydrogen,
10:37and last of all, compressed it over charcoal
10:40to absorb any remaining impurities.
10:42After several months, Onnes had about 25 party balloons
10:46of pure helium, an estimated 80% of the world supply.
10:52Onnes quickly conducted careful experiments to try
10:55and tease out whether helium
10:56atoms attract each other at all.
10:59- Kamerlingh Onnes did a lot of work
11:02and I think he was becoming more
11:04and more convinced that helium can in fact be liquefied.
11:08- It seemed like the helium atoms did attract each other
11:11just ever so slightly.
11:13So how do two neutral atoms that don't react
11:16with anything attract each other?
11:18Well, a helium atom is just a nucleus with two electrons
11:21around it, but in reality you never know exactly
11:24where those electrons are.
11:26So a more accurate picture looks like this, a nucleus
11:29with an electron cloud around it.
11:31But that cloud isn't always perfectly evenly distributed.
11:35At any particular instant,
11:36there's slightly more negative charge
11:38on one side than the other.
11:40And now if there's another helium atom close enough, well,
11:43this charge imbalance can induce a charge imbalance
11:46on that atom.
11:48And now the two exert a force of attraction on each other.
11:53This is known as a van der Waals force,
11:56and it's why every atom attracts every other if
11:59you get it close enough.
12:01But helium holds its two electrons very tightly,
12:04so the cloud hardly budges
12:05and the van der Waals force is tiny, the smallest
12:08of any element, but perhaps it's just enough.
12:13The second problem with helium is that no one knew exactly
12:16how cold it would have to get to liquefy, only that it had
12:20to be colder than liquid hydrogen, possibly much colder.
12:23And that's the issue
12:24because the rate at which heat transfers in from the
12:28environment depends on the temperature difference.
12:30The bigger the gap, the more heat flows in
12:33and cooling something down to possibly a few degrees
12:36above absolute zero while the room it's in is 290 degrees
12:40above it, well, that's a humongous gap.
12:44Fortunately, Dewar had already invented one
12:46key piece of apparatus.
12:49Essentially, one glass flask nested inside another
12:53with a vacuum in between.
12:55This limited heat conduction
12:56to a small connection point at the top.
12:59Convection got ruled out entirely due to the vacuum
13:02and optionally by covering the insides
13:04with a silver coating, you could also reflect radiation.
13:08This reduced heat losses by a factor of 30,
13:11and it's the basic design behind every thermos flask today.
13:15But for Onnes, it still wasn't good enough.
13:20So he nested several Dewar flasks inside each other.
13:24The helium would go in the inner one, a bath
13:27of liquid hydrogen would sit around that,
13:29and a bath of liquid air around that.
13:32So each layer acted as a heat shield for the next,
13:35and the helium would see a wall of 20 Kelvin instead of 300,
13:39cutting the radiated heat coming in by a factor of 50,000.
13:44To measure the temperature inside the inner vessel,
13:47Onnes inserted a small glass bulb filled
13:50with dilute helium gas held at a constant volume.
13:54A thin capillary connected this to
13:56what was essentially a pressure meter outside the liquefier,
14:00because for a gas at a constant volume,
14:02the temperature is directly proportional to the pressure.
14:06So if you first measure the baseline pressure at a
14:08temperature you know, like the freezing point of water
14:10around 273 Kelvin, you could set that to 100%.
14:16Then if you later measure a pressure that's only 10% of
14:19that amount, then the temperature must also be 10%
14:21of the original or around 27 Kelvin.
14:26By using this trick,
14:27Onnes could measure the temperature even at a few degrees
14:30above absolute zero.
14:32Now in their case, they read off a level of mercury,
14:35but for simplicity, we'll animate it as a temperature scale.
14:39Onnes finally had everything ready.
14:42At Professor Weinfurtner's gravity laboratory, we got
14:45to see the setup we'd use.
14:46- Is this also what it would have looked like back in the
14:49day when Onnes was doing it?
14:51- Yeah, so this is actually a really similar setup
14:53to Kamerlingh Onnes.
14:54- The biggest difference is that we only have two
14:57Dewars nested inside each other.
14:59The outer one will be filled with liquid nitrogen
15:01and the inner one is where the helium will go.
15:04By the 10th of July 1908, Onnes decided to go for it.
15:08He arrived at his lab around 5:30 AM
15:11where his technicians were already hard at work.
15:15They prepared 75 liters of liquid air the day before
15:18and Onnes now ordered them to start making liquid hydrogen.
15:22By 1:30 PM they had 20 liters, which was enough
15:25to launch their attack on helium.
15:31They started by filling the outer vessels.
15:34- First step, we're going to fill the outer
15:38Dewar with liquid nitrogen.
15:40That's going to precool the whole system down
15:42to about 77 Kelvin.
15:44Once we're full, we'll start filling with helium.
15:47So we've used about 25
15:52liters of nitrogen to get to that point.
15:54Now consider how big the Dewar is, that probably holds,
15:59you know, less than 10 liters.
16:02- With the liquid air and hydrogen in place, they started
16:04to cool the liquefier.
16:07After 30 minutes, it had cooled to 93 Kelvin.
16:11Now it was time to start cycling the helium.
16:14A pump compressed it to 100 atmospheres
16:16before it went to the liquefier, where it got precooled to
16:19around 15 Kelvin.
16:21The gas was then rapidly expanded into the inner vessel,
16:25greatly increasing the distance between atoms.
16:29But remember, helium atoms do slightly attract each other,
16:32so moving them apart costs energy
16:35and they pay for this with some of their kinetic energy,
16:38causing the particles to slow down
16:40and their temperature to drop.
16:44The gas then got pumped back to the compressor,
16:46but the smart thing was that they let this colder gas cool
16:49the incoming helium,
16:51slightly lowering its temperature before it expanded.
16:54And by doing this over
16:56and over, the incoming helium got colder and colder.
17:01- It's so faint. - Yeah.
17:06- It feels like a ghostly fluid or something.
17:08- You know, it's not liquid that's escaping.
17:10It's more of kind of a super dense vapor.
17:13It's only 10 degrees inside, but it's dropping fast.
17:18- A little after 6:30 PM, Onnes
17:20and his team had gotten down to six degrees
17:22above absolute zero.
17:24That is closer than anyone had ever gotten
17:26before, and they kept going.
17:29But just below five Kelvin, the temperature stopped falling,
17:34so they started to worry that perhaps something was wrong.
17:39But then a chemist in the room remarked, "It's so stable,
17:42it's almost as if the thermometer is submerged in a bath
17:45of liquid." So they rushed to the apparatus,
17:50but still saw nothing until Onnes grabbed a torch
17:54and shone it from a low angle, and they saw a reflection.
17:59After the surface had once been seen,
18:01it was no more lost sight of, Onnes wrote,
18:04"It stood out sharply defined like the edge
18:06of a knife against a glass wall."
18:10They had just made liquid
18:13helium.
18:15- That's really cool.
18:18It's just hitting the light on it while it's yeah.
18:21So beautiful. It's a, it's a helium kettle.
18:24- Four degrees above absolute zero.
18:28- You can see that really faint liquid level on the
18:31inside. That's the liquid helium.
18:33- Oh wow. Okay. So the, the refractive index
18:36of liquid helium is very close to air, is it?
18:38- 1.025. Okay.
18:40- So yeah, really, really difficult to see by eye. Wow.
18:43- Isn't it crazy to think that that is about four
18:47degrees above absolute zero? It doesn't look like.
18:51- It doesn't, there's nothing that would tip you off.
18:53- Yeah. Back in Leiden, everyone was elated.
18:58For the next hour, they ran basic experiments until
19:01by 8:30 PM only 10 millilitres were left.
19:05But then Onnes thought, "Well, we've made liquid helium.
19:08Why not make solid helium?" So he was going to cool it down,
19:12and the way you can cool down any liquid
19:15is by pumping on it.
19:16- When we pump on it, we're removing all
19:19of the hottest atoms from the system,
19:20which takes about a third of the fluid away.
19:22So we initially take the level way higher than we need it
19:25so it can drop down.
19:27- So Onnes started his pumps and so did we.
19:31- Are we ready? - Three, two, one, go.
19:36- Oh, - Wow. Okay.
19:41- Oh wow. It's gone down so much.
19:46It's gone down like, what, five centimeters?
19:49- Yeah, immediately. - Immediately.
19:51- Because we're removing all the
19:52hottest atoms from the system.
19:55So we're just shy of 3.5 K.
19:57- This rapidly evaporating helium is quickly cooling down.
20:01- So we're about 2.6 K. We're below 2.3 K.
20:06- Okay. - Oh,
20:07- It's going super fast, super - Fast.
20:09- It keeps boiling until right at 2.17 Kelvin,
20:13it abruptly stops
20:16and the liquid goes eerily quiet. There it is.
20:21- Ooh. It's completely still again.
20:24Man, you can hardly see it. It feels like a mirror surface.
20:28It's crazy how smooth that is.
20:30- Onnes pumped his helium down to 1.7 Kelvin,
20:35colder than outer space, but it didn't freeze.
20:39Instead, it just sat there, remarkably still.
20:43Onnes concluded it must just freeze at some lower
20:46temperature, and since everyone was exhausted,
20:49they called it a day.
20:50Little did he know what sat there evaporating in his flask.
21:02Onnes still wanted to freeze helium,
21:05so over the next two years, he tried several more times,
21:08getting all the way down to one Kelvin,
21:11but all his attempts failed.
21:14This led him to wonder whether helium will not remain
21:16perhaps liquid even if it is cooled to the absolute zero.
21:21And indeed, scientists later discovered this is true.
21:24If you take any substance
21:26and keep cooling it down, eventually it always freezes,
21:29except for helium.
21:32Somehow it's the only substance that never freezes
21:35by just cooling it down.
21:37So Onnes was wise when he gave up on
21:40trying to freeze helium.
21:42Instead, he set out to finally test Kelvin's prediction
21:44that the electrical conductivity
21:46of metals should stop near absolute zero.
21:49In other words, that their electrical resistance should
21:51become effectively infinite.
21:53To do this, he submerged different metals in liquid helium
21:57and measured their resistance.
21:58He started with gold and platinum,
22:01but their resistance just kept slowly decreasing
22:04until finally settling on a steady value.
22:07Then on the 8th of April, 1911, he measured mercury
22:11and wrote down just three words
22:17mercury practically zero.
22:20As you cooled down, its resistance slowly decreased until
22:24around 4.2 Kelvin, where it suddenly plummets to
22:28what appeared to be zero.
22:30Electricity that flowed with no apparent losses,
22:34the exact opposite of what Kelvin had hypothesized,
22:37Onnes had just discovered superconductivity.
22:41- So that's the story most people know.
22:44- Okay. - What most people don't know is that later
22:47that same day, he also cooled down the helium more
22:51and he wrote, "Right
22:52before the helium reached the coldest temperature,
22:56which was 1.8 Kelvin, the boiling suddenly stopped."
23:01- He noticed.
23:03- He noticed. - So what did Onnes do
23:07after seeing this sudden change?
23:09Almost nothing. He guessed at an ordinary
23:12explanation and moved on.
23:15But helium kept dropping clues.
23:18When they measured its density,
23:19they discovered it had a curious peak around 2.17 Kelvin.
23:23And when they measured the latent heat, they found that
23:26around the same temperature, something happened
23:28to the helium, which was perhaps even discontinuous.
23:32Yet the biggest smoking gun
23:34of all happened when they measured a property known
23:36as the specific heat, which is how much energy it takes
23:39to heat up one gram by one kelvin.
23:42They found that as the temperature dropped,
23:44the specific heat declined
23:46until they approached 2.17 Kelvin,
23:48where it suddenly shot up.
23:51This was as clear a sign as any
23:53that something was happening at this point.
23:56- But they knew what this was.
23:58This was condensation happening, you know,
24:01experimental error.
24:03So they had these points,
24:04but they, they wrote like,
24:06"We felt justified in disregarding these irregular points,
24:10and they only published the part above 2.61 Kelvin,
24:14which neatly fit the formula they had for it."
24:18- Onnes brushed off hint
24:20after hint, even while seeing it happen with his own eyes.
24:25One day, Onnes put one container of liquid helium
24:28inside another, and he noticed that below 2.17 Kelvin,
24:32if one level started higher, it always quickly evened out.
24:37He even remarked how strikingly fast it happened.
24:40He thought it was too fast for distillation,
24:43but since he had no better explanation, he stuck with that.
24:47If only he had raised his container,
24:49he might have discovered the truth.
24:51Here, we've got a polymer cup. This is our container.
24:55- So we got the bowl loaded up.
24:59It's way at the bottom, and I think what we're going to try
25:01and do is lift it up.
25:03- Yeah. The colder we get, the better this effect is going
25:05to be, and it's a super subtle effect at the best of times.
25:09We want to get down to about 1.5, 1.55 K.
25:13I'm going to try and raise it just above the fluid surface.
25:17- Whoa. - Can you see that?
25:20- Yeah, yeah, - Yeah. Yeah.
25:21- Yeah, no, you see like waves almost.
25:25- Yeah. - Yeah. Ah, you can really see a pulse down there.
25:29- If you look at the bottom, you can see it's dripping.
25:34And if we speed it up, you can see it keeps dripping.
25:38More and more liquid helium is going to the bottom.
25:41What's happening is that somehow the helium is climbing up
25:45and over the walls, literally emptying itself out.
25:50This was not distillation.
25:52Something far stranger was happening.
25:57Onnes's successors grew suspicious
25:59of the specific heat data, so they remeasured
26:02and they became convinced it wasn't an error.
26:06This was real. Something was happening at 2.17 Kelvin.
26:10Since the curve resembled the Greek letter Lambda,
26:13they called this the Lambda point
26:15and they suggested that a phase transition must be happening
26:18here, but not an ordinary phase transition.
26:22This would be a phase transition to a more exotic kind
26:25of matter, from a normal liquid to something else.
26:29But what?
26:30Figuring out what's really happening when there are so many different clues can be confusing.
26:36If not, outright impossible. The same is still true today.
26:40Recently, I came across this story where scientists used AI
26:44to design viruses not found in nature,
26:46but depending on where you read the story,
26:48you might have a completely different reaction.
26:50If you read IFLS first,
26:53you might think this is an exciting scientific breakthrough,
26:55AI designing a virus that can kill bacteria.
26:59If you'd seen this first, you'd get a more complete,
27:02albeit complex picture, 16 viruses from scratch,
27:05but with possible downsides.
27:07And if you saw this New York Post article first,
27:10you'd most likely focus on what could go wrong.
27:13So three headlines,
27:14but three completely different conclusions.
27:16Most people just get their news from one source,
27:19and as a result, their takeaway is a little bit like the
27:22luck of the draw, which outlet did you happen to see first?
27:25That's why we use today's sponsor, Ground News.
27:28They're a website, an app that brings a data-driven
27:30and objective approach to reading the news.
27:32They pull thousands of stories daily,
27:35and for each, they break down the political leaning,
27:37how factual the reporting is, and who owns the reporting.
27:40For example, with the AI virus story,
27:42it immediately tells you
27:43that over 200 news outlets reported on it
27:46and that it got covered fairly evenly
27:48across the political spectrum.
27:50Below that, you can also see the factuality, ownership,
27:53and even who broke the news first, Forbes in this case.
27:56The bias comparison feature highlights specific differences
27:59in the contents of the articles,
28:01but my favorite feature might be their blind spot feed.
28:03It shows you stories that are under-reported by one side
28:06of the political spectrum or the other.
28:08Trying to figure all these differences out by yourself
28:11for every story would be a full-time job.
28:13That's what makes Ground News so valuable.
28:15Their data-driven approach helps make these stories
28:18digestible in seconds.
28:19So if you, like us, care about getting to the truth, go
28:22to ground.news/ve or scan this QR code.
28:26Our link gets you 40% off their vantage plan.
28:29I'd like to thank Ground News for sponsoring this part
28:31of the video, and now back to
28:33what was happening when you keep cooling down helium.
28:36The answer started to take shape throughout the 1930s.
28:39- So in 1945,
28:41there was this graduate student called Don Misener,
28:45and he needed to do some experiments
28:47for his coursework to get enough credit.
28:50And he was aware of everything that was happening in helium,
28:53but he realized that no one had measured its viscosity.
28:56So he's like, "Oh, that's
28:57weird. I will measure it." - So how do you measure the viscosity?
29:02- There are two ways. You have a disc
29:08and you try to see what the resistance of the liquid is.
29:13Another way is to have a small
29:18capillary, and then you push the liquid through that,
29:23and the viscosity you measure
29:27between the two methods should be exactly the same.
29:32- Misener chose a variation of the first method.
29:36He submerged a cylinder that was connected
29:38through a torsion fiber in liquid helium.
29:41He then rotated the cylinder
29:42and let go, causing the cylinder to swing back
29:45and forth to oscillate.
29:47Now, by watching how long it oscillated,
29:49he could determine the viscosity.
29:50The longer it oscillated, the less friction it must
29:53experience, so the lower the viscosity.
29:55And what he found was that at 4.2 Kelvin,
29:58the viscosity was about 14%.
30:00That of air, already tiny.
30:02Then, as he decreased the temperature,
30:04the viscosity slowly increased
30:07until at the transition point, it suddenly plummeted, not
30:10to zero, but to about 5% that of air.
30:14This caught the attention of Jack Allen, a
30:1627-year-old physicist who'd just come to Cambridge.
30:19Allen had inherited a helium liquefier from someone
30:22who could no longer use it,
30:23and now he resolved to use it
30:25to unravel the mysteries of helium.
30:27Of course, he could use some help,
30:29and Misener seemed like the perfect PhD student,
30:32so Allen convinced him,
30:33and together, they continued studying helium's viscosity.
30:37Only now they turn to the second method of studying it,
30:40which works something like this.
30:42I've got a tube filled with two layers of glycerol.
30:45Now, if I let it flow, what's going to happen?
30:47Well, you'd expect the layers to drop, which they do,
30:51but not in a straightforward way.
30:53It doesn't maintain its shape.
30:54Instead, it stretches out in the middle.
30:57What's happening is that if you zoom in, you'd find
30:59that the molecules near the wall get stuck,
31:02they experience friction,
31:03and then the molecules sliding past them experience
31:05friction with those molecules.
31:07The internal friction is basically what viscosity is.
31:11In general, if you make the channel smaller,
31:13the flow rate should decrease,
31:15and if you push on the liquid, it should increase.
31:18Now, by carefully keeping track of
31:19how things change when you alter these parameters,
31:22you can determine the viscosity.
31:24In November of 1937, Allen
31:26and Misener did a version of this experiment
31:28with liquid helium at around one kelvin.
31:30They let it flow through one wide capillary
31:33and one narrow one, and they found that the flow speed
31:36through both was almost the same,
31:38despite the narrow one having a surface area a
31:40thousand times as small.
31:42Even weirder was
31:43that the speed hardly changed when the pressure increased.
31:47They concluded that any known formula cannot, from our data,
31:50give a value of the viscosity,
31:51which would have much meaning.
31:53As far as they could tell, the viscosity was non-existent.
31:57- They know they've got gold on their hands here, right?
32:00They know this is a, like a bombshell of a result.
32:04- So they spent the next three weeks making sure they hadn't
32:07made any mistakes.
32:09They repeated all the experiments
32:10and did all the other measurements they
32:12needed, ready to write it up.
32:14- But then on the last day, one
32:16of Kapitsa's old students walks in
32:19with a letter from Kapitsa.
32:20- "Who's Kapitsa? Well, remember that Allen had inherited
32:24that liquefier from someone who couldn't use it anymore?
32:27Well, that was Soviet physicist Pyotr Kapitsa.
32:31He built the liquefier and ran the lab before Allen.
32:35But then during a trip back home in the summer of 1934,
32:38Stalin had his passport confiscated.
32:42He was to work and stay in the Soviet Union, where
32:44for the past few years, Kapitsa had been rebuilding his lab,
32:48telling almost no one exactly what he was working on.
32:51But Allen and Misener were about to find out.
32:54- He had done the same sort of experiment,
32:59found the viscosity was vanishingly small,
33:02and send a note to the editor of Nature
33:05that he would be very grateful if they could publish it
33:08as soon as possible.
33:10Don't bother to send the proofs to me here to Moscow.
33:13It takes too much time. He had just scooped his old lab.
33:17- He knew what he had and he raced to publish.
33:20Allen and Misener were freaking out,
33:23but over the next four days, they wrote up all their results
33:26and also sent it to Nature.
33:29On the 8th of January, 1938, the two papers appeared side
33:32by side, reporting the vanishingly small viscosity,
33:37perhaps even zero.
33:39And then in analogy to superconductors,
33:41Kapitsa called it a superfluid.
33:45This was the breakthrough everyone had been waiting for
33:48because it finally began to explain some
33:50of the strange things they'd been seeing.
33:53Above 2.17 Kelvin, helium is just a normal fluid
33:56and it has internal friction
33:58or viscosity, so when it flows through tiny cracks, part
34:01of it gets stuck to the walls.
34:04Now, in the case of the bucket, the bottom is made
34:06of a polymer aerogel, which is filled
34:08with microscopic channels up to tens of nanometers across.
34:12That's so small
34:13that the helium gets fully clamped to the walls.
34:16It can't flow through.
34:18But as you cool down below 2.17 Kelvin,
34:21the helium transforms,
34:24and now it can suddenly pass through tiny cracks
34:26with no friction, so it leaks straight through the bottom.
34:33It also explained what was happening with the little cup.
34:36How? Well, remember those van der Waals forces
34:39that cause every atom to attract every other?
34:42Well, between helium atoms, they're very weak,
34:45but the atoms in the walls
34:46of the cup have many more electrons in their outer shell,
34:49so the charge imbalances become larger.
34:52And as a result, the helium atoms are much more strongly
34:55drawn to the wall than to each other.
34:57So they climb up the wall
34:59and within seconds wet the entire surface with a layer
35:02that's around 30 nanometers thick.
35:05And because the bottom of this thin sheet is
35:08below the liquid layer, it now acts as a siphon.
35:12Normal fluid can't flow through
35:14because it experiences friction,
35:16but superfluid can, so it flows up
35:20and over the walls until it drips out the bottom.
35:31But for all the idea of a superfluid explained,
35:34it also raised a much more fundamental question.
35:37How do you get a superfluid?
35:38- Like if it is actually zero, I want you to think about
35:42how crazy that is, right?
35:43Because if we think back to, okay, what is this viscosity?
35:47It's internal friction between the molecules.
35:49So if you have actually zero viscosity, it means
35:53that you have like a billion,
35:55billion particles all traveling through this pipe
35:58and not a single one is bouncing off of anything
36:01and losing some energy or bouncing into
36:04each other and losing energy.
36:06They all must flow perfectly
36:09and somehow not lose a single bit of energy.
36:12It seems preposterous.
36:15- Within two months of the nature papers,
36:17Fritz London proposed a possible mechanism.
36:20There was this forgotten paper from 13 years earlier.
36:23It predicted that if you just kept cooling down a certain
36:25type of particle, then the atoms would fall
36:27into an exotic state.
36:29And the author of that forgotten paper was none other
36:32than Albert Einstein.
36:34To understand what Einstein had predicted,
36:36consider this plane of particles.
36:38They're not interacting
36:39and they're free to move around in every direction
36:42with different velocities and different kinetic energies.
36:44Now at room temperature, you can just think of them
36:47as point particles,
36:49but if you start cooling them down,
36:51what Einstein realized is that, well,
36:53this is no longer the case
36:55because now quantum theory starts to play a role.
36:57And according to that theory, particles aren't just
36:59particles, they also have a wave-like character.
37:02- So-called wave particle duality, the quantum
37:05wave function tells you the likelihood of
37:08where it would be found if you were
37:09to try to measure its location.
37:11- The important thing here is that the wavelength
37:13of these waves or how spread out they are is equal
37:16to Planck's constant over their momentum
37:18or mass times velocity.
37:20- So if you're cooling these particles down,
37:22they on average have less
37:23and less momentum per particle, their quantum wavelength
37:27or the de Broglie wavelength gets longer.
37:30And so eventually the sort of quantum state associated
37:33with each of those seemingly independent particles starts
37:36to smoosh out and overlap.
37:38This distinguishability in space starts to melt away.
37:42- What happens next depends on which kind
37:45of particle you're cooling down.
37:47Every particle in the universe is one of two kinds.
37:50It's either a boson or a fermion.
37:53Fermions are most of the particles you're familiar with,
37:55like protons, neutrons and electrons,
37:57and bosons are particles like photons.
38:00You can see the difference between bosons
38:02and fermions by looking at two indistinguishable particles.
38:06Now, one particle just has its own wave function,
38:10but if you take two of them together,
38:12something special happens.
38:13They get a combined wave function
38:15that describes both of them together.
38:18In the case of bosons, if you could see that wave function,
38:21you'd see it's the same on both sides,
38:24that is it's symmetric.
38:26But if those particles are fermions,
38:28then their combined wave function is the
38:30opposite on both sides.
38:32It's anti-symmetric.
38:35Now I want you to think about what this means if you try
38:37to move those particles together.
38:39In the case of the bosons, the two sides
38:41of the wave function add up
38:43and you just get two bosons in the same place or state.
38:46But now try doing the same for fermions.
38:49You get one peak to overlap with one valley
38:52and they'd cancel out.
38:54But that would suggest that all
38:55of a sudden there's a 0% probability
38:57of finding the particles anywhere,
38:59which doesn't make any sense.
39:00They have to be somewhere. So
39:02what this is really telling us is
39:04that two indistinguishable fermions can never occupy
39:07the same quantum state.
39:09This is known as Pauli's exclusion principle,
39:12and it's why particles like electrons can never be in
39:14exactly the same state.
39:16If the particles you're cooling down are fermions,
39:18then they start filling up the energy states like a ladder
39:21from lowest to the higher states,
39:24with only one fermion per quantum state.
39:27But bosons don't have this restriction.
39:29They're very happy to share the same quantum state.
39:32When you keep cooling them down, more
39:34and more of them start to occupy lower
39:36and lower energy levels.
39:39And Einstein realized there comes a point
39:41where a macroscopic portion suddenly condenses into the
39:45lowest energy state
39:46or ground state, which in this case is just when
39:49they're completely still.
39:51- These little particles could actually act in a way
39:55where they would sort of lose, so to speak, their,
39:57their individuality, that they could act
40:00as a single coherent state of matter, a whole new state
40:04of matter that had no analog.
40:07- This became known as a bose Einstein condensate
40:10after Bose whose work Einstein was building on.
40:13But Einstein doubted this could happen in real life.
40:16He wrote, "The theory is pretty,
40:18but is there also some truth to it?" 13 years later,
40:23London believed there might be
40:24and that perhaps superfluid helium was a
40:27bose Einstein condensate.
40:29At first, that might seem impossible
40:31because helium is made up of protons, neutrons,
40:33and electrons, and every one of those is a fermion.
40:36So you'd expect helium to be a fermion too.
40:39And remember, fermions can't share the same state,
40:42but helium is not a fermion.
40:46bosons and fermions differ in one other fundamental way,
40:49their intrinsic angular momentum or spin.
40:52For bosons, this is always an integer number, zero, one,
40:56two, three, and so on.
40:57But fermions always have a half integer spin, like a half.
41:01So what happens if you have multiple fermions
41:04together, say in one atom?
41:06Well, now you have to add their spins together
41:08to get an overall spin for the atom,
41:10and then use that to see whether it's a boson or a fermion.
41:14In the case of helium, which has six half spin particles,
41:18then no matter how you add them up,
41:20you always end up with an integer number.
41:22As a result, helium atoms behave as bosons.
41:27And as you cool them down below 2.17 Kelvin,
41:30a macroscopic fraction
41:31of the atoms condense into the lowest energy state.
41:34They lose their individuality
41:37and about a trillion trillion atoms all behave as one.
41:42Normally, you think of wave functions
41:44as these teeny tiny things,
41:45but here it's literally the size of the container.
41:49It's as if after the transition, the helium has turned into
41:53one big atom with all these atoms behaving as one.
42:00- This is quantum mechanics with your own eyes.
42:04- And it makes you look at matter in a different way.
42:08It allows us to explore the boundary of quantum
42:11and regular matter.
42:14So it seems like we've solved it.
42:16The atoms are in the lowest energy state.
42:19They've lost their individuality
42:20and act as one, so they can't scatter
42:22and they flow with zero internal friction.
42:28That's how I first understood superfluids
42:30and how I'd explain it to anyone who wanted to know.
42:33I mean, it made sense and we were all happy.
42:36The only problem is that it's wrong.
42:38I mean, the first issue is that in a BEC,
42:41all the atoms occupy the state
42:43where the particles are at rest,
42:45but in a superfluid, well, they flow.
42:48Maybe that issue could be excused
42:50because you could imagine moving along with the superfluid
42:53and finding a frame in which it effectively stands still,
42:56but there's an even bigger problem, which is
42:58that in a non-interacting Bows Einstein condensate,
43:02even the tiniest push can excite any of the atoms.
43:05So if you'd try to let it flow,
43:07the smallest disturbance would knock
43:09atoms out of the ground state.
43:10They would scatter and the flow would lose energy.
43:14So clearly a BEC alone isn't enough.
43:17You need something else, but what?
43:20Fortunately, Kapitsa knew
43:22exactly who could solve the problem.
43:24Unfortunately, he had been locked
43:26inside Stalin's prison for the last year.
43:28- This was right around the time of Stalin's great terror
43:32where he was executing
43:34or imprisoning all the enemies of state,
43:37whether they were real or imaginary.
43:39- But in the case of Lev Landau,
43:41the physicist Kapitsa was thinking about,
43:44it seems like there was some truth to it.
43:46Landau was known as a genius theorist
43:49who cared deeply about elevating Soviet physics.
43:52He was the head of the physics department
43:53and personally trained some
43:54of the best physicists of the century.
43:57But around the mid 1930s,
43:59he became disillusioned with Stalin's regime.
44:02He reportedly told one colleague that Soviet officials were
44:05"born subhuman." And according to KGB Files, he was accused
44:09of heading a counter-revolutionary organization.
44:13Landau was arrested on the 28th of April 1938
44:16and taken to the Lubyanka Prison.
44:20The first day he was forced to stand for seven hours,
44:23followed by six days of 12-hour persuasion sessions.
44:27After two months, Landau broke
44:29and wrote a six-page confession.
44:31It reads, "We came to the conclusion
44:33that the party had degenerated
44:34and that the Soviet government no longer acted in the
44:37interests of workers, but in the interests
44:39of a small ruling group.
44:41The interests of the country demanded the overthrow
44:43of the existing government."
44:45- That's a pretty, pretty strong confession.
44:48- This confession at a time where people were being executed
44:52for no reason should have meant the end for Landau.
44:56But fortunately for him, he had someone defending him.
44:59The morning Landau got arrested,
45:00Kapitsa immediately wrote to Stalin.
45:03His character, to put it simply, is nasty.
45:05He is a bully and a brawler,
45:07but I find it very hard to believe that Landau was capable
45:10of anything dishonest.
45:11Landau is young and he still has much
45:14to accomplish in science.
45:16No one else can write about all of this,
45:17which is why I'm writing to you.
45:20Now, Stalin never replied.
45:22So a year later, Kapitsa wrote to one
45:24of Stalin's closest allies.
45:26He told him how he had discovered superfluidity
45:29and that no theorist other than Landau could explain it.
45:33Three weeks later, Kapitsa was summoned to the prison.
45:36As he recalled, "I was summoned at 1:00 AM.
45:38They escorted me to a big office occupied by two men.
45:42Do you understand for whom you are pleading?
45:44He's a most dangerous criminal, a spy who confessed
45:47to everything." We talked till four in the morning.
45:50Towards the end, one of them said, "All right, Kapitsa,
45:53if you pledge your word for Landau,
45:54then give us a written guarantee.
45:56If anything happens, you will be held responsible."
46:00Kapitsa wrote the letter
46:02and two days later, Landau turned up at the institute, ready
46:05to get back to what he loved doing most, physics.
46:11Kapitsa had put his neck on the line.
46:13Now it was up to Landau to prove he was up to the task,
46:16and superfluid helium had only gotten more interesting.
46:20Back in January 1938, just days
46:23after the nature papers, allen was studying
46:25how superfluid helium conducts heat.
46:27To do this, he packed some Emery powder,
46:30basically fine sand, in a glass bulb with an open bottom.
46:33To see what was happening, he shone his pocket torch at it.
46:37And according to him, as the Emery powder tube came into
46:40view, helium spurted out the top of the tube.
46:44It continued to spurt as long
46:45as the light shone on the powder
46:47and ceased when a card cut off the light.
46:50Allen later showed this fountain with no moving parts
46:53to the Royal Society, during which famous writer H.G.
46:56Wells crawled under the table looking
46:58for a hidden pump, but found nothing.
47:01- Yeah, so we had this kind of specialist made.
47:04It's a single piece of glass.
47:05There are no seams in it whatsoever.
47:07The filtered disc is fused in,
47:09and then it's just filled with carbon stones,
47:11and we just hit it with a bright light
47:13that heats it up, drives the fountain flow.
47:15- Amazing. Fingers crossed. You guys ready?
47:20Three, two, one. Torch it.
47:24Woah! Yeah.
47:31Let's go. What?
47:36That's absurd.
47:37- That's awesome. Woo. Let's go.
47:43- So what was happening?
47:47How could light cause the helium to shoot out?
47:51- Any theory now had to take
47:55into consideration all these phenomena.
47:59So for theoreticians was a really heavy task.
48:03- Landau spent the next eight years working on it.
48:07He started by considering a superfluid flowing at absolute
48:10zero, and he realized what London had left out,
48:13the forces between atoms.
48:16In a Bose Einstein condensate, there are no forces
48:18between particles, and as a result,
48:20a push can knock a single atom out
48:22of the lowest energy state, resulting in easy friction.
48:26But in liquid helium, this is not the case.
48:28The atoms interact, so if you push on one,
48:31you're pushing on a bunch of them.
48:33Landau realized that this rigidity meant
48:35that the only way you can create excitations in a superfluid
48:38and lose energy in the flow is
48:40through the collective motion of the atoms.
48:43He argued that these excitations come in two main flavors.
48:47The first is that if you push hard enough, you get the atoms
48:50to bump into each other
48:51and create ripples throughout the fluid,
48:53basically sound waves, just a quantized version
48:56of them called phonons.
48:58Similar to how photons are particles of light,
49:00these are particles of sound.
49:03The second kind of excitation is something known as a roton,
49:06because originally people thought of them as tiny whirls,
49:10but now they're seeing more like a patch of atoms
49:13that suddenly orders themselves into a crystal lattice just
49:15for a split second.
49:17You can think of them almost as tiny ghosts of a solid.
49:20And just like a phonon creating a roton costs both
49:23energy and momentum.
49:25So Landau argued that these phonons
49:27and rotons were the only way
49:29that the flow could lose energy.
49:31- And you're like, okay, well, it's quite easy,
49:34you know, to make a sound wave.
49:36Like all I have to do is knock on it, right?
49:38That in terms of like how much energy it takes
49:41to create a phonon, it can be very little energy.
49:44It can be as little energy as you want,
49:46but you don't just have to conserve energy.
49:48You also have to conserve momentum.
49:50So when you push on it
49:52or when the liquid bumps into something, it both needs
49:55to satisfy conservation of energy
49:57and conservation of momentum.
49:59And there is a very nice relationship that tells you exactly
50:04how the momentum of a phonon corresponds to the energy
50:06of a phonon, and it's connected with the speed
50:10of sound in a material.
50:12And so the only way you can satisfy both constraints is if
50:15the flow is going at least the speed of sound,
50:19which in helium two is very fast.
50:21It's about, I think, 220 or 240 meters per second.
50:25Rotons, he realizes, can get created a little bit easier.
50:29I think the modern value is about 60
50:31meters per second, but it's still a lot.
50:33- So in Landau's theory, below about 60 meters per second,
50:36the flow can't create a single phonon or roton.
50:40The atoms all stay in the ground state
50:42and no energy is lost.
50:43So it's not that the friction is vanishingly
50:45small in a superfluid.
50:47Landau proved that it's strictly zero.
50:50Friction is forbidden.
50:52In practice, other excitations are created at lower speeds
50:55and therefore the actual flow rate at which superfluidity
50:58breaks down is lower.
51:00But if the friction is actually zero,
51:02that implies something else.
51:04You should be able to get a superfluid flowing in a loop,
51:07and once it's going, it should just keep going like a
51:10perpetual motion machine.
51:12In 1964, two physicists at Yale tested this.
51:15They got superfluid helium circling in a stack
51:18of thin rings and waited.
51:20And what they found was
51:21that no matter whether they waited five minutes
51:24or more than 12 hours, the flow was just as strong.
51:28- But then, you know, if the friction is zero,
51:32how is it possible that Misener, you know,
51:34when he did his like torsion
51:35experiment, he didn't measure zero.
51:37- The liquid wasn't just passing over liquid,
51:40like you had like a cylinder of something in there?
51:42- Yep. - Yep. I don't know. Is it coming from the wire?
51:45Is it coming from the cylinder?
51:47Like the different materials? What is it coming from?
51:49- Your instinct is
51:51that Misener must have done something wrong.
51:54He didn't. Both experiments are true. How can both be true?
51:58- You're measuring different things.
52:00I don't know, but how are you measuring different things?
52:04This is where the second part of Landau's work comes in.
52:06So far, Landau had only looked at a fluid at absolute zero,
52:10but every real experiment happened at higher temperatures.
52:14So next he considered what if it flows at
52:16some finite temperature?
52:18To do this, he built on the work of Tisza, who suggested
52:22that helium below 2.17 Kelvin acts
52:24as a mixture of two fluids.
52:26One is the superfluid. It's effectively at absolute zero.
52:30It carries no heat and flows without friction.
52:33You can think of this as the background fluid.
52:36And the second is the normal fluid, which is made up
52:38of the phonons and rotons.
52:40It carries all the heat and has viscosity.
52:43At absolute zero, the whole fluid is superfluid,
52:46but as it heats up, part
52:48of the superfluid turns into normal fluid.
52:50The superfluid fraction decreases
52:52and the normal fraction increases until at 2.17 Kelvin,
52:56the superfluid component is zero.
52:59- Now, if you were to look at one atom in the fluid
53:02and say, "Is that superfluid
53:03or normal fluid?" Well, it's hard to tell
53:06because the superfluid atoms are constantly exchanging
53:09energy with the normal fluid atoms.
53:11- Sure. - So any one atom is constantly bouncing up
53:13and down between these two modes.
53:15- Now, only the superfluid can flow through tiny channels.
53:19So when Allen and Misener pushed helium
53:21through their capillaries, they only measured the super
53:23fluid part and they found a zero viscosity.
53:26But Misener's torsion experiment sat in a
53:29mixture of both fluids.
53:30So the cylinder felt the viscosity from the normal fluid.
53:34Misener hadn't made a mistake,
53:36they were just measuring different things.
53:39And once you know what you're looking for,
53:40you can also physically see the effect.
53:43Just below the transition temperature, say at 2.15 Kelvin,
53:46only around 13% of the helium is superfluid.
53:49So only that 13% can leak
53:52through the bottom and you get a drip.
53:54But if you cool it down to 1.5 Kelvin, well now
53:57around 89% is superfluid.
53:59That's almost seven times as much helium that can flow
54:02through, so it just pours out the bottom.
54:06It even explains how light can power a fountain.
54:10When the light hits the rocks, the rocks absorb some
54:12of the photons and heat up a little
54:14- Bit.
54:15- This causes part of the superfluid
54:16to turn into normal fluid,
54:19but now the superfluid density is lower
54:21inside the fountain, so more superfluid immediately rushes
54:24in through the bottom to restore the proportions.
54:27That bottom, however, is just like the bottom of the bucket.
54:30It only allows superfluid through.
54:32So the normal fluid can't go out that way.
54:35The only way the excess helium can go is up and out.
54:39And so you end up with this,
54:41a fountain powered solely by light.
54:43The heat from the light gets turned into the mechanical
54:46energy of the spouting helium.
54:49But the two fluids do more than make fountains.
54:52They change how helium transfers heat.
54:55Take a bath of liquid helium below the lambda point
54:57with a heater on one side.
54:59- The helium atoms that are in contact with that hot face -
55:03- Yeah. - Are going to gain energy. They're
55:04going to jump up this ladder.
55:05- Yeah. - So they're going to be part of the normal fluid.
55:07- Right. - So my normal fluid is going
55:12to flow away from the heater, carrying all of the heat
55:15and the entropy that's imparted into the fluid away.
55:17- Yeah. - But as the normal fluid flows away - Yeah.
55:21The superfluid flows back to replace it. Right.
55:24And the overall density of the fluid remains constant.
55:28- Sure. - So this is a two-fluid
55:31thermomechanical counterflow.
55:32They basically flow through each other
55:34like they weren't even there.
55:35- But you can take this one step further.
55:38Imagine changing the temperature
55:39of the heat source periodically.
55:41What is going to happen?
55:43Well, the amount of normal fluid created will
55:45also change periodically.
55:47So you get these fluctuations in the normal fluid
55:49and superfluid densities.
55:51They spread out and travel away,
55:54almost like a sort of sound.
55:56- When we think of first sound, regular sound - Yeah.
55:59That's density waves in the air driven
56:02by tiny pressure gradients.
56:04- But with a superfluid, you can get density waves in the
56:07normal and superfluid components.
56:09And since the normal fluid carries all the heat,
56:11that means you get temperature waves.
56:13- And they can spread out like sound waves.
56:16- Yeah. - So we call temperature waves
56:17in helium second sound.
56:19- The crazy thing about it is it's a completely new mode
56:23of heat transport, unlike anything else in the uni - right?
56:27Yeah. When I heard that, I was like,
56:29"That's crazy." So you always think of conduction,
56:32convection, and radiation.
56:34And now helium gets a fourth.
56:37- This is a combination of second sound
56:40and thermal counterflow, and it is effective,
56:43around a thousand times more conductive than copper,
56:45which is one of the best thermal conductors we know of.
56:48It finally helps explain one last thing you've been seeing
56:51this whole time, that is why does the boiling suddenly stop?
56:56Well, above 2.17 Kelvin,
56:58helium boils just like a normal liquid.
57:01- Say, for example, I've got a bucket of water
57:05and I take a red hot poker and I dump it in.
57:08The bubbles aren't going to form anywhere in the bucket.
57:10They're going to form around that poker
57:12because you've got this high temperature region
57:15where you've dumped in enough energy locally
57:19to vaporize the water
57:20and then that's where the bubbles are going to form.
57:22The amount of energy it takes
57:24to do this is the latent heat of vaporization.
57:26In water, that is about
57:292,200 joules per gram.
57:33In helium, it's about 23 joules per gram.
57:37So if you've got helium in its liquid state,
57:42a tiny thermal fluctuation is enough
57:45to locally vaporize a small pocket,
57:48which then forms a bubble.
57:49And that's why boiling is so constant,
57:52because tiny thermal fluctuations will form
57:54bubbles very, very easily.
57:56- But as you cool helium down below 2.17 kelvin,
58:00thermal counterflows take over.
58:02They transfer heat so effectively
58:04that the entire liquid almost immediately
58:06reaches the same temperature.
58:08So there are no hotspots
58:10and no places for bubbles to form,
58:12the boiling suddenly stops.
58:16This lack of boiling paired
58:17with its unparalleled heat transfer also makes superfluid
58:21helium the ideal coolant at one or two kelvin,
58:24and that's why CERN uses around 700,000 liters of it
58:27to cool the LHC's 9,000 superconducting magnets.
58:31- There is still one thing that bothers me though, Chris,
58:37which is that, you know, if you take any substance
58:40and you keep cooling it down, it always freezes.
58:44But helium never does, yet
58:47it does liquefy, which is the annoying part.
58:50So because it liquefies, it means
58:51that there is some attraction between the atoms.
58:54And so you'd expect, you know,
58:55I can just keep cooling it down more and more
58:57and more until eventually the kinetic energy becomes
59:01arbitrarily small
59:02and it must, like the attractive energy must be much larger
59:05and so it must form a solid.
59:07- Yes. So this is the kind of
59:11intuitive picture that you would expect.
59:13Well, you can't answer that in an intuitive way.
59:17You have to bring in quantum mechanics.
59:19- Because according to quantum mechanics,
59:21the more precisely you know a particle's position,
59:24the less you know about its momentum and vice versa.
59:27This is known as Heisenberg's uncertainty principle,
59:29and mathematically it tells you
59:31that the uncertainty in position delta x times the
59:33uncertainty in momentum delta p is greater than
59:36or equal to h-bar over two.
59:38For simplicity, let's just say it's around h-bar.
59:41- Let's say I've got an atom in a liquid
59:45and it has some freedom.
59:46It can move around,
59:48but you know, it's very closely packed in
59:50with all of its neighboring atoms.
59:51Let's say that it occupies some spherical volume
59:55with a radius r.
59:57Well, we can say that that r
59:59is the uncertainty in its position.
1:00:02So if the uncertainty in the momentum is going to look like
1:00:06h-bar, Planck's constant, over r.
1:00:10- But if it's got an uncertainty in momentum,
1:00:13it means it must be jiggling around inside that sphere.
1:00:16- We know if it's jiggling, we can say it's got an energy.
1:00:20Well, we know what it's momentum is.
1:00:21It's Planck's constant over, over r.
1:00:25So how do we get the energy? Well, it's easy.
1:00:27We say it's the momentum squared over twice the mass.
1:00:31Now, helium being such a light atom,
1:00:35that mass turn is very small.
1:00:38So that means that the energy that it has,
1:00:40because it's confined, is very, very large.
1:00:44Now, this is what we call the zero point energy.
1:00:47This is the energy it cannot get rid of.
1:00:49It's nothing to do with thermal energy.
1:00:51It's not because there's a temperature in the liquid.
1:00:53In fact, when you're in the liquid state,
1:00:56that zero point energy accounts for more than 98%
1:01:00of the total energy in the fluid.
1:01:02- Wait, what? - It is, it is enormous.
1:01:05- That is crazy. I had no clue.
1:01:07- This zero point energy is too large for the atoms ever
1:01:11to lock down in place.
1:01:13So helium never freezes just by cooling it down.
1:01:16- So when you're thinking of this kind of ladder of states,
1:01:20this kind of BEC picture, like, you know,
1:01:22everything's macroscopically occupying this bottom state,
1:01:25we think we, the, the intuition is to think of it
1:01:28as like ladder where each rung is nice and evenly spaced.
1:01:33But the way to think of it in terms
1:01:34of this zero point energy is that
1:01:35that first rung is 10 stories off the floor.
1:01:40And then the thermal states stack up on top of that.
1:01:42- Oh, that's so good. - But there is one way in which you
1:01:46can freeze helium, which was discovered by Onnes's successor.
1:01:50That is by cooling helium down to about one Kelvin
1:01:52and by increasing the pressure to 25 atmospheres.
1:01:56If you do that, helium finally freezes.
1:01:59As for Onnes, he died in 1926 at the age of 72.
1:02:04He had been making superfluid helium for 18 years,
1:02:07looking at it, seeing hint after hint,
1:02:10and yet he never realized
1:02:11what he'd been staring at the entire time.
1:02:14Now, you might look at this
1:02:15and think Onnes was a little stupid or ignorant to miss it
1:02:19and to not put the pieces together,
1:02:21but I don't think that's the case.
1:02:22Instead, I think it reveals a fundamental
1:02:25feature of being human.
1:02:26I think it goes to show a lot
1:02:28of people think you got to see it to believe it.
1:02:31And I think often you have to believe it in order to see it.
1:02:35It makes me think a lot about Kuhn's, the structure
1:02:39of scientific revolutions,
1:02:41because it focuses a lot on, you know, you have the
1:02:43existing science, you have the existing paradigms,
1:02:45you have the existing formulas
1:02:46that these things are supposed to follow.
1:02:49And how often would the
1:02:52data point going off the line just be some error,
1:02:54someone messed up, condensation, something happening?
1:02:58It's easy to write those things off.
1:03:00So I have a lot of sympathy
1:03:01and I don't think, "Oh, how could they miss it?" I kind
1:03:04of think like, "No, that's actually
1:03:05how we operate." Especially, I
1:03:08don't know if scientists are supposed
1:03:09to be a little bit more observant
1:03:11or a little bit more skeptical,
1:03:12but at the same time, you're up against
1:03:14so much chaos in the world that you do kind
1:03:18of sometimes just see, "Yeah, yeah, that fits that.
1:03:22Yeah, that fits the data." And they sort of ignore things
1:03:26that don't fit because you're looking
1:03:28for something. I don't know.
1:03:29- Yeah. But I think probably especially if you are a
1:03:33scientist, it's these unexpected things
1:03:36that can end up having the biggest breakthrough
1:03:38because those are the things that disconfirm
1:03:42your view of reality.
1:03:43- And the thing is, often all the signs
1:03:46for the breakthrough are there.
1:03:48It's just that we're not ready to see them.
1:03:50So it makes you wonder what signs are we missing right now?
1:03:54I used to think that the reason someone didn't make the
1:03:57breakthrough was because maybe they weren't smart enough
1:03:59or they didn't work hard enough or didn't get lucky enough.
1:04:02But now I feel like I've always missed something.
1:04:05Just look at Onnes. He had all of those things.
1:04:07He was an experimental genius, had an immense work ethic
1:04:10and even discovered superconductivity, one
1:04:13of the biggest breakthroughs of the 20th century.
1:04:15He got three out of three.
1:04:17So perhaps sometimes making the breakthrough doesn't require
1:04:21any extra intelligence, hard work or even luck.
1:04:24Maybe all it takes is for us to let go
1:04:27of the way we believe the world to be
1:04:29and instead, just for a moment, try
1:04:31to see it as it really is.
1:04:34What if that is all it takes?
1:04:36And Onnes, well, he may have missed superfluidity,
1:04:39but he changed cold temperature physics forever.
1:04:42And in 1913, he received the Nobel Prize in physics
1:04:45for his achievements at finding out
1:04:47what happens when you just keep cooling down.
1:04:54Hey, two quick final things.
1:04:56First, this video would not have been possible without the
1:04:58help of Professor Weinfurtner's Gravity Laboratory at
1:05:00Nottingham University and the funding
1:05:02provided to them to run their superfluid experiments.
1:05:05So I want to give a big thank you to all of them
1:05:07for helping us recreate these experiments.
1:05:10The second thing is that we didn't even get
1:05:13to show you their research
1:05:14because while it might seem like we've solved all the
1:05:17mysteries of superfluids,
1:05:18it turns out we're just getting started
1:05:20because really weird things happen when
1:05:23you start rotating them.
1:05:24And this rotation
1:05:25or the physics of it can play a role in everything from
1:05:28neutron stars to simulating rotating black holes
1:05:31to a black hole bomb.
1:05:34And because if you look closely enough, you can kind
1:05:36of see the entire universe in a drop of superfluid.
1:05:40So let us know if you want us to make a part two on that.

About this transcript

This page contains the full transcript of I finally saw quantum mechanics with my own eyes... (It's spookier than I expected) by Veritasium, generated from the public captions YouTube serves with the video. The transcript has 10,726 words across 1,407 segments, with the original timestamps preserved so you can click any line to jump to that moment in the embedded player.

What you can do with it

Use the transcript to take notes, quote the speaker, build a study guide, generate a summary with ChatGPT or Claude via the YouTube Summary tool, or export it as a timed subtitle file with YouTube to SRT. You can also re-open it in the transcriber to choose a caption track available on the video.

Free YouTube transcript tool

YouTube2Text is a free YouTube transcript generator — no signup, no daily limit. Paste any YouTube link and get the full transcript instantly, with timestamps, click-to-jump, caption language selection, AI prompts for ChatGPT, Claude, and Gemini, and exports to TXT, SRT, VTT, or Markdown.