Why We Need Zip Lines On The Moon

Why would a zip line be the best form of transport on the Moon? Why exactly can your feet still feel other textures right through your socks? Hannah and Michael tackle the spectacular physics of extreme commutes and everyday biomechanics. They unpick the orbital chaos and terrifying vacuum of space, proving why a lunar theme park ride is essentially a brilliant, fiery death trap. Back down on Earth, they dive into the hypersensitive neurology of touch, revealing how your brain decodes microscopic vibrations through layers of cotton to perfectly map the floor beneath you. To top it all off, Hannah shares her very old school, steam punk esq, mechanical calculator. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton + Oli OakleyVideo & Social: Bex TyrrellProducer: Simona RataSenior Producer: Lauren Armstrong-CarterHead Of Digital: Samuel OakleyExec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Hannah Fry?

  Welcome to The Rest Is Science. This is Field Notes, which is our Thursday edition, our podcast expedition diary, as it were.

• 0:08Michael Stevens?

  That's right. We answer questions from you, which you can submit by emailing [email protected].

• 0:15Hannah Fry?

  Now, after the break, I have got what I like to describe as a steampunk iPad. That's what I've got for you today.

• 0:31Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 0:34Michael Stevens?

  If you wanted to type out the entire human genome, you would have to type at 60 words a minute for eight hours a day for about 50 years. Okay, 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.

• 0:53Hannah Fry?

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

• 0:59Michael Stevens?

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

• 1:08Hannah Fry?

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

• 1:18Michael Stevens?

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

• 1:26Hannah Fry?

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

• 1:31Michael Stevens?

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

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• 2:47Hannah Fry?

  First, we're going to go to some of your questions. And we're going to begin with this one from Chris and Oliver Hornsby. Me and my lad Oliver are thinking of starting a zipline business from the moon to earth. Assuming the budget was infinite, what problems might occur? How can we overcome them? And what other considerations should we take into account? Good luck and we'll offer you 5% of the business for free each, which is very generous.

• 3:12Michael Stevens?

  That's pretty generous because I feel like we are not going to solve their lunar zipline business problems on this podcast. And yet we each get 5% of it. That could be a very valuable business. It certainly could. Sending something on a zipline between the earth and the moon would save so much money. You could just build a robot that climbed the zipline to get from the earth to the moon. And it would take so much less fuel, so much less energy, and it could carry such heavy payloads. That would really change human society. So this is not a new idea. This isn't necessarily just a silly idea. There are some silly things that we need to avoid, right? Like, first of all, the moon and the earth are always moving, right?

• 3:57Michael Stevens?

  The earth is turning, the moon is orbiting the earth, which means that they're not always facing the same points to the same points. If you had a tower on the earth and a tower on the moon connected by a cable,

• 4:11Michael Stevens?

  well, sometimes the moon's at the horizon and that cable's coming straight across and it might hit your roof. But then as the moon climbs up higher, that cable on the earth tower goes up higher and then it comes down. It lays flat on the ground, cuts the earth in half. No, really, the cable is just going to snap. But just for the moment, let's assume that we fix that. We get the earth and the moon to just face each other. Because the budget is infinite, he said. If the budget is infinite, then let's change the moon's orbit so that it's just always at the same point in the sky. So it becomes geostationary? So it's geostationary, which means that half of the world never sees the moon. For half of the world, it's always right above.

• 4:54Michael Stevens?

  And for people on the edge, it's always right at the horizon. You get it. We could, we could string a cable between the two. And if you're on the moon's side, you could, you really could zipline down to the earth. I mean, you'd have to have a little bit of a push to escape the gravity of the moon, but the earth has a lot of gravity. That's why the moon doesn't go away. The earth's always pulling even that far away. So you're gonna fall to earth. And problem is that it's a long distance. Like, you're not gonna just zip on down. It takes, you know, five minutes and what a ride. It's gonna take more than five days. Okay. Five full days, like night and day, you're gonna be accelerating the whole

• 5:39Michael Stevens?

  time, you know? That's one good thing, except it's not really that great, because by the time you reached earth, you'd be going seven miles a second. Wowza. So you're gonna need protective gear for re-entry, or I guess entry into earth, and you're gonna need to slow down. So it's gonna probably, I think it would take like a week, just ballparking, like you gotta speed up and then you gotta slow back down for a safe arrival. It would be very boring. I think that you can't just hang on to like a T-bar and zip down. You're gonna need to build a gondola that the passenger can sit in and, you know, watch TV and read books and bring friends with them, because again, this is like a week-long trip.

• 6:22Michael Stevens?

  But that's assuming we change the moon's orbit and steal views of the moon from half of the earth, so that the moon is just always in one place in the sky. Funny enough, this is still a serious idea, because again, like I said, it would, it would change everything about how accessible the moon was. Of course, we wouldn't attach a tower on the moon to a tower on earth. We would attach a tower on the moon with a cable, with a counterweight somewhere up above earth, like near high earth orbit. So you could, you would only get that close to the earth, and then from there, you could enter like space shuttle style, maybe. I don't know, I'm not an astrophysicist or an astronaut, but... Um, my head is in the clouds.

• 7:08Michael Stevens?

  But I'm, I'm shooting for the stars. Point is, this is a real proposal because it's not as hard to get things into these earth orbits. It's still expensive, but you could couple a zipline that went almost, almost to the earth with a space elevator that went from the earth up really high.

• 7:28Hannah Fry?

  Which is a serious proposal.

• 7:30Michael Stevens?

  Which is a serious proposal, and I think it's absolutely shameful that we aren't already planning such a thing because that's going to be our number one chance to start mining asteroids and just changing the entire world. I mean, what are asteroids made out of? Like literally platinum, gold, iron, nickel, like it's all just sitting right there. But the problem is that we have to burn tons and tons of fuel to get there. And it's so expensive, but with a space elevator that went up from earth or a zipline from the moon close to earth, you could, you could literally just put stairs on it and you could walk all the way up into earth orbit. And you'd have to take a couple of breaks, but you wouldn't need giant fuel tanks or anything.

• 8:16Hannah Fry?

  Michael, I had to take a couple of breaks going up the Eiffel Tower. I think it's more than a couple of breaks.

• 8:20Michael Stevens?

  Okay, well, look, Alex Honnold can be employed by NASA to do all of this asteroid mining, back and forth climbing. But the point is that a space elevator, also, I just want to say, like, it sounds a bit ridiculous because what the Burj Khalifa is the tallest building. It's not even a kilometer tall. But if you build tall enough, the centrifugal force that, that fictitious force that pulls things away from a spinning object starts to help you out. You build a tall enough building and its top is actually flying away from earth. And if you build it just right, it doesn't even have to touch the earth. It can just float above it, you know? You can put a counterweight up at the top that's swinging and lifting the thing up

• 9:04Michael Stevens?

  so that its foundation doesn't have as much weight on it. That's why, you know, we're also not doing it because this is a huge project. We're talking an enormous amount of material. And where do you build it? Because you don't want it to fall during construction. It would literally be as wide across as like Kansas, you know? So it's got to be, you know, it's a hard problem.

• 9:24Hannah Fry?

  I mean, this is the reason why satellites are possible, that they can stay in the same level of orbit. I mean, that's the effect that you're talking about. They are these massive objects effectively. And they are, I mean, depending on what frame of reference you're talking about, sort of suspended in the sky, right? So it is possible, you can have, you know, ladders hanging down from them. If you get it perfectly right, it is possible. It's just hard.

• 9:50Michael Stevens?

  Yeah. And it would be really a great way to teach physics as well, because you could take a field trip with your like fifth grade class up the steps of the space elevator. And even when you got up as high as the International Space Station orbits, Earth's gravity is still like 89% of its strength at the surface. So you could still walk around. You might feel a bit lighter, but you would not feel like, whoa. You wouldn't be floating. You wouldn't be experiencing zero G because the International Space Station astronauts feel it because they are falling. Luckily, they're also moving to the side so quickly that by the time they fall a little bit, the Earth is curved away from them, so they never hit. But if you climbed the space elevator stairs, you really could just step off it and you

• 10:35Michael Stevens?

  would fall. You would fall to Earth. It would take a while, especially if you climbed all the way to the moon. It's going to take, like we said, five days. I want to mention, though, that zip lines in the moon are even more seriously considered as a method of transportation on the moon from one place to another. Because the moon is very dusty and using wheels or anything that's going to contact the surface is this whole problem. The, the moon dust is really abrasive. It clogs things up. It makes things dirty. It cuts things up. It's, it's like really, you know,

• 11:14Michael Stevens?

  nasty stuff. And the moon's surface is, you know, it's got all this like rough terrain and there's all these landmarks. And of course, if we start driving all over it, we change the way it looks so that research can't be done anymore on how the moon was before we messed it up. But if we connected bases and launch pads with zip lines, you don't touch the ground. And it's not like you whiz by the ground and you kick up a breeze that messes it up. There's no breeze. There's no atmosphere. There's no friction from the atmosphere. And you can just zip around the moon on zip lines. So maybe we should put this in the description. I found an entire proposal from NASA, digging into the advantages

• 11:57Michael Stevens?

  of zip line transportation on the lunar surface as we try to spend more and more time there due to the Artemis missions. Very exciting stuff. But overall, to answer your question, I think what you need to be worried about is how boring the trip would be if you ever figured it out. So Chris and Oliver Hornsby, great question. And good luck.

• 12:19Hannah Fry?

  What you also need to be worried about now is, is how you're gonna sign over that 10% to me and Michael. That's, that's the other main, main issue you should be concerned about because, you know, my lawyers can be pretty feisty.

• 12:32Michael Stevens?

  Look, Hannah, don't worry about it. It's already in writing. Like, we've got the email.

• 12:37Hannah Fry?

  This is true. This is very true.

• 12:39Michael Stevens?

  All right, let's do another question. This one comes from Jade. Is it just me or do some numbers give off better vibes than others? For example, the number 12 feels like a benign grandfatherly number. If 12 was a person, it would smell of peppermint and hand out Werther's Originals. 17, on the other hand, feels like a three pin plug to the soul of a barefoot. What do you think?

• 13:05Hannah Fry?

  But well, actually, I think Jade, you've probably got a type of synesthesia. Um, the most common version of this that you hear is that, uh, people think that different words have colors, or there was one guy who I spoke to once who said different tube stations were associated with a different smell in his mind, and not just because the smell that he remembered when he was in those places, but Notting Hill smelled like sort of summer roses, you know, that kind of thing. It's called ordinal linguistic personification. And it's something that just happens where a certain number of people, quite a small number of people, end up sort of their brain assigns character or some characteristic to things that ordinarily wouldn't. So for me, I don't have this at all, right? I, I don't

• 13:48Hannah Fry?

  have, um, I don't really feel like some numbers are nicer than others. Do you have it?

• 13:53Michael Stevens?

  Um, I think that what Jade is describing is more relatable than, than synesthesia to me. I'm always like, five is brown? Give me a break. But this whole like, yeah, 12 is like a powerful number. And, um, 17, see, it gets personal. I used to live on the 17th floor of a building, so I love 17. I think it's also my sister's favorite number, but uh, I think the vibes of a number I definitely get, but I don't have sensory associations with the numbers.

• 14:24Hannah Fry?

  Okay. So I think I could understand it in terms of what the numbers do and how the numbers are. I mean, like 12 is such, is a really, 12 is a cool number. It's what we call highly compound, because it's got so many different factors. I mean, we absolutely, we've mentioned this before, but we absolutely should be working in base 12. The fact that we are stuck on lame-o base 10 is a travesty for all humanity.

• 14:46Michael Stevens?

  Yeah. I mean, look, 10 is pretty cool, right? I can divide it in half. I can divide it by five. Um, oh, shoot. Besides one and 10, we're done. But 12 is like, divide me into thirds, divide me in half, divide me into quarters, divide me into sixths. I'll do it all. Almost all.

• 15:05Hannah Fry?

  Yeah. Which is a bit grandfather-like in a lot of ways, you know? There's a, that is quite, you know, being, being flexible and wise. I like that.

• 15:13Michael Stevens?

  Yeah.

• 15:14Hannah Fry?

  17, meanwhile, prime number, extremely obtuse, refuses to go into anything. Uh, so I, I can, I can see that. I can see that sort of the, what the numbers themselves are doing can relate to their personalities.

• 15:25Michael Stevens?

  It does seem, and I think this might be the words we use for the numbers, but it seems like 17 is a sharper number, whereas like, Twelve is kind of a blob. It's like Kiki and Bobo.

• 15:37Hannah Fry?

  Mm hmm. So Kiki and Bobo is this, um, this very famous experiment where you draw, uh, two shapes, one of which is very spiky, uh, sort of like a misshapen star. And the other one is, is more like a cloud, very round around the edges. And then you go across the world and you say, one of these is called Kiki, one of these is called Bobo. And, uh, and almost universally, people think that Kiki should be the sharp shape. Um, there's no real reason why that should be the case, but it's sort of, it just, it's something that feels right among all of us.

• 16:10Michael Stevens?

  Yeah, just the sound of Kiki.

• 16:13Hannah Fry?

  Hmm.

• 16:14Michael Stevens?

  Is very percussive, whereas Bobo is very blobbed out and soft. The actual physical motion of your mouth, regardless of your culture or language, they're parallel.

• 16:26Hannah Fry?

  One thing I will say, Jade, is that you, uh, if you do have this, this ability to see characters in numbers, uh, to assign characters to numbers, you're in good company because some really amazing mathematicians had exactly the same thing. Notably, Ramanujan, who we did an entire podcast about a few months ago now. Um, this is, uh, an incredible Indian mathematician, self-taught. But he had this incredible ability to sort of feel the character of numbers. And for him, it would make sense in the way that numbers fit together. So there's the amazing story about, uh, a conversation he has on his deathbed of the number seventeen twenty-nine, and he immediately comes up with the way that it's constructed

• 17:11Hannah Fry?

  from other numbers, a sort of sequence of, of numbers cubed and so on. And, uh, he was doing that ultimately because his brain was able to assign character to each of these numbers. And then in the way that they related to each other, it meant that his mental arithmetic essentially had this gigantic shortcut.

• 17:28Michael Stevens?

  Yeah, that, which makes sense. Like, I feel like it's hard to remember bare facts, but it's easy to remember people's personalities. So if numbers have a personality because of their properties, then sure, you'd remember them better, right?

• 17:43Hannah Fry?

  Yeah, yeah, absolutely. Feynman, Richard Feynman, also appears to have had this. He said that when he sees equations, he can see letters and colors, essentially. He's seeing pictures of vessel functions with light tans and violet bluish-ies and dark brown Xs and all of that floating around. And it meant that he had more of an emotional connection to the things that he was manipulating than maybe the rest of us do.

• 18:10Michael Stevens?

  Wow.

• 18:11Hannah Fry?

  So yeah, maybe, maybe you should really lean into that, Jayce. That's, uh, that's, I think, where I'm going with this one.

• 18:17Michael Stevens?

  Definitely embrace it.

• 18:19Hannah Fry?

  Yeah. All right. Another question we had in, this is from Elliot, who says, "I read The Curious Incident of the Dog in the Nighttime about 20 years ago, and I remember the Monty Hall problem being described in it, and I could not get my head around the maths of it. Is there any chance that you could explain it like I'm five?" Michael, over to you.

• 18:36Michael Stevens?

  There is. And look, I, I chose this question, Elliot, even though, like, before we recorded, Hannah was like, "Oh, no, the Monty Hall problem. Everyone's talked about this over and over again." And I was like, Hannah, look, this is a joint project. And yes, even I have made a 14-minute-long video on the Monty Hall problem. And I'm one of 18 billion people who have done videos on it, and mine is not even the best. But I feel like 14 minutes was an obnoxious amount of time to spend on it.

• 19:07Hannah Fry?

  Yeah, I know. Well.

• 19:08Michael Stevens?

  I now feel like the Monty Hall problem is one of the least mysterious things, and we keep making it mysterious because of language. We just describe the problem too imprecisely, and it makes it confusing. But I think all the confusion simply comes down to the, the rules about what the host can do. And it's never made clear when you read a description. So here we go. What, what I'm going to do is I'm going to actually read the famous Monty Hall. And again, I don't want to spend a lot of time on this because it doesn't need a lot of time. This is the Monty Hall problem as it was proposed, as it was written by Marilyn Vasavant in her famous Ask Marilyn Parade magazine column in 1990. I'll tell

• 19:49Michael Stevens?

  you, why was it famous? Because no one thought she was right. I mean, some people did. But the vast majority of people, when shown this problem, make the wrong decision, and they cannot be convinced that they're wrong. Here's what's going on. I'm gonna read you what she wrote.

• 20:04Hannah Fry?

  Let me just say very, very briefly, for any of you who have not come across the Monty Hall problem, because I believe that there is still a small percentage of people

• 20:12Michael Stevens?

  Yes.

• 20:12Hannah Fry?

  who fit into this category. This is a puzzle about a game show host, some goats and some cars, that has become infamous, I would say, because everybody, when they first hear it, gets extremely confused about the answer. And there have been untold arguments about this very problem. But anyway, over to Michael, who's about to explain it.

• 20:39Michael Stevens?

  Here's the problem. And again, I'm quoting from Marilyn Vas Savant's article. So this is like an official, famous way it's been described. Suppose you're on a game show and you're given the choice of three doors. Behind one door is a car. Behind the others, goats. You pick a door, say, door number one, and the host, who knows what's behind the doors, opens another door, say, number three, which has a goat. He then says to you, do you want to pick door number two? Is it to your advantage to switch your choice? Even that description, I think, is confusing. Because here's the answer. It is always best to switch. Many people say, well, it doesn't make a difference.

• 21:23Michael Stevens?

  It's still, you know, at that point, it's, you know, it's the same. Nothing's changed. And then when you tell them, no, no, no, if you switch, you will win two thirds of the time. They're like, what? No, I mean, it seems like he's revealed where one goat is, and then there's the door I picked, but it's just a 50/50 now, right? Whether I picked the money or the goat, or, you know, whatever. But that's not true. You should always switch. And the reason you should always switch is, very simply, that the host not only knows what's behind each door, but the host is required to always open a door that has a goat behind it. And that is an assumption that Marilyn Vas Savant later on admits is required, because

• 22:08Michael Stevens?

  what's happening is not that you're choosing a door, and then the host is randomly opening one of the other two doors. The host will never open a door to show a car and say, nah, okay, I guess it's a draw, you know, you lose. Basically, when you, when you choose a door, the host is then required to look at the two remaining closed doors and open one that has a goat behind it. One third of the time, you have already picked the car. You've already picked the winning door. And so the host can just randomly open one remaining door or the other. But most of the time, two thirds of the time, you have not picked the car. And the host is faced with two doors, one with a car, one without. And the host goes, well, I gotta open the one that has a goat behind it.

• 22:49Michael Stevens?

  So two thirds of the time, the one door that remains unchosen was unchosen because there's a car behind it. So choose that one, switch.

• 22:58Hannah Fry?

  I think that's exactly right. The, the only way that you will not win by switching is if you were lucky enough to do it, to have picked the car right at the very beginning.

• 23:09Michael Stevens?

  And that will happen a third of the time.

• 23:12Hannah Fry?

  One third of the time. Every other scenario, you're better off switching, which happens two thirds of the time. So actually, once it boil it down in that way, it is really super duper simple. But my goodness me, you know, actually, right, like, so let's say maybe 2011, 2012. I went for a screen test at a production company, which is this thing that they do when they're trying to work out if they're gonna include you in a documentary or whatever it might be. Anyway, they showed me this clip of how they were like doing lots of cool maths. And the clip that they had was James May. Did, did you ever come across him? He was like super duper famous.

• 23:54Michael Stevens?

  I've worked with him on a Vsauce video.

• 23:55Hannah Fry?

  Oh, he's great. He's a really great guy, I like him a lot. But he was explaining the Monty Hall problem. And, and then he had an explanation at the end, which was absolutely categorically wrong.

• 24:08Michael Stevens?

  Oh.

• 24:09Hannah Fry?

  And they had shown this, they had like filmed this, they'd put it out on TV already. They were sort of showing me the clip to kind of demonstrate the type of stuff that, that this production company were doing and how, you know, interesting it might be for me to work with them. And then I was like this quite young, I was like 28 years old or something. I'd just finished my PhD. And I was sort of sitting there in this production company and had no idea what to do. I had no idea whether to tell them that they were wrong or not. So I didn't say anything, but then when I got back to, um, you know, my office later, I was like, you know what? I need to tell them that they, they just.

• 24:45Michael Stevens?

  Yeah.

• 24:46Hannah Fry?

  They absolutely butchered that explanation. That was terrible. Like that was, they've just completely misunderstood it. So I wrote them this email just saying, in the kindest way possible, I'm really sorry, but the thing, the clip that you showed me is wrong. Like, you shouldn't show that on TV ever again. The like head of the production company replied to me telling me that they had fact checked it by.

• 25:07Michael Stevens?

  Oh, no.

• 25:08Hannah Fry?

  Very serious and important. And I was completely mistaken. And then I never worked with them.

• 25:13Michael Stevens?

  So was their explanation wrong? Or was it just suboptimal?

• 25:19Hannah Fry?

  No, I think it was wrong.

• 25:21Michael Stevens?

  Dang, wow.

• 25:22Hannah Fry?

  I can't remember exactly in what way they got it wrong, but I think that they had said that the chances, I think they said the chances of winning by switching were a half because at that point you have two doors. Maybe that's why I don't like doing the Monty Hall problem because actually it's, I think it delayed my TV career by about five years.

• 25:43Michael Stevens?

  It's personal with you.

• 25:45Hannah Fry?

  It's personal. It's personal. It really demonstrates how much it confuses people though, right? That you could have TV level production, someone as amazing as James May, who will himself be the first to admit that he's not a mathematician, right?

• 25:58Michael Stevens?

  Oh, sure. And I'm, I guarantee you, there are going to be a lot of comments on this video saying, oh, no, I still don't believe you. Or endless analogies of like, but what if they're not doors? What if they're marbles in a bag? Or what if there's a hundred doors and 99 of them are opened and blah, blah, blah, blah. And it's just like 98 of them are opened. And it's like, guys, we could talk about this for 14 minutes. I already did. And after that, my like go to when I'm driving, when I was commuting in LA from work to home, it was to talk out loud about the Monty Hall problem. And I just realized, man, a lot of breath is being wasted on what is otherwise pretty clear if you. Describe it well.

• 26:36Hannah Fry?

  Hmm.

• 26:37Michael Stevens?

  Look, let's just move on. Here's a question from Lucas. How many calories do we burn while giving blood?

• 26:46Hannah Fry?

  All right, so I've done some calculations as I take every opportunity to do every single week on this stuff. The thing is, is that when you're just sitting there, kind of none. I mean, you're just, it's sort of like your base rate calorie usage. But the problem is, you've got to go away and remake all of the stuff that's in your blood. There's four kind of main components here. You've got your red blood cells, which are like these, I mean, they're basically like little inner tubes effectively that are carrying around hemoglobin, hence oxygen. In 500 mil of blood, you've got two and a half trillion red blood cells. You've got to make all of those from scratch. It's a lot of protein. It's a lot of lipids. It's a lot of work to do.

• 27:29Hannah Fry?

  You've got platelets as well, which is essentially, they're like little irregular cell fragments that that break off from these giant cells in your bone marrow. And these are the things that are waiting to stick together to clot if there's a tear in your blood vessel. You're going to need 150 billion of them. So far, far, far less than your red blood cells, but that's still, I would say, quite a lot.

• 27:50Michael Stevens?

  These are big numbers, but they're small things.

• 27:53Hannah Fry?

  These are big numbers, but small things are great. You're also going to need white blood cells, that's a rarer mix. You need about a cuppa billion of them. Barely any, really, they're quite rare in comparison. And then you've got the plasma, which is all the water and so on. So in total, all of those together, you're losing about 110 grams of protein. And manufacturing proteins from from raw amino acids, that's, that's really where the energy intensive work is going to come in. So, so in total, that's about 500 calories. And then probably an extra 150 or so. Um, to get all of the blood glucose that you've lost, the kind of cellular energy

• 28:35Hannah Fry?

  reserves, the lipid bilayers. So that's what you're looking at, about six hundred and fifty calories. Okay. That's a lot more than I expected. It is quite a lot, isn't it? The thing is, it's spread out over a period of time. So in the first twenty four hours, you're going to get the volume replacement. That's really just where your body's going to pull water from other tissues. Okay. Um, mostly to replace your, your lost blood plasma. And then it's over the next one to four weeks, which is when these blood cells uh start being made. Um, that's really when you're, when you're kind of spending those, those six hundred and fifty calories. But what I will say is they give you sort of, you know, a glass of orange juice and a biscuit, um, to say thank you very much. You're, you're kind of already there, really.

• 29:19Hannah Fry?

  Just on eating those, you're kind of back up to zero. Well, yeah, especially if it's sugary juice. Exactly. So how, wait, how long does it take for your body to replenish everything that was donated? About four weeks. Four weeks? Yeah, which is quite a long time. Oh, man. Here's what I was thinking. If you, um, if you were sitting there and instead of giving you some orange juice to drink, what if instead you sort of became a bit of a vampire and drank a pint of blood instead? Ah. Because here's what's interesting, uh, half a litre of blood, it might take six hundred and fifty kilocalories for you to make it, but it only contains about four hundred and fifty calories.

• 30:04Hannah Fry?

  That is fascinating. Isn't it? So gulping down blood isn't a good way to bulk up? Gulping down blood is not a good way to bulk up. So here was what I was thinking was like, okay, if you, I mean, sure, there's like, that's still some good calories in there. But what if you connected yourself up? So you basically made yourself a sealed, a sealed tube. Oh, yeah. Where you were continually drinking your own blood, right? How would that work out? Probably not well. The answer is definitely not well. So there's a few issues here. The first is that you would, you would almost certainly die from heavy metal poisoning.

• 30:46Hannah Fry?

  Because a single pint of blood also contains 250 milligrams of pure iron. And if you are drinking that continually, you're going to be forcing your digestive tract to, to absorb these massive toxic doses of iron. Which is going to overwhelm your liver.

• 31:03Michael Stevens?

  Oh, wow. So it's obviously that iron got in there naturally and healthily, but to have it so concentrated.

• 31:10Hannah Fry?

  Exactly. It's generally not a good idea.

• 31:12Michael Stevens?

  Wow.

• 31:14Hannah Fry?

  What you could do though, is if you were, if you're allowed to drink water, I'm going to assume that you're allowed to drink water. How long would it take you? You're running this calorie deficit. It takes you 650 calories to make the blood, but you're only getting 450 calories from drinking it. How long would it take you to waste away, to wither away? So we're talking here, you know, you've got your, you've got your base metabolic rate as well. That's all going on. And then you've got this deficit that you're running at.

• 31:45Hannah Fry?

  Essentially, I work it out as you're, you're running at a deficit of about 2,190 kilocalories a day. I reckon you would wither away in 87.6 days.

• 31:57Michael Stevens?

  Almost three months. That is also longer than I expected. So if you tell someone to go eat themselves, it's like telling them to go die in three months.

• 32:08Hannah Fry?

  Yeah. Yeah. Now, what I will say is I'm not a medical professional and I wouldn't, I wouldn't recommend this as a way to be. And it does also assume a steady supply of water. Okay. Cause you're kind of losing water at every stage of this process.

• 32:23Michael Stevens?

  Yeah, you got to replenish that. But wow. Okay. So donating blood means that over the next month, you will have an extra 650 calories. You're going to have to burn just to make back that blood. How often can you give blood?

• 32:40Hannah Fry?

  I think it's about every three months for men and every four months for women.

• 32:45Michael Stevens?

  Oh, interesting. It's different.

• 32:47Hannah Fry?

  I guess because if you're menstruating, presumably that comes into it too.

• 32:51Michael Stevens?

  Gosh, only every like three or four months and a pound of fat, roughly everyone's body is different, but it takes like, that contains like 3,500 calories. So for your body to eat a pound of its own fat, to compensate for the blood loss from the donations would take how many months? Seven times four, like 28 months, basically two years to lose a pound just by donating blood at a healthy rate. Not worth it, guys. Now, this is reminding me of a question that I'd always wondered, and I finally tried to look it up this morning, and it was, you know, I don't grow hair up on top of my head anymore. And I'm like, how many, how many

• 33:33Michael Stevens?

  extra calories am I not burning because my body isn't physically producing hair out of these follicles? Like, is that something to be, you know, worried about? And it turns out probably not. I found a forum on the Straight Dope where this user called Squink, which how can you not trust Squink? Squink is like, really loved this question and got obsessed with it, but the only data Squink could find was the amount of energy sheep need to grow their wool. And so Squink's conclusion is that if, if a person is like a sheep, well, actually Squink is talking about beards specifically,

• 34:14Michael Stevens?

  to grow a beard for a day, just a day's worth of beard growth is only 2.28 calories.

• 34:24Hannah Fry?

  Is that it?

• 34:25Michael Stevens?

  That seems really low, but I know our bodies are really efficient. I mean, if that's true, then it kind of explains why when people go bald, they don't suddenly go, wow, my metabolism is so different.

• 34:40Hannah Fry?

  That sort of feels worth it, actually. I mean, if you're thinking of the cost benefit analysis.

• 34:46Hannah Fry?

  I will pay you 2.2 calories for a day of beard. That seems worth it.

• 34:53Michael Stevens?

  Depends how much you want to have a beard, I guess. Balding is a better comparison because you can't stop growing a beard. And if you pluck or shave all your hair off, it keeps growing. You're still spending those calories every day. But once you actually go bald or have the follicles destroyed, then that's when the weight loss begins, guys.

• 35:13Hannah Fry?

  Over time, it starts to build up.

• 35:15Michael Stevens?

  Yeah, I mean, you can also shave off all your hair for like a weigh in. If you want to weigh like an ounce less, you could remove some weight that way. Speaking of which, if you look behind me, there's a Ziploc bag on the board. Let me bring this closer.

• 35:30Hannah Fry?

  Yeah.

• 35:33Michael Stevens?

  Here's some of my beard hair.

• 35:36Hannah Fry?

  Honestly, honestly, Michael, I think, I think sometimes a tour around your office, you would be confused as to whether it was the office of the, the great Vsauce or a serial killer, but go on.

• 35:48Michael Stevens?

  Or both. Yeah, it is weird, but you know, I shaved it all off once for charity and we had more than we needed. So I'm like, I can't throw this out. It's like a reminder of my beard before it turned completely white, which will happen one day soon. And so I just, I just have it.

• 36:04Hannah Fry?

  When I was, I don't know, maybe about eight years old or something, I got into a big fight with my sister. Um, and she pulled out a great big handful of my hair.

• 36:14Michael Stevens?

  Ooh, ouch.

• 36:15Hannah Fry?

  And, um, my mom, who has a sort of strange approach to punishment. She kept it in a Ziploc bag. Uh, and put a little note in it saying, this is what happened on this day. And, uh, she still has it, still has this little, this handful of hair. And the two things that I've noticed, um, in viewing it as an adult is one, my goodness me, my sister has got a good, good grip strength because it was an astonishing amount of hair to pull out of a child's head. Um, but secondly, my hair color has not changed at all my entire life.

• 36:49Michael Stevens?

  Oh, see, that's why it's so great that your mother did that. It's, it's, that's not punishment, that's, that's being an archivist. Like I, I understand, I've done that too. I've still got my daughter's umbilical cord, the little bit that like you had to wait for it to fall off. And I've got clippings from her first haircut. I've got the wristband that they put on to her and us at the hospital when she was born. And I've given them all dates. I'm trying to remember. I might have like her first fingernail clipping. And they're all in like envelopes in my bedside table. It's like a whole, you know, Hannibal Lecter drawer, but it's actually beautiful.

• 37:26Hannah Fry?

  Yeah, it is. It is. A life through offcuts.

• 37:31Michael Stevens?

  Yeah.

• 37:32Hannah Fry?

  Okay, should we go to a break?

• 37:33Michael Stevens?

  Let's take a break and when we come back, I want to see this steampunk iPad.

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• 39:19Michael Stevens?

  And we are back. Hannah, you've got some kind of, I'm assuming like a mechanical calculator, but not an abacus, something in between then and now.

• 39:30Hannah Fry?

  I do have, I do have a mechanical calculator. So this was, um, my sister gets really good gifts for me for Christmas, and this was my Christmas gift this year. Um, and it's a thing of, it's very beautiful. Look at this. So it's, um, it's brass or looks like brass anyway. And, um, it is, it sort of fits in your hand and, uh, it's got a fractimater written across the top. It has feet, inches and sixteenths. And basically it's an addition system with a subtraction system hiding on the back.

• 40:08Michael Stevens?

  Right. It's quite flat. It's definitely iPad scale kind of a thing, like a very thin little book, but it's made of metal and it's got grooves on it with what look like divots or something.

• 40:22Hannah Fry?

  Little holes that you can use.

• 40:25Michael Stevens?

  I cannot wait to see how this works because believe it or not, there's something similar to that at the thrift store next door to my office and I could not figure out how to move the numbers or the, or the pieces inside of it. Do you need to have a stylus or a pencil?

• 40:40Hannah Fry?

  A pencil. Exactly. Right. Okay. So I just want to, um, I just want to sort of like picture the world at this point. So this particular one was in, is made by a German company. They started making them in the 1920s, but these continued to be popular even after the invention of the calculator. Essentially, this is like a pre calculator world, right? And you have to imagine that if you are doing complicated sums, so either you are an accountant and you're totting stuff up all day long, um, or worse, you're an engineer trying to build a bridge or an aircraft. Actually, do you know what? I don't know when the actual digital calculator was invented.

• 41:24Michael Stevens?

  I know that my grandparents got one, I think in the early seventies. They got it for free for looking at some property, like real estate. And they just, they drove like all day just to do this tour with no intention of buying because they wanted the free digital calculator.

• 41:40Hannah Fry?

  Do you know what? Your, your grandparents are my kind of people, frankly. So the first electronic desktop calculator was in nineteen sixty one. But as you said, they were not widely available. Texas Instruments, who are sort of the kind of the calculator kings, they weren't releasing them until the late nineteen sixties, nineteen seventies. So we're talking about, there are Sputniks in space. We've got, you know, attempts to get to the moon while people are running the calculations on all of the orbits, on how to build everything.

• 42:16Hannah Fry?

  They're running them on these kind of things here, these like, these mechanical systems where uh, there's no electronics involved, right? When you stop to think about it, they're like constructing nuclear bombs with mechanical calculators. They're sending people into space with mechanical calculators. Just, they're discovering general relativity and quantum mechanics with mechanical calculators. This is just wild to me. I can't even, I can't even do five times eight anymore without checking it on an electronic calculator.

• 42:55Michael Stevens?

  Yeah. Show me, show me how this works.

• 42:59Hannah Fry?

  Okay, let me show how it works. All right. So this particular one, the fractometer, uh, the thing that makes this special and the thing that made it outlast electronic calculators is because it can handle fractions of an inch. Okay. So if you go run along the top here, so this is all about distance.

• 43:16Michael Stevens?

  Can you use this for simple addition and subtraction, or is it for working within the imperial system of feet and inches?

• 43:23Hannah Fry?

  So you can use it for integer addition and subtraction if you want to. But when it comes to the decimal points, then you're, then you're, you're locked into the imperial system. Okay. So let me show you like an additional, a, a, a really straightforward addition on this. Okay. So what you do, so, so, um, for those of you who are listening, let me try and describe what you're looking at here. So you have, uh, You've got this panel of numbers and they're arranged as you might expect. So you've got some units, tens, hundreds, et cetera, et cetera, et cetera. And then over to the far right hand side, you have inches and then you have fractions of an inch right over on the far right hand column. Okay, so if you want to add, the thing is, if you just want to add, let's, let's go here, right?

• 44:07Hannah Fry?

  So in the ones column, if you want to add, say, um, four.

• 44:13Hannah Fry?

  So you slide it down to the number four, you see the number four appears here.

• 44:17Michael Stevens?

  Right, you've stuck your pencil into the hole for the number four and you've dragged an internal rail downwards.

• 44:26Hannah Fry?

  Exactly. So if you want to do something simple like four plus two, I've done the four and then I can do the exact same process for two. I put my pencil in the two and I drag it down. And then you've moved the internal mechanism six spaces. Okay, that's really super easy. Where things become difficult is when you take a number that then requires you to carry over into the next column.

• 44:50Michael Stevens?

  Ah.

• 44:50Hannah Fry?

  This has always been the thing that makes it tricky. So what you do in that case, so I now have six, uh, if I want to add a number to six, let's say you want to add eight to six. Instead of going down, you go up and over, which effectively

• 45:07Michael Stevens?

  Oh.

• 45:07Hannah Fry?

  moves this block, moves the block that you were originally in by, um, eight spaces. But it also carries over to the following column. So it's like you're effectively like minusing two from this column and adding 10 to the next. And it's doing it all simultaneously. And so this is the bit that makes this super clever is that you can, rather than the rail just going up and down and up and down, when you carry over, you, your pencil physically moves the next, the next column of units.

• 45:43Michael Stevens?

  Right. I was wondering why they had that candy cane shape. And it's because you have to go up and then drag over to notch one more on the next place. So the slider that you're sticking your pencil into, it's mainly silver, but eventually you start seeing red numbers and if it's red, you need to pull up, I guess is the rule.

• 46:07Hannah Fry?

  Exactly. Because adding eight is the same as saying minus two plus 10.

• 46:13Hannah Fry?

  Or minus two to this column, add one to the following column.

• 46:16Michael Stevens?

  This is why I think devices like this should be in schools. Not because I love old fashioned things, but because it shows you how numbers work with each other and what the goal is and the different ways to get there. When you just are taught, memorize five times eight, or memorize this particular strategy for multiplying two-digit numbers, it just becomes, I don't know, like you do it by rote and you don't go, ah, place value matters and I get why we're carrying over or whatever.

• 46:51Hannah Fry?

  Yeah, because you are literally carrying it. I mean, you're physically carrying it over to the next column. I think that's exactly right, exactly right, that you get this kind of intuitive, physical feeling for it that you don't get when you're just dealing with an electronic calculator alone. The really nice thing about this one, so all of these, where you're in the feet columns, you're just going one to 10. So this is all makes, you know, perfect sense. Because it's in imperial, once you get over to the inches, it no longer is one to 10, or naught to nine, I should say, it suddenly becomes naught to 11 in the inches column.

• 47:27Michael Stevens?

  Right.

• 47:28Hannah Fry?

  It still allows you to carry over, but it takes into account the fact that there are 12 inches in a foot.

• 47:33Michael Stevens?

  Right, so it's the 12th that moves into the next place.

• 47:38Hannah Fry?

  The next column, quite right. When you're over here on the sixteenths of an inch, this is where it has an advantage. If you're having to deal in feet and inches and sixteenths of inches and so on all the time in imperial metrics and you're building bridges and whatnot, this is where this has an advantage to an electronic calculator. Because let's say that you want to do, okay, three eighths of an inch. Hang on, let me do a little reset, clear all, thank you.

• 48:04Michael Stevens?

  Oh, wait, show us how it resets. It's got a little like bar on top?

• 48:07Hannah Fry?

  It does have a little bar and you pull it up and it resets all of the numbers. Isn't that cute?

• 48:11Michael Stevens?

  I love that.

• 48:13Hannah Fry?

  So let's say that you had three eighths of an inch. And you wanted to add to it eleven sixteenths of an inch. If you're doing it on a calculator, you first got to convert everything to sixteenths. You've got to add it up. It's just, it gets a bit complicated. Then you've got the imperial to metric conversion, or sorry, the imperial to decimal conversion. It just, it's all really complicated. But with this situation, you can just go, okay. Actually, it's just fifteen sixteenths. Easy peasy.

• 48:43Michael Stevens?

  That's really nice.

• 48:44Hannah Fry?

  So these actually, as I say, lasted on in time. These were still being used well into sort of late seventies, early eighties, just because they had this advantage to it.

• 48:55Michael Stevens?

  I wanted to ask how subtraction works. Do you just reverse the rule? You move silver up and you move red down?

• 49:02Hannah Fry?

  Don't just reverse the rule, my friend. You reverse the side.

• 49:05Michael Stevens?

  What?

• 49:06Hannah Fry?

  It's the exact same internal mechanism. It's just you're moving it up on one side, down on the other effectively, because it's, it's, it's flipped over.

• 49:14Michael Stevens?

  Right.

• 49:15Hannah Fry?

  So you don't need a new thing. You can just add and subtract, add and subtract. And it can tally the entire thing as you go. So you could be adding some things together, flip it over, subtract them.

• 49:25Michael Stevens?

  So you flip it over. It's, it's got the answer you currently have, the running total. And then you can start subtracting from it on the other side.

• 49:32Hannah Fry?

  Exactly.

• 49:33Michael Stevens?

  That's beautiful.

• 49:34Hannah Fry?

  Isn't it gorgeous? And this is the thing, okay, so just going back to the point that you made earlier about how having these old school mechanical objects actually give you a much more intuitive feel for what the numbers are actually doing. Even when you're doing simple things like addition and subtraction and multiplication. I think the same thing of good old fashioned slide rules. You know, we're taught, we're in boom at tech now.

• 50:01Hannah Fry?

  I love a good slide rule, but I love no slide rule more than the slide rule, Michael, that also happens to be a pair of chopsticks, which was a gift from you.

• 50:13Michael Stevens?

  Look at those. They look familiar. Yeah, we designed those and they were in a curiosity box a few, a couple of years ago. And yeah, you can, you can eat some food with your chopsticks, but then they also slide by each other and you can do. Slide rule calculations. You can do addition, subtraction, multiplication, division, as well as exponents.

• 50:33Hannah Fry?

  Yeah, to the power of two, to the power of three.

• 50:35Michael Stevens?

  We don't have time to go through it today, but learn how a slide rule works. Learn how that kind of, what the heck is that thing that you, what's it called in general? What you brought today? It's a mechanical calculator, but is it called like a slider or a?

• 50:50Hannah Fry?

  This one is called an addiator.

• 50:52Michael Stevens?

  Ah, yes, a slide calculator, also known as an addiator, after the best-known brand. So the generic is slide calculator. I don't want the addiator people coming after us, they're their competitors. But the point is that learning how these work gives you such a much deeper understanding of number theory. It's amazing.

• 51:11Hannah Fry?

  The reason why I think slide rules are really interesting is because they use this mathematical trick to make multiplication way easier. The thing that makes these unusual, the slide rule and the addiator, is that they're not using any turning, because most mechanical calculators would be based on a system of cogs, where you would have one cog that would turn and turn and turn and turn and turn, and then on the 10th would flip the next cog. That was sort of how they managed to handle carrying over a digit effectively. And there are all kinds of these that exist at various levels of complexity. One of them, which I think is probably the most beautiful, is called a curta.

• 51:53Michael Stevens?

  Oh, yeah.

• 51:53Hannah Fry?

  And these things, they get found in people's lofts, like stashed away in the stuff your grandparents owned, thinking, they often get sort of chucked away, really, because people think that they're musical boxes that don't work anymore. But they look like an old-fashioned sort of pepper mill. They have a handle on top that you can turn around to do the calculations.

• 52:15Michael Stevens?

  Like a pepper mill, yeah.

• 52:16Hannah Fry?

  Like a pepper mill, exactly.

• 52:17Michael Stevens?

  I know, I've seen videos about them and I want one so badly, but look on eBay, they're all over a thousand dollars.

• 52:24Hannah Fry?

  Over a thousand dollars. They're wildly expensive. And fair play, because they are an unbelievable masterclass in precision engineering.

• 52:34Hannah Fry?

  But the story about these Kertas is that it was actually invented by a prisoner of war during the Second World War, Nazi prisoner of war. And were it not for his clear talents in this area of engineering, then he almost certainly would have been murdered. But instead, he managed to escape the camps and after the war, launch this brand. And it was, you know, one of the most popular mechanical calculators for a number of decades.

• 53:04Michael Stevens?

  Wow.

• 53:05Hannah Fry?

  I want one of those so bad. I want one so bad.

• 53:08Michael Stevens?

  All right, I'll put that on the Christmas list for Hannah. Kerta mechanical calculator.

• 53:14Hannah Fry?

  The fractometer, by contrast, I think is worth about 20 quid.

• 53:17Michael Stevens?

  Yeah, I was gonna say the slide calculator next door is $3.

• 53:23Hannah Fry?

  Yeah, buy it though, I wanna see a picture of it.

• 53:26Michael Stevens?

  Yeah, I'll see if I can find it.

• 53:28Hannah Fry?

  You've inspired me though, I think I'm gonna go and play with that with my kids. I was in the House of Lords last week, talking about adult numeracy. This is the kind of stuff I do on days when I'm not with you, Michael. And they were saying, how can we improve numeracy in young people and adults in the general population? And it's really hard, but I think that you're right, that actually having a physical connection when you're manipulating numbers is one of the really good ways to do it.

• 53:55Michael Stevens?

  Well, yeah, I'm sure you've seen people build those mechanical counters that go up with binary. And like, that shows how binary works so much better than reading the Wikipedia page. And you can build calculators with falling marbles and just little levers in the same way, in base 10. And your understanding of what the heck numerals and our notation, or what a place-value system, what that means, it makes everything so much clearer. I have a book that is, I haven't read it yet, but it's one of my ideas for like, a long video in the future. And it's a book of all the different ways there are to multiply numbers. And add them and subtract them. And by that, it means all the methods.

• 54:39Michael Stevens?

  And in school, you usually learn the method that your country at the time has said, this is what we should teach. So for like me multiplying, we did the whole like, okay, so you start with the ones digit and you go to the ones times the ones, the ones times the tens and you label them out and add them up. But then you see these viral TikToks about in Japan, they use diagonal lines and in China, they use dots or whatever. And everyone goes, what? Math should only go one way. And it's like, no, these are the methods for working with numbers. Sometimes it's more convenient to do it one way, sometimes the other. But if you learn them all, you suddenly are like a wizard of numbers.

• 55:17Hannah Fry?

  I have a similar book downstairs, actually. It's speed arithmetic techniques.

• 55:23Michael Stevens?

  Oh.

• 55:24Hannah Fry?

  As you can tell by the fact that I need your calculator to do four times eight, I haven't read it, but, um, but one day, Michael, one day I'm going to read it and then I'm going to blow you away.

• 55:34Michael Stevens?

  Oh yeah, I found that fell down a rabbit hole of watching a like math, a speed math person on Instagram or something. And I loved it because the guy was using very different methods. He was like, don't do the whole thing you learned in school. Like just take every number and break it apart. 72 times 48, you can do 70 times 40 really quickly. And you can do two times eight really quickly. And then adding them together in your mind, way easier to do mentally.

• 56:00Hannah Fry?

  Do you know what? I think we should do another Field Notes episode where we just shout numbers at each other.

• 56:05Michael Stevens?

  Yes. I do definitely think an episode on like mental math tricks could be useful. Like there are some, you know, like finding the square roots of certain numbers can actually be really easy. Squaring numbers and cubing them, there are shortcuts. And then the listeners will walk away going, hey, name a four digit number. I can cube, I can find its cube root in like a second. And your life will be the same.

• 56:31Hannah Fry?

  Okay. Well, you can write to us and tell us if that is an episode that you would like to hear or the very idea if it horrifies you. As ever, send us anything you would like us to answer to [email protected].

• 56:43Michael Stevens?

  Yes, please do. And join our newsletter at therestis.com/science. Until next time, stay curious.

• 56:50Hannah Fry?

  Bye.

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• 58:00Michael Stevens?

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• 58:06Speaker 09

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• 58:27Speaker 10

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