21:48
TED2012

Brian Greene: Is our universe the only universe?

Filmed:

Is there more than one universe? In this visually rich, action-packed talk, Brian Greene shows how the unanswered questions of physics (starting with a big one: What caused the Big Bang?) have led to the theory that our own universe is just one of many in the "multiverse."

- Physicist
Brian Greene is perhaps the best-known proponent of superstring theory, the idea that minuscule strands of energy vibrating in a higher dimensional space-time create every particle and force in the universe. Full bio

A few months ago
00:15
the Nobel Prize in physics
00:17
was awarded to two teams of astronomers
00:19
for a discovery that has been hailed
00:21
as one of the most important
00:24
astronomical observations ever.
00:26
And today, after briefly describing what they found,
00:28
I'm going to tell you about a highly controversial framework
00:30
for explaining their discovery,
00:33
namely the possibility
00:36
that way beyond the Earth,
00:38
the Milky Way and other distant galaxies,
00:40
we may find that our universe
00:43
is not the only universe,
00:45
but is instead
00:47
part of a vast complex of universes
00:49
that we call the multiverse.
00:51
Now the idea of a multiverse is a strange one.
00:53
I mean, most of us were raised to believe
00:56
that the word "universe" means everything.
00:58
And I say most of us with forethought,
01:01
as my four-year-old daughter has heard me speak of these ideas since she was born.
01:04
And last year I was holding her
01:07
and I said, "Sophia,
01:09
I love you more than anything in the universe."
01:11
And she turned to me and said, "Daddy,
01:14
universe or multiverse?"
01:16
(Laughter)
01:18
But barring such an anomalous upbringing,
01:21
it is strange to imagine
01:24
other realms separate from ours,
01:26
most with fundamentally different features,
01:28
that would rightly be called universes of their own.
01:30
And yet,
01:33
speculative though the idea surely is,
01:35
I aim to convince you
01:37
that there's reason for taking it seriously,
01:39
as it just might be right.
01:41
I'm going to tell the story of the multiverse in three parts.
01:43
In part one,
01:46
I'm going to describe those Nobel Prize-winning results
01:48
and to highlight a profound mystery
01:50
which those results revealed.
01:52
In part two,
01:54
I'll offer a solution to that mystery.
01:56
It's based on an approach called string theory,
01:58
and that's where the idea of the multiverse
02:00
will come into the story.
02:02
Finally, in part three,
02:04
I'm going to describe a cosmological theory
02:06
called inflation,
02:08
which will pull all the pieces of the story together.
02:10
Okay, part one starts back in 1929
02:13
when the great astronomer Edwin Hubble
02:17
realized that the distant galaxies
02:19
were all rushing away from us,
02:22
establishing that space itself is stretching,
02:24
it's expanding.
02:26
Now this was revolutionary.
02:28
The prevailing wisdom was that on the largest of scales
02:31
the universe was static.
02:34
But even so,
02:36
there was one thing that everyone was certain of:
02:38
The expansion must be slowing down.
02:41
That, much as the gravitational pull of the Earth
02:44
slows the ascent of an apple tossed upward,
02:47
the gravitational pull
02:50
of each galaxy on every other
02:52
must be slowing
02:54
the expansion of space.
02:56
Now let's fast-forward to the 1990s
02:58
when those two teams of astronomers
03:01
I mentioned at the outset
03:03
were inspired by this reasoning
03:05
to measure the rate
03:07
at which the expansion has been slowing.
03:09
And they did this
03:11
by painstaking observations
03:13
of numerous distant galaxies,
03:15
allowing them to chart
03:17
how the expansion rate has changed over time.
03:19
Here's the surprise:
03:22
They found that the expansion is not slowing down.
03:25
Instead they found that it's speeding up,
03:28
going faster and faster.
03:30
That's like tossing an apple upward
03:32
and it goes up faster and faster.
03:34
Now if you saw an apple do that,
03:36
you'd want to know why.
03:38
What's pushing on it?
03:40
Similarly, the astronomers' results
03:42
are surely well-deserving of the Nobel Prize,
03:44
but they raised an analogous question.
03:47
What force is driving all galaxies
03:51
to rush away from every other
03:53
at an ever-quickening speed?
03:56
Well the most promising answer
03:59
comes from an old idea of Einstein's.
04:01
You see, we are all used to gravity
04:04
being a force that does one thing,
04:06
pulls objects together.
04:09
But in Einstein's theory of gravity,
04:11
his general theory of relativity,
04:13
gravity can also push things apart.
04:15
How? Well according to Einstein's math,
04:18
if space is uniformly filled
04:21
with an invisible energy,
04:23
sort of like a uniform, invisible mist,
04:25
then the gravity generated by that mist
04:28
would be repulsive,
04:31
repulsive gravity,
04:33
which is just what we need to explain the observations.
04:35
Because the repulsive gravity
04:38
of an invisible energy in space --
04:40
we now call it dark energy,
04:42
but I've made it smokey white here so you can see it --
04:44
its repulsive gravity
04:47
would cause each galaxy to push against every other,
04:49
driving expansion to speed up,
04:51
not slow down.
04:53
And this explanation
04:55
represents great progress.
04:57
But I promised you a mystery
04:59
here in part one.
05:02
Here it is.
05:04
When the astronomers worked out
05:06
how much of this dark energy
05:08
must be infusing space
05:11
to account for the cosmic speed up,
05:13
look at what they found.
05:15
This number is small.
05:24
Expressed in the relevant unit,
05:26
it is spectacularly small.
05:28
And the mystery is to explain this peculiar number.
05:30
We want this number
05:33
to emerge from the laws of physics,
05:35
but so far no one has found a way to do that.
05:37
Now you might wonder,
05:40
should you care?
05:43
Maybe explaining this number
05:45
is just a technical issue,
05:47
a technical detail of interest to experts,
05:49
but of no relevance to anybody else.
05:52
Well it surely is a technical detail,
05:54
but some details really matter.
05:57
Some details provide
05:59
windows into uncharted realms of reality,
06:01
and this peculiar number may be doing just that,
06:03
as the only approach that's so far made headway to explain it
06:06
invokes the possibility of other universes --
06:09
an idea that naturally emerges from string theory,
06:12
which takes me to part two: string theory.
06:15
So hold the mystery of the dark energy
06:18
in the back of your mind
06:22
as I now go on to tell you
06:24
three key things about string theory.
06:26
First off, what is it?
06:29
Well it's an approach to realize Einstein's dream
06:31
of a unified theory of physics,
06:34
a single overarching framework
06:37
that would be able to describe
06:39
all the forces at work in the universe.
06:41
And the central idea of string theory
06:43
is quite straightforward.
06:45
It says that if you examine
06:47
any piece of matter ever more finely,
06:49
at first you'll find molecules
06:51
and then you'll find atoms and subatomic particles.
06:53
But the theory says that if you could probe smaller,
06:56
much smaller than we can with existing technology,
06:58
you'd find something else inside these particles --
07:01
a little tiny vibrating filament of energy,
07:04
a little tiny vibrating string.
07:07
And just like the strings on a violin,
07:10
they can vibrate in different patterns
07:12
producing different musical notes.
07:14
These little fundamental strings,
07:16
when they vibrate in different patterns,
07:18
they produce different kinds of particles --
07:20
so electrons, quarks, neutrinos, photons,
07:22
all other particles
07:24
would be united into a single framework,
07:26
as they would all arise from vibrating strings.
07:28
It's a compelling picture,
07:31
a kind of cosmic symphony,
07:34
where all the richness
07:36
that we see in the world around us
07:38
emerges from the music
07:40
that these little, tiny strings can play.
07:42
But there's a cost
07:45
to this elegant unification,
07:47
because years of research
07:49
have shown that the math of string theory doesn't quite work.
07:51
It has internal inconsistencies,
07:54
unless we allow
07:56
for something wholly unfamiliar --
07:58
extra dimensions of space.
08:01
That is, we all know about the usual three dimensions of space.
08:04
And you can think about those
08:07
as height, width and depth.
08:09
But string theory says that, on fantastically small scales,
08:12
there are additional dimensions
08:15
crumpled to a tiny size so small
08:17
that we have not detected them.
08:19
But even though the dimensions are hidden,
08:21
they would have an impact on things that we can observe
08:23
because the shape of the extra dimensions
08:26
constrains how the strings can vibrate.
08:29
And in string theory,
08:32
vibration determines everything.
08:34
So particle masses, the strengths of forces,
08:37
and most importantly, the amount of dark energy
08:39
would be determined
08:42
by the shape of the extra dimensions.
08:44
So if we knew the shape of the extra dimensions,
08:46
we should be able to calculate these features,
08:49
calculate the amount of dark energy.
08:52
The challenge
08:55
is we don't know
08:57
the shape of the extra dimensions.
08:59
All we have
09:02
is a list of candidate shapes
09:04
allowed by the math.
09:06
Now when these ideas were first developed,
09:09
there were only about five different candidate shapes,
09:11
so you can imagine
09:13
analyzing them one-by-one
09:15
to determine if any yield
09:17
the physical features we observe.
09:19
But over time the list grew
09:21
as researchers found other candidate shapes.
09:23
From five, the number grew into the hundreds and then the thousands --
09:25
A large, but still manageable, collection to analyze,
09:28
since after all,
09:31
graduate students need something to do.
09:33
But then the list continued to grow
09:36
into the millions and the billions, until today.
09:38
The list of candidate shapes
09:41
has soared to about 10 to the 500.
09:43
So, what to do?
09:48
Well some researchers lost heart,
09:51
concluding that was so many candidate shapes for the extra dimensions,
09:54
each giving rise to different physical features,
09:57
string theory would never make
10:00
definitive, testable predictions.
10:02
But others turned this issue on its head,
10:04
taking us to the possibility of a multiverse.
10:08
Here's the idea.
10:10
Maybe each of these shapes is on an equal footing with every other.
10:12
Each is as real as every other,
10:15
in the sense
10:17
that there are many universes,
10:19
each with a different shape, for the extra dimensions.
10:21
And this radical proposal
10:24
has a profound impact on this mystery:
10:26
the amount of dark energy revealed by the Nobel Prize-winning results.
10:29
Because you see,
10:32
if there are other universes,
10:34
and if those universes
10:37
each have, say, a different shape for the extra dimensions,
10:39
then the physical features of each universe will be different,
10:43
and in particular,
10:45
the amount of dark energy in each universe
10:47
will be different.
10:49
Which means that the mystery
10:51
of explaining the amount of dark energy we've now measured
10:53
would take on a wholly different character.
10:55
In this context,
10:58
the laws of physics can't explain one number for the dark energy
11:00
because there isn't just one number,
11:03
there are many numbers.
11:06
Which means
11:08
we have been asking the wrong question.
11:10
It's that the right question to ask is,
11:13
why do we humans find ourselves in a universe
11:15
with a particular amount of dark energy we've measured
11:18
instead of any of the other possibilities
11:21
that are out there?
11:24
And that's a question on which we can make headway.
11:26
Because those universes
11:29
that have much more dark energy than ours,
11:31
whenever matter tries to clump into galaxies,
11:33
the repulsive push of the dark energy is so strong
11:36
that it blows the clump apart
11:39
and galaxies don't form.
11:41
And in those universes that have much less dark energy,
11:43
well they collapse back on themselves so quickly
11:46
that, again, galaxies don't form.
11:48
And without galaxies, there are no stars, no planets
11:51
and no chance
11:54
for our form of life
11:56
to exist in those other universes.
11:58
So we find ourselves in a universe
12:00
with the particular amount of dark energy we've measured
12:02
simply because our universe has conditions
12:05
hospitable to our form of life.
12:08
And that would be that.
12:12
Mystery solved,
12:14
multiverse found.
12:16
Now some find this explanation unsatisfying.
12:18
We're used to physics
12:23
giving us definitive explanations for the features we observe.
12:25
But the point is,
12:28
if the feature you're observing
12:30
can and does take on
12:33
a wide variety of different values
12:35
across the wider landscape of reality,
12:37
then thinking one explanation
12:40
for a particular value
12:42
is simply misguided.
12:44
An early example
12:47
comes from the great astronomer Johannes Kepler
12:49
who was obsessed with understanding
12:52
a different number --
12:54
why the Sun is 93 million miles away from the Earth.
12:56
And he worked for decades trying to explain this number,
13:00
but he never succeeded, and we know why.
13:03
Kepler was asking
13:06
the wrong question.
13:08
We now know that there are many planets
13:10
at a wide variety of different distances from their host stars.
13:13
So hoping that the laws of physics
13:16
will explain one particular number, 93 million miles,
13:19
well that is simply wrongheaded.
13:22
Instead the right question to ask is,
13:25
why do we humans find ourselves on a planet
13:27
at this particular distance,
13:30
instead of any of the other possibilities?
13:32
And again, that's a question we can answer.
13:35
Those planets which are much closer to a star like the Sun
13:38
would be so hot
13:41
that our form of life wouldn't exist.
13:43
And those planets that are much farther away from the star,
13:45
well they're so cold
13:48
that, again, our form of life would not take hold.
13:50
So we find ourselves
13:52
on a planet at this particular distance
13:54
simply because it yields conditions
13:56
vital to our form of life.
13:58
And when it comes to planets and their distances,
14:01
this clearly is the right kind of reasoning.
14:04
The point is,
14:08
when it comes to universes and the dark energy that they contain,
14:10
it may also be the right kind of reasoning.
14:13
One key difference, of course,
14:17
is we know that there are other planets out there,
14:20
but so far I've only speculated on the possibility
14:22
that there might be other universes.
14:25
So to pull it all together,
14:27
we need a mechanism
14:29
that can actually generate other universes.
14:31
And that takes me to my final part, part three.
14:34
Because such a mechanism has been found
14:37
by cosmologists trying to understand the Big Bang.
14:40
You see, when we speak of the Big Bang,
14:43
we often have an image
14:45
of a kind of cosmic explosion
14:47
that created our universe
14:49
and set space rushing outward.
14:51
But there's a little secret.
14:54
The Big Bang leaves out something pretty important,
14:56
the Bang.
14:59
It tells us how the universe evolved after the Bang,
15:01
but gives us no insight
15:04
into what would have powered the Bang itself.
15:06
And this gap was finally filled
15:10
by an enhanced version of the Big Bang theory.
15:12
It's called inflationary cosmology,
15:14
which identified a particular kind of fuel
15:17
that would naturally generate
15:21
an outward rush of space.
15:23
The fuel is based on something called a quantum field,
15:25
but the only detail that matters for us
15:28
is that this fuel proves to be so efficient
15:31
that it's virtually impossible
15:34
to use it all up,
15:36
which means in the inflationary theory,
15:38
the Big Bang giving rise to our universe
15:40
is likely not a one-time event.
15:43
Instead the fuel not only generated our Big Bang,
15:46
but it would also generate countless other Big Bangs,
15:49
each giving rise to its own separate universe
15:55
with our universe becoming but one bubble
15:58
in a grand cosmic bubble bath of universes.
16:00
And now, when we meld this with string theory,
16:03
here's the picture we're led to.
16:05
Each of these universes has extra dimensions.
16:07
The extra dimensions take on a wide variety of different shapes.
16:09
The different shapes yield different physical features.
16:12
And we find ourselves in one universe instead of another
16:15
simply because it's only in our universe
16:18
that the physical features, like the amount of dark energy,
16:21
are right for our form of life to take hold.
16:24
And this is the compelling but highly controversial picture
16:28
of the wider cosmos
16:31
that cutting-edge observation and theory
16:33
have now led us to seriously consider.
16:35
One big remaining question, of course, is,
16:39
could we ever confirm
16:43
the existence of other universes?
16:46
Well let me describe
16:49
one way that might one day happen.
16:51
The inflationary theory
16:54
already has strong observational support.
16:56
Because the theory predicts
16:58
that the Big Bang would have been so intense
17:00
that as space rapidly expanded,
17:02
tiny quantum jitters from the micro world
17:05
would have been stretched out to the macro world,
17:07
yielding a distinctive fingerprint,
17:10
a pattern of slightly hotter spots and slightly colder spots,
17:13
across space,
17:15
which powerful telescopes have now observed.
17:17
Going further, if there are other universes,
17:20
the theory predicts that every so often
17:23
those universes can collide.
17:25
And if our universe got hit by another,
17:27
that collision
17:29
would generate an additional subtle pattern
17:31
of temperature variations across space
17:33
that we might one day
17:35
be able to detect.
17:37
And so exotic as this picture is,
17:39
it may one day be grounded
17:42
in observations,
17:44
establishing the existence of other universes.
17:46
I'll conclude
17:49
with a striking implication
17:51
of all these ideas
17:54
for the very far future.
17:56
You see, we learned
17:58
that our universe is not static,
18:00
that space is expanding,
18:02
that that expansion is speeding up
18:04
and that there might be other universes
18:06
all by carefully examining
18:08
faint pinpoints of starlight
18:10
coming to us from distant galaxies.
18:12
But because the expansion is speeding up,
18:15
in the very far future,
18:18
those galaxies will rush away so far and so fast
18:20
that we won't be able to see them --
18:23
not because of technological limitations,
18:26
but because of the laws of physics.
18:28
The light those galaxies emit,
18:30
even traveling at the fastest speed, the speed of light,
18:32
will not be able to overcome
18:35
the ever-widening gulf between us.
18:37
So astronomers in the far future
18:40
looking out into deep space
18:42
will see nothing but an endless stretch
18:44
of static, inky, black stillness.
18:47
And they will conclude
18:51
that the universe is static and unchanging
18:53
and populated by a single central oasis of matter
18:55
that they inhabit --
18:58
a picture of the cosmos
19:00
that we definitively know to be wrong.
19:02
Now maybe those future astronomers will have records
19:05
handed down from an earlier era,
19:08
like ours,
19:10
attesting to an expanding cosmos
19:12
teeming with galaxies.
19:14
But would those future astronomers
19:16
believe such ancient knowledge?
19:18
Or would they believe
19:21
in the black, static empty universe
19:23
that their own state-of-the-art observations reveal?
19:26
I suspect the latter.
19:30
Which means that we are living
19:32
through a remarkably privileged era
19:34
when certain deep truths about the cosmos
19:37
are still within reach
19:39
of the human spirit of exploration.
19:41
It appears that it may not always be that way.
19:43
Because today's astronomers,
19:48
by turning powerful telescopes to the sky,
19:50
have captured a handful of starkly informative photons --
19:53
a kind of cosmic telegram
19:56
billions of years in transit.
19:59
and the message echoing across the ages is clear.
20:01
Sometimes nature guards her secrets
20:05
with the unbreakable grip
20:08
of physical law.
20:10
Sometimes the true nature of reality beckons
20:12
from just beyond the horizon.
20:16
Thank you very much.
20:19
(Applause)
20:21
Chris Anderson: Brian, thank you.
20:25
The range of ideas you've just spoken about
20:27
are dizzying, exhilarating, incredible.
20:29
How do you think
20:32
of where cosmology is now,
20:34
in a sort of historical side?
20:36
Are we in the middle of something unusual historically in your opinion?
20:38
BG: Well it's hard to say.
20:41
When we learn that astronomers of the far future
20:43
may not have enough information to figure things out,
20:46
the natural question is, maybe we're already in that position
20:49
and certain deep, critical features of the universe
20:52
already have escaped our ability to understand
20:55
because of how cosmology evolves.
20:58
So from that perspective,
21:00
maybe we will always be asking questions
21:02
and never be able to fully answer them.
21:04
On the other hand, we now can understand
21:06
how old the universe is.
21:08
We can understand
21:10
how to understand the data from the microwave background radiation
21:12
that was set down 13.72 billion years ago --
21:15
and yet, we can do calculations today to predict how it will look
21:18
and it matches.
21:20
Holy cow! That's just amazing.
21:22
So on the one hand, it's just incredible where we've gotten,
21:24
but who knows what sort of blocks we may find in the future.
21:27
CA: You're going to be around for the next few days.
21:31
Maybe some of these conversations can continue.
21:34
Thank you. Thank you, Brian. (BG: My pleasure.)
21:36
(Applause)
21:38
Translated by Jenny Zurawell

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About the Speaker:

Brian Greene - Physicist
Brian Greene is perhaps the best-known proponent of superstring theory, the idea that minuscule strands of energy vibrating in a higher dimensional space-time create every particle and force in the universe.

Why you should listen

Greene, a professor of physics and mathematics at Columbia University, has focused on unified theories for more than 25 years, and has written several best-selling and non-technical books on the subject including The Elegant Universe, a Pulitzer finalist, and The Fabric of the Cosmos — each of which has been adapted into a NOVA mini-series. His latest book, The Hidden Reality, explores the possibility that our universe is not the only universe.

Greene believes science must be brought to general audiences in new and compelling ways, such as his live stage odyssey, Icarus at the Edge of Time, with original orchestral score by Philip Glass, and the annual World Science Festival, which he co-founded in 2008 with journalist Tracy Day.

More profile about the speaker
Brian Greene | Speaker | TED.com