The Elegant Laminar Flow Of Moroccan Tea

Summary

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The Rest Is Science – Episode Summary

Episode Title: The Elegant Laminar Flow Of Moroccan Tea

Host: Goalhanger
Guests: Professor Hannah Fry, Michael Stevens (Vsauce)
Date: April 15, 2026

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Overview

In this engaging and curious episode of The Rest Is Science, Professor Hannah Fry and Michael Stevens take listeners on a fluid yet profound exploration of the science behind Moroccan tea – from the intricate design of the teapot to the wonders of laminar flow, the chemical origins of that signature foamy head, and why folk design can outpace formal scientific modeling. The second half of the episode pivots into fielding fascinating listener questions—ranging from how altitude and sea-level shifts affect boiling points, to estimating how much of Earth's surface humans have truly set foot on, and even how we sense the world through layers like socks or sticks.

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Key Discussion Points & Insights

1. Moroccan Tea Ritual and the Science of the Teapot

[03:28–10:51]

• Hannah’s Marrakech Inspiration:
  Staying at La Mamounia in Marrakech, Hannah is captivated by the unique shape and pour technique of Moroccan teapots.
• Teapot Design Breakdown:
  • The body is a section of a cone; spout starts unusually low, curves upward, then out, forming an S-shape, and ends with a sharp tip.
  • The design expertly applies principles of fluid dynamics—particularly laminar flow, and the acceleration of the tea stream.
  • Quote – Hannah Fry [04:33]:
    "It is basically a masterclass in fluid dynamics, chemistry, and the evolution of design ... it's like the whole world in a teapot."
• Foamy Head Purpose:
  • Pouring from a height produces a foam, practical for desert conditions: the bubbles trap sand and are a sign of good hospitality and proper preparation.
  • Foam is enabled by saponins in green tea (amphiphilic molecules, like soap) and stabilized by added sugar (increased viscosity) and mint oils.
  • Quote – Hannah Fry [06:03]:
    "It looks like a bubbly lager ... the idea is that if there is sand floating around ... it will get trapped inside these glossy bubbles."
• Laminar Flow Explained:
  • Pouring from high requires a smooth, cohesive stream—laminar flow—to avoid tea splatter:
    • The low-starting, narrowing S-shaped spout accelerates tea and straightens fluid flow, minimizing turbulence, making a controlled, "well-behaved" stream possible.
    • The sharp tip is critical—proven by a Nobel-prize winning study on drip prevention—that, incidentally, Morocco had solved generations earlier.
  • Quote – Hannah Fry [11:13]:
    "You need the tea to leave the spout in what's called laminar flow. It needs to be extremely neat, extremely slippy, extremely well behaved."
• Folk Physics Versus Scientific Modeling:
  • All this was achieved by centuries of trial and error, not formal calculation—a triumph of folk engineering.
  • Quote – Michael Stevens [14:58]:
    "It emerges through natural selection in a way, except the pressure is our desire to have laminar flow, foamy tea ... but you never had the equations."
  • Diverse cultures often intuit optimal engineering long before formal science explains it (examples: Roman arches, Japanese steel forging).
  • Quote – Hannah Fry [16:37]:
    "The sum total of human knowledge, even without science, is just really profound ... in a single cup of tea, you'll eventually discover the truth of 10,000 forms in the universe."

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2. Listener Q&A: Boiling Water, Altitude, and Sea-Level Change

[20:04–28:47]

• Q1: How Does Changing Sea Level Affect Boiling Points at Altitude?
  • At higher altitudes, water boils at lower temperatures (e.g., 95°C/203°F in Boulder, Colorado vs. 100°C at sea level).
  • If sea level rises, your "altitude above sea level" technically reduces, but the actual effect on air pressure (which dictates boiling point) is incredibly minor compared to the sheer mass and thickness of Earth’s atmosphere.
  • Both hosts riff on global-scale physics, using metaphors (cake and frosting) and toy models; conclude the effect is so miniscule compared to other planetary variables that it’s not practical.
  • Quote – Michael Stevens [27:40]:
    "If all that happened was that Earth technically had a smaller volume, I think that the boiling point of water would go up, regardless of the fact that your altitude above sea level went down."

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3. Listener Q&A: Human Footprint on Earth’s Surfaces

[28:47–34:33]

• Q2: What Percentage of Vertical and Horizontal Surfaces Have Been Touched by Humans?
  • Urban infrastructure covers only ~1% of land; including agriculture and managed forests, the human-used portion is a minority.
  • Wilderness (uninhabited, undisturbed) covers ~52% of terrestrial land.
  • Rough estimate: Only about 15% of land, and ~5% of Earth's total surface (factoring oceans), has ever had a human footprint.
  • For vertical surfaces (e.g., rock faces), Hannah estimates using rock climbing routes: the total is insignificantly tiny.
  • Quote – Hannah Fry [31:41]:
    "Only 5% of the Earth's total surface has ever had a human footprint on it. Isn't that amazing?"

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4. Listener Q&A: Feeling Through Socks, Shoes, and Tools

[34:45–40:44]

• Q3: How Do We Perceive Texture Through Socks, Shoes, or Even Objects?
  • Our nervous system is remarkably adept at inferring tactile details through layers or intermediaries (like socks or sticks).
  • Michael shares the example of sensing stone textures through shoes, referencing studies on vibration feedback and the brain's interpretation of sensory input.
  • The brain "extends" the sense of touch to prosthetics, tools, and worn objects, treating them as part of the body, much as visually it ignores glasses frames or the nose in one's field of vision.
  • Quote – Michael Stevens [38:14]:
    "It's very trippy to think that it's an extension of our body, really. We learn how to feel through sticks and walking sticks and socks and shoes and gloves, as though they were part of our bodies with sensory receptors on them."
  • Quote – Hannah Fry [39:08]:
    "Or ... your nose is permanently in your field of vision. You just don't bother noticing it. ... Reality is not reality. It's just your brain's interpretation of it."

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Notable Quotes & Moments

• On the elegance of Moroccan teapots:
  "This teapot ... is one of the most gorgeous demonstrations of how the evolution of design lands on totally optimal physics solutions without ever having touched an equation."
  — Hannah Fry [13:28]
• On the universality within a cup of tea:
  "A single cup of tea gives us thermodynamics, fluid dynamics. It gives us history and botany and human culture and tastes and flavors. It’s all there."
  — Michael Stevens [17:05]
• On human hubris and exploration:
  "We think of ourselves as an invasive species. We've barely got started." — Hannah Fry [33:44]

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Timestamps for Key Segments

• [03:28] — Hannah introduces the science of Moroccan tea and teapots
• [06:03] — Why Moroccan tea is poured from a height, and the chemistry of the foam
• [10:51] — Laminar flow and the fluid dynamics of teapots
• [13:28] — Folk engineering vs. formal physics
• [20:04] — Listener Q: Altitude, boiling points, and sea-level rise
• [28:47] — Listener Q: Percentage of Earth's surfaces with human contact
• [34:45] — Listener Q: How we feel through socks/shoes/tools
• [39:08] — On how the brain interprets reality

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Tone & Style

True to Fry and Stevens' playful, accessible, yet intellectually rigorous style, the conversation swings between real-world anecdotes, historical and cultural context, and foundational scientific principles. Their curiosity is palpable; their explanations vivid; and their respect for the limits—and creativity—of human knowledge is a recurring thread.

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For the Curious:

If you love learning how everyday objects embody centuries of trial-and-error science—or want to rethink what it means to "experience" the world—this episode is as refreshing and layered as a cup of minty Moroccan tea.

Transcript

[Michael]

Hello and welcome to Field Notes here at the Rest Is Science. Field Notes is our little exploration diary. Do you like calling it that, Hannah?

[Hannah]

You know, exploration of the mind. I'm okay with that.

[Michael]

For me, it's more of a show and tell. Because you are here today to show us something very cool about tea. And I don't know what it is. It's not my exploration, it's your. It's your reveal.

[Hannah]

It's your exploration into my mind.

[Michael]

That's right. I will be exploring your mind while you spill the tea, as they say, or used to say. And later on, we'll answer some questions from you guys, which you can always send to [e-mail] Please keep them coming in because there's some awesome stuff.

[Hannah]

Yeah, we've switched the episode upside down today because, I'll be honest with you, I'm on holiday in Marrakech, staying at La Mamouna, this beautiful hotel, and I have become completely obsessed with Moroccan teapots. And. And so I've insisted. I've insisted basically that that's where we start. Let me just. Let me just go get this. Go and get this tea sitting behind me. Hold on one second. This episode is brought to you by Cancer Research uk.

[Michael]

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[Hannah]

Okay?

[Michael]

That's the scale of the DNA rulebook inside each one of your cells, telling it when to grow, when to divide, and when to stop.

[Hannah]

And different tissues read that same rulebook in different ways. So a skin cell doesn't behave like a lung cell.

[Michael]

And cancer can begin when those instructions change not one dramatic moment, but through small, gradual edits over time.

[Hannah]

Now, cancer isn't one disease. It is more than 200 types shape by where those changes to the rule book happen and how cells respond.

[Michael]

Cancer Research UK is the world's largest charitable funder of cancer research, backing studies across all types of cancer work that

[Hannah]

takes years of very careful, steady progress to deliver each breakthrough.

[Michael]

For more information about Cancer Research uk, their research breakthroughs and how you can support them, visit cancerresearchuk.org the rest is science girl.

[Hannah]

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[Hannah]

Visit Red Bull.com BrightSummerAhead to learn more.

[Michael]

See you this summer.

[Hannah]

So have you been to Morocco, by the way?

[Michael]

I've never been to Morocco.

[Hannah]

Oh my gosh. It's absolutely beautiful, right? But one of the things that they serve here is this very delicious mint tea. And it comes in a teapot that is absolutely scorching hot that looks like this. What I'm holding is this absolutely beautifully ornate. I think it might even be plated in silver. But it's the shape of it that is so incredibly interesting. So it is. It's like a section of a cone. The body of the teapot is like a section of a cone. And the spout starts really unusually low, almost at the very bottom of the of where the liquid would be sitting. And then it curves upward, but. But then crucially curves outward again. It makes this sort of this beautiful S shape and then finishes with this very sharp little point that sits above the level of where the fluid would actually be. And I think by the end of this, you're going to know why I'm obsessed with this teapot. Okay. Because it is basically a masterclass in fluid dynamics, chemistry, and the evolution of design. All right? So it's like the whole world in a teapot.

[Michael]

Nice.

[Hannah]

Okay, now I'm going to try and do this without burning myself. The key thing about morale Moroccan tea is it's like, it's like a green tea that they stuff with mint leaves and like an insane amount of sugar. But crucially, when they pour it, they do this really, really big pour. Now, that was. Anyone who is actually Moroccan is going to look at what I did there and think it was a pathetic. A pathetic version of what the professionals do. I've got a video for those of you who are watching on YouTube of an actual professional doing this for me at breakfast.

[Michael]

Okay, so we're outside and we're watching a waiter pouring tea into a cup that's on a tr and he starts low, but Then he lifts the pot up all the way above his head and this stream is just falling, falling, falling right into the cup. And I can see that sloshing, the air getting mixed in. By the way, we're outdoors, this is beautiful. We got a nice blue sky. Looks delicious, I should say.

[Hannah]

If anyone watching or listening to this is themselves a pro at pouring Moroccan tea, then send us in your video and we can judge it. So the key thing to notice is that when you pour this tea, you end up like this. Down here is all like, you know, tea, tea colour, fine, boring. But what you have on the top is these really foamy bubbles. Can you see that? There is a good reason why you would want this on your tea.

[Michael]

It looks like the head of a beer. It's a foamy, but it's the tea like air has been mixed in. Right. Is that what causes it?

[Hannah]

Exactly. It looks like. It looks like a border lager, essentially, like a bubbly lager. And the story goes that the reason why you would have this in Morocco is because if you are out in the desert and you're drinking tea and there's sand flying around all over the place, what this does. I mean, this shows how pathetic mine is when, when the professionals do it, it's a really foamy head of. Head of tea that stays there for absolutely ages. Mine is already popping. Demonstrates. I'm not very good at this. But the idea then is that if there is sand floating around in the wind and it gets onto your tea, it will get trapped inside these glossy bubbles on top and then you can either blow them off to get rid of the sand or if you kind of drink in a particular way, then the. The sand will. Will stay within the bubbles and you can liquid below. But even though it starts off having this really practical reason behind it, it's also. This foam is now considered as this visual indicator that it's good hospitality and it's good quality tea and that you've been served correctly. Because where it comes from, I mean, if you pour normal black tea that you get in, in, in Britain, for example, you don't end up getting this foam. And where this comes from is this, like, chemical process that is going on inside the tea. Because green tea leaves, they contain this thing called saponins, which is like this natural soap like molecule, and essentially they are amphiphilic, I think is the word. Basically, this molecule ends up half the molecule. One end of the molecule loves water, the other half hates it, so it ends up orienting itself to make these Kind of bubbles, the sort of water hating tail stick out into the air bubble while the water loving heads of this molecule stay inside the tea. And that creates this kind of protective skin around the air which effectively lowers the surface tension and makes these bubbles form in the first place. But also you add a shed load of sugar to increase the viscosity of this, of this liquid so that the bubbles stay for a while. And then because you've got loads of peppermint in there and loads of mint, you have these, these kind of essential oils which then stabilize the bubbles even further. So. Okay, it's good to keep sand out and it lets you know that it's sweet enough and that. And you, you're not, you're not cheating. You're not using like mint, basically, you're using like the fresh good stuff. Okay, hang on one second. I'm actually just gonna have a little sip of tea. Hold on.

[Michael]

Is this only true for teas that are made with things like mint leaves? Would you not get this from a traditional black tea, an English breakfast tea, for example?

[Hannah]

I don't know, actually, I'm not sure that's a good question because black tea and green tea are from the same plant, aren't they? They're just the way that they, the way that they process them ends up being quite different. So I'm not sure. But definitely the aim of black tea is not to create these bubbles, right?

[Michael]

No, it isn't. But it's cool that the, the purpose, the, the stated original purpose of the bubbles is to keep out debris. Because the first thing I thought was, oh, that's how soap works. Soap creates bubbles. Surfactants create these bubbles that then lift out debris like particles and dust. And that's literally what they're being used for in the tea.

[Hannah]

You're literally drinking King Sophie tea. I mean, it is a surfactant that you're, that you're doing.

[Michael]

Yes.

[Hannah]

Okay. The real reason why I'm obsessed with this, right. Is when, I mean, it tastes delicious. Sure, that's great. But when I saw the teapot, I immediately spotted the fluid dynamics that's going on inside this thing because this is a very particular shape.

[Michael]

It's like the size of a pretty big coffee mug. But of course it's a, it's a jar shape with a, like a bell shaped kind of lid. But the spout is starting from very low.

[Hannah]

The other thing to notice about this spout, because this doesn't look like a British teapot. Okay. Like a British teapot, you have the Spout starts maybe halfway up the pot, sometimes a little lower, but, but around about halfway and the spout just curves upwards. It's like a U shaped spout. This spout is S shaped and that is absolutely critical to getting these bubbles to form. If you want to get proper bubbly tea, what you need to do is you need to be able to pour this tea from a real height, like arm's length away from the cup. You want it to gather up all of the speed and momentum and when it plunges into the liquid, you want it to take in all of the air from around it with it. So it's kind of like plunging right to the bottom of the glass and creating these big bubbles and foamy surface. Now, in order to do that, in order to be able to pour this tea from a. Which by the way has the added advantage of cooling it down to make it sippable.

[Michael]

Oh, sure, yeah, yeah.

[Hannah]

You know, even though in the pot it's absolutely boiling, what you need is you need the tea to leave the spout in what's called laminar flow. It needs to be extremely neat, extremely slippy, extremely well behaved tea so that you can get this stream that will behave all the way down and you can direct it perfectly into your cup.

[Michael]

If it wasn't laminar, it would spread out too much over this long falling distance and you would just get, it would rain tea all around the cup.

[Hannah]

It would be boiling tea. Soap rain. Yes, exactly. Okay. Doesn't that sound delicious? All right, so to get laminar flow, then what you have got here is all of the things that you notice. So for starters, the fact that this, the spout starts right at the very bottom of the teapot. This means that you are at the point of the liquid that is, that's under pressure, right? You've got like the highest kind of dense, you've also got a lot of dense tea leaves down there. Sort of the, the, the, the flavor of the tea is going to be best down there, but it means that you've got the weight of all of the tea, all of the liquid sitting on this, on this exit as you're pouring so that you're kind of pushing down on it as much as possible. Then what happens is this spout starts off reasonably wide and then it gets thinner and thinner and thinner and thinner and thinner all the way to the end, which means you're, you're reducing the aperture, you're speeding up effectively, you are accelerating this tea all the way through the spout to get it? Go faster and faster.

[Michael]

And what is that? Is that like Bernoulli's principle or something? Like the liquid speeds up as it's. The cross sectional area of the tube shrinks.

[Hannah]

Yeah, you've got Bernoulli, which is talking about the pressure, talking about the size of the aperture. There's like Reynolds number stuff going on here. There's the Venturi effect, there's like the. There's all sorts of like really delicious fluid dynamics going on here. The thing is, is that normal teapots with the U shape, they're doing the same thing. They're accelerating the T out of the end. But this S shape is absolutely critical because if you imagine that you were going on a slide, like a water slide, and you kind of come round and the slide bends in one direction, you're going to go up the slide, you're going to be slopping about all over the place as you exit that slide. So crucially, this S bend, so it curves one way and then curves the other, it essentially straightens out all of these water molecules so that when they exit from the tip, they're all pulling in the same direction. You haven't got any little tiny little eddies or like bits of. Bits of turbulence that are hiding inside

[Michael]

of the sprout, right?

[Hannah]

All essential for laminar flow. Then you've also got this extremely sharp tip. So in the 90s, there was this, this group of fluid dynamicists who won the IG Nobel Prize for working out how to stop a teapot from dripping, did all of this unimaginable, rich mathematical analysis. And the answer that they came to essentially was what the Moroccans had already found hundreds of years ago, which is to just have this sharp corner on the end. And this, I think, brings me to what I really, really like. The reason why I find this kind of design so interesting and exciting, because I could literally spend maybe four years writing the equations for this teapot, right? I could, I could work out these extremely sophisticated computer simulations demonstrating that this is the optimal way to create laminar flow that behaves in this way to, to. To create bubbles exactly like that. But here's the thing. This teapot, there was literally no maths or physics that was involved in the design of this. And so I think this is one of the most gorgeous demonstrations of how the evolution of design manages to land on totally opt physics solutions without ever having touched an equation. I think that's really cool.

[Michael]

It's beautiful. It just, it emerges through natural selection in a way, except the Pressure is our desires to have laminar flow. Foamy tea. And you try things out and then, boom, you've solved the equations, but you never had the equations, but you never had them.

[Hannah]

And you see this. I mean, you do see this over and over again, right? Roman arches is a really good example of, like, finding this optimal way to support a structure. You see this in Japanese steel in the way that they heat the. That when I'm making knives and blades, the way that they heat it is actually this. You know, to understand what they're doing requires this incredibly deep understanding of the material science of the structure of the materials that they're working with. But actually they just chanced upon it through many, many, many, many, many iterations of design over numerous generations.

[Michael]

Yeah, that's right. I love the folk physics of it. The like, look, if you warm the metal up and then you cool it down quickly, it's going to be softer and we call that annealing. And today you can look up a bunch of videos about exactly what's happening at a molecular level and why. But before that was really understood, before atomic theory, before molecular theory, it was just like the metal has a personality and you have to treat it this way. You have to be mean and then tender. And I can see how much more alive the world was when you had to just make everything an anim. And yet it still gave you the right answer. I mean, maybe it didn't allow you to fiddle with numbers a little bit and develop whole new, like, hardnesses of steel, but you would discover them on accident and it was like a blessing.

[Hannah]

Exactly. I think this is it, actually. You know, the sum total of human knowledge, even without science, is just really profound. There's this Japanese idea, actually, that in a single cup of tea, you'll eventually discover the truth of 10,000 forms in the universe. Right. Like this idea that you can observe all of humanity in a single cup of tea.

[Michael]

And it's not that far off. A single cup of tea gives us thermodynamics, fluid dynamics. It gives us history and botany and human culture and tastes and flavors. It's all there.

[Hannah]

It's all there. There you go, Michael. That's me spilling the tea. Maybe less juicy than you might have been expecting, but more fluid dynamics. All right, should we go to a break? And then we'll come back to some of your questions after. This episode is is brought to you by Cancer Research uk.

[Michael]

We often think of beating cancer as treatment, but imagine stopping it before it begins. After years of work, Cancer Research UK Scientists are launching a clinical trial of lungvax, the first vaccine designed to prevent lung cancer.

[Hannah]

It builds on Tracer X, the world's largest cancer evolution study, which tracked lung cancer cells over many years to uncover the disease's earliest warning signs. Lungvax is designed to train the immune system to spot these signs early on, destroying faulty cells before cancer develops.

[Michael]

So it's not treatment, but preventative, with the potential to stop lung cancer before it starts. The first stage of the trial starts this year. Focusing on people at higher risk, it

[Hannah]

shows what long term research makes possible.

[Michael]

For more information about Cancer Research uk, their research breakthroughs and how you can support them, visit cancerresearchuk.org TheReestisScience this episode is brought to you by Prime Obsession is in session and this summer Prime Originals have everything you want. Steamy romances, irresistible love stories, and the book to screen favorites you've already read twice off campus. Ellen every year after the Love Hypothesis, Sterling point and more slow burns, second chances chemistry you can feel through the screen. Your next obsession is waiting. Watch only on Prime.

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[Michael]

And welcome back. Hannah and I are now going to delve into questions that you all have sent us.

[Hannah]

We certainly are. Do you want to go first, Michael? I took the first half.

[Michael]

Yeah, I'll go first because I love this question from Lewis Taylor. The question is the boiling point of water is affected by your altitude, which we tend to measure as a height above sea level. But if sea level changes outside of its normal range, will that change the time it takes to boil water and therefore eggs? And I love this because one, it's very personal to me because I spend time pretty much at sea level in LA, but also up over 5,000ft in Boulder, Colorado. And you notice a difference. I have to now remember two different times for soft boiled eggs. Like here in la, it's like five minutes. We're getting, we're gonna, it's too far. But I need to do six and a half in Boulder and you can measure the water temperature and it's like, oh wow, this Boiling water is not 212. You know, it's like, let me see. Temp of boiling water. Boulder, Colorado. Yeah, the water boils in Colorado, in Boulder, at 203 Fahrenheit.

[Hannah]

Translate that into a senseful system for me, could you?

[Michael]

In C is 95. Whoa. So water boils at 95 degrees Celsius in Boulder. And that means that to cook an egg, like to soft boil or hard boil it, it just needs to be in that water a lot longer. Like a couple minutes longer at least. And pasta is the same way. You know, normally pasta instructions would be like 10 to 12 minutes. Boil this to be soft and I would go just to like 10 or even under the minimum. But in Boulder, I've got to go past the maximum and it's still not even al dente. It's still like too hard.

[Hannah]

I think that you would. I think you'd particularly notice that too, given that for some strange reason, Americans don't have kettles. I've always found that the weirdest thing about your country.

[Michael]

I can't say too much about it though, because my mother had a kettle. Oh, my wife, I married a Kiwi who lived in England for decades. So she's got an electric kettle in our house and we use it all the time. So. Look, I get what you're saying, though. It takes even longer to boil water when you don't have like a super fast turbo electric kettle. But yeah, I think constantly about how my altitude is the reason the water's taking longer to boil. Long story short, there's just less air above you when you're already up in a mountain. And it's that water's. Sorry, it's the air's weight that's pushing down on the water molecules, saying, no, you cannot leave your liquid state. You cannot leave this container. And if you go up high enough, there's so much less weight from that air squeezing on top that the water molecules can get out more easily. They don't have to have as much energy to leave the liquid state so that it boils with a lower temperature with less kinetic energy.

[Hannah]

And

[Michael]

I always think of my altitude in terms of a number like 5,000 beats or whatever, and that's from sea level. But if sea levels go up, like let's say I live at 5,000ft. If the sea levels rise by a foot, do I Now live at 4,999ft? Technically I do. But what happens to the actual air pressure effect on boiling? And I think that's a lot more complicated And I think, first of all, the ice. And tell me if I'm wrong, because this is just me kind of noodling through it. The ice that melts that then causes the sea level to rise, took up more volume than the water does. I know that a lot of glacial ice is much more dense than the typical ice that we'll put in a drink, but I think that overall, the amount of space taken up by solid matter on Earth goes down when sea levels rise because you've exchanged some liquid water. Well, you've exchanged some solid ice for some liquid water, which is a smaller volume.

[Hannah]

But crucially, you have a hell of a lot of water in glaciers on land that could be added to that total.

[Michael]

Yes, but if a glacier on land melts, the total volume of solid material on Earth has gone down because it's turned from ice to water. And so in that way, the average height of Earth, like what we call mean sea level, would. Well, but mean sea level is based on where the oceans are floating. But you got to imagine that all the solid stuff on Earth is displacing the air. And when the solid stuff that's displacing the air gets smaller, you now have, what, a thicker atmosphere or a thinner.

[Hannah]

I see where you're going with this. Isn't there an additional complicating factor, which is that there is a huge amount of gas also trapped within glaciers, so that once that ends up melting, there's a higher amount of air, of air overall. Right.

[Michael]

Let's just do like a toy model. And let's imagine that the. The Earth becomes smaller, like half the diameter, but it has the same amount of air. Now, we're really exaggerating this change, but that same amount of air is now going to be much thicker everywhere the atmosphere is thicker because it's got a smaller surface to cover.

[Hannah]

Hang on, let me make sure I'm following you. You take the sphere of Earth and you shrink it so that it's half the size.

[Michael]

Yeah. And you keep all the same air around.

[Hannah]

All the same air. So, air. Are we saying that the outer, like the Karman line, as it were, effectively is like. Is in the same place or is the whole thing shrinking?

[Michael]

I think that the. Only the solid part of Earth shrinks. So I think the Karman line would go up because you've got the same amount of air. Okay, so imagine you've got, like, a cake that's really big and you've got one jar of frosting. You can cover the whole cake, but it's A thin layer. Now, you make the cake half the diameter, but you have the same amount of frosting, it's going to be a thicker layer. So air pressure, frosting pressure in our example here is going to be greater now at each point on the cake surface because there's a higher column of frosting above you than when you were a big cake with a thin layer. Let us know in the comments below what I'm getting wrong, because I am just noodling on this and I'm thinking that if the total volume of solid stuff that Earth is made of shrinks, like the total amount of mass I'm imagining stays the same, the volume gets smaller, the air pressure would increase. But I think there's also so many complicating factors, like could the atmosphere become thicker or is it going to start getting picked off more quickly by solar wind or something? Thing like, I think when we talk about sea level rising one foot, we're probably talking about such a small change that other consequences might compensate. Extra air release from melting glaciers, thicker atmosphere being pulled off by solar wind, there might not be much of a change. But if all that happened was that Earth technically had a smaller volume, I think that the boiling point of water would go. Go up regardless of the fact that your altitude above sea level went down.

[Hannah]

I like this so much. This feels like. This almost feels like a. You know, when they do those incredibly hard interview questions.

[Michael]

Yes.

[Hannah]

They. Yeah.

[Michael]

How many ping pong balls are here in New York City? Yeah, right.

[Hannah]

Exactly. Or like, if you were a flea trapped in a blender, how would you get out like that? If you were shrunk down to the size of a flea and put into a blender? Right. That kind of thing, you know. Yeah.

[Michael]

This is.

[Hannah]

I want to think about this. I want to think about this. Maybe, maybe in the comments as well, you can, you can tell us your answers and maybe we'll come back to this in another episode of Phil Notes and we'll compare. Because I think there's an argument that it's the opposite. But, you know, there's. I want to. I want to get my equations done before I. Before I commit.

[Michael]

Yeah, yeah, yeah. Using no equations or anything, just straight up what I know about stuff and comparing stuff to cake sometimes very helpful. I think that all else being equal, if sea levels rose because of melting glaciers and ice caps, technically the boiling point of water everywhere would be a little higher, despite the fact that your altitude above sea level will have gone down because mean sea level will be closer to you.

[Hannah]

I'LL tell you what, there was a question I really like the look of by Ol Stearne. That's my best possible attempt at the pronunciation. And I think this links in quite nicely with this, because here's a question. I was watching some climbing videos on YouTube and it made me wonder what percentage of, of naturally occurring vertical surfaces on land that are on land have actually been traversed by humans? What Great question. So I had a little go at this at calculating it. And a sort of related question that I have wondered so many times is how much of horizontal surface has ever seen a human footprint? Like when you go out and you're kind of walking the land, how much of Earth has no human footprint ever laid upon? This is sort of, I mean, this question about earlier is like the vertical version of this, I guess, and that one does have a clear answer. Right. So I'll tell you this. Well, clear ish answer, because I think that humans, you sort of. I think it's very easy to feel like we're spread out all over the place, that we've come, dominated the entire planet. But the reality is we're actually a really huddled species. So if you take all urban infrastructure, everything that we've built and live in, it's actually only 1% of land. It's tiny, teeny, teeny tiny, which is

[Michael]

an even smaller percentage of Earth's surface.

[Hannah]

Exactly. So everything, every number I'm going to say here, you have to basically cut it by 30. You know, cut it to 30% of the number because as you say, 70% of the earth's surface is, is ocean. Even our agriculture, which is way, way, way, way, way bigger than the urban infrastructure, is only 37% of Earth's land mass managed forests about 10%. But wilderness dominates. I mean, if you think about the Sahara Desert or, you know, the Arctic, the Antarctic, it's 52% of the, of the land mass. So if you go through and you make some assumptions about, you know, I think you can, you can assume that in an urban landscape, 100% of the land, you can't, you know, you're not going to find a single patch where people haven't stepped on it, which decreases as you go further down. So in agriculture, some bits of agriculture, maybe it's going to be 80%, some bits, maybe slightly less, more like 20%. But in the wilderness, I mean, almost none of it has been stepped on by humans. So when you kind of go through and calculate this, it's about 15% of the Earth's surface have ever Seen a human footprint. Footprint, which as you say, sorry, that's of land. And then when you consider that that's only a third of the surface because of the oceans, 5% of the Earth's total surface has ever had a human footprint on it. Isn't that amazing?

[Michael]

That's amazing, yeah. Because it's easy to get so sad about how there's no new frontiers, there are no far off, distant lands that, that we've never visited. I was thinking about that today. You're coming from Marrakesh. Just 200 years ago, I would have read about it in a book, but I would never even probably correspond with anyone from there. Now it's like, oh, we're both gonna be talking live from Los Angeles and Marrakech simultaneously. Piece of cake. Now, are you factoring in like the actual surface area of a footstep? Because I could walk in a field, but I haven't walked on the entire field. But I have, you know, been near and seen a lot.

[Hannah]

I was cheating slightly. I did, I did some, I did some rule of thumbs. So I said that if it's a field, then it's about 80. 80 of that would have been covered. Interesting.

[Michael]

Okay.

[Hannah]

I mean, I'm getting, basically, I'm guessing, Michael, at this point. Oh, you get down deep enough, you're always guessing at everything, frankly. So there's some estimation going on. But to do this for the vertical surface, which is. Was the question, right? This, this, I mean, if we're, if we're talking about 5% of the horizontal surface of the Earth, the vertical surface is almost nothing. So I, I looked it up. There's about a million established rock climbing routes around the world. I mean, that is, I like, I'm doing one significant figure here that is like, again, this is some serious estimation going on here. And then when you think about them, I was like, what do you reckon? About 30 meters? I mean, most of them are not going to be on average about 30 meters, maybe about 2 meters wide. So once you run these numbers, you're talking 60 square meters of climbing surface for each of these million routes. I mean, this is a surrounding error. We've done none of it. Literally none of it.

[Michael]

Wow. We could have not climbed anything and still pretty much touched the same amount of earth. It's so small in comparison. Wow.

[Hannah]

We think of ourselves as an invasive species. We've barely got started.

[Michael]

Michael, it really depends on what you mean by invasive, doesn't it? Because we've got satellite imagery of so much our emissions surround and touch so much, but yet our flesh has touched so little.

[Hannah]

Almost nothing. Yeah.

[Michael]

And I use the word flesh just there. But usually we're wearing shoes. And when you're climbing, well, you'll have the chalk on your hands, but, you know, I'm saying shoes and gloves are different, too. I've always been like, no one's really touched the moon. No one's run on it barefoot yet. Does it count if there's fabric in between your skin and the moon's surface? You're counting contact we've made wearing shoes with the earth.

[Hannah]

You are absolutely right. Let's get out there and touch grass, everybody.

[Michael]

Yeah, for real.

[Hannah]

Because you're right. You'd have to, I mean, split this by a massive fraction.

[Michael]

This is actually a perfect segue to a question that came in from Andrew. So, Andrew asked, while lying in bed, I ran my foot along my bed cover and noticed I could feel the fluffiness of it through my sock, but I could also feel my sock. It made me think about all the times I was able to discern a texture through my socks or my socks and my shoes. How am I able to feel and discern all these separate textures? I know exactly what you're talking about, Andrew, because I really tripped out when I was at Derek Muller's wedding in Portugal.

[Hannah]

This is Veritasium.

[Michael]

Veritasium got married. And I was there. I was very honored to be there. And I was walking around the streets of Lisbon, and the stones there that line the streets are very smooth, but sometimes they weren't. And I could tell the texture, like the micro texture of the stones through my socks and shoes. I don't know if this. If anyone else has experienced this, but I could, like, immediately tell, wow, these stones are different, but they looked identical. And I'd reach down and feel them, and I'd be like, oh, these aren't polished as smooth. But I couldn't tell with gross motor movements whether it was slippy or not. It wasn't obvious. It was something different. And I think it might have been even like the vibrations, like the micro vibrations. And that reminded me of a study I saw that looked into how we measure how heavy things are, and that it's not as simple as we hold it and we just look at how much our muscles are having to work. We really do feel like thousands of little micro motions were sensitive to when we reach out and grab something before we even lift it that tell us how easy it's going to be to change this thing's velocity, how easy it's going to be to accelerate, to pick up, and we can tell that before we even hold it. And as I dove deeper into this, Andrew, I found that there's all kinds of ways to trick your sense of weight. You can actually make things vibrate so that they feel heavier. If you take a little device and you have it vibrate side to side, not even up and down, but side to side, people will think it's heavier than it really is because it's. Well, we don't know why. One hypothesis is that it's using more muscles, and the brain goes, ooh, it's taking a lot more muscle activity to hold this. It must be heavier. Even though, of course, it isn't. We know for certain that you can have something vibrate up and down, especially asymmetrically, like a bigger vibration. Down, down. It has to be pretty fast. Like 30 hertz, I think, is like the real sweet spot for making something feel heavier because of its vibrations. And so basically, we, I think, learn through experience how to tell the textures of things through other things. You can poke an object with a stick and learn if it's rough or smooth, even if you're blindfolded. And I think we learn that through time. And it's very trippy to think that it's an extension of our body, really. We learn how to feel through sticks and walking sticks and socks and shoes and gloves, as though they were part of our bodies with sensory receptors on them, though of course they're not. They're inert. And our bodies don't have to be this big. They can be large.

[Hannah]

You do hear that about blind people in particular, where the stick essentially becomes an extension of their body, exactly as you describe. But in a way, I mean, it's the same story in the sense that your brain is receiving signals. It's receiving some data, some input, and it's interpreting. It's making an interpretation based on. I sort of feel like almost every week we come back to this idea that, like, reality is not reality. It's just your brain's interpretation of it. But it sort of feels like it's the same thing.

[Michael]

Yeah, it's a little show your brain puts on for your awareness. Like my glasses, right. I wear these all the time, and I don't think I'm looking through glasses. This is just the world. So these have become part of my eyes. As far as my brain is concerned. They're not there. They are a prosthetic that is almost all the time completely fused with my

[Hannah]

body, or I Mean, you could say for people who don't wear glasses, this kind of the opposite way around, that your nose is permanently in your field of vision. You just don't bother noticing it.

[Michael]

Yeah, we ignore it. Just like you can ignore that the sock is there and go, oh, yeah, I can feel the fluffiness of my duvet through the sock. No one ever looks out at a beautiful view and says, oh, let me describe this for you. So first of all, there's this, like, nose down here, and there's, like, the bar of my glasses. So imagine that's going over the top. You know, second, fifth. No, no, no. They just described the view. We can ignore these things.

[Hannah]

I feel like the classic painters. They really should. They really missed a trick. You know, I think it would have been so much more beautiful if you saw the Mona Lisa and you just had da Vinci's nose in the middle

[Michael]

of the canvas right there. Yeah, yeah, he erased himself.

[Hannah]

He did.

[Michael]

You know, let's embrace that. Let's put in the, like, weird overgrown eyelashes that are kind of down there and the little dust on the glasses that you keep ignoring. Or the floaters for those of you who don't use. I mean, I see floaters, too, but, you know, even without glasses, you've still got artifacts in your vision.

[Hannah]

Maybe I'll stop into the Louvre on the way home. On the way home from Marrakesh and just break in and at midnight, paint da Vinci's nose on top of the.

[Michael]

Thank you. I'd appreciate that. I'm sure there's some famous works of art in Morocco that would be easier for you to vandalize.

[Hannah]

Do you know what? That's true. That's also true. I'm also obsessed with Moroccan tiles, by the way. Expect to see me do a video on that for the Internet very shortly.

[Michael]

I can't wait to see you talk about that. So, anyway, I'm glad that we get to touch you guys. All through your ears, every single. Send us in your questions. Like I said, the rest is scienceolehanger.com

[Hannah]

or you can sign up for our free newsletter, therestis.com science and we will see you next week.

[Michael]

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[Hannah]

You're welcome.

[Michael]

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[Hannah]

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[Michael]

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[Hannah]

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[Michael]

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[Hannah]

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