Does physics have to be causal and local?

Daniel and Kelly talk to Sean Carroll about where some of the basic assumptions we make about the Universe come from, and whether we need them.

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Episode Transcript

Speaker 1 (00:07):
The big picture task.

Speaker 2 (00:09):
The ultimate quest of physics is to make sense of
the universe, to sort through our amazing and bonkers experience
and find as compact and simple an explanation as possible
for why what happens happens. We've made a lot of progress,
but we've also struggled together. Do quantum particles really collapse

(00:30):
together across space time? Why is gravity so difficult to
unify with quantum theory? Why does time flow forward? Sometimes?
I wonder if the reason we feel stuck is that
we started from the wrong place. What if there's a simple,
basic assumption that we're making that has led us down
the wrong path. It's happened to us before, like when

(00:52):
we assumed that time flows the same way across the universe,
or that we were at the center of the Solar system,
or that light needed medium to propagate. When we removed
the mistaken assumption, the veil was pulled from our eyes
in a simpler, clear if weirder explanation emerged, Could that
be the cause of our current head scratching? Is there

(01:13):
some requirement we've imposed on physics that's keeping us from
seeing the simple explanation staring us in the face. Today
on the podcast, will knock on the doors the most
basic intuitive foundational concepts in physics and ask do we
really need them? We'll question locality, do things have to
be in the same place in order to interact? And

(01:34):
its cousin causality, do causes have to come before effects?
Welcome to Daniel and Kelly's extraordinary counterintuitive Universe.

Speaker 3 (01:56):
Hello. I'm Kelly Readersmith. I study parasites and and after
today's conversation, I feel really lucky that locality and causality
just make a lot more sense in biology.

Speaker 1 (02:07):
Hi, I'm Daniel.

Speaker 2 (02:08):
I'm a particle physicist, and I like to pretend sometimes
to be a philosopher.

Speaker 3 (02:13):
Ooh yeah, I enjoy philosophy too.

Speaker 4 (02:16):
Well.

Speaker 2 (02:16):
One of the things I love about physics is that
it butts right up against philosophical questions. We're asking about
the deep, fundamental nature of the universe, and so the
philosophical questions are like immediate and obvious. I hear a
little bit less about like the philosophy of biology. You know,
what does it mean to be a parasite?

Speaker 1 (02:33):
This sort of stuff?

Speaker 3 (02:35):
Well, I mean, we have plenty of arguments about that
at the conference as we go to. But I feel
like for us philosophy, Well, what's the difference between philosophy
and ethics? Is ethics a subset of philosophy? Because we
talk a lot about ethics associated with the biology stuff
that we're working on. So I'm more used to tackling
ethical questions than like, you know, what does causality even mean?

Speaker 2 (02:59):
Well, I think you can your finger on it, because
anytime you get bogged down in a conversation by defining
your terms, that's when you know you're doing philosophy.

Speaker 3 (03:06):
Ah, there we go. All right, Well, we spend a
glorious amount of time talking about how to define terms today,
and we've got Sean Carroll to help us do that,
and I love all of the stories that he tells.
But before we jump into our interview with Sean, we
should hear what our audience thinks about what it means
to be local and causal.

Speaker 2 (03:26):
That's right, as usual, I went out there to our
group of volunteers to see what they thought about these
fundamental concepts in physics and philosophy. So think about it
for a moment before you hear these answers. Do you
think physics has to be local and causal. Here's what
our listeners had to say.

Speaker 5 (03:45):
Physics used to be local to me years ago when
I dated a woman whose father was a physicist. Splash, Yes,
chemist at Bell Labs. But it's not anymore.

Speaker 2 (03:57):
Observations and experiments at much larger scale, much smaller scales
seem to show that both of those concepts break down
in the right circumstances.

Speaker 1 (04:05):
Most physicists assume causality.

Speaker 6 (04:08):
I don't think physics have to be local, but I
think they have to be caucal to have causality.

Speaker 7 (04:16):
I think classical physics is local and causal, but I
think quantum mechanics, with its spontaneous random activity and its
ability to have spooky action at a distance, breaks that.

Speaker 4 (04:29):
A little bit.

Speaker 6 (04:30):
Physics has to be local in causal, at least in
our quarter of the universe. Otherwise Einstein's spooky action would
have your lamp turned on before you hit the switch,
Newton would drop his apple in reverse, and all hell
would have broken loose yesterday.

Speaker 4 (04:43):
I would think physics has to be local in order
to be testimal, so that the results are always predictable.
But I'm not sure what causal means. Maybe cause and effect.

Speaker 7 (04:55):
I think It's really interesting to think of physics as
not being local, because the implications would kind of crazy,
and maybe it would explain some of the stuff that
we don't really understand yet.

Speaker 6 (05:04):
How about it has to be at least as local
as gravity and at least as causal as quantum decay.

Speaker 8 (05:13):
What an interesting question? Does physics have to be local
and or casual? Well, if it's a dating app I
was supposed to be local, and because I had a
casual relationship with physics in high school, I really don't
get the question. But I would say, isn't physics universal
and cosmological?

Speaker 6 (05:34):
Maybe things change based on locality and conditions.

Speaker 7 (05:38):
I say, yes, physics has to be local and clusal.

Speaker 2 (05:41):
Love these answers both insightful and hilarious, especially the person
who misinterpreted causal to be casual.

Speaker 3 (05:50):
Are you sure you spelled it right in your email, Daniel.

Speaker 1 (05:53):
I am not sure. Maybe that was the cause.

Speaker 3 (05:57):
Of Oh yeah, See most things in my world there's
a cause and effect relationship and there's no going back
in time.

Speaker 2 (06:06):
Well, you know, this really is a fundamental issue in philosophy.
We are just discussing on the discord this morning, Like
does the universe have to make sense? Does there have
to be an explanation for everything? I think as humans,
we are curious and we want to know answers to
questions about the universe because we assume that there are
answers right, that there is a thing that is happening

(06:26):
and we can figure it out somehow. And sometimes these
discussions of like locality and causality tell me that the
universe could be very, very different from the way that
we assume that it is, and in a way that
might never make sense to us.

Speaker 3 (06:40):
And you know, the more we learn about quantum mechanics,
the more the more it breaks my brain, to be honest,
but the more yeah interesting questions, I realize that there
are left to ask like questions about locality and causality
at the quantum mechanic level. That is what I will
be thinking about tonight when I'm trying to fall asleep
and I'm not sure if it's going to keep me
up or make me fall asleep more quickly. So we'll see.

Speaker 2 (07:03):
And to dig deep into these topics, we invited an
expert in physics and philosophy on the show and a
friend of the podcast, Sean Carroll, who's not afraid to
get bogged down defining terms.

Speaker 3 (07:14):
Yeah, things got heated between you two at a couple points,
so let's jump in.

Speaker 1 (07:19):
All right, it's my pleasure to welcome to the podcast.
Shawn Carroll Sean is a.

Speaker 2 (07:23):
Theoretical physicist who's done impouring work on the foundations of
quantum mechanics, cosmology, and the arrow of time. He holds
a joint appointment between physics and philosophy at Johns Hopkins.
He's also a prolific science communicator, the host of the
mind ski podcast, which I hardly recommend for its impressively
deep dives, and author several books such as The Biggest
Ideas in the Universe. Shawn, Welcome to the podcast.

Speaker 4 (07:46):
Thanks very much for having me.

Speaker 9 (07:47):
This is the first time I have been on a
podcast hosted by two previous guests of my podcast, So
I like how we are closing the triangle.

Speaker 3 (07:56):
This is exciting.

Speaker 2 (07:57):
We are accelerating towards the podcast singularity. Well, speaking of Singularity,
today we're going to die really deep on something that
I've always wanted to understand better in physics, and I
can't imagine a better person to ask hard questions about
issues on the boundary of philosophy in physics. I mean,
I know a lot of people who are physicists to

(08:18):
interest in philosophy. I know a lot of philosophers who
do some physics as well, But I don't think I
know anybody else who literally has an appointment in physics
and in philosophy. So congratulations on straddling that barrier.

Speaker 4 (08:29):
Thanks.

Speaker 9 (08:29):
So it's not easy to make it happen, you know,
as you know academia, we love our little silos, and
I mean there's plenty of people in universities who are
cross appointed between departments, but a humanities and a science
thing getting together is really hard to pull off.

Speaker 3 (08:45):
Yeah, and really awesome.

Speaker 9 (08:47):
I think it's awesome. I'm having a good time, so
you know, it makes me feel special.

Speaker 2 (08:51):
Well, I don't have a joint appointment philosophy. I do
have a courtesy appointment in the philosophy department, which is
just because I showed up to enough philosophy seminars to
ask awkward questions that they were like.

Speaker 9 (09:02):
Who are you?

Speaker 1 (09:03):
And then they're like, oh, do you want to courtesy appointment?
It gives you nothing. That's very yeah, exactly.

Speaker 3 (09:10):
It's like they're encouraging you to keep coming back though.

Speaker 2 (09:12):
That's nice, Yeah, exactly, yeah, yeah, although I did learn
there's a big difference between the two fields. And physics seminars,
it's totally normal to interrupt with questions. You don't understand something,
raise your hands, speak up, start a discussion. If I
give a physics seminar, I feel like it's a failure
if nobody's interrupted to discuss something. First time I asked

(09:32):
a question in the middle of a philosophy seminar, everybody
in the room turned to me with horror. So it's
like objecting in the middle of a wedding or something.

Speaker 1 (09:40):
You know, like hold your question to the end, sir.

Speaker 9 (09:43):
But then also at a philosophy colloquium, you know, often
they will let the speaker talk for an hour, take
a five minute break, and then come back for an
hour of questions, which physicists would never do. Physicists be like,
what you expected us to listen to? What was happening
into the seminar? What's going on here?

Speaker 2 (10:00):
All right, Well, let's dive into the topic for today.
We're talking about locality and causality, and these of course
are intermingled, but we're going to take them one at
a time if we can. So let's start with locality, Sean,
how do you interpret the concept of locality? What does
locality mean in physics? It doesn't mean like, get your
donuts from the store around the corner.

Speaker 9 (10:20):
It kind of does. I mean, it's pretty close to that.
It's the idea that the donuts aren't that far away,
the donuts that you're actually going to want. You know,
when I get donuts, I'm more likely to go to
the place a couple blocks away than to the place
three thousand miles away. And fundamentally that's because of locality.

Speaker 4 (10:36):
You know.

Speaker 9 (10:36):
Locality comes in different ways in different stretches of physics.
But the basic idea is that there's this thing called space,
and indeed we can promote it to space time and
we can talk about that too. But there are places
where we're located in the universe. And that kind of sounds,
you know, obvious. So yes, there are places where we're
located in the universe, but it's not so obvious. It's

(10:59):
a very strong claim that what the universe is made
out of is space and things inside of space. Right,
Like I have a location, you have a location. This
electron has a location, et cetera. And then furthermore that
when these things bump into each other. This sort of
the basic ideal locality is there is space and things

(11:20):
in it. The next level ideal locality is that when
different things interact, they do so at the same point
in space time, or at least at neighboring points. And
you might say, well, wait a minute, Like the Sun
exerts a gravitational field on the Earth, even though it's
very far away, but there is a field in between

(11:40):
the Sun and the Earth. And it's more like the
Sun effects it's gravitational field right at the Sun, and
that affects the gravitational field right next to that. And
you work your way up to what's going on here
on Earth.

Speaker 2 (11:51):
But let me back you up because you said something
which already blew my mind, Yeah, which is that it's
a strong claim essentially to say that locations exist before
we get to locality requirements, that things have to interact
at the same location. You're saying, it's a big idea
that there is space and there are locations, right, And
if that's a strong claim, like, what's the opposite, Like,

(12:12):
are you suggesting it's possible to have a universe without
locations or.

Speaker 3 (12:16):
A physicists just making things complicated.

Speaker 7 (12:19):
No.

Speaker 9 (12:19):
In fact, what I was just about to say is
the thing about that version of locality I just said
that the world is made of things located in space,
is that it's false. It is clearly not true, because
there's this thing called quantum mechanics, and quantum mechanics says
that's not what the world is made out of. In
again various different levels of precision. Even if you just

(12:40):
have one electron, there's a wave function for the electron,
and that wave function has some profile throughout space, so
there's no such thing as the point at which the
electron is located. But you might say, like, okay, but fine,
there is something called the value of the electrons wave
function at every point in space. It's kind of like
the value of the electric field or the gravitational field.

(13:03):
Is still a kind of locality, which is fine until
you have two electrons. When you have two electrons, now
there's not the wave function of electron one and the
wave function of electron two. There's the wave function quantum
mechanically for the system of both electrons at once, and
that's what the world is. And that's not things located

(13:27):
in space. It's something much weirder than that. And so
quantum mechanics, and I'm sure we'll get into it, but
quantum mechanics just makes the world look really not local
at all. And then the question I would argue is
why does it kind of look local?

Speaker 4 (13:43):
Right?

Speaker 9 (13:43):
You know, why do we get along so well thinking
that the world looks local if quantum mechanics is trying
to tell us something different?

Speaker 3 (13:49):
Does this question about what local means? You know, does
it not make sense at the quantum level, but when
you go past the quantum level, it totally makes sense.

Speaker 1 (14:00):
Yes.

Speaker 9 (14:00):
But I will just like be I got to be
a stickler in this whole conversation, right because where this
is one of those things where we think we know
what the words mean, and we're digging deeply into what
they mean, and so we can't be beholden to our
folk wisdom about what these words mean. So when you say,
like the quantum level and not the quantum level, I

(14:20):
just want to remind everyone everything is the quantum level.
There's no non quantum level. The world is quantum. It's
not like you get quantum when you look at things
that are very small it's the eppisode way around. You
get classical when you look at things that are very big,
So classical mechanics, allah, Isaac Newton, et cetera becomes a

(14:42):
good approximation when things are big and ponderous and macroscopic
and so forth. And that's the world in which, yes,
you're completely right. The world looks in the classical limit
as if it's made of objects with locations bumping into
other objects when they are at the same location.

Speaker 2 (15:00):
Okay, and before we get quantum, like Isaac Newton's theory
of gravity's already non local right, like.

Speaker 9 (15:07):
We already got quantum, Daniel, it is too late. You've
been quantum this whole time. Why are you listening?

Speaker 2 (15:13):
Yes, and I acknowledge that Newton is made of quantum objects. Yeah,
in that sense, he is quantum, but he didn't know
about quantum. And when he was developing his theory of gravity,
he invented a concept which had instantaneous communication across space
and time. And so if people think about non localities,
this new fangled thing that emerge from quantum mechanics, isn't

(15:34):
it sort of the old fangle thing that we sort
of accepted for a while and then gave up.

Speaker 9 (15:37):
And now we're returning to we had to be very
very careful. Once again, Sorry for being so careful, but
please be careful. There are once again two different levels
of locality that we have to distinguish between. One is
just what I said, like, the equations of physics are local.
Means that you write down the things the world is

(15:59):
made of, and the whole equation is a function of position. Right,
so in Newtonian gravity, you're right, that's not quite true.
Like when Isaac Newton literally wrote down the inverse square
law for gravity, the rule that says that the gravitational
force is weaker when you're further away, stronger when you're

(16:19):
close by a factor of one over the distance squared.
And he worried that this seemed non local, that here's
the Earth. The Earth is being pulled on by the
Sun and the Moon and everything in the universe instantaneously.
Because he's Isac Newton, he doesn't know about relativity. It
doesn't know about special relativity anyway. So there is absolute space,

(16:40):
absolute time. And Newton in the principia said like, yeah,
this bothers me, the fact that somehow the information about
the gravitational field of everything in the universe is conveyed
to the Earth immediately, and he literally said, I'm going
to leave this for future generations to sort out. I
don't like it. It bugs me. I'm not smart enough.
I'mder Isaac Newton. You know, eventually we will figure it out. Now,

(17:03):
there's a sort of little known thing that happened, but
I think is really crucial. In a circa eighteen hundred,
Pierre Simone Laplace solved that problem. And no one ever
gives them credit for this, but Laplace pointed out that
you can write down Newtonian gravity as a field theory.
He invented the idea of the gravitational potential. And by

(17:27):
a field theory, we just mean rather than just saying
there's a force acting on the Earth, and to know
what the force is, you have to know what everything
in the universe is doing. He says, there is something
called the gravitational potential field, and it's a field, so
it has a value at every point in space, and
the equation that that field obeys is entirely local. What

(17:47):
is happening through the field right here at some point
only depends on what's happening at right next door points.
And it's exactly like we said at the beginning, that
the Sun creates a dimple in the gravitational field at
where thus is, and that pulls on the field next
to it, and that pulls on the field next to it,
and it works its way out to the Earth. So,

(18:07):
in that sense, because there is a local field theory
description of Newtonian gravity, Lutonian gravity is entirely local.

Speaker 1 (18:15):
But is it still instantaneous?

Speaker 9 (18:17):
Well, you've skipped ahead to the world of relativity, where
there's a speed limit. So when relativity comes along, which
is long after either Newton or laplace, now we have
an idea that even if the laws of physics are
local in space time, which they are, like Einstein's equations
and Maxwell's equations and whatever, still, if something happens at

(18:41):
a particular point in space time, the effects of that
thing only ripple out slower than or at the speed
of light. Right, You do not affect things instantaneously far away.
And what that means is that we can have a better, stronger,
more satisfying version of locality, which is that not only
do the equations simply exist at every point in space

(19:05):
or every point in space time, but that you're not
being affected by things infinitely far away instantaneously. So we
have ex post facto retrofitted our notion of locality to
demand not instantaneous communication between two different points of space.
For Newton, that would have been fine. For in the

(19:25):
post Einstein world, that's no longer okay, because the speed
of light is a fundamental limit.

Speaker 3 (19:30):
Okay, So the biologist who lives in the world where
locality works, fine, thank you very much. Is so if
I can sort of summarize what's happening. So locality is
a problem because at the quantum level it doesn't really
work and you scale up. But it sounds like you
were just saying that locality is fine when you're talking
about things like the Sun impacting the Earth because you've

(19:52):
got a field and you're connected every point along the way,
And so is our conversation then mostly about the quantum level.

Speaker 9 (19:59):
Now am I following, Well, there's lots of conversations. Okay,
you know, there's definitely that. The distinction we were just
drawing is between pre relativity classical physics and post relativity
classical physics, where we have glommed onto the notion of locality.
The idea that signals don't travel faster than light, so
there cannot be any instantaneous communication across distances. You know,

(20:24):
in Isaac Newton's world, I could send a signal to
the Andromeda Galaxy instantaneously by taking a planet and shaking
it a little bit right. Its gravitational force in principle,
although not in practice, would change instantaneously in the Andromeda Galaxy.
And so is it local, You know, kind of it's
fading away, but still strictly speaking, it's hard to wrap

(20:47):
your brain around. But now, in Einstein's universe, if you
shake a planet, it's going to take you a million
light years for that signal to get to the Andromeda Galaxy.
But still that's all within the limit that we called
the classical limit of quantum mechanics. And so one place
where our notions of locality are challenged and need to
be updated is in the quantum realm. But by the way,

(21:10):
another place that their challenge need to be updated is
in things like biology. Oh no, yes, because you know
in biology what happens is most of the biological organisms
that you and I know and love are moving very
slow compared to the speed of light. Yeap so even
though they're big and they're in the classical world and

(21:31):
they bump into each other, when things do happen, they
happen essentially instantaneously, right. The signals can get to, you know,
across the organism very very fast, and so it can
often be useful at that higher emergent level of biology
to have both local things like here's a little cell

(21:52):
or there's a little bacterium or whatever, but also global things, right,
global variables. What is the pH of your solution that
you're in, or you know, what is the gradient of nutrients?
Or is the human being happy or sad? Like, that's
not a local thing. Where's the happiness, where's the sadness?

Speaker 4 (22:12):
Right?

Speaker 9 (22:12):
So, both at the microscopic quantum level and at the
emergent non physics y higher levels, the strict notions of
locality that we have in classical relativistic physics become a
little shaky.

Speaker 2 (22:27):
All right, So let's trace it historically. We start with
these concepts where you can have instantaneous interaction across space
and time. Then we get special relativity, which gives us
a cool concept of locality. You can only be influenced
by things in your past like cone and influence things
in your future like cone, because signals take time to propagate,

(22:47):
which is still a cool concept of locality. It means that,
like very much, something interacts with something else, which interacts
with something else, so you have to have like a
chain of interactions or you know, wiggle or propagation of
this interaction. But what about general relativity? Is general relativity
local in the same way that special relativity is.

Speaker 9 (23:06):
General relativity is almost as local as special relativity is,
so special relativity came along in nineteen oh five. This
is Einstein's idea that you can reconcile the non existence
of any preferred frame of reference in the universe. There's
no ether or anything like that, with the fact that
everyone thinks that the speed of light is the same.

(23:27):
All you have to do, says Einstein, is give up
your conventional notions of what space is and what time
is and marry them together. Yeah, that's why he's Einstein,
you know, marry them together to make space time. And
then ten years later in general relativity he says, you
know what I forgot to tell you, but space time.
This arena in which the game of physics is played

(23:50):
has a life of its own, it's dynamical, it has
a geometry, it can be curved, it can respond to
matter and energy and their motion in the universe. They're
a player, They're not just the arena in which the
game is played. So that makes things much trickier when
it comes to saying, you know, what depends on what,

(24:13):
like the future of the universe, the future of the
curvature of the universe, et cetera. In general relativity, in principle,
this is a deterministic consequence of what's happening in the
universe right now. But there can be limitations on that.
There can be weird global structures in the universe. You know,
there can be closed time like curves where the timelike

(24:34):
trajectories that a person can travel on in a rocket
ship can loop back on themselves. Space can be curled
up on itself. Space can be a tourist. There can
be extra dimensions of space that are very tiny, and
things like that. So the fundamental equations of general relativity
are still one hundred percent local, but there are global

(24:55):
consequences of those equations that get things a little more subtle.

Speaker 2 (24:58):
Well, what about concepts like work? I mean, if we're
talking about locality. General relativity gives us more flexibility and
what locality means because it changes which points of space
are near each other. Right, So in principle, you open
a wormhole between our galaxy and Drameda. Now some part
of Andrameda is local. Is that concept of locality so

(25:19):
to preserved through the wormhole.

Speaker 9 (25:21):
There is absolutely a concept of locality that is preserved
even in the presence of wormholes. Like if you say,
if I take a limit where I look at creatures
or points of light or particles or whatever that are
small compared to the curvature and the topology of space time,
everything is local. All of the equations are local, all

(25:42):
the dynamics are local. But you're right, you can imagine.
General relativity gives you this freedom to imagine. Okay, I'm
going to make a shortcut in space time. I'm going
to construct a wormhole that attaches two different parts of
space that I thought were far away. But if I
go from point A to point B three the wormhole,
I'm going to say, it doesn't take that long at all.

(26:03):
They're actually much closer. And this was an idea that
has been around a long time. Einstein, as usual was
one of the first people to talk about wormholes. John
Wheeler gave them the name. We have enough time to
tell this one, very very amusing story here. It was
when Carl Sagan wrote this novel Contact that wormholes became
important in physics because Sagan wrote his book and he

(26:26):
wanted to get his hero Ellie across the galaxy very
very quickly. And Sagan, you know, was a great scientist,
but he was a planetary scientist. He was not a
fundamental theoretical physicist. He knows general relativity very well, so
he had her fall into a black hole in the
original draft of the novel. The good news is that
Carl Sagan was close friends with Kip Thorne, who does
know his general relativity very well, and had him read

(26:50):
the draft and Kip said, you can't have her fall
into a black hole. She'll get spaghettified and smushed, and
that's not what you want. What you want is for
her to fall into a wormhole and that will get
her across the galaxy in a very short period of time.
And so that's what happens in the novel and in
the movie. But then Kip was thinking about it, and
you know, he thought about it, and he said, look,
you know, usually we say it's bad if you can

(27:14):
travel faster than the speed of light, because just change
your reference frame and now it looks like you're traveling
backward in time. That's one of the reasons why conventional
physics we say you probably can't travel faster than the
speed of light. It enables time travel. And he thought
about it and his students and post talks and they
chatted about it, and they realized that's because you can
travel backward in time if you have a wormhole. And

(27:36):
he wrote a series of papers with his collaborators on
building time machines using wormholes. So it's a perfect example
of how locally everything looks local in a small north
region of space time everything looks local, but globally in
the presence of that wormhole, your conventional notions of locality
or all.

Speaker 2 (27:53):
That stuff, Joe relativity is mine.

Speaker 1 (27:55):
Benning.

Speaker 3 (27:57):
Well, I hope we are acting on you at a
distance in your head, having fun at whatever location you
find yourself in, and when we get back we will
dig more into locality.

Speaker 1 (28:25):
All right.

Speaker 2 (28:25):
We are talking to Sean Carroll by the concepts of
locality and causality, and I think it's time to dig
into the thing we've been dancing around, which is locality
in quantum mechanics. So we've been talking about how you
have to be the same location to have interactions. But
now we have this concept in quantum mechanics of extended states.
You can have particles that interact and that are entangled

(28:48):
with each other, so their fates are connected, and yet
they're moving apart from each other. And we know from
Bell's experiments that if you interact with one, it can
collapse the wave function of the other. How do we
need gel locality and quantum mechanics or does quantum mechanics
require us to give up on locality? Is the universe
fundamentally non local?

Speaker 9 (29:08):
So the answer you're going to get to this question
is the universe fundamentally non local because the quantum mechanics
will be completely different. If you ask a philosopher and
a physicist and they don't even they don't even disagree
with each other, they're just caring about different things.

Speaker 4 (29:21):
Okay.

Speaker 9 (29:22):
So, and this is the fundamental weirdness of quantum mechanics
is that when we tell you the rules for quantum mechanics,
it's kind of cludgy and awkward.

Speaker 4 (29:32):
Right.

Speaker 9 (29:32):
In classical mechanics, we say, here's a thing like a
planet or an electric field or something like that, and
here are the equations that govern how that thing behaves
at equals M for Newton's laws or Maxwell's equations or whatever.

Speaker 4 (29:45):
That's it.

Speaker 9 (29:45):
That's your theory, that's all you need. In quantum mechanics,
we say, here's a thing. We call the thing the
wave function of an electron or of a field or whatever.
Wave functions are the things. Here's an equation. Just like
classical mechanics, the equation of this case is the Shroding equation,
but you can rewrite it in different forms finement path
integrales or another way of doing it, et cetera. But

(30:06):
then we don't stop there. Right, Like classically we would
have stopped. In quantum mechanics, we say, oh, but there's
more rules. And the rules have to do with what
happens when you measure or observe the system. Right, you
can measure certain quantities and you can't predict the outcome
you're going to get. You can predict the probability of
getting certain outcomes, and there are rules for how that happens.

(30:28):
And when you make the measurement. The state the wave
function changes dramatically in something we call the collapse of
the wave function. So there's basically two sets of rules
that apply inner different circumstances. The set of rules, which,
if you want to be technical and impress your friends
at parties, are called unitary evolution. That's the part where

(30:49):
you're just obeying the Shortener equation and not being observed.
And then there's the measurement process, where you actually measure something.
And so the measurement process in quantum mechanics, well, let
me get it exactly right. The state of things in
quantum mechanics, the wave function is just manifestly nonlocal. It
just isn't local. It's not a thing with a value

(31:11):
at every point in space and time. That's not what
it is, and everyone knows that's not what it is.
But what is the importance of that. Well, on the
unitary evolution side, the laws of physics, the Shortener equation
or what have you still look perfectly local as far
as we can tell. Indeed, the whole discipline of quantum

(31:32):
field theory is based on the idea that the thing
that you're quantizing to make your theory are fields. That
have values in space and time and only interact with
each other at the points in space and time where
they overlap with each other. So the particle physicists, bless
their hearts, Daniel, that's what they care about. They care

(31:53):
about that unitary evolution part and they know that at
the end of the day they're going to measure and
the wave functions going to collapse. But who cares. They
have a lot of work to do with Fieman diagrams
and things like that, just calculating the unitary.

Speaker 2 (32:04):
Evolution, and we spend a lot of money getting those
particles to the same location in space so that they
do interact with each other.

Speaker 9 (32:10):
There's a picture right behind you on your zoom background
of exactly that happening. Yes, getting them to the same
point really matters. So the physicists are like, locality is
crucially important. What are you talking about? It's like the
most important thing. It's what makes quantum field theory go.
And the philosophers will say, like, who cares about all
your integrals and your Fineman diagrams and whatever. I care
about what the measurement process is about and what the

(32:33):
implications of that are, because that's the part of quantum
mechanics that is not very well understood and needs deeper explication.
And guess what, it's wildly non local. That is the
implication of the EPR thought experiment from Einstein, Podolski, and Rosen,
who say that particles can be entangled and the measurement
outcome you get on one particle can instantaneously affect the

(32:56):
allowed measurement outcomes on another particle. And then John Bell
comes along and makes that very rigorous. And again, if
I have a couple of minutes to tell an amusing
story here, it was David Boehm, who was a young
assistant professor at Princeton the nineteen fifties, who wrote a
textbook on quantum mechanics, and he says in the textbook

(33:16):
he quotes a theorem on John von Neumann, famous physicist
who says you can't reproduce the predictions of quantum mechanics
used in what are called hidden variables, saying that there's
a wave function but also extra variables that are being
pushed around. And Einstein reads Boehm's book and calls Boem
into his office right because he's also at the Instruhravan
study right there in Princeton and says this is wrong.

(33:38):
Like I speak German. Von Neuman's book was only in German,
and so like then, the Americans knew what was in
his book. They just quoted it because they thought he
was a genius. And Einstein said this, he made a mistake.
He doesn't cover a lot of the important possibilities that
you might want to consider here. So you have not shown,
or you're not even given a good argument that you
can't have hidden variable theories. So he was inspired by

(34:00):
this and he goes off and invents a hidden variable theory.
The only way to make it work, though, is to
make it non local, so the dynamics of that theory
are explicitly non local. And then everyone ignores him, because
everyone ignores the foundations of quantum mechanics. By this point
in time, it's the fifties, and then in the sixties
it's discovered by John Bell, who reads Boehm's papers, notices

(34:23):
that it's non local and wonders to himself, like, is
there a way of doing this hidden variable thing without
being non local? And he basically proves that the answer
is no. You cannot reproduce the predictions of quantum mechanics
without being non local, and he hid that from his friends.
He was working at CERN at the time. He didn't

(34:44):
tell anybody because it was considered disreputable. But a couple
of years ago people won the Nobel Prize for testing
his predictions and showing that they're correct.

Speaker 3 (34:51):
So it's non local. But we discussed earlier that these
interactions can't happen faster than the speed of light, which
Daniel has also we've talked about on the show right,
that they can't communicate or whatever it is that they're
doing faster than the speed of light. So what is
happening then if it's not local and it's not traveling
faster than the speed of.

Speaker 9 (35:09):
Light, Daniel, do you know what's happening? Does anyone know
what's happening? Like, if you know what's happening, you win,
You save quantum mechanics. Quantachanics has been around for one
hundred years. Just a couple of weeks ago, Nature the
Journal came out with a poll that they did of
working physicists who care about quantum mechanics, showing there's no
consensus whatsoever about what's going on at the deep levels. So, Kelly,

(35:32):
you're just you're right, Like, I mean, you didn't. You're
not right because you asked a question. But you're you know,
charmingly adorable about thinking that we have the answer to
that question, because this is exactly what we don't have
the answer to. And and most working physicists, by the way,
just you know, use the strategy called denial to deal

(35:53):
with this, like they just say, just it works, okay, Like,
don't bug me about this. I can make the predictions,
the predictions come true. But above Einstein, that's why he
wrote the CPR paper back in nineteen thirty five. You know,
he basically it's very unfair to Einstein when I'm about
to say because he had a much more sophisticated argument.
But basically he points this out. He says, I can
get two particles. They're entangled. They're very very far away.

(36:16):
I observe one, and apparently the formalism is telling me
that instantly changes the state of the other particle very
far away. I'm Einstein, I know that's not possible. You
can't change things instantaneously very far away. What's up with that?
And we still know what's up with that?

Speaker 2 (36:35):
And I read that bell, you know, he interpreted his
thought experiments in the later actual experiments to mean, as
you say that there is no local hidden variable, right,
the particles are not carrying with them some details created
at the moment of their entanglement which actually determined the outcome,
But that there is this loophole that you could have
global hidden variables. So I think it's widely misunderstood that

(36:59):
Bell's mus tell you there's no hidden variables. It just
tells you there's no local hidden variables. And as you say,
a global theory is possible. But tell me about the
different interpretations of quantum mechanics. I mean, Kelly asks like
the question what is real? What is happening? And now
essentially we feel like there is no local realism. But
do the different interpretations of quantum mechanics tell different stories

(37:20):
about how to accommodate this? And I know we have
like Boemian mechanics where you have a pilot wave which
is explicitly non locally. You have a global theory, as
you say, But with the Copenhagen interpretation, is that a
non local theory or does Copenhagen essentially shrug away this
question the way it does most of the important.

Speaker 9 (37:38):
Issues yeah, I mean, look, it really is fascinating. I
do encourage anyone with a little bit of quantum mechanics
knowledge to actually read the original EPR paper Einstein, Podolski,
and Rosen, because you know, Daniel can back me up
on this. But we physicist don't read the original papers
like that's you know, we have a textbook and that
tells us what's going on. But the original papers are

(37:59):
fascinating because they don't know the answer, right, They're struggling
with figuring out what's going on. So Einstein and Podolski
and Rosen, but I get the impression that the ideas
were mostly from Einstein. They didn't just say, look, there
is this spooky action at a distance that bugs us.
They were much more careful than that. They tried their
best to construct an argument that says there should be

(38:22):
what they called local elements of reality. And I think
that it was Bell who later called these things be
a bles be ables in the sense of things that be,
things that are things that exist.

Speaker 2 (38:34):
Right, philosophers, and they're creating phrases for.

Speaker 9 (38:38):
John Bell is a car carrying physicist. I gotta say.

Speaker 1 (38:42):
He's doing philosophy. Though when he invents a new meaning
for the word b.

Speaker 9 (38:46):
He's doing philosophy, so he invents this word beable, and
it's exactly what Einstein wanted to be the case. Einstein
wanted it to be the fact that at every point
in space time there's a fact of the matter about
what is physical going on in the universe. And as
we said, quantum mechanics doesn't say that. Bell wanted, you know,

(39:07):
to really understand this locality issue. And so the way
that Boem solves the problem is there are local beables,
like in Boehm's theory, there are particles. When you see
at the LHC the track of a particle, what Boem
would say is you're not seeing the wave function. You're
seeing the particle. The particles there in addition to the

(39:29):
wave function. But the equation that the particle follows is
non local. It depends on what all the other particles
everywhere else are doing, which is which is really weird.
And so Bell asked this question, can you come up
with any theory where everything is one hundred percent local
and none of the dynamics are local, none of the
things are non local or non local issues. I hope

(39:50):
I said that correctly both times and he proves the
answer is no. So the different strategies for solving this
just take very different points of view. In Bomi mechanics,
despite the bullet there's a non local evolution rule. There
are what are called objective collapse models of quantum mechanics,
where the wave function just suddenly changes all over space,

(40:13):
all at once. That's very non local in its own
in everready, in quantum mechanics, in the many worlds theory,
you kind of sidestep the question. One of the axioms,
one of the assumptions of premises of Bell's theorem is
that measurements have definite outcomes. When you measure the spin

(40:33):
of a particle, it will either be spin up or
spin down, And whatever it says is well, it's spinned
up in one universe and spin down in another universe.
So that's not quite what Bell had in mind. So
people have huge arguments over whether or not many worlds
is local or not. The answer, of courses depends on
your definitions, but that is one of the reasons to

(40:54):
preserve that kind of dynamical locality that you might like
many worlds Copenhagen. I'm just I'm going to like boycott
any question about the Copenhagen interpretation from now on, because
it's giving it too much credit. It's not well defined,
it's not a theory like many worlds. Bomian mechanics, spontaneous
collapse models, these are theories. They have equations and they

(41:17):
make predictions. Copenhagen just won't answer certain questions, which I
don't think we should reward it by taking it seriously.

Speaker 3 (41:24):
Can we give a little more information about what the
Copenhagen stuff means for the biologists in the room.

Speaker 9 (41:29):
Yes, yes, Copenhagen is a city in Denmark and I
got that sean full of people doing quantum mechanics and
it was a great time. You know, is again fascinating
to read the original papers. We're here in twenty twenty five.
It's the it's been dubbed the Year of Quantum because
the International Year of Quantum, because exactly one hundred years

(41:50):
ago the first papers came out by Heisenberg and Schrodinger
et cetera setting up quantum mechanics, and we still understand it.
But into like the ten years after nineteen ten twenty
five Nils Borr and Werner, Heisenberg and Wolfgang Powley, who
all were sort of either affiliated with or spent a
lot of time at Bor's Institute in Copenhagen, promulgated this

(42:13):
way of thinking about quantum mechanics. And it's exactly what
I already said. It said that you have a wave
function and it solves the Schortener equation when you're not
looking at it, and then when you do look at it,
it collapses and you get a probability. But the philosophical
side of the Copenhagen interpretation is the claim that there's

(42:33):
no such thing as what is happening when you are
not looking at the quantum system. And this is very
explicit in Heisenberg's papers from nineteen twenty five, and it's
one of the reasons why it's very hard to understand
these papers. Stephen Weinberg, who is one of the most
brilliant physicists of the twentieth century, said like he tried

(42:53):
very hard to read Heisenberg's papers and he has no
idea what was going on there. But the big philosophical
move was stop asking about where the electron is. There
is no such thing as where the electron is. There
is only where you will see it when you measure it.
And that's really the fundamental ethos of the Copenhagen interpretation.

(43:14):
And by that, by itself, that's fine, but it leaves
a whole bunch of questions unanswered. That's the bad part.
Number one, What is a measurement? What counts as doing
a measurement? Can a video camera do a measurement? Do
you have to be a conscious observer to do a measurement?
What if you don't have good eyesight, does that count
as a measurement? Number two, The Copenhagen interpretation says, the

(43:39):
classical world exists and is real. You and I are
not quantum things. You know, Daniel made the joke earlier
about Isaac Newton being made of quantum mechanical things. Werner
Heisenberg didn't think that. He thought that Isaac Newton was classical,
and that the things that you look at in a
microscope or quantum And there's literally an idea called the
Heisenberg cut. And this is somehow in the space of

(44:02):
all things happening in the world, there's one side of
the cut where there's the quantum stuff, and the other
side of the cut where it's the classical stuff. And
like who invented that where does that go? Like, no
one knows what is going on with any of this,
and so the Copenhagen interpretation is kind of just what
we teach students in our quantum mechanics courses. But it's
hilariously ill defined and as a as a starting point

(44:25):
as the kind of conjectural hypothesis that we throw out
to do physics. It's great, it's amazing, it makes perfect sense.
The weird thing is we've been pretending for one hundred
years that it's somehow a satisfactory final answer.

Speaker 2 (44:38):
I think you've been slightly unfair to the Copenhagen interpretation, and.

Speaker 9 (44:42):
I would be much more unfair if you wanted me to,
I would be much harder.

Speaker 2 (44:45):
And I can't believe I'm in the situation of defending it.
I mean, I agree that there's a fundamental issue at
the heart of it, which is that they don't define
the distinction between you know, what causes a collapse and
what doesn't, what's a classical object and what's a quantum object. Absolutely,
and that's a fatal error. But you know, there's these
other issues of like are you conscious or you know,

(45:06):
does a human do it? Or an eyeball do it
or a video camera do it. I think those are
maybe side issues, but I agree at the core of it,
Copenhagen is ill defined.

Speaker 3 (45:13):
Things are heating up, things are eating up.

Speaker 9 (45:15):
So I gotta wait, I gotta I gotta bump in here.
You gotta interrupt, because I just gave a talk at
the American Association of Physics Teachers where I was talking
about the foundations of quantum mechanics, and for that talk,
I made a slide in which I appealed to a
th arty. So I'm going to quote some people who
are much smarter than me talking about the Copenhagen interpretation.
Albert Einstein says the theory is apt to beguile us

(45:38):
into error in our search for a uniform basis for physics,
because in my belief, it is an incomplete representation of
real things. Erwin Schrodinger says, I don't like it, and
I'm sorry I ever had anything to do with it.
Hugh Everett says this is a philosophical monstrosity. And then
Carl Popper, who invented false fae as the demarketse mean

(46:01):
science and nonscience, says the Copenhagen interpretation is a mistaken
and even vicious doctrine.

Speaker 1 (46:07):
Okay, So if you think.

Speaker 9 (46:09):
I'm being unfair, like the people who care about this
a lot, I think are are pretty hardcore that this
is not acceptable.

Speaker 1 (46:15):
Well, I'm glad you didn't hold back.

Speaker 2 (46:18):
My own personal anecdote about Copenhagen is I got to
spend a year at the Boer Institute, and when I
got there, they gave me an office, and I noticed
that the office next door to mine was quite different
in that it had a bathtub in it, and I thought,
why is there a bathtub.

Speaker 1 (46:32):
In this office?

Speaker 9 (46:33):
And then that can't be good, Nothing good can happen.

Speaker 2 (46:35):
I learned the story that in the old days, when
you had an institute, you lived at the institute the
way like the president lives at the White House. And
so there was an apartment. And then later, after Boor
was no longer there, they're like, well, let's just turn
these into offices. And so somebody got the office with
the bathtub in it.

Speaker 4 (46:51):
Why didn't they?

Speaker 2 (46:53):
He had important thoughts in that tub. You know, you
can't just get rid of it.

Speaker 3 (46:57):
Are you're gonna crawl in when you need to solve
your next big NB?

Speaker 2 (47:01):
All right, So on that note, let's solve the next
big problem. I want to talk about quantum gravity and
the implications, but first let's detour two causality. We've talked
about locality a lot, but let's take a break, and
when we come back, we're going to talk about causality. Okay,

(47:36):
we're back, and we're talking to physicists and philosophers. Sometimes
when you measure him, he collapses into one state or
the other. About causality and locality in physics, So we
started the conversation about locality with the definition, and we've
already touched on some of the concepts of causality, But
could you give us a crisp definition for what we
mean by causality in physics.

Speaker 9 (47:57):
I'm glad you said it in physics, because just like locality,
the causality is a notion that physicists and philosophers both
talk about, but they mean entirely different things. So let's
first just mention like what someone who is neither would mean. Right,
You know, we talk about causes and effects. If you say, like,
why were you late for work? Someone might say, well,

(48:18):
because the traffic was unusually bad, there was an accident, right,
and that's a meaningful statement. There's a cause there's an accident,
there is an effect that there was traffic, and that
there's another cause there was traffic, and there's another fact
you're late for work, And there's a chain of causes
and effects that ripples throughout all of our existence. And
this goes back to you know, Aristotle thinking about this

(48:38):
kind of thing. So it's very, very common that we
take that kind of word that we use in everyday
language and repurpose it for some rigorous idea in physics
or philosophy, et cetera. So what the physicists have done
post Einstein is to notice this feature of relativity that
we've already talked about that when you do something at

(49:01):
some location, some event in space and time, the implications
the effects of that doing something can only ripple forward
in time, and they can only ripple forward in time
slower than the speed of light. So, really physicists, by
the word causality, what they usually mean is that signals

(49:21):
travel slower than the speed of light, or at the
speed of light, certainly no faster than the speed of light.

Speaker 2 (49:26):
So an alien shoots their death ray at us from
Andromeda if they can't kill us today or tomorrow or yesterday.
They can only kill us in a million years.

Speaker 9 (49:34):
Or so unless they shot at a million years ago, right, yes,
which is not well defined in relativity. So yeah, I'm
not gonna I'm worried about the aliens. So, but there's
already a tension there in that physicist's way of thinking,
because there's a super important feature of the everyday notion
of causality, which is that the cause always happens before

(49:58):
the effect. Right, You would not convince anyone by saying
there was an accident this morning because I was going
to be late for work, Right, That's just not how
causes and effects work. You're late for work as there
was an accident, not the other way around. But the
fundamental laws of physics, whether it's relativity or quantum mechanics
or anything, are time reversal invariant. Are invariant going forward

(50:21):
and backward in time. So really, when the physicists talk
about causality, they kind of mean signals propagate to the
future at or slower than the speed of light. But
also they get to us from the past at or
slower than the speed of light. Both are equally good
features of what physicists call causality and philosophers want to know. Okay,

(50:43):
but if that's true, why in the macroscopic world of
our everyday existence do we have this strong feeling that
causes precede effects. And probably that has something to do
with entropy in the hour of time. And it's a
whole long story, all right.

Speaker 2 (50:57):
So in physics we have this concept that the past
determines the future. Essentially, it's a statement on top of
the existing laws of physics, which, as you say, are
mostly invariant with respect to time, that there's a directionality
to time. Is that a compact way to understand causality
in physics?

Speaker 9 (51:16):
It might be a little bit too compact, So let
me be let's just expand it out a little bit.
You know, back in circa eighteen hundred, again our hero
Piersimone Laplace points out this existing feature of classical mechanics.
You know, Newton invented classical mechanics more or less in
its modern form in the sixteen hundreds. It took a
while for Laplace to realize the implication of Newton's laws,

(51:40):
which is that information about what is happening in the
universe is conserved over time. So if you live in
a purely Newtonian universe, purely classical, if you know what
happens at any one moment of time, if you know
that perfectly, you know the position and the velocity of
every single atom in the universe. Okay, then according to Laplace,

(52:03):
the laws of physics determine everything that will happen in
the future one hundred percent reliability. And it also determines
everything that happens in the past, because there is no
distinction between past and future in Newtonian mechanics. So that's
what we mean by information being conserved from moment to
moment in time. So this is already kind of different

(52:25):
than our conventional notion of causality, that the knowledge of
what is happening in the present determines equally well the
future and the past. And so to recover from that
description our folk wisdom about causes preceding effects, the fundamental
laws of physics are not enough. You also need to
put in some boundary conditions. And usually what we do

(52:48):
is we say the early universe, the Big Bang fourteen
billion years ago, had very very special conditions. They were
low entropy, they were a very very tiny kind of
configuration in the space of all possibilities. And because we
know that, because we know the conditions that were there
in the early universe, we have a better handle on

(53:10):
what things were like in the past than we do
in the future. We say the past is fixed, right,
We informally think that the past is just in the books.
There's no decision I can make now that will change
the past. But we think there's a decision I can
make now that will change the future. How to reconcile
that with laplace? The answer is, you know, I don't

(53:31):
know the position and velocity of every atom in the universe.
I have some incomplete macroscopic information, and that might be
enough to really fix what happened in the past, like
I have a photograph or a video record of it.
It is never enough to completely fix what happens in
the future.

Speaker 2 (53:48):
I think that's a really subtle and underappreciated point, that
the present determines the past in the way you say
it in some sense. Another way to think about it
is that from the pre you can recover the past
because there's a unique moment, like the details of the
universe now can only have come from a certain history

(54:10):
of the past, and so in that sense you can recover.
It's not like if I change the present, the past changes,
but there's a unique path which I can recover from
knowing enough about the present. I don't know if you
saw that not great show Devs where they had this
whole concept that try to see an image of the Crucifixion,
for example, by measuring the motion of air particles over

(54:30):
Malaysia or whatever.

Speaker 9 (54:33):
Well, yes, So one of the reasons why that is
a TV show and not reality is because again one
cannot emphasize enough that you don't have information about position
and velocity of every particle in the universe. If you
wanted to just to drive it home, if you wanted
to recover something that was happening in an event two

(54:54):
thousand years ago, even in principle, you would need to
know everything that is going on in the universe now
to within a two thousand light year radius, because there
were photons emitted from the Earth back two thousand years
ago and they've been moving away from you to speed
of light. So if you don't have those photons, you
cannot recover what was going on back there in the past.

(55:17):
But at the same time, in principle you could, except
there's quantum mechanics that gets in the way of this,
But classically you could absolutely do this. So it's an
interesting back and forth about the rigidity of the laws
of physics. What's amazing to me is that we can
make as much progress as we can talking about both
the past and the future given such amazingly limited information
about the state of the universe.

Speaker 2 (55:37):
So let's dig into the quantum mechanics of it. I mean,
you describe something like a clockwork universe where things are deterministic,
and in principle, if you knew all the information, you
could predict the future, et cetera. But we know that's
not our universe. We know there is fuzziness, and we
talk a lot in physics, especially in popular science, about
quantum fluctuations, which are treated as if they have no cause.

(55:58):
You know, why does there a guy lexi here? Oh
there was a fluctuation in the early universe. Why is
there no galaxy there? Oh, there was a fluctuation. Do
those fluctuations in quantum mechanics do they really have no cause?
Is there nothing that determines them?

Speaker 9 (56:10):
Well, I, at the risk of yet again being careful
and pedantic. We have to distinguish between not having a
cause and not being determined. Those are slightly two different things.
Of course, you're right. In the macroscopic world, when you
make a quantum measurement, the outcomes are not determined by
the quantum state of the universe. As far as we know,

(56:34):
there is no hidden information that would determine them. In
theories like Bomian mechanics, there literally is hidden information that
does determine them. So Boei mechanics is one hundred percent deterministic,
but we literally have no access to that information. So
you know, what good is it to think that this
is a determined event even though we don't know, we

(56:54):
cannot know what is the thing determining it. But also,
you know, we're cheating a little bit because we're taking,
we're borrowing this notion of causality that has kind of
been handed down since Aristotle, the idea that you know,
for every event we can assign a cause, which was
never really very fundamentally rigorous, like, Okay, I'm late for work,

(57:16):
and I assigned the cause to that that there was
traffic that morning, But what about assigning the cause to
that that space time is four dimensional? Like if it weren't,
I wouldn't have been late for work, or you know,
I mean like there's a whole bunch of facts about
the universe on which this result depends. And this is
full employment for philosophers to figure out exactly what you
mean by the cause. But in physics we've sidestepped that

(57:40):
question by replacing the ideas of cause and effect with
a much more clear, rigorous framework, which is patterns, essentially
differential equations that tell you from one thing another thing
is going to follow. Or maybe you have some discrete
version of physics or whatever it is. But an if
then statement, you know, if this happens and that happens,

(58:01):
it's not quite cause and effect, even though we speak
that way sometimes, Like the example I like to use
is the real numbers, right, zero, one, two, three minus
one minus two minus three. There's a pattern there. If
I tell you the number n, you can figure out
what the number N plus one is. If I tell
you five, you can figure out six. But five is

(58:21):
not the cause of six, right, it's just the previous
thing in the pattern. That's how the laws of physics work.
It's one damn thing after another, and it's okay. If
some of those laws are stochastic, right, Like, maybe they are,
since we don't understand the foundations of quantum mechanics perfectly well.
Classical mechanics was deterministic, but maybe quantum mechanics just isn't

(58:45):
even at the most fundamental level. Or maybe it is,
as I said, Bomi mechanics many worlds. These are deterministic theories,
but we don't know which one, if either one of
those is right. So I wouldn't say that quantum events
don't have a cause. Events follow the patterns given to
us by the laws of physics. As to whether those

(59:06):
patterns are deterministic or stochastic, we just don't know at
the fundamental level. We do know that as observers in
the universe, they seem stochastic to.

Speaker 2 (59:15):
Us, right, And I didn't mean to imply that like
anything goes in quantum mechanics, you do an experiment and
like anything can happen. Obviously, we construct conditions and quantum
mechanics gives us predictions for probability distributions. In that sense,
it determines those distributions, but not the individual experiment. That's
an important distinction, thank you.

Speaker 9 (59:34):
No, it's actually super important because a lot of people,
when you tell them that the way that fundamental physics
works isn't exactly in line with our informal, twenty five
hundred year old notion of cause and effect, they instantly
leap to, oh, then anything goes. But that's really not
what it is. There's still laws, there are still patterns.

(59:55):
Maybe there's a stochastic element to them. We're just not sure.

Speaker 2 (59:58):
So then let me ask you to speculate why, Because
we're peering into the future where maybe folks smarter than
us are going to unravel the nature of space time
and gives us their quantum gravity that lets us understand
all of this stuff. Do you think that theory is
going to be non local and causal only in the
way the quantum mechanics is. Or do we have no

(01:00:19):
idea about what's going to happen with quantum gravity?

Speaker 9 (01:00:22):
So your second guess is probably more accurate, more more fair,
more legitimate, more honest to say we have no idea
what's going to happen. I have my favorite ideas, and
my favorite idea is just taking quantum mechanics really super
duper seriously. So what do I mean by that? I

(01:00:44):
already said that when it comes to locality, the way
that we represent the state of a physical system in
quantum mechanics with a wave function or whatever is non local. Right,
you have a wave function that depends on all the
particles in all the fields, not on just one local
in space. You know, that's a sort of particular specific

(01:01:06):
version of a more general abstract statement that quantum states are,
you know, elements of some abstract mathematical space. And space,
good old space, good old three dimensional x, y z
space is not there in the fundamental description of quantum mechanics.
Time is, which is weird because the Shortinger equation has

(01:01:28):
a T in it, but the general form of the
Shortinger equation does not have an x in it. And
that's intension with the spirit of relativity. And that's just true,
and okay, we're gona have to deal with that, et cetera.
But to me, the real deep question is not how
do we reconcile ourselves to the apparent non locality of

(01:01:49):
quantum measurement, the apparent spooky action at a distance and
we measure one particle and we see an instantaneous effect
on its quantum state. That's not the question. The question
is because remember, there's this two sides of quantum mechanics.
What happens when you're not looking at it, where everything
looks local, and what happens when you look at it
when there are these non local correlations. I'm interested in

(01:02:11):
why things ever look local at all? You know, if
you just think that your starting point is this abstract
quantum mechanical state vector of the universe, Hilbert space is
the mathematical space in which these quantum states live, why
does it look like we live in space and have
approximately local interactions at all? And in fact, so I've

(01:02:35):
written papers about this, like you can actually be a
working physicist and try to ask this question. It's a
little bit hard to make too much progress because the
question is so grandiose. So we don't know a lot
about it. But I think that it's a different angle
on quantum gravity than the traditional ones. You know, Traditionally,
in all of physics, what we do is we invent

(01:02:56):
a classical theory of something like electromagnetism or this in
barmonic osclator or a propagating string and ten dimensions and
all these are classical descriptions, and then we quantize them.
We have some rules for turning that quantum theory into
a classical theory, and we keep bumping into problems when
applying these rules to the questions of quantum gravity. But

(01:03:17):
nature doesn't work that way. Nature doesn't start with the
classical theory and quantize it. Nature is just quantum from
the start. So maybe the obstacle defining the right theory
of quantum gravity is that we keep wanting to start
with a classical theory and quantizing it. Maybe you should
start with a quantum theory of nothing at all and
asking under what conditions might it look like the classical

(01:03:41):
three spatial dimensional universe with certain particles and fields and
stuff like that. So I think that thinking about locality
in this way might very well turn out to be
absolutely crucial to making progress in quantum gravity.

Speaker 2 (01:03:55):
So I think the description you're suggesting here is some
concept in which space it'sself is not fundamental.

Speaker 9 (01:04:01):
It's emergent absolutely, And that's actually remarkably common among people
who think about quantum gravity, that space itself is not fundamental.
What's not really well understood is what that means. Like
there's only one way to be fundamental, and there's many
ways to not be fundamental. So okay, space is emergent,
it's not fundamental. What does that mean?

Speaker 4 (01:04:21):
What is it?

Speaker 9 (01:04:21):
Where did it come from? And different people have different
opinions about that.

Speaker 2 (01:04:24):
And it's a real struggle because we think geometrically. I mean,
if you tell me the universe is a bunch of
wave functions and they're entangled together, I try to think
where are they? I imagine them in my head. Yeah,
because I think about where do I put it? And
in my head I have a space and that space
is three dimensional because I have lived in three dimensional space,

(01:04:45):
so I think in three D. So it's a real
struggle to counter our intuitions and to try to come
up with a conception of the universe that doesn't make
these assumptions.

Speaker 9 (01:04:53):
So one question I have that is well beyond my
pay grade, But could you model a virtual reac environment
in which space is four dimensional? Could you retrain your
brain to move and live and perceive things in four
dimensional space? Or is there something in the structure of

(01:05:14):
our brain itself that only makes sense in three dimensions?
You know, people like a manual cont to the philosopher
you know, had an argument that three dimensional space was
kind of necessary. People argue about exactly how necessary he
thought it was. But it was almost an anthropic argument,
you know, you tried to argue that this is the
only way to make sense of the world is if
you have this spatial arena in which things have locations

(01:05:36):
and things like that. It's a what is it? A
cautionary tale because philosophers should be careful that their ideas
won't be overthrown by later advances in physics. But I
don't know, and I have ideas that are very vague
in hand wavy about why space should have emerged in
the first place. Maybe not really anthropic, but maybe something

(01:05:57):
about locality is very helpful if you just want complexity
at all, Like if literally every particle in the universe
could instantaneously affect every other particle, Like how do you
get through the day? Like how do you make a
living in a world like that? I don't know. So
I do think that these questions are very deep and
certainly not understood, but maybe understandable.

Speaker 2 (01:06:19):
I have a suspicion about whether we could have four
dimensional VR. I mean, I remember trying to play video
games with my teenager, and I played a lot of
video games as a kid, But you know, the controller
was simple. There was ab you know, there was a
little directional thing you could jump on whatever. So then
I'm trying to play Halo with my teenager and there's
like a knob for the direction of your gun, is

(01:06:39):
a knob for the direction you move, and a knob
for the direction.

Speaker 1 (01:06:42):
Your head goes.

Speaker 2 (01:06:43):
And this kid is moving through essentially six dimensional space,
you know, and he's controlling it and it's totally intuitive
to him. And I'm like, I have to put this
finger on this joystick and that thing on that joystick,
And of course he know, headshotted me instantaneously every single time.
So I think the human brain is probably plastic enough
to be able to adapt to those kind of environments,
but not yours mine.

Speaker 9 (01:07:04):
That's funny. You don't look that old Daniels. It's kind
of weird, but I guess sorry, we're learning.

Speaker 2 (01:07:08):
That's the zoom filter. But I want to take our
brains out universal and think, of course, about how aliens
experience the universe. Do you think if we get to
talk to aliens that they will have gone through a
similar trajectory, you know, where they imagine the universe is
local and causal and then they discover, oh, actually, at

(01:07:28):
its foundations, these are just intuitive assumptions we're making in
the universe doesn't respect them. Or do you think it's
possible that they grew up natively to imagine a non
local universe.

Speaker 9 (01:07:40):
I think that it's. Uh, there's a tension there, because
I do think that everything is possible when you ask
these possibility questions, but some things are easier to imagine
than others. I do think the embodiedness of we beings
in three dimensional space is pretty natural to imagine as
a universal feature, even of alien life, because cause, you know,

(01:08:01):
the classical world is really helpful to you know, making
predictions about what's going to happen, and it's a very
good approximation to the world. So I suspect that whatever
trajectory of scientific understanding the aliens take, it will start
with classical three dimensional physics and and move on from there.
But you know, if the aliens get really good at

(01:08:22):
either VR or uploading into the matrix or whatever, maybe
they're very used to thinking and perceiving things in different
numbers of dimensions. Maybe that's just a switch they flip
when they when they go in there. I remember one
more completely amusing but irrelevant story. I was a science
consultant for the movie Tron Legacy.

Speaker 3 (01:08:44):
Fun.

Speaker 9 (01:08:45):
And you might remember Tron, those of us who are
old enough. I remember Tron when it came out. It was,
you know, one of the first early eighties Disney movies
that had a lot of computer graphics in it, right,
Jeff Bridges was in it, and it was you know,
it was not greats and by any stretch, but it was.

Speaker 1 (01:09:01):
Fun, deeply influential.

Speaker 9 (01:09:03):
I well, it was great ish. It is a certain
kind of great.

Speaker 1 (01:09:07):
Yeah.

Speaker 9 (01:09:08):
But the sequel that they had later in the two thousands,
Tron Legacy, and I think they've had another one, right,
but or they're making it. But so Tron Legacy was
not as successful cinematically, And part of it was when
you made Tron, computer graphics were terrible, right, Like it was,

(01:09:30):
it was amazing you could do it at all, and
you were like, oh my god, this is so mind
blowing that you were in this thing that was obviously
a bunch of people on scooters with neon taped to them, right,
but you know they had some computer graphics going on.
But by you know, twenty ten or whatever, you can
just make everything look perfectly realistic, and so they did
so it looked perfectly realistic, like inside the video game,

(01:09:53):
things look like the real world. But my attitude was like,
but we live in the real world, wants to see that.
What we want to see is something that doesn't look
anything like the real world, because you can make any
world you want. And they did not go down that road.
But I think that that movie still remains to be
seen where people like are literally living in six dimensional

(01:10:15):
space and fighting their motorcycle battles.

Speaker 3 (01:10:18):
Accordingly, trum Legacy would have done so much better if
they took your advice.

Speaker 9 (01:10:22):
So many things in life. Yeah that could be said about.

Speaker 2 (01:10:25):
Yeah, all right, Well, thank you for joining us today
on this element of our struggle to understand the real
world and what is real about the world.

Speaker 1 (01:10:33):
I appreciate your thoughts and comments. Sean, thanks very much
for having me.

Speaker 3 (01:10:43):
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