NARRATOR: 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 (Columbia University): Why don't
we ever see events unfold in reverse order?
According to the laws of physics, this can
happen.
NARRATOR: 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.
BRIAN GREENE: I'm going to have what he's
having.
NARRATOR: 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.
BRIAN GREENE: But how could this be?
How could we be so wrong about something so
familiar?
DAVID GROSS (University of California, Santa
Barbara): Does it bother us?
Absolutely.
STEVEN WEINBERG (The University of Texas at
Austin): There's no principle built into the
laws of nature that says theoretical physicists
have to be happy.
NARRATOR: It's a game-changing perspective
that opens up a whole new world of possibilities.
Coming up: what if new universes were born
all the timeâ€¦
ALEX VILENKIN (Tufts University): In this
picture, the big bang is not a unique event.
NARRATOR: â€¦and ours was one of numerous
parallel realities?
BRIAN GREENE: Somewhere there's a duplicate
of you
and me and everyone else.
NARRATOR: Are we in a universe or a multiverse?
The Fabric of the Cosmos, right now on NOVA.
BRIAN GREENE: New York City: they say there's
nowhere else like it, home to 8,000,000 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.
ALAN GUTH (Massachusetts Institute of Technology):
There will be other places where there will
be Alan Guths who look and think and act exactly
like me, as well as many where there will
be Alan Guths who look and think almost exactly
like me, but with some small differences.
LEONARD SUSSKIND (Stanford University): 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.
BRIAN 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 (University of California,
Davis): I'm very uncomfortable with the multiverse.
To become solid science it's got a lot of
growing up to do.
DAVID GROSS: You know, it exists in the same
way that, you know, angels might exist.
STEVEN WEINBERG: We have to make our bets,
and I think, right now, the multiverse is
a pretty good bet.
ALAN GUTH: 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.
BRIAN GREENE: So where did this idea come
from, and what's the evidence for it?
Well, several surprising discoveries suggest
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 14 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.
ALAN GUTH: It's called the Big Bang theory,
but the one thing it really says nothing about
is the bang itself.
It says nothing about what banged, why it
banged, or what happened before it banged.
BRIAN 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 M.I.T., but back
in 1979, Guth and a colleague, Henry Tye,
were pursuing a new idea about how particles
might have formed in the early universe.
ALAN GUTH: Henry suggested to me that we should
maybe look at whether or not this new process
we were thinking of would influence the expansion
rate of the universe.
BRIAN 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.
ALAN GUTH: I stayed up quite late that night
and went over the calculations very carefully,
trying to make sure everything was correct.
BRIAN GREENE: As the night wore on, Guth discovered
something extraordinary in the equations describing
how new particles might have formed in the
early universe.
ALAN 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.
BRIAN 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.
ALAN 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.
BRIAN 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.
ALAN GUTH: 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, so I went on, actually, a pretty
long trip, giving talks about this.
STEVEN WEINBERG: Suddenly this idea caught
on.
ANDREAS ALBRECHT: Talks about inflation were
packed with people from all areas of physics.
STEVEN WEINBERG: Lots of astrophysical theorists,
including me, got very enthusiastic.
ANDREAS ALBRECHT: It was a very, very exciting
time.
STEVEN WEINBERG: If you have a really good
idea that allows other people to move the
field forward, people are going to pay attention.
ALAN GUTH: â€¦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.
BRIAN 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 10 years before the
technology was sensitive enough to test this
prediction.
Then, in 1989, NASA launched the Cosmic Background
Explorer satellite, followed by a second satellite,
W.M.A.P., 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.
W.M.A.P. found what the math of inflation
predicted.
That is enormously convincing.
ANDREAS ALBRECHT: 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.
BRIAN GREENE: Guth's work on inflation, along
with that of other physicists, was hailed
as a milestone toward understanding the origin
of the universe.
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 Russian 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.
So I went home greatly impressed.
BRIAN GREENE: Alex Vilenkin was so impressed
that, for months afterward, he couldn't stop
thinking about inflation.
ALEX VILENKIN: Usually, I have my thought
of the day in the shower, which I tend to
take long.
BRIAN 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?
ALEX VILENKIN: As I thought about it, it turns
out that the end of inflation doesn't happen
everywhere at once.
BRIAN 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.
ALEX 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.
BRIAN 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 just
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, 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 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.
ALEX VILENKIN: I thought I had 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 are
different in different regions of spaceâ€¦
I thought he would be excited about it, but
this encounter didn't go as planned.
â€¦inflation will last longer than in
others.
As I was describing to him my new picture
of the universe, inflating regions and so
forth, ahem, expansion, I noticed that Alan
is beginning to doze off a little bit.
Actually, I was, of course, very unhappy about
that, so I thought that I probably should
go.
BRIAN 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.
ALEX 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.
BRIAN 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.
ANDREAS 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.
STEVEN WEINBERG: 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?
PAUL STEINHARDT (Princeton University): 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?
BRIAN 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.
ALEX VILENKIN: Many people thought 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.
BRIAN 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 (Stanford University): 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 then, if somebody doesn't want to hear
it, that's their problem.
BRIAN 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."
RAPHAEL BOUSSO (University of California,
Berkeley): It's among the most important experimental
discoveries ever, in the history of science.
ANDREAS ALBRECHT: It took most of us completely
by surprise.
CLIFFORD JOHNSON (University of California,
Berkeley): And so, we're still trying to come
to grips with that.
BRIAN 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 122 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.
LEONARD SUSSKIND: We have tried everything
to explain why the dark energy is as small
as it is.
We have tried everything, and everything fails.
STEVEN WEINBERG: Hopeless.
I once called this the worst failure of an
order of magnitude estimate in the history
of science.
DAVID GROSS: Does it bother us?
Absolutely!
BRIAN GREENE: 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.
Hmmm.
Thanks.
DESK CLERK: Enjoy your stay.
BRIAN GREENE: 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 rumber 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 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 just 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 galaxies.
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.
So 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 this might not be
the end of the line.
These sub-atomic 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 everything we see in the world
around us.
LEONARD 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.
BRIAN 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 (University of California,
Santa Barbara): 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.
BRIAN GREENE: 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
exists, 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.
LEONARD 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.
BRIAN 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 the fundamental features
of our universe may be determined by the shape
of the extra dimensions.
LEONARD SUSSKIND: The way those extra dimensions
of space are put together is, in many respects,
like the D.N.A. of the universe.
They determine the way the universe is going
to behave, just exactly the same way as D.N.A.
determines the way an animal is going to look.
BRIAN 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 (Stanford University): I think
the consensus, right now, is that that number
seems to be astronomical.
There are published papers suggesting upwards
of 10-to-the-500—that's 10 followed by 500
zeroes—different possible shapes.
BRIAN GREENE: Ten-to-the-five-hundred 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.
LEONARD SUSSKIND: Ten-to-the-500 different
string theories—this sounded like a complete
disaster.
What good is it to have a theory that has
10-to-the-five-hundred 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."
BRIAN 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.
CLIFFORD JOHNSON: To everyone's surprise,
string theory was actually quite readily describing
huge numbers of different kinds of solutions
whichâ€¦each of which corresponds to a
possible universe.
ANDREI LINDE: So we just got this multiverse
for free.
DELIA SCHWARTZ-PERLOV (Tufts University):
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.
BRIAN 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.
RAPHEL BOUSSO: String theory, without even
trying, solved that problem.
BRIAN 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 Gross is going to tell us, "No, no,
no."
But multiverse advocates, like Alex Vilenkin,
Alan Guth and Andrei Linde are no longer alone.
ALEX VILENKIN: The tide appears to be turning.
And now these ideas are accepted to a much
larger degree.
ANDREI LINDE: The genie is out of the bottle.
You cannot put it back.
BRIAN GREENE: So, what would it be like, if
we could travel to some of these other universes?
What would we see?
Some would 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 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.
SECOND BRIAN GREENE: That's because, with
52 cards, there's a limited number of different
hands you can deal.
BRIAN GREENE: So, if you deal the cards an
infinite number of times, then repeating hands
are inevitable.
THIRD BRIAN GREENE: And in the multiverse,
a similar principle applies.
BRIAN GREENE: 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,â€¦
SECOND BRIAN GREENE: ...somewhere there's
a duplicate of you and meâ€¦
ALL THREE BRIAN GREENES: ...and everyone else.
ALAN GUTH: This can be shocking.
SHAMIT KACHRU: 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.
CLIFFORD JOHNSON: That means all those things
that I've never found time to do are maybe
being done by some copy of me somewhere else.
ALEX VILENKIN: I was rather depressed actually.
This picture robs us of our uniqueness.
LEONARD SUSSKIND: It is a consequence of the
ideas, and the ideas seem very well motivated.
BRIAN 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.
PAUL 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.
DAVID GROSS: I don't find that satisfactory.
You can apply this kind of reasoning anytime
you don't have a better explanation.
BRIAN 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 1500s, 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.
LEONARD SUSSKIND: You might think it was an
extraordinary accident.
It's not.
It's just that there are a lot of planets
out there.
BRIAN 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.
ALAN GUTH: Clearly, if we had a good physical
reason, that would be great, and we would
understand it.
We'd be much happier.
STEVEN WEINBERG: 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.
LEONARD SUSSKIND: It's a hypothesis.
It's the leading hypothesis, because nobody
has another hypothesis which makes as much
sense.
BRIAN 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.
CLIFFORD JOHNSON: 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 that it's about evidence; it's not about
belief.
BRIAN GREENE: And some scientists now think
we might just be able to find evidence for
a multiverse.
They propose that if our universe and another
were born close together, the two might have
collided.
That collision could have left its own fingerprint:
ripples in the cosmic background radiation,
the heat left over from the Big Bang.
LEONARD SUSSKIND: My guess is yes, that in
100 years, we will know one way or another
whether these ideas are right.
STEVEN WEINBERG: 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.
BRIAN 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.
DELIA SCHWARTZ-PERLOV: Regardless, I think
it's more important is that we're so lucky
that we can understand the universe.
ANDREAS ALBRECHT: 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.
BRIAN 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.
