Which Universe Are We In? Horizon

There's an idea, once thought

so radical that just mentioning it was considered pure insanity.

But now, these scientists are daring to believe it's actually true.

They think that our universe is not alone.

It's just one of an infinite number of weird and wonderful worlds.

Some, where life is familiar.

Others, where things turned out a little differently.

The dinosaur-killing asteroid, that was our lucky break,

missed Earth, so there are no humans, just dinosaurs in Winchester today.

Some of these worlds are so strange that the laws of nature no

longer apply.

So these students might, for example, be going to

class in five dimensions, rather than four dimensions.

Or they might be talking about a whole different force,

the blue force, that we don't have in our universe.

In others, infinite copies of you are playing out every

possible storyline of your life.

So, I every time I flip a coin, say heads or tails, that is

just some little quantum accident.

The universe is splitting into two worlds.

It sounds like a plot stolen straight from Hollywood,

but some scientists think they've actually found the evidence

to prove the theory is true.

I was so elated and happy and couldn't believe my eyes that

I allowed myself for a few minutes to jump up and down.

And if these scientists are right, the question isn't

whether multiple universes exist, it's which one are we in?

Ever since we've been studying the night sky, we've been able to

rely on one simple idea to describe everything around us,

everything on Earth and beyond, all the planets, all the stars

and all the galaxies.

This idea is what we call "our universe".

The universe as one beautiful unique thing, the sum total of all

the stuff we can see and everything we know about.

And for a long, long time, we've been pretty happy with this idea.

It makes total sense.

But recently, a few inconvenient

scientists are finding flaws

with this long-cherished idea.

In fact, they think it's time to throw the whole notion out

the window.

For cosmologists, the universe extends to the furthest point from

which light has had time to reach us, since the beginning of time.

It's what we call the Observable Universe,

beautifully captured in this one image.

This is what we affectionately call "our universe",

this spherical region of space from which light has the time to

reach us so far, during the 13.8 billion years since our Big Bang.

You can ask - is that really everything that is or is this

just everything we can see?

And we've come a long way in cosmology to a point where we

have pretty strong evidence that the actual universe,

the whole universe, is much, much bigger than this.

It's hard to imagine how we cannot ask the question - what is

beyond the walls of this object and what was there before the Big Bang?

So, although I think this is everything that we can observe,

I don't think this is everything that exists.

So, this may really only be just

a small part of something that,

you know, is really much, much bigger.

So this universe, stretching out 13.8 billion light

years into space, is a beautiful thing, but it's not the only thing.

So I'm afraid, universe,

it's time for you to retire as the only thing out there.

You've had a good run, given us a lot of good times,

but it's time to go.

Why don't you just go down to Florida and buy a condo?

A very large condo.

Sending the universe into retirement might seem like a bad joke,

but for these scientists,

the idea of just one universe simply doesn't make sense.

They are convinced that for different reasons,

our universe is just one of an infinite number of others.

One universe in a vast, vast multiverse.

# If I was a flower growing wild and free

# All I'd want is you to be my sweet honey bee

# And if I was a tree growing tall and green

# All I'd want is... #

The maths is devilishly complicated, but they stem from questions

so simple, a child could ask them.

So, where does the universe really end?

Max Tegmark is a professor of cosmology.

When he isn't playing Lego, he spends his time contemplating

some of the big questions about life, the universe and everything.

# All I'd want is you to shade me and be my leaves... #

And there's one particular question that's been bothering him.

Is there an end to space? Or does it go on for ever?

When I was a little kid,

I used to wonder whether space went on for ever and I used to think

- it has to be infinite, because it would be silly for it to have an end.

Would there be a sign there, saying 'Warning, space ends here.

'Mind the gap'? And if so, what's on the other side?

So we don't have a shred of evidence suggesting that space actually

ends here, exactly at the edge of what we can see

and I don't have a single colleague in physics either who believes that.

It would be a little bit like believing if you're

in the boat in the ocean, that the ocean ends exactly at your horizon.

Why should it?

The idea that space goes on for ever seems simple enough.

But this relatively straightforward concept has profound implications.

Just as this house is made out of fundamental building

blocks that we call Legos, everything in our world is

made of fundamental building blocks we call elementary particles.

And if you have some random process,

arranging elementary building blocks in a finite volume, there

are of course very many different ways in which it could do this.

And that means that if this process repeats,

and an infinite number of other volumes of the same size,

then we're guaranteed that eventually,

it's going to create every possible arrangement.

According to Max, and the hard and fast laws of probability,

our universe is one of an infinite number of others,

each one about 90 billion light years across

and each containing a finite number of particles.

And just like Max, if you assembly these Lego bricks enough times,

you'll create every possible variation of them,

eventually ending up with two model houses exactly the same.

Likewise, rearrange the particles in the universe often enough

and you end up with an identical universe and an identical Earth.

And even a Max over there who is identical to me, not just

in his physical appearance, but in that he actually feels that he is me.

So, the answer to Max's question of what's at the edge of space

leads unavoidably to a world where other universes are not only

likely, but are a mathematical certainty.

But there's another idea that questions our unique

place in the cosmos.

This time, it's based not on a question of where space ends,

but rather, how did it all begin?

Professor Anthony Aguirre has been grappling with the sticky

matter of the origins of our universe.

And his attempts to find answers lead to a completely

different sort of multiverse.

This picture is actually something pretty amazing.

It's a picture of our observable universe,

just a couple of hundred thousand years after what

we call the Big Bang and it's a picture that's been

taken in what's called the cosmic microwave background radiation.

This is radiation that's come to us to telescopes like this one

and many others since and it gives us an actual image

of what the state of the universe was like at incredibly early times.

The image, which depicts both the edge of the universe as well as the

earliest light we can see, revealed that all the matter in the universe,

all the stars and all the galaxies, were very evenly distributed.

It suggested something happened to make it that way.

And that something is a process called inflation.

The theory of inflation is that early on,

the universe didn't just expand, but it expanded exponentially,

meaning it doubled in size over and over again in a very small

fraction of a second.

Now, what this did was it took a pattern of variations

in the density of the universe, the same pattern we see now, and it took

it from a tiny size and stretched it over the entire observable universe.

According to inflation, while our universe was just a hot

pool of fire, the very fabric of space inflated.

It was so rapid that that the uniformity of the baby

universe was preserved.

But for Anthony, inflation was more than just a method of expansion.

It was a driving force that created our universe in the first place and

if it could happen once, there was nothing to stop it happening again.

And again.

And again.

This is eternal inflation.

So inflation was a little bit like a genie that you let out of a bottle.

You open the bottle and you ask the genie, "Make me a universe,"

and the genie does a spectacular job of it,

but then the genie says, "Well, I'm going to make another universe."

"Wait a minute, I just wanted one."

"Nah, I'm going to make ten more universes." "No, I just wanted one."

"I'm going to make an infinite number of universes."

That's what we're talking about with eternal inflation.

Once the genie gets out of the bottle, it just never stops.

So, asking two simple questions have, for different reasons,

led to the same conclusion.

What we see when we look up at the night sky is just a tiny

fraction of the story of our existence.

However, things get even stranger

when you consider the hardest question of all.

How does the universe actually work?

Professor Seth Lloyd resides in the totally weird world of quantum

physics, where nothing is quite as it seems.

And where things can be in two places at the same time.

HE CHUCKLES

The important thing to remember about quantum mechanics is it's weird.

So, stop, stop, stop, stop!

I don't understand that,

but I console myself with the fact that nobody understands that.

It was from an attempt to make sense of this strange quantum

world that the idea of many universes was born.

It all began in the 1950s,

when maverick genius Hugh Everett tried to explain weird

phenomena at the heart of the now infamous Double Slit experiment,

where electrons can be waves and particles at the same time.

The famous Double Slit experiment in quantum mechanics where

a beam of electrons go through space and then they go through two slits.

Now, the wave goes through both slits at once and on the far side,

the wave interferes with itself and then hits a screen

and makes an interference pattern. You might say - come on!

There's lots of electrons.

Like some of the electrons have waves, big deal.

But in fact, if you attenuate this beam of electrons,

so there's only one electron going through at a time,

you still see this interference pattern, even though

there's only one electron,

so the wave for one electron goes through both slits at once.

Ends up on the screen, interferes and makes this pattern.

In the experiment,

when single electrons are fired through two slits,

you'd expect them to create two vertical stripes on the screen

behind, but in fact, they mysteriously create three.

The pattern is only possible

if the individual electrons behave as waves,

passing through both slits at the same time.

It's completely counterintuitive

and simply doesn't make sense.

The trouble is it seems to be true.

It's a problem that even the finest minds in physics have battled with.

Actually, there's a lot of resistance to quantum mechanics.

The most famous resistor of quantum mechanics was Einstein,

who famously got his Nobel prize for work he did on quantum mechanics,

but he nonetheless didn't like it, "God doesn't play dice," he said.

But he was wrong. Suck it up, Albert!

And like Einstein, Hugh Everett was also unhappy with the existing

interpretation of the experiment.

And so, he came up with a radical new theory.

In the mid 20th century, Hugh Everett came up with what

he originally called the Many Worlds theory of quantum mechanics.

So, the idea here is that

when you make a measurement of a particle that's here

and there at the same time, and you find the particle over here,

then there's a you which finds the particle over here in this world,

but at the same time, there's another world over there where another

you has found the particle over there.

And both of these worlds are equally real.

Hugh Everett's big idea was that at the point when the particle can go

through one slit or the other, the universe literally splits in two.

The particle goes through both slits at once,

but it does so in two separate universes.

It was both ingenious and terrifying,

and at the time, it seemed totally bonkers.

Despite the fact that now it really is a widely accepted

theory of quantum mechanics, at the time, it got a very frosty reception.

And he couldn't get a position in physics.

Everett's extreme idea set him at loggerheads

with the establishment, and sadly, he died

before ever receiving the recognition he deserved.

But in recent years, there's been a remarkable turn-around.

Everett's idea of many universes,

bizarre and counter-intuitive as it seems,

is now considered by many to be the only way to explain

how the world really works.

Everybody's intuition about quantum mechanics is wrong

and so if you're going to demand that your intuition be right,

you're just going to be unhappy. On the other hand,

if you can just accept that your intuition is wrong, you know,

grab your quantum surfboard and surf that quantum wave,

then life can be good.

In spite of the weight of evidence now pointing towards a multiverse,

until very recently, anyone dabbling in this field risked career suicide.

I couldn't get a job to save my life.

When I was a grad student,

I used to secretly print out my multiverse related papers

when my adviser was far from the laser printer

and I didn't even show these papers to him

until after he'd signed my dissertation

because it was considered mostly science fiction

and speculation back then.

This job at MIT was the only job I was ever interviewed for.

I was on the verge of having to drive a taxi cab.

HE CHUCKLES

Three entirely different questions have all led to the same conclusion,

the multiverse is now impossible to ignore.

Beyond the realm of our most powerful telescopes,

Max believes infinite universes to be a mathematical certainty.

He thinks the universe simply cannot end...

..while for Anthony, the quest to understand our origins

provides a tantalising glimpse of a time before,

when inflation brought countless other universes into existence.

And, as if that wasn't enough, Seth's strange quantum world

suggests the universe splitting into multiple others all around us.

The notion of one universe is clearly resting

on perilously shaky foundations.

It's a dramatic turn of events that could fundamentally change

the way we view ourselves for ever.

But while these scientists might agree that the universe needs

to be retired, deciding what the multiverse actually looks like

is an entirely different matter.

The first and perhaps most straightforward model is

the infinite patchwork multiverse.

Arguably, it's the least controversial idea,

but nonetheless, it has some pretty astounding consequences.

Even if the multiverse, all of space, is infinite,

the part of space that we can observe,

our universe, is finite, with a finite amount of stuff.

Only about 10 to the power of 80 atoms,

which can only be arranged in a finite number of ways.

So if you start considering all the other regions of space,

if you roll the dice infinitely many times,

eventually, we are guaranteed to find an identical copy

of our whole universe, as well as countless variations

where things are similar to here but still different.

The infinite multiverse is a bit like an endless patchwork quilt.

Each patch is another universe, the same size as our own,

each one containing a finite number of particles,

each with its own configuration that forms a universe.

So what might these other universes be like?

We know exactly what our universe looks like, the familiar everyday.

And yet another one,

the dinosaur-killing asteroid that was our lucky break

missed Earth, so there are no humans,

just dinosaurs in Winchester today.

There is another one where the powers that be decided

to film this interview not here in Winchester

but at Niagara Falls,

and yet another one, I didn't make it as a physicist,

but I'm actually enjoying life working as a bartender.

Then there's a whole bunch which are very similar

where I just chose to dress a little bit differently.

In this multiverse, every single possibility is played out somewhere.

There are infinite copies of Earth, some familiar,

others where history took an entirely different course.

One where, actually, Germans won World War II

and wir reden alle Deutsch jetzt,

and finally, if we go a bit over a googolplex meters away, where -

a googolplex is 1 with a googol zeroes,

and a googol is 1 with 100 zeroes -

then we come to a universe that looks exactly like this one.

It sounds like fantasy, but this is exactly what the maths predicts.

Somewhere right now, you're being attacked by aliens.

Another you has just won Olympic gold.

In one world you're behind bars,

in another you've just been elected president.

The possibilities are only limited by your imagination.

An infinite multiverse with infinite copies of you

is probably more than enough to be getting on with.

But this is just the first stop on the magical multiverse tour.

Anthony has a very different vision of the multiverse.

It's a place of even more mind-bending diversity,

where not even the laws of nature are the same.

Imagine this lake is that inflationary substance

that existed before our Big Bang.

But this medium has the property that it inflates the universe,

it doubles its volume in size every tiny fraction of a second.

That's the inflating background, but then within that background,

bubbles can form.

And although these bubbles start small, they grow.

They grow, in fact, infinitely big, and so within one of these bubbles

could reside our entire observable universe and even a whole lot more.

As this process goes on, you end up with a huge

and infinite, even, number of these bubbles.

Some could be our observable universe,

some could be other universes with potentially different properties.

And it's these different properties

that mark this multiverse out from the rest.

Unlike the others, in the inflationary multiverse,

the laws of physics vary from one universe to the next,

making it a very strange place indeed.

What's fascinating about this sort of multiverse is that

these universes could have incredibly diverse properties.

Some of these might be like our universe.

They might have very low energy,

they might have electromagnetic force,

they might have about the same strength of force as we have.

Atoms, planets, stars, galaxies,

everything we see could be in some of these other bubbles as well.

But they could be radically different.

For example, some of the bubbles

might not have an electromagnetic force.

Some of them might have an electromagnetic force

but it's much stronger than ours and atoms wouldn't exist

because they would all collapse or explode.

In these other universes,

there might be physics students taking physics class

but as well as learning different things in history class,

they learn different things in physics class

than physics students in our universe.

So these, these students might for example be going to class

in five dimensions rather than four dimensions,

or they might be talking about a whole different force,

the blue force, that we don't have in our universe.

They might not have atoms.

They might have other strange collections of quarks

that combine in some strange way and create more complicated forms

that can form life in physics students.

There might be... boy, I haven't thought about this very much.

I'm just making it up!

HE LAUGHS

Infinite bubble universes bobbing around

on an inflationary sea where the laws of physics run riot.

Maybe it's making you feel like

you've consumed too much bubbly yourself.

But this is nothing compared to life in the quantum multiverse.

In a world where things can be in two places at once,

there are multiverses lurking right under your nose.

Great!

Mmm.

Pan-fried dumplings, my favourite.

The quantum multiverse could be all around us,

but we can't see it, because it operates

according to the utterly bizarre laws of quantum mechanics.

So the quantum multiverse is not separated from us by distance.

It's not some place very, very far away.

It's some place that's effectively here in space,

but the complexity of the quantum dynamics

prevents these different branches from talking to each other.

The quantum multiverse comes from Hugh Everett's idea

that every single event that can happen does happen.

They just take place in parallel worlds.

It's like an infinite garden of endlessly forking paths.

How do we make sense of this wacky quantum multiverse

in which all possibilities exist simultaneously?

So every time I flip a coin, say heads or tails,

that is just some little quantum accident.

The universe is splitting into two worlds every time it comes up

heads or tails, so our experience of this splitting

is like the experience of walking through a garden of paths that fork.

When we come to a fork in the path, we take one or the other,

heads or tails.

But both forks exist at the same time.

We only experience one of them.

Every time I flip a coin, the universe splits into two worlds.

Heads I stick around, tails I'm out of here.

OK, it's heads, I stick around.

So in that other universe, where I got tails, the interview is over.

Sorry, lady. HE CHUCKLES

To understand what this might be like,

imagine Seth's next few hours

determined by the toss of a coin alone.

I'd had enough of that interview. Let's see.

I flip the coin again, heads I go left, tails I go right.

Already, the universe has split into two,

a Seth in a restaurant and another Seth wandering the streets.

Maybe I should take a short cut over to the Bowery

through this little alleyway here. Should I do it? Looks a little dark.

Let's let the coin decide.

Another flip and the universe splits again.

In this one, Seth makes the fateful decision to walk down the alleyway.

Hey!

Hey, hey, hey, hey!

But in another universe,

the Seth who didn't take the short cut is safely taking a taxi home.

So we can continue, so the me that went left went home,

back to the hotel, good night's sleep.

The me that went right said, "Let's see what Chinatown has to offer.

"Here's an arcade. Shall I go in or not?"

Another split, and one Seth decides to try his luck.

MACHINE PINGS

Yes, yes, yes!

The other Seth chooses to walk home,

which results in the great New York coffee disaster.

After a few hours, multiple Seths occupy multiple universes.

Each one irrevocably separated, existing in their own reality.

Quantum accident, like setting down this alleyway...

Each with its own independent future.

This bewildering quantum multiverse

is what's known as the Hilbert space.

It's a place of endlessly forking paths and parallel realities.

A place where every version of every event

for every living organism on Earth is happening, somewhere.

The number of possibilities is growing exponentially,

doubling every time I flip a coin.

There are literally gajillions of universes out there

of which ours is only one.

Actually, I calculated one day, that if you look at the total

number of bits there could possibly be in the universe,

so let's ask how many quantum coins could have been tossed since

the universe began 13.8 billion years ago, a simple calculation

tells you that the maximum number is ten to the 120.

There have been ten to the 120 quantum coin tosses,

which means that they're two to the ten to the 120

different possible universes.

It's a large number but it's finite.

I just told you what it was.

It's not infinite.

The quantum multiverse feels like something

straight out of a science fiction story.

But, for some physicists, it's an inescapable reality.

Whether you like it or not, the fact that we live in a multiverse

is the dominant scientific paradigm.

Suck it up, if I may say so again!

This magical multiverse tour

has taken us to some weird and wonderful places indeed.

We've journeyed across an infinite multiverse quilt

and dived into a giant inflating sea,

and as if that wasn't enough,

we've wandered through a thoroughly dizzying quantum maze.

So, if you were starting to feel like you'd fallen down a rabbit hole

into Wonderland, it's worth pausing for a moment to ask...

Which theory is right?

And can anyone actually prove it?

For its most vociferous critics, the multiverse is unscientific

because it can't be tested...

..even in principle.

So, without proof, doesn't this make the whole idea of the multiverse

simply a waste of time?

Well, not necessarily.

Exciting new discoveries

now mean that evidence for the multiverse

might not be as far away as we think.

In fact, one scientist thinks she may have already found it.

# She's a true original... #

Professor Laura Mersini-Houghton has a radically new vision

of the multiverse.

It's bold and daring and, even by the standards of the multiverse,

it's considered highly controversial.

Truth goes through three stages.

First it's ridicule,

then opposed strongly,

and finally, it becomes self-evident.

Perhaps now we are reaching the stage of self-evident.

Laura's major breakthrough was to take two big ideas

and put them together.

She combined the physics of string theory

with those of quantum mechanics.

The mathematics involved is fiendishly complicated,

and Laura is probably one of just a handful of people

who can even begin to comprehend it.

But, for us mere mortals,

one way to picture it is as a landscape and a wave.

Before our universe went through the Big Bang,

we can think of the pre-Big Bang era as a space which is abstract,

it's an energy space, and various places on this landscape,

on this energy field can produce different universes.

We can think of these waves leaping over the rocks as the wave function

of the universe, trying to travel through this landscape structure.

If I think of the rocks as the energy field,

and each pocket on these rocks representing an energy valley

on the landscape, as the waves come through,

many of them will be trapped in different pockets,

rather than travel any further.

Each of these little pockets can be a potential birthplace

for a universe similar to ours.

Laura's idea was to represent space

a bit like a mountain range of different energies.

She thinks that our universe started out as a wave.

As it crossed the landscape, some energy got trapped,

creating different universes with different properties.

It is without doubt a radical notion...

..but Laura also predicted a series of anomalies,

which she believed could actually be observed

in our own night sky.

One of these would even reveal how our universe

was once entangled with another, through a process called cross-talk.

These universes are not only producing space time,

but they are also separating from one another.

Each one of those is taking its own identity.

However, traces of that infancy,

of that cross-talk between all the surviving universes

and the landscape structure,

those are imprinted forever in our sky

because, after all, what we look at today in the sky

is just a blown-up version, a re-scaled version,

of what once was in our universe when it was in its infancy.

Laura predicted that this cross-talk would leave an imprint

on our early universe, a bit like a birthmark,

and we could see this as a cold spot in the cosmic microwave background.

The detection of the cold spot was one of the signatures

that we predicted by tracing forward the quantum entanglement

of our universe with all the other surviving universes.

We predicted that there should be a large area in the sky

of about ten degrees, and indeed that's what was observed

about seven months after we made the prediction.

Remarkably, all Laura's predictions have since been observed,

including this cold spot,

which she claims is a trace of another universe

once entangled with our own.

It's a discovery beyond anything she dared hope for.

That felt incredibly good.

It was unbelievable.

I really got excited, and allowed myself for a moment

to think that there might be something more to this idea,

and when I heard the list of anomalies,

I was so elated and happy

and couldn't believe my eyes,

that I allowed myself for a few minutes to jump up and down.

I was jumping on the balcony.

I wonder what the neighbours thought?!

# She's a true original... #

It's a thrilling thought, that somewhere up there in our own sky

could be a clue to the presence of another universe.

Laura's ideas are considered radical

and she's yet to convince many of her critics,

but it's a major breakthrough for an idea most people dismissed.

Then, in 2014,

scientists claim to have made another important discovery.

NEWS REPORT: Space scientists hail a great advance,

claiming the first direct evidence

of what happened in the first moments of the universe.

An international team of leading space scientists say they've

found the first direct evidence of how the universe was born.

Scientists had been scanning the sky,

looking for evidence of gravitational waves.

The news that they thought they had found them caused a sensation,

because, if confirmed, it offered yet more tantalising clues

to the existence of the multiverse.

So this has been a really exciting time

to be studying the theory of inflation

because inflation predicts that there would be

gravitational waves formed during the inflationary process,

these are ripples in space time.

It turns out that those gravitational waves

leave a telltale signature

in a pattern on the microwave background radiation,

and that signature has been searched for for a long time

because it's a prediction of inflation.

The data in 2014 turned out to be a false alarm.

But the theory is solid.

The idea is that the violent nature of inflation

created gravitational waves.

These waves would have warped the fabric of space

and produced a particular pattern of ripples in the cosmic

microwave background, enabling us to observe them, even today.

The hunt is still on for evidence of gravitational waves.

But, if they're discovered, it would be a huge leap forward

for the idea of the multiverse.

Gravitational waves, I think,

makes the multiverse more likely in two ways.

The first is that the multiverse is a prediction really of inflation,

and so because this makes inflation more likely,

it then makes the multiverse more likely.

Second, I would say that the sorts of models,

the simple models that people have been thinking about for 30 years

that give rise to eternal inflation, are precisely the sort of models

that are compatible with these theories.

Scientists are still looking for proof of gravitational waves,

but if found, it will be a major step forward in cosmology.

So, this really makes me feel excited to be a cosmologist

and be alive and working now.

This is a spectacular time

that we're never going to have in human history again.

We've learned what the history of the universe is,

and now we're learning where the universe came from,

how was it born, were there other universes born?

And we're actually making progress.

We're not just talking, we're learning real things,

and we've come incredibly far in that quest.

These new ideas have given great support

to theories of the multiverse,

and to those critics that suggest the idea of the multiverse

is a waste of time, it's given them something to think about too.

While some scientists are looking for evidence

for the multiverse in the distant regions of space,

others believe our best hope of detecting the multiverse

might lie much closer to home in the dizzying world of quantum computing,

where today this team at MIT have switched on their latest machine.

The type of evidence we can get for the quantum multiverse

is much more immediate than that that we get

for the inflationary multiverse, for instance.

So, for the inflationary multiverse

we're never going to actually have access to these other worlds,

so we're just going to have to believe in inflation

and thereby believe that these exist.

But, we could indeed do some quantum virtual reality experiment

which demonstrated the existence of the quantum multiverse,

at least in the context of virtual reality.

In the quantum multiverse,

each universe that splits apart is permanently severed,

but quantum computers might have an extraordinary ability

to access them.

So, a quantum computer is like a regular computer,

but really, really small.

That is to say, the bits of the computer,

so the places where you store information,

are individual atoms,

or individual elementary particles, like photons or electrons.

An ordinary digital computer processes information

by busting up the information to its smallest pieces

which are called bits, so a bit is a small chunk of information.

It only has two possibilities which are usually called zero or one,

but they could also be yes or no, heads or tails, true or false.

So, a quantum computer also divides up information into bits,

but now they're quantum bits,

so if I'm an electron that can be over here,

we call that zero, or over here we call that one,

so this quantum bit or Q-bit,

in some weird funky quantum way that nobody really understands

registers zero and one simultaneously.

And this gives quantum computers a power

that ordinary classical computers don't have.

Because quantum computers have the ability

to operate in many states at once,

performing a staggering number of calculations at the same time,

some believe these calculations are taking place in parallel worlds.

Some people believe that quantum computers already prove

the existence of the multiverse.

They say that the quantum computer

is doing all these different computations in different worlds,

and then to get the answer to the question, it recombines all this

information from these different worlds to give you the answer.

And there's one famous maths problem

that if a quantum computer could solve,

could one day prove the existence of the quantum multiverse.

The killer app for quantum computers is factoring large numbers,

so factoring is taking a number and dividing it up into its factors

so four is two times two,

15 is three times five,

21 is three times seven.

Now that's pretty easy to do,

and you and I can factor small numbers pretty easily,

but when the numbers get very, very large, so I have a 512 digit number

that's a product of two 256 digit numbers, it starts to get very hard.

And, in fact, there's no known method on a classical computer

to factor such numbers without taking a very, very large amount of time,

maybe the age of the universe.

However, a very small quantum computer with only a few

tens of thousands of quantum bits could, in fact,

factor a 512 digit number very rapidly,

and how does it do that?

It basically breaks up all the possible ways

of factoring the number,

and then all of these different parts get tried out together,

and then recombined to give you the answer to the problem,

to give you the factors of the number.

These colossal calculations would take a classical computer

more than the age of the universe to arrive at the correct answer,

but a quantum computer could, in theory,

tap into the multiverse,

doing all the calculations in different universes

at the same time.

We're on the verge of a quantum computing revolution.

The fact that these engineers might be building the first

machines to access these hidden worlds is a spine-tingling thought.

I am excited.

I am, now that the thing is up and running as of this morning,

I am going to come up with all kinds of fiendish

and nefarious uses for this beautiful quantum computer.

Wa-ha-ha!

The multiverse is, admittedly, more than a little bewildering.

A dizzying array of inflating bubbles,

split personalities,

and undulating landscapes.

But, if you're struggling to choose

which multiverse model you are actually in,

you might not have to

because we could exist in all of them at once.

Max believes all the multiverses could happily live together...

..in one extraordinary, humungous mathematical multiverse.

The idea that all of this is completely mathematical

sounds pretty nutty at first since it doesn't seem mathematical at all,

but actually, when we look closer, there is mathematics everywhere.

Here we see some beautiful mathematical circles forming,

and whose spreading over the water is perfectly described

by a mathematical equation called the wave equation.

This tree here doesn't look very mathematical at first sight,

but if we look really closely at this bark,

it's made of cells, that are made of molecules,

that are made of atoms,

that are made of quarks and electrons,

and as far as we can tell,

neither the electrons nor the quarks,

nor any of the other particles that make up all of this

have any properties at all, except for mathematical properties.

Understanding how our physical world of stuff can come from

something as intangible as maths, is hard to get your head round.

But we experience worlds of numbers all the time.

Hi, my name is Shawn Robertson.

I'm the animation director at Irrational Games,

and I'd like to introduce you to Elizabeth.

She's curious about the world, I mean, you know,

from a personality standpoint, so she wants to touch everything, she

wants to take a look at everything, she wants to see everything.

Elizabeth's world is a representation of our own,

but look closer and it all boils down to sets of numbers.

So, here we're looking deep under the hood at just one of the objects

that's in the game. So the initial, look at it, it looks

fairly realistic. It's got shape to it, it's got textures to it.

But if you look at what this is really built out of at its barest

level, it's just a bunch of vertices and polygons and, you know, we can

go in and we can manipulate these as artists and really change everything

that's, you know, that the end user is going to see in the game.

The only properties left here that this object has is a bunch of

numbers, an X, Y and a Z coordinate here for each of your points.

It's like you can put in some more numbers here

and they would specify colour and texture

and so on, but ultimately both this world and our physical world

seems really at the bedrock level of description to be just numbers.

Max believes our world, like Elizabeth's, is also made of maths.

Maths is as real as the ground under your feet.

And he also believes that equations are the foundations

on which the multiverse is built.

If we accept that our physical reality is actually

a mathematical structure and nothing more, then we have to accept

that any mathematical structure is a physical reality and nothing less,

which makes me wonder, what about all these other mathematical

structures that mathematicians have discovered and classified, and can

make a whole atlas containing one after the other, what about them?

This is Max's mathematical multiverse,

home to all the other theories.

These weird and wacky worlds have all been given life

by the equations that describe them.

Every equation is a multiverse in its own right,

and each is a part of one giant mathematical structure.

By his own admission, it's a radical idea.

Whenever in the history of physics someone puts forward an idea

which sounds kind of radical,

a lot of other people are going to jump up and say, "Oh, this is nuts,

"this is crazy, I can't believe our world is so weird,"

but our job as scientists is not to tell our world how to be to conform

with our aesthetic preconceptions for how...

For how it ought to behave.

Our job is to simply follow the evidence wherever it leads us

and try to, with an open mind, determine how our world actually is.

And I think it's abundantly clear already from the history of physics

that the world is vastly stranger than we ever thought it was.

So, on another world in another universe, could there be

another Hugh Everett who is finally getting the recognition he deserves?

One who is witnessing how the ideas once dismissed as the ramblings

of a crazy eccentric are now part of the scientific mainstream?

There are still many more questions for future generations to answer...

but these ideas, and the people advocating them,

are bringing about a scientific revolution.

It's a truly thrilling place to be.

Now we've come to a remarkable time where observational

evidence about the universe combined with our theories of what's going on

at the most microscopic quantum mechanical level

give very strong evidence for the existence of these other worlds

and these other universes in the multiverse.

We used to think we were unique.

We believed that the Earth, our home,

was at the centre of the universe.

But little by little, we've been forced to change

our perspective, learning that we are just one planet revolving

around the sun, which is just one star amongst countless others.

Now it seems we may have to give up the long-cherished notions

of the universe altogether,

accepting instead that it could be just one

of an infinite number of others,

just a humble part of a truly infinite multiverse.

When I think of our universe as a humble member in the vastness,

I can only marvel at the beauty of nature.

I am not surprised because, of course, nature always has new

and beautiful surprises in store for us that are out there to be

discovered, but I also feel very fortunate that I am

at the right time and place where I can take part in that discovery,

even if it's just one small step beyond what's already known.

And there is an awe inspired by that beauty,

complexity, and yet simplicity of nature.

Some people feel that when they think about

how big even the observable universe is, it makes

them feel tiny and insignificant and that's right, physically -

we're just really specks -

but mentally, when you think of our understanding of the universe, that

we have been able to conceive of the laws that govern the evolution

and creation and complexity of the universe, we're huge.

We're giants in that sense,

and I feel excited to be a part of that process and it makes the

universe, the interior universe, the mental universe, feel vast to me.

Some people don't like multiverses cos they say it makes them

feel insignificant, but I think it's actually good news,

because we humans have again and again and again underestimated

not only the size of our cosmos, realising that everything

we thought existed was just a small part of a much grander structure -

a planet, a solar system, a galaxy, a universe and a whole hierarchy

of parallel universes - but we also repeatedly underestimated the power

of our human minds to understand our cosmos and that's a wonderfully

empowering thing, which shouldn't make us feel insignificant at all.