Size Matters with Hannah Fry (2018) – BBC – Documentary

Everything in the universe has a size Planets are big Insects are small People are somewhere in-between Everything has a place in the grand order and we take it for granted that things are as they should be But are they? Does size matter? What would it be like if things were a little bigger? Would it be better? After all, big things are generally more impressive than small things And there are other advantages, too Bigger species tend to live longer Stats even show that taller people get paid more And you probably know that the biggest building in the world is the Burj Khalifa But what if I asked you to name the smallest? All of which makes me wonder whether bigger isn’t just fundamentally better somehow, so we are going to put that theory to the ultimate test Using the power of science, we’re going to set about super-sizing the world around us and everything in it, including even us, to see whether a bigger world really is a better world We’ll learn why megastars can’t last, we’ll see just how far plants can be super-sized, and we’ll find out why the human body may already have reached its limits Along the way, we’ll discover just how much size really matters You’ll never look at yourself, or your world, in quite the same way again So here we are, at Joe’s house And this is Joe BEEPING An average human, on an average morning, going about his business just like he does any other average day But this isn’t an average day because, as Joe’s about to discover, he’s woken up in a parallel universe, a universe just like our own, but with one important difference In this universe, we can change the size of things just to see what happens So, with Joe’s help, we’re going to go on a bit of a journey, scaling up to dig down into the very nature of our universe, and witnessing the surprising effects that changes to the size of things around us can make, which means ordinary, average Joe here, is about to have a very extraordinary day And that’s going to help us understand whether size is just an accident, or whether there’s a reason that the universe and everything in it is the size it is Thought experiments are vitally important for science because, with thought experiments, we have a way to quickly explore the realm of possibilities, before we then engage in very detailed calculations So, whenever we start any question in science, we begin with a thought experiment So, in the spirit of this episode, I figured it was fitting that we start big – by changing the size of Planet Earth itself 12,756 kilometres across, with a mass of six billion trillion tonnes – that’s six followed by 21 zeros – Planet Earth is already pretty large, but, in fact, it’s only the fifth biggest of the eight planets in our solar system Compare that with the largest planet, Jupiter, over 11 times the width of Earth So what if Earth were bigger? Could life thrive if it were twice as big, or even the size of Jupiter, 143,000 kilometres across? Or, is the way we are and the way we live intrinsically linked to Earth being around 13,000km across? There’s only one way to find out We’re going to grow earth in size, bit by bit, and watch what happens to Joe The first thing Joe would notice, as we begin to grow our earth, is that the infrastructure around him – buildings, bridges and roads –

which are all tailor-made for our normal-sized earth, would begin to struggle Now, bridges are actually designed to have some give, so Joe might just make it across just in time to watch his whole city start to collapse CRASHING So it would be fair to say that we’ve got off to a bad start and we haven’t even begun to feel the effects of, you guessed it, gravity Gravity is the key force that governs the universe and nature on the largest scales and, at very basic levels, every object in the universe is attracting every other object according to its mass The more mass, the stronger the force of gravity With the planet now twice its normal size, Earth’s gravity is far stronger, which is bad news for satellites Higher gravity would upset the equilibrium of their orbits, severing communications across the globe, meaning this revolution will not be televised, and there’ll be no phoning it in either And as low earth orbit satellites hit the upper atmosphere, the increased drag literally pulls them out of the sky EXPLOSION But, even if Joe avoids being obliterated by the showers of falling satellites, his problems are just beginning With the width of the planet doubled, surface gravity would be doubled as well. If you want to know how that feels, then you don’t need to go to a parallel world – you can do your own thought experiment Just imagine giving yourself a piggyback permanently Back in the real world, there are some people who regularly experience something that’s equivalent to double gravity, and then some – fighter pilots To do his job, RAF Typhoon fighter pilot Mark Long has to deal with extreme G-forces, so he knows just what the human body can and can’t take G-force is like an increase in the gravitational effect Your arms feel heavier, you find that breathing’s a little bit more difficult because you’re working harder against the force that you wouldn’t normally be exposed to So, it’s like gravity, but just more intense Today he’s flying a display routine, which will take his body and his plane to the limit The forces he experiences during some of his manoeuvres are the same that Joe would experience on his higher G planet I’ve just bottomed out from a looping manoeuvre here and I’m turning the jet into a barrel roll manoeuvre It’s quite slow, so the G-force is around 2.5 at this point As I get towards the bottom manoeuvre, it starts to increase Which is pretty much bang on what Joe would be feeling on his twice as big, double gravity planet So, 2G actually feels all right You can look around fine, you can move your head, but everything is just a bit more of an effort Moving around under 2G conditions would be tiring Life would be very slow, it would be lethargic, moving your legs would feel heavy – it would just take longer to do stuff and you’d feel tired at the end of every activity you tried So you’d need to take regular breaks, but for now at least it seems like Joe would still be able to function And where there’s a will there’s a way So while it would take some time to achieve, humanity would survive Rebuilding new gravity resistance cities and infrastructure, and adapting the way we live so that 2G living would become the new normal For starters, standing up would largely need to be avoided If I was lying down, now the heart hasn’t got to work so hard and life would be a lot easier So the best physique is actually fairly muscular, quite short The distance between your heart and your head is the most crucial thing in the vertical sense

because your heart has to pump the blood against gravity So, the fact that we evolved to stand upright on two legs is actually a disadvantage in higher gravity because our brains have ended up much further above our hearts And had the earth been bigger, and had higher gravity all along, who knows if upright humans would have even evolved Unfortunately, 2G world has even more pain in store Changing the size of Earth also upsets the cosmic applecart Our moon sits in perfect equilibrium with the Earth You make the Earth twice as large, in radius, and eight times more massive, it has a dramatic effect on the moon’s orbit The moon would be pulled into a strange new orbit that would pass much, much nearer to the Earth Not close enough to hit, but you’d soon see the difference This moon also has a dark side In its normal orbit, the moon causes tides in our oceans that are rarely more than ten metres high If we had tides operating with this moon coming very close to the Earth then, in fact, the heights of the tides would be extremely high So, obviously there are some cities around the world, many cities, which would become uninhabitable under those conditions because you’d have these great tidal waves sweeping around the planet With the moon now passing so much closer, tides could be hundreds of metres high Just doubling the width of our planet is enough to completely overturn life as we know it And we’re nowhere near being 11 times wider to match the size of Jupiter Now, as we increase the width of the planet further, and gravity gets even stronger, anyone in our thought experiment who has a choice in the matter might want to start thinking about getting out because things are going to get seriously dangerous Oh, not you, Joe! But just what is the limit for the human body? Centrifuge studies show that untrained people suffer badly from 3G onwards Above that, people begin to lose vision and black out But pilots build up resistance to the effects of G-force through their regular exposure to it Their bodies adapt and they learn to tense muscles in their legs and lower body to keep blood flowing to their brain So, if the planet did get bigger, we’d all get better at coping – up to a point Doing 350mph at this point and the jet is accelerating all the time, so I’m going through 4G, 5G, up to 6G, as I turn onto the crowd line At 6G, I don’t think the human body would be able to function on a bigger planet It would be incapacitated You would just be spending your whole time trying to fight against the G-force. You wouldn’t be able to do anything In fact, you’d probably spend your entire life lying down But lying down won’t help with what comes next because, by the time Earth passes half the width of Jupiter, something very strange would happen The size of the planet affects the very air we breathe and, at six times normal size, Earth’s atmosphere becomes toxic As the increasing gravity of massive Earth pulls the atmosphere closer to the surface, the oxygen is packed more tightly, so Joe would be getting more oxygen molecules in every lungful That may sound like a good thing, but the problem is oxygen is a highly reactive gas We’re only designed to cope with a certain amount Too much of it can result in a whole host of unpleasant side-effects, from breathing problems, to seizures, and even death There’s a chilling irony about oxygen toxicity Too much oxygen causes irritation in the chest and a cough that, over time, will get worse and worse, until, eventually, it gets so bad you can’t breathe So, the end result from too much oxygen

is that you die from lack of oxygen JOE COUGHS Sorry, Joe And we’re not finished yet I’m preparing my body right now for this onslaught of Gs So, as soon as I enter that turn, I’m at 9-G I can barely move my head, so I’m forcing my body against the edge of the seat to look round for my reference point ready for pitching up for the loop When I pitch up for the loop, I know there’s going to be another 9G manoeuvre, so I’ve pre-tensed my body and now, as I get to the top of the loop, relaxing into the G-force I’ve had the Typhoon up to 10.4G and it was painful I would hate to be on a planet that’s at 11 G In fact, I’d look for a different planet As the planet nears the size of Jupiter, Joe and the rest of humanity would be long gone Insects and other small invertebrates probably the only land creatures still coping, but even they have got no chance to make it through the grand finale With gravity 11 times what it was, the moon would be pulled from its orbit So we make the Earth 11 times bigger, essentially the size of Jupiter, the moon can no longer go around the Earth any more It will crash into the Earth and it will crash into the Earth in about three hours The result would be catastrophic Things haven’t worked out particularly well for our bigger Earth, have they? Well, even without being hit by the moon, big planets don’t work very well for us humans So, if we had to leave Earth on some galactic Noah’s Ark one day, loaded with all of the Earth’s animals and looking for a new world, we humans would be one of the hardest to re-home because we are so sensitive to gravity Our soft-bodied upright design, our ability to breathe the air and not be at the mercy of colossal tidal waves is all tied in with the fact that our planet is 12,000-odd kilometres across, with a mass of six billion trillion tonnes and no more So, it’s time to put the planet back to the size it has always been With super-sizing Earth clearly off-limits, maybe we’d be better off trying to super-size something that wasn’t so big in the first place, like some of Earth’s inhabitants Just like planets, living things come in a huge variety of sizes So, if we decided as part of our grand thought experiment to make some of them bigger, surely there’s no harm in that? Well, it depends When it comes to living things, size works in mysterious ways Take insects As animals go, they’re pretty small Even the very biggest insects weigh no more than 115g But 300 million years ago, insects were much, much bigger Take Meganeura, a dragonfly-like insect with a wingspan approaching three quarters of a metre So, if they existed once, surely there’s no reason why giant insects like Meganeura couldn’t take to the skies again? Well, as it turns out, there is Insects breathe through tiny, little openings in their body called spiracles, which connect to even finer tubes called tracheals, which permeate the entire body This network transports oxygen to each and every single cell and removes carbon dioxide It’s a system that works well, but only for small bodies You might think of the way that insects breathe as letting oxygen sink in through their tissues Now that’s fine for everything that’s very close to their body surface, but it becomes harder and harder to reach structures that are deep inside the body And if you become larger and larger, you have more and more volume relative to your surface area, and therefore it becomes harder and harder to actually reach the structures deep inside your body If the way that insects breathe limits their size today, why were they so much bigger in the past? 300 million years ago, the oxygen content in the atmosphere was higher – around one and a half times as high as now. We now have about 21% oxygen concentration and there used to be 30 to 31% In today’s atmosphere, insects this big couldn’t get anywhere near enough oxygen to function, so it’s sayonara, Meganeura, I’m afraid For an insect these days, it’s small or nothing

But for other living things, the rules are different and some people are already making giant happen My twin brother, we lived and breathed pumpkins – we always have done Stuey and Ian Paton have devoted their lives to making pumpkins into giants When Ian and I were little kids, we took one of these seeds and we planted before we went on holiday And since then, we haven’t stopped, and that’s 42 years ago, and it’s just our total passion, really This seed is – they’re like racehorses We can look year-to-year, we can see if they get a pedigree and you know that this seed in particular will grow lots of pumpkins to 2,000 pounds It’s not just the brothers who care about the size of their pumpkins – people all over the world compete to grow the biggest It’s a competitive business and the brothers but plenty of time and money into their passion But however much work the brothers put in, there’s one thing that limits how big a pumpkin can get After around four months, pumpkins are genetically programmed to stop growing as their skins thicken and harden It’s physically got to grow that fast, so next year we’ll be putting the heating up at night to 22 Celsius We need to get this thing growing faster than 57 pounds a day, so we need to get it going as fast as we can, and it’s just getting everything perfect and speed, speed, speed really It’s a nerve-racking time for us, you know That’s a year and a massive amount of work The biggest one, yeah? I told you There. That’s good news Bit more. Whoa! We need to get the world record We’re getting close to it and we’ve grown some big ones this year, so we’ll see how we do Hopefully our season’s going to get rewarded with something good VOICEOVER: Well, a very, very good morning, everyone, and welcome to this Autumn Pumpkin Festival Today there is a pumpkin weigh-in at a nearby country fair and, for the brothers and their gigantic squash, it’s their moment of truth Nice and flat, this one Our goal is to beat our personal best – that’s the first thing we must do Our next goal is, Stuart and I, we really want a 2,000-pound pumpkin There’s a top few people in the world that have managed to do that How will their pumpkins stack up in the world rankings? Size is the only thing that matters in this game Heaviest pumpkin wins – that’s the rule So, what do we reckon, guys? Do we think we have a new UK record? Yes! You do? 2,251 THEY CHEER The brothers’ winner is a staggering 200 times heavier than an average pumpkin They are pumpkin legends 71 pounds short of the world-record It is basically the second biggest fruit on the planet ever weighed You two won it! 42 years and…how cool is that? So, if it works for pumpkins, why not humans? Could we super-size ourselves to weigh in at around a tonne? And, hey, why stop there? What about making ourselves bigger than a blue whale and becoming the biggest animal of all time? Could the giants of fairy tales be made flesh? ALARM SOUNDS So, let’s say we make Joe three times his normal size Five metres tall Way taller than any human has ever been before How would he get on? Well, as it turns out, not very well at all And, to get a unique perspective on why that is, we need to go to Turkey TRANSLATION: When I go to a hotel, to a friend’s, the biggest problem is finding a bed I fit in The standard bed is just two metres I usually put two beds together As well as having oversized feet, Sultan Kosen has an oversized

pituitary gland that has produced too much human growth hormone throughout his life As a result, Sultan is spectacularly tall 2.51 metres tall, to be precise, which makes him the world’s tallest man which, let’s face it, is pretty cool, but also comes with problems As a consequence of his size, Sultan has suffered repeated fractures in his legs Today, Sultan has travelled from his home town to Ankara – to visit his doctor HE SPEAKS IN TURKISH This is the femoral fracture that has occurred six years ago It was a mid-shaft fracture, which has displaced all the fragments, and this long rod has been inserted with nails to stabilise the fragments There’s a significant relationship with the length of the bone because you know that, if you apply a force on a very long rod, you can easily break it If it is relatively short, it’s very It’s not so easy to break it because of the physical properties of the bone To make matters worse, Sultan’s other problem is staying balanced and it’s a hard landing from his height He cannot walk as we walk – that’s his basic problem That’s why he’s so prone to easy falls TRANSLATION: Size doesn’t matter What matters is health and overcoming obstructions When obstructions are overcome, it doesn’t matter whether you are short or tall Sultan’s health issues reveal one of the main problems faced by large living things, something that’s crucial to Joe’s chances of success at his new height The square cube law is a simple law with huge implications when it comes to making things bigger Take a statue. Now, let’s make it double the height Same proportions, made of the same stone, just scaled up So then you’d have a bigger statue What’s not to like? Well, even though it’s exactly the same shape and material, the bigger statue breaks under its own weight Let’s run that again, doing the maths as we go The square cube law tells us that, as something gets bigger, it gets heavier a lot faster than it gets stronger When we double the height, the weight doesn’t double Because weight relies on three dimensions, it actually goes up by two times two times two equals eight times Strength relies on cross-sectional area, which is just two-dimensional and doesn’t increase as fast Result? Arms that fall off Just ask Venus – she knows all about it And there’s a dramatic real-life example of the effect of the square cube law in humans This here is Robert Wadlow as a baby But, by the time that he was eight, he was the same height as me Aged ten, he was nearly two metres tall – six foot five in old money And he kept on growing until, by the time that he died, at aged 22, he was an incredible 2.72 metres tall That’s just shy of nine feet – the tallest person ever recorded That made him just over one and a half times the height of the average male But the story of his weight is even more astonishing because, at his peak, he tipped the scales at a staggering 223kg – that’s almost four times the weight of an average man – and that is because, as the square cube law tells us, if you double a person’s height, their weight increases by a factor of eight Now, all of that extra mass caused a huge strain on his body because the fact is, if you want to be a giant, our body shape just doesn’t cut it So it seems our quest to scale Joe up to the size of the blue whale has hit a serious snag Maybe we should pop him back to normal while we figure out a better way For a little inspiration, we’re visiting California

to take a look at the tallest living things on the planet I love these trees because of their massive size, their old age, and because of the way they build the forest that they do Professor Todd Dawson and his team of scientists have studied every inch of these trees right to the very top If anyone knows how they get so tall, it’s him So, redwoods are a remarkably long-lived tree and the coast redwood can live more than 2,500 years because they’re plants, they’re constantly growing And because they’re very old, as long as they keep growing and they have that old age, they’re just going to keep getting bigger and bigger and bigger So, a tree like the one we’re standing next to here, it probably weighs thousands and thousands and thousands of pounds, many tonnes One of the things that we notice about these trees as they grow is they begin to add more at the base So you go from a very small tree that just goes right into the ground, like a carrot in the Earth, and as they grow larger and get heavier and heavier, they begin to buttress, they begin to add mass at the base, cos they have to have a way to support themselves Eight metres, five centimetres Nice-sized tree They sort of build some sort of a structure like an elephant’s foot, if you will, at the base Remarkable set of compensation mechanisms The redwood strategy of shape-shifting, getting more triangular as they grow taller, means that their trunks could support the weight of taller trees So is this the limit? Well, there’s no point in having a parallel universe if you don’t push the boundaries here and there Let’s say we made a giant, giant redwood What could possibly go wrong? So, one of the factors that limits how tall tree is get is their ability to get water to the very top of the tree itself And while a lot of trees may stop their growth, redwoods have a special way of dealing with that water limitation as they get taller For example, we’ve got leaves from the lower part of the crown and, uh These much smaller leaves, they come from the very top of the tree As the leaf area goes down, the amount of water used goes down as well So they become more thrifty in their water use at the top of the tree than they do at the bottom of the tree Moving water upwards several hundred feet needs pressure Fluid migrating into the roots creates pressure at the bottom, pushing water up. At the same time, water evaporating from the leaves leads to lower pressure at the top, which pulls water upwards Having smaller leaves higher up means less water needs to make the journey But, with our giant, giant redwood, not even this special adaptation could solve the water supply problem The leaves would dry out and die, and the tree would go no further Despite their problems getting water to the top, giant redwoods still show us just how big you can get if you change your shape as you grow What goes for flora goes for fauna, too Bigger creatures can’t be scaled up versions of smaller creatures They need profound shape changes to overcome the challenges of size Take Joe’s dog Max, for example Max’s skeleton makes up 8% of his total body weight But if we doubled his size, we’d have to beef it up a bit to support his extra weight At twice the height he’d need thicker bones, so his skeleton now makes up 14% of his body weight – which is a lot of dog to shift So far so good, but make him any bigger and you start to hit other problems Look at elephants They evolved to be large

so that they could dominate their environment, be safe from predators and reach food more easily But big animals have less surface area per kilogram of weight than small ones, so they have proportionately less skin to lose heat from – which means they can overheat So elephants needed to evolve a way to stay cool in a hot environment, and they did Elephants don’t just have large ears because they look good – they also help them to control body temperature Billy, our eight-year old, is being very energetic right now, but we’re still seeing, because of the colder temperatures, his ears are much colder than the rest of his body, so it really shows how efficient they are at controlling their body temperature by using their ears In the summer times, we would see the opposite – their ears would be much warmer The veins would be full of blood, using as an air conditioner, helping cool down the rest of the body temperature Maybe giant Max could learn a lesson from his elephant cousins Grow a pair of big ears to increase surface area and lose the fur, since hair traps heat So if we keep making Max’s bones stronger and increasing the size of his ears to keep him cool, how max could Max get? How about as large as a medium-sized dinosaur? Well, that’s just big enough to run into the next problem It’s a problem with size that only rears its head for true giants – they’re just too slow Herve Bocherens is a palaeontologist who knows exactly why big animals are so slow Large animals have an issue with the distance the signal has to travel along the nerves Of course, if you are already about 20 metres long, the signal has to be both ways and therefore it’s already a significant fraction of a second that it takes time for the signal to travel and therefore the reaction time of big animals will be slower than the reaction time of smaller animals, and this will make a difference in their life So, today for instance, the largest land animals are elephants, and it has been observed that elephants have to be quite careful when they walk because the signal has to travel quite a long distance And when they hit something with their foot, for instance, it takes a couple of milliseconds to reach the brain and, therefore, they have to wait until the signal comes back to know if they have hit something with their foot before they make the next step Good news from next-door’s cat, but bad news for maxi Max In the wild, a giant dog would probably starve to death But, luckily for Max, he’s a man’s best friend It worked on a dog, so it should be straightforward then to scale Joe up to match Well, let’s see Making Joe five times normal height would make him 125 times normal weight So he’d need much thicker bones Then, of course, he’ll need big ears like an elephant to get rid of all that heat But, before he gets any bigger, he has to face one challenge that maxi Max didn’t – humans walk on two legs Herve Bocherens is studying another primate that tried to get giant in the past It’s a story that begins in an unusual place It started in a Chinese pharmacy in Hong Kong, where a palaeontologist was looking at what he called dragon teeth And among this fossil material there were some fossil human teeth, but also teeth that were much bigger and looked like humans – and these were the first discoveries of Gigantopithecus Of course, they weren’t dragons’ teeth. They were, in fact, teeth from the biggest ape to ever live – Gigantopithecus Estimating the size of an animal based on just teeth and lower jaw is a difficult thing,

but we can say that at least it was twice as heavy as a gorilla, so the weight could be 500kg, which is the maximum estimate That makes it the largest primate that has ever existed But estimating its height is even harder Most probably, it was not standing so much because, with such a heavy weight, it was probably, essentially, quite drupedal And therefore, it is more a comparison with a gorilla, who is mostly walking on four legs rather than walking erect Gigantopithecus was just too big to stand tall like a human We have many examples in the past of animals that started bipedal and had to go back to their four legs and, when they grew bigger, it means that they had to carry their weight on four legs and not any more on two legs So, as we continue our quest to make Joe a blue whale beating human, should he take a step back and resort to walking on all fours like Gigantopithecus? Perhaps not If it’s going to be a giant human, it has to be bipedal because bipedalism is the thing, the one major thing, that sets our genus, or our group, the hominins, off from all of the other primates We are habitually bipedal Being bipedal is a major evolutionary advantage and it’s one of the main reasons that we humans made our way to the top of the food chain Your head is raised, so you can see danger much further away, and you can also reach higher to get food But perhaps the biggest evolutionary advantage was that it kept our hands free for other things, like using tools To borrow a phrase, two legs good, four legs bad If Joe’s like Gigantopithecus, he’s not a human and we’ve failed So if we’re going to make a giant human, Joe has to support all of that weight on two legs If you’re a giant and you’re really, really tall, well, you’ve got to get that blood pumped away from the heart and up to the brain – very important So you need a big pump, a big heart, and that means a big chest to put the heart in And it’s got to go uphill, and so I might put the heart higher up within the chest The heart’s job is to pump blood around the body and most importantly to the brain Pumping uphill is harder work, so a giant human needs a big ticker and as short a distance as possible between heart and brain So here’s giant Joe with his gigantic heart, but chances are it’s going to be a lonely heart and not just because of the way that he looks Another consequence of being large is the population will be small because each one needs a lot of food and so giants would probably not be very numerous Even with his four-legged friend by his side, life as a giant would be no fairy tale for Joe He’d be a very strange creature, indeed, to overcome all the problems that being big creates for a human being And that’s the thing – your size determines how you’re built, how you look, how you live, what you eat Size is an intrinsic part of making every species what it is I don’t know about you, but I don’t think that is a human any more Change size too much and you simply get a different species Time to put things back to how they were ALARM SOUNDS We’ve tried changing the size of the Earth and pretty much everything on it, and I think it’s fair to say that it was a bit of a mess When it comes down to it, things on Earth are just far too interconnected to make any of them that much bigger But there is something that’s already seriously massive that might just have power to add Given that it’s just sitting there at the centre of the solar system, it’s almost begging for it Surely we’d all love a little more sunshine in our lives? 150 million kilometres across space sits the sun, our nearest star And when it comes to stars, does size matter?

As astrophysicist Volker Bromm can tell us, it’s absolutely crucial because the size of a star determines how it behaves When you think about stars, then size is absolutely essential because you could say that the size of the star, or more precisely the mass of a star, is destiny If a star’s size is its destiny, just how big is ours? Well, it’s 109 times wider than Planet Earth and weighs 330,000 times as much, accounting for 99.86% of all mass in the solar system The sun is, for us, the essential star The sun really provides us with a perfect cosmic environment to enable life on Earth If the sun is so perfect the way it is, there’s probably a really good argument for not doing what we’re about to do in our parallel world But where’s the fun in that? Let’s start by doubling the amount of gas in our sun What would that mean for the Earth? How a star reacts to increasing the mass is highly non-intuitive, so you double the mass and you increase the energy output tenfold The effects would be staggering and, for Joe, it would be hell on earth The equilibrium temperature on Earth would go up by a significant factor, so we would have a much hotter Earth, which would then suddenly become completely inhospitable First, the ice caps would melt Then the oceans and rivers would boil off, and the surface temperature would eventually stabilise at around a somewhat uncomfortable 200 Celsius But all is not lost because there is one way we could survive a bigger sun without having to resort to slapping on the Factor 50,000 A key question in modern astronomy is that we think about the region around any star where life in principle could be sustained Life as we know it can only exist if the temperature of a planet is right for water to exist as a liquid With a bigger sun, Earth would be way out of its element, so to speak You basically have a very, very special region around any star, and this Goldilocks zone of not too hot, not too cold, what we call the habitable zone, is a very precious, precarious zone because, if you play with the conditions in the central star, everything changes With our double size sun burning ten times hotter, the Goldilocks zone would be much further away – three times further away, in fact So all we need to do is move Planet Earth to win new orbit, well past Mars, 450 million kilometres from the bigger sun In its new resting place, life would carry on as normal. Well, sort of, because of course the new wider orbit would mean that a year would be much longer and so would the seasons In fact, winter would last 16 months, which might make growing crops difficult But I’m sure we could get round that somehow So, if moving Earth outwards is a goer, what’s the biggest sun possible? Well, ever since mankind invented the telescope, we’ve been wondering about the size of stars – and this is what we found Now, we like to think of our sun as pretty big But actually, in the universe’s Hall of Fame, it’s a bit of a nobody because there is a star that is almost twice as wide as our sun Sirius, the dog star And things don’t end there Pollux here, eight times the width of our sun Rigel, almost 80 times bigger And Antares is around 800 times as big But this star itself is dwarfed by the biggest star that mankind has ever observed – UY Scuti This star is more than 1,700 times the width of our sun But, even that is a tiddler if you look a bit further afield, not in distance, but in time In astronomy, we have a great privilege We can do something that historians and archaeologists only dream of We can directly take images of the distant past and we do this by looking at objects that are far away in the universe Objects that are far away, they have sent out the light that we receive

billions of years later, and light travels with a limited speed, the speed of light, and therefore looking far out into space means also looking far back into time By looking far enough away and far enough back in time, scientists were able to see that the very biggest stars to ever exist were born when the universe was in its infancy If one thinks about the early universe, then you also discover possibilities of really pushing star formation to the extreme Professor Volker Bromm has built theoretical models that give us an idea of just how big these early stars would have been His visualisation lab simulates how massive clouds of gas in the early universe collapsed to form a giant stars So there are special regions in the universe where conditions are ripe, that you can have extraordinary large clouds of primordial gas, collapsing in one go, if you like So then we have a very, very extreme case of star formation, a million solar mass cloud collapsing in one go And again, this is a very special condition and we believe this only could have happened in the early universe Volker’s work tells us that in the early universe there may have been stars a million times more massive than our own Far, far bigger than anything around in the modern universe If his theory is correct, then our sun is just a speck by comparison So could our planet orbit the biggest star in the history of the universe? Could Earth survive if the sun was a million times more massive than it is now? Well, for starters, a star that size would envelop the inner planets so the Earth needs to head to a safe distance far beyond Neptune to the outer edge of our solar system, and the centre of the new Goldilocks zone Then we would have our planet sitting out at this huge distance, and we can also ask – how long would then one orbit take? You would calculate that one orbit would roughly take 30,000 years Not great for getting birthday presents and you will only get to sing Auld Lang Syne once every 1,200 generations But, otherwise, what’s the harm? Well, there is a twist in this tale that means none of that really matters The big problem then that such a hypothetical planet would encounter is that the whole star, the supermassive star, would have a very short lifetime because our sun lives for another five billion years, but the total lifetime of the supermassive stars is just a few hundred thousand years, maybe up to a million years, so this is, in astronomical terms, a blink of an eye Put simply, if our sun had been a massive early star, life as we know it would never have existed Before we’d even had time to evolve, things would have come to a, well, stellar end EXPLOSION If these early superstars were to explode in a supernova explosion, then we could say that these would be the biggest explosions ever to happen in the history of the universe since the Big Bang The Big Bang, of course, would still beat everything else But, otherwise, those would be truly colossal explosions So there we have it The death of our super-sized sun would bring our experiment to an end, taking Planet Earth with it When you think about the repercussions of a bigger sun, it is blindingly obvious that, like everything else we’ve messed with, it’s best left well alone As Joe’s found out the hard way, a bigger world is a completely different world and one he didn’t have much luck trying to survive in because the laws of nature conspire against bigger meaning better The fact is, we are all far better off being human-sized beings living on an Earth-sized planet orbiting a slow burning, sunny little star We set out to discover whether size matters and I think we can now mostly agree that it does It matters hugely If you’re looking at bodies just sustaining life in different thermal habitats, if you’re looking at the brain, yes

So size matters tremendously TRANSLATION: Size doesn’t matter I’ve met the shortest people in the world and also some of the tallest I even once met something taller than me – it was a giraffe! Today, Ian and Stuart have produced a pumpkin weighing 2,252.3 lbs That’s a new UK record THEY CHEER Size is the only thing that matters It don’t have to be pretty, it don’t have to look nice, the only thing that matters is size So size definitely matters for the redwood forest I think it really creates the structure, the scaffolding, that makes the redwood forest such a beautiful place to spend time So what have our thought experiments taught us? You may think that this is all just fun because we’re never actually going to make any of these things any bigger, but the truth is we’re making things bigger all the time – sometimes intentionally, sometimes not So, as we do, it’s worth remembering bigger things behave differently Crowds behave differently to groups, cities to villages, oceans to lakes And so, as our population grows, our cities continue to swell and the ever more ambitious scale of our buildings and infrastructure rise up to match, we mustn’t forget that bigger isn’t necessarily better – or we might end up helping ourselves to a bit more than we bargained for EXPLOSION In the next show, we will be turning our attention to the other end of the spectrum – to find out whether small is beautiful And, as Joe will be discovering, it turns out small is by no means just the opposite of big Small has a completely different set of problems We’ll see how our world would cope with a smaller sun, how small creatures are ruled by a different set of natural laws, and how small changes can have earth-shattering consequences and bring out the superhuman in all of us Everything in the universe has a size Planets are big Insects are small People are somewhere in between Everything has a place in the grand order and we take it for granted that things are as they should be But are they? Does size matter, or could things be different? What if things were smaller? What would be the harm in reducing the size of things here and there? After all, being smaller has its perks Smaller stars burn far, far longer than big stars Smaller things are, relatively speaking, stronger And it turns out smaller people actually live longer Using the power of science, we’re going to do the ultimate thought experiment – shrinking our world and everything in it, including us, to find out if there’s a reason things and people don’t exist on an even smaller scale, or whether evolution might have taken a completely different path Along the way, we’ll discover just how much size really matters, and to what extent the size of something determines the very nature of the thing itself You may never look at yourself, or the world around you, in quite the same way again You might think this looks like an ordinary house on an ordinary sunny morning But, if you watched the last programme, then you’ll know

that this is, in fact, a parallel universe One just like our own, but with one important difference ALARM In this universe, we can change the size of things just to see what happens And that means our old friend, Average Joe, here, is about to have some very strange days indeed In the last programme, we tried to improve Joe’s universe by making everything bigger The Earth, the sun and even Joe himself But it all turned out that bigger isn’t better In fact, it’s very often, if not always, cataclysmically worse So, in this programme, we’re hoping we’re going to have a bit more luck by shrinking things instead, because, as it turns out, small isn’t just the opposite of big In fact, small throws up a whole world of other challenges As in the last episode, we’ll be putting Joe through three different thought experiments – resizing people, stars, and starting with our very own planet. So, here goes Earth is the fifth-largest planet in our solar system And it’s also, of course, the fourth smallest – 12,756 kilometres across, with an atmosphere 100km deep, all the way around Below that is the surface, a thin layer of solid rock, sitting on top of 5,000 kilometres of rock and molten metal And finally, at the core, a 2,400km-wide ball of solid iron The question is – how much do those numbers really matter? How important is it to our lives on Earth that our world is exactly the size it is? What would happen if we halved its width, meaning that instead of being a shade bigger than Venus, Earth was suddenly a shade smaller than Mars? Well, there’s only one way to find out And there it is, a half-sized Earth All made of the same stuff, same proportions, just a little bit smaller Well, as it turns out, Earth’s vital statistics are, well, vital, because changing Earth’s size messes with a few other things that you don’t necessarily want to be messing with For instance, gravity As we discovered when we made things big, the gravity of a planet is in proportion to its width So, a half-sized planet would mean half the normal gravity at the surface Enough of a change to put a spring in your step And if you’re athletic, like Joe here, who knows what you might be capable of So, half gravity would mean you can jump higher and only fall at half the speed Which would take a little bit of getting used to, but, by and large, sounds rather fun So, you might ask, what’s the catch? Joe’s fun wouldn’t last long The universe is a system, basically, so size is important in the sense that if you change the size of one single element, with respect to the others, then the whole thing breaks You see, gravity doesn’t just affect people and objects It also affects the air we breathe, which would get thinner because it would be less strongly attracted to the ground The air at sea level would now be as thin as it used to be two-thirds of the way up Everest There would be less oxygen in every breath Headaches, nausea and shortness of breath would soon follow, as Joe found himself with a nasty case of altitude sickness Luckily for him, humans can acclimatise to thinner air

After a few days, Joe’s red blood cell count would increase enough to compensate for the reduced oxygen So, he’d be back up and about in the low gravity of a half-sized Earth, just in time to notice that it’s beginning to look a lot like Norway The spectacular phenomenon that is the aurora isn’t usually seen beyond the polar regions, but our new half-size Earth would be lit up all over To get the inside track on how and why, we’re going to the University of Maryland Daniel Lathrop has spent 20 years building models of the inside of planet Earth, to help understand how our planet generates its magnetic field And that’s given him a unique insight into the workings of the aurora Dan’s model has a solid metal ball at its centre, surrounded by a thick layer of molten metal Just like planet Earth As the Earth rotates, the currents of molten metal generate a magnetic field Dan built his model to study how this happens, and along the way, he’s discovered just how important the Earth’s magnetic field is So, the Earth’s magnetic field serves as a shield against the worst parts of bad, uh, solar weather So, the sun has storms that occasionally give large amounts of radiation aimed at the Earth, and the Earth’s magnetic field inflates something like a bubble around the Earth, the magnetosphere, that acts as a primary barrier to the worst of the radiation The shape of the planet’s magnetic field funnels this cosmic radiation towards the poles, where it hits the upper atmosphere, causing gases to glow and giving us the beautiful, ethereal lights of the aurora Of course, Dan never set out to study what would happen on a smaller Earth, but it just so happens that he’s been studying just that all along, because over the years, he’s built several versions of his model Earth at different sizes So these, actually, were the first three sodium experiments we built to try to understand the Earth’s magnetic field. So the first one, a 20cm-diameter model, rapidly rotating Next came the 30cm experiment There’s an inner sphere deep inside there that you can’t see And the third experiment, at 60cm, here’s the bottom half of the outer sphere and then a solid copper model of the inner core that independently rotates Then the whole thing would be filled with liquid sodium, in the experiments Thinking about what it would be like if the Earth were half size, we could then examine data between the different-size experiments to see how the magnetic fields are different Instead of iron and nickel like the Earth, Dan’s model is filled with sodium, because of its low melting point But it still takes three days before it’s all molten and ready to spin So here we see magnetic field data from the 30cm smaller experiment, and comparing it then to more recent data from 3m, it’s very evident that as the experiments have gotten larger, we have much more magnetic induction, much stronger magnetic fields overall A smaller Earth would have a weaker magnetosphere, but that’s not all Dan’s experiments reveal When we go from larger to a smaller experiment, the magnetic fields’ strengths have both become weaker and have changed pattern And if you look at, you know, the data of the larger model, there’s kind of well-defined north-south magnetic poles, where in the smaller experiment, at these parameters, we had like a ring of south poles around the equator, and then two magnetic norths at either end So, it is possible for the shape of the magnetic fields to change when you change its size Half-sized Earth would likely have many magnetic poles spread around its surface, which would explain how Joe has ticked seeing the northern lights off his bucket list without ever leaving his back garden The bad news, however, is that these multiple auroras are a sign that Earth’s weaker magnetic field is being overwhelmed by the barrage of solar energy And from here on in for Joe, it’s all going to get a bit dark If you have a smaller planet with a weaker magnetic field, there will be more problems with telecommunications The sun still has these big, erm, anger things –

they’re called coronal mass ejections, where it really sends a burst of radiation in space We have systems on Earth which are so big, depending on electricity, that when the sun is angry, potentially you have bigger ejections and it causes problems And with every blast of solar wind, a weaker magnetic field also wobbles more, which induces surges in electrical systems down on Earth, and generally sending us back to the Dark Ages SHE LIGHTS MATCH Even on a normal-sized Earth, sometimes the radiation from solar storms can punch through the magnetic field and cause big problems On March 13th in 1989, the entire province of Quebec in Canada suffered a power cut that lasted 12 hours The cause? Radiation from a solar storm tripped circuit breakers at a hydroelectric power station With only a weaker magnetic field to protect us, these things could happen all the time. Ow! And that’s just the start A weak magnetic field would also double down on a problem that Joe was already struggling with Reduced gravity would mean gases were finding it easier to escape into space Increased cosmic radiation would supercharge that process And, for Joe, that combination would be a major problem Now, cosmic radiation has a particularly bad effect on oxygen Quite quickly, it would cause all the precious oxygen in our atmosphere to escape off into space, leaving only an unbreathable mixture of heavy gases like nitrogen and CO2 And that, of course, would be a death sentence for Joe and all other animal life Still, this should keep him going for a bit But, over time, half-sized Earth would end up as a barren, uninhabited wasteland, just like Mars It’s the curse of small planets So far, so apocalyptic In pursuit of a smaller world, we sentenced poor Joe to a slow, lingering death, and turned Earth into an uninhabitable desert I think it’s probably time to put it back to normal And there we are. 12,756km across, planet Earth, a shade bigger than Venus, and just as it should be ALARM SOUNDS Maybe it’s time to try something a little less ambitious, something that might work out a bit better for Joe What about us? As mammals go, we’re pretty big Maybe we could stand to lose a little There are about 4,000 mammal species in the world and they come in all different shapes and sizes The largest, of course, are the whales The blue whale is absolutely enormous, the size of several school buses put together And the smallest mammal is very small, two grams It’s essentially the size of your thumb And the typical size of a mammal, however, erm, is not sort of in the middle Instead, it’s much closer to the smallest size, about 40g, which is the size of a rat Humans are about 65 kilos, on average, give or take, erm, and so, that makes us enormous When we look at an elephant, we may feel small. But, in fact, humans are around 1,600 times heavier than the average mammal But that’s not necessarily good news Aaron Clauset is a data scientist who studies the relationship between size and extinction What we found is that the larger an animal is, the more likely that species is to go extinct in the long run And there are various reasons for this Typically, species that are larger have smaller populations, and so, if there happened to be a few bad years in terms of reproduction or food, then their population could crash, and as a result, they could become extinct Whereas, much smaller animals typically have much larger populations, and so they are robust to these kinds of events So, in general, the larger the animal is, the faster it goes extinct Bye-bye. And, when you think that in recent times,

the average human has been getting bigger and bigger, that might give us cause to worry Does this mean then that we are accelerating towards extinction? Well, possibly not, if you take a wider view Back in the Stone Age, when humans were hunter-gatherers, the average male height wasn’t that far off what it is today But about 12,000 years ago, during the Neolithic Revolution, when we started farming, we quite quickly became considerably smaller, as our new grain-based diet had a lower nutritional value And it’s really only much more recently that we have finally got back to hunter-gatherer size, thanks to modern improvements in food and medicine Well, that’s a relief. Being a big species means a long lifespan But not getting bigger means we don’t have to worry about extinction A big win all round, then Well, not necessarily You see, big species might have longer lives than small ones, but within each species it seems to be the other way around Generally, smaller individuals of a species live longer For instance, smaller dogs live longer than big ones Could this also be true for us? Geneticist Diana van Heemst has the answer I guess, you know, in a lot of species, and if we look at dogs, horses, elephants, it’s actually the smaller variance of that species that seem to live longer, and of course, then, the real question is, does it also apply to humans? Well, that’s not so easy to answer There are so many factors that influence human lifespans that it’s hard to tell what’s down to size and what’s down to, say, diet or exercise To get to the bottom of this, Diana has been re-examining a remarkable 1970s study, which homed in on a group of people with very similar lifestyles, but varying heights – professional athletes For example, American baseball players, there’s a nice encyclopaedia which is a rich source of information, not only for the baseball fans about, you know, all the details about performances and nicknames, but also, actually, it contains date of birth, date of death, the adult’s height and their weight If you know their height and age of death, you can start looking for a pattern Wally Burnette – 1.83m Murry Dickson – 1.78m The original study used data from hundreds of players, but we can see what they discovered by looking at just a few I took from the Encyclopaedia of Baseball nine representative examples of baseball players, and we have, you know, attached to them to the wall based on the height and the age at death, and this mimics the original study, which made use of the full sample of the encyclopaedia, which found this negative correlation between height and the age at death The 1970s study found that size DID matter Being five centimetres shorter meant, on average, you would live for two years longer The big question is, why? And it’s only now, four decades later, that researchers like Diana van Heemst have come up with a plausible explanation In order to grow, our body makes growth hormone, which, you know, stimulates growth, but at the same time, it also influences lots of other processes in our body And if we look at the data that has been derived from work on other animals, they stimulate the body to grow, and this is kind of a signal that there’s enough food, that – you know, there’s favourable conditions, that it would be wise to invest as much as possible energy in growth and reproduction, and this may come at a cost, because it means there’s less energy available to invest simply in maintaining our bodies in good shape And actually, when conditions get worse, or become less favourable, like when there is a food shortage, or a lot of toxins, then, as a consequence, as a response to that, we don’t grow, we kind of stop growth, and we really invest the available energy in maintaining our body and trying to kind of, you know, survive these periods of hardship until things get better It seems like being big comes at a big price. But for tall people, there is some light at the end of the tunnel However, size is not the only thing that matters There’s lots of things that people can do themselves to adopt a healthy lifestyle, like, you know, not smoking, healthy food, lots of exercise So, it matters, but it’s not the only thing that matters So, just how small could we go?

Well, let’s start with what we know The smallest adult humans known to science are just over 50cm tall I used to wonder what it would be like to be a bit shorter, mainly because being this tall, finding clothes to fit is a bit of a pain But today, I’m wondering what it would be like to be substantially shorter Like, a third of my current height And that is because I’m about to meet the one person in the world who can tell me what it’s like 23-year-old student Jyoti is on a sightseeing trip to London Wherever she goes, she gets as much attention as the biggest attractions TRANSLATION: When I go outside, then everyone gathers together and stares at me Then I feel a bit strange And at the same time, it feels good that they all look at me Did you have this one specially made, then, or did you just happen to find a small spoon somewhere? TRANSLATION: I haven’t had anything specially made for me I can easily find things in India in the market, in the baby section In our last episode, we met Sultan Kosen, the world’s tallest man Sultan’s incredible stature is down to his body producing too much growth hormone, a condition which has gone on to cause him considerable health problems in his adult life For Jyoti, though, the story is a happier one TRANSLATION: The doctors told me I have hormone deficiency This is the reason I can’t grow taller I don’t have any other health problems The only complaint that Jyoti has is a practical one The world is simply too big for her TRANSLATION: One thing which I can’t do because of my height is drive cars And when I want to go out, I can’t go out alone I always have to have help from my family, like my sisters and brothers I always need help These are the problems I face But in an environment that’s made to measure, she fares much better TRANSLATION: In my house, everything is specially made for me In my bedroom, I have a small bed, cupboard, chair, table, and everything made in my size, all my furniture I don’t have any problems in my house So, what if we were all smaller? In fact, there’s a group of scientists who think that smaller humans would solve some of humanity’s biggest problems Food would go further, we’d all live longer, and what’s more, they reckon there’d be less disease and fewer wars A tiny utopia So, let’s just say they’re right How much further could we shrink Joe? Let’s go from the size of the smallest human down to the size of the smallest mammal This is an Etruscan shrew In the wild, they weigh just two grams, which makes them the smallest mammal, by weight, in the world Potentially a good role model, then, for a miniature human being Professor Michael Brecht has studied them for years Ah, here they are Cool. And I want to chase them into this So, now, here we have them Let me show you what we do for gender determination So, the sexes, they look quite similar The really foolproof sex testing is what I’m going to do now So we actually use this box here, and what you do is, carefully, you put the shrew into the little box and you carefully sniff on it Now, if it’s very, very stinky, it turns out it’s a female If you sniff on it and you pass out, it’s a male So, let me do this here Female OK. Now, let’s figure out how much she weighs This is on the higher side for these animals Many of the adults are just two grams

They have perfectly the same mammalian equipment, it’s all there, it’s just very tiny Like, uh, it’s very difficult to circulate blood through such a small body The circulation system of mammals is much more suitable for bigger bodies, and both the respiration and the blood supply are a huge challenge for such a small body So what we would see is they have a giant heart, yeah, 5% of the body weight or so, a really big heart What we also see is that they have unheard of respiration rates one would see a breath per minute go up to about 1,000 breaths per minute, an absolutely unheard of rate in mammals It’s really also difficult to understand how a mammalian brain and lung could do that 1,000 breaths a minute is hard to fathom, but their hearts push things even further, beating up to 1,500 times a minute That’s 20 beats for every beat of a human heart It’s clearly hard work for a mammal to be so small The question is, why bother? The idea that ecologists have about these animals is that they are specialists for small spaces, yeah? For tunnels, and they go into small spaces where no other predator can go, and then, paradoxically, they are, again, big predators Matching your size to your environment is an important part of evolution, but filling this particular niche has its difficulties The biggest problem they face is heat loss You see him in a thermal camera and you see how much heat he gives off, how much he lights up And this is actually a central problem of their life Uh, the immense heat loss they have, or energy loss they have, is a result of their unfavourable surface-to-volume ratio If you peeled me, and please don’t, by the way, but if you did, and then measured my skin, you would find that I have around a quarter of a square centimetre of skin for every gram of my body weight But if you peeled an Etruscan shrew, and likewise, don’t, you would find that he’s got a lot more, around 20 times more for every gram Now, this is all down to the square-cube law, which states that as a shape grows or shrinks in size, its volume changes much faster than its surface area, and one consequence of having a proportionally larger surface area is smaller animals lose heat much faster Losing heat is particularly bad news if you’re a mammal like Joe, because unlike insects and reptiles, mammals have to keep their bodies at a constant temperature of around 37 degrees centigrade And if you think these furry little fellows have problems keeping their body temperature up, try being one of their babies But somehow, with the help of their parents, they survive The newborn shrews are incredibly small, inconceivably small 0.2 grams is just absolutely incredible, and they look kind of unreal I mean, their whole body is totally transparent They huddle together very heavily The mother is very protective and, obviously, also supplies a lot of energy So, it’s feasible, then, that a human could exist at just five centimetres tall But, of course, we’d face all the same problems as the shrew, an insane heart rate and a constant battle to keep warm Life at their size requires a totally different lifestyle It’s not just about huddling together for warmth If you’re losing energy fast, you need to be very good at replacing it In fact, scientists have discovered that small animals have to have a completely different relationship to food than big animals More than 100 years ago, a scientist named Kleiber observed empirically that the amount of food that an animal requires increases, of course, with how big the animal is, so, an elephant eats more than a deer does, but that the relationship doesn’t go up proportionally So, an elephant eats a little bit less than you’d expect than an equal number of deer would An Asian elephant weighs about 5,000kg, but how much does it eat? Just ask a zoo keeper

This is the amount of hay one of our male elephants gets every day, 43 kilos to 45 kilos About 1% of its body weight For the dik-dik, their mass is about seven kilos This is the amount of alfalfa our dik-dik get on a daily basis, 0.5 kilos Which works out as 7% of its body weight In fact, if you map out the amount of energy animals need to eat relative to their body weight, a very clear pattern emerges that holds true for just about all creatures It’s called Kleiber’s law and it shows that as you get smaller, the amount of food you need, relative to your size, increases rapidly Or, to put that another way, the smaller you are, the hungrier you get Applied to five-centimetre-tall Joe here, Kleiber’s law tells us that he’d have to eat his own body weight every day. Most of his life would be spent looking for, and eating food Just like the Etruscan shrew But what if we went smaller still? So, if you were to take a mammal and, um, to make it smaller than the smallest current mammal, then it would cease to be a mammal as we know it, because the rate at which it would lose heat into the environment would be so great that it couldn’t maintain its internal temperature to be warm-blooded, and so it would have to change its physiology, it would have to become cold-blooded and use different strategies in order to regulate its temperature Going smaller means saying goodbye to being a mammal From here on in, we’ll need to be cold-blooded with organs more like an insect. But it’s worth it, because incredible things start happening once you get down to the size of a wasp At Cambridge University, they’re finding that for very small creatures, the world is a completely different place to the one we experience It’s almost as if they’re ruled by different laws of physics In terms of their relative strength, you might almost say that insects are superheroes So, some of the strongest ants can easily carry four or five times their own body weight, which, for us, is the equivalent of almost a small car, if you’re a relatively big human This is all because volume, area and length change by different amounts when you make things smaller The overall effect is to make small creatures much stronger than big ones, relatively speaking An ant supports the weight of a paintbrush, which is roughly 2.5 grams That corresponds to around 500 times its own weight, which would be the equivalent of me for 40 tonnes, which is probably about six, seven lorries So that’s really quite impressive But there’s more to being small than strength Now, it turns out that our old friend, the square-cube law, which means that small creatures lose heat more easily because of their large-surface-area-to-weight ratio, also has an upside if you find yourself falling from height A five-millimetre human falling from a countertop is equivalent to a normal-size human falling 300m So, Joe can be forgiven for thinking that it’s not going to end well But it does So, one of the very first studies that thought about the question, how size matters and what the right size for an animal is, thought about a problem of why you can drop an ant down a shaft and the ant just falls on the ground and walks away, but if you would do the same to a human, the human would break Because the ant is so small, air resistance is much more important for the ant, so the velocity with which the ant hits the ground is much slower than what would happen to a human Taken to an extreme, it’s why rock dust floats in the air, but rocks don’t, even though they’re made of the same stuff And there are even more advantages to being so small So, one of the things we’re interested in is how well insects stick, and one of the techniques we use to measure that is a centrifuge So, we take a little ant, put it on a centrifuge, and start spinning it around And we then try and measure at what acceleration these ants actually fall off the centrifuge Wow G-force, or G, is the name for the feeling you might get on a roller-coaster In the tightest turn, you might experience 6G, but even with much higher G-forces, somehow the ants hang on And during these measurements, we’ve seen ants withstand 500G, 1,000G for 10, 20, even 30 seconds,

and then they fall off, and walk off as if nothing happens It’s this stickiness that allows insects to walk up the smoothest of walls, even hang on to the ceiling So, how does it work? Insects, or geckos, or any animal that climbs with adhesive feet, they can’t use a glue, because it will take a long time to activate and deactivate So, as far as we know, climbing animals use intermolecular forces to stick to surfaces It’s still under debate what exactly these intermolecular forces are If you have two molecules and they attract each other, then you have to convince them to split apart, and that’s what helps these animals to stick We experience these intermolecular sticking forces, too, but at our normal human size we’re not even aware of them because they’re tiny compared to gravity But once more, being small changes the rules A five-millimetre human could climb a wall just like an ant But it’s not all good news for small creatures A five-millimetre human may be good at climbing, but he might not understand why he’s doing it It’s a problem that affects all very small creatures, brain size Our brains rely on our neurons and it looks like that neurons remain relatively constant in size across different animals So, whether you’re a very small animal, or a very big animal, the neurons are approximately the same size Now, this immediately means that if you’re very small, you have fewer neurons, and that might present you with a problem regarding your cognitive abilities The bottom line is, if you’re going to get really small, you’re going to lose brainpower A five-millimetre-tall Joe would only have around two million neurons, which would put his ability to do crosswords somewhere between a cockroach and a small fish He’d be smart enough to spot food, but probably not smart enough to worry about the puddle of coffee in the way For normal-sized people, surface tension is barely noticeable, but when you’re tiny, it’s suddenly deadly Surface tension is a force that becomes very, very powerful if you’re very small, and really unimportant if you’re really big So, for very small animals, a droplet of water can be very dangerous, while very large animals will hardly notice the droplet So, great – tiny humans might be able to climb walls, and carry things 100 times their own body weight, but what’s the point if you’re too stupid to tie your own shoelaces, constantly looking for food, and if the wasps don’t kill you, the coffee will? Because if that’s what being a human is, you can count me out, and I think that’s the point There is a sweet spot for human sizes that works for the way the world is now, and it’s no coincidence that we are all in it Other conditions would have provoked other sizes, because that’s how evolution works So, having established that, we should probably put Joe out of his misery. So, back to normal with you In fact, it turns out that the size we are now is a perfect fit for the way we live, and the world we live in Lifespan, health, food, society, resources, it all goes hand in hand with our size We’ve tried shrinking the planet, and even ourselves, but so far, smaller has not proved to be any more beautiful But there’s one thing we haven’t tried something so big that surely we could make it a little smaller, without ending life as we know it the sun Perhaps a smaller sun would be a good idea Our current sun is a kind of star known to astronomers as a yellow dwarf But, of course, it’s not really much of a dwarf by Earth’s standards In fact, it’s 1.4 million kilometres wide That’s 109 times wider than Earth Just to put that in perspective, flying around the world nonstop in a commercial airliner would take round about two days But if you were circumnavigating the sun nonstop, apart from the whole being burned alive thing, just to do one loop would take you six months Guess I’d better settle in, then! But does it have to be so large, or would everything work out fine with a smaller, gentler sun? One that wouldn’t damage our skin One that we could safely look at with the naked eye Well, the key to this question is understanding what makes a star shine in the first place A star is actually just a big ball of gas, which is pulled together by gravity At a certain point, the mass of the ball gets so large that superheated gas at the centre begins to fuse together

This is, of course, nuclear fusion, which generates such enormous amounts of energy that we’ve been trying to replicate the process on Earth ever since it was discovered The problem is that it’s incredibly hard to do This is one of the world’s best attempts JET – Joint European Torus. The first place on Earth they managed to achieve controlled nuclear fusion It’s a phenomenally complicated and expensive facility, which uses as much power as a small town, in very short bursts, to superheat gases until they fuse The fusion takes place inside this chamber We actually create a highly ionised gas, or plasma The centre of the plasma, which is probably roughly just above my head, would be where the temperature and the density are at the maximum The temperature there could be something in the region of between 100 and 150 million degrees centigrade Conditions inside the machine are so extreme that they can only run it for 30 seconds at a time before it becomes unstable Today they’re running the fusion test at even higher power levels than they’ve tried before Ten, nine, eight, seven, six, five, four, three, two, one, zero! SIRENS BLARING What you are looking at right now is fusion, the very same process that happens at the heart of a star The challenge of making fusion happen on Earth is far greater than the challenge of making it happen inside a star, because we can’t resort to the simple technique that stars use, being massive The main requirement, if you want to trigger fusion, anywhere in the universe, inside of stars or in a laboratory on Earth, is that you have to create conditions of very high temperature, and the problem then is that if you have high temperature, you also basically have the problem that this high-temperature ball of material wants to be pushed out by the pressure So you have to somehow overcome the pressure, and the stars do this by having all the gravity of the overlying material The gravity of the star is confining the pressure of the hot material For stars, then, size very definitely matters If there’s not enough material creating pressure to contain and superheat the gas inside, fusion can’t happen If we make stars smaller, less massive, the temperature of the star in the centre will also go down, and at some point, the temperature is not sufficient any longer to ignite nuclear fusion And this really is the fundamental limit, if you like, for stardom So, how small a dwarf still packs enough punch? Well, before we get to that Here’s a quick bit of star terminology for you Now, a star that can’t do fusion is called a brown dwarf, although it’s not always brown and technically, it’s not actually a star A red dwarf is bigger than a brown dwarf, although still small by star standards, and is red, unless, of course, it’s yellow. So that’s easy to remember! Then we have the white dwarfs Now, these are collapsed stars, the size of a planet, and they no longer do nuclear fusion So they’re very faint and should probably be called really small, dark-grey dwarfs. Finally, there’s black dwarfs Now, these are white dwarfs that have completely run out of energy, but since that takes longer than the current age of the universe to happen, they aren’t technically even possible yet And that’s just the dwarfs Don’t even get me started on the supergiants Anyway, the main thing to remember is that stars behave completely differently depending on their size, and that their names don’t always make sense Anyway, if we were changing the size of our sun, there’s a limit to how small we could go before fusion ceased In fact, the very smallest is just under one tenth of the width of our current sun ALARM So, with a sun this size, what kind of Earth would Joe wake up to? Well, for starters, he’d be seeing red The reason that this lunar star would be red, is that this star would have a much-reduced gravity, and therefore, also, it would have a much-reduced temperature It would shift the peak wavelength of your photons that the star is emitting to a longer and longer wavelength This means that we’ve shifted from the yellow, that our sun has, into the red, that those red dwarf stars would have

The smallest stars, like this one, give off much less light, and less light means less heat Now, to put that into some kind of perspective, the energy that our current sun sends us on average is equivalent to 24 60-watt light bulbs per square metre, which keeps Earth nice and toasty But if we swapped our sun for a tiny star like this one, it would only be sending the equivalent of half a glow-worm of light for every square metre And I don’t much fancy trying to stay warm huddling around half of him! Within hours Joe, would find that things were getting a bit nippy, to say the least What would happen to a planet around such a red dwarf central star? The most dramatic thing is that because of the very, very much reduced temperature, we would basically experience a deep freeze Within a week, temperatures would plummet because a star this size gives off just one six thousandth of the heat of our sun All the liquid water would be converted into ice Even our atmosphere would begin to freeze out We would enter into a state of complete cold, deep, desperate freeze All water on the planet would freeze As temperatures dropped further, the air itself would turn solid, causing the atmosphere to collapse So, how would we save the world? The answer may seem obvious, move the planet closer to the sun, so that things warm up again But would that actually solve things? Earth’s normal orbit is about 150 million kilometres from the sun, which is in the middle of what’s known as the Goldilocks Zone, the habitable belt around the sun, where it’s not too hot and not too cold With the sun ten times smaller, that zone would now be 100 times closer So that’s where we’d need to move the Earth Now Joe would be getting the same energy from the sun as he was used to, but there’d be a few changes The new sun may be much smaller, but we’d be so close to it that it would look much bigger in the sky, ten times bigger than he’s used to But other than that, would it be business as usual? What kind of a world have we made? Well, that’s actually a question that scientists have been trying to answer, because it may have huge implications for the future of our species Looking up at the night sky, you probably wouldn’t even notice Proxima Centauri, but it’s actually our nearest neighbour, the closest star to us outside of our solar system The reason you might not notice it is because it’s very small just one-seventh the size of the sun, but it’s up there if you know where to look And at Queen Mary University of London, astronomers have been looking very hard at the faint light it gives off, to see what they can discover about the sun’s tiny neighbour Proxima Centauri is the nearest star to the sun This is where astronomy begins So it’s really the first spot in the next frontier So the first place to go when we go beyond our solar system So that makes it very special In August 2016, they made an astonishing discovery by analysing the light that Proxima Centauri gives off So, basically, what we do, we go to a telescope, the telescope has an optical fibre sitting at the focus, and then the light from the star goes through the optical fibre to the basement of the observatory, where there’s a spectrometer, and what the spectrometer does, it takes the light coming from the optical fibre and these two elements here, a prism and a grating, separate the light into wavelengths And we see that there are these dark spots in the middle of the traces These are the footprints of molecules and atoms of the atmosphere of the star As they continued to observe the star, they saw that the spectrum of Proxima Centauri was changing So, we come here to the telescope two months later and we take more data and we see that the measurements start to trend Something is happening, but we don’t know what We get more measurements, more measurements, more measurements, and after two years, then we see that it reaches a peak, and then you have this signal And it’s repeating also If we keep observing the star, we see the same thing over and over again These wobbles in the spectrum reveal that Proxima Centauri is being pulled backwards and forwards

It’s the telltale sign of a planet orbiting close by The newly discovered planet was given the name Proxima b, but what got the scientists excited is that this planet has a lot of similarities to our own It is roughly Earth-sized, and mostly made of rock However, unlike Earth, which takes 365 days to go around the sun, the spectrum patterns reveal that Proxima b takes a mere 11 days, meaning it must be very close to its star In fact, the maths shows that Proxima b is also at the perfect distance for supporting life, not too hot, and not too cold So could this planet one day be a viable next step, as we branch out of our own solar system? Might life even exist there already? It’s only right with a solar system that we can expect to actually start to search for evidence of life in planets like this one in Proxima Centauri and also some very nearby stars If the locals proved to be friendly, could we one day, in the distant future, make Proxima b our new home? What can our thought experiment tell us about this newly discovered world? Well, it wouldn’t just be the red light and the huge sun in the sky that would make things look different Plants on Earth are green because they use specific wavelengths of light But if our sun was red like Proxima Centauri, those crucial wavelengths would be missing, and our green plants couldn’t survive To stand any chance of absorbing enough sunlight, they’d have to be black, so it would all start to look seriously alien And there is another surprising side effect of orbiting so close to a star that literally stops the world going round tidal locking So, basically, you have a small star, the small star makes very little energy, it’s also faint So you need to be warm, you need to be close to it, and the fact of being close to it means that you always have very strong tidal forces then, and most likely, what will happen is like what happens with the moon to the Earth, the rotation of the planet synchronised to your width of the planet, so the same side faces the star In a fully tidally-locked planet, one side is frozen in perpetual night, but the other basks in 24-hour sunlight Hence they’re sometimes called eyeball planets So, have we finally done a thought experiment Joe can at least survive? Probably not. Don’t rush out to Proxima b, because there are some big downsides to this small-star scenario Most of the planet is uninhabitable, too hot on the light side, too cold on the dark side Oh, and I should mention that close to a star there are huge levels of radiation So, at the very least, you’d have to live underground OK, that didn’t go so well after all, much like it didn’t go well expanding the sun, or making the planet bigger, or smaller, or shrinking Joe, or making him into a giant And it all goes to show just how narrow and fragile the universal balance is, that allowed us to exist in the first place If our sun, or our planet had been a different size, there’d be no us And if we were a different size, well, we just wouldn’t be us Size does matter Size determines on the one hand, you know, the height we will achieve, but on the other hand, size determines our lifespan, because size determines how much energy we invest in maintaining our body in good shape A very large animal and a very small animal are from completely different worlds. So, they face completely different problems Evolution has produced completely different solutions to these problems and for scientists, it’s really interesting to try and understand how things work at small scales and big scales How big you are determines the scale of the world around you and how you interact with it. The smaller you become, the kinds of things that are dangerous to you change But size is also a very mutable variable, it’s flexible, and mammals have found a way to live very successfully at all different sizes I think size is important in the universe It’s a clockwork, really. You have your clock and everything works perfectly, but if you change the size of one of the cogs, then it doesn’t fit with the rest any more and the whole system will collapse We set out to examine the nature of our universe by changing the size of things in it, but in doing so, we’ve also examined the very nature of size itself Size isn’t like colour or pattern, it’s not arbitrary It’s absolutely intrinsic to the nature of a thing itself Change the size of something, and what you end up with

is something else. Planets are big, insects are small, and people are somewhere in between Mess with that at your peril