MALE SPEAKER: Welcome everybody
to one more "Talks at Google"
event.
Today our guest is Sean Carroll.
It is my distinct honor
to welcome him today.
He is one of the greatest
humanist thinkers
of this generation.
His new book is titled "The
Big Picture-- On the Origins
of Life, Meaning, and
the University Itself."
It's available in
your fine bookstores.
Alan Lightman, author of
"The Accidental Universe"
and "Einstein's Dreams,"
said the following,
"Sean Caroll is a leading
theoretical cosmologist
with the added ability to
write about his subject
with unusual clarity,
flare, and wit."
Sean Caroll is a theoretical
physicist at Caltech.
He received his
PhD from Harvard.
He has worked on the foundations
of quantum mechanics,
the error of time, and the
emergence of complexity.
He has appeared on "The
Colbert Report," PBS's
"NOVA" and "Through
the Wormhole,"
and has been interviewed by NPR,
"Scientific American," "Wired,"
"The New York
Times," and Google.
Please welcome option Sean too.
SEAN CARROLL: Thank
you very much.
Let's see.
So thanks very much
for having me here.
It's good to be back at Google.
And I appreciate
whenever people come out
for a talk on this completely
crazy title, "The Big Picture--
On the Origins of Life, Meaning,
and the Universe Itself."
Usually a response I
get when people first
see the title is who
do you think you are?
How presumptuous must
you be to think that you
can talk about these things?
So I want to get the
disclaimer right on the board
right away, which is
that I do not know
what the origin of life is.
I do not know what the
origin of the universe is.
I do not know the
meaning of life.
What I do think is
that we have a way
of talking about
these things now
that is sort of better
than we had before,
if before I means 500 years ago.
And so this is not so
much a set of answers
to difficult questions
as it is encouragement
to continue the conversation
within a particular framework.
And I'd like to start by telling
you the story of Lucia de Berk.
She was a Dutch nurse.
In 2004, she was convicted to
a sentence of life imprisonment
for murdering several
infants under her care.
Now this is a sad story.
But if you look into it,
there is an interesting thing
about the legal
proceedings, which
is there essentially wasn't
any evidence she had done it.
And you might ask,
how is it possible
that someone gets convicted of
basically being a mass murder
without any direct evidence?
There were no
eyewitnesses that saw
her do anything, no
poison in her handbag
or anything like that.
And the answer is mathematics.
The prosecution
asked statisticians
to estimate the likelihood
that this number of children
would die when a certain
nurse was on duty.
And they said oh, it's
one in a million chance
or hundreds of
millions of chance.
That was the primary reason
why she was convicted.
Now later, other
mathematicians looked at it
and realize that it had
been bad mathematics.
The argument was it was
more like a one in 25 or one
in 100 chance that something
like that would happen,
which as I'm sure you all
know, those chances happen
all the time.
In fact someone pointed
out that the total death
rate for children in
this hospital-- I mean,
it's a pediatric care hospital,
there's sick children there--
the death rate went down
after she was hired,
which is not the
effect you would
expect as the hiring of a
serial killer to really have.
But the other question is,
besides the bad mathematics,
why were the people on
the jury so easily swayed,
even though there was no direct
evidence that she had actually
done it?
And I'd like to point
to the possibility
that really it was part
of our very human desire
to blame something
when something happens.
Rather than to think that
just things happen and there's
a little bit of irrationality
and randomness in the world,
we like to think there's a
reason why things happen.
So when a whole bunch
of children die,
more than we might expect,
we want someone to blame.
And once that starts,
we convince ourselves
that we are on the
right track, finding
this reason for this happening.
The picture on the left is
what Lucia de Berk actually
looks like.
The picture on the right
is the courtroom sketch
of what she looks like.
Once you decide that this
person is the evil one who
was responsible for this,
then you look at her
in a slightly different way.
Fortunately the
bad math went away.
The good math took over
and she was released,
she was exonerated, and
found innocent later on.
But the idea that we find
reasons why things happen
is not necessarily a bad one.
It's an ancient one
and it goes back
to some of the greatest
thinkers in history.
Aristotle very
famously had what he
called the four
causes for things
happening, which we
might really think
of as four kinds of
explanation why things occur.
And one of them was the
final cause for something.
The final cause for Aristotle is
the reason for which something
exists, the goal for which it
is created in the first place.
The final cause of a seed is to
grow into a tree, for example.
And this kind of reasoning,
this kind of metaphysical view
that the world at
its deepest levels
is a story of
causes and effects,
went through to these guys.
This is Spinoza and
Leibniz, Baruch Spinoza
and Gottfried Leibniz.
And they promulgated something
we call the principle
of sufficient reason.
They also promulgated
a principle
that philosophers' hair gets
better and better over time,
as you see.
I think Leibniz got
artificial help.
But overall, there's
certainly a progress in nature
that we can observe here.
The principal of
sufficient reason
is simply the statement
that everything
happens for a reason.
You can find this on bumper
stickers and greeting cards,
but Leibniz in
particular raised it
to the level of a
metaphysical principle.
For everything that happens,
there is a cause or reason why.
And again it's not crazy.
In our everyday experience,
that is kind of what we see.
Things do not just happen.
The book is not going to
see just fly off into air.
There seems to be reasons
why things happen.
If the book moves, it's
because I moved it.
And for Aristotle and
for many other people,
this metaphysical claim
that things that happen
do so because something
causes them to happen,
influenced their
ideas about physics.
So for Aristotle, if
things are moving,
it implies that
something is moving them.
There is a reason why
things are moving.
And his reasoning is
quite straightforward.
If I start pushing on
the book, it will move.
And if I stop, it stops.
There you go.
That's the basis for a way
of thinking about physics,
that if you see things
moving in the world,
you need to explain that.
You need to find the reason
why, what the mover is.
And this extends from
individual objects like books
to everything in the universe.
The universe is full of motions.
But to Aristotle, the
natural state of being
was for things to
stay stationary.
So the existence of all these
motions and transformations
of all sorts
implies that there's
something behind the scenes
causing those kinds of motions.
That is not how we think about
fundamental physics today.
Over the course of improved
explanation and experimentation
and theorizing, we have a
very, very different way
of thinking about
how the world works
at its most fundamental level.
But it's not a way that
we make a big deal of.
There's many things that
popularization of science
will talk about
over and over again.
You can't go faster
than the speed of light.
You can't know your position
and velocity at the same time.
But we don't talk about the
fact that the whole principle
of cause and effect as
a fundamental organizing
principle for the universe
is no longer part of our best
theories of physics.
Some people talked about it.
Bertrand Russell
liked to emphasize it.
He was primarily a philosopher,
but a mathematical philosopher
who knew a lot of physics.
And he said, "The law
of causality I believe,
like much that passes
muster among philosophers,
is a relic of a bygone age,
surviving, like the monarchy--"
he just had to
get that in there.
He couldn't stick to just
the philosophy and science--
"only because it is erroneously
supposed to do no harm."
So this should be, I
hope, in your minds,
quite an extraordinary claim.
The law of causality, that you
have a cause for every effect,
that the cause precedes
the effect, that's
not part of our understanding
of the world anymore.
What is going on?
Well it's not that
anything could happen.
It's not that because
there's no cause and effect
at the deep level, the
book can in fact just fly
all over the place.
The point is that we've
replaced the principal that
causes precede effects
with a principle
that the world is
governed by patterns.
And this happened slowly.
Here's one example, sort
of the tipping point,
if you will, from
one way of thinking
about the world to another
one, was the principal we
call conservation of momentum.
If any of you in the room
have taken physics classes,
you have been tortured by
the principle of conservation
of momentum to sort of
find the solution when
different balls bump into
each other and so forth.
But in fact this
principle is part
of a whole new way
of thinking about how
reality works at a deep level.
It took hundreds of years
and many, many smart people
to think of it.
One of the primary
people was Ibn Sina,
who was a Persian polymath.
To a modern physicist
like me, Ibn Sina
is extremely annoying, because
he wasn't even a physicist,
primarily.
He was a doctor.
He was a medical doctor.
He wrote a lot about the
human body and anatomy.
He did physics in
his spare time,
and he invented
conservation of momentum.
This is very annoying to me.
But he wouldn't have said
conservation of momentum.
Again, it took hundreds
of years to get it right.
The basic idea that Ibn
Sina put his finger on
is that if you could
remove friction,
if you could remove dissipation,
if you imagine something
moving through the vacuum,
it would keep moving forever.
This was the invention
of that other device
that physics teachers like
to use to torture people,
the frictionless surface.
If you imagine something
moving in the complete absence
of friction, it
would not slow down
or require a cause
to keep moving,
it would just keep moving.
And this principle was improved
upon by people like Galileo who
did experiments, and
finally Christiaan Huygens
was the one who
actually formulated
our modern mathematical notion
of conservation of momentum.
So why is conservation of
momentum such a big deal,
over and above
the fact that it's
a tool for physicists to use?
Because it implies that
there's a different way
that the world naturally is.
If you're Aristotle, the
natural way for things to be
is to kind of sit there
in their happy place,
and you need to do something
to get them moving.
In a world with
conservation of momentum,
the natural thing for the
world is to move and change,
and implies you don't need to
explain why things are moving.
Things just naturally move.
And this was developed over
time and probably reached
the pinnacle with Pierre-Simon
Laplace, a French mathematician
and physicist around
the year 1800.
He did not invent
classical mechanics.
It was Newton, as we all know,
who really put the finishing
touches on classical mechanics.
But you can make the
argument that Laplace
was the first person to really
internalize what it meant,
the deep implications of this
Newtonian clockwork universe
worldview.
So you know that if you
do a physics problem,
and again you ignore
friction and dissipation
and so forth, you play
physics billiards,
physicist billiards where
balls just bump into each other
and so forth, you can solve
the problem of these two balls
are moving with
certain velocities.
They scatter and they go
off in another direction.
What is the direction
and the speed
at which the balls
are going to go?
Laplace was the
first to point out
that that process is reversible.
That if you started
up here saying
that the balls are
moving apart, what
were they doing in the past?
And Newton's laws make an
absolutely clear prediction
for what they were
doing in this world
where you can ignore
friction and dissipation.
If you made a movie
of this whole process
and played it backward, it
would look completely plausible.
So Laplace invented what we
now call Laplace's demon,
or what he called
a vast intellect.
Laplace's demon
is something that
has the ability-- what
we'd now really call
it is a really good computer.
Maybe you have a Laplace's demon
in one of the other buildings.
If Laplace's demon
knew everything
about the state of the
universe at one moment in time,
the position and the momentum
of every particle moving
in the universe,
then Laplace says
that vast intellect would
know the future and past just
as surely as the present.
If that vast intellect knew
all the laws of physics
and was able to calculate
what would happen,
there would be
nothing that would
be uncertain to that
intellect about what
would happen in the future,
what had happened in the past.
So this is, even though
it's a subtle difference,
a crucially different way
of thinking about the world.
It's not that this configuration
of stuff causes this one
and therefore causes that.
All of them are
related by a pattern
called the laws of physics.
Just like the integers 0, 1, 2,
3, minus 1, minus 2, minus 3,
the number two happens
after the number one
and before the
number three, but we
don't say that the number one
is the cause of number two,
or two is the cause of three.
We just say that every
number is one bigger or less
than the numbers next to it.
It's just a pattern
that follows,
and you can go forward
or backward equally well.
And Laplace says that
is what the world is
like at a deep level.
Now these things we know better
than Newtonian mechanics.
We've had quantum mechanics,
statistical mechanics,
general relativity,
and so forth.
But the basic Laplacian
principle remains the same.
It's just the actual laws
that we have are different.
So one of the claims
that I make in the book
that I would like to defend
is that this audacious
sounding idea, that
the laws of physics
underlying everyday life
are completely known.
Now when I say this,
people like to stop
listening when I say
underlying everyday life, so I
want to emphasize that.
I'm not saying the laws of
physics are completely known.
I'm not also saying
that everyday life
is completely known.
There's plenty of things
about everyday life
that I don't know about,
plenty of things about physics
we don't know about--
dark matter, dark energy,
black holes, the Big Bang,
plenty of physics that is not
about the underlying
laws that we don't know--
high temperature,
superconductivity,
turbulence, and so forth.
I'm making quite a specific
claim, that you and I
and all this stuff
right in front of us,
literally the stuff we can touch
and see in our everyday lives,
these are made of things--
atoms, electrons, protons,
neutrons.
Those protons and neutrons
are made of quarks.
And all these particles
interact in a certain way.
And we know both what
those particles are
and how they interact.
When I say we know, not
only do we have a good idea,
but a thousand years from now
or a million years from now
this idea will still be right.
Hopefully we will learn
more-- I mean, maybe
the quarks and
electrons and so forth
are made of even tinier things.
That's great.
But there won't stop being
quarks and electrons,
and we won't be wrong
about how they interact.
And there are deep reasons
for believing this is true.
Let me just tell you
what the ingredients are.
This is an atom, right?
This a neutron and a
proton, an electron,
so it's a deuterium
isotope of hydrogen.
The electron is held
together to the nucleus
by electromagnetism.
The individual
protons and neutrons
are made of quarks, up and down
quarks, which are held together
with the strong nuclear force.
Occasionally an up
quark or a down quark
can convert into the other
one by the weak nuclear force
and give up a neutrino
in the process.
Everything is pulled towards
everything else by gravity.
And everything's swims in the
background of the Higgs field.
The Higgs boson
particle is something
we discovered just in 2012,
the Large Hadron Collider.
The particle is
what happens when
you start this field
vibrating, but the field
itself, the Higgs field
pervades all of space,
affects the properties
of the other particles
that we're made up.
There are more
particles than this.
These matter particles,
the electron, the quarks,
neutrinos, they all
have heavier cousins.
But they decay away
very, very quickly
if you try to make them.
They do not affect
our everyday lives.
We know them.
We can complete
them in the theory.
But you don't actually
need to know them to know
what you and I are made out of.
So that's it, as far as our
everyday world is concerned.
The point is that there's no
new particle, no new field,
no new force that we
will ever discover
that will have an impact on
our literal every day biology
or environment, like
what holds this table up.
We hope to discover
a lot more physics.
It will not affect you or
what you're made out of.
So I know this is
a-- I keep being
told this is a technically
inclined audience.
So you don't like the picture.
The picture makes you nervous.
You want an equation.
So here it is.
This is what Nobel
laureate Frank Wilczek
has called the Core Theory.
And he invented the name to
emphasize that we usually
distinguish between the standard
model of particle physics
and general relativity,
our best theory of gravity.
And the reason we do that is
because general relativity is
a classical theory,
not a quantum theory.
We don't have a full
and complete theory
of quantum gravity yet.
What gets lost in
that true statement
is that we have a
pretty good every day
theory of quantum gravity.
We know quantum gravity in the
regime where fields are weak.
We know quantum
gravity perfectly well
if you want to use it to
calculate the Moon orbiting
around the Earth, for example.
So if you're literally
only interested
in the regime of everyday life,
this is it, including gravity.
This is basically the Feynman
path integral, the probability
to go from one amplitude in a
field theory to another one.
I'm not going to go
through all the details,
but basically you see how
all of the different pieces
of modern physics get involved.
There's quantum mechanics,
space time, gravity,
this is Einstein's general
relativity right there,
all the other forces,
electromagnetism
and the nuclear forces,
the matter particles
of which we are made, and
the Higgs in the background.
If you want know more
about the details,
I did manage to squeeze
it into the book.
But I was told it should
be put into an appendix,
and the font size is very
small in the appendix.
But nevertheless, every term
here is explained, briefly.
It takes a year long
quantum field theory course
in graduate school
to get the details,
but at least say what
every term means,
including the i for
example and including
the k less than lambda.
What you don't see are causes,
purposes, or reasons why.
It's just Laplacian calculation
over and over again.
This is the modern
version of what
you need to program
into Laplace's demon
so that starting from the
position and configuration
of the world at one point, it
can find out what will happen
next or what happened before.
The final criterion
you need for this
to be a good, successful
theory is that it
should fit on a T-shirt.
So we did the experiment.
There it is.
It totally fits on the T-shirt.
You can buy them on my website.
I don't make any money off them,
but you can buy the T-shirt.
So I want to do at least
one minute of justification
for this grandiose claim.
I mean, it's one thing
to have a theory.
We have lots of theories.
Our theories are never complete.
Our theories are never things
we should have 100% credence in.
We should always,
as scientists, be
willing to improve
upon our theories.
So what gives me the
right to say that
a million years from
now, this is still
going to be the
theory underlying
the particles and forces of
which you and I are made.
The answer is something
called crossing
symmetry, which is a feature
of quantum field theory.
I mention fields.
Fields are in fact what
you and I are made out of.
You might have taken a
physics course and been asked,
is light a particle or a wave,
or is an electron a particle
or a wave?
Probably you were
not told the answer.
It's a wave.
That's what it is.
According to quantum mechanics,
the world is made of waves.
The world looks like particles
when we look closely enough.
But really the way that
we talk about the world
in modern physics is through
quantum field theory.
And quantum field theory
uses these little pictures
called Feynman diagrams.
My personal claim to fame
in the world of physics
is that the desk I have
in my office at Caltech
used to be owned
by Richard Feynman.
So I sit at Feynman's old desk.
I leave blank pieces of paper in
there hoping some diagrams will
appear, but it never happens.
So what Feynman did is to
invent a way of talking about
what happens in particle physics
and quantum field theory,
and also how likely
it is to happen.
So if we have a particle we
know about, like a proton,
and we imagine there's
a new particle.
Maybe there's a
new particle that
really does affect how
you choose your food
or how plants photosynthesize
or how you think,
well then there must be
a Feynman diagram that
says that that
particle can interact
with a proton via
some new interaction.
And Feynman's rules say how
you can use this diagram
to calculate how likely
that is to happen,
the amplitude or the
probability of that process.
And then crossing
symmetry says--
this is a diagram that time
evolves from left to right.
So X comes in, P comes in.
They just scatter
off of each other.
Crossing symmetry says that
if this diagram exists,
I can rotate it
clockwise by 90 degrees
and get another
diagram that exists.
So I'm glossing over the
difference between particles
and antiparticles here.
Really if this is a proton,
this is still a proton.
This diagram talks about a
proton and an antiproton coming
together to produce
an X particle
and an anti-X particle.
And what crossing
symmetry says is
if you know how big
this diagram is,
if you know how likely
that process is,
you know how likely
this process is.
So if this new X particle
interacts with protons
or with neutrons or
quarks or neutrinos
or whatever, strongly enough
to affect your everyday life,
then we could make it by
smashing together the particles
out of which we are created.
And the punchline
is we have looked.
We have smashed together
all the particles.
We've smashed together
protons and protons.
That's what the LHC is doing.
We've done protons and
antiprotons, electrons
and electrons, electrons and
positrons on down the line.
We would have loved to find
a new particle like this,
and we have not done so yet.
There are no
particles like this.
The closest we have
is a tiny little bit
of a hint at the
Large Hadron Collider,
as of May, 2016,
that there might
be a new particle that is 800
times the mass of the proton.
It may or may not be true.
There's a little bump.
Maybe it's there.
We're still looking.
But even if it is, it
decays away in less than a
zeptosecond.
It is not something that affects
your everyday life in any way.
So in terms of the particles
that actually matter to you
and me that make us up, we
know the complete collection.
So the question is, the
big question ahead of us
today is, if that's true,
why does the manifest world
of our everyday experience
seem so different
than the underlying laws of
quantum mechanics and quantum
field theory and
particle physics?
And the answer is this
tricky idea called emergence.
You can have an
underlying layer of
microscopic fundamental physics
made of particles, forces,
and differential Equations.
It can do what Laplace said.
Information is conserved from
moment to moment over time.
The rules of physics are
patterns written down
in differential equations.
And yet, when you
collect together
many of these particles, there
can be collective behavior
that is implicit but
not at all obvious,
in this microscopic rules.
This collective behavior could
emerge into wholly new concepts
and vocabularies.
So the idea that there are
tables and chairs and people
and planets, that's
nowhere obvious
in this underlying description.
But the two levels can be
compatible with each other.
This is the world of cause
and effect, reasons why,
dissipation, and most
importantly the arrow of time,
the difference between
past and future.
So our task today is to see
how this one level can be
compatible with the other one.
Why can we think that there are
reasons why causes and effects,
and for that matter
right and wrong
and truth and
beauty in the world,
even though at some
level, deep down, it's
just stuff happening
according to that equation
that I showed you.
So the arrow of time, one
of my favorite topics,
is simply the fact that
the past and future
are different from each other.
This is not a surprising
fact if you're Aristotle.
Motion through time, the
evolution of the universe
is obviously something profound.
The past is different
from the future
because the past
already happened.
But according to Laplace
or to modern physics,
there's no difference between
moving toward the past
and moving toward the future
at the microscopic level.
Only macroscopically
is there a difference,
and there are many differences.
You can remember the past, but
you can't remember the future.
We were all younger in
the past and will be older
in the future.
Sorry to break that to you.
Most importantly or
most fundamentally,
entropy increases over time.
Entropy is the way
that we have of talking
about the disorderliness,
the randomness,
the disorganization
of stuff over time.
And there's a general
principle that organized things
like unbroken eggs can easily
evolve into disorganized things
like broken eggs, but never
backward the other way,
or at least very, very
rarely, or at least
you need to do a lot of
work to make it happen.
If you live in a room or you
have an office and it's clean,
it will naturally happen that
it becomes messy over time.
If your office or room
is messy, it will never
clean itself up all by itself.
You need to do work.
That's because
entropy is increasing.
If you'd like to think of it as
the working out of a great law
of physics, be my guest.
And the reason why
this is true is
because there are more
ways to be high entropy
than to be low entropy.
There are more
arrangements of stuff
that are messy
than are organized.
This was the brilliant
insight of Ludwig Boltzmann,
the 19th century physicist.
And so therefore if you start in
a configuration of low entropy,
entropy naturally increases.
The problem was,
therefore, why isn't
entropy at its maximum value?
There are many, many
more ways for entropy
to be high than for
entropy to be low.
Why is it true that
entropy was ever low?
So Boltzmann, in
other words, explains
why, given the entropy
of the universe today,
it will be higher tomorrow.
There are more ways to be high
entropy than to be low entropy.
But he does not explain
why it was lower yesterday.
I'm here to tell you the answer.
The reason the entropy
of the universe
was lower yesterday than today
is because it was even lower
the day before yesterday.
And the reason why
that's true too
is because it was even
lower the day before that.
And this reasoning goes
back 13.7 billion years
to the Big Bang.
The reason why the universe
has had a low entropy all along
is because it started that way.
Nobody knows why.
This is a profound question
for modern cosmology.
But once you give me that, I
can explain all the differences
between the past and the future.
So one way of thinking
about it is, we all agree
that there's no arrow of space.
If you're out there
in a space suit,
there would be no difference
between up and down,
left, right, forward, backward.
But here in this room there is.
If I drop the laser pointer,
I know it's going to go down.
There's an arrow of
space pointing down.
Nobody thinks that this is
some profound consequence
of the fundamental
laws of physics.
It's because we live in the
vicinity of a very influential
object, namely the Earth.
The point of this discussion
is that time is like that.
There is no intrinsic arrow of
time in the laws of physics,
but we think that there is
in our observable universe
because we live in the aftermath
of a very influential event,
the Big Bang.
I'm not going to explain
why the Big Bang was low
entropy, because nobody knows.
Don't believe anyone who
comes in here and tells you
they know.
It's a good topic
for conversation.
But given that, we can try
to explain other features
of the arrow of time.
For existence, the existence
of memories and causes, right?
Memories are something
where we know something now.
It implies something
about the past.
The cause is something
that we do something now,
it implies something will
happen in the future.
Where does this
asymmetry come from?
So here's a memory.
Here's a picture of
a record of an event.
This is an egg that was broken.
You're walking down
the street, you
see a broken egg
on the sidewalk.
Ask yourself, what does the
future hold for this egg?
I don't know why you're
asking yourself this.
You're in a reflective mood.
What is the future of this
poor egg going to hold?
You don't know.
I mean, there's many
different possibilities.
It could just sit
there for a long time.
It could wash away
in a rainstorm.
Someone could clean it up.
But if you ask yourself what
does the past of the egg
probably experience, what
are things like for the egg
recently, with
overwhelming probability
that egg used to be unbroken,
and someone dropped it.
Why is it that this
single record-- this
isn't even moving, right?
The macroscopic information
is not changing in time.
The egg is just sitting
there stationary.
Why are you able to draw
such different conclusions
about its past than its future?
The answer is
because secretly you
know that the Big Bang
had a low entropy.
You don't use that in your
everyday life, but that's why.
If all you knew is physics and
the macroscopic information
about the egg, the
number of things
that could happen
in the future would
be exactly equal to
the number of things
that happened in the past.
But the extra thing you
know is that the universe
started with low entropy.
That ties the possible
histories in the past.
And what that means is
that you know something
about the past
condition of the egg.
Unbroken eggs lead
to broken eggs,
because that's the
easiest way to get
to broken eggs given the low
entropy past of the universe.
And causes and effects
work the same way.
Just like an egg is
something-- a memory
is something that if it were
a little bit different now,
it would imply something
different about the past.
Think about what a cause does.
If I say I move my hand
and the book moves,
if I move my hand a
little bit differently,
like I missed the book, then
it wouldn't have moved, right?
So if my hand moving is the
cause of the book moving,
that's because if my hand had
done something very different,
it would have implied
something different about what
comes next.
If I'm waving my
hand over here, I
could wave in a
slightly different way
and it doesn't imply anything
different about the book.
And therefore this hand moving
is not the cause of the book.
It's the thing that came
right next to it that is
the cause of the book moving.
The idea that causes
proceed effects
emerges in our macroscopic world
because of the arrow of time.
So that is the first
little baby step
towards reconciling
our everyday world
with this impersonal,
calculational underlying
laws of physics.
The next step is if
the universe is just
a story of stuff
becoming more and more
disorderly and entropic
over time, why are we here?
Why is anything complex and
intricate and organized exist
in the universe?
This is another good
question to which
we don't know the
complete answer,
but it's interesting
that there's
a big part of the answer
which is that simplicity
versus complexity is a whole
different axis on which
to think about the
world than low entropy
versus high entropy.
If you think about the classic
example of entropy increasing,
mixing cream
together with coffee.
You know that in this picture,
this picture, this picture,
time moves left to right.
It's easy to mix things.
It's hard to unmix them.
This is low entropy.
That's high entropy.
But this low entropy
configuration
with the cream on top, coffee on
the bottom is also very simple.
Cream's on the top,
coffee's on the bottom.
Towards computer
thinking people,
it's algorithmically
compressible.
A small file sizes
necessary to tell you
what happens microscopically
in that picture.
But the same thing
is true over here.
It's high entropy.
It's all mixed up,
but still simple.
It's all mixed up.
That's all you need to know.
The file size is
also small here.
It's in the middle where the
cream and coffee are beginning
to get mixed together, where
the tendrils of cream and coffee
are reaching into each other
and a fractal pattern develops,
that's where it's complex.
This file size to show
you that picture is bigger
than the one on the left,
the one on the right.
So while entropy in the universe
just increases monotonically,
complexity first increases
and then goes away.
When entropy is
very, very small,
it's impossible to be
complex because there's not
that many possible
arrangements you can be.
But when entropy
is very, very large
it's impossible to be
complex, because everything
is smooth and homogeneous.
It's only in between that
complexity is possible.
And therefore it's not only
compatible with the increase
of entropy to see complex
forms arise in the universe.
It's because entropy
is increasing
that it can possibly happen.
And this behavior, complexity
going up and going down,
is not just cream and coffee.
The universe is the same way.
The universe started
very simple and low
entropy, hot dense expanding
universe near the Big Bang.
It will end very simple
and high entropy.
Eventually all the
stars will burn out.
All the black holes
will evaporate
and we'll have nothing
but empty space.
We'll once again be very,
very simple but high entropy.
The last black hole will
evaporate 10 to the 100 years
from now.
Yes, that's right, one
google years from now.
Before you guys stole the word
from us, this was a google.
The entropy of the universe
increases monotonically
through its history, but the
complexity comes and goes.
The universe became
more and more complex
up to the present
day, and will start
becoming less and less complex
as those stars stop shining.
The stars stop shining
about 1 quadrillion years
after the Big Bang.
So it's today when gravity
has pulled things together,
made the universal lumpy,
brought into existence planets
and stars and galaxies,
biospheres and people
that the universe is
interesting and complex.
As small as we are compared
to the vastness of the cosmos,
we live in the interesting part
of the history of the universe
for exactly that reason.
And this kind of
reasoning can help
us explain even questions
like why life itself exists.
So I like to tell the story,
I was once on a plane flight
going to a conference
to give a talk.
And as often happens, if you're
a physicist or a cosmologist,
people find that
out and they want
to tell you their theories.
Everyone it seems has a
theory about the universe.
I was reading some papers
about statistical mechanics
and the origin of life.
The guy sitting next
to me on the airplane
says, oh yes, I've
read those papers.
So I'm a little bit skeptical.
But he says in fact I can
tell you the purpose of life.
And I'm very skeptical.
But he says the purpose of
life is to hydrogenation
carbon dioxide.
This is not the response
I was expecting to get.
It turns out that I was seated
next to Dr Michael Russell, one
of the world's experts
in abiogenesis,
the origin of life.
He works at JPL, just down
the street from Caltech.
And he writes papers
with graphs like this.
And he was very serious about
the hydrogenation business.
What he means-- and again, we
don't know whether this is true
or not.
We don't know how life begins.
This is one of the theories
people are advancing.
But you can see in
all these theories,
you can sort of see
the hints of how
a really, really
difficult problem suddenly
seems to be a lot more soluble.
So what Russell's
pointing out, that there
are many environments
in the early Earth
where there's a lot of carbon
dioxide and a lot of hydrogen.
And that is a low
entropy configuration.
And that's what we
call high free energy.
So this is free energy versus
different compound structures.
If you took that carbon,
removed the oxygen,
and palled them all up with
a bunch of hydrogen atoms,
the carbon would
now be in methane,
and the entropy
would be much higher.
In some sense,
the carbon dioxide
wants to become methane.
The problem is that
there's a barrier,
that all the ways to
get from CO2 to CH4
involve going through even
lower entropy configurations--
higher free energy
configurations.
And that can't
happen all by itself.
It's not like lighting
a match on a candle.
But what Russell
points out is that it
can happen if there's a
complicated network of chemical
reactions brought together
in just the right way
with the right
catalysts and so forth.
And that kind of network
in the right conditions
could be the precursor of the
metabolism of modern life.
So in the 1980s on the basis
of this kind of reasoning,
Russell predicted the
existence of a certain kind
of underwater geological
formation, what
we call warm alkaline
hydrothermal vents.
And after he made the
prediction, they found one.
This is the lost city
configuration, lost city--
I don't know what it is.
It's a bunch of stuff happening
under the mid-Atlantic Ocean.
And it has exactly
the properties
that you would need to get
this kind of reaction starting.
There might be many of
them underneath the ocean.
This is something that
we think will probably
last there for tens of thousands
of years before it washes away.
And you make new ones.
So we don't know how life began.
But this way of
thinking about it
is interesting because rather
than looking at it as life
exists, how did it possibly
start, this point of view
is saying we have a
puzzle, how to increase
the entropy of the
early Earth, and life
is the solution to that puzzle.
Of course you need
to get it together
with other things like cell
walls and replication and RNA
and so forth.
Putting all those
pieces of the puzzle
together is full employment
for abiogenesis researchers
for the next hundred
years probably.
But then once that gets
started, once you have life,
then things get interesting.
What is life anyway?
Nobody knows that either.
I like the definition given
by the physicist Erwin
Schrodinger.
Schrodinger said that
life is something
that keeps moving long
after it should've stopped.
What does he mean by that?
He means if you put a dead
thing in a bowl of water,
it will just sit there.
It won't do anything.
If you put a living thing like
a goldfish, in my experience
it will also just die
and then it will float
to the bottom of the thing.
But if you give it
food, the living thing
can last for a long time.
What is food?
Food is energy in
a low entropy form.
That's exactly what
we get from the sun.
The sun gives us
energy, and you might
think that's what's important.
The sun gives us energy.
But if the whole sky were
the temperature of the sun,
none of us would be
here talking about it.
You would come to
thermal equilibrium.
What the sun gives us
is low entropy energy.
For every one photon of
light we get from the sun,
we give back 20 photons
back to the universe.
But we get visible light.
We radiate infrared.
We radiate photons with
1/20 of the energy each.
So we get back the same
energy we get, but only
after increasing its
entropy by a factor of 20,
by photosynthesizing,
chewing our cud,
having meetings, writing
software, et cetera.
Then we radiate back
into the universe
having increased its disorder
by a considerable fraction.
And that explains how an
individual organism can persist
and survive and sustain itself.
But of course the
great thing about life
is that it reproduces
and there are mutations.
And therefore evolution
gets off the ground.
So if our goal is
to understand how
ideas like cause and effect
and even purposes can arise,
evolution is a
wonderful mechanism
for making that happen.
Why do giraffes have long necks?
Well one answer is because
of the state of the universe
and the laws of physics.
That's not a very
helpful answer, is it?
Another answer is to reach these
leaves up there in the tree.
Evolution can be thought
of as a search strategy
for this different
genetic information,
for the genome that
passes down to try
to maximize the chances
of reproductive success
in this particular environment.
And given that strategy, it's
perfectly OK to say the reason
why the giraffe has a long
neck is to reach those leaves.
You can also see something
like this in simple computer
examples.
This is a cellular automaton
invented by computer scientist
Melanie Mitchell.
She calls it Robby the Robot.
Robby had a party last night.
There are beer cans
scattered all over his house.
What's the best algorithm
to pick up the beer cans?
Many of you are
probably familiar
with genetic algorithms.
You just pick some random
strategies, let them evolve.
That is to say, find out which
ones are most successful,
cull them.
Randomly mutate them.
Find out which ones of
those are most successful.
Repeat this.
And in a very few
generations, Robbie
finds a better strategy than
its human designers ever found.
And once it has that
strategy, are you
allowed to say Robbie
is quote, unquote,
trying its best to
pick up the cans?
Sure, that's what
I'm trying to say.
There's no such thing as
a real true purpose that
goes above and beyond a
way of talking about what
happens in the physical world.
Evolution, laws of physics,
and the arrow of time
make it perfectly sensible
that such ways of talking
would become convenient and
useful as complicated organisms
adapt and go on.
And that even
counts for thinking,
for consciousness itself.
We again do not know how
consciousness arose either.
We don't even know
what consciousness is
or how it works.
But we can see little
steps that might
have happened along the way.
So Malcolm MacIver, who's
an engineer at Northwestern,
likes to talk about the first
fish to flap up onto land.
The evolutionary
pressures on land
are very, very different
than those under the water.
If you're under the water,
you can't see very far.
The attenuation length of
the photons is a few meters.
So you're swimming around
at a few meters per second,
you need to be able to
instantly react to what you see.
But then you climb up
onto land, and now you
can see for kilometers.
So there's a whole new
evolutionary pressure which
is to make a smart decision.
You had the time to
think about what to do.
And what that means is that
developing the capacity
to contemplate different
hypothetical futures
becomes a smart thing to do.
And we can look back.
We can do the neuroscience
and look in your brain.
What are you doing when
you're contemplating
hypothetical futures,
when you're really
sort of consciously
imagining different things?
It's not a whole new
module of your brain.
You're using the same
part of your brain
that gets used when
you recall a memory.
This is exactly what
evolution likes to do,
to repurpose all parts of
the functional organism
to do new tasks.
Imagining the future is one
part of being conscious.
It's not the whole
thing obviously.
But again you could see
how it would happen.
We don't need to invoke
anything beyond the particles
and fields of the Core Theory
to explain our consciousness.
Here is a picture of my head.
It's not to scale.
But this was a map
made in the laboratory
of David Poeppel at NYU.
It's evidence that
I actually have
a brain inside my skull,
which I was happy to see.
When you have a
thought, in your neurons
there are literally
charged particles jumping
from one neuron to another.
Any physicists will tell you,
following the Core Theory,
that charged particles in
motion create magnetic fields.
So this is an MEG, a
magnetoencephalograph.
It is literally an image
of the tiny magnetic fields
that stick outside of my skull
while I was hearing some sounds
and my neurons were
going ah yes, you
were hearing some sounds.
Now this isn't evidence
of anything very strong,
just a reminder of
the obvious fact
that your thoughts
and your dreams
and your aspirations
and your emotions
are correlated with physical
goings on inside your brain.
We don't need to go
beyond that to explain
what is it that is happening
when you are thinking.
This is again an
ancient argument.
It goes back to these folks.
This is Renee Descartes, famous
physicist, famous mathematician
also.
And this is Princess
Elizabeth of Bohemia,
considerably less famous.
But they became friends and they
carried on a long conversation,
basically because
Descartes was always
looking for a potential patron.
And even an exiled royal family
like Princess Elizabeth's was
is better than no
royal family at all.
Unfortunately she did
not become his patron,
and she didn't give him a
hard time about his ideas.
One of Descartes' favorite
ideas was mind body dualism.
He felt that the
mind or the soul
was something immaterial
and separate from the body.
And Elizabeth pressed
him on how in the world
could something immaterial
and without any location
could possibly influence the
physical reality of our body.
So he had a theory.
This is the pineal
gland in your brain.
It's the one part
of your brain that
is not broken into two,
two different hemispheres.
So Descartes drew this
picture and literally proposed
that the soul communicated
through your pineal gland
with your body.
Nobody ever bought
this explanation.
But the point is that if
Elizabeth were alive today,
she would point at the
equation for the Core Theory
and say if you want to
believe in something
over and above the
physical world,
how does that change the
behavior of the particles
in your brain as it is predicted
by that Core Theory equation?
It's not enough to
say, well there's
things we don't understand.
If you don't think that
the brain is simply
the workings out of physical
matter at some level,
then you're saying
that equation is wrong.
And saying how that
equation is wrong
is a daunting
obstacle to overcome
if you think that
the world is more
than just the physical stuff.
And you don't need
to think that.
Even if you think the world
is just physical stuff,
it's perfectly OK
to talk about things
that we like to
talk about when we
discuss human beings like
choice and responsibility
and morality, for exactly
the same reason it's
OK to talk about temperature and
density and pressure in a fluid
even though we know it's
really made of atoms.
These are emergent
features of the world.
If you're Laplace's demon,
you can predict the future.
That is true.
There's no such thing as
free will in the world
of Laplace's demon.
But we're not in that world.
None of us has that ability.
None of us knows the
requisite information.
Therefore, the best
way we have to model
the behavior of real
human beings, real agents
in the world, is as creatures
who are able to make decisions.
The problem only arises when
you mix up the vocabularies.
There are different vocabularies
that can both be true,
people making choices and atoms
obeying the laws of physics.
But you have to pick
one or the other.
You walk up to your
closet and say,
am I going to wear the red
shirt or the blue shirt?
Oh, I'll just do whatever my
atoms say the laws of physics
are going to tell them to do.
That does not make sense.
You can talk about
what your atoms do
or what you're going to do.
You can't talk
about both at once.
So this picture of the
world as just governed
by the laws of
physics isn't as bad
as you might think in terms
of recovering the human scale
world of meaning and
mattering and so forth.
But there is a downside, namely
that you're going to die.
If you're made of the
stuff of the Core Theory,
if you're made out of the atoms
and particles, then when you
die there's no place
for the information
that was in your brain to go.
There's no known
forces or particles
that could carry that away if
your atoms are actually still
there in your brain.
So one way of
driving that home is
to look at this
plot that was made.
There's some
complexity theorists
who like to study
scaling laws in biology.
It turns out that
for mammals, there's
a scaling law that relates
your heartbeat to your mass,
and also your life
expectancy to your mass.
And they cancel out.
So everywhere along this
curve, the total number
of heartbeats in the lifespan
of a mammal is about the same.
It's about 1.5
billion heartbeats.
Now humans are the exception.
Because we invented
medicine, Obamacare, and now
we live for about
twice as long as you
would have predicted on the
basis of this scaling relation.
But then again before
we had modern medicine,
we lived for 30 or 40
years, right on the line.
So that means that we get
about 3 billion heartbeats
in our lives.
There's no law of
physics that says this.
Biological progress can
certainly extend our lifespan
way, way longer than this.
But we're not there yet.
And if you believe that you
are a bunch of particles that
are interacting
under that equation,
this is the span of your life.
You do not continue
on after that.
And the 3 billion is
kind of a big number.
3 billion is pretty
big, but it's not
limitless by any stretch
of the imagination.
To me, claims like
well, if I'm just stuff
obeying the laws of physics
removes all the meaning
and mattering from my
life don't hold water,
because to me the
fact that I only
have this small
period of time makes
every little bit of that
period much more precious.
Every one of your heartbeats
should be used to good effect.
One way I like to
drive this home
is the last slide is the pale
blue dot image of the Earth.
So the "Voyager" spacecraft,
one of the first spacecraft that
left the solar system, when
it was 4 billion miles away,
Carl Sagan and his
team convinced NASA
to turn it around
and take a parting
shot, a photograph of the Earth
and the rest of the planets
in the solar system.
So that little dot is us.
That's the Earth,
the pale blue dot.
Every human being who has
ever lived is in that picture,
or at least their atoms
are in that picture.
So on the one hand,
it makes you feel
very small to think that all of
us are just in that little bit.
And this is not anywhere
near the whole universe
that we're looking at right now.
On the other hand, we
did take the picture.
That's pretty good.
It's a selfie for
the whole Earth.
"Voyager's" just
an elaborate selfie
stick out there letting us
take a picture of ourselves.
But you know what, the
ability to take a selfie
shouldn't be underestimated.
It is a reflection of the
fact that even though we
are very tiny compared
to the universe,
both in space and in time,
we are in that complex phase.
We are part of the universe
that has gained the ability
to think about ourselves, to
be self aware, to make choices
for ourselves on the basis
of rational reflection,
to create technological
marvels that
can help us look at ourselves
and think about what to do next
rather than just moving
from moment to moment.
That's both the world we
live in, which is true,
and it's a world that we can
try to work towards making
the best we can, which is good.
Thank you very much.
[APPLAUSE]
AUDIENCE: So you
connected the Big Bang
to the arrow of
time and the fact
that the past seems
different from the future.
Can you also sort
of use it to give
an explanation for why it seems
impossible to go back in time?
SEAN CARROLL: Well, yes and no.
So this is a good question.
The question is,
there's an arrow of time
that we attribute to low
entropy of the Big Bang.
What about going
backward in time?
Mostly the fact that we
can't go back in time
is because time is
only one dimensional.
That's the real reason.
We can go around in
space because space
is three dimensional.
If time were two
dimensional, there'd
be no trouble to go
back to the past.
Basically the fact that time
is one dimensional and the fact
that the world happens once
at every moment in time
means there's just no way
to get there from here.
It is actually not
in any direct way
related to what we
call the arrow of time.
Now that's complicated
by the fact that
in Einstein's theory of general
relativity, space and time
are flexible.
And you can imagine building a
wormhole or something like that
that would get into the past.
And remember I said
just a few slides ago
that our impression that
there is free will and choice
is ultimately because entropy
is increasing toward the future.
So if you were able to hop in a
wormhole like an "Interstellar"
and go into the past,
then your personal future
would be the past
of the universe.
And which one wins?
And that's a very
good question to which
no one has the right answer.
Probably the simple
answer is you can't do it,
you can't go backward in time.
AUDIENCE: The
emergent properties
that come out of
atoms or of us, do
you think they're determined?
You said they're implicit.
Do they have to
happen, or could there
be other emerging properties
from the same particles?
SEAN CARROLL: Well this
is a good question too.
Are the emergent properties
determined in some way
from the underlying stuff?
Yes and no.
I mean ultimately the
answer is yes in the sense
that if you believe
everything I've said,
then given that Core
Theory equation,
you could put it on a computer
and uniquely find out what
would happen in the future.
So in that sense
everything is determined.
There's the one
very large footnote
to that, which is of
course quantum mechanics
introduces some
uncertainty of the game.
As I personally am an
advocate of the many worlds
interpretation where
everything's still
is 100% deterministic,
but not everyone
agrees with me about that.
So that doesn't give you any
help whatsoever in the emergent
properties business.
It's still a rule.
It's just a boring
rule for probabilities
rather than some
deterministic rule.
So on the one hand,
if you knew everything
and had Laplacian demon
level intelligence
about the underlying stuff,
the future is determined.
The emergent properties
are determined.
That's the whole story.
On the other hand, you
do gain new knowledge
by figuring out what those
emergent properties are.
You have a way of talking
about the system that
is extremely
algorithmically compressed
compared to the microscopic
way of talking about it.
So even though the
behavior is determined,
you do learn something new by
figuring out what the higher
level laws really are.
And the higher level
laws could be true even
with different underlying stuff.
The underlying stuff determines
the higher level laws,
but not vice versa.
The way to become a successful
biologist or psychologists
is not to study
particle physics.
That's the
fundamental rule here.
AUDIENCE: So you said that the
Core Theory sort of explains
like-- and it will remain
true for forever basically.
So at different
moments in our history
we've had this sort
of view of physics
or mechanics or whatever.
What makes this moment in
human history different,
like whatever we know
will never change?
SEAN CARROLL: Well again,
it's not that whatever we know
will never change.
It's that very particular way
of talking about the universe
will remain accurate.
Like Newtonian gravity
was well established.
Of course now we know
that the vocabulary
used by Newtonian gravity is
not the best vocabulary to use.
Einstein's theory of general
relativity came along,
gave us a very good,
better way of talking.
And the whole
vocabulary is different.
Einstein talks about curved
space time and energy momentum.
Newton talks about absolute
space and time and forces
and so forth.
But it remains true
that if you want
to get a rocket from
the Earth to the Moon,
you put Newton's equations
into the computer.
So there is a certain
level of establishedness
that a scientific
theory has passed
which it might be improved upon,
but it will not be discarded.
If you're thinking about
things like the phlogiston
model of combustion or the
plum pudding model of the atom,
those were never
accepted as correct.
The Core Theory
is just as likely
to remain true as this statement
that this table is made
of atoms is likely to be true.
You might have a
better understanding
of what atoms are,
but the table's
not going to stop
being made of atoms.
AUDIENCE: What are you
working on these days?
SEAN CARROLL: What am I
working on these days?
I'm working on doing
exactly this thing, which
is making a better version of
the underlying laws of physics.
We do have this problem that
quantum mechanics and gravity
do not play well together
in extreme conditions,
near the Big Bang, near the
black holes and so forth.
So there are some
of us who think
that space and time themselves
are not fundamental,
that we need to do better at
a quantum mechanical level
of figuring out how space
and time themselves are
emergent from a deeper
level of description.
So I actually am
ambivalent about time.
Time may or may
not be fundamental.
I'm 99% convinced that
space is not fundamental,
and it's somehow a
good approximation
just like the fluid description
of the air this room is
a good approximation, even
though it's ultimately
made of atoms.
AUDIENCE: To a lay
person, you say
that the evolution of particles
is pretty well described
by the Core Theory.
But the process of
statistical mechanics
seems a little bit
more hand wavy.
It seems as though the way you
enumerate states of the world
and group them into
categories really
determines the predictions of
your theories to a wild extent.
Where do we stand
on the evolution
of statistical mechanics then?
SEAN CARROLL: You point
our a very important point.
It's actually related
to the earlier question.
Statistical mechanics speaks
a language of core screening
and macroscopic states.
So if you have a bunch
of air in this room
like we have right now
and it's made of air atoms
and molecules, you want to
describe it in a course grained
way as a fluid
with a temperature
and a pressure and a density.
Well that's one way
of describing it.
That's one sort of
macroscopic description.
But there are many ways.
For example when we
really coarse grain,
we really take a
little region of space,
like one cubic
millimeter in size.
We take the average
number of particles
and the average motion
of those particles
and we relate them to
macroscopic features
of temperature and density
and pressure and so forth.
But who says that's
what you can do?
You could imagine taking a
little tiny cube in momentum
space rather than
physical space.
You could coarse grain that way.
You could reduce your number
of variables that way.
What you would end up
getting is a horrendous mess.
And that's because given the
actual configuration of stuff
in the world and
the laws of physics,
there's some
reductions from many,
many particles to much smaller
macroscopic descriptions that
work nicely, and
some that don't.
So even though it in principle
sounds very arbitrary,
in practice this gain in
knowledge and understanding
that you get from moving
to the macro level
is highly, highly constrained.
We don't know all the details of
how it should work in general.
It's usually something where
we know it when we see it,
which is not nearly
satisfying enough.
We want to do better.
But there does not seem to
be multiple incompatible
macroscopic descriptions of
the same realistic underlying
stuff.
There might be in
special circumstances,
but in the real world that
doesn't seem to be a problem.
So I think there's
an interesting issue
at the intersection of
philosophy and physics
here about why that is the case,
but in practice that really
does seem to be the case.
AUDIENCE: You spoke a little bit
earlier about the Higgs field,
and when you excite it
you get the Higgs boson.
And then you also mentioned
that there might even
be a new particle
on the horizon as we
add more and more
energy to the systems
or simulate our collisions.
Do you foresee that
we'll just keep
discovering that there's an
infinite number of particles?
And actually I have an even
more fundamental question,
which is what is the meaning
of these particles that
exist almost for
less than an instant?
SEAN CARROLL: Right.
And although these
are good questions,
I don't think there are an
infinite number of varieties
of particles.
This is just my guess.
What do I know?
We don't know is the only
correct and humble thing
to say.
But I think that given
the existence of gravity,
you can't have particles
that are infinitely heavy.
They become black
holes at some point.
So there's probably
some finiteness
to a list of all the
different kinds of particles
we can imagine having.
Sorry, what was
the other question?
AUDIENCE: What was
the point of those?
SEAN CARROLL: Oh, what's
the point of them?
Oh, they're not a point.
They're just there.
Not everything needs
to have a point.
Things can just be.
AUDIENCE: And how
can you be so sure
that those pointless
particles don't
have a point in our
everyday existence?
SEAN CARROLL: Well,
that's a good question.
In the six hour
version of this talk
or in the book that you can
buy right now, I explain why.
Basically there's different
ways that other particles
could exist.
They could be so heavy
that you need a $10 billion
particle accelerator
to make them.
They could be so short lived
that even if you bring them
into existence, they
disappear almost instantly.
Or they could be so
weakly interacting
that even if they were
there, they would go right
through your body.
All these are ways that
particles could have avoided
detection, but in
every case, they
will not be interesting or
useful in your everyday life.
They would not have a point.
AUDIENCE: But would they have a
point to any important feature
or fact about the universe?
SEAN CARROLL: Yeah,
they might play a role
in sort of understanding how
the different forces of nature
unify at high energies
or something like that.
But again, the idea
that things have points
is not part of the
fundamental nature of reality.
This level of meaning
and purpose and causality
is a higher level
emergent thing.
It's not something
we have the right
to demand from the fundamental
architecture of reality.
AUDIENCE: You mentioned that
you were somewhat certain
that space was
not a fundamental.
What gave you that certainty
or what gave you that evidence?
SEAN CARROLL: So why do I think
that space is more likely to be
nonfundamental than time?
For one thing, quantum
mechanics intrinsically
treats time and space
very differently.
The fundamental equation
of quantum mechanics,
Schrodinger's equation,
has time in it,
but it doesn't have
space, in general.
So there's a chance that time
really is fundamental just
for that reason.
The Schrodinger
equation might not
be right, might not be
the right description,
so that's why we are
still not certain.
But there's at least
a fighting chance.
Whereas space is just
obviously not fundamental.
Space is something where, when
you go from classical mechanics
to quantum mechanics, space
more or less disappears.
In classical
mechanics, what do you
have-- some particles
moving through space
with some velocity.
In quantum mechanics,
you have a wave function
of all those particles.
And that wave function,
we tend to talk a language
that the wave function is a
function of all the particles
in their locations in space.
But we don't have to
talk that language.
We can use what is called the
momentum space description.
We can completely
describe the particles
by how fast they're
moving instead of where
they are in the universe.
And for that matter,
we don't need
to use any description at all.
We can just use these
quantum mechanical states
in their own right, with no
reference to space whatsoever.
So the kind of thing
I'm doing right now
is trying to figure out ways
to answer the question, someone
hands you a wave function,
the quantum mechanical state.
Can you figure out
what it is describing
at the classical level,
how many particles moving,
what kind of dimensional
space, et cetera?
So everything we know about
quantum mechanics, quantum
gravity, et cetera,
denigrates space
into something that is
just a good approximation
of low energies.
Thank you.
[INAUDIBLE]
