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]
This always gets me. There's nothing "common sense" about quantum physics, our brains and/or experiences just aren't designed to really grasp this stuff, at least given the information we have so far. So when people pull the "obviously, there had to be a beginning" or "obviously something had to start it all off" stuff, I just chuckle. Who the hell can say!?! I don't even understand how the universe doesn't have a center, I'm not going to armchair-physics my way into the origin of all things.
I simply play their game and go with "everything has a cause", which is their core logic they are using as an argument for God.
Me: Everything has a cause...okay so what caused God?
Theist: God doesn't need a cause. He is uncaused, he is the first cause.
Me: But your logic here is that "everything has a cause". According that that logic, there can be no such thing as a "first" cause. So why is God exempt from the very same logic that you're using as an argument for God's existence? It's self-refuting.
Theist: Because otherwise you just end up with infinite recursion, that that would make no sense.
Me: Why not? Isn't your God also infinite? You seem to have no problem accepting that. Either way we run into infinites that make no sense.
Theist: Well God just sounds more likely.
Me: More likely? Is your religion built around a probability? There are two possibilities here: a) Either absolutely everything has a cause (your own logic) which includes God himself, at which point he can no longer be called God and we get infinite recursion. OR b) everything doesn't need a cause, and therefore the universe could simply be uncaused, removing a need for God. Which one is it?
I loved his debate with William Lane Craig. Especially the parts where WLC tried to explain cosmology to a professional cosmologist, and the part where he admits he denies relativity.
I use this rebuttle to the unmoved mover argument all the time.
That. Was. Amazing.
There is so much in this talk, well worth watching 3, 4 or 5 times, carefully, to get it all.
Brilliant. I love his books, but there's another dimension of him when he gives a lecture.
William Lane Craig refuted this claim in their debate when he said this.