The Infinite Monkey Cage - Brian and Robin's Infinite Inbox

Episode Date: February 25, 2016

Brian Cox and Robin Ince answer The Infinite Monkey Cage listeners' questions.

This is the BBC. This podcast is supported by advertising outside the UK. Hello and welcome to Brian and Robin's Infinite Inbox. I'm Robin Ince, I'm one of the presenters of The Infinite Monkey Cage on Radio 4. Across from me is Professor Brian Cox. I'm the other presenter of The Infinite Monkey Cage on Radio 4. And I have lots of questions for him which were sent by listeners. I presume they're listeners

or they're just people a shot in the dark imagining what's on in the show and have somehow emailed us. They're not just for me though, they're for you as well. I think you should feel qualified to answer some of them.

How many, we've done 100 monkey cages now. 100 monkey cages. Some of it should have seeped in. But I don't want the illusion. You're an English graduate, what is it, sept or seeped?

Seeped. Seeped in, yeah. Seeped in is if something seeps in and then actually creates an infection. That's when it's sept in. Yeah, just so you know. So, no, we've received lots of questions based on the last series and also some general questions. And so Brian's mainly going to do the talking

and I'll do different voices for each one of the letters in the style of all of the we've received a letter program such as points of view etc and if you could sing a little bit because we are in the BBC Radio 3 studio we are listeners there's a grand piano so this is the first letter that we've received this is from James Davis he's from Garstang in Lancashire I'm not going to attempt to do an accent for that because you get a lot of complaints for that kind of thing. We'll do a Radio 3 accent then. Well, this is from James Davis in Garstang in Lancashire. His question, Brian, is this,

and if you get this right, it means the rest of the show we don't have to do and it just goes straight into playing Rites of Spring. Could you summarise all the laws of physics on a stamp such that it could be read with moderate reading glasses? So? I think you could. I mean, this is, in some sense, a notational question

because, of course, mathematical notation is extremely compact and hides a lot of complexity. But if I was to do that, I would write down the standard model of the Grandian, which is usually presented as four lines of mathematical symbols and not too many symbols. The top two lines describe how the forces interact with the particles of matter that we know of.

So 12 particles of matter interacting through three forces of nature, the strong nuclear force, the weak nuclear force and electromagnetism. And the bottom two lines, as usually presented, describe the Higgs mechanism, which gives mass to the other particles. So that's all the quantum mechanics that we know summarised in this thing called the Standard Model Lagrangian, about four lines. And then general relativity, which is a very simple one line equation, which is Einstein's theory of gravity, but also includes in that notation special relativity as well. So that's essentially it as far as fundamental physics is concerned. Standard-Malagrandian, Einstein's theory of general relativity, and that would fit on a

stamp, and you could just about read it in compact notation, certainly with a magnifying glass. So we'll probably be looking not at a bog standard stamp, but maybe a commemorative stamp for, say, a British person who's won the Nobel Prize or something involving a Doctor Who commemorative first day cover. Yeah, but the sense of the question is, yes, you could, with a very fine pen, a human being could write down the standard model Lagrangian and Einstein's theory of general relativity

on the back of a standard-sized stamp. The danger, of course, would be as you go to lick the stamp, not realising that you are licking off all the laws of the universe, then briefly giving you a God complex. Well, it is true that if you only licked off the Higgs sector from the standard model Lagrangian, then you'd immediately fall to bits because you'd become massless.

So you wouldn't want to do that. See, that is a beautiful image now that there are people at home. But obviously they're also having to use historical stamps because I think they've all got glue on the back now. But anyway... Glue-ons! Right, glue-ons pun. He's done a glue-ons pun. Right, we'll have to go straight to the next letter.

But if you would like to play a god at home, then you know all the rules. We've just given them to you. So here's another one. This is from Tim Bowers, and he says, Hi, Brian and Robin. Brian, you're my idol. And that's enough of that boring question.

You're Brian. It's just rude. Why not just say Hi, Brian, you're my idol. Hi, Brian and Robin. Just so you know, Robin, just to remind you, you're far less a human being. Anyway, so, Tim continues. Brian, you're my idol. Hi, Brian and Robin. Just so you know, Robin, just to remind you, you're far less a human being. Anyway, so Tim continues. Brian, you're my idol. Self-loathing comedian.

No, I don't have to do it myself. There's lots of others who are getting involved now. I hope one day to also play a keyboard and stand inside a giant satellite dish. Anyway, when talking about relativity, you mentioned that gravity or the effects thereof are caused by the curvature of space-time. Does this in fact mean that gravity as a force that pulls things together simply doesn't exist? There is no force. The first bit is absolutely correct. Well, the you're his idol. No, the second bit. Right. So there is no force in the conventional sense in Einstein's theory of general relativity.

As the writer says, what's his name? It's Tim. As Tim says, the space and time are curved. And the rule is that everything moves in a straight line through that curved space time. And the analogy that I always think of is to imagine standing on the equator of the earth with a friend and agreeing to walk due north. So you set off, so you walk parallel to each other, you don't deviate from your straight line, but you find as you approach the North Pole that you get closer and closer to your friend, because you would bump into them at the pole, because the parallel lines on the surface of the earth are lines of longitude and they meet at the pole.

So if you didn't know you were on a curved surface, you would conclude that there is a force pulling you together, a force of attraction. But in fact, it isn't. It's an artefact of the geometry of the surface on which you're moving in a straight line. And that is exactly what happens in Einstein's theory of general relativity.

Do you find it problematic that as human beings have a greater understanding of the curvature of space-time, we are currently having a boom in flat-earth societies? I'd not noticed. Of course I'd noticed, I guess. So you're still not sure if it's real or if it's all a kind of darderist prank? That's what I'm trying to work out. It can't be real.

What is interesting about the flat-earthersthers and i think they are real actually and they're they're what the interesting there's one thing they've got right which is they explain in inverted commas the feeling of the force of gravity by saying that the ground is accelerating and they are correct in that that is absolutely the foundation of einstein's theory of general relativity what they're missing in that that is absolutely the foundation of Einstein's theory of general relativity. What they're missing is that the ground is accelerating in a particular way, which is essentially caused by the curvature of space and time. And we are standing on a sphere. So they miss that bit.

You don't have to. It's actually quite a deep and interesting thing to think about. The ground can be accelerating upwards at 9.8 metres per second squared and yet the Earth is not expanding. And that's the wonder of the curvature of space and time. But that's the bit where they get

confused. So the Earth is spherical. So you say. This is an interesting thing where i've noticed that there have been various articles written saying this starts from a very positive position it starts from position of human beings not wishing to take on received wisdom uh as they see it from a position of authority but it seems that maybe flat earth is not the right place to that to actually work out where you should be questioning and what

should be the things to question and why the idea of a flat earth even if you just go back to you know the greek philosophers who who look at the sails disappearing on the horizon one of the key things about science is that um it's an integrated picture of nature so the the this the the big theories that we have today the standard model of particle physics, general relativity, or indeed Newton's laws, in a more approximate sense, describe the action of gravity. Those theories describe a vast range of phenomena. They describe a lot of things. So if, for example, you say, I don't like the idea of a force of gravity which is what the standard kind of flat earth conspiracist would say then you have to explain for example what a pulsar is so in the

nebula there's a radio source and we measure the pulses from the radio source with radio telescopes like jodrell bank so you have to explain what it is now we, we know what it is. It's a neutron star, which is a spherical, super compressed remnant of a dead star, which is spinning very fast because of the conservation of angular momentum. And because of the way the magnetic fields interact with these pulses of radio waves. So there's all sorts of physics in there, 20th, actually 19th, 20th and 21st century physics that describes a thing that we see so if you pull away at one thread and you say well i don't believe that there is such a thing as the force of gravity i think the earth is flat and the whole thing is accelerating upwards you you can say with precision

actually well that's locally if i in this room now not looking outside the room it is true that i can't tell the difference between the floor accelerating me upwards and a force of gravity pulling me downwards in Newton's language. But if you remove that idea of a force of gravity or general relativity, you have to explain the observation of, let's say, a pulsar. Or you have to explain the motion of the planets against the fixed stars. Or you have to explain the observation of the rotation of galaxies. You have to explain things like the cosmic microwave background radiation. Where did it come from? Again, our understanding of that observation, that the universe glows at

a temperature of 2.7 degrees with absolute zero, the explanation comes from Einstein's theory of general relativity, that there was a time when the universe was hot and dense and it's been expanding and cooling ever since. So you've got to explain that. So that's the problem with all conspiracy theories, or most of them, tend to be able to explain something if they focus only on that and not the consequences of the explanation for the bigger picture. And the picture we have of nature is tremendously successful, wide-ranging and self-consistent we don't need to buy a telescope and why would we be i mean maybe i haven't understood the flat earth thing but why would we be the only planet that uh we're observing standing on it that

happens to be flat and we just happen to be surrounded by or is that are we then meant to believe that somehow the whole sky has been projected it's a kind of evil hologram perhaps and why why the point is don't point debate in particular conspiracies but i think the interesting thing is that the wider point that that science is a is a is an integrated and self-consistent picture of a lot of different things so you can't prod away a little bit so you can't say the Earth's 6,000 years old. If you say the Earth's 6,000 years old, you've got to explain a lot of things away. You've got to understand what's wrong with radioactive dating of rocks.

You've got to understand what's wrong with carbon dating. That says you've got to understand what's wrong with nuclear physics. And nuclear physics also explains the way the sun shines. It explains the way nuclear reactors work. So you've got to be very careful that if you prod away at one thing and nuclear physics also explains the way the sun shines and explains the way nuclear reactors work. So you've got to be very careful that if you prod away at one thing because you don't like it, then the consequences for explanations of other things are often devastating to your argument. But anyway, so points of view never gave anyone this amount of homework.

So you've got about seven different things to do. I'm taking a mouthful of pasty. Carry on. This is very exciting, by the way, for the listeners who were just before the show, Brian bit into a pasty and discovered that the entropy had not been great enough within the pasty for him to be able to swallow it without going hot, hot, hot. There's a lovely thing, which is you've just heard.

What is high entropy in a pasty? High temperature. That's what I mean, yeah. A lot of available face space. The heat death of the pasty had not got to the point where... A lot of face space for motion of the molecules in the carrots. But there's a real joy in seeing someone who can so elegantly describe the universe

also just go, hot, hot, hot, not understand food, hot. Hot come from oven, but still hot. Do you know what? I don't think we're going to have time on this show unless there's a brilliant edit, which there may well have been already, to fit in any more questions apart from those about the general relativity specials.

We won't even get to the series. I'm going to have another bite of pasta. You have another bite of pie. I'll try and drag this letter out for long enough for you to... Oh, hot, hot, hot, hot. Pie's still hot. So we'll move on. If you would like to know more about general relativity,

the two specials, I think, are still available on the BBC website. It was just called A Monkey's Guide to General Relativity. Or you can do a degree in theoretical physics. Yeah, if you haven't got time to do a degree in... Third year before you even get on to... Yeah, third year, fourth year. If you have the patience, the integrity,

then go for it. I'm sure it's very rewarding. So this is, well, this is actually probably a very good thing to say because this letter is from an 11 year old who obviously we would like to, and he feels reasonably inspired already.

Hello, Brian. Hello, Brian. Not even mentioned, not even ridiculous. This is getting, I'm not enjoying this show at all.

Hello, Brian and Robin, and Robin, Eve. Hello, Brian. I hope you are well. Hello, Brian and Robin. That's what it says. There's nothing wrong with that. It's alphabetical. Hello, Brian and Robin. And Robin, Eve. Hello, Brian. I hope you are well. It's just said hello, Brian and Robin. That's what it says, isn't it?

There's nothing wrong with that. It's alphabetical. Hello, Brian. And Robin. No, I've added the and Robin. I thought that might have been nice. I'm going to blame Eve Mogridge because even though it's not, it's from her account, it's actually her son who sent this.

Anyway, so hello, Brian. I hope you are well. That's the first question. Are you well? I'm very well, thank you. Good. My name is Theo King. I'm writing through my mum's account because I'm an

11-year-old boy. Last Bonafide night I was talking to a friend of my mum's about my understanding of science stuff. I've known him since I was two. He was impressed by my knowledge of the subject. He has studied to degree level physics and chemistry. He has now challenged me to understand strong

nuclear force by Glastonbury 2016 for £50. So this is a... I don't even know if we're allowed to on the BBC because this has now become a money-based challenge. I have gone to both my science teachers, family, friends, Siri and Google search. I often listen to The Infinite Monkey Cage and learn a lot from your explanations.

I thought you might be able to help. Strawberries die when they're digested, and there's now such a thing as an Infinite Monkey Cage because a cage has edges. So, can we therefore... Strong nuclear force. £50 resting on this. It depends what you mean by understand, I suppose. The theory of the strong nuclear force.

Yeah, I suppose beyond mere words, but to have some sense that as you hear those words and as you think about them in your mind, you also have some sense of the reality they create. If you think about electromagnetism first, then the way a particle physicist would describe it, let's say that the force between two electrons, they have negative charge. That means, in particle physics language that they can exchange photons, particles of light between them.

And those are the force carriers of the electromagnetic force. And that's our quantum picture of electricity and magnetism. It's called quantum electrodynamics. So you picture two electrons exchanging photons between them because they're electrically charged. And that causes them, in this case, because they've got the same charges, to be repelled away from each other. The strong nuclear force is the same conceptually. It operates between things that have a different kind of charge, which we call colour charge. So quarks, for example, carry both electric charge and colour charge.

And the force is carried by particles called gluons. There happen to be eight of them rather than one of them for a photon. And that's because of three different colour charges. And gluons, the difference in behaviour, most of the difference in behaviour between the two forces, is called because gluons can interact with each other. So they carry also the charge so it's like having an electrically charged particle of light and imagine what that would be like it'd be strange because as the great richard feynman always said one of the wonderful things

about light is that i can see you across the room even though there's all sorts of other light whizzing in every direction and they don't bump into each other the photons don't bump into each other which is why we can see long distances um but what we can see to the edge of the universe in fact because light travels in straight lines um through curved space it should be said it's a caveat but photons don't bump into each other gluons do bump into each other so i suppose the answer would be it's a conceptually the same as electromagnetism it's carried by a force carrier called a gluon uh but it's different because there are three different charges rather than one and the that means there are eight different gluons which is a consequence of that and the gluons can talk to each other as well as to the things that carry the charge because they

also carry charge well i hope that's uh one new 50 pounds uh theo uh or eve mogridge pretending to be theo and pretend to be 11 years to win 50 pounds i should say it's one of the great triumphs of modern physics the quantum chromodynamics it's a very it's a difficult theory to calculate in um and it has these strange properties i said asymptotic freedom where they are so another strange property is if you which you see in particle physics all the time is it it so i said asymptotic freedom where they are so another strange property is if you which you see in particle physics all the time is it it so i said that the force kind of when things what happens when things separate away from each other if you knock a quark out of a proton what happens and it's like having an elastic band between the quarks that stretches and stretches

and stretches and then snaps and makes more quarks And so you get this shower of new particles created when you try to separate quarks from each other, which is very unusual. The force gets stronger and stronger and stronger the further away the quarks are from each other. So it's the net effect is kind of the opposite. It's like an elastic band rather than being a charge. In the usual way, you expect as you move things apart the force gets weaker but it goes straight straight yeah the quantum dynamics is a difficult

theory so how old theo he's 11 years old so he's got time on his hands uh what how i mean that point that you've just been talking about now and your level of understanding how long did that take you at what point do you think in terms of going through education uh you thought right now this is a new area of of the big the beginning hints of understanding well i mean you learn that uh in in just superficially very superficially the introduction to it is a postgraduate degree course it was was when I did it. And then you have to be an expert to really do calculations in it. And I couldn't do it because that was not my thing. So to do calculations in quantum chromodynamics

is a really specialist area that specialist physicists do. Theo's watching that 50 pounds drift further and further away. It's an expert field because it's difficult well now we move on to quickly move on to the actual series itself our first episode was about artificial intelligence and the question there was can you tell me if there is a difference between artificial intelligence and computing power this is from alan deaden well it seems to me you could have a computer of arbitrary power but if you've got no software running on it then it's not going to be doing anything well let's find out because uh we went to annal seth who was one of our guests and we

asked him and he says having more computing power can certainly help build more powerful ai systems but this is only part of the story the real challenge for ai is to figure out the principles the algorithms that enable artificial systems to do smart things. It's not what you've got, it's what you do with it. And then Alan Winfield, who was also on the programme, where we talked about some very interesting small robots he'd made. Although I'm basically saying exactly the same thing as Anil, I would answer it in a different way. To build a cathedral, you need a lot of marble.

But having a huge pile of finest Italian marble isn't enough to build a cathedral. You also need the design, the architect's drawings. Computing power is like marble, and AI is like a cathedral. Computing power is the raw material needed to build AI. Yes, you need lots of it, but without the architect's drawings, you'll never build a cathedral. So, there's more homework for you.

But it's a metaphor, isn't it? It is a metaphor, but you can also use it. He doesn't really mean build a cathedral. That doesn't mean we can't take the information he's given us and go, do you know what? I don't think I'm going to get very far with AI, but there's lots of cathedrals, so I reckon I've got a higher chance. Look, the choice of what you want to do with your own marble is up to you.

You can either build a marble giant that you hope is artificially intelligent or a cathedral. All I'm saying is I think you'll finish cathedral first. But I don't think there are any cathedrals built out of marble. I mean, what's York Minster built out of? It's not marble, is it? It's stone. Well, that's...

Well, marble is a form of stone, but you're probably right. So the... What we're saying is that Alan's answer was very good as a metaphor. What is York Minster made of? I didn't find any matching places. Do you know what I think is delightful? It's here we are dealing with artificial intelligence and you ask your artificially intelligent phone

a simple question like, what is York Minster made out of? And he goes, do you want to see Forrest Gump? So let's move on to the next question. This is from our show that we did about the nature of race and understanding of it. So this is from David Davison. He says,

listening to the show about genetics and how everyone in the world can be traced back to a common ancestor about 6,000 years ago, just worried that sounds like creationist religion. No, it isn't. It's statistics. It was actually, I must say that I think...

6,000, I think, initially, the gut's reaction to that is 6,000 years for us to have a shared common human ancestor. What would be... So that's not a mitochondrial Eve, is it? No, the first time you hear it, it's surprising. And I was surprised, really. But then you think about it, and it was explained then. What you're saying is that I have two parents and four grandparents

and eight great-grandparents, and you go back and back and back. And so obviously you're doubling every time. At some point, you get to a point where your ancestor number exceeds the population of the Earth. Very quickly, in fact. And so you just ask the question, well, at what point does that happen? And then, of course, it is true that there's inbreeding and all those things. And so that's not the way it works. But broadly speaking, you can make a statistical statement that you can't have an independent family tree

from anyone else on the planet if you go back so far that the number of ancestors would be 10 billion, which is not that very far, actually. In fact, it's 2 to the n, where 2 to the n equals 10 billion. What is that number? Siri.

Siri. Well, that is one part of the joy, isn't it, of when you are wandering around. the N equals 10 billion. What is that number? Siri. No. What is it? Well, that is one part of the joy, isn't it, of when you are wandering around. N log. If you see someone who has facial characteristics very similar to your own facial characteristics, then that's an early sign to suggest

that you won't have to go that far back in that person's family tree to find that you have common ancestors. You just can't do that because they interlock and overlap not too far back and that's where this number of a few thousand years comes from because any further back than that and all everyone's family tree overlaps with everybody else's which is that's the statement essentially it's just a purely statistical statement. That's why you get that thing when people go if anyone marries the royal family who appears to be a commoner and they go and yet if you only they're actually related to henry the eighth yeah yeah

yeah the point is that we're all related to anyone who had any offspring a thousand years ago basically and that that's that was the surprising thing that when you first say that it took me by surprise but then you think about it and you first say that, it took me by surprise. But then you think about it and you go, yeah, of course, it must be the case. Because there weren't that many people in Britain a thousand years ago. Even if we just confined the thing to Britain, we're related.

We're all related. This is from our episode, What Is Reality? It's a question from Mark. And he says, if I am in a football stadium and listening to the clapping and wish to join in, how come I'm in time with the other clappers

and not half a second late? Is this my perception of reality or am I out of sync with the crowd? How did they manage to get in sync? Now, one of the most interesting things in the reality programme i thought was that the speed of your conscious experience of your perception of the world is surprisingly slow so yeah i didn't know that you recreate reality but with something like a half a second delay

so you have kind of it takes a long time to sync up that what you see with your eyes, what you hear with your ears, what you feel with your fingers. It takes a long time to synchronize all that, even with the processing power in the brain. But, and I did ask that very question to David Eagleman and here's what he said.

So it turns out your unconscious brain, which is most of what's happening, can do extremely sophisticated things. So for example, you know, people hit fastballs and baseball all the time. Those are traveling, you know, at least 92 miles an hour. The ball travels from the pitcher's mound to home plate in four tenths of a second, which is faster than your conscious mind can keep up with. And, you know, as we automatize things, that's what what our life becomes when you ride a bicycle or walk or eat or do 100 other things you don't even have access anymore to how you're doing it uh but nonetheless your brain can take care of it so that means that

you can if you say a baseball is coming towards the bat then that that timing of the hit which we can time very accurately if you're a good baseball player, that's unconscious. It takes precedence. So somehow your brain prioritizes that information and says, do this first and I'll build the model of reality and experience it afterwards. Exactly. There are shortcuts where you can have visual information coming in, making decisions, hitting the motor cortex, signals go down your spinal cord to your muscles you're making um feedback corrections on the swing all of that happens and this just you know underscores the point that putting together the story of your conscious experience is a separate

process that's very slow there we go and that uh excerpt that david egan was taken from the extended version of the infinite monkey cage what What Is Reality? Is it extended because we just take the radio from which it emanates and accelerate it at high speed past the stationary observer? Is it a relativistic extension? No, we have actually

there is more content, there are more words within it, but if you would like to do that with your radio as well and extend it in your own particular manner, that's another little bit of homework once you've got your helios, seismograph, whatever in your cathedral. If you put your radio on a pendulum

and swung it backwards and forwards, it would be moving relative to you, the stationary observer. So therefore time would pass in more slowly for the radio. So that would extend it as well. So you can extend it even if you haven't got... That's the answer when people say, well, it's all right very well in these podcasts, but I have no computer or way of downloading them.

And you say, well, you can extend it another way by just moving your radio around. And the closer to the speed of light you move it, the longer the broadcast will be. Can I just make it clear, by the way, that anyone who would now like to send in the bills for their broken radio after they have accidentally swung it, not having worked out that their radio, much like a cat, their broken radio after they have accidentally swung it, not having worked out

that their radio, much like a cat, may well occasionally hit the walls when swung. All of those bills could go directly to Professor Brian Cox, not to the Infinite Monkey Cage or me. It was Brian's idea for you to put your radio on a pendulum, and I have no idea why he did it, merely because he's a particle physicist does not mean he knows much about pendulum radio work.

Can I also say that if you do that experiment with your cat to check what Robin said, then don't send the pieces of cat in to the BBC. What we would say is if you do do that, get someone else to do it with the cat and then never open the door of the room where the man has swung the cat. Yeah.

We refer you to some earlier thought experiments from the 1930s. That's actually a unification of relativity and quantum theory, that. So you're accelerating or moving a cat around very quickly relative to the station you observe it to test relativistic effects, but also the possibility you bump it into the wall and kill it will not be realised

unless you open the door. So this is probably, other than Hawking radiation I'm not aware of any other unification of special relativity and, well, of relativity in quantum mechanics. And like all Radio 4 Extra shows, we will be publishing this as a paper sometime in the next couple of years. So we now have, we move on to the more general...

Well, in fact, I would like to go back, if I'm allowed to by a producer who silently sits there, well, she silently sits there in the edit, but she's been babbling all the way through this quite cross with us. I would like to just very quickly go back to general relativity and just ask Hannah's question. Hannah Greenwood sent in the edit, but she's been babbling all the way through this quite cross with us. I would like to just very quickly go back to general relativity and just ask Hannah's question. Hannah Greenwood sent in three questions, and see if we can do this as quickly as

possible. In just a minute? Yes, just a minute. Without hesitation, deviation, or repetition? The idea of you doing it without deviation, even though in many ways it remains within the subject of particle physics or cosmology when you deviate. Nevertheless, I'm going to remove deviation because I've talked to you before. Which is geodesic.

There we go, there's deviation. So, hi there, monkeys. I enjoyed the Infinite Monkeys' Guide to General Relativity. However, I have three questions. If gravity doesn't exist, one, how can the Earth be rushing towards people on all sides of the planet at the same time? Surely it can only rush in one direction. It's a very good question.

The answer in one sentence is's a it's a very good question the answer in in one sentence is because space-time is curved brilliant two why is the fact of gravity still being taught in school so that people like me are convinced it does exist now that's an issue that i had as well the fact that we're continually taught the idea it's a force if you want to be a purist then what you should say is that we have models for the way the universe behaves. And a model is Newton's law of universal gravitation, which is a good enough model to send spacecraft to the outer edge of the solar system, land people on the moon, etc. Right, it works. We have another model, which is Einstein's model, which is more accurate. It

produces more accurate descriptions or more accurate predictions which can be tested against the way nature behaves. For example, in the orbit of Mercury. It's also the framework for cosmology. So it's a more accurate model. Whether there is a yet more accurate model in which, for example, there are force-carrying particles of gravity, which are called gravitons. That would be a quantum theory of gravity. Whether that exists or not most people believe that it does but we don't have it yet so so i would the pure answer

is that these are models and you apply you choose the model that best allows you to make some calculation that you want to know the answer to or make some prediction so if you want to fly to jupiter you wouldn't use general relativity you don't need to you'd use newton's laws it's easier and question three from hannah is if the earth is spinning through space really fast and there is no gravity to hold us on why aren't we all falling off it well um it's probably question one again isn't it as i say i mean i think it's interesting isn't it that that you you're asking the question, which one is the real description? Is there a force or is there essentially a fictitious force which is due to the geometry of space-time? And it's true that the most accurate theory tells you there's a geometry of space-time.

It works. So Einstein's theory doesn't fall to bits when you say, well, why am I fixed to the surface of the Earth spinning as it orbits the Sun? The theory perfectly well describes why you stay fixed to the surface of the Earth as it orbits the Sun. So it works. It's just a rather more counterintuitive description, which is based on the curvature of space and time. You can still do it, though. You can still take Einstein's theory and work it all out, and you will find that you don't go flying off the Earth. If you did, according to Einstein's theory, then it would be wrong because we don't go flying off the Earth. So you can rest assured that the predictions of Einstein's theory match all observations that we've made of simple things like everyone standing on the surface of the Earth or flying to the moon or whatever it is. whatever it is but they also match all the rather more exotic observations we've made of the universe like pulsars orbiting around each other black holes all those things that we can we can see

or infer the existence of in the universe uh general relativity explains those as well so if it didn't it would fail there is a delightful image of that moment where the scientist as he disproved einstein he looked out his window to see his family flying off the earth and everything else with it. Here's a question returning to something we've talked about before. This is from Ken Ryan, who is somewhere in Canada. I have a problem with the title of your show that I'm sure has been previously addressed. I believe a cage would be defined as an area bounded by bars. This side of the bar is not in the cage. That side of the bar is in the cage. The bars define the boundary of the cage. If the cage is infinite, then there is no boundary. That is the essence of infinity. No ending. Therefore, without boundaries, there cannot be bars. So,

there cannot be an infinite cage, only infinite space and a whole lot of monkeys. Now, I should, of course, refer to the fact that we have realised that it's not an infinite cage. It's actually an infinite monkey and then the cage around it. That probably doesn't help things, does it, Ken? Well, this being the BBC, we've managed to make a phone call and we've managed to draft in Hannah Fry, mathematician, in order to answer a very specific question. So, Hannah, this is from Ken,

and this is actually not relating to the show we did with you on statistics of love and sex. We had to just put and sex like that because it was on Radio 4. And we were, of course, slightly embarrassed by that. David Spiegelhalter really wasn't embarrassed enough, as you can hear. No, Radio 4 listeners will know. I mean, they're all from the home counties and things and within the confines of their own house.

I'm sure they're all at it. They can do whatever Robert Winston tells them to do on one of his programmes. We received a question from Ken Ryan. Now, we've had this come up many times about the title of the show, The Infinite Monkey Cage. How do you feel about Ken's... May I just say that you could phrase this question.

We also noticed there was a grammatical ambiguity. So infinite monkey cage can either mean there's an infinite monkey in a cage or an infinite cage in which there is a monkey. So you could phrase the question, can you fit an infinite monkey into an infinite cage? Well, so you can certainly contain infinite things within a finite space so a really good example of this is fractals so there's something called the Koch curve which you can make by you can imagine making it with a piece of string which you halfway along the string you pull upward so that it creates almost like a triangle and you continue doing that on every straight section of string

you continue creating another triangle another another triangle, another triangle. Now, if you drew that on a piece of paper, what you would have would be a line that was infinitely long with infinite bends and bumps on it, but that's contained within a single piece of paper. So it's certainly possible that you could have something that's infinite that can have boundaries. So I think that's kind of fine.

The infinite number of monkeys, though, well, it depends what type of infinite you mean. There are think that's kind of fine. The infinite number of monkeys, though, well, it depends what type of infinite you mean. There are lots of different types of infinity. Do you want to be able to count the number of monkeys that you have? You want to be able to count... OK, so Aleph naught is a countable infinity, right? So it's the number of hours in forever, if you like, or also the number of seconds in forever.

It's something that... Or the number of integers. also the number of seconds in forever it's something that the number of integers are the number of integers exactly so the number of integers is an infinite number but it's one that you can write a list you can start at the beginning and you can you can know that you're not missing any of those out you can also do the same with uh with fractions you can you can write out all the fractions and in particular ways but if you want to include all of the irrational numbers you can't count the number of irrational numbers. You can't be sure that you've got all of them. So these are the numbers, the infinite decimal,

like pi is an example. Exactly. Yeah, the argument's called Cantor's diagonal slash, isn't it? It is indeed. Oh, my gosh. I'm about to be tested on Cantor's diagonal slash. Yes, you're right. So the idea is you can write down all the integers.

Imagine writing the list of integers, all of them down. And then next to each integer, you're writing down one of the irrational numbers. You can always show, essentially by going down the diagonal of all your irrational numbers and adding a one on the diagonal to each one, then you create a number that's not in the list

because it differs by one in at least one place in every number so you can make one more so there's at least one more of those than there is uh than there are the infinite numbers of integers and so would it be easier if we go back to hilbert's hotel which we referred to in an earlier series, rather than have one cage, if we put the monkeys in Hilbert's Monkey Cage Hotel, where each monkey has a separate cage, will that make it easier rather than just having one cage? I'm just trying

to think of the best way of housing the monkeys. So if we go for Hilbert's Monkey Cage. Yeah, I think so. I think, yeah, having an infinite number of rooms. Well, the problem of course is you have to have an integer number of monkeys. Right. So it's clear that they're countable. They have to be, right?

Yeah, they have to be. We can't have a non-integer number of monkeys. Well, unless it's a bit gory. If we just have this big cage, we go for Hilbert's Monkey Cage Hotel. And then each time the new monkey arrives, we just move them all along the monkey cage. If you have an infinite... This is the Hilbert's Cage argument, isn't it?

The question is, if you've got an infinite number of monkeys in a cage of infinite volume, is there room for another monkey? Absolutely. That's the answer then. Thanks very much. I hope that's helped you, Ken. Thank you, Hannah.

You may go back to making your other BBC programmes now. Why, thank you. Now, of course, the strawberry question comes up again, Hannah. You may go back to making your other BBC programmes now. Why, thank you. Now, of course, the strawberry question comes up again, Brian, as for those of you who've never listened before, some years ago we ended up in a peculiar debate about when is a strawberry really dead, and Obi has sent in this. Can't we consider the strawberry both dead and alive

until we attempt reviving it? Brian? Well, that's Schrodinger's cat that is true that if you believe you can write down a wave function for a strawberry and there's not some kind of process that the some kind of we call it decoherence but there's some kind of way that you could isolate it from the rest of the universe then you have to write the thing

as a linear superposition of alive and dead as we keep saying Or you can just ask a botanist Yes Which is what we did, in fact I think the final episode of the previous series was a botany special so if you go back to that again that's still available on the BBC website,

you will hear someone give a very lengthy, specific and botanical answer. Because I think sometimes, I know you think physicists should answer everything, but just occasionally when it comes to berry-based questions, botanists have an angle. This is from Michael Hancock. How does it make any scientist on the panel feel to know that in a few hundred years your average school child might know more and understand the universe

deeper than you can? They'd be delighted. That's what I think because Michael says sobering thought but I think it's sobering in a very delightful sobering way. No, the best expression of this way of thinking that all scientists subscribe to was given, not surprisingly, I think, by Richard Feynman, who called science a satisfactory philosophy of ignorance, by which he means that you are to be delighted when you don't know the answer to something and you will search for it. And if you don't find the answer, you will be curious and happy that you don't find the answer you will be curious and happy that you

haven't found an answer um it's the there's a it's a misnomer that that scientists like to know everything it's completely wrong a research scientist will go and seek out the things that are not known and spend all their time trying to know them and if they find out they will go and find something else that's not known and have a go at that so the the we would be out of a job if the known were saturated if we knew everything there would be no job for scientists to do so uh now finally at the end of our what is reality show brian listed a number of things which he decided were not real and no no i listed a number of things that don't exist you okay don't exist not real okay... No, no, I listed a number of things that don't exist. You... OK, don't exist, not real. OK, so...

I didn't decide. They may well exist as ideas in people's minds, but you believe that they don't have a physical existence. No, no, in reality, they don't have a physical existence. OK, well, that's what I'm saying, is they may exist in people's minds, but they may not exist in a physical reality. They don't exist. Well, let's find out, because not all the listeners agreed. Brian,

dear Robin and Brian, correct order, well done. I enjoy your shows immensely and listen to all of them via the Miracle Podcast. I must, however, take issue with the list of things that Brian stated are not real at the end of last week's What Is Reality? Can I just say that podcasting isn't a miracle? It's based essentially on quantum mechanics. Right, you can say that, but don't try and shy away from a very important question about Bigfoot. Clearly, Bigfoot does exist. Well, I take issue with myself then,

because it's clearly not against the laws of nature that Bigfoot can exist. If you define Bigfoot as a species of ape, let's say, or some species of hominid, a hominin, whatever these people mean, one of those, if you describe it as a species which has not yet been discovered, then of course that could exist. So I disagree with myself if I said Bigfoot doesn't exist.

Excellent. That's very good. It might exist. I doubt it, but it's not impossible. If a biologist were here, I suspect what they would say is the same as a Loch Ness monster. If you're going to have some species that exists on Earth, you need enough of them to have a viable breeding colony. You can't have one. Obviously you can't have one because it will die

and then there won't be any. So you've got to have enough to breed and to have a gene pool that can maintain itself and that there's a species there that's viable. So you need a lot of them. So you can't just have big foot. You need big feet. And that's probably what I meant when I said big foot, singular, big foot, singular Loch Ness monster. Impossible.

Cannot exist. Bigfoot, singular, Bigfoot, singular Loch Ness monster, impossible, cannot exist, with, I suppose, the caveat for the pedantic listener that it could be the last. So you can have... I suppose there will be a time when, if a species becomes extinct, there is one left, like the dodo on Pirates and an Adventure with Scientists, which is a great film, I think. So you can have one, but that would be the last one.

So you can't have one that just persists. You've got to have a breeding colony of them. And that's why I think it's unlikely that there are very big animals out there that have not been discovered, because you can't just have one or two of them. Especially land mammals, big land mammals. You need a thousand of them or something like that.

I should say, of course, that the reason that Alan sent this in is possibly because he works for Bigfoot Programme and Project Support Limited. So there, there's certainly one form of Bigfoot that does exist. Now, here's another thing on the list. This is from Chris. He says, Brian's ideological response that there are no ghosts is very simply unscientific.

Rubbish. A logical rather than ideological view might be, one, current science has defined no actual mechanism for ghosts to exist. Two, yet many people report ghost experiences. Three, I can't dismiss them all as hallucinating subhuman knuck. No, I probably can't say that, but can I? I can't all dismiss them as hallucinating subhuman fools. Well, nobody said that.

Richard Feynman, we've mentioned Richard Feynman many times on the series. On this episode alone. What he would say is that he actually used this argument Richard Feynman many times on the series. On this episode alone. On this episode. What he would say is that he actually used this argument to describe the sightings of aliens and why it's unlikely that UFOs and alien sightings are actually due to the efforts of an extraterrestrial civilisation. And people said it's very unscientific. You can't say that.

You can't rule it out as a scientist. And ultimately he said, no, I can't rule it out so but i'm dealing with things that are more likely or less likely so it is more likely that what we see when we are studying the phenomena of people seeing ghosts and apparitions spirits what we are seeing are malfunctions in human perception which doesn't mean the people who see them are abnormal in any way. We make many, many observations all the time. We all misinterpret them from time to time. It is more likely that's the case

than having to completely tear up the laws of physics as we know them and rewrite them in order to permit the existence of life after death. That's the key point. Now, that's going to get loads of letters, isn't it? Because that steps on many philosophical and intellectual toes. Though Chris N's letter trumps you with a bit of Wittgenstein.

Whereof one cannot speak, thereof one must be silent. Wittgenstein. Wittgenstein, yeah. Who is it? Oh, Wittgenstein. Wittgenstein, yeah. Who is it? Who's Wittgenstein? So quoting a philosopher is winning an argument. Well, no, he's got a lot of other points

but it's quite a long letter. I can win lots of arguments by just quoting philosophers. OK, then go on. That's why you're there. Go on, quote philosophers. As Plato once said. Come on.

You're round. Exactly, exactly. You are quoting Monty Python's apparent quoting of philosophers. I'm not. I'm talking about the platonic solids. Right. Anyway, so that's Bigfoot's general relativity and

the nature of infinity dealt with in half an hour or longer if you've listened to the extended podcast. Thank you very much for listening to Brian and Robin's Infinite Inbox. You can listen to all of the episodes that we've talked about and others on the BBC website for, well, for a very long time. Not necessarily infinity, but probably some definition of infinity,

or at least to the heat death of the universe or the heat death of the BBC, whichever comes first, or indeed the heat death of Brian as he yet again attempts a pie in an unruly and foolish manner. Remember, just because you understand physics and cosmology generally doesn't necessarily mean you understand the nature of pies. Thank you.

Is that a thought for the day? Your version of thought for the day? My version. In the infinite monkey cage. That naughty monkey. In the infinite monkey cage. Without your trousers. In the infinite monkey cage. Turned out nice again.