How To Fall To Earth (Without Burning Up)

Summary

The Rest Is Science: "How To Fall To Earth (Without Burning Up)"

Episode Date: February 26, 2026
Host: Professor Hannah Fry
Guest: Michael Stevens (Vsauce)

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Episode Overview

This "Field Notes" edition of The Rest Is Science brings together mathematician Hannah Fry and science creator Michael Stevens for a deep dive into the physics and ingenuity behind surviving re-entry from space. The episode cleverly weaves listener questions into explorations of scientific history, superhero physics, and the marvel of the Space Shuttle’s thermal tiles—the ultimate thermal insulators. The tone is energetic, irreverent, and filled with accessible, mind-bending science.

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Key Discussion Points and Insights

1. Landmark Experiments: The Little Albert Controversy

[03:41-13:23]

• Listener Question: "What landmark experiment do you wish you’d witnessed?"

• Michael’s Pick: The controversial Little Albert experiment (1920), conducted by John Watson and Rosalie Rayner at Johns Hopkins, where a nine month old was conditioned to fear lab rats—essentially inventing classical fear conditioning in humans.

  • Ethics and Scientific Flaws:

    • "I wouldn't have wanted to be there... I would have just gone there to stop it." (Michael Stevens, 04:16)
    • "They introduced this irrational fear in a human that they never undid... We didn't actually gain any serious or interesting scientific knowledge." (Hannah Fry, 09:53)

  • Origins and Aftermath:

    • Inspiration from Pavlov's dog experiments—showing that responses can be trained.
    • Led to more humane follow-up work by Mary Cover Jones ("the mother of behavioral therapy"), applying desensitization techniques.

  • Memorable Moment:

    • Michael shares his own "consenting subject" modern version from Minefield:
      • "I got electrically shocked every time a purple square was shown on a screen... It did the same thing... but I consented to it." (13:14)

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2. Superpowers & Energy: The Physics of Being a Superhero

[13:23-18:59]

• Listener Question from Dr. Sunny Macon: "If superhero powers had to obey conservation of energy, what’s the most expensive in calorie terms: strength, flight, or super-speed?"

  • Super Strength:

    • Lifting a tank (~60 tons) requires only 280 food calories—“one Snickers bar.”
    • "Super strength is very cheap and easy." (Hannah Fry, 14:47)

  • Flight (by hovering):

    • Requires force against gravity—~100 kilowatts, or 300 burgers/hour.
    • "I'm not using aerodynamics... I'm just saying if you can just move yourself upwards like a rubbish helicopter." (15:55)

  • Super-Speed:

    • Energy requirements scale dramatically (V² for kinetic energy, V³ for air resistance).
    • At even 1% speed of light, air friction could melt lead; at near-light speeds, it would cause “nuclear explosions.”
    • "Every time I do one of these calculations, it ends in nuclear explosions." (Hannah Fry, 18:33)

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3. The Fry-Stevens Origin Story

[18:59-24:21]

• Listener Question: "When did you first meet?"
  • Met in 2015 at YouTube’s London office, by chance in an elevator.
  • Michael: “You were like an actual smart person. I was just a guy who was like, look at this cool thing.” (20:35)
  • Their first collaboration was for the Vsauce Banach–Tarski video, where Hannah's feedback marked a turning point:
    • “Every video after that is more than 20 minutes long. Every video before... is like eight minutes.” (21:05)
  • Hannah highlights the mathematical mind-bendiness and accessibility of the Banach–Tarski paradox video.

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4. The Coolest Thermal Insulator: Space Shuttle Tiles

[27:15-44:38]

• Main Exploration: The evolution of heat-shield technology and the paradoxical genius behind how spacecraft survive re-entry velocity and temperature.

• History & The Problem:

  • In the 1950s space race, Harvey Allen realized streamlined, needle-like shapes were counterproductive for heat protection.
  • Key Insight: “What if, instead of making it streamlined, you just deliberately make it as unstreamlined as possible?... the aerospace equivalent of a manhole cover.” (Hannah Fry, 29:25)

• The Solution - Space Shuttle Tiles:

  • Material: Pure silica fibers, ~94% air, called “LI 900.”

  • “It messes with your mind because... it sort of looks like a piece of chalk... but when you pick it up, it’s unbelievably light. It’s essentially solid air.” (Hannah Fry, 34:05)

  • Unmatched heat resistance: Can be glowing red-hot on one side, safe to touch on the other.

  • Engineering Nightmares:

    • More than 2,400 unique fragile tiles per shuttle, each custom-shaped.
    • Tiles would soak up rainwater and needed waterproofing agents injected via needle.
    • Fun fact: “If you broke one... you’ve got to grab that exact shape, that custom replacement for that specific coordinate.” (Michael Stevens, 39:04)

  • Duality of Materials:

    • Tiles are “fragile enough to be crushed by a toddler, but strong enough to survive... over a thousand degrees.” (Hannah Fry, 40:41)

  • Asbestos Comparison:

    • Michael draws a parallel to asbestos’ insulating properties but notes the different (and less carcinogenic) risk profile.

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5. Space Conspiracies and Engineering Triumphs

[41:20-44:08]

• Moon Landing Skepticism Debunked:

  • Hannah points out the difficulty was always the return, not the landing.

  • Michael references an analysis showing how faking the broadcast would have been harder technologically than actually going to the moon.

  • “If they had solved that, that would have been more unbelievable and more of an achievement than the moon landing.” (Michael Stevens, 43:12)

  • “How many thousands of people were involved... Not one... accidentally told everyone that it was a ruse. I just don’t believe in humans that much.” (Hannah Fry, 43:36)

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Notable Quotes & Memorable Moments

• On classical conditioning:

  • “Can I entrain a behavioral response to an originally neutral stimulus?” (Michael Stevens, 05:12)

• On super-speed heroics:

  • “If you run at 1% of the speed of light... you’re generating enough heat from air compression to melt lead.” (Hannah Fry, 17:36)

• On irrational fears:

  • “We should also maybe focus on how fears can be unlearned.” (Michael Stevens, 11:46)

• On scientific progress:

  • “Sometimes out of trauma, growth can occur or at least... sometimes it takes despair for there to be triumph.” (Michael Stevens, 10:33)

• On shuttle tiles:

  • “It’s wild. There’s this video of people doing that... picking them up glowing hot.” (Hannah Fry, 35:03)

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Timestamps for Key Sections

| Segment | Speaker(s) | Timestamp | |----------------------------------------------|----------------------|-----------------| | Landmark Experiments: Little Albert | Michael & Hannah | 03:41-13:23 | | Physics of Superpowers | Hannah & Michael | 13:23-18:59 | | Fry & Stevens First Meeting Story | Hannah & Michael | 18:59-24:21 | | Shuttle Re-Entry & Insulation Tiles | Hannah & Michael | 27:15-44:38 | | Moon Landing Skepticism & Engineering Feats | Hannah & Michael | 41:20-44:08 |

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Final Thoughts & Tone

The conversation is playful, candid, and rich with concrete, quirky science. Alongside the technical explanations, there’s a constant thread of human curiosity and wonder. Both Fry and Stevens share a passion not just for facts, but for the stories and people that drive scientific progress—even when they involve mistakes, paradoxes, or tiles that are “solid air.”

Further Engagement

Listeners are invited to send questions (and even shuttle tiles) to the show, and to subscribe for upcoming Field Notes and full-length episodes—"until then, stay curious."

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(Summary skips all advertisements, intros, outros, and non-content interludes.)

Transcript

[Hannah Fry]

Welcome to the rest of science. This is Field Notes, sort of podcast expedition, if you will, where Michael and I, we trade the strange and curious objects that are filling our shelves and occupying our minds and we answer the

[Michael Stevens]

questions that are troubling yours.

[Hannah Fry]

Every week, one of us is going to bring a sort of strange and spectacular object onto the show. And together, well, we're going to see what sort of uncharted territory it takes us to.

[Michael Stevens]

And we want to hear your questions, your theories, your thought experiments. So send them in and stay tuned to see where we end up.

[Hannah Fry]

Yeah, absolutely. And later on, I mean, there's not many spaces on the Internet where I could use this as a hook and tease, but I think the rest of science is one. Because, Michael, later on, I'm going to be showing you the coolest thermal insulator that I own. Okay?

[Michael Stevens]

The coolest. Like, does it skateboard and wear sunglasses? And I'm trying to think of another cool thing. I don't know.

[Hannah Fry]

Neither of us know cool things, let's be honest.

[Michael Stevens]

Does it have ripped jeans and a bad attitude?

[Hannah Fry]

Hey, maybe you'll just have to tune in to find out. Or tune in. Stay tuned, I guess.

[Michael Stevens]

This episode is brought to you by Cancer Research uk.

[Hannah Fry]

The word cancer comes from the Greek karkonos, meaning crab. And Hippocrates used that word because tumors can spread out like crabs legs.

[Michael Stevens]

For a long time, cancer was poorly understood, and so I think because of that, it was almost scarier and people didn't even say its name. But what science has done since is replace uncertainty with understanding.

[Hannah Fry]

But that understanding isn't instant, because cancer isn't just one disease. It's hundreds of different diseases, each behaving differently depending on where it is and its genes. And that complexity is why progress in cancer research can feel like it's slow. But step by step, research is saving and improving lives.

[Michael Stevens]

That's why Cancer Research uk, the world's largest charitable funder of cancer research, supports work across more than 200 types of cancer, from the tiny changes inside cells that start the disease to better ways to spot it earlier and treat it more precisely.

[Hannah Fry]

For more information about Cancer Research uk, their research and breakthroughs and how you can support them, visit Cancer Research uk. This episode is brought to you by State Farm. Listening to this podcast. Smart move. Being financially savvy. Smart move. Another smart move. Having State Farm help you create a competitive price. When you choose to bundle home and auto bundling. Just another way to save. With a personal price plan. Like a good neighbor, State Farm is there Prices are based on rating plans that vary by state. Coverage options are selected by the customer. Availability, amount of discounts, and savings and elig vary by state. You said you were over him, but his hoodie's still in your rotation. It's time. Grab your phone, snap a few pics, and sell it on depop. Listed in minutes with no selling fees. And just like that, a guy 500 miles away just paid full price for your closure. And right on cue. Hey, still got my hoodie? Nope. But I've got tonight's dinner paid for. Start selling on depop. Where taste recognizes taste list. Now with no selling fees, payment processing fees and boosting fees still apply. See website for details.

[Michael Stevens]

All right, well, look, our first discovery doesn't even come from Hannah or me. It comes from you.

[Hannah Fry]

Certainly does, because Anna has sent us in a question. This one's for you, Michael. What's one landmark experiment you wish you'd been at?

[Michael Stevens]

Oh, okay. I. I thought of a lot of experiments when I heard this question. I think that ultimately there are a lot of results I would have loved to have witnessed. But I'm gonna have to say I would like to have been present at the Little Albert experiments.

[Hannah Fry]

Ooh. These are some of the most controversial experiments in the history of science.

[Michael Stevens]

Exactly. And I wouldn't have wanted to be there, to be a part of it or to watch it happen. I would have just gone there to stop it.

[Hannah Fry]

Oh, okay. We should probably tell people what they are. For those that haven't.

[Michael Stevens]

Yeah. So the. The Little Albert experiments were done sort of early, mid, early 20th century. The results were published in 1920. The experiments were conducted at John Hopkins by John Watson and his grad student, Rosalie Rayner. The goal of the experiment was to see if they could teach a new fear to an emotionally stable child. Like, already, it sounds like. Let's just assume you can and not try. But they both did this. Rosalie is the woman that you see in the videos. If you really want to have a bad time, go to Wikipedia, look up the Little Albert experiment, and you can watch the videos. And she's the one there with the child.

[Hannah Fry]

It's horrible.

[Michael Stevens]

The research was based on Ivan Pavlov's work with dogs. Okay. And I don't know, I'll go on and on about this because I love psychology. But the point is, we've heard of the Pavlovian reactions. What that is about is called classical conditioning. Can I entrain a behavioral response to a originally neutral stimulus? And what that means in the case of the most Famous experiment Pavlov did was when dogs are fed, when they smell food, they salivate at the mouth. So what he did is he rang a bell every time he fed his dogs. And when I say his dogs, I mean dogs in a laboratory. And he did this for many days until eventually he could just ring the bell and the dogs would start salivating even if he didn't feed them. And he's like, wow, look at this. So this is a huge part of what became behavioralism, the study of human behavior as essentially inputs and outputs. Like we learn things and we can be programmed like a machine. We've got buttons and levers and a human is kind of just a, like a steam engine.

[Hannah Fry]

There's no evolutionary reason why it should provoke that, that, that involuntary response.

[Michael Stevens]

Exactly. And so going off of this ringing bell associated with food, we start to combine them and we salivate. When we hear a bell, suddenly, that might explain why we're afraid of things, why we love each other, why we decide to do the things we do, why we're greedy, why we're kind. All of this became less popular starting in the 50s. The cognitive revolution changed and now even today we focus more on how we think because ultimately we know too much based on how many inputs we've received. Point is, let's get back to the experiment. Here's what they did. They somehow, they being Watson and Rainer, they found a nine month old child. We don't know how, we don't know the actual identity of the child, but in the paper they called the child Little Albert wasn't its real name.

[Hannah Fry]

Was Albert adopted or were his parents involved in the experiment too? Do we know that?

[Michael Stevens]

We don't know. But all of the theories are that the mother was in the picture and aware of what was going on. The extent to which she really had a choice is unclear. She may have worked at the hospital and sort of felt like, I can't really say no because these people are like, you know, they have power over me in the hierarchy of John Hopkins research, or they're my boss, you know, or we don't know. There are some guesses as to who the person might have been, the person, the baby, little Albert may have been. I think the most popular theory is that it was this individual who died at the age of six of a condition of the brain unrelated to the experiment. But that throws into question the validity of the results because this infant wasn't a, A, a completely, you know, healthy infant anyway. What did they do? Okay, so they took this child and they presented it with all kinds of different objects, literally, you name it, they showed it to the baby and the baby wasn't scared of any of it. The baby was curious. And then they decided, okay, let's associate something scary with one of these. And the one they chose was a white lab rat. They allowed the child to play with this lab rat, but every time they introduced the rat, the white rat, they hit a steel bar behind the child with a hammer really loud. And the child started crying and needed to be consoled and was frightened. And then they would present the child with a rubber duck or a shoe, and they wouldn't hit the steel. Only when the white rat was there would the child be frightened. And sure enough, they were able to teach the child, little Albert, to be afraid of fuzzy white cute things. They could present the child with a rabbit and it would get scared. They were even able to present the child with a Santa Claus mask that had a cotton ball beard. And the child would freak out and cry and scream. And they were like, wow, great job, everybody. We've learned that the human body is a machine that can be programmed and can learn. And like, that's just every behavior we have is some kind of learned thing. Now, despite just being like unethical by today's standards, it also didn't give us any good results. Like literally, it was one subject with no control subjects. And they didn't do follow ups. They didn't then see if they could unlearn the fear from the child. They were just like, excellent. That was cool. Um, and it's very sad.

[Hannah Fry]

This is really the reason why this is considered such a sort of bad experiment in the history of science. Of course there were, I mean, look, worse things have happened to people on earth, right. Than them being, than them being jump scared. But the key point about this was that they spent days after days after days deliberately frightening a child. Right. Which is just already quite a horrible image. But they introduced this irrational fear in a human that they never undid, that they had no understanding of how that might go on to impact his life going forwards. And that exactly as you describe. We didn't actually gain any serious or interesting scientific knowledge from.

[Michael Stevens]

Yeah, it was not a scientific method experiment. It was a let's poke around and see what happens. Let's poke it and see if it bleeds and then we'll move on and go catch a movie. I want to say the only, only sort of silver lining is going to come from that thought of like, sometimes out of trauma, growth can occur or at least, like sometimes it takes despair for there to be triumph. And Watson, after these experiments, did a lot of weekend lectures and he did one. And in the audience was a woman named Mary Cover Jones. And she was very interested in fear as something that could be learned. And so of course she thinks the kind of more kinder thought, which is, well, fine, but we should also maybe focus on how fears can be unlearned. And she happened to know a child that had been brought to her because the child was afraid of fuzzy white things. Strangely, it was not Little Albert. This was like, I think a 6 year old. So she says, well, I'm going to do the opposite of Little Albert. I'm going to teach a to not be afraid of furry white things. And she developed a method that involved something that I think a lot of us are familiar with today. Gradual exposure. Right. So just slowly introducing fuzzy things to the child and also having the child hang out with other children who were not afraid of furry things. And she was able to essentially cure the child of this irrational fear that was making the kid's life worse. And to this day, she's consider considered the mother of behavioral therapy. And so I don't know, maybe Little Albert's sacrifice brought us more quickly to behavioral therapy that helped people's lives. That's the only silver lining I can find in it. But I still think I'd go back and stop it. Because if it wasn't Mary Cover Jones, it would be someone else.

[Hannah Fry]

Yeah, I mean, I think science was on that path. Right. Of like learned behaviors. Sort of feels like it's only the adjacent possible to that is unlearning behaviors. Great answer though. And yeah, great answer. I haven't thought about Little Albert for a while. I'm not going on the Wikipedia to watch the videos. I remember watching them and finding them. I think maybe I was pregnant at the time or had a very small baby when I first watched them.

[Michael Stevens]

Oh yeah. Terrible time to do it. I saw them in high school in a psychology class and I was like, what the heck? Let's, let's make adults scared of things. I did that in a Minefield episode. I got electrically shocked every time a purple square was shown on a screen.

[Hannah Fry]

Oh yeah.

[Michael Stevens]

And it did the same thing. Like, like just. It created an involuntary panic react whenever I saw purple squares. But I consented to it.

[Hannah Fry]

Has it subsided over time? Can you see, can you see purple squares now?

[Michael Stevens]

Yeah, yeah. It, it evaporated very quickly. How?

[Hannah Fry]

Pink squares.

[Michael Stevens]

No, no, look away, Michael. No, I'm fine. I'M not a nine month old baby. So I knew what I was and I. And I was prepared to accept a fear of purple squares for life. So, you know, Yeah, I knew what I was getting into. Let's get into something else, though. Here's a question from Dr. Sunny Macon. If superhero powers obeyed basic physics and conservation of energy, which powers are actually the most expensive? In energy terms, I'm thinking of things like super strength, flight, and super speed.

[Hannah Fry]

Okay. You better believe I got out my calculator, Michael.

[Michael Stevens]

Great.

[Hannah Fry]

I've run some numbers maybe. Okay, right. Super strength. I mean, this is incredibly cheap. There's almost no energy involved in this. So let's say you're trying to lift up a tank. Okay. Tank, about 60 tons. All right. The work that done. Right. I'm going back to a level physics here.

[Michael Stevens]

Good.

[Hannah Fry]

Mass times gravity times height. Okay. That comes out about 1.2 million joules, which sounds like a lot, but in dietary terms, it's about 280 calories. You're talking one Snickers bar, essentially half a milkshake. Easy peasy. You can do that. It's no big deal.

[Michael Stevens]

Like one can of full sugar soda. Yeah. I mean, first of all, I want to take a break. That's like how efficient the human body is, right? That it's like, yeah, oh, you know, you, you drank one can of soda. You better go lift. What. What was this that we're lifting?

[Hannah Fry]

A. A tank.

[Michael Stevens]

A tank. Go lift a tank and then you can earn yourself another. I don't want to say earn. Basically, let me just leave it at that. The human body is really efficient.

[Hannah Fry]

Well, it's also, it's also the wrong way to think about it. Like, you know, if you go on a treadmill and you're counting calories on your treadmill and you finish and you've, you've like exhau up and it's like 60 calories. You can have half a peanut. Yes. That's not the exact. It's extremely, extremely depressing. But the key point is that just being alive, just breathing, just being upright, just existing and thinking uses up a lot of calories. So you shouldn't ever think of calories as a sort of an equation in terms of your, your physical exertion. But I'm doing it purely for the purposes of this question. And super strength is very cheap and easy. Okay. Flight. I'm gonna go medium level for flight. Okay.

[Michael Stevens]

Okay.

[Hannah Fry]

So if you are, I'm gonna just, you know, ignore the aerodynamics and just, you know, and just Think of you pushing against gravity, keeping yourself up. That essentially means that you need to be a very, a human inefficient helicopter. Okay. So you're, if you're gonna hover, you've got to accelerate air downwards. And for human sized objects, I, I work that out to be about, let's say 100 kilowatts of power. Well, that's quite a lot, actually. That is quite a lot. You're gonna have to, to eat. Please check my calculations by the way, on this. You can, you can write to me, tell me that I've got it wrong, but I think that's about 300 burgers per hour, which is quite a lot actually. You've gotta, you've gotta, you've got to pump through quite a lot there. I'm not, because I'm not using aerodynamics. If you were going forwards, if you were gliding, it would be more efficient. But I'm just saying if you can just move yourself upwards like a, like a rubbish helicopter. So that's kind of medium, but by a long way. The thing that is going to bankrupt you energy wise is super speed. Really Superspeed is off the scale, so because for starters you've got kinetic energy. Kinetic energy, by the way, is the equation for it is half MV squared. Notice that, V squared. So the faster you get, if you run twice as fast, you need four times the amount of energy. So super speed is going to scale very, very quickly. If you run at 1% of the speed of light, which is compared to like a cosmic ray, for example, it's nothing, snail's pace, it's not moving.

[Michael Stevens]

Yeah.

[Hannah Fry]

You've also got air resistance here that you've got to consider.

[Michael Stevens]

I was wondering if you were going to factor that in.

[Hannah Fry]

Yeah, here it is, here it is. Air resistance scales with V cubed, my friend. It's the cube law. Oh, lordy. So if you're going to run twice as fast through air, you need eight times the power to push the air out of the. So you know, let's make it slower. Let's, let's just say you're running at the speed of sound. You're not just burning calories, you are, you're, you're generating heat from air compression. You, you, you know that that's enough to, to melt lead. And you know, if you're trying to run at the speed of light, which I think is what Superman does, you know, I've seen, I've seen the videos of him circulating the earth. I don't think it was documentary But. But, yeah, I mean, you're. You're. You're going to be causing nuclear explosions all over the place. Obviously, every time I do one of these calculations, it ends in nuclear explosions.

[Michael Stevens]

As it should. As it should. So super speed. I mean. Yeah. Listening to you explain it, it makes sense. Super speed also requires super strength because you're having to push an enormous amount of air out of the way very quickly.

[Hannah Fry]

Yeah, you really are.

[Michael Stevens]

All right, so we went from a snickers bar to 300 hamburgers an hour to constant nuclear bomb explosions.

[Hannah Fry]

Exactly. Just a simple, simple scale. Simple linear, hop, skip, and a jump from one to the other. Okay. All right, here's another question. Oh, okay. This is nice. This is from Owen, who wants to know when we first met, Michael. Oh, yeah, I remember. Do you remember?

[Michael Stevens]

I think I remember. I think we've talked about this before. I mean, I was aware of you because of your appearances on Numberphile.

[Hannah Fry]

Correct.

[Michael Stevens]

I don't know how we got in touch, but we went to lunch. Why? Why did we do that, though? Like, I did. I have your email address.

[Hannah Fry]

Let's think about what year this was. This was probably. What do you reckon, 2014?

[Michael Stevens]

Maybe that year would have been 2015.

[Hannah Fry]

2015. There you go. You were working at YouTube in London.

[Michael Stevens]

Yeah.

[Hannah Fry]

And you had been running Vsauce for a few years by then already. So I had obviously watched every single one of your videos. Yeah, of course, of course. As most of the Internet had. And I was in YouTube recording a video, and then as I was leaving, you happened to be in the lift.

[Michael Stevens]

Right, that makes sense. Okay. So we just ran into each other at the YouTube office in London.

[Hannah Fry]

Yeah, yeah. You were very cool and I was not.

[Michael Stevens]

What do you mean cool?

[Hannah Fry]

I remember the entire conversation, Michael. You were like. I was like, oh, my God, it's be sauce. And didn't want to say anything and stood in the corner. And then you were like, oh, hang on, you're Hannah. And I was like, oh, my God, no, kid.

[Michael Stevens]

I had really. Because I was really nervous because you were like an actual smart person. I was just a guy who was like, look at this cool thing.

[Hannah Fry]

And then I was like, oh, my God, you know who I am. And you were like, well, yeah, it's kind of my job to know. I work for YouTube. Right. It's kind of my job to know.

[Michael Stevens]

True.

[Hannah Fry]

And then you said that you were working on a video for Banach Tarski and you needed a proper mathematician, and I didn't know any of those.

[Michael Stevens]

We had lunch and you were gracious enough to listen to me basically do the Banach Tarski video live, which was great. And I just needed someone to tell me, does this make sense? Can it be followed? Are there any egregious mistakes? And your interest in it and also the fact that you weren't immediately like, okay, no, this is obviously wrong. Gave me so much confidence. Oh, that's nice that I made the video. And if you look at the history of Vsauce, that's a turning point in the channel where I went from, ugh, could you eat your own poop if you cooked it first? To I'm gonna take a long time to really dive into something, to explain something that I can't understand, that hasn't been taught, I think just in the right way yet. And I'm not gonna do it right either, but I'll at least add a different voice to it. And so every video after that is like more than 20 minutes long. Every video before that one is like eight minutes long. So yeah, you played this like pivotal role in the evolution of the channel and myself. So that's how we met.

[Hannah Fry]

The Bannock Tarski video, by the way, I should tell everybody, is talking about a extremely obscure and mind bending mathematical theorem where when you take the surface of a sphere and rearrange in a particular way, you can end up with two of the original object without adding or deleting anything. It's extremely mind bending. And I think up until that point really nobody had delved into anything that complicated in a public forum, in a sort of, in a something that was to be consumed by everybody. And those of you who haven't seen Michael's video on Banik Tarski, I think it's got something like tens and tens of millions of views, right?

[Michael Stevens]

Yeah, something like that.

[Hannah Fry]

It is wild to the entire mathematical world that you could take something that mind bendy and bring it to totes to so many people.

[Michael Stevens]

I know there's an appetite for it online and I think ever since then, not that I started it, but I knew these things would be popular. And now thankfully we've got three blue, one brown. We've got all these channels that are like, no, let's do it, let's, let's get into this. And now it's like, no matter how deep of a question I have, I'm like, what exactly is energy? Or why did chimneys break in the middle when they fall? There's always some like Indian YouTuber who's like, let's get into it. And they draw out every single thing and what each. Each formula means. And it's just. It makes me feel so great for the human race.

[Hannah Fry]

Yeah.

[Michael Stevens]

That we want this kind of content. So, yeah. With Bonak Tarski, there's, like, one thing I wish I had done differently, which is the way I describe choosing the first set of points from the sphere I could have been more clear about. So one of these days I'll revisit it. But yeah, the point is that this very paradoxical dissection puzzle requires removing points in very precise ways and then putting them back together and like, oh, crap, I have two now. Wait, where did all this extra stuff come from? But that's how we met.

[Hannah Fry]

There you go. Oh, that was a really nice. That was a really lovely question. Thank you. Yeah, thank you very much, Owen. Should we take ourselves to a break after that?

[Michael Stevens]

Yeah. This episode is brought to you by Cancer Research uk.

[Hannah Fry]

Radiotherapy is over a century old, but it is still changing. Cancer Research UK helped lay the foundations of radiotherapy in the early 20th century and has driven progress ever since.

[Michael Stevens]

Radiotherapy remains one of the cornerstones of cancer treatment today. Every year, millions of people worldwide benefit From Cancer Research UK's work to make it more precise.

[Hannah Fry]

Scientists are still refining how radiotherapy is delivered and one example is an experimental treatment called flash radiotherapy, which delivers radiation in fractions of a second, up to a thousand times faster than standard radiotherapy.

[Michael Stevens]

And early studies suggest that speed could make a real difference. Flash radiotherapy may cause up to 50% less damage to healthy cells, but scientists

[Hannah Fry]

don't yet know why healthy cells seem to be spared. So Cancer Research UK are working to answer that, understanding it could be key to reducing side effects in the future.

[Michael Stevens]

For more information about Cancer Research uk, their research and breakthroughs and how you can support them, visit cancerresearchuk.org thereestisscience.

[Hannah Fry]

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[Michael Stevens]

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[Hannah Fry]

So go build your dream team today with Indeed. Get a $75 sponsored job credit@ Indeed.com podcast. Terms and conditions apply. Welcome back, everybody. I promised you some thermo insulation and I'm going to deliver. Oh, I'm going to deliver on the most exciting thermal insulation that there is. Not just in the world, but I'm going to say in the solar system. Okay, that's where I'm going with this because I'm talking space here. I want to bring you back, Michael, to the 1950s when the space race was in full force and there was an incredibly difficult problem that people were trying to solve, which is how on earth do you get vehicles from space back to Earth, the re entry vehicles for NASA. Now, the, the, the thing is, is that getting rockets out of, out of the atmosphere, actually that bit's kind of easy. I mean, maybe easy is a bit strong, but it's not too difficult. When you're coming back down, though, you are traveling at such an unimaginable speed that you have so much air that needs to get out of your way. You're squashing this air so rapidly with such a force that the temperatures that you start reaching are you way higher than basically any human made product could withstand. So back in the 1950s, there's a guy called Harvey Allen and he set the task of designing these re entry vehicles for NASA. And the first ideas, everyone was like, well, we just need it to be like a rocket shape, basically. We need to make it as streamlined as possible.

[Michael Stevens]

Like slice through the air.

[Hannah Fry]

Yeah, like a needle, right? Pierce through, get through the atmosphere quickly. Minimize the drag, minimize the friction forces on the side. Get in. But these temperatures, these speeds, I mean, they are so massive that it's just, it just wasn't. Nothing would work. So Harvey had this absolutely genius idea because he realized that when something is streamlined, all of that heat is going to happen right next to the Skin of the vehicle, Right. It's like literally on the surface. And really what you want, you want some kind of like a buffer, like a protective blanket that you can sort of wrap around the re entry vehicle and take some of that heat away. So his idea was what if, what if instead of making it streamlined, you just deliberately make it as unstreamlined as possible? Right. What if we make the aerospace equivalent of a manhole cover? Right. That's essentially what he was thinking. And the point about this is that if you re entered the atmosphere with like a massive flat wall, then the air that's getting crushed, it can't get out of the way quick enough. So it ends up forming this, this basically a cushion at the front of the vehicle. You get a much bigger shock wave, you get way more friction, but it happens away from, from the skin of the vehicle so that then it's not in direct contact with the sort of human made substances around the side, but it's kind of directed around it. It. Right, right. I love this idea, I love this idea of backwards thinking. And this is exactly what they ended up doing. So even, I mean they started off with these sort of these, these round shapes. The Russians copied the ideas. They used spheres. The Americans sort of settled on that cone shape which would come in, it's sort of belly flopping into the atmosphere. And then even the space shuttle, which looks more like a plane. Right. But when it would come into the atmosphere, it wouldn't go nose first and dive through it. Belly flops. Right, right. It kind of comes in. So this was the general idea, but still, even still, the temperatures on the surface could still get up to 1,250 degrees centigrade. Right. Still vast, vast, vastly higher than anything that would be. You know, you can't have like a metal skin on the outside, for example,

[Michael Stevens]

because it would melt.

[Hannah Fry]

Because it would melt. Exactly. So I should tell you, actually I did a video on this, I did a little social media video about this. Just I really love the aerodynamics of re entry vehicles. They just think it's. There's something deeply amusing to me about there being such a difficult problem and people turning it upside down and going for the most unaerodynamic space possible. So I did this video about it and then I was in California and I was in Google actually, and someone came up to me and they were like, oh, I knew you were going to be here today. I just finished working for NASA, now I work for Google. And I have for you a tile of the particular material that was used on the space shuttle. This is the material that they came up with that was the workaround for these absolutely insane temperatures on the space shuttle.

[Michael Stevens]

Yeah, we're looking at a tile of. It looks like white foam. It's about the size of a domino or maybe two dominoes on top of each other stacked hamburger style. But it's also striated on the sides like a cake made of very thin layers, all stacked up with black and white lines. It's a glossy black paint on one face, one of the large faces, but then it's white everywhere else, except for those thin black lines on the sides. And on the back, though, it looks like it's made almost differently of transversely organized sections.

[Hannah Fry]

Basically, what you have is these many, many, many layers, both horizontally and vertically. And this is actually. It's pure silica. It's 100% silica. But it's these fibers that are laid in such a way that this block is 94% air. Okay. So it's. It messes with your mind because you look at it, it sort of looks like, I don't know, like a piece of chalk perhaps. It's sort of white and it's got. Almost powdery.

[Michael Stevens]

Yeah, I was gonna say like ceramic or chalk.

[Hannah Fry]

Yeah. But when you pick it up, it's. It's way lighter than you would expect than. Than your brain thinks it's going to be. It's unbelievably light. It's. It's essentially solid air. That's what they've created. Solid. Solid air in a cage of silica. It's. It was invented by Lockheed missiles and space company, this one, specifically for the space shuttle. It's called LI 900. The. The 900 in it stands for its density. It's nine pounds per cubic foot. Okay, so there you go. Americans. This is. You can tell this is made in America.

[Michael Stevens]

Yeah.

[Hannah Fry]

Right. For context, by the way, Styrofoam is three pounds per cubic foot. So this is slightly heavier than Styrofoam, but. But not by much. It's sort of that. That's the sort of weight that it is. The thing about this stuff is that it is because it is, you know, sort of solid air in a lot of ways. You could easily crush this. You could, like, just put your hand through it. I mean, it sort of feels quite fragile under. Under your fingers. Now the thing is, all of these little pockets of air in here, air is this unbelievably good insulator. You know, heat doesn't really like traveling across air gaps yeah. So when you have, you know, an unimaginable number of tiny little caged air bubbles inside a material like this, it means that it is so poor at conducting heat that you can literally put this in a kiln, heat it to well over, you know, a thousand degrees centigrade, 2,000 degrees Fahrenheit, like, so it's glowing hot. Glowing hot. Literally red hot. And yet you can still pick it up with your bare hands because it is so bad at conducting heat that that it's not gonna burn your skin. Right. It's wild. It's wild. There's this video of people doing that.

[Michael Stevens]

I've seen the video, but I never really knew what they were made of. But now hearing your description of these silica fibers makes me think that it's almost like a possibly safer version of asbestos. Both of them are like woven rock fibers made into a material. They're both bad to get in your lungs. But I don't know, that silica stuff might be a little bit less carcinogenic. Only one way to find out, though.

[Hannah Fry]

Hey, come back in five years.

[Michael Stevens]

Let's see.

[Hannah Fry]

So let's probably put it back in a little box, shall I? I didn't know that's how asbestos works. Asbestos, like trapping loads of. Just traps loads of air molecules in it. Does it?

[Michael Stevens]

Yeah, it's like. It's also like, it's rock, so it's flame proof, fire resistant and it traps in air. So it insulates really well. It. Yeah, it doesn't burn. It's really marvelous stuff. Except for that one problem, that one tiny thing.

[Hannah Fry]

There was asbestos in my house, actually, when we did up my house, there was asbestos everywhere. And I got really scared because I'd had my babies in this house sort of sleeping next to asbestos walls. And they said that actually, until you disturb it, it's completely fine.

[Michael Stevens]

It's fine. It's a rock. It's sitting there. But once you decide to, like, remove it, that's when you've got to do a lot of safe precautions because it breaks into little tiny. Little tiny, like needles that float in the air and you breathe them in, they get stuck in your lungs. They don't dissolve or digest or anything because it's rock. And for reasons I'm not really clear of, it also accelerates cancer. It's not just like it clogs up your lungs. So. Yeah. What is the disease called? Mesothelioma.

[Hannah Fry]

Something like that. We should talk to our friends at Cancer Research uk.

[Michael Stevens]

We should, yes.

[Hannah Fry]

So here's the Thing. Okay, so, so the, this idea of LI 900, they sort of, they knew that they'd solved it, that they'd, that they'd found something that was just unbelievably good thermal insulator. But these tiles are so incredibly fragile. You can't have them as big, they can't be large. They couldn't just create these massive tiles. Put them in and off you go. The space shuttle wasn't covered in this uniform blanket. It was covered in 2,400 tiny individual tiles.

[Michael Stevens]

Oh, wow.

[Hannah Fry]

And because the space shuttles bend and flexed during flight, you can't just glue all of these tiles directly to the metal skin underneath, because they would snap. So there was like this strain isolation pad, which was then glued to the ship. And then every single tile on top had a unique shape, Right? Which means that if you broke one of them, you couldn't just go and grab one off the shelf.

[Michael Stevens]

You've got to grab that shape, that

[Hannah Fry]

exact shape, that custom replacement for that specific coordinate. These were an engineering nightmare, these things, because the other issue is that, you know, these are 94% empty space, right? Like, they're, they're, they're solid air, as it were, which means it's incredibly poorer. So if it's raining, if it's raining while the shuttle's sitting on the launch pad, the tiles would absorb, like, hundreds of kilos of water, right?

[Michael Stevens]

They would become solid liquid water.

[Hannah Fry]

Solid liquid water, exactly. And then, and then, I mean, then you've got problems of, you know, maybe that water could freeze, it could expand, it would shatter all the tiles. It would boil off in orbit. You know, it would blow them all apart.

[Michael Stevens]

A lot of extra weight. Suddenly you've got to launch. Yeah.

[Hannah Fry]

Right. So then they're having to, like, inject these tiles with this waterproofing agent, basically, you know, Scotchgard on steroids, essentially, via a needle. They've got to do it in all of these gaps and all of these holes. But they, they. One of the ways around this to sort of seal the outside is, especially on the belly, is they have this, this glaze that they put on it. This, this reaction cured glass, this borosilicate, and that, that helps to seal it, but also maximizes the heat emissibility, sort of sheds heat back into space. So these are on the bottom and then the white tiles, these are the ones on the top.

[Michael Stevens]

Yeah.

[Hannah Fry]

Cool, huh? Isn't that cool?

[Michael Stevens]

Yeah. Because the bottom of the space shuttle is black. And so I was wondering. So is that what I'm looking at there? Yeah. The black side is the side that faces out during the belly flop.

[Hannah Fry]

Yeah. Isn't that amazing? There, like this complete paradoxical material. Something that's like fragile enough to be crushed by a toddler, but strong enough to survive this plunge through the atmosphere at like over a thousand degrees.

[Michael Stevens]

Yeah, I think that describes a lot of people I know. I love it too, because we talk so much about to leave Earth, how hard it is to leave. To have the Delta V required to like leave our cradle. And yet. Yet coming back home is its own engineering problem. It's like the Earth doesn't want to accept us back readily.

[Hannah Fry]

I always thought that was the hardest bit. The whole thing about whether the moon landing was real or not. I always thought that the argument for me was that getting to the moon and landing on the moon actually, I mean, difficult, but not anywhere near as difficult as the problem of getting back off the moon, getting back down to Earth and landing safely. And that was always the thing, is that you had this moment in time. These, well, unbelievable geopolitical pressure for the space race combined with, you know, launching missiles and nuclear weapons from space and all of the sort of defense side of things that came with that. But you also had incredibly gung ho pilots. I don't think it would have been difficult to find somebody who was willing to do a suicide mission to the moon and just stay there. Like that bit wasn't, wasn't hard. So it always seemed to me that the showy bit wasn't the hard bit, which makes me believe that they did the entire thing.

[Michael Stevens]

Have you seen that video where the guy shows that in 1969 it would have been easier to have gone to the moon and come back than it would have been to create a broadcast of that length. Like a pre recorded film segment. Yeah, he's like, actually that feed of the moon landing is so long that if you had done it in a studio beforehand on film, and then you had to play it back like it was live and not have a single piece of dust or a hair on it, not allow any mistake to show that you're just playing back a reel like that would have been an even bigger technological miracle than actually going to the moon and back. It's a very fascinating video.

[Hannah Fry]

Yeah, I think, you know, I sort of vaguely do remember he was talking about the size of the reels, like the physical size of the reel of tape. Right. If I remember rightly.

[Michael Stevens]

Yeah. Cause he's talking about like hours and hours of footage that is like streamed out onto televisions and Somehow it needs to never look that way. It needs to look like a live broadcast. So it probably was, because if they had solved that, that would have been more unbelievable and more of an achievement than the moon landing.

[Hannah Fry]

The other argument that I find compelling is that how many thousands of people were involved, right? How many? The cleaners, the sort of the janitors, the people who are painting the walls all the way through to the engineers and the astronauts and the people in politics. Not one of them, you know, not one of them, however many decades later, slipped up and accidentally told everyone that it was a ruse. I mean, I just don't believe in humans that much. I just don't believe in humans abilities to keep secrets that much.

[Michael Stevens]

Yeah. Oh, yeah.

[Hannah Fry]

Okay. Well, we're hearing from our producer that Li900 would need exposure to 1450° centigrade for hours, then crush rushed to have any carcinogenic potential whatsoever. So live to fight another day, Michael.

[Michael Stevens]

Wow.

[Hannah Fry]

You're safe. Okay. Important that you recognize I was willing to put my life into your hands for the purpose.

[Michael Stevens]

I know, I know. But now we know that you were safe all along, so I feel better all along.

[Hannah Fry]

All right, well, I guess we'll leave it there for this week. As ever. If you have any questions, ideas, tiles off the space shuttle that you want to send us, you can email us. The rest is scienceoalhanger.com and you can

[Michael Stevens]

join our newsletter at thereestis.com science we're

[Hannah Fry]

going to be back next Thursday with another edition of Field Notes and on Tuesday with our normal episode.

[Michael Stevens]

So until then, stay curious. It's tax season, and at Lifelock, we know you're tired of numbers, but here's a big one you need to billions. That's the amount of money and refunds the IRS has flagged for possible identity fraud. Now, here's another big number. 100 million. That's how many data points LifeLock monitors every second. If your identity is stolen, we'll fix it, guaranteed. One last big number. Save up to 40% your first year. Visit lifelock.com podcast for the threats you can't control. Terms apply. At blinds.com, it's not just about window treatments. It's about you. Your style, your space, your way. Whether you DIY or want the pros to handle it all, you'll have the confidence of knowing it's done right. From free expert design help to our 100% satisfaction guarantee, everything we do is made to fit your life and your windows. [e-mail], the only thing we treat better than Windows is you. Visit blinds.com now for up to 45% off with minimum purchase plus a professional measure at no cost. Rules and restrictions apply.