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Sean Carroll: Distant time and the hint of a multiverse

January 14, 2011

Cosmologist Sean Carroll attacks -- in an entertaining and thought-provoking tour through the nature of time and the universe -- a deceptively simple question: Why does time exist at all? The potential answers point to a surprising view of the nature of the universe, and our place in it.

Sean M. Carroll - Physicist, cosmologist
A physicist, cosmologist and gifted science communicator, Sean Carroll is asking himself -- and asking us to consider -- questions that get at the fundamental nature of the universe. Full bio

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Double-click the English subtitles below to play the video.
The universe
00:15
is really big.
00:17
We live in a galaxy, the Milky Way Galaxy.
00:19
There are about a hundred billion stars in the Milky Way Galaxy.
00:22
And if you take a camera
00:25
and you point it at a random part of the sky,
00:27
and you just keep the shutter open,
00:29
as long as your camera is attached to the Hubble Space Telescope,
00:31
it will see something like this.
00:34
Every one of these little blobs
00:36
is a galaxy roughly the size of our Milky Way --
00:39
a hundred billion stars in each of those blobs.
00:41
There are approximately a hundred billion galaxies
00:44
in the observable universe.
00:47
100 billion is the only number you need to know.
00:49
The age of the universe, between now and the Big Bang,
00:51
is a hundred billion in dog years.
00:54
(Laughter)
00:56
Which tells you something about our place in the universe.
00:58
One thing you can do with a picture like this is simply admire it.
01:01
It's extremely beautiful.
01:03
I've often wondered, what is the evolutionary pressure
01:05
that made our ancestors in the Veldt adapt and evolve
01:08
to really enjoy pictures of galaxies
01:11
when they didn't have any.
01:13
But we would also like to understand it.
01:15
As a cosmologist, I want to ask, why is the universe like this?
01:17
One big clue we have is that the universe is changing with time.
01:21
If you looked at one of these galaxies and measured its velocity,
01:24
it would be moving away from you.
01:27
And if you look at a galaxy even farther away,
01:29
it would be moving away faster.
01:31
So we say the universe is expanding.
01:33
What that means, of course, is that, in the past,
01:35
things were closer together.
01:37
In the past, the universe was more dense,
01:39
and it was also hotter.
01:41
If you squeeze things together, the temperature goes up.
01:43
That kind of makes sense to us.
01:45
The thing that doesn't make sense to us as much
01:47
is that the universe, at early times, near the Big Bang,
01:49
was also very, very smooth.
01:52
You might think that that's not a surprise.
01:54
The air in this room is very smooth.
01:56
You might say, "Well, maybe things just smoothed themselves out."
01:58
But the conditions near the Big Bang are very, very different
02:01
than the conditions of the air in this room.
02:04
In particular, things were a lot denser.
02:06
The gravitational pull of things
02:08
was a lot stronger near the Big Bang.
02:10
What you have to think about
02:12
is we have a universe with a hundred billion galaxies,
02:14
a hundred billion stars each.
02:16
At early times, those hundred billion galaxies
02:18
were squeezed into a region about this big --
02:21
literally -- at early times.
02:24
And you have to imagine doing that squeezing
02:26
without any imperfections,
02:28
without any little spots
02:30
where there were a few more atoms than somewhere else.
02:32
Because if there had been, they would have collapsed under the gravitational pull
02:34
into a huge black hole.
02:37
Keeping the universe very, very smooth at early times
02:39
is not easy; it's a delicate arrangement.
02:42
It's a clue
02:44
that the early universe is not chosen randomly.
02:46
There is something that made it that way.
02:48
We would like to know what.
02:50
So part of our understanding of this was given to us by Ludwig Boltzmann,
02:52
an Austrian physicist in the 19th century.
02:55
And Boltzmann's contribution was that he helped us understand entropy.
02:58
You've heard of entropy.
03:01
It's the randomness, the disorder, the chaoticness of some systems.
03:03
Boltzmann gave us a formula --
03:06
engraved on his tombstone now --
03:08
that really quantifies what entropy is.
03:10
And it's basically just saying
03:12
that entropy is the number of ways
03:14
we can rearrange the constituents of a system so that you don't notice,
03:16
so that macroscopically it looks the same.
03:19
If you have the air in this room,
03:21
you don't notice each individual atom.
03:23
A low entropy configuration
03:26
is one in which there's only a few arrangements that look that way.
03:28
A high entropy arrangement
03:30
is one that there are many arrangements that look that way.
03:32
This is a crucially important insight
03:34
because it helps us explain
03:36
the second law of thermodynamics --
03:38
the law that says that entropy increases in the universe,
03:40
or in some isolated bit of the universe.
03:43
The reason why entropy increases
03:45
is simply because there are many more ways
03:47
to be high entropy than to be low entropy.
03:50
That's a wonderful insight,
03:52
but it leaves something out.
03:54
This insight that entropy increases, by the way,
03:56
is what's behind what we call the arrow of time,
03:58
the difference between the past and the future.
04:01
Every difference that there is
04:03
between the past and the future
04:05
is because entropy is increasing --
04:07
the fact that you can remember the past, but not the future.
04:09
The fact that you are born, and then you live, and then you die,
04:12
always in that order,
04:15
that's because entropy is increasing.
04:17
Boltzmann explained that if you start with low entropy,
04:19
it's very natural for it to increase
04:21
because there's more ways to be high entropy.
04:23
What he didn't explain
04:26
was why the entropy was ever low in the first place.
04:28
The fact that the entropy of the universe was low
04:31
was a reflection of the fact
04:33
that the early universe was very, very smooth.
04:35
We'd like to understand that.
04:37
That's our job as cosmologists.
04:39
Unfortunately, it's actually not a problem
04:41
that we've been giving enough attention to.
04:43
It's not one of the first things people would say,
04:45
if you asked a modern cosmologist,
04:47
"What are the problems we're trying to address?"
04:49
One of the people who did understand that this was a problem
04:51
was Richard Feynman.
04:53
50 years ago, he gave a series of a bunch of different lectures.
04:55
He gave the popular lectures
04:57
that became "The Character of Physical Law."
04:59
He gave lectures to Caltech undergrads
05:01
that became "The Feynman Lectures on Physics."
05:03
He gave lectures to Caltech graduate students
05:05
that became "The Feynman Lectures on Gravitation."
05:07
In every one of these books, every one of these sets of lectures,
05:09
he emphasized this puzzle:
05:12
Why did the early universe have such a small entropy?
05:14
So he says -- I'm not going to do the accent --
05:17
he says, "For some reason, the universe, at one time,
05:19
had a very low entropy for its energy content,
05:22
and since then the entropy has increased.
05:25
The arrow of time cannot be completely understood
05:27
until the mystery of the beginnings of the history of the universe
05:30
are reduced still further
05:33
from speculation to understanding."
05:35
So that's our job.
05:37
We want to know -- this is 50 years ago, "Surely," you're thinking,
05:39
"we've figured it out by now."
05:41
It's not true that we've figured it out by now.
05:43
The reason the problem has gotten worse,
05:45
rather than better,
05:47
is because in 1998
05:49
we learned something crucial about the universe that we didn't know before.
05:51
We learned that it's accelerating.
05:54
The universe is not only expanding.
05:56
If you look at the galaxy, it's moving away.
05:58
If you come back a billion years later and look at it again,
06:00
it will be moving away faster.
06:02
Individual galaxies are speeding away from us faster and faster
06:05
so we say the universe is accelerating.
06:08
Unlike the low entropy of the early universe,
06:10
even though we don't know the answer for this,
06:12
we at least have a good theory that can explain it,
06:14
if that theory is right,
06:16
and that's the theory of dark energy.
06:18
It's just the idea that empty space itself has energy.
06:20
In every little cubic centimeter of space,
06:23
whether or not there's stuff,
06:26
whether or not there's particles, matter, radiation or whatever,
06:28
there's still energy, even in the space itself.
06:30
And this energy, according to Einstein,
06:33
exerts a push on the universe.
06:35
It is a perpetual impulse
06:38
that pushes galaxies apart from each other.
06:40
Because dark energy, unlike matter or radiation,
06:42
does not dilute away as the universe expands.
06:45
The amount of energy in each cubic centimeter
06:48
remains the same,
06:50
even as the universe gets bigger and bigger.
06:52
This has crucial implications
06:54
for what the universe is going to do in the future.
06:57
For one thing, the universe will expand forever.
07:00
Back when I was your age,
07:02
we didn't know what the universe was going to do.
07:04
Some people thought that the universe would recollapse in the future.
07:06
Einstein was fond of this idea.
07:09
But if there's dark energy, and the dark energy does not go away,
07:11
the universe is just going to keep expanding forever and ever and ever.
07:14
14 billion years in the past,
07:17
100 billion dog years,
07:19
but an infinite number of years into the future.
07:21
Meanwhile, for all intents and purposes,
07:24
space looks finite to us.
07:27
Space may be finite or infinite,
07:29
but because the universe is accelerating,
07:31
there are parts of it we cannot see
07:33
and never will see.
07:35
There's a finite region of space that we have access to,
07:37
surrounded by a horizon.
07:39
So even though time goes on forever,
07:41
space is limited to us.
07:43
Finally, empty space has a temperature.
07:45
In the 1970s, Stephen Hawking told us
07:48
that a black hole, even though you think it's black,
07:50
it actually emits radiation
07:52
when you take into account quantum mechanics.
07:54
The curvature of space-time around the black hole
07:56
brings to life the quantum mechanical fluctuation,
07:59
and the black hole radiates.
08:02
A precisely similar calculation by Hawking and Gary Gibbons
08:04
showed that if you have dark energy in empty space,
08:07
then the whole universe radiates.
08:10
The energy of empty space
08:13
brings to life quantum fluctuations.
08:15
And so even though the universe will last forever,
08:17
and ordinary matter and radiation will dilute away,
08:19
there will always be some radiation,
08:22
some thermal fluctuations,
08:24
even in empty space.
08:26
So what this means
08:28
is that the universe is like a box of gas
08:30
that lasts forever.
08:32
Well what is the implication of that?
08:34
That implication was studied by Boltzmann back in the 19th century.
08:36
He said, well, entropy increases
08:39
because there are many, many more ways
08:42
for the universe to be high entropy, rather than low entropy.
08:44
But that's a probabilistic statement.
08:47
It will probably increase,
08:50
and the probability is enormously huge.
08:52
It's not something you have to worry about --
08:54
the air in this room all gathering over one part of the room and suffocating us.
08:56
It's very, very unlikely.
09:00
Except if they locked the doors
09:02
and kept us here literally forever,
09:04
that would happen.
09:06
Everything that is allowed,
09:08
every configuration that is allowed to be obtained by the molecules in this room,
09:10
would eventually be obtained.
09:13
So Boltzmann says, look, you could start with a universe
09:15
that was in thermal equilibrium.
09:18
He didn't know about the Big Bang. He didn't know about the expansion of the universe.
09:20
He thought that space and time were explained by Isaac Newton --
09:23
they were absolute; they just stuck there forever.
09:26
So his idea of a natural universe
09:28
was one in which the air molecules were just spread out evenly everywhere --
09:30
the everything molecules.
09:33
But if you're Boltzmann, you know that if you wait long enough,
09:35
the random fluctuations of those molecules
09:38
will occasionally bring them
09:41
into lower entropy configurations.
09:43
And then, of course, in the natural course of things,
09:45
they will expand back.
09:47
So it's not that entropy must always increase --
09:49
you can get fluctuations into lower entropy,
09:51
more organized situations.
09:54
Well if that's true,
09:56
Boltzmann then goes onto invent
09:58
two very modern-sounding ideas --
10:00
the multiverse and the anthropic principle.
10:02
He says, the problem with thermal equilibrium
10:05
is that we can't live there.
10:07
Remember, life itself depends on the arrow of time.
10:09
We would not be able to process information,
10:12
metabolize, walk and talk,
10:14
if we lived in thermal equilibrium.
10:16
So if you imagine a very, very big universe,
10:18
an infinitely big universe,
10:20
with randomly bumping into each other particles,
10:22
there will occasionally be small fluctuations in the lower entropy states,
10:24
and then they relax back.
10:27
But there will also be large fluctuations.
10:29
Occasionally, you will make a planet
10:31
or a star or a galaxy
10:33
or a hundred billion galaxies.
10:35
So Boltzmann says,
10:37
we will only live in the part of the multiverse,
10:39
in the part of this infinitely big set of fluctuating particles,
10:42
where life is possible.
10:45
That's the region where entropy is low.
10:47
Maybe our universe is just one of those things
10:49
that happens from time to time.
10:52
Now your homework assignment
10:54
is to really think about this, to contemplate what it means.
10:56
Carl Sagan once famously said
10:58
that "in order to make an apple pie,
11:00
you must first invent the universe."
11:02
But he was not right.
11:05
In Boltzmann's scenario, if you want to make an apple pie,
11:07
you just wait for the random motion of atoms
11:10
to make you an apple pie.
11:13
That will happen much more frequently
11:15
than the random motions of atoms
11:17
making you an apple orchard
11:19
and some sugar and an oven,
11:21
and then making you an apple pie.
11:23
So this scenario makes predictions.
11:25
And the predictions are
11:28
that the fluctuations that make us are minimal.
11:30
Even if you imagine that this room we are in now
11:33
exists and is real and here we are,
11:36
and we have, not only our memories,
11:38
but our impression that outside there's something
11:40
called Caltech and the United States and the Milky Way Galaxy,
11:42
it's much easier for all those impressions to randomly fluctuate into your brain
11:46
than for them actually to randomly fluctuate
11:49
into Caltech, the United States and the galaxy.
11:51
The good news is that,
11:54
therefore, this scenario does not work; it is not right.
11:56
This scenario predicts that we should be a minimal fluctuation.
11:59
Even if you left our galaxy out,
12:02
you would not get a hundred billion other galaxies.
12:04
And Feynman also understood this.
12:06
Feynman says, "From the hypothesis that the world is a fluctuation,
12:08
all the predictions are that
12:12
if we look at a part of the world we've never seen before,
12:14
we will find it mixed up, and not like the piece we've just looked at --
12:16
high entropy.
12:18
If our order were due to a fluctuation,
12:20
we would not expect order anywhere but where we have just noticed it.
12:22
We therefore conclude the universe is not a fluctuation."
12:24
So that's good. The question is then what is the right answer?
12:28
If the universe is not a fluctuation,
12:31
why did the early universe have a low entropy?
12:33
And I would love to tell you the answer, but I'm running out of time.
12:36
(Laughter)
12:39
Here is the universe that we tell you about,
12:41
versus the universe that really exists.
12:43
I just showed you this picture.
12:45
The universe is expanding for the last 10 billion years or so.
12:47
It's cooling off.
12:49
But we now know enough about the future of the universe
12:51
to say a lot more.
12:53
If the dark energy remains around,
12:55
the stars around us will use up their nuclear fuel, they will stop burning.
12:57
They will fall into black holes.
13:00
We will live in a universe
13:02
with nothing in it but black holes.
13:04
That universe will last 10 to the 100 years --
13:06
a lot longer than our little universe has lived.
13:10
The future is much longer than the past.
13:12
But even black holes don't last forever.
13:14
They will evaporate,
13:16
and we will be left with nothing but empty space.
13:18
That empty space lasts essentially forever.
13:20
However, you notice, since empty space gives off radiation,
13:24
there's actually thermal fluctuations,
13:27
and it cycles around
13:29
all the different possible combinations
13:31
of the degrees of freedom that exist in empty space.
13:33
So even though the universe lasts forever,
13:36
there's only a finite number of things
13:38
that can possibly happen in the universe.
13:40
They all happen over a period of time
13:42
equal to 10 to the 10 to the 120 years.
13:44
So here's two questions for you.
13:47
Number one: If the universe lasts for 10 to the 10 to the 120 years,
13:49
why are we born
13:52
in the first 14 billion years of it,
13:54
in the warm, comfortable afterglow of the Big Bang?
13:57
Why aren't we in empty space?
14:00
You might say, "Well there's nothing there to be living,"
14:02
but that's not right.
14:04
You could be a random fluctuation out of the nothingness.
14:06
Why aren't you?
14:08
More homework assignment for you.
14:10
So like I said, I don't actually know the answer.
14:13
I'm going to give you my favorite scenario.
14:15
Either it's just like that. There is no explanation.
14:17
This is a brute fact about the universe
14:20
that you should learn to accept and stop asking questions.
14:22
Or maybe the Big Bang
14:26
is not the beginning of the universe.
14:28
An egg, an unbroken egg, is a low entropy configuration,
14:30
and yet, when we open our refrigerator,
14:33
we do not go, "Hah, how surprising to find
14:35
this low entropy configuration in our refrigerator."
14:37
That's because an egg is not a closed system;
14:39
it comes out of a chicken.
14:42
Maybe the universe comes out of a universal chicken.
14:44
Maybe there is something that naturally,
14:48
through the growth of the laws of physics,
14:50
gives rise to universe like ours
14:53
in low entropy configurations.
14:55
If that's true, it would happen more than once;
14:57
we would be part of a much bigger multiverse.
14:59
That's my favorite scenario.
15:02
So the organizers asked me to end with a bold speculation.
15:04
My bold speculation
15:07
is that I will be absolutely vindicated by history.
15:09
And 50 years from now,
15:12
all of my current wild ideas will be accepted as truths
15:14
by the scientific and external communities.
15:17
We will all believe that our little universe
15:20
is just a small part of a much larger multiverse.
15:22
And even better, we will understand what happened at the Big Bang
15:25
in terms of a theory
15:28
that we will be able to compare to observations.
15:30
This is a prediction. I might be wrong.
15:32
But we've been thinking as a human race
15:34
about what the universe was like,
15:36
why it came to be in the way it did for many, many years.
15:38
It's exciting to think we may finally know the answer someday.
15:41
Thank you.
15:44
(Applause)
15:46

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Sean M. Carroll - Physicist, cosmologist
A physicist, cosmologist and gifted science communicator, Sean Carroll is asking himself -- and asking us to consider -- questions that get at the fundamental nature of the universe.

Why you should listen

Sean Carroll is a theoretical physicist at Caltech in Pasadena, California, where he researches theoretical aspects of cosmology, field theory and gravitation -- exploring the nature of fundamental physics by studying the structure and evolution of the universe.

His book on cosmology and the arrow of time, From Eternity to Here: The Quest for the Ultimate Theory of Time, was published in 2010. He keeps a regular blog at Cosmic Variance.

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