Original subtitles

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.