Lee Smolin: Quantum Gravity and Einstein’s Unfinished Revolution
嘉宾：Lee Smolin
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§01Transcript · 00:00–09:12
The following is a conversation with Lee Smolin.
He's a theoretical physicist,
co-inventor of loop quantum gravity,
and a contributor of many interesting ideas
to cosmology, quantum field theory,
the foundations of quantum mechanics,
theoretical biology, and the philosophy of science.
He's the author of several books,
including one that critiques the state of physics
and its string theory called "The Trouble with Physics,"
and his latest book, "Einstein's Unfinished Revolution,
"The Search for What Lies Beyond the Quantum."
He's an outspoken personality in the public debates
on the nature of our universe,
among the top minds in the theoretical physics community.
This community has its respected academics,
its naked emperors, its outcasts and its revolutionaries,
its madmen and its dreamers.
This is why it's an exciting world to explore
through long-form conversation.
I recommend you listen back to the episodes
of Leonard Susskind, Sean Carroll, Michio Kaku,
Max Stegmark, Eric Weinstein, and Jim Gates.
You might be asking, "Why talk to physicists
"if you're interested in AI?"
To me, creating artificial intelligence systems
requires more than Python and deep learning.
It requires that we return to exploring
the fundamental nature of the universe and the human mind.
Theoretical physicists venture out into the dark,
mysterious, psychologically challenging place
to force principles more than almost any other discipline.
This is the Artificial Intelligence Podcast.
If you enjoy it, subscribe on YouTube,
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@LexFriedman, spelled F-R-I-D-M-A-N.
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And now, here's my conversation with Lee Smolin.
What is real?
Let's start with an easy question.
Put another way, how do we know what is real
and what is merely a creation
of our human perception and imagination?
- We don't know.
We don't know.
This is science.
I presume we're talking about science.
And we believe, or I believe,
that there is a world that is independent of my existence
and my experience about it and my knowledge of it.
And this I call the real world.
- So you said science, but even bigger than science.
- Sure, sure.
I need not have said this is science.
I just was warming up.
- Warming up.
Okay, now that we're warmed up,
let's take a brief step outside of science.
Is it completely a crazy idea to you
that everything that exists
is merely a creation of our mind?
So there's a few, not many, this is outside of science now,
people who believe perception
is fundamentally what's in our human perception,
the visual cortex and so on,
the cognitive constructs that's being formed there,
is the reality.
And then anything outside
is something that we can never really grasp.
Is that a crazy idea to you?
- There's a version of that that is not crazy at all.
What we experience is constructed by our brains
and by our brains in an active mode.
So we don't see the raw world.
We see a very processed world.
We feel something that's very processed through our brains
and our brains are incredible.
But I still believe that behind that experience,
that mirror or veil or whatever you wanna call it,
there is a real world and I'm curious about it.
- Can we truly, how do we get a sense of that real world?
Is it through the tools of physics
from theory to the experiments?
Or can we actually grasp it in some intuitive way
that's more connected to our ape ancestors?
Or is it still fundamentally the tools of math and physics
that really allow us to grasp it?
- Well, let's talk about what tools they are.
What you say are the tools of math and physics.
I mean, I think we're in the same position
as our ancestors in the caves
or before the caves or whatever.
We find ourselves in this world and we're curious.
We also, it's important to be able to explain
what happens when there are fires,
when there are not fires,
what animals and plants are good to eat and all that stuff.
But we're also just curious.
We look up in the sky and we see the sun
and the moon and the stars and we see some of those move
and we're very curious about that.
And I think we're just naturally curious.
So we make, this is my version of how we work.
We make up stories and explanations.
And there are two things which I think
are just true of being human.
We make judgments fast because we have to.
We're to survive, is that a tiger or is that not a tiger?
And we go.
Act.
We have to act fast on incomplete information.
So we judge quickly and we're often wrong,
or at least sometimes wrong,
which is all I need for this.
We're often wrong.
So we fool ourselves and we fool other people readily.
And so there's lots of stories that get told
and some of them result in a concrete benefit
and some of them don't.
- So you said we're often wrong,
but what does it mean to be right?
- Right, that's an excellent question.
To be right, well, since I believe
that there is a real world,
I believe that to be, you can challenge me on this
if you're not a realist.
A realist is somebody who believes
in this real objective world
which is independent of our perception.
If I'm a realist, I think that to be right
is to come closer.
I think, first of all, there's a relative scale.
There's not right and wrong.
There's right or more right and less right.
And you're more right if you come closer
to an exact, true description of that real world.
Now, can we know that for sure?
No.
- And the scientific method is ultimately
what allows us to get a sense
of how close we're getting to that real world?
- No on two counts.
First of all, I don't believe there's a scientific method.
I was very influenced when I was in graduate school
by the writings of Paul Feyerabend
who was an important philosopher of science
who argued that there isn't a scientific method.
- There is or there isn't?
- There is not.
- There's not.
Can you elaborate?
Sorry if you were going to,
but can you elaborate on the,
what does it mean for there not to be a scientific method,
this notion that I think a lot of people believe in
in this day and age?
- Sure.
Paul Feyerabend, he was a student of Popper
who taught Karl Popper.
And Feyerabend argued both by logic
and by historical example that you name anything
that should be part of the practice of science,
say you should always make sure that your theories
agree with all the data that's already been taken.
And he'll prove to you that there have to be times
when science contradicts, when some scientist
contradicts that advice for science to progress overall.
So it's not a simple matter.
§02Transcript · 09:18–19:56
I think that, I think of science as a community.
- Of people.
- Of people, and as a community of people
bound by certain ethical precepts, percepts, whatever that is.
- So in that community, a set of ideas they operate under,
meaning ethically, of kind of the rules of the game
they operate under.
- Don't lie, report all your results,
whether they agree or don't agree with your hypothesis.
Check the training of a scientist,
mostly consists of methods of checking,
because again, we make lots of mistakes,
we're very error prone.
But there are tools, both on the mathematics side
and the experimental side, to check and double check
and triple check.
And a scientist goes through a training,
and I think this is part of it.
You can't just walk off the street and say,
"Yo, I'm a scientist."
You have to go through the training,
and the training, the test that lets you be done
with the training is, can you form a convincing case
for something that your colleagues
will not be able to shout down,
because they'll ask, "Did you check this,
"and did you check that, and did you check this,
"and what about a seeming contradiction with this?"
And you've gotta have answers to all those things
or you don't get taken seriously.
And when you get to the point where you can produce
that kind of defense and argument,
then they give you a PhD.
And you're kind of licensed.
You're still gonna be questioned,
and you still may propose or publish mistakes,
but the community is gonna have to waste less time
fixing your mistakes.
- Yes, but if you can maybe linger on it a little longer,
what's the gap between the thing that that community does
and the ideal of the scientific method?
The scientific method is you should be able
to repeat an experiment.
There's a lot of elements to what construes
the scientific method, but the final result,
the hope of it, is that you should be able to say
with some confidence that a particular thing
is close to the truth.
- Right, but there's not a simple relationship
between experiment and hypothesis or theory.
For example, Galileo did this experiment
of dropping a ball from the top of a tower,
and it falls right at the base of the tower.
And Aristotelian would say, "Wow, of course it falls
"right to the base of the tower.
"That shows that the Earth isn't moving
"while the ball is falling."
And Galileo says no weight is a principle of inertia
and it has an inertia in the direction
with the Earth isn't moving and the tower
and the ball and the Earth all move together.
When the principle of inertia tells you it hits the bottom,
it does look, therefore my principle of inertia is right.
And Aristotelian says no, Aristotle's science is right,
the Earth is stationary.
And so you've got to get an interconnected bunch of cases
and work hard to line up and explain.
It took centuries to make the transition
from Aristotelian physics to the new physics.
It wasn't done till Newton in 1680-something, 1687.
- So what do you think is the nature of the process
that seems to lead to progress?
If we at least look at the long arc of science,
of all the community of scientists,
they seem to do a better job of coming up with ideas
that engineers can then take on and build rockets with
or build computers with or build cool stuff with.
- I don't know, a better job than what?
- Than this previous century.
So century by century, we'll talk about string theory
and so on and kind of possible,
what you might think of as dead ends and so on.
- Which is not the way I think of string theory.
We'll straighten it out, we'll get on string straight.
But there is, nevertheless, in science,
very often at least temporary dead ends.
But if you look through centuries,
you know, the century before Newton
and the century after Newton,
it seems like a lot of ideas came closer to the truth
that then could be usable by our civilization
to build the iPhone, right?
To build cool things that improve our quality of life.
That's the progress I'm kind of referring to.
- Let me, can I say that more precisely?
- Yes.
(laughing)
It's a low bar.
- 'Cause I think it's important to get
the time places right.
- Yes.
- There was a scientific revolution that partly succeeded
between about 1900 or late 1890s
and into the 1930s, 1940s,
and maybe some, if you stretched it, into the 1970s.
And the technology, this was the discovery of relativity
and that included a lot of developments
of electromagnetism.
The confirmation, which wasn't really well confirmed
into the 20th century, that matter was made of atoms.
And the whole picture of nuclei
with electrons going around,
this is early 20th century.
And then quantum mechanics was from 1905,
took a long time to develop to the late 1920s
and then it was basically in final form.
And the basis of this partial revolution,
and we can come back to why it's only a partial revolution,
is the basis of the technologies you mentioned.
All of, I mean, electrical technology
was being developed slowly with this.
And in fact, there's a close relation
between the development of electricity
and the electrification of cities in the United States
and Europe and so forth.
And the development of the science.
The fundamental physics,
since the early 1970s,
doesn't have a story like that so far.
There's not a series of triumphs and progresses
and there's not any practical application.
- So just to linger briefly on the early 20th century
and the revolutions in science that happened there,
what was the method by which the scientific community
kept each other in check about
when you get something right, when you get something wrong?
Is experimental validation ultimately the final test?
- It's absolutely necessary.
And the key things were all validated.
The key predictions of quantum mechanics
and of the theory of electricity and magnetism.
- So before we talk about Einstein,
your new book before string theory, quantum mechanics,
let's take a step back at a higher level question.
What is, that you mentioned, what is realism?
What is anti-realism?
And maybe why do you find realism,
as you mentioned, so compelling?
- Realism is the belief in an external world
independent of our existence, our perception,
our belief, our knowledge.
A realist, as a physicist, is somebody who believes
that there should be possible
some completely objective description
of each and every process at the fundamental level,
which describes and explains exactly what happens
and why it happens.
- That kind of implies that that system,
in a realist view, is deterministic,
meaning there's no fuzzy magic going on
that you can never get to the bottom of.
You can get to the bottom of anything
and perfectly describe it.
- Some people would say that I'm not that interested
in determinism, but I could live with the fundamental world,
which had some chance in it.
- So do you, you said you could live with it,
but do you think God plays dice in our universe?
- I think it's probably much worse than that.
- In which direction?
- I think that theories can change
and theories can change without warning.
I think the future is open.
- You mean the fundamental laws of physics can change?
- Yeah.
- Okay, we'll get there.
(laughs)
I thought we would be able to find some solid ground,
but apparently--
- Well, the ground is pretty solid.
- The entirety of it, temporarily so.
Okay, so realism is the idea that while the ground is solid,
you can describe it.
What's the role of the human being,
our beautiful, complex human mind in realism?
Do we have a, are we just another set of molecules
connected together in a clever way?
Or the observer, does the observer,
our human mind, consciousness, have a role
in this realism view of the physical universe?
- There's two ways, there's two questions
you could be asking.
Does our conscious mind, do our perceptions
play a role in making things become,
in making things real or things becoming?
That's question one.
Question two is, does this, we can call it
a naturalist view of the world
that is based on realism, allow a place
to understand the existence of and the nature
§03Transcript · 19:58–29:51
of perceptions and consciousness in mind?
And that's question two.
Question two, I do think a lot about,
and my answer, which is not an answer, is I hope so.
But it certainly doesn't yet.
- So what-- - Question one,
I don't think so.
But of course, the answer to question one
depends on question two.
- Right.
- So I'm not up to question one.
- So question two is the thing that you can
kind of struggle with at this time.
What about the anti-realists?
So what flavor, what are the different camps
of anti-realists that you've talked about?
I think it would be nice if you could articulate
for the people for whom there is not
a very concrete real world, or there's divisions,
or it's messier than the realist view of the universe.
What are the different camps?
What are the different views?
- I'm not sure I'm a good scholar
and can talk about the different camps and analyze it.
But some, many of the inventors of quantum physics
were not realists, were anti-realists.
And there are scholars, they lived in a very perilous time
between the two world wars.
And there were a lot of trends in culture
which were going that way.
But in any case, they said things like
the purpose of science is not to give an objective
realist description of nature as it would be in our absence.
This might be saying Niels Bohr.
The purpose of science is as an extension
of our conversations with each other,
to describe our interactions with nature.
And we're free to invent and use terms like particle,
or wave, or causality, or time, or space,
if they're useful to us and they carry
some intuitive implication.
But we shouldn't believe that they actually have to do
with what nature would be like in our absence,
which we have nothing to say about.
- Do you find any aspect of that,
'cause you kind of said that we human beings tell stories.
Do you find aspects of that kind of anti-realist view
of Niels Bohr compelling?
That we're fundamentally are storytellers,
and then we create tools of space and time,
and causality, and whatever this fun
quantum mechanics stuff is to help us
tell the story of our world.
- Sure, I just would like to believe
it is an aspiration for the other thing.
- The other thing being what?
- The realist point of view.
- Do you hope that the stories will eventually lead us
to discovering the real world as it is?
- Yeah.
- Is perfection possible, by the way?
- No.
You mean, will we ever get there and know that we're there?
- Yeah, exactly.
- That's for people 5,000 years in the future.
We're certainly nowhere near there yet.
- Do you think reality that exists outside of our mind,
do you think there's a limit to our cognitive abilities,
as again, descendants of apes,
who are just biological systems?
Is there a limit to our mind's capability
to actually understand reality?
There comes a point, even with the help
of the tools of physics, that we just cannot grasp
some fundamental aspects of that reality.
- Again, I think that's a question
for 5,000 years in the future.
- We're not even close to that limit.
- I think there is a universality.
Here, I don't agree with David Deutsch about everything,
but I admire the way he put things in his last book.
And he talked about the role of explanation.
And he talked about the universality of certain languages,
or the universality of mathematics,
or of computing, and so forth.
And he believed that universality,
which is something real, which somehow comes out
of the fact that a symbolic system,
or a mathematical system, can refer to itself,
and can, I forget what that's called,
can reference back to itself.
And build, in which he argued for a universality
of possibility for our understanding,
whatever is out there.
But I admire that argument.
But it seems to me we're doing okay so far,
but we'll have to see.
- Whether there is a limit or not.
For now, we've got plenty to play with.
- Yeah.
There are things which are right there in front of us,
which we miss.
And I'll quote my friend Eric Weinstein in saying,
look, Einstein carried his luggage.
Freud carried his luggage.
Marx carried his luggage.
Martha Graham carried her luggage, et cetera.
Edison carried his luggage.
All these geniuses carried their luggage.
And not once before, relatively recently,
did it occur to anybody to put a wheel on luggage
and pull it.
(laughing)
And it was right there waiting to be invented
for centuries.
- So this is Eric Weinstein.
Yeah, what do the wheels represent?
Are you basically saying that there's stuff
right in front of our eyes that once we,
it just clicks, we put the wheels in the luggage,
a lot of things will fall into place?
- Yes, I do, I do.
And every day I wake up and think,
why can't I be that guy who was walking through the airport?
- What do you think it takes to be that guy?
Because like you said, a lot of really smart people
carried their luggage.
What, just psychologically speaking,
so Eric Weinstein is a good example of a person
who thinks outside the box.
- Yes.
- Who resists almost conventional thinking.
You're an example of a person who,
by habit, by psychology, by upbringing, I don't know,
but resists conventional thinking as well, just by nature.
- Thank you, that's a compliment.
- That's a compliment?
Good, so what do you think it takes to do that?
Is that something you were just born with?
- I doubt it.
Well, from my studying some cases,
'cause I'm curious about that, obviously,
and just in a more concrete way,
when I started out in physics,
'cause I started a long way from physics,
so it took me a long, not a long time,
but a lot of work to get to study it and get into it,
so I did wonder about that.
And so I read the biographies,
and in fact, I started with the autobiography of Weinstein
and Newton and Galileo and all those people.
And I think there's a couple things.
Some of it is luck, being in the right place
at the right time.
Some of it is stubbornness and arrogance,
which can easily go wrong.
- Yes.
- And I know all of these are doorways,
if you go through them slightly at the wrong speed
or in the wrong angle, they're ways to fail.
But if you somehow have the right luck,
the right confidence and arrogance, caring,
I think Einstein cared to understand nature
with a ferocity and a commitment
that exceeded other people of his time.
So he asked more stubborn questions,
he asked deeper questions.
I think, and there's a level of ability,
and whether ability is born in or can be developed
to the extent to which it can be developed,
like any of these things, like musical talent.
- You mentioned ego.
What's the role of ego in that process?
- Confidence.
- Confidence, but in your own life,
have you found yourself walking that nice edge
of too much or too little?
So being overconfident and therefore leading yourself astray
or not sufficiently confident to throw away
the conventional thinking of whatever the theory of the day,
of theoretical physics?
- I don't know if I, I mean, I've contributed
where I've contributed, whether if I had had
more confidence in something, I would have gotten further.
I don't know.
Certainly, I'm sitting here at this moment
with very much my own approach to nearly everything.
And I'm calm, I'm happy about that.
But on the other hand, I know people
whose self-confidence vastly exceeds mine,
and sometimes I think it's justified,
and sometimes I think it's not justified.
- Your most recent book titled
"Einstein's Unfinished Revolution,"
so I have to ask, what is Einstein's unfinished revolution?
And also, how do we finish it?
- Well, that's something I've been trying
to do my whole life.
But Einstein's unfinished revolution
is the twin revolutions which invented relativity theory,
§04Transcript · 29:54–40:25
special and especially general relativity,
and quantum theory, which he was the first person
to realize in 1905 that there would have to be
a radically different theory which somehow realized
or resolved the paradox of the duality
of particle and wave for photons.
- And he was, I mean, people, I think,
don't always associate Einstein with quantum mechanics,
'cause I think his connection with it,
founding, as one of the founders, I would say,
of quantum mechanics, he kind of put it in the closet.
Is it--
- Well, he didn't believe that the quantum mechanics,
as it was developed in the late 19th, middle late 1920s,
was completely correct.
At first, he didn't believe it at all.
Then he was convinced that it's consistent but incomplete,
and that also is my view.
It needs, for various reasons I can elucidate,
to have additional degrees of freedom,
particles, forces, something, to reach the stage
where it gives a complete description of each phenomenon,
as I was saying, realism demands.
- So what aspect of quantum mechanics bothers you
and Einstein the most?
Is it some aspect of the wave function collapse discussions,
the measurement problem?
Is it the--
- The measurement problem.
I'm not gonna speak for Einstein.
(Lex laughing)
The measurement problem, basically, and the fact that--
- What is the measurement problem, sorry?
- The basic formulation of quantum mechanics
gives you two ways to evolve situations in time.
One of them is explicitly when no observer is observing
or no measurement is taking place,
and the other is when a measurement
or an observation is taking place,
and they basically contradict each other.
But there's another reason why the revolution
was incomplete, which is we don't understand
the relationship between these two parts,
general relativity, which became our best theory
of space and time and gravitation and cosmology,
and quantum theory.
- So for the most part, general relativity
describes big things, quantum theory describes
little things, and that's the revolution
that we found really powerful tools
to describe big things and little things,
and it's unfinished because we have
two totally separate things, and we need to figure out
how to connect them so it can describe everything.
- Right, and we either do that,
if we believe quantum mechanics, as understood now,
is correct, by bringing general relativity
or some extension of general relativity
that describes gravity and so forth
into the quantum domain that's called quantized,
the theory of gravity, or if you believe
with Einstein that quantum mechanics
needs to be completed, and this is my view,
then part of the job of finding the right completion
or extension of quantum mechanics
would be one that incorporated space-time and gravity.
- So, where do we begin?
So first, let me ask, perhaps you can give me a chance,
if I could ask you some just really basic questions,
well, they're not at all, the basic questions
are the hardest, but you mentioned space-time.
What is space-time?
- Space-time, you talked about a construction,
so I believe that space-time is an intellectual construction
that we make of the events in the universe.
I believe the events are real,
and the relationships between the events,
which cause which are real, but the idea
that there's a four-dimensional smooth geometry
which has a metric and a connection
and satisfies the equations that Einstein wrote,
it's a good description to some scale,
it's a good approximation, it captures
some of what's really going on in nature,
but I don't believe it for a minute is fundamental.
- So, okay, we're gonna, allow me to linger on that.
So the universe has events, events cause other events,
this is the idea of causality, okay, so that's real.
- That's in my-- - In your view, is real.
- Or hypothesis, or the theories that I have been
working to develop make that assumption.
So space-time, you said four-dimensional space
is kind of the location of things,
and time is whatever the heck time is,
and you're saying that space-time is,
both space and time are emergent and not fundamental?
- No. - Sorry, before you correct me,
what does it mean to be fundamental or emergent?
- Fundamental means it's part of the description
as far down as you go, we have this notion.
- As real. - Yes.
- As real as real could be.
- Yeah, so I think that time is fundamental,
and quote goes all the way down, and space does not,
and the combination of them we use in general relativity
that we call space-time also does not.
- But what is time, then?
- I think that time, the activity of time
is a continual creation of events from existing events.
- So if there's no events, there's no time.
- Then there's not only no time, there's no nothing.
- So-- - So I believe the universe
has a history which goes to the past.
I believe the future does not exist.
There's a notion of a present and a notion of the past,
and the past consists of, is a story about events
that took place to our past.
- So you said the future doesn't exist.
- Yes.
- Could you say that again?
Can you try to give me a chance to understand that
one more time?
So events cause other events.
What is this universe?
'Cause we'll talk about locality and non-locality.
- Good.
'Cause it's a crazy, I mean, it's not crazy,
it's a beautiful set of ideas that you propose.
But, and if causality is fundamental,
I'd just like to understand it better.
What is the past, what is the future,
what is the flow of time, even the error of time
in our universe, in your view?
And maybe what's an event?
Right? - Oh, an event is
where something changes, or where two,
it's hard to say because it's a primitive concept.
An event is a moment of time within space,
this is the view in general relativity,
where two particles intersect in their paths,
or something changes in the path of a particle.
Now, we are postulating that there is,
at the fundamental level, a notion,
which is an elementary notion,
so it doesn't have a definition in terms of other things,
but it is something elementary happening.
- And it doesn't have a connection to energy,
or matter, or exchange of any--
- It does have a connection to energy and matter.
- So it's at that level.
- Yes, it involves, and that's why the version
of a theory of events that I've developed
with Marina Cortes, and by the way,
I want to mention my collaborators,
because they've been at least as important
in this work as I have, as Marina Cortes
in all the works since about 2013, 2012, 2013,
about causality, causal sets, and in the period
before that, Roberto Manguibara Unger,
who is a philosopher and a professor of law.
- And that's in your efforts together
with your collaborators to finish the unfinished revolution.
- Yes. - And focus on causality
as a fundamental-- - Yes.
- As fundamental to physics.
- And there's certainly other people we've worked with,
but those two people's thinking
had a huge influence on my own thinking.
- So in the way you describe causality,
that's what you mean of time being fundamental,
that causality is fundamental.
- Yes.
- And what does it mean for space to not be fundamental,
to be emergent? - That's very good.
That there's a level of description
in which there are events, there are events
create other events, but there's no space.
They don't live in space.
They have an order in which they caused each other,
and that is part of the nature of time for us.
But there is an emergent approximate description,
and you asked me to define emergent, I didn't.
An emergent property is a property
that arises at some level of complexity,
larger than and more complex than the fundamental level,
which requires some property to describe it,
which is not directly explicable or derivable,
is the word I want,
from the properties of the fundamental things.
- And space is one of those things
in a sufficiently complex universe,
space, three-dimensional position of things emerged.
- Yes, and we have this, we saw how this happens in detail
in some models, both computationally and analytically.
- Okay, so connected to space is the idea of locality.
- Yes.
- So we talked about realism.
So I live in this world, I like sports.
Locality is a thing that you can affect things close to you
and don't have an effect on things that are far away.
§05Transcript · 40:29–50:08
It's the thing that bothers me about gravity in general,
or action at a distance.
Same thing that probably bothered Newton,
or at least he said a little bit about it.
Okay, so what do you think about locality?
Is it just a construct?
Is it us humans just like this idea
and are connected to it because we exist in it,
we need it for our survival, but it's not fundamental?
I mean, it seems crazy for it not to be
a fundamental aspect of our reality.
- It does.
- So can you comfort me, sort of as a therapist?
- I'm not a good therapist, but I'll do my best.
- Okay.
- There are several different definitions of locality
when you come to talk about locality in physics.
In quantum field theory, which is a mixture
of special relativity and quantum mechanics,
there is a precise definition of locality.
Field operators corresponding to events in space-time,
which are spaced like separated commute
with each other as operators.
- So in quantum mechanics, you think about
the nature of reality as fields,
and things that are close in a field
have an impact on each other more than farther away.
- That's, yes.
That's very comforting.
That makes sense.
- So that's a property of quantum field theory,
and it's well-tested.
Unfortunately, there's another definition of local,
which was expressed by Einstein,
and expressed more precisely by John Bell,
which has been tested experimentally and found to fail.
And this setup is you take two particles.
So one thing that's really weird about quantum mechanics
is a property called entanglement.
You can have two particles interact
and then share a property without it being a property
of either one of the two particles.
And if you take such a system,
and then you make a measurement on particle A,
which is over here on my right side,
and particle B, which is over here,
somebody else makes a measurement on particle B,
you can ask that wherever is the real reality of particle B,
it not be affected by the choice
the observer at particle A makes about what to measure.
Not the outcome, just the choice
of the different things they might measure.
And that's a notion of locality,
because it assumes that these things
are very far space-like separated,
and it's gonna take a while for any information
about the choice made by the people here at A
to affect the reality at B.
But you make that assumption, that's called Bell locality.
And you derive a certain inequality
that some correlations,
functions of correlations have to satisfy.
And then you can test that pretty directly
in experiments which create pairs of photons
or other particles.
And it's wrong by many sigma.
- In experiment, it doesn't match.
So what does that mean?
- That means that that definition
of locality I stated is false.
- The one that Einstein was playing with?
- Yeah, and the one that I stated,
that is, it's not true that whatever is real
about particle B is unaffected by the choice
that the observer makes as to what to measure
in particle A, no matter how long they've been propagating
at almost the speed of light,
or the speed of light away from each other.
- No matter, so like the distance between them?
- Well, it's been tested, of course,
if you want to have hope for quantum mechanics
being incomplete or wrong,
and corrected by something that changes this.
It's been tested over a number of kilometers.
I don't remember whether it's 25 kilometers
or 100 and something kilometers.
So in trying to solve the unsolved revolution,
in trying to come up with a theory for everything,
is causality fundamental and breaking away from locality?
- Absolutely.
- A crucial step.
So in your book, essentially,
those are the two things we really need
to think about as a community,
especially the physics community has to think about this.
So I guess my question is, how do we solve,
how do we finish the unfinished revolution?
- Well, that's, I can only tell you what I'm trying to do,
and what I have abandoned.
- Yes, exactly.
- As not working.
- As one ant, smart ant in an ant colony.
- Or maybe dumb, that's why, who knows?
But anyway, my view of the,
we've had some nice theories invented.
There's a bunch of different ones,
both relate to quantum mechanics,
relate to quantum gravity.
There's a lot to admire in many of these different approaches
but to my understanding,
none of them completely solve the problems that I care about.
And so we're in a situation
which is either terrifying for a student
or full of opportunity for the right student,
in which we've got more than a dozen attempts,
and I never thought, I don't think anybody anticipated
it would work out this way, which worked partly,
and then at some point, they have an issue
that nobody can figure out how to go around
or how to solve.
And that's the situation we're in.
My reaction to that is twofold.
One of them is to try to bring people,
we evolved into this unfortunate sociological situation
in which there are communities
around some of these approaches.
And to borrow again a metaphor from Eric,
they sit on top of hills in the landscape of theories
and throw rocks at each other.
And as Eric says, we need two things.
We need people to get off their hills
and come down into the valleys and party and talk
and become friendly and learn to say
not no but, but yes and.
Yes, your idea goes this far,
but maybe if we put it together with my idea,
we can go further.
- Yes.
So in that spirit, I've talked several times
with Sean Carroll, who's also written
an excellent book recently.
And he kind of, he plays around,
is a big fan of the many worlds interpretation
of quantum mechanics.
So I'm a troublemaker, so let me ask,
what's your sense of Sean
and the idea of many worlds interpretation?
I've read many, the commentary back and forth.
You guys are friendly, respect each other,
but have a lot of fun debating.
- I love Sean and he, no, I really,
he's articulate and he's a great representative
or ambassador of science to the public
and for different fields of science to each other.
He also, like I do, takes philosophy seriously.
And unlike what I do in all cases,
he's really done the homework.
He's read a lot, he knows the people,
he talks to them, he exposes his arguments to them.
And I, there's this mysterious thing
that we so often end up on the opposite sides
of one of these issues.
- It's fun though.
- It's fun and I'd love to have a conversation about that,
but I would want to include him.
- I see, about many worlds.
Well--
- No, I can tell you what I think about many worlds.
- I'd love to, but actually on that, let me pause.
Sean has a podcast, you should definitely figure out
how to talk to Sean.
I actually told Sean I would love to hear you guys
just going back and forth.
So I hope you can make that happen eventually,
you and Sean.
- I won't tell you what it is,
but there's something that Sean said to me
in June of 2016 that changed my whole approach to a problem.
But I have to tell him first.
- Yes, and that'll be great to tell him on his podcast.
So--
- I can invite myself to his podcast.
- I told him, yeah, okay, we'll make it happen.
So many worlds.
- Anyway.
- What's your view?
Many worlds, we talked about non-locality.
Many worlds is also a very uncomfortable idea
or beautiful, depending on your perspective.
It's very nice in terms of,
I mean, there's a realist aspect to it.
I think you called it magical realism.
- Yeah.
(laughing)
- It's just a beautiful line.
But at the same time, it's very difficult
to for our limited human minds to comprehend.
So what are your thoughts about it?
- Let me start with the easy and obvious
and then go to the scientific.
§06Transcript · 50:10–1:01:02
- Okay.
- It doesn't appeal to me.
It doesn't answer the questions that I want answered.
And it does so to such a strong case
that when Roberto Manguibar-Anger and I
began looking for principles,
and I want to come back and talk about
the use of principles in science,
'cause that's the other thing I was gonna say,
and I don't want to lose that.
When we started looking for principles,
we made our first principle, there is just one world,
and it happens once.
But so it's not helpful to my personal approach,
to my personal agenda.
But of course, I'm part of a community.
And my sense of the many worlds interpretation,
I have thought a lot about it and struggled a lot with it,
is the following.
First of all, there's Everett himself,
there's what's in Everett.
And there are several issues there
connected with the derivation of the Born Rule,
which is the rule that gives probabilities to events.
And the reasons why there is a problem with probability
is that I mentioned the two ways
that physical systems can evolve.
The many worlds interpretation cuts off,
one, the one having to do with measurement,
and just has the other one, the Schrodinger evolution,
which is this smooth evolution of the quantum state.
But the notion of probability is only in the second rule,
which we've thrown away.
So where does probability come from?
You have to answer the question,
because experimentalists use probabilities
to check the theory.
Now, at first sight, you get very confused,
'cause there seems to be a real problem.
Because in the many worlds interpretation,
this talk about branches is not quite precise,
but I'll use it.
There's a branch in which everything that might happen
does happen, with probability one in that branch.
You might think you could count the number of branches
in which things do and don't happen,
and get numbers that you can define
as something like frequentist probabilities.
And Everett did have an argument in that direction.
But the argument gets very subtle
when there are an infinite number of possibilities,
as is the case in most quantum systems.
And my understanding, although I'm not as much of an expert
as some other people, is that Everett's own proposal failed,
did not work.
There are then, but it doesn't stop there.
There is an important idea that Everett didn't know about,
which is decoherence, and it is a phenomenon
that might be very much relevant.
And so a number of people post-Everett
have tried to make versions of what you might call
many worlds quantum mechanics.
And this is a big area, and it's subtle,
and it's not the kind of thing that I do well.
So I consulted, that's why there's two chapters on this
in the book I wrote, chapter 10,
which is about Everett's version, and chapter 11.
There's a very good group of philosophers of physics
in Oxford, Simon Saunders, David Wallace,
Harvey Brown, and a number of others.
And of course, there's David Deutsch, who is there.
And those people have developed and put a lot of work
into a very sophisticated set of ideas
designed to come back and answer that question.
They have the flavor of, there are really no probabilities,
we admit that, but imagine if the Everett story was true
and you were living in that multiverse,
how would you make bets?
And so they use decision theory
from the theory of probability and gambling and so forth
to shape a story of how you would bet
if you were inside an Everettian universe and you knew that.
And there's a debate among those experts
as to whether they or somebody else has really succeeded.
And when I checked in as I was finishing the book
with some of those people, like Simon,
who's a good friend of mine, and David Wallace,
they told me that they weren't sure
that any of them was yet correct.
So that's what I put in my book.
Now, to add to that, Sean has his own approach
to that problem in what's called self-referencing
or self-locating observers.
And it doesn't, I tried to read it
and it didn't make sense to me,
but I didn't study it hard, I didn't communicate with Sean,
I didn't do the things that I would do,
so I had nothing to say about it in the book.
I don't know whether it's right or not.
- Let's talk a little bit about science.
You mentioned the use of principles in science.
What does it mean to have a principle
and why is that important?
- When I feel very frustrated about quantum gravity,
I like to go back and read history.
And of course, Einstein, his achievements are a huge lesson
and hopefully something like a role model.
And it's very clear that Einstein thought
that the first job when you want to enter a new domain
of theoretical physics is to discover and invent principles
and then make models of how those principles
might be applied in some experimental situation,
which is where the mathematics comes in.
So for Einstein, there was no unified space and time.
Minkowski invented this idea of space time.
For Einstein, it was a model of his principles
or his postulates.
And I've taken the view that we don't know the principles
of quantum gravity.
I can think about candidates and I have some papers
where I discuss different candidates
and I'm happy to discuss them.
But my belief now is that those partially successful
approaches are all models which might describe indeed
some quantum gravity physics in some domain,
in some aspect, but ultimately would be important
because they model the principles
and the first job is to tie down those principles.
So that's the approach that I'm taking.
- So speaking of principles, in your 2006 book,
The Trouble with Physics, you criticized a bit
string theory for taking us away from the rigors
of the scientific method or whatever you would call it.
But what's the trouble with physics today
and how do we fix it?
- Can I say how I read that book?
- Sure.
- Because I, and I'm not, this of course has to be my fault
because you can't as an author claim
after all the work you put in that you were misread.
But I will say that many of the reviewers
who are not personally involved and even many
who were working on string theory
or some other approach to quantum gravity
told me, communicated with me and told me
they thought that I was fair
and balance was the word that was usually used.
So let me tell you what my purpose was
in writing that book, which clearly got diverted by,
because there was already a rather hot argument going on.
And this is--
- On which topic, on string theory specifically
or in general in physics?
- No, more specifically than string theory.
So since we're in Cambridge, can I say that?
We're doing this in Cambridge.
- Yeah, of course, Cambridge, just to be clear,
Massachusetts and on Harvard campus.
- Right, so Andy Strominger is a good friend of mine
and has been for many, many years.
And Andy, so originally there was this beautiful idea
that there were five string theories
and maybe they would be unified into one.
And we would discover a way to break that,
the symmetries of one of those string theories
and discover the standard model
and predict all the properties
of the standard model particles,
like their masses and charges and so forth, coupling constant.
And then there was a bunch of solutions
to string theory found, which led each of them
to a different version of particle physics
with a different phenomenology.
These are called the Calabi-Yau metaphors,
named after Yau, who was also here.
Not, certainly we've been friends
sometime in the past anyway.
And then there were, nobody was sure,
but hundreds of thousands of different versions
of string theory.
And then Andy found there was a way
to put a certain kind of mathematical curvature
called torsion into the solutions.
And he wrote a paper, "String Theory with Torsion,"
in which he discovered there was
not formally uncountable, but he was unable
to invent any way to count the number of solutions
or classify the diverse solutions.
And he wrote that this is worrying
because doing phenomenology the old-fashioned way
by solving the theory is not gonna work
because there's gonna be loads of solutions
for editing proposed phenomenology
for anything in the experiments.
This hasn't quite worked out that way.
But nonetheless, he took that worry to me.
We spoke at least once, maybe two or three times about that.
And I got seriously worried about that.
And this is a little--
- Sounds like an anecdote that inspired
§07Transcript · 1:01:05–1:09:44
your worry about string theory in general.
- Well, I tried to solve the problem,
and I tried to solve the problem.
I was reading at that time a lot of biology,
a lot of evolutionary theory,
like Lin-Margulis and Steve Gould and so forth.
And I could take your time to go through the things,
but it occurred to me maybe physics
was like evolutionary biology,
and maybe the laws evolved.
And there was, the biologists talk about a landscape,
a fitness landscape of DNA sequences
or protein sequences or species or something like that.
And I took their concept and the word landscape
from theoretical biology and made a scenario
about how the universe as a whole could evolve
to discover the parameters of the standard model.
And I'm happy to discuss,
that's called cosmological natural selection.
- Cosmological natural selection.
- Yeah, and I--
- Wow, so the parameters of the standard model,
so it's the laws of physics are changing.
This idea would say that the laws of physics
are changing in some way that echoes
that of natural selection,
or just it adjusts in some way towards some goal.
- Yes, and I published that,
I wrote the paper in '88 or '89,
the paper was published in '92.
My first book in 1997,
The Life of the Cosmos was explicitly about that.
And I was very clear that what was important
is that because you would develop an ensemble of universes,
but they were related by descent through natural selection,
almost every universe would share the property
that it was, its fitness was maximized to some extent,
or at least close to maximum.
And I could deduce predictions
that could be tested from that.
And I worked all of that out,
and I compared it to the anthropic principle
where you weren't able to make tests
or make falsifications.
All of this was in the late '80s and early '90s.
- That's a really compelling notion,
but how does that help you arrive--
- I'm coming to where the book came from.
- Yes.
- So what got me,
I worked on string theory.
I also worked on loop quantum gravity,
and I was one of the inventors of loop quantum gravity.
And because of my strong belief in some other principles,
which led to this notion of wanting a quantum theory
of gravity to be what we call relational
or background independent,
I tried very hard to make string theory
background independent,
and ended up developing a bunch of tools
which then could apply directly to general relativity,
and that became loop quantum gravity.
So the things were very closely related,
and have always been very closely related in my mind.
The idea that there were two communities,
one devoted to strings and one devoted to loops,
is nuts and has always been nuts.
- Okay, so--
- So anyway--
- There's this nuts community of loops and strings
that are all beautiful and compelling
and mathematically speaking,
and what's the trouble with all that?
Why is that such a problem?
- So I was interested in developing that notion
of how science works based on a community and ethics
that I told you about.
And I wrote a draft of a book about that,
which had several chapters on methodology of science,
and it was a rather academically oriented book.
And those chapters were the first part of the book,
the first third of it,
and you can find their remnants
in what's now the last part of "The Trouble with Physics."
And then I described a number of test cases, case studies,
and one of them, which I knew,
was the search for quantum gravity
and string theory and so forth.
And I was unable to get that book published.
So somebody made the suggestion of flipping it around
and starting with the story of string theory,
which was already controversial.
This was 2004, 2005.
But I was very careful to be detailed,
to criticize papers and not people.
You won't find me criticizing individuals,
you'll find me criticizing certain writing.
But in any case, here's what I regret.
Let me make your program worthwhile.
- Yes.
- As far as I know, with the exception of not understanding
how large the applications to condensed matter,
say, of ADS-CFT would get,
I think largely my diagnosis of string theory,
as it was then, has stood up since 2006.
What I regret is that the same critique,
I was using string theory as an example,
and the same critique applies to many other communities
in science and all of, including,
and this is where I regret my own community,
that is a community of people working on quantum gravity
outside string theory.
But, and I considered saying that explicitly.
But to say that explicitly,
since it's a small, intimate community,
I would be telling stories and naming names of,
and making a kind of history that I have no right to write.
So I stayed away from that, but was misunderstood.
- But if I may ask, is there a hopeful message
for theoretical physics that we can take from that book,
sort of that looks at the community,
not just your own work on,
not with causality and non-locality,
but just broadly in understanding
the fundamental nature of our reality,
what's your hope for the 21st century in physics
that we can take?
- Well, that we solve the problem.
- That we solve the unfinished problem of life science.
- That's certainly the thing that I care about most,
and hope for most.
Let me say one thing.
Among the young people that I work with,
I hear very often and sense a total disinterest
in these arguments that we older scientists have.
And an interest in what each other is doing,
and this is starting to appear in conferences
where the young people interested in quantum gravity
make a conference, they invite loops and strings
and causal dynamical triangulations and causal set people.
And we're having a conference like this next week,
a small workshop at Perimeter,
and I guess I'm advertising this,
and then in the summer,
we're having a big full-on conference,
which is just quantum gravity.
It's not strings, it's not loops.
But the organizers and the speakers
will be from all the different communities.
And this to me is very helpful.
- That the different ideas are coming together.
At least people are expressing an interest in that.
- It's a huge honor talking to you, Lee.
Thanks so much for your time today.
- Thank you.
- Thanks for listening to this conversation,
and thank you to our presenting sponsor, Cash App.
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And now, let me leave you with some words from Lee Smolin.
One possibility is,
God is nothing but the power of the universe
to organize itself.
Thanks for listening, and hope to see you next time.
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