Mathematical Paradoxes (Understanding Infinity Part 1)

Is infinity actually a real number, or just a brilliant mathematical hallucination? Professor Hannah Fry and Michael Stevens (VSauce) tumble down the numerical rabbit hole to explore the mind-bending origins of infinity. They unpick exactly how humanity managed to trap the endless void. From ancient paradoxes to endless hotel rooms, they dive into the bizarre history of our universe's most impossible idea. ------------------- 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 Oakley Video & Social: Bex Tyrrell Assistant Producer: Imee Marriott Researcher: Lucy Lipscomb Producer: Simona Rata Senior Producer: Lauren Armstrong-Carter Head Of Digital: Samuel Oakley Exec 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.

• 0:00Hannah Fry?

  Welcome to The Rest Is Science. I'm Hannah Fry.

• 0:02Michael Stevens?

  And I'm Michael Stevens.

• 0:03Hannah Fry?

  Okay, Michael, I'm gonna start with an easy question for you today.

• 0:06Michael Stevens?

  Okay.

• 0:07Hannah Fry?

  Would you want to live forever?

• 0:09Michael Stevens?

  No.

• 0:10Hannah Fry?

  No, me neither. I think I think life only has meaning because it's finite.

• 0:14Michael Stevens?

  Yeah, I agree.

• 0:15Hannah Fry?

  Here's another question for you, though, that's related. If you had the choice between dying in the next five minutes or living another week, what would you what would you choose?

• 0:30Hannah Fry?

  This episode is brought to you by Cancer Research UK.

• 0:32Michael 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.

• 0:52Hannah Fry?

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

• 0:58Michael Stevens?

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

• 1:06Hannah 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.

• 1:16Michael Stevens?

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

• 1:24Hannah Fry?

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

• 1:29Michael Stevens?

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

• 1:42Speaker 02

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• 2:13Speaker 03

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• 2:48Michael Stevens?

  Well, both of those are finite, but I will pick the larger of the two. I would pick a week.

• 2:52Hannah Fry?

  You're right. So now, now let's imagine I ask this question again in a week's time. Would you rather live another five minutes or, or survive for another week? What would you pick?

• 3:01Michael Stevens?

  I imagine that a week from now, I would still prefer one more week.

• 3:04Hannah Fry?

  Hmm, what about, what about the week after that?

• 3:06Michael Stevens?

  Another week, please.

• 3:08Hannah Fry?

  You may see where I'm going with this.

• 3:11Michael Stevens?

  Right.

• 3:11Hannah Fry?

  Because here's the thing, if you continue on with the argument forever, this is like a classic philosophical experiment. This is originally put forward by Thomas Nagel in The View from Nowhere in 1986. And he said, "Given the simple choice between living for another week and dying in five minutes, I would always choose to live for another week. Thus, I conclude, I would be glad to live forever."

• 3:30Michael Stevens?

  I don't agree with that conclusion. I mean, I'm not, I'm not saying that Nagel's a liar. I just think that eventually you will not prefer a week over five minutes. Eventually, in my life, I, I imagine I will reach a point where I say, "Yeah, give me just five more minutes. That's all I need, and that's all my loved ones and friends need of me. Like, I'm done, and it's their turn."

• 3:56Hannah Fry?

  You're happy with a finite life, effectively.

• 3:58Michael Stevens?

  I'm happy with it, and I desire it, yeah. I think that if I was granted immortality right now, my first emotion would not be, "Whoa!", it would be an immense anxiety and claustrophobia, a feeling of being so trapped. I'm trapped here in this universe. And that would be terrifying. It would not be freedom at all.

• 4:21Hannah Fry?

  No. The concept of infinity is just a bit too much to bear.

• 4:25Michael Stevens?

  And so today, we're gonna bear it. We're going to wade into and grasp infinity.

• 4:34Hannah Fry?

  Or at least try.

• 4:35Michael Stevens?

  First things first, let's set the ground rule about what infinity is. When, when I say infinity, and I want to hear what you think too. Infinity, to me, is a number. It can be, it can be a number. It can be an amount. And that can be an amount that infinity means is unending. Okay, there's, there's no end to it. There's no final member. And so it's not on the number line. But if you come, if you think of the entire number line and you ask how many numbers are there? The answer is infinity.

• 5:07Hannah Fry?

  Yeah, see, I think I'm gonna slightly disagree with you because, because I don't think it's a number in the traditional sense. Purely because, you know, you can't do number like things with it. You know, you can't like, it doesn't really lend itself to addition or multiplication or like subtraction. I think that infinity, and by the way, we're gonna do two episodes on infinity. But I think that for the purposes of this episode, I think it is something that you can approach but never reach. I think it is something that is boundless, that is endless, that is, that is larger than any other measurable quality.

• 5:45Michael Stevens?

  Excellent. And I completely disagree. I think that we reach it every day multiple times. I think that infinity is an amount. I think that there are different kinds of numbers. Yes, you, it's hard to do certain kinds of arithmetic with infinity. But try taking the square root of a negative number, you know? I think that we just have to admit that there are different kinds of numbers. There's rational, there's irrational, there's imaginary, and there's infinite. They're all numbers, though.

• 6:17Hannah Fry?

  Can we just talk a little bit about how weird infinity is? Because it is so much weirder than any of the other numbers that you've described.

• 6:23Michael Stevens?

  Yeah, I grant that. So let's, let's, let's talk about it.

• 6:26Hannah Fry?

  Okay, so I think one of my favorite explanations about how weird infinity is comes from the mathematician Hilbert, German mathematician. He imagined this hotel, okay, called Hilbert's Hotel, and it's a wonderful demonstration. So essentially, this hotel, it's got an infinite number of rooms, all right? It's just sort of wild to imagine. But also, every single one of those rooms, right now, happens to be fully booked. It is an infinite hotel that is infinitely full. You turn up to the hotel, looking for somewhere to stay for the night. And how do you get in? I mean, the hotel's completely full.

• 7:04Michael Stevens?

  It's booked. Yeah, it's booked. And I just wanted to say, just, just for clarity's sake, this hotel does not have a lot of rooms. It has an unending number of rooms. You think you found the last room, there's always one after it, always.

• 7:16Hannah Fry?

  Yeah.

• 7:17Michael Stevens?

  And yet, it's fully booked. So like, it's, there's no vacancy, or is there?

• 7:22Hannah Fry?

  Right, there is, there's no vacancy or is there? Because, you're very clever, so you say, well, it's fine. You could make room for me in this infinite hotel. Because all you need to do is, everybody needs to leave the room that they're currently in, and then go to the room that is one along. So if you're in room two, go to room three. Room, room seven, go to room eight. And then, all of a sudden, there's always a room that you can move down to, even though it's infinitely full. But now, the very first room, room one, is now vacant, because nobody has gone into it. So you, as the additional guest, have suddenly found space in an otherwise completely, completely booked hotel. So, okay, addition doesn't really work in the same way for, for infinity.

• 8:05Hannah Fry?

  But, but there's, I mean, it gets way weirder than this. Because now, imagine that a bus with an infinite number of seats on it, which is also full, turns up to your infinite hotel, which is also completely full. I mean, this is much more of a puzzle. How on earth do you fit infinity inside infinity? This is just infinity plus one. How do you do it? But there is, in fact, a way to do this. You, it's a similar trick to before. All you do is, you say, okay, if you are in... Room N, you double the number of your room, you go to 2N. So if you're in room two, you move to room four. If you're in room seven, you move to room 14. If you're in room 25, you move to room 50.

• 8:50Hannah Fry?

  But because everybody has doubled their room number, suddenly all of the odd numbers are completely vacant. And there is an infinite number of odd numbers, which means you can fit the infinite busload of people into this infinitely full, infinite hotel.

• 9:07Michael Stevens?

  Yeah, infinity plus infinity is just still infinity. It's still a fully booked hotel. You had room, you can always make room. You can always make room for one more because infinity is unending.

• 9:17Hannah Fry?

  There are, there are layers to this, right? Because, I mean, you can go further with the weirdness of this infinite hotel. And we should, and we shall, no less. Because now imagine that you've got this infinite car park outside. And now, instead of one person or one bus, or one bus with an infinite number of people. Now, this infinite car park is full of an infinite number of buses, which are full of an infinite number of people. How can you fit an infinity of infinities inside of infinity? And it, and it turns out you can do that too. That's not even a problem. You can do it because you start off with the same trick as before. Every person doubles their room number, leaving all of the odd number rooms completely free and open. There's this infinite number of prime numbers.

• 10:01Hannah Fry?

  So the first bus, you say, okay, you can be bus number three, all right? The first real useful prime, you know, the first odd prime. You'll bus number three. The first person goes into, uh, three to the power of one, the third room. The second person goes in three to the power of two, the ninth room. The third person goes in three to the power of three, and so on, and so on, and so on. You go through and you fit that infinite number of people, all into odd rooms, by the way.

• 10:27Michael Stevens?

  All powers of three.

• 10:29Hannah Fry?

  All powers of three, but you have completely left untouched. Room five, room seven, room 11. All of the other prime numbers, you can repeat this trick for all of those. So the second bus is bus five. The first person goes in five to the one. The second person goes in five to the. The two and so on and so on and so on. I mean, a bit difficult to follow this on a podcast, I'll admit, but you can trust me. You can trust me that this works perfectly. An infinite number of buses with an infinite number of people can fit into an infinite hotel with an infinite number of rooms that are all booked. Like, this is extremely strange.

• 11:08Michael Stevens?

  So, yes, this is all very strange behavior, but it's still numbery behavior, in my opinion. I mean, infinity, what does it even mean? It just means in, which means not, finity, finite. So not finite, not ever coming to an end. And, you know, today, we're very familiar with the concept of infinity. The word is very popular, you know, and the symbol.

• 11:36Hannah Fry?

  It's a flopped over eight, let's be honest.

• 11:40Michael Stevens?

  The limniscape, yeah, it's a flopped over eight. It's a lazy eight. And it represents infinity, but we actually don't know why that symbol came to mean without end. The very first, the very earliest use of the limniscape to mean unending was by John Wallace in 1655. And he just uses it, but does not explain what it means or why he chose that symbol. So maybe people were already talking about using this symbol. I think the best guess is that, my favorite at least, is that it came from Roman numerals. Because, yes, there was a, like, one of the earliest forms of Roman numerals

• 12:25Michael Stevens?

  did this thing where parentheses were used to change the value of a number. And so 500 in Roman numerals, we all think of as being the letter D. The capital letter D is what it looked like. But before it was a D, it was an I followed by a backward C, which is, I'll do it for the mirror, yeah, which you combine them, that's a D. So the D for 500 may have come from an I followed by a backward C and they got squished together. So. Before the whole D thing, the parenthetical arrangement of Roman numerals worked where for every backward C you added, the value went up by a thousand more.

• 13:10Michael Stevens?

  So I backward C, backward C wasn't 500, it was 5,000. But if you put a C on either side of the I, it didn't represent 1, it represented 1,000. So C, I, backward C represented 1,000. And it's believed that that might have been where the M came from, that eventually came to be, and today, we usually think of as being the Roman numeral for 1,000. That it was originally regular C, I, backward C, and then they not only melded together, but the bottom broke open and it became an M, alright? But, this I that's encapsulated in a circle gets us really close to that sideways 8.

• 13:51Michael Stevens?

  In John Wallace's time, 1,000 was often used hyperbolically to mean can't even count, thousands and thousands. So 1,000 in Roman numerals could mean never-ending, is what I'm really saying. Not exactly 1,000, not 10 hundreds, but just, you know, infinite.

• 14:10Hannah Fry?

  I really like that a lot. I really like that. I'd always sort of assumed that it was related to the number 0. 0 being this symbol from Indian mathematicians that's very related to this idea of like a state of nothingness and like no beginning, no end. So I'd always, I think, assumed that it was something like that, like a twisted 0. But you know, if you're saying 1655, right, that's simple. I mean, 0 wasn't even, I mean, was it even widely used in as far away as Britain at that point? I think it's around about the same time, right? That makes much more sense that it comes from the Roman.

• 14:46Michael Stevens?

  It could have, it could have come from Greek, right? So another theory is that the letter omega, which in lowercase looks like a little W, it could have gotten closed up and made the lemonscape. And the omega is the last letter, you know, the end. So, I don't know, it could be something like that. Thank goodness we started using 0. Like, Roman numerals are a joke. I don't even care. If the ghost of some Roman soldier comes to

• 15:10Hannah Fry?

  Julius Caesar comes to kill you.

• 15:12Michael Stevens?

  Right. I'll be like, dude, don't even try. You don't even understand positional notation. Like, give it a break. Because I just, just yesterday, I finished Ursula Le Guin's short story, The Masters. And it's this really, I don't know if you've read it, but I'll tell you, the story is fantastic because the premise is that there was some terrible apocalyptic thing caused by human technology. Right? You could imagine that it was like a nuclear war. They just call it the hellfire. And it happened 14 generations ago. Because it was so traumatic and all these famines occurred, there's been this taboo created around rational thinking, the scientific method, using numbers. No one's allowed to do them.

• 15:53Michael Stevens?

  But they still have all the technology that we have, you know, they have steam engines and cars and all this stuff, but everything's built just with comparing sticks. You're not allowed to have numbers like Western Arabic numerals, you know, one, two, three, four, zero through nine, those things. You can't use those. Everyone's been forced to revert back to Roman numerals because they're so hard to do math with. And you've got, you know, you've got the apprentices who were building the steam engines being like, hey, what's, uh, what's X I V plus X X V I? And they can't do it mentally. They have to just memorize this stuff. But there's a guy who draws a circle in the sand and he's like, guys, that's the symbol for nothing. And they're like, stop it. That's one of the black letters. We're not allowed to talk about that.

• 16:39Michael Stevens?

  And it's just very cool. And all the examples of people trying to do math with Roman numerals is so hilarious that Julius Caesar come at me. I am not afraid.

• 16:48Hannah Fry?

  Yeah. I don't know. I think this will come to in the second part of this, this episode. Zero and infinity, they're sort of sisters, you know, they, they really do go hand in hand. And so I think it is quite interesting, actually, that it's around about the same time that English mathematicians at least are writing about infinity. As zero is, is coming into common usage, you know, I think that's, that's probably not a coincidence. I mean, the, the folks on the rest is history can, can correct us on a whole wild, wild guesses as to what might have happened in the past. But, um, but I think that's probably not a coincidence. Okay. Just picking up on that idea of Omega. Because the ancient Greeks, they did have this like inkling about infinity. They weren't like, they were sort of, I mean, they didn't like it.

• 17:33Hannah Fry?

  It didn't feel comfortable. They sort of wanted to avoid it as much as possible. But they definitely had this inkling that there might be something there. So the particularly famous story is about Pythagoras and his disciples. So by the way, Pythagoras is like, he's kind of survived through history as this like, this genius figure, this lone genius. Not true at all. He's basically a cult leader. He had some, he had some absolutely wild ideas about, well, notably about beans. I remember I've mentioned this in this podcast before, but he was absolutely terrified of beans. He thought that they were essentially little humans.

• 18:09Michael Stevens?

  Why, why, do you know why he thought beans were little humans?

• 18:12Hannah Fry?

  I think he, I think he sort of thought that they looked like them. I mean, once you get this far back, the number of interpretations and original source materials that you have to work on. There's many, many different ways to go. I think also that there's a few different reports that Pythagoras was trying to persuade bulls not to eat farver beans. You know, it's, it's, it's commitment. The thing about Pythagoras, his cult, and actually the ancient Greeks more generally, is they had this really deep belief that the universe was made up of exquisite order. You know, that the, the movement of the planets in the sky, that, you know, the, the shape of circles, of triangles.

• 18:56Hannah Fry?

  It was just, you know, unending beauty in fractions and whole numbers. So in music, for example, this whole idea of harmonics, it was the Pythagoreans who worked out that frequency of certain notes, they sound good together when that you mix frequencies that are an exact multiple of one another. You know, that's how you get sort of harmonic and beautiful sounding music. And so the Pythagoreans, they had this list, this table of opposites, they called it. In one side, they would be good, the good things, the things they liked, things they were happy with. And on the other side, there'd be sort of the bad things. So they had odd and even.

• 19:38Hannah Fry?

  Um, one and many, uh, right and left, male and female. See if you can guess which column female went in.

• 19:45Michael Stevens?

  The bad side. The evil side.

• 19:49Hannah Fry?

  Yeah, it was joined by good and evil. Evil's also in the, uh, the, the, the bad column. Uh, square and oblong. Ugh, oblong, disgusting. Oblongs and females, get over there.

• 19:59Michael Stevens?

  Light and darkness.

• 20:00Hannah Fry?

  Light and darkness, exactly. But also finite and infinite. And infinite being like this horrible, disgusting thing that kind of go belongs over there. There is this story that Hippasus, who was an early follower of Pythagoras, and he was playing around with triangles and he had a, a, a right angled triangle with two sides that were both equal to one, right? So they're kind of equivalent to each other. And then he was working out how long the hypotenuse was, how long the, the diagonal that cut between those would be. Or essentially, if you take a square and you split it across the diagonal, how long is that line? And it's equal to the square root of two, which Hippasus was like, okay, hang on a second, the square root of two, there's no order to it. There's no natural

• 20:44Hannah Fry?

  beauty. There's this, these numbers that continue on indefinitely. There is infinity contained within this beautiful geometric shape. And, uh, the, the, the, the story goes, and we can never be quite sure. The story goes that they took Hippasus, um, out on the ocean and then just chucked him off the boat, drowned him at sea for arguing.

• 21:06Michael Stevens?

  Wait, for, for the sin of, of what entertaining and irrational number?

• 21:11Hannah Fry?

  I think he had a proof for it. I think he even managed to prove that it was irrational. And they were like, don't you come at me with this, with this sacrilegious nonsense. This is, this is despicable.

• 21:21Michael Stevens?

  Despicable, almost a little female.

• 21:26Hannah Fry?

  Almost a little female, exactly.

• 21:28Michael Stevens?

  What if, what if, what if it turned out that I was a closet Pythagorean and I, I believed all these things and I was part of his cult, like a modern day neo Pythagorean cult? And I was like, look, Hannah, infinity is evil and so is darkness and so is, so are odd numbers and women. Just, did you not know that about me?

• 21:49Hannah Fry?

  There's, um, there's, there's a very easy way to test this. I'll just, I'll just make you bake beans on toast to see what you do.

• 21:54Michael Stevens?

  Ugh, beans, more like human beans. See, now the language at least matches our weird belief.

• 22:01Hannah Fry?

  I think we should bring back the idea of, um, Being a closet Pythagorean is an insult, you know? I think we should start throwing it around.

• 22:07Michael Stevens?

  Well, I know, but the problem is that they also had some really great things. I mean, I love their

• 22:12Hannah Fry?

  They did.

• 22:13Michael Stevens?

  Their like worship of numbers and ratios as being something really fundamental, something that was so timeless, it was outside of time. It didn't change. It was godlike. Maybe numbers and math literally were god, right? I don't know why they also hated beans. But look, this is just, it's just too long ago and we can't ask them and we don't have a lot of what they wrote. Speaking of which, we don't have anything that that famous guy Zeno wrote.

• 22:43Hannah Fry?

  Hmm, because he was also tackling infinity, right?

• 22:46Michael Stevens?

  Well, kind of, you know? It's unclear to what extent. Zeno was around in like the fifth century BCE. And Zeno was a follower of a guy named, first of all, here's a cool, here's a cool trick. People bring up Zeno and especially Zeno's paradoxes all the time. And whenever people bring them up, I like to be like, like I act confused at first and I go, oh, Zeno of Ilya. Right, yes, of course, go on. As though there are like other Zenos I know and I need the clarification. So the, the Zeno of Ilya is the kind of the full name and Ilya is the Greek colony that he lived in. The mathematicians and well, really the philosophers who lived there were the Iliadics.

• 23:29Michael Stevens?

  And they had a very, to me, impenetrable belief that motion was impossible. Like I, I cannot describe this in a way to convince you, but they believed that there was no change, that it was all an illusion, that everything was just one. Everything was the monad, that's it. When we think that something's changing or moving, we're being fooled. And so the, the righteous, the wise thing to do was just to sit and do nothing and chant, it is, it is. That's it, it, everything. And of course, there were other thinkers at the time who were like, you guys are ridiculous. What does this even mean? Obviously, things can move. Obviously, things can change and it's real.

• 24:12Michael Stevens?

  And so Zeno was like, guys, I know you think that we're silly, but you're just as silly. Listen, if I believed in motion. Then here's a contradiction. Let's have Achilles race a tortoise. The tortoise is obviously slower. So Achilles says, oh, I'll give you a head start. I'll let you, you know, go for, you know, a minute before I even start. And the question is, who wins? And you might think, well, I guess it kind of depends. Like Achilles can clearly outrun the tortoise eventually. But Zeno says, logically, no. Because sure, when Achilles starts running, the tortoise is already somewhere up ahead.

• 24:54Michael Stevens?

  And Achilles has to first run to where that tortoise was when he started. But in that period of time, the tortoise has already moved a little bit. So the tortoise is now ahead of him. And Achilles now has to run from where the tortoise used to be to where the tortoise is now. But by the time Achilles gets there, the tortoise will be yet a little further ahead. And Achilles has to close that gap. But by the time Achilles has closed that gap, the tortoise will be a little bit further still. So it's impossible, since we can divide this out forever, it's impossible for Achilles to ever outrun the tortoise. And the other thinkers were like, yes, but he does. It does happen. And Zeno was like, but you can't explain why. And so that's the paradox. And Zeno wasn't making a joke.

• 25:39Michael Stevens?

  He wasn't necessarily arguing, here's proof that motion is impossible. He was just saying that if you believe in motion, you have to embrace contradictions, just like we are perhaps embracing some as the Iliadics.

• 25:52Hannah Fry?

  Well, that was the conclusion that he was drawing, right? Was that like this idea that motion is real really obviously breaks down once you consider this paradox. And thus, and thus, motion can't be real. Motion must be this illusion.

• 26:05Michael Stevens?

  Yeah, how do you explain that? And, and Zeno and, and the people that we, that we do have existing writings from never really used the word infinity to talk about what Zeno was describing here. But they, they did have to dismiss it in some way, usually just by literally getting up and walking and saying, eh, what do you think about this? How am I possibly doing this? But, you know, Zeno would say, guys, I don't, but how does it make sense that you can? Move because in order to move, you have to, you know, first cover like half the distance that you're going to cover. But before you can do that, you have to cover a quarter of it and an eighth of that first, but then a 16th of that first. So wait, you have, there's, how do you even start? How does a journey even begin? And they were like, yeah, where is the problem?

• 26:47Hannah Fry?

  At the heart of it is infinity there, right? But infinity is this philosophical monster that nobody could, could quite get their head around that was, I mean, essentially breaking human logic. And that was the case for many, many hundreds of years. I mean, thousands of years, frankly, until

• 27:02Michael Stevens?

  Until?

• 27:03Hannah Fry?

  Until Newton, the big fat baby came along.

• 27:07Michael Stevens?

  The big, the big fat baby.

• 27:09Hannah Fry?

  And Started squealing about it.

• 27:12Michael Stevens?

  Started squealing about it and like seemingly resolved the paradoxes, right? Today we have a very powerful tool. I would argue though, that really we haven't solved Zeno's paradoxes. We have constructed a bunch of great answers about them. But I think we should look at those answers because they approach infinity in a really different way, where instead of just dancing around it, they hold it.

• 27:35Hannah Fry?

  Okay. I'll tell you what, let's do this after the break. I am gonna do a little story about why Newton is a big fat baby. What he did that resolved Zeno's paradox. And then Michael and I can argue till the end of the program as to whether it actually is resolved or not. Sound good?

• 27:52Michael Stevens?

  Good. Let's do it.

• 27:53Hannah Fry?

  Okay.

• 28:01Michael Stevens?

  This episode is brought to you by Project Hail Mary, the new spectacular space adventure movie coming to cinemas from the author of The Martian, Andy Weir, and the directors of the Spider-Verse movies, Phil Lord and Christopher Miller. But here's an even better combination? Teachers in space.

• 28:18Hannah Fry?

  Hello. Thank you. Project Hail Mary stars Ryan Gosling as science teacher Ryland Grace, who is sent unexpectedly on an impossible mission into space to discover why the sun and the stars are dying. And he teams up with an unimaginable ally to defy all odds and save the universe from extinction. Okay. Here's a question for you, Michael. What, what kind of prep would you hope? Ryan Gosling had done for this role in order to play the role of a of a science teacher.

• 28:44Michael Stevens?

  He should have spent a bunch of time with cool teenagers and tried to teach things to them. So that it wasn't just like a good explanation, but also kept their interest and made them want to hear more and understand it so that they could share it to be cool, too.

• 29:02Hannah Fry?

  See Project How Mary now in cinemas and IMAX everywhere.

• 29:07Speaker 02

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• 29:37Speaker 04

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• 30:13Hannah Fry?

  Okay, the thing that we're hinting at here is calculus. You know, we don't, it's not mandatory in the UK, probably rightly so. You don't really learn it until you're 17 or 18 and you're doing A-levels. There is, there are huge swathes of the, of the British population who just have never, have never come across this absolutely beautiful subject. I think it's my favorite area of mathematics, by the way.

• 30:34Michael Stevens?

  I think calculus has become like slang for difficult math. And that's, that's somewhat unfair. I, I felt that way up until I was in my 30s and I got a copy of Calculus by Michael Spivack, which is literally a textbook. But I don't know if you've Read it. I once recommended this on Twitter that this was like the best way to understand calculus. And some of the most brilliant people that I follow on Twitter who are mathematicians were like, no, that's a terrible book to recommend to people. And I'm like, no, well, okay, so take this with a grain of salt, but it changed my life. Because Spivak doesn't just say, look, here's how to solve problems. He says, what the heck is change? And can we make sense of change in smaller and smaller increments?

• 31:20Michael Stevens?

  Or even change at an instant? He like breaks down what all these terms mean so fundamentally that he's like, look, we need to define even what like an ordered pair is, what is a function? And suddenly it just unlocked an understanding of of everything that mentions calculus. So I love that book.

• 31:38Hannah Fry?

  We should start from the beginning here then, because because calculus, it is the mathematics have changed. Everything that came before it, you know, geometry, number theory, they're all sort of standing still, as it were. But but in the real world, everything's constantly changing. Everything is moving, speeding up, slowing down, moving on curves, fluctuating. And so calculus was this really big idea that is what we use to measure and understand anything that is moving or changing effectively. The idea of it is is actually incredibly simple. And essentially what you say is, let's say that you've got this this really wibbly wobbly line, right? Like a kind of really strange little curve.

• 32:24Hannah Fry?

  You can't measure it with a ruler. I mean, you can't really can't really sort of say anything proper about it if all you've got to work on is a ruler. But what Newton and Leibniz, I'm gonna study about in a second. What both of them realized is that if you zoom in closer and closer and closer on any section of this really curvy line, if you zoom in close enough, it will look straight.

• 32:49Michael Stevens?

  Yeah.

• 32:50Hannah Fry?

  And so this was the, this was the really big idea is that if you cut up any line into an infinite number of chunks, each one of those chunks you can handle yourself, you can handle it. There's no, there's no problem there. So if you wanna look at the area under a curve, if you wanna work out the shape of anything, no matter how kind of crazy the outside of it is, you can use calculus to chop it up and then not actually have to do the infinite number of steps, because what this method gives you is a shortcut of doing an infinite number of things all in one go, essentially.

• 33:28Michael Stevens?

  And this is by using the concept of a limit.

• 33:31Hannah Fry?

  Of a limit. Yeah, exactly.

• 33:33Michael Stevens?

  Describe for me what's happening here, because if I am trying to walk from here to the door, I have to cover half the distance.

• 33:41Hannah Fry?

  You do.

• 33:41Michael Stevens?

  Then I have to cover half of what's left and then half of what's left and then half of what's left. And this goes on forever. It's an infinite number of things. And yet I cover them all in a finite number of time. How?

• 33:53Hannah Fry?

  Because the key thing that was missing from the original formulation of Zeno's paradox is that the amount of time that it takes for you to do that, it's a certain amount of time for your first step, it's a smaller amount of time for the next step, smaller and smaller and smaller. So you are doing an infinite number of things, but once they get infinitely small, they take you an infinitely small amount of time too. And so you can sum up an infinite number of things and end up with a finite number.

• 34:26Michael Stevens?

  And calculus formalized this and made it not just an idea, but a mathematical, logical, just demonstrable thing.

• 34:38Hannah Fry?

  Exactly. And one with, which, I mean, it's, it's in, in, at the heart of it, it's an incredibly simple idea, right? Zoom in close enough and everything is, you can handle everything. Um, but the power of it is just phenomenal. I mean, You know, there's no, like Newton's orbital mechanics, roller coasters, right? Like race cars, I mean, everything you can imagine, anything that moves or changes, the stock market, right? Like, anything at all is gonna have calculus in there somewhere, that this mathematics have changed. I've sort of been saying Newton so far, right? But the thing about Newton is he did come up with this. He wrote it down and then he stuck

• 35:19Hannah Fry?

  it in a drawer for forty years, didn't tell anyone about it. And then a little while later, this, this German diplomat, actually, this diplomat and philosopher called Gottfried Wilhelm Leibniz or Leibnitz, he completely independently, he came up with his own version of exactly the same thing. And Newton, Newton had been doing it looking at motion and time, and Leibniz had been looking at geometry and space. I mean, Newton's a big deal, right? So Leibniz knew, he'd heard some rumors that maybe Newton had done a bit of this stuff already. So he writes this letter to Newton saying, oh, I've heard this rumor that you're, that you're working on this. I don't want to tread on your toes. But Newton,

• 36:00Hannah Fry?

  who, if you've listened to our previous episodes, we know was like a right little whiner. He was not happy about this at all. And so what he did is he sent back, Leibniz, this, this really bizarre Latin anagram, where if you unscramble it and translate it, basically, it was, it was very cryptic. It was essentially the kind of equivalent of sending a sort of very cryptic subtweet and timestamping it. So that when you look back at it later, it said something like, given any equation involving any number of affluent quantities to find the fluxians and vice versa, right? That's just sort of what he was saying. He was proving that he knew how this thing worked without telling him.

• 36:45Michael Stevens?

  But he could prove later, look, I described it all in this letter that's got a date on it, so I deserve the credit.

• 36:52Hannah Fry?

  Totally. So, anyway, Leibniz is just like, oh, okay, I'm not really sure what to do about Newton. This is a bit of a weird guy, but I'm gonna publish it anyway. Everyone loves it, it goes great. And then Newton forms one of the deepest, most vengeful attacks.

• 37:11Hannah Fry?

  On Leibniz for the rest of his life. So it starts off and Newton gets this Scottish mathematician called John Keel. He sets him off as his attack dog and Keel writes this article publicly accusing Leibniz of being a thief. Leibniz is not a thief, to be absolutely clear.

• 37:28Michael Stevens?

  What's the the evidence that he's a thief? Is it that he received this cryptic letter and then solved it and stole the idea?

• 37:35Hannah Fry?

  No. So I mean, this is, I mean, evidently not, evidently not. He'd already come up with a theory by that point. But Newton had been sending letters to people in Europe that had inklings of this idea. And he had done it. He had done it a few decades earlier, right? We know now for sure that Newton did come up with it independently and Leibniz came up with it independently. But Newton just didn't want, he wasn't happy that this, that this other guy had come in and got the credit for an idea that he'd already had. So it's just basically a smear campaign bluntly. So there's all these public articles. Leibniz is furious about this, like, you know, this, this, this Scottish mathematician accusing him of, of whatever. So he, he writes a formal letter of complaint to the Royal Society, who's sort of the ultimate arbiter of science in

• 38:20Hannah Fry?

  the day. And Newton, who was the president of the Royal Society, okay, it's like, I'm sorry, I'm, I'm not going to censor any of these members. They can say what they want to. But secretly was going behind everybody's back and whispering in the ear of John, John Keel, being like, here's what I need you to say next. Here's how you can attack him more. So this argument like extends and extends and extends until the early 1700s, when Newton's like publicly accusing him of being a fraud and a plagiarist. And this fight gets so bad that the Royal Society decides that they're going to assemble this independent impartial committee of like the greatest minds in order to work out once and for all who actually came up with the idea of calculus. Now, I mean, I've

• 39:03Hannah Fry?

  mentioned that, that Newton was the president of the Royal Society. So what he does is he secretly handpicks every member of the independent jury. When they release their, their official report, which destroyed Leibniz's, or Leibniz's reputation, we now know that actually Newton secretly wrote that report himself. Okay. And then to like really twist the knife, Newton then anonymously publishes the glowing review of his own secret report in the Royal Society's journal, saying about how undeniable, undeniably correct and thorough it was. And then Leibniz, his reputation is genuinely ruined, like he's over. And he ends up dying, you know, a few years later, he's impoverished,

• 39:47Hannah Fry?

  he's out of favour with the royal courts. And then Newton, in his private notes, writes how proud he was that he'd broken Leibniz's heart. Goodness gracious. I mean, Newton, honestly, is not a nice guy. He's horrible.

• 40:02Michael Stevens?

  This is like, the whole time I've been listening, I've been imagining Tina Fry, if you're out there listening, let's do a prequel to Mean Girls and have it set in the time of Newton and Leibniz. And the Royal Society can be a little clique.

• 40:17Hannah Fry?

  Yeah.

• 40:18Michael Stevens?

  Luckily, we got the knowledge.

• 40:20Hannah Fry?

  We got the knowledge. We did. And here's the thing, right? So while Newton did come up with the idea before, and Leibniz came up with it independently, the thing is, is that Newton's, the way that he writes it, Newton's notation, as we describe it, was kind of rubbish. It just doesn't, it was way more clunky. It's like the Roman numerals of calculus, effectively. Leibniz's is one is way neater, makes way more sense. And because of this loyalty, that sort of the national loyalty that the British mathematicians had to Newton, they refused to use Leibniz's notation. Whereas on the continent, in France in particular, they're quite happy they were using Leibniz's notation. And because

• 41:05Hannah Fry?

  it was so much better, so much clearer, so much easier to work with, actually, British mathematics on that front kind of stalled for a few hundred years. So now, if you do learn calculus in school, or even in the Spivak book that you were recommending earlier, almost certainly, they'll be using Leibniz's notation. And the word calculus itself was Leibniz, not Newton. He wanted to call it method of. Fluxians.

• 41:33Michael Stevens?

  Okay, so first of all, Fluxians is a pretty cool word.

• 41:37Hannah Fry?

  It's a pretty cool word.

• 41:38Michael Stevens?

  Like, I gotta admit that. But I guess that's a nice little, like, happily ever after story that at least Leibniz got to name it and decide on the symbols and the way we learn it today.

• 41:54Hannah Fry?

  I mean, it's probably not worth the horrible death, but, you know.

• 41:57Michael Stevens?

  Yeah, I bet if you asked him, he would have said, you know, hey, give me one more week, please.

• 42:01Hannah Fry?

  Yeah, exactly. By the way, to be absolutely clear, this is gonna be a continual running theme in the rest of science about what a massive bitch Newton was.

• 42:11Michael Stevens?

  Yeah, as it should be. This is one of my favorite parts of the podcast. Because I think, yeah, it's like yin and yang. You know, Newton did a lot, gets a lot of accolades, gets an entire unit of force named after him. I think it's about time we applied some force, too.

• 42:27Hannah Fry?

  Quite right. He was still a human and had lots of human flaws. It did resolve Zeno, though. It did resolve Zeno. Or at least, I think many generations of mathematicians are comfortable that it resolves Zeno.

• 42:38Michael Stevens?

  That's right. We all feel like we've moved on from Zeno's paradoxes. We say, look, it's possible, and, you know, you can, you can use the calculus concept of a limit to show that you arrive at the door, even though you've got an infinite number of little pieces.

• 43:01Michael Stevens?

  This segment is brought to you by Cancer Research UK.

• 43:04Hannah Fry?

  In England, every year, around 4,300 people begin treatment with a drug that is designed to switch off a very specific cancer-driving signal that appears inside their cells. That, by the way, is enough people to fill 10 jumbo jets.

• 43:18Michael Stevens?

  And those drugs are now used worldwide, boosting the impact of existing cancer treatments and giving millions of people more time.

• 43:25Hannah Fry?

  And that idea of finding the signal that is driving a cancer and then, and then blocking that specifically, it traces back to research that Cancer Research UK was funding back in the 1980s.

• 43:38Michael Stevens?

  That's right. And so today, we are asking, how did a clue? From chickens lead to targeted cancer drugs. So a long time ago, like in 1916, scientists noticed that chickens that were infected with a specific virus developed cancer, that this virus produced cancer-causing molecules. But it was also noticed that this molecule called epidermal growth factor would cause cells to grow out of control in Petri dishes. Now, epidermal growth factor, EGF, was was pretty well understood. But what we didn't know a lot about was the EGF receptor on the cells. And so that's what Julian Downward needed to collect a lot of.

• 44:17Michael Stevens?

  And as it turns out, placentas are a great place to collect EFGR, epidermal growth factor receptors. So then he started working out the sequence of amino acids that made them up and to find out how they worked. And then he compared those sequences to other known proteins and was doing this late at night, just like waiting for the X to appear to have say, oh, we found a match, we found a match. And suddenly, boom, a match was found with proteins created by a virus that was known to cause cancer in chickens. And so this, this was huge news, right? He famously like called his boss, middle of the night. The boss comes over and they spend all night working on this because this was the beginning of much more targeted cancer drugs.

• 45:01Hannah Fry?

  Right, because you have to realize the idea that it could be your own internal cellular machinery that could just simply go a bit haywire and cause cancer. At this point in time, it was, it was, you know, still a barely formed thought. But what Julian Downward had done here in looking at stuff that is naturally occurring in humans, this, this EGFR, this cell surface receptor that sits on the outside of your cells and helps to regulate cell growth and division, by noticing that it had this link to chicken cancer, it changed the entire game. It meant that people started realizing that it could be something in your own body that

• 45:43Hannah Fry?

  causes your cells to, to, to just go completely overactive. But having that knowledge, having that groundwork, that is what has led To this gigantic shift in treatment because if you know exactly which signal is faulty, which naturally occurring signal in the human body is going wrong, then you can design a drug to block just that, which then in turn, hopefully, results in far fewer side effects. And so by the early 2000s, the very first generation of EGFR inhibitors, things that block this protein, ended up reaching the clinic. And now there are 13 different drugs out there that target EGFR, which are used to treat six different types of cancers,

• 46:28Hannah Fry?

  including certain types of lung cancer, of bowel cancer, of head and neck cancers. And all of this can be traced to that exact moment, late night in the lab, when Julian, the Cancer Research UK funded PhD student, discovered this incredible link. And I mean, this is an entire wave of innovation that got sparked from that moment. In the UK today, more than eight in 10 people who receive cancer drugs are receiving a treatment that was developed either by Cancer Research UK or with their involvement.

• 46:59Michael Stevens?

  Yeah, so the story here is about the question changing. It's not just about what causes cancer, but which signal inside the cell is driving it. And once you can identify that, that specific signal, you can try to interrupt it and you can tailor treatments to the biology of each cancer, which makes treatments kinder and more effective.

• 47:22Hannah Fry?

  And for the 11 people every day who are beginning these treatments in England and many more people across the world, that change is shaping the care that they receive.

• 47:31Michael Stevens?

  So for more information about Cancer Research UK, their research, their breakthroughs, and how you can support them, visit cancerresearchuk.org/restisscience.

• 47:49Michael Stevens?

  There still remain a lot of metaphysical puzzles around here. And I think we should kind of leave everyone with these. Let's imagine that I get up and I walk to the door. Yes, I have to cover half the distance. And then I have to cover half of what's left and then half of what's left and half of what's left forever. But now imagine that I've got two flags. Like I've got a green and a red flag. And after each half of the journey that I cover, I switch which flag I'm holding. So I walk halfway there, put up the red flag. Then I cover half of what's left, put up the green flag. Half of what's left, put up the red. Half of what's left, put up the green. And I keep alternating. Eventually, I get to the door. As we know, I do. Which flag am I holding up when I reach the door?

• 48:32Hannah Fry?

  Last one.

• 48:33Michael Stevens?

  That's right. Is the number of halves I've covered even or odd? I'm asking for the last number, the biggest number. And how do you feel about these kinds of questions? Because they're not resolved.

• 48:45Hannah Fry?

  They're not resolved. Because I think that this is when it really becomes clear that infinity doesn't act like a normal number. We did that episode on really large, finite numbers, like Graham's number that we know ends in a three. You know, infinity is not like that. We don't know what it ends in because it doesn't ever end.

• 49:05Michael Stevens?

  Right. And yet, my flag question presupposes that there's an end.

• 49:11Hannah Fry?

  Mm hmm.

• 49:12Michael Stevens?

  Because we reach the destination, so it ends. I should be able to figure out, you know, what the state there is. Another famous, very similar thought experiment is called Thompson's lamp. Where I'd say you're gonna, you're gonna play with a lamp for a minute. And what you do is you wait half a minute and you turn the lamp on. It starts off. And then you wait 15 seconds and you turn it off. And then you wait seven and a half seconds and turn it back on. So as you can see, after half of the time that's left has passed, you've switched the state of the lamp. Okay? So it's going off, on, off, on, off, on in this accelerating rate. A minute eventually passes. When that happens, is the lamp on or off?

• 49:56Hannah Fry?

  And in exactly the same way, it's like, what's the last step? What is the last step of an infinite number of steps in a finite amount of time?

• 50:04Michael Stevens?

  Surely, a minute can pass, but yet, the lamp cannot be on because every time it's on, it's immediately turned off because infinity is unending. But every time it's off, it's turned back on right afterwards. There can't be an end, and yet, the sequence itself can end. So, um, the, the best I've heard as a response to these paradoxes is, is, I, I don't even know if it totally answers them, but it basically says, "We don't have enough information. There's a trick to these questions where we're being asked to say something about the state of a lamp or a flag, but we've only been given rules

• 50:47Michael Stevens?

  about its behavior before that moment. You've told me when a certain flag will be raised or not, and I can tell you, um, at which step, which flag will be up. I can always tell you that. But now you're asking me a question about when you reach the door, but that's not part of the flag rules, so I can't know. I don't know if that just kind of sidesteps the problem.

• 51:11Hannah Fry?

  So what you would do, um, in a mathematical sense with this, because I think ultimately, the problem here comes in when you are trying to make this ultimately imaginary concept of infinity fit into reality, right? Because, because planting the flag suddenly makes it real, right? Suddenly, you're, you're physically interacting with the world. Cutting something into an infinite number of pieces that you never actually do, you're just using that as a shortcut to get you to the answer. Sort of works, right? But but once you're planting flags and then saying, what's the last one? That's where you end up coming into this problem.

• 51:56Hannah Fry?

  So what you do in this situation, I'm thinking about like in fluid dynamics, for example, right? You have this concept of this perfectly smooth fluid that you can cut and cut and cut and cut and cut an infinite number of times. But once, of course, you actually get to reality, reality isn't made like that. You can't cut up reality an infinite number of times, or maybe you can. We'll discuss that more in the next episode, maybe. But, you know, there comes a point where you're down to atoms and then you're down to quarks. And so what you can do mathematically is you can say, okay, we're going to draw a line in the sand. Right? And anything that is below this order of magnitude can't be included.

• 52:38Hannah Fry?

  Um, because there's it comes a point where your infinitesimally small steps no longer match with reality.

• 52:47Michael Stevens?

  Yeah, they don't have any physical meaning anymore. So it's like a clash between our ability to think and reason and the world we've been given. The lamp, I can imagine turning it on and off in this accelerating fashion. But yet, at a certain point, and I'd love to know when, actually, at a certain point, I am having to push this lamp switch faster than the speed of light in order to continue following the rules. And so are we just actually prohibited from ever actually doing these? And so they're what, like little made up fictitious paradoxes? If reason can create the paradox, why can't it also resolve it? What what's the problem? I get that the real world won't allow us to experiment and and just observe the answer.

• 53:32Hannah Fry?

  Doesn't feel very satisfying there, does it?

• 53:34Michael Stevens?

  Doesn't feel very satisfying. There's another famous one called the Ross-Littlewood paradox. And this one is is pretty fun because it doesn't even involve having to cut space up into small pieces. Here's the experiment. You you think in your mind that you're going to do this. You're going to take a big jar and you put 10 balls in there. Let's say they're ping pong balls.

• 53:53Michael Stevens?

  Then you take 10 more balls and you put them in. But when you do that, you also remove one ball. Then you put in 10 more and remove one. You put in 10 more and you remove one. You put in 10 more and you remove one. And you do this an infinite number of times. Which because of the tasks we've already described, we can imagine doing in a finite amount of time. You know, you put the first balls in after half a minute. You put the second batch in after just 15 seconds and remove one. After you've done this process an infinite number of times, 10 balls in, one out, 10 balls in, one out. How many balls are left in the jug? I think a lot of people out there listening are gonna say, "Well, it's an infinite number." Because yeah, you put in 10 and then remove one. That means you really just put in nine.

• 54:35Michael Stevens?

  And nine plus nine plus nine plus nine forever is infinity. However, let me, let me put it to you this way. Let's say that the balls have numbers on them. 1, 2, 3, 4, going all the way up, right? And I put in balls 1 to 10. And then I put in balls 11 to 20. And I remove ball number 1. And then I put in the next 10 balls and I remove ball number 2. And then I put in another 10 balls and I remove ball number 4. Or 3, I forget where I was. But you see what I'm saying here, right? If that's what I do, then the answer is, at the end of an infinite number of steps, the jug's empty. There are no balls in it. Because ball number 1 was removed at, you know, step 1.

• 55:19Michael Stevens?

  Ball number 10 was removed at step 10. Ball number 18 bajillion was removed at step 18 bajillion. For every single ball I put in, there is a moment when it was removed. So there are no balls? Or are there an infinite number of balls? There's that zero and infinity are kind of like twins thing again, where it's like, it's one of those. It's either none or unending. Not anywhere in between.

• 55:43Hannah Fry?

  I mean, what you're describing here essentially is, it does take us back to calculus. You're, you're essentially describing what happens in the limit of something. But you're describing sequences that have no limit, that don't converge. You know, the, the sequence half plus a quarter, plus an eighth, plus a sixteenth, and so on and so on and so on, which is the Zeno paradox one in, in one of the formulations that you described it. That's fine. That equals 1. No big deal. We can deal with that. But on off, on off, zero one, zero one, zero one, zero one, it just carries on oscillating forever. It doesn't have a limit. You know, likewise, the thing that you're describing there, because there are other sort of strange paradoxes, which that are similar to the

• 56:23Hannah Fry?

  one you're describing, that do have limits, that can have resolution. So one that I really like is, imagine that you have an ant that is on an elastic band. This elastic band is pretty special. You can stretch it as much as you like, right? You can can carry on stretching it forever. So this elastic band, in the first second, it's 10 centimeters long. In the first second, the ant can crawl one centimeter along the length of the band. But at the end of one second, you stretch the elastic band to 10 kilometers long. Now, the ant crawls another one centimeter, you stretch it again to another 10 kilometers.

• 57:05Hannah Fry?

  And this goes on and on. Every second, the ant traverses one centimeter, and the rubber band gets stretched an additional 10 kilometers. Question is, does the ant ever reach the end of the elastic band? I reckon we should leave this one as a as a as something that the the listeners can work on. I reckon we leave that because it does have a solution. It does have a solution.

• 57:29Michael Stevens?

  It has a solution. And and I mean, this is a bit of a spoiler, but it's surprising. The answer.

• 57:35Hannah Fry?

  It's surprising. It's really surprising.

• 57:37Michael Stevens?

  The ant covers one centimeter of distance every second, but the entire string or band becomes 10 kilometers longer every second. Can he reach the end? And I want to I want to add too, that in the next episode, we will tackle another question, which I really love, which is, okay, fine, Hilbert's Hotel. To go back to the beginning, Hilbert's Hotel can always accept more guests. But we're having to move the guests from the front and on, right? But what if another hotel opened next door? And it needs to buy some number plates to put on its doors. But Hilbert's Hotel annoyingly has bought all the plates that there are. It's got every numbered plate.

• 58:20Michael Stevens?

  What should the first room in this new hotel be numbered? What number plates does it use? Or even better, let's imagine you're running a race and you're really slow. An infinite number of people finish the race before you do. What place did you get? Now, we're not talking about rearranging infinity or approaching infinity or completing infinity. We're talking about after infinity. What numbers are there? And that's where we're going to go next week.

• 58:49Hannah Fry?

  It certainly is. And we're going to come to, I think, the even more mind boggling conclusion that some infinities are larger than others. It's going to hurt.

• 58:59Michael Stevens?

  Yeah.

• 59:00Hannah Fry?

  Prepare your brains for some more mind bending infinity strangeness.

• 59:05Michael Stevens?

  And if you'd like to ask us a question, we might answer it in our Thursday Field Notes episode. So send those questions to [email protected].

• 59:14Hannah Fry?

  And in the meantime, you can also sign up to our free newsletter, therestis.com/science. See you next time.

• 59:21Michael Stevens?

  Bye.

• 59:33Speaker 05

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