Nova (1974–…): Season 38, Episode 24 - The Fabric of the Cosmos: Universe or Multiverse? - full transcript

Hard as it is to swallow, cutting-edge theories are suggesting that our universe may not be the only universe. Instead, it may be just one of an infinite number of worlds that make up the multiverse. In this show, Brian Greene takes us on a tour of this brave new theory at the frontier of physics, explaining why scientists believe it's true and showing what some of these alternate realities might be like.

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---
Lying just beneath
everyday reality

is a breathtaking world,

where much of what we perceive
about the universe is wrong.

Physicist and best-selling
author Brian Greene takes you

on a journey that bends the
rules of human experience.

BRIAN GREENE:
Why don't we ever see events
unfold in reverse order?

According to the laws
of physics, this can happen.

It's a world
that comes to light

as we probe the most extreme
realms of the cosmos,

from black holes
to the Big Bang

to the very heart
of matter itself.



I'm going to have
what he's having.

Here, empty space teems
with ferocious activity.

The three-dimensional world
may be just an illusion,

and there's no distinction

between past, present
and future.

GREENE:
But how could this be?

How could we be so wrong
about something so familiar?

Does it bother us?

Absolutely.

There's no principle

built into the laws of nature

that say that theoretical
physicists have to be happy.

It's a game-changing
perspective

that opens up a new world
of possibilities.



Coming up...

What if new universes
were born all the time...

MAN:
In this picture, the Big Bang
is not a unique event.

...and ours was one of numerous
parallel realities?

GREENE:
Somewhere there's a duplicate
of you and me

and everyone else.

Are we in a universe
or a multiverse?

Right now on NOVA.

Major funding for NOVA is
provided by the following:

And...

And by the Corporation
for Public Broadcasting

and by contributions
to your PBS station from:

Major funding
for "The Fabric of the Cosmos"

is provided by
the National Science Foundation.

And...

Supporting original research
and public understanding

of science, technology,
engineering and mathematics.

Additional funding
is provided by...

And the George D. Smith Fund.

BRIAN GREENE:
New York City.

They say there's
nowhere else like it--

home to eight million people,
countless structures, monuments

and landmarks,
every one of them unique.

Or so we think.

Uniqueness is an idea
so familiar,

we never even question it.

Experience tells us people
and objects are one of a kind.

Why else would we visit museums
and collect great masterpieces?

Yet a new picture of the cosmos

is coming to light
in which nothing is unique.

Not that the world's great
masterpieces are fakes.

Instead, I'm talking about
something far more profound:

a new picture of the cosmos

that challenges the very notion
of uniqueness,

one in which duplicates
are inevitable.

And that's just the beginning.

There might be duplicates
not just of objects,

but of you and me
and everyone else.

But if this new picture
is right,

where are these duplicates

and why haven't
we ever seen them?

The answer may lie
outside our universe.

There was a time
when the word "universe"

meant "all there is,"
everything.

The notion of more
than one universe,

more than one "everything,"
seemed impossible.

But perhaps if we could go
beyond our solar system,

beyond the Milky Way, even
beyond other distant galaxies,

past the end of the
observable universe,

we'll find that there's more,
a lot more,

that our universe is not alone.

There may be other universes.

In fact, there might be new ones
being born all the time.

We may actually live

in an expanding sea
of multiplying universes:

a "multiverse."

If we could visit
these other universes,

we'd find that some might have
basic properties of nature

so foreign that matter as we
know it couldn't exist.

Others might have galaxies,
stars, even a planet

that looks familiar, but with
some surprising differences.

And if there are an infinite
number of universes

in the multiverse,
somewhere there's a place

where almost everything
is identical to ours

except for the slightest
details.

Like maybe there's
another Brian Greene

who ends up in a different
line of work.

STEVEN WEINBERG:
If the multiverse
is indeed infinite,

then one is going to have to
confront a lot of possibilities

that are very hard to imagine.

There will be other places
where there will be Alan Guths

who will look and think and act
exactly like me,

as well as many where
the Alan Guths look and think

almost exactly like me, but
with some small differences.

LEONARD SUSSKIND:
Is it science?

Is it a part of metaphysics?

Is it just philosophy?

Is it religion?

Physicists tend not to ask
those questions.

They just say,
"Let's follow the logic."

And the logic seems
to lead there.

GREENE:
However unfamiliar and strange
the multiverse might seem,

a growing number of scientists
think it may be the final step

in a long line
of radical revisions

to our picture of the cosmos.

That is, there was a time
when we thought that the Earth

was at the center of the cosmos

and that everything else
revolved around us.

Then, along came scientists
like Galileo and Copernicus,

and they showed us

that it's the Sun,
not the Earth,

that's at the center
of our solar system.

And our solar system?

It's just a little neighborhood

in the outskirts
of a gigantic galaxy.

And our galaxy?

It's one of hundreds
of billions of galaxies

that make up our universe.

Now, all of these ideas
sounded outrageous

when they were first proposed,

but today, we don't even
question them.

The idea of a multiverse
may be similar.

It simply may require
a drastic change

in our cosmic perspective.

On the other hand,

some scientists think
that the multiverse

is nothing but
a dead end for physics.

ANDREAS ALBRECHT:
I'm very uncomfortable
with the multiverse.

To become solid science, it's
got a lot of growing up to do.

You know, it exists
in the same way that,

you know, angels might exist.

We have to make our bets,

and I think right now the
multiverse is a pretty good bet.

I think there's a good chance
that the multiverse is real

and that a hundred years
from now,

people might be convinced
that it's real.

GREENE:
So, where did this idea
come from,

and what's the evidence for it?

Well, several surprising
discoveries suggest

that we really may be
part of a multiverse.

The first of these discoveries

has to do with the generally
accepted theory

of the origin of our universe:
the Big Bang.

According to this theory,

our universe began
some 1 4 billion years ago

in an intensely violent
explosion.

Over billions of years, the
universe cooled and coalesced,

allowing the formation of stars,
planets and galaxies.

As a result of that explosion,

the universe is still
expanding today.

But if you could run the history
of our universe in reverse,

all the way back
to the beginning,

you'd find that the Big Bang
theory tells us nothing

about what sent everything
hurtling outward

in the first place.

GUTH:
It's called the Big Bang theory,

but the one thing that it really
says nothing about at all

is the bang itself.

It says nothing
about what banged,

why it banged, or what
happened before it banged.

GREENE:
So, what fueled
that violent explosion?

What force could have driven
everything apart?

The quest to figure that out
would bring scientists

face to face
with the multiverse.

One physicist whose work

unexpectedly helped
lay the foundation

for the multiverse idea
is Alan Guth.

Today he's a professor at MIT.

But back in 1 979, Guth
and a colleague, Henry Tye,

were pursuing a new idea about
how particles might have formed

in the early universe.

GUTH:
Henry suggested to me
that we should maybe look

at whether or not this new
process that we were thinking of

would influence the expansion
rate of the universe.

GREENE:
Guth and Tye hadn't set out
to investigate

the expansion rate
of the universe

in the first moments
after the Big Bang.

But Henry Tye's question

caused Guth to review their
calculations one more time.

GUTH:
I stayed up quite late
that night

and went over the calculations
very carefully

trying to make sure everything
was correct.

GREENE:
As the night wore on,

Guth discovered something
extraordinary in the equations

descrmight have formedticles

in the early universe.

GUTH:
I came to the shocking
conclusion

that these new-fangled
particle theories

would have a tremendous effect

on the expansion rate
of the universe.

The kind of process
Henry and I were talking about

would drive the universe

into a period of incredibly
rapid exponential expansion.

GREENE:
What Guth found in the math

was evidence that
in the extreme environment

of the very early universe,
gravity can act in reverse.

Instead of pulling things
together,

this "repulsive gravity" would
repel everything around it,

causing a huge expansion.

GUTH:
I immediately became
very excited about it

and scribbled out the
calculation in my notebook,

and then at the end I wrote
"spectacular realization"

with a double box around it,

because I realized
that if it was right,

it could be very important.

GREENE:
By discovering
this "repulsive gravity,"

Alan Guth had unintentionally
shed light

on the very beginning
of the Big Bang.

Described mathematically,
this force was so powerful,

it could take a bit of space
as tiny as a molecule

and blow it up to the size
of the Milky Way galaxy

in less than a billionth
of a billionth of a billionth

of a blink of an eye.

After this incredibly short
outward burst,

space would continue to expand
more slowly and cool,

allowing stars
and galaxies to form,

just as they do
in the Big Bang theory.

Guth called this short burst
of expansion "inflation,"

and he believed it explained

what set the universe expanding
in the first place.

The powerful, repulsive
gravity of inflation

was the "bang" in the Big Bang.

But despite having made
a momentous breakthrough,

Alan Guth had an even more
pressing concern.

I had no idea
what my employment might be.

I was really looking
for a more permanent job.

The inflationary universe
scenario looks very exciting.

GUTH:
So I went on actually
a pretty long trip,

giving talks about this.

WEINBERG:
Suddenly this idea caught on.

ALBRECHT:
Talks about inflation
were packed with people

from all areas of physics.

WEINBERG:
Lots of astrophysical theorists,

including me,
got very enthusiastic.

ALBRECHT:
It was a very,
very exciting time.

WEINBERG:
If you have a really good idea

that allows other people
to move the field forward,

people are going
to pay attention.

GUTH:
An amazing feeling

that suddenly I had crossed
that gap

from being an unknown post-doc

to being one of
the major players,

and it was very hard to absorb,
but it certainly felt good.

GREENE:
One reason inflation
was so exciting

was that it made predictions

that could be tested
through observation.

Scientists realized that
if the theory were correct,

evidence for it should be found
in the night sky.

Imagine that we could
shut off the Sun

and take away all the stars.

If our eyes could detect

the rest of the energy
that's still there,

we'd see a warm glow
everywhere in the cosmos.

This sea of radiation is called
the cosmic microwave background.

It's the last remnants of heat
from the Big Bang itself.

Theory predicted that the
violent expansion of space

during inflation would leave
an imprint on this radiation.

These telltale "fingerprints"
would form a precise pattern

of temperature variations--

slightly hotter spots
and slightly colder spots--

that would look
something like this.

But it would be about ten years

before the technology
was sensitive enough

to test this prediction.

Then in 1 989,

NASA launched the Cosmic
Background Explorer satellite,

followed by a second satellite,
WMAP, in 2001

that would put inflation
to the test.

The missions measured
the radiation

with tremendous precision,
and the results were stunning.

The temperature variations found
in the cosmos

were an almost identical match

with the predictions
of the theory of inflation.

It's just a theory,
mathematics on the page,

until it makes predictions
that are confirmed.

WMAP found what the math
of inflation predicted.

That is enormously convincing.

So inflation has had a number
of chances now to fail.

It made predictions,
data came in,

and inflation has come through
with flying colors.

GREENE:
Guth's work on inflation, along
with that of other physicists,

was hailed as a milestone

toward understanding the origin
of the universe.

In the process, its expanse...

GREENE:
But soon, two Russian physicists
would discover

that the equations of inflation
held a shocking secret:

our universe may not be alone.

One of these physicists was
Andrei Linde,

who had already made
pivotal contributions

to inflationary theory.

The other was Alex Vilenkin,

who happened to attend
one of the talks Alan Guth gave

during his road trip.

ALEX VILENKIN:
He gave a wonderful talk.

I hadn't met him before,

but what I heard
was rather unexpected.

In one shot,
inflation explained very well

many features of the Big Bang

and was quite remarkable.

...why the universe is
the way it is.

VILENKIN:
So I went home
greatly impressed.

GREENE:
Alex Vilenkin was so impressed
that for months afterward,

he couldn't stop thinking
about inflation.

VILENKIN:
Usually I have my thought
of the day in the shower,

which I tend to take long.

GREENE:
The more Vilenkin considered
the process of inflation,

the more he wondered
about what would make it stop.

How would a region of space
transition out of inflation?

What exactly would happen
at the moment inflation ends?

VILENKIN:
As I thought about it, it turns
out that the end of inflation

doesn't happen everywhere
at once.

GREENE:
Vilenkin suddenly realized

that if inflation doesn't end
everywhere at once,

then there's always
some part of space

where it's still happening.

VILENKIN:
So in this picture,

the Big Bang is not a unique
event that happened.

There were multiple bangs
that happened before ours,

and there will be countless
other bangs

that will happen in the future.

GREENE:
It was a striking
and unexpected new picture

in which inflation
would stop in some regions,

but always continue
somewhere else.

New big bangs
are always occurring

and new universes are always
being born,

yielding an eternally
expanding multiverse.

Linde and Vilenkin in particular

pushed the idea
that inflation might never end,

that this ballooning process

could happen over
and over again,

giving one universe
after another after another.

So was this
a revolution in science

or a theory that's
full of holes?

The idea became known
as "eternal inflation,"

and you can picture it
something like this.

Imagine that this block
of cheese is all of space

before the formation
of stars and galaxies.

Now, according to inflation,

space is uniformly filled
with a huge amount of energy,

and that energy causes space
to expand at an enormous speed.

As it does, here and there
the energy discharges,

sort of like a spark
of static electricity.

But this is like static
electricity on a cosmic scale,

and when it discharges...

(explosion)

...all that energy is rapidly
transformed into matter

in the form of tiny particles.

That process is the birth
of a new universe,

what we have traditionally
called "the Big Bang."

Inside these new universes,

which are like holes
in the cheese,

space continues to expand,
but much more slowly.

And sometimes, galaxies,
stars and planets form,

much as we see
in our universe today.

Meanwhile, outside
of these new universes,

the rest of space is still
full of undischarged energy

and is still expanding
at enormous speed.

And more expanding space

means more places where
the energy can discharge

into more big bangs
and create more new universes.

And that means this process
could go on forever.

In other words, when it comes
to eternal inflation,

that cheese is more like
Swiss cheese,

in which new universes endlessly
form, creating a multiverse.

The multiverse--
a profound implication

of eternal inflation.

But, as Alex Vilenkin
would soon learn,

one that would not be
easily accepted.

VILENKIN:
I thought I realized
something important

about the universe,

and I wanted to share this
with my fellow physicists,

and one of the first, of course,
had to be Alan Guth.

Now, we know that quantum
fluctuations in the scalar field

are different in different
regions in space.

VILENKIN:
I thought he would be
excited about it.

As a result,
in some regions...

But this encounter
didn't go as planned.

...inflation will last
longer than in others.

The delay of inflation...

VILENKIN:
As I was describing to him my
new picture of the universe,

inflating regions
and so forth...

(clears his throat)

...expansion.

VILENKIN:
I noticed that Alan is beginning

to doze off a little bit.

(snoring)

VILENKIN:
Actually I was of course
very unhappy about that,

so I thought
that I probably should go.

GREENE:
One problem with the concept
of a multiverse

was that there seemed to be
no way to detect it.

Not only is each universe
expanding,

but so is the space
in between them.

That means that nothing,
not even light,

can travel from any of the other
universes to reach us.

VILENKIN:
Physicists did not really
respond very well

to this idea
of eternal inflation.

Once I said that
I'm going to tell them

something about things
beyond our horizon

that cannot in principle
be observed,

most of them just lost interest
right there.

GREENE:
Alex Vilenkin thought he was
on to something big,

but others were skeptical.

So Vilenkin reluctantly tried

to put his work on eternal
inflation out of his mind.

ALBRECHT:
Who wants to talk
about a universe

you're never going to see?

The multiverse can't make
predictions, it can't be tested.

You could make the case
that it's not really science.

How can you ever
be confident of it

when you can't see the other
parts of the multiverse?

We can only see
our little patch,

our little expanding cloud
of galaxies.

How are we ever going to know?

You can't prove
the multiverse exists.

It's not wrong.

You can't prove
that it doesn't exist.

So why should we believe it?

GREENE:
Alex Vilenkin tried to stop
thinking about the multiverse.

With no hard evidence
to support it,

the idea seemed to have hit
a dead end.

VILENKIN:
Many people thought
that it's just not science

to talk about things
that you cannot observe.

So I did not return to the
subject for almost ten years.

GREENE:
Meanwhile, Vilenkin's
Russian colleague Andrei Linde

kept the flame alive.

He had independently come up

with his own version
of eternal inflation,

but unlike Vilenkin,
he would not be deterred.

ANDREI LINDE:
Maybe I am a little bit
more arrogant.

When I got the idea
for this multiverse,

I understood that this may be
the most important thing

which I ever do in my life.

And if somebody
doesn't want to hear it,

that's their problem.

GREENE:
Linde published
more than a dozen papers,

but his work would meet
an equally chilly reception.

It seemed no one wanted to hear
about the idea of a multiverse.

If the equations
of eternal inflation

were the only clues
pointing to the multiverse,

that's where the story
might have ended.

But the multiverse idea would
gain some unexpected support

from two completely unrelated
areas of science.

One was an idea called
string theory,

designed to explain
how the universe works

at the tiniest scales.

The other was
an astounding discovery

made by astronomers
exploring the universe

on the largest scale,

a discovery that's
utterly mysterious

if there's only one universe.

But if we're part
of a multiverse,

it's a whole new ballgame.

It has to do with the expansion
of the universe,

and it's easy to explain
using a baseball.

Now, if I toss this ball
up in the air,

we all know what will happen.

As it rises, it slows down
because of gravity.

Now, astronomers knew that
the universe was expanding,

and they assumed that the
expansion would slow down

because of the gravitational
pull of stars and galaxies,

just as the ball slows down

because of the gravitational
pull of the Earth.

But when they actually did
the measurements,

they found something
astonishing,

something that rocked
the foundations of physics.

They found that the expansion
is not slowing down.

It's speeding up.

It's as if I took this baseball
and when I throw it...

...instead of slowing down as it
rushes away, it speeds up.

Now, if you saw a ball do that,

you'd assume there's
some invisible force

that's counteracting gravity,
pushing on the ball,

forcing it to speed away
ever more quickly.

Astronomers came to the same
conclusion about the universe:

that some kind of energy
in space

must be pushing
all the galaxies apart,

causing the expansion
to speed up.

Because we don't see
this energy,

the astronomers
called it "dark energy."

It's among the most important
experimental discoveries ever

in the history of science.

It took most of us
completely by surprise.

And so, we're still trying
to come to grips with that.

GREENE:
Discovering that dark energy

is pushing every galaxy
in our universe

away from every other
at an accelerating rate

was shocking enough.

But even more surprising was the
strength of that dark energy.

For over a decade, scientists
have been unable to explain

why such a peculiar amount of it
exists in empty space.

But that mystery
seems easier to resolve

if we're part
of a much larger multiverse.

Now, the idea that space

contains any energy
at all sounds strange.

But our theory of small things
like molecules and atoms,

the theory called
quantum mechanics,

tells us that there's
a lot of activity

in the microscopic realm,

activity that can contribute
an energy to space.

And according to the math,

the amount of energy generated
by that microscopic activity

is enormous.

The problem is, when astronomers
measured the amount of energy

that's actually out there,

the amount of energy required
to force the galaxies apart

at the accelerating rate
that's observed,

they get a number like this:

a decimal point followed by
1 22 zeroes, and then a one.

An incredibly tiny amount,
very close to zero,

and nothing at all like what
the theory predicted.

In fact, it's trillions
and trillions

and trillions and trillions
of times smaller,

a colossal mismatch.

We have tried everything

to explain why the dark energy
is as small as it is.

We have tried everything,
and everything fails.

Hopeless!

I once called this
the worst failure

of an order
of magnitude estimate

in the history of science.

Does it bother us?

Absolutely.

Finding that the amount
of energy in space

is so much less
than our theory predicts

is not just an academic problem.

The precise strength of that
repulsive gravity, well,

that has profound implications
for all of us.

For example,

if I were to increase the
strength of the dark energy

just a little bit by erasing
four or five of these zeroes,

I still have a tiny number,

but the universe would be
radically different.

That's because a slightly
stronger dark energy

would push everything apart
so fast

that stars, planets
and galaxies

would never have formed.

And that means we simply
would not exist.

And yet here we are.

So, why is the amount
of dark energy

so much less
than our theory predicts

and also just right
to allow the formation

of galaxies, stars, planets
and life?

We just don't know.

The mismatch between
the theoretical predictions

of dark energy and what
astronomers have observed

is one of the great mysteries
that science faces today.

But consider this:

If we do live in a multiverse,
then the mystery of dark energy

might not be so mysterious
after all.

In fact, if we're part
of a multiverse,

the value of dark energy
we've measured

might actually make total sense.

Hi.

Reservation for Greene.

To see how the multiverse might
solve the dark energy puzzle,

imagine you're checking
into a hotel

and you get a room number
like this:

ten million and one.

Hmm.

Thanks.

Enjoy your stay.

Ten million and one

would seem like
a pretty strange room number,

and getting a room number like
this would be surprising,

much as the value of dark energy
in our universe

is a number that scientists
have found surprising.

But here's the thing:

if this hotel had
a huge number of rooms...

say, billions and billions,

then getting room number
ten million and one

wouldn't be so surprising.

In a hotel this big, you expect
to find a room with that number.

Similarly, if we're part
of a multiverse

with a huge number of universes,

each with a different value
of the dark energy,

then you'd expect to find one

with the value as small
as what we've measured.

If you think of each of these
rooms as a universe,

and each universe has
a different value

for the dark energy,

then most of these universes
won't be hospitable

to life as we know it.

The reason is that the value of
the dark energy wouldn't allow

the formation of galaxies,
stars and planets.

Universes with much less
dark energy than ours

would collapse in on themselves.

And universes with much more
dark energy than ours

would expand so fast that matter
would never have the chance

to coalesce into clumps
and form stars and planets.

So, of course we find ourselves
in a universe

where the value of the dark
energy is hospitable to life.

Otherwise we wouldn't be here
to talk about it.

If we're part of a multiverse,

the mystery of dark energy
becomes not so mysterious.

But there's a piece
of the puzzle missing.

How do we know if there's enough
diversity within the multiverse

so that every value
for dark energy,

including the strange value
we observe in our universe,

can be found somewhere?

The answer would emerge

from an entirely different area
of physics.

I'm talking about
a ground-breaking theory

that comes from investigating
the universe

on the tiniest scale.

We know that inside atoms are
even tinier bits of matter,

protons and neutrons,

which are made of still smaller
particles called quarks.

But physicists realized

that this might not be
the end of the line.

These subatomic bits

might actually be made
of something even smaller--

tiny vibrating strands or loops
of energy called strings.

This set of ideas,
called string theory,

says everything that exists

is made of this one kind
of ingredient.

And just as a single string
on a cello

can produce many different notes
depending on how it vibrates,

strings can take on
different properties

depending on how they vibrate,

creating many kinds
of particles.

From this theory came the
promise of elegant simplicity:

a single master equation that
would explain what we see

in the world around us.

SUSSKIND:
String theory would be
beautiful, it would be elegant,

and calculation from
that very simple theory

would produce the world
as we know it.

GREENE:
But for this beautiful theory
to work, there was a catch.

The math of string theory

required something
that defies common sense,

a feature that would open
the door to the multiverse:

extra dimensions of space.

We're all familiar with
three dimensions of space:

height, width and depth.

But the math of string theory

says these aren't
the only dimensions.

JOSEPH POLCHINSKI:
The mathematics works only if
the strings move and vibrate,

not just in the three directions
that we see,

but in those and, say,
six more--

nine space dimensions in all.

So if string theory is right,

where are these
extra dimensions,

and why can't we see them?

Think about the cable supporting
a traffic light.

From a distance, it looks like
a line, one-dimensional.

But if you could shrink down
to, say, the size of an ant,

you'd find another
dimension,

a circular dimension
that curls around the cable.

And string theory says
that if we could shrink down

billions of times smaller
than that ant,

we'd find tiny
extra dimensions like this

are curled up
everywhere in space.

SUSSKIND:
At every point of space,

there's extra dimensions
of space

that are curled up
into little tiny knots

that you can't see
because they're too small.

GREENE:
And the shape
of those extra dimensions

determines the fundamental
features of our universe.

Just the way the air streams

that are going through an
instrument like a French horn

have vibrational patterns

that are determined
by the shape of the instrument,

the shape of the extra
dimensions

determines how the little
strings vibrate.

Those vibrational patterns
determine particle properties,

so all of the fundamental
features of our universe

may be determined by the shape
of the extra dimensions.

SUSSKIND:
The way those extra dimensions
of space are put together

is in many respects
like the DNA of the universe.

They determine the way the
universe is going to behave,

just exactly the same way

as DNA determines the way
an animal is going to look.

GREENE:
The problem was, the more
string theorists looked,

the more ways they found
that extra dimensions

could be curled up.

And the math provided no clues

as to which shape
was the right one

corresponding to our universe.

SHAMIT KACHRU:
I think the consensus right now

is that that number
seems to be astronomical.

There are published papers

suggesting upwards
of 1 0 to the 500--

that's 1 0 followed
by 500 zeroes--

different possible shapes.

GREENE:
Ten to the 500
different possible shapes

for the extra dimensions,
each appearing equally valid.

It seemed preposterous.

Especially for a theory

that was looking for one,
single master equation

to describe our universe.

But then it occurred
to some string theorists

that perhaps there was
a different way

to look at the problem,
and this different perspective

would breathe new life
into the idea of a multiverse.

Ten to the 500
different string theories.

This sounded like
a complete disaster.

What good is it to have a theory

that has ten to the 500
solutions?

You can't find
anything in there.

Well, that left string theorists

somewhat unhappy,
somewhat depressed.

My own reaction to it
at the time is, "This is great.

"This is fantastic.

"This is exactly what the
cosmologists are looking for:

"enormous diversity
of possibilities.

"Don't be unhappy about this.

"This says that string theory

"fits extremely well
with cosmology

and with all the interesting
ideas about multiverses."

GREENE:
Turning what seemed like
a vice into a virtue,

some string theorists
became convinced

that the multiple solutions
of string theory

might each represent a real
and very different universe.

In other words, string theory
was describing a multiverse--

and an extremely diverse one
at that.

JOHNSON:
To everyone's surprise,

string theory was actually
quite readily describing

huge numbers of different
kinds of solutions,

each of which corresponds
to a possible universe.

So we just got
this multiverse for free.

DELIA SCHWARTZ-PERLOV:
Both from string theory
and from inflation,

you have these universes
that are produced.

These different universes
would all naturally have

different amounts
of dark energy.

GREENE:
In fact, according to the math,
the amount of dark energy

would span such a wide range of
values from universe to universe

that the strange amount we've
measured would surely turn up.

String theory, without even
trying, solved that problem.

GREENE:
So, over a decade
after Linde and Vilenkin

had come up with their ideas
about eternal inflation,

the multiverse was revived.

Three lines of reasoning
were now all pointing

to the same conclusion:

eternal inflation, dark energy
and string theory.

Just the way it takes three legs
to support a stool,

these three ideas taken together
support the argument

that we may live
in a multiverse.

When different lines of research
all converge on one idea,

that doesn't mean it's right,

but when all the spokes of the
wheel are pointing at one idea,

that idea becomes
pretty convincing.

Today the multiverse
is hotly debated.

Many critics remain.

David Grace is going
to tell us, "No, no, no."

GREENE:
But multiverse advocates

like Alex Vilenkin, Alan Guth
and Andrei Linde

are no longer alone.

VILENKIN:
The tide appears to be turning.

Now these ideas are accepted
to a much larger degree.

The genie is out of the bottle.

You cannot put it back.

GREENE:
So, what would it be like?

If we could travel to some
of these other universes,

what would we see?

Some might be vastly different
from our own,

with properties unlike anything
we've ever seen.

In fact, some universes
in the multiverse

might not have light or matter
or anything recognizable at all.

And there might be other
universes with features

not unlike the familiar ones
we know,

but where life takes
a completely different form,

perhaps communicating in ways
we'd find utterly bizarre.

And the math shows

that if we were able to visit
enough of these universes,

we might eventually find
ones like ours,

with a Milky Way galaxy,
a solar system and an Earth.

Except with some
slight differences.

In one, maybe the asteroid

that killed off the dinosaurs
65 million years ago missed,

and evolution charted
a new course.

In another,
there might be an Earth

with people similar to us...

(phone ringing)

...but better at multitasking.

But there's something
even stranger.

Somewhere out there,

we should find exact copies
of our universe

with duplicates of everything
and everyone.

How could this be?

How could there be exact
duplicates of ourselves

out there in the multiverse?

To see how, take this deck
of cards.

It's made up
of 52 different cards,

and if I deal them, everyone
will get a different hand.

But, over the course
of many, many rounds,

eventually some
of the combinations

will start to repeat.

That's because with 52 cards,

there's a limited number of
different hands you can deal.

So if you deal the cards
an infinite number of times,

then repeating hands
are inevitable.

And in the multiverse,
a similar principle applies.

That's because, according
to the laws of nature,

the fundamental ingredients
of matter, or particles,

are kind of like
a deck of cards:

in any region of space,
they can only be arranged

in a finite number
of different ways.

So if space is infinite,

if there are an infinite
number of universes,

then those arrangements
are bound to repeat.

And since each one of us

is just a particular
arrangement of particles,

somewhere there's a duplicate
of you and me

and everyone else.

This can be shocking.

It could be that
in another universe

I was a rock star
and my life is much better.

Or much worse, depending on
your opinion of rock stars.

It means all those things that
I've never found time to do

are maybe being done by some
copy of me somewhere else.

I was rather depressed,
actually.

This picture robs us
of our uniqueness.

It is a consequence
of the ideas,

and the ideas
seem very well motivated.

GREENE:
Yet critics argue

the multiverse is just too
convenient an explanation

for things we don't understand,

like the tiny value
of dark energy in our universe

and the huge number
of possible shapes

for the extra dimensions
in string theory.

STEINHARDT:
The problem with that kind
of reasoning

is that it doesn't explain

why the dark energy is
the way it is.

It just says it's random chance.

I don't find that satisfactory.

You can apply this kind
of reasoning

any time you don't have
a better explanation.

GREENE:
On the other hand, supporters
of the multiverse

point out that sometimes
a better or deeper explanation

for the way things are
simply does not exist.

Take, for example, the Earth's
orbit around the Sun.

We find ourselves at a distance
of 93 million miles,

perfect for life.

If we were much closer
to the Sun,

our planet would be too hot
for life as we know it to exist.

And if we were much farther
from the Sun,

it would be too cold for life.

So, why are we
in this sweet spot?

Well, starting
in the late 1 500s,

the famous astronomer Johannes
Kepler asked that very question,

and he spent years trying
to find a physical reason,

some law of nature

that requires the Earth to be
93 million miles from the Sun.

But Kepler never found it,
because it doesn't exist.

There isn't any physical law
requiring the Earth to be

93 million miles from the Sun.

It's simply one possibility
of the many you'd expect to find

in a universe we know is full
of solar systems.

SUSSKIND:
You might think it was
an extraordinary accident.

It's not.

It's just that there are
a lot of planets out there.

GREENE:
Similarly, some suggest
that the true explanation

for many of the fundamental
features of our world

will elude us if we don't
consider the possibility

that we live in a multiverse.

GUTH:
Clearly if we had
a good physical reason,

that would be great
and we would understand it.

We'd be much happier.

We may have to live with that.

There's no principle built
into the laws of nature

that say that theoretical
physicists have to be happy.

It's a hypothesis.

It's the leading hypothesis

because nobody has
another hypothesis

which makes as much sense.

GREENE:
The multiverse,
a tantalizing possibility.

But with no experimental
evidence, should you believe it?

We can't believe in anything

until there's observational
or experimental support.

But what we have found
over the last few centuries

is that mathematics provides
a sure-footed guide

to the nature of things

that we haven't yet been able to
see, observe or experiment with.

Math predicted things
like black holes

and certain subatomic particles

long before we ever
observed them.

And math is suggesting

that there may be
these other universes.

That doesn't mean it's right,
but often it's leading you

to a deeper understanding
of reality.

If you choose not to believe it,
that's perfectly fine,

because we have not given you
any evidence yet,

and one of the wonderful
things about science

is it's about evidence;
it's not about belief.

GREENE:
And some scientists now think
we might just be able

to find that evidence.

They propose that
if our universe and another

were born close together,

the two might have collided.

That collision could have left
its own telltale sign

in the form of a pattern
of temperature differences

that we might detect in the
cosmic background radiation,

the heat left over
from the Big Bang.

My guess is yes,
that in 1 00 years

we will know one way or another
whether these ideas are right.

A hundred years from now

it may be an amusing
historical episode.

We don't know.

But if you only work
on the things

that are already
well established,

you're not going to be part
of the next big excitement.

GREENE:
If we do verify the multiverse,
it would change our perspective

much as Copernicus did 500 years
ago when he showed

that the Earth is not
the center of the cosmos.

And some might say that if our
universe is just one of many,

our descent from the center
would be complete.

SCHWARTZ-PERLOV:
Regardless, I think
it's more important

just that we're so lucky that we
can understand the universe.

I think it's a great ride,

and I think it's
really good for physics

that we have this tension.

I don't know where
we're going to end up.

GREENE:
So, what does this all mean?

Are there infinite duplicates

of you and me and everything
existing right now

in an infinite number
of other universes?

Is the multiverse the next
Copernican revolution?

We don't know,
at least not yet.

But if the idea that we live
in a multiverse proves true,

we'd be witnessing one
of the most exciting

and dramatic upheavals

to our understanding
of the fabric of the cosmos.

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