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The Very Weird Reason Hot Water Freezes Faster
The Very Weird Reason Hot Water Freezes Faster
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0:00
- Thank you to Displate for supporting PBS.
0:03
One day as part of a cooking class,
0:05
a boy named Erasto Mpemba was given the assignment
0:08
of making some ice cream.
0:10
He boiled some milk, added some sugar.
0:14
That's probably about enough.
0:16
Then he poured that mixture into a container.
0:20
Of course, he had to wait for it to cool
0:22
before he could put it in the freezer.
0:27
Who has time for that? I want ice cream.
0:30
On that day in 1963, Mpemba was in a hurry too,
0:34
so he put his hot ice cream mixture in the freezer while it
0:37
was still hot.
0:40
Now, his classmates left theirs out on the counter
0:43
to cool first before putting them in the freezer,
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but later, to everyone's surprise,
0:50
even though everyone else's mixture had started at a lower
0:54
temperature, Mpemba's ice cream mix froze first,
0:58
faster than everyone else's.
1:00
All of this happened at a secondary school in
1:03
Tanzania when
1:03
Mpemba was just 13 years old.
1:06
When he found his oddly expeditiously frozen
1:09
treat waiting
1:10
for him the next day, he could have shrugged it off
1:12
as some sort of fluke,
1:14
a weird unexplained event, but he didn't.
1:16
Instead, he asked why.
1:21
That question turned out to be a lot harder
1:24
to answer than anyone expected.
1:26
60 years after the ice cream incident,
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physicists are still arguing about this phenomenon,
1:32
now appropriately called the Mpemba Effect.
1:35
Maybe you've heard about this trick before.
1:37
But what you probably don't know is
1:39
how this strange observation,
1:41
watching hot water freeze faster than cooler water,
1:45
has begun to shake up very different corners of physics.
1:48
From how we heat and cool machines
1:50
to how we design materials,
1:52
even quantum computing.
1:54
Today, we're going to dig into this deceivingly simple
1:57
phenomenon to learn why taking the longer route can
2:01
sometimes be faster.
2:03
After I finish my ice cream.
2:12
Hey, smart people, Joe here.
2:14
It turns out that 13-year-old Erasto Mpemba wasn't the
2:17
first person to notice this.
2:19
Scholars have been pointing out this weird effect
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for thousands of years.
2:23
The first known reference came from Aristotle.
2:25
Doesn't it always? It was all the way back in 350 BCE.
2:29
He wrote this book called Meteorologica,
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in which he mentions how in certain places it was common
2:34
practice to heat up water before freezing it.
2:36
He points, for example, to ice fishermen in Pontus,
2:39
in what's now Turkey.
2:41
They would pour hot water on their rods
2:43
because it freezes quicker
2:45
and then use that ice to hold them in place.
2:48
Over the centuries, the effect was noted again
2:50
and again by scholars.
2:52
Here's one from Roger Bacon in the 13th century,
2:55
or René Descartes in the 17th century.
2:58
Lots of pretty smart people have been fascinated by this.
3:02
And historians have since found numerous accounts of people
3:05
around the world making the same claim.
3:08
Hot water sometimes freezes faster than cooler water.
3:12
But all this seems
3:13
to go against our intuition about how heat works.
3:16
So people have been pretty skeptical of claims like these.
3:19
Some even outright dismissing them as flawed experiments.
3:23
When Mpemba asked his physics teacher why his hot ice cream
3:27
froze first, he was told, "That is Mpemba's physics,
3:30
not the universal physics." Skepticism
3:33
is a good thing, of course.
3:34
It's a necessary ingredient in science,
3:36
but luckily that didn't stop Mpemba.
3:40
Years later, a physicist named Denis Osborne visited his
3:44
school and Mpemba asked him the same question.
3:47
Of course, his classmates laughed at him,
3:49
but Osborne didn't.
3:51
He went back to his lab and ran the experiment himself
3:54
and got the same seemingly impossible
3:57
result that Mpemba got.
3:58
And they published it together with Mpemba as first author.
4:02
My name is Erasto B
4:03
Mpemba, and I'm going to tell you about my discovery,
4:07
which was due to misusing a refrigerator."
4:10
Let's take a second to appreciate
4:11
how weird this actually is.
4:13
When you freeze a cup of water,
4:15
it cools continuously from room
4:17
temperature down to freezing.
4:19
If you start hotter, you've just got farther to travel
4:22
before getting to freezing.
4:23
So it should always take longer, right?
4:26
Well, this was captured in math by Isaac Newton
4:31
and his law of cooling from 1701.
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It describes how things change temperature continuously.
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Compared to a room temperature glass, a hotter glass
4:40
of water will drop in temperature faster at first,
4:44
but then it'll just trail behind the other glass
4:47
as they keep on cooling.
4:49
I mean, that makes sense, right?
4:52
Picture two curiously handsome runners
4:55
racing to a finish line.
4:57
One starts closer to the line,
4:58
the other one starts way back here.
5:00
Obviously the one that starts closer
5:01
to the finish line wins, right?
5:03
Well, that's not always the case.
5:05
Sometimes the runner
5:06
who starts farther back crosses the finish line first,
5:10
but only if he runs the race in a very certain way.
5:13
That's the Mpemba Effect.
5:15
It's actually not hard to show
5:17
that hot water can freeze really fast.
5:20
On a super cold day, throw some boiling water into the air
5:24
and it'll instantly freeze into these beautiful
5:26
icy rainbow shapes.
5:28
We try that with lukewarm water, just get cold rain.
5:31
But it has turned out to be incredibly challenging
5:34
to demonstrate the Mpemba Effect rigorously
5:37
and repeatedly in the lab.
5:38
In the decades after Mpemba
5:40
and Osborne published their paper, physicists had attempted
5:44
to replicate the effect with, honestly, pretty mixed results.
5:47
I mean, some studies seemed to replicate the findings,
5:50
but others directly contradicted them.
5:53
Now, there are two big reasons for this controversy.
5:56
One reason is that these experiments are really tough
5:58
to control perfectly.
6:00
Whenever you compare when two liquids freeze, what tips
6:04
that balance to one or the other can be so many things.
6:07
The shape of the container, dissolved gases, how smooth
6:11
or rough the surface is
6:12
or if something disrupts a super cool liquid,
6:15
because even tiny ice crystals can start a chain
6:19
reaction of freezing.
6:20
There's an absurd number of variables to control.
6:22
The second reason is that water itself is very weird.
6:26
It's actually most dense at four degrees Celsius,
6:30
just above freezing.
6:31
So as it cools, the coldest layer sinks
6:34
and warmer water rises to the surface
6:36
where it evaporates faster.
6:38
Once ice does actually start forming, those crystals float
6:41
and insulate the water underneath,
6:43
slowing the heat loss back down.
6:46
Water is just surprisingly strange stuff.
6:48
And for that matter, what does freezing even mean?
6:51
Is it when the first ice crystal forms?
6:53
Or when the average temperature hits zero?
6:55
When it's fully solid?
6:57
Different labs have used different definitions
6:59
and gotten different answers.
7:01
This is tricky stuff.
7:02
So maybe the Mpemba Effect happens just
7:05
because water is uniquely weird, right?
7:08
Well, not quite.
7:09
Over the last few decades, that exact same signature effect,
7:13
starting hotter and getting to frozen faster,
7:16
keeps showing up in materials that they have nothing to do
7:19
with water, which means that whatever's going on
7:21
is bigger than just one strange liquid.
7:25
Take these molecules, for example. Polylactic acid,
7:28
or PLA.
7:29
It's what many compostable cups are made of.
7:32
And if you got a 3D printer,
7:33
you've almost certainly used this stuff.
7:35
But to form it, you have to get the molecules
7:38
to crystallize.
7:40
They lock into an ordered structure,
7:42
which is sort of like freezing.
7:44
And guess how you can get them to freeze faster?
7:46
By heating them up first.
7:48
But it gets even weirder than that.
7:49
The effect doesn't just apply to temperature either.
7:52
You can find a magnetic Mpemba Effect too.
7:55
If you look at the magnetic field of certain materials,
7:58
you'll find that objects
7:59
with a stronger magnetic field can be demagnetized faster.
8:04
Again, winning the race
8:05
by starting farther from the finish line.
8:08
It seems like the Mpemba Effect isn't some one-off quirk.
8:12
It's a real feature of nature.
8:14
Now, the thing that they all have in common,
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the more extreme it is where you start,
8:19
the faster you can find your way back to equilibrium.
8:22
A state where everything is settled
8:24
and stable, done changing.
8:27
You see, nature loves equilibrium.
8:31
Leave out a cup of ice in a warm room,
8:33
and eventually it melts.
8:35
It settles at the same temperature as the room.
8:38
A leaf falls onto the ground and eventually it'll decompose
8:41
and become indistinguishable with the soil.
8:44
Everything, given enough time,
8:46
ultimately seeks out sameness.
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Why does the world crave equilibrium?
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Well, it's simple statistics, really.
8:53
Basically, there's just many more ways for things
8:55
to be randomly mixed up than to stay clumped together.
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The lukewarm water is vastly more probable than an ice cube.
9:04
Mixed up soil is more probable than an intact leaf.
9:07
So what does this have to do
9:08
with Mpemba's ice cream experiment?
9:11
Well, inside of a freezer, the equilibrium state is,
9:15
well, frozen solid.
9:17
Both the hot and cold cup of water are racing to
9:20
that finish line.
9:22
The question is, how does the hot cup get there first if the
9:25
cold cup has a head start?
9:27
Does it somehow want it more?
9:29
It turns out, well, sort of, yes.
9:32
See, scientists have worked out a mathematical
9:34
explanation for this shortcut.
9:36
They showed that the fastest path to equilibrium
9:39
isn't always the most direct one,
9:42
especially when systems start with more energy.
9:46
Imagine firing a ball into a maze.
9:49
The harder that you shoot it, the more that it can ricochet
9:52
around and maybe find a path to the target.
9:54
A system with more energy and more things inside, jittering
9:58
and moving in all sort of quantum and atomic-y ways.
10:00
They can explore more pathways to equilibrium,
10:04
potentially finding faster shortcuts along the way.
10:07
Picture the road to equilibrium as a downhill journey.
10:10
For a ball that starts here, it'll roll sort
10:13
of sluggishly to the finish.
10:15
But for a ball that rolls from here at a higher energy
10:18
starting point, it has more energy to carry it
10:21
to the end faster.
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If there are intermediate valleys along the way,
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the faster ball might be able to find shortcuts
10:27
to the lowest energy state.
10:28
That's how it reaches equilibrium faster.
10:31
One group of scientists tested the effect
10:33
by building this energy landscape in real life.
10:36
When they drop tiny glass beads into this landscape,
10:39
they could stop at this first energy hump,
10:42
or they could roll through that first one
10:43
to this more stable resting place.
10:46
They found that the beads that started out hotter tended
10:48
to find the lowest resting place faster,
10:51
just like the math predicted.
10:53
And it goes even smaller than that.
10:55
Down at the scale of individual atoms,
10:57
physicists have found a quantum version
10:59
of the Mpemba Effect too.
11:01
Picture a row of trapped ions, kind
11:03
of like tiny compass needles all pointing the same way.
11:07
That's a state of symmetry. Now tilt them out of alignment.
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If you just leave them alone,
11:12
they'll eventually drift back into sync.
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But if you tilt them further out of alignment,
11:17
sometimes they snap back into sync faster than ions
11:20
that were only disturbed a little.
11:22
Physicists confirmed this quantum Mpemba Effect in the lab
11:26
for the first time in 2024.
11:28
And this isn't just a curiosity.
11:30
Researchers have started using this exact effect
11:33
to quickly reset the memory in quantum computers.
11:37
It's sure starting to seem like this is all the same beast
11:40
in different forms.
11:42
From freezing ice cream to wiping out magnets
11:44
to quantum alignments.
11:46
Now across all these different materials
11:48
and different scales, there's one consistent theme.
11:52
The shortest route is not always the fastest one.
11:55
And we still don't fully understand why the Mpemba Effect
11:58
happens in every case,
11:59
but that doesn't stop people from putting it to work.
12:01
Physicists are already exploring Mpemba-inspired tricks
12:04
to make cooling and heating systems more efficient,
12:07
which not a huge surprise.
12:10
But Mpemba's turning up in some much weirder places too,
12:12
like improving the manufacturing of ceramic materials.
12:16
Now some very special ceramics have
12:18
to be compressed really tightly to be formed.
12:21
Instead of using a big squish machine,
12:23
one group used the expansion of freezing ice
12:26
to squeeze the material.
12:28
And sure enough, they found
12:29
that the freezing happened faster if they
12:31
started with hot water.
12:33
Thanks, Mpemba. Who knows where else Mpemba will show up?
12:36
I mean, that's the beauty of discoveries like this.
12:39
You start with this tiny little oddity, something
12:41
that doesn't seem quite right.
12:43
And if you keep poking,
12:45
you might end up exposing a new way of looking at the world.
12:48
In this case, the mystery comes from assuming
12:51
that everything is close to equilibrium.
12:53
Whether or not we realize it, much
12:55
of our intuition about the world is
12:57
built on that assumption.
12:59
I mean, we imagine that the air in a room is exactly the
13:02
same temperature everywhere.
13:04
We assume that flipping a coin a hundred times will land us
13:07
exactly 50 heads or tails,
13:09
or that the balls in a ball pit are randomly
13:12
distributed by color.
13:13
But if you think about it, the things
13:15
that are far from equilibrium are often
13:17
the most interesting.
13:19
Take fire. If you leave it in an empty room,
13:22
it'll eventually peter out and just blend in with the air.
13:25
But if you keep feeding it oxygen
13:27
and fuel, if you keep pushing it out of equilibrium,
13:30
it can light up your room, even drive your engine.
13:34
And you and I
13:35
and all life as we know it, we are all out
13:38
of equilibrium too.
13:39
All the work that we do in the world is only possible
13:43
because we draw energy from the sun
13:45
and from our food to constantly push ourselves away
13:48
from equilibrium.
13:50
When we fall into equilibrium, we die. We decompose.
13:54
Let me just blend in with the soil, just like that leaf.
13:57
Far from equilibrium is
13:58
where all the exciting stuff happens.
14:00
But it's also where our intuition about the world
14:02
kind of goes out the window.
14:03
It's where nature can play all sorts
14:05
of surprising tricks on us, like the origin of life
14:08
or earthquakes or the Mpemba Effect.
14:11
This story is a wonderful reminder of the power
14:14
of questioning the world around us.
14:15
Even things as trivial as a bowl of ice cream.
14:18
You never know what it will turn up. New questions?
14:21
Faulty assumptions, deep insights,
14:23
just hiding in plain sight.
14:25
I think Mpemba and Osborne put it best in their very first
14:29
paper about this effect:
14:31
"No question should be ridiculed.
14:33
Everyday events are seldom as simple as they seem."
14:37
Or in other words, stay curious.
14:40
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We'll see you in the next video.
17:04
Sorry, Carl, I win.
17:06
I don't need, I don't need to eat more ice cream at the end.
17:10
Director: "rolling"
17:11
Joe: "cool drumming noises"
