All language subtitles for BBC.Size.Matters.with.Hannah.Fry.1of2.Big.Trouble.1080p.HDTV.x264.AAC.MVGroup.org.eng

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.