Pedro Vieira on a theory of all quantum field theories

Episode Transcript
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(00:00):
(upbeat music)

(00:08):
- Hey everyone and thanks for joining us
for Conversations at the Perimeter.
I'm Colin, and as alwaysI'm here with Lauren.
- Hi, everyone.
- And on this episode we had
the pleasure of chattingwith Pedro Vieira,
who holds the ClayRiddell Paul Dirac Chair
in theoretical physics here at Perimeter.
And Pedro is also an expertin quantum field theory,
which is something thatI am most definitely not.

(00:30):
As you'll hear,
both Lauren and I had some apprehensions
about discussing sucha complicated subject,
but Pedro immediately put us at ease.
- I've actually worked withPedro a few times over the years
to create courses for graduate students
and I even took one of his courses back
when I was a student.
So I've seen him explain technical
mathematical concepts many times,
but in this conversation you'll experience

(00:52):
something pretty different from that.
Pedro takes some of those same concepts
from these graduate courses,
but he paints some amazingnon-technical mental pictures
for us with no mathematicalbackground required.
- Yeah, you'll hear Pedro describe
some really esoteric ideas in physics
like quantum field theory andholography and black holes,
but he describes themin terms of sculptures

(01:13):
and maps and bouncing red balls.
And as he was describing
how our universe could be a hologram,
I could actually see a mentalpicture forming in my brain
where there hadn't beena mental picture before.
- Pedro also talks abouthow he finds great joy
in sharing science with others,
both here at Perimeterand in Brazil at the ICTP,
South American Institute forFundamental Research or SAIFR,

(01:35):
which he helped to launch.
And he even gives a theoreticalphysicist perspective on
why you may or may not wantto keep your room messy.
We talk about some complicated stuff here,
but Pedro really makes it fun.
So let's step inside theperimeter with Pedro Vieira.
Hi Pedro, thank you somuch for joining us today
on Conversations at the Perimeter.

(01:55):
- Thank you.
Thank you for having me.
It's fun to be here.
- I wanna start by admitting
I was a little bit nervousto interview today,
you today at first becausea lot of your work relies
on quantum field theory and as a teacher
of some subjects like that,
I'm may be used to adifferent pedagogical approach
where one might first takean undergraduate degree

(02:17):
in physics study some classical mechanics,
special relativity,
take a graduate coursein quantum mechanics
before even mentioning theterm quantum field theory.
But of course, we're not gonna walk
through all of those prerequisites today.
So I thought, how are we gonna talk
about Pedro's work with all of those
things that usually come before?
But you know,

(02:38):
we had a conversationwith you and you were
so great at explaining what you do.
So now I'm not nervous,
I'm just excited to hearhow you're gonna explain
all of these complicated concepts.
And so maybe we canjust start by asking you
what is a quantum field
and what is quantum field theory?
- In physics, the two main theories
that describe physics as you said,
are quantum mechanics and the relativity.

(02:59):
So those if you want,
are the very basicprinciples of all of physics.
So relativity tells us about space
and time and how thingsbehave in space and time.
It even tells us what is space and time.
I mean how space canbecome time and how time
can become space and howdo we measure distances,
how do we measure times,

(03:21):
and what are points in space and time?
Where do things happen and whendo things happen and so on.
And quantum mechanics is thetheory that describes particles
that describes the most fundamentalobjects of the universe,
our microscopic particlesthat move in this space-time.
And what we understood isthat one way of understanding

(03:41):
what are particles thatmove in this space-time
is by imagining there thereis this fluid like quantity,
this field that permeatesall of space-time.
So a field is just a way of assigning
to each point of spaceand time, some quantity.
That quantity could bethe temperature in a room

(04:02):
to each point in the roomthere is a temperature.
It can be the velocity ofwater inside a swimming pool.
To each point inside the swimming pool,
there is a velocity of water there.
It can be the magneticfield in the universe.
To each point around aroundus there is a magnetic field,
stronger close to the sunand weaker farther away.
And in particular,

(04:22):
particles themselves areexcitations of fields.
You can imagine that all
our fundamental particles are understood,
they're small waves of fieldsthat permeate the universe.
And so field theory is thelanguage that puts together
quantum mechanics and relativity.
It's all about space-time.
It's all about thisarena where things move.

(04:45):
And it describes not onlythe arena where things move,
but the things that move themselves
as excitations of some kindof a field like object.
We can picture it as like amembrane that can be still.
And then there are some smallripples of this membrane,
of this shape that move around.
And these small ripples are particles
that make us and the universe.

(05:07):
- And what is quantumabout this description?
- So quantum mechanics isthe theory of the world,
of the world of particles.
And the very basic featureof nature and of quantum
mechanics is the idea thatmany things can happen at once.
When a particle movesfrom a point to another,

(05:29):
what's actually happeningis that the particle
is going through all possibletrajectories at once.
And that's very surprising
because that's not what wesee in our day to day, right?
We throw a ball and the ball goes along
some trajectory and I throwthe ball, you catch the ball,
you throw the ball back to me,
we don't see the ballgoing in all direction.
And the reason is just thatthe most important trajectories

(05:52):
dominate the physics,they dominate the process,
they are more important.
And when you compute indeed,
you realize that thereare some trajectories,
some classical things that are happening
that are much more important than others.
But strictly speaking,
and in particular when yougo to the microscopic world,
all these things that can happen
are happening at the sametime and they matter.
And so when you have these fields

(06:14):
that describe these particles,these fields are not quiet,
they are not just some boringmembrane that is still.
And then there is a particle here
that's moving andfollowing a straight line.
These fields are vibratingand these vibrations
are what we call quantummechanical vibrations.
Many things are happening at once
and in fact everything ishappening at the same time.
And these particles are allinteracting with each other,

(06:35):
moving in all possible trajectories,
throwing particles at each other.
That's how particles interact.
Particles deflect becausethey throw stuff at each other
and so they are coming into collision,
but they throw stuff ateach other and they deviate
and they deflect and theyinteract with each other.
And that's how nature works.
This might start to look very complicated.
How are we going todescribe things that happen?

(06:56):
If I tell you that todescribe what happens,
you have to describeeverything that can happen.
- That sounds impossibly complicated.
- It sounds complicated.
And the way out is because as I said,
there are things thatmatter more than others.
When you throw a particle inthe middle of empty space,
the thing that matters mostis when the particle goes
in a straight line frompoint A to point B.

(07:19):
Then there are otherthings that can happen.
The particle can emit some other particle
that can be absorbed later and so on.
But that, it's a little bit less likely
and it can emit two or three particles
and that's even less likely.
And there is an notion inphysics of the coupling
and the coupling is it's what quantifies,

(07:39):
how much of these quantumfluctuations are going on?
Are you studying a system where the,
where if you want the coupling is small,
where this quantum effects are small
and where things are notas bubbly as they could be?
Or are you studying somethingwhere the coupling is big
and really everything ishappening at the same time?
And most things we study,
the coupling is small andnot much is happening.

(08:02):
And yet that describesmost of what we see.
Take for example, light.
Light for the most partjust goes straight.
We turn on a flashlightand what's happening
is that a gazillion photons are going
from the flashlight to the wall.
- Is that the actual number?
- Yeah.- Give or take.
- Maybe three or four.
- Got it.
- So three or four gazillion
go from the flashlight to the wall.

(08:25):
And what do they do as they travel?
They go all together, right?
Quiet, like very respectfulphotons all by each other.
And they just go and they go
They are not struggling,
they are not fighting witheach other as they go.
They're not like a bunch of hooligans.
They're really going calmly

(08:46):
And so that's an examplewhere not much is going on.
So it's true that to describe the photons,
we don't describe just a straight line
We describe the other trajectories,
but they really matter very little.
And so that's an example of asystem that is weekly coupled.
So the photons, they also interact.
They interact with the air,
they heat the light ones in the air.
That's why we see thelight when the light,

(09:08):
because it's from time totime heating some particles
in the air and beingdeflected into our eyes.
But for the most part,
most of the light goes from the flashlight
and we see just a spotof light on the wall.
And so when things don't interact much,
this taking into account allthe possibility in practice
means taking account a fewpossibilities because not man,
the craziest ones don't matter.

(09:30):
We don't consider thetrajectory of a photon
where the photon and instead
of going photon are particles of light.
When the particles of light go
from the flashlight to the wall,
if instead of going directly it first goes
around the room and then it goes,
that's going to be anirrelevant contribution
to what's going on.
Now that's not always the case.
So sometimes there are situationsin physics where really

(09:55):
this craziness of quantum mechanics
where everything is going onat the same time matters a lot.
And an example is whathappens inside the nucleus.
Inside the nucleus.
We also have particles like photons.
What do I mean by like photons?
It's particles that don't have mass
that are very, very light.
They are called gluons instead of photons.
And the main difference between the gluons

(10:15):
and the photons is thatthe gluons, when they move,
if I had a flashlight ofgluons and even if I would turn
on my flashlight insteadof the gluons propagating
from the flashlight to the wall,
they would start fighting with each other.
They would start of having these brawls
and fighting and makingballs of energy of gluons.
And they would end up beingstuck in this big, big fight,

(10:37):
and this by this fight,
I mean all these quantum effects going on.
- I want that gluon flashlight,that sounds fascinating.
- That's what keeps us,
that's what makes usalive because the gluons,
the name gluons comes from glue.
And what they do is this crazyfight of the gluons is what
keeps the constituentsof the nucleus together.

(10:58):
So you could imagine that you put many,
many protons together atthe center of our atom,
and the protons, theyhave all positive charge.
Why would they want to be together?
They don't, right?
They hate each other.
Particles with positivecharge, they repel.
So you put a bunch of protons together,
the first thing theywant to do is each one,
they want to go apart.
And yet our nucleus is fullof a bunch of protons, right?

(11:20):
So what is happening?
Who is keeping them?
They want to fly apart,
but all these gluons are therefighting with each other.
And so the protons cannot get away
because they're in themiddle of this fight.
They're just looking, oh my God,
let me stay here.
'Cause there is all thesegluon fight around them.
And by this fight I mean that the gluons
are behaving all possible ways.
Everything is happening.
They are moving left,they're moving right.
They're moving up, downall at the same time.

(11:42):
And we really need to take
all these quantum effects into account.
- Another way to say this isthat they're strongly coupled.
- And that's another way,they're strongly coupled.
So the probability ofthe gluon going straight
is as likely as the gluonsplitting into two gluons
or turning right or turning left.
And everything matters at the same time.
And it's not true like withthe photons that you just
consider something simplelike going straight

(12:03):
and the rest doesn't matter.
That's not true with gluons.
And that means that when you stay,
the world is describedby quantum field theory.
That's totally true.
But quantum field theory gets split
into two quantum field theories.
If you want, you can call itthe easy one and the hard one.
- I'll go for the easy one, please.
- Yeah.
When the coupling is small,you get the easy one.

(12:23):
Somethings happen but not much.
You can control what'sgoing on and you can compute
what's going on and improveslowly your computation.
You can say the particlesof light go straight plus
a small deviation plus a small deviation,
plus a small deviation.
And step by step youimprove your calculation.
So in school you learn some, you learn,
then you go to graduate school,
you learn how to correctit a little bit more

(12:45):
and you keep improving.
And this is fantastic.
It works amazingly andin many, many situations.
It's what allows us to test physics
with this crazy number of precisions
where we have all this analogies
that we measure instances
in particle physics within the precision
of an error and stuff like this.
But sometimes when quantumeffects are strong,
sometimes we have a qualitativepicture of what's going on.

(13:08):
We kind of understand incartoonish terms what's going on.
We understand the protonsthey need to be stuck there
because all these gluons arefighting with each other.
But this is a cartoonish picture, right?
I'm speaking with my hands literally
right in in saying this.
Now if you want to ask me, okay,
given that you know that gluonsinteract in this crazy way
and that they hold the protonstogether, can you from that,

(13:30):
and even they allow the proton to exist
because the proton itself is made out
of these quarks and quarks,
they also like to get away from each other
and it's the gluons that keep
the constituents of the proton together.
So given that gluons are so important
in maintaining the stability of matter,
can you from the dynamics of the gluons,
tell me what's the mass of the proton?

(13:50):
Tell me about this fundamental properties.
And the answer for the most part,
for these very tough questionsthat involve controlling
strong coupling is no, ourmathematics is not good enough.
I cannot sit with a pieceof empty paper and start my
computation, step one,the gluon, da, da, da,
compute, compute, compute.
And at the end give youthe mass of the proton

(14:11):
at the end of the page
or at the end of 20 pages or 50 pages.
That's not possible.
We don't know how todo these computations,
and that means we need todevelop these new tools.
So we need to understand quantumfields when they are easy,
but that we kind of understand
it's just about computing more and more.
So you suffer and youget three decimal places

(14:32):
and you suffer more and you get
four decimal places and you suffer
even more and you get five decimal places.
And the more you suffer, themore decimal places you get.
And then you have thehard quantum field theory
that is not even about suffering.
It's that you don't know where to start
because everything matters.
I need to compute everything.
How do I compute everything?
I dunno how to compute everything.
And you need new tools.

(14:52):
And some of these tools are, for example,
using computers likewhat you learn and do.
And some other toolscould be trying to develop
what could be the new ways of thinking
about quantum fieldsthat allow me to develop
some techniques forstudying what could happen
in these crazy situations
where quantum mechanics is so strong.

(15:15):
And by the way,
typically that also means
that relativity effects are very important
because when things are happening
a lot at these very high energies
and things are vibrating a lot,
they're moving very fast.
And when things are moving very fast
is when relativity is important,
when space and time getentangled with each other.
- So both quantum theoryand relativity are at play.
- Both quantum theory andrelativity are at play.

(15:35):
Everything is happening at the same time.
We need new rules, weneed new ideas to think.
And I would say that's oneof the key things we try
to do at PI is understand
what are these new ideas that we need?
How do I describe quantumnature when quantum effects are
the dominant thing and wheneverything is happening at once,
do we just give up or what do we do?

(15:56):
- So what do you do?
What would you see as the eureka moment
if you could suddenlycalculate these things,
where would that take us?
- So another way of saying it is
when do we care aboutthese very strong effects?
So I told you already one example
which is to understand the matter,

(16:20):
to understand the stability even of matter
and what holds us together
and what makes nucleus andfundamental particles stable.
So understanding matterand particle physics
is one of the ultimate goals,
but maybe more conceptually,

(16:42):
another very importantsituation where we would need
to tame this very quantum effects
is when we try to understand how
to merge quantum mechanics and relativity
into what is called atheory of quantum gravity.
We understand very well therules of quantum mechanics
when quantum mechanics is important.
We understand very wellthe rules of relativity

(17:04):
and of Einstein's theoryof relativity of gravity.
When we try to put both together,
we don't know how how that works.
We don't know what arethe rules of the game
when I need to use at thesame time quantum mechanics.
That's very important especiallywhen things are very small.
And then when things are very small,
everything that can happen will happen
and you have to take all that into account

(17:24):
and gravity that describeshow the space-time
can itself be deformed
and how because that isactually what gravity is.
Now, normally we don't care
because normally when space-time
is deformed is when youhave some kind of huge star
that is bending the space-timeor a black hole or something.
And typically when objects are huge,

(17:45):
quantum effects are irrelevant.
And when quantum mechanics is important is
when we are studying electronsor protons or photons,
but then they are very light
and they don't deform space-time.
On the other hand,
when you are close tosingularities of black holes
or at the beginning of the universe
where everything was squeezedtogether in a big bang,
then you cannot get away withoutusing both at the same time
because things are both veryheavy but also very small.

(18:09):
And that's a key thingwe want to understand
what are the ultimate rules of the game?
What describes really our universe
and what's the ultimate theory of physics?
And that ultimate theory needsto deal with strong coupling.
So understanding, developingthese mathematical tools
is useful both for real world physics,
for understanding how do protons behave,

(18:30):
how do some materials behave?
Because not all materialsare weakly coupled.
Sometimes we have in regular materials
what are called phase transitions.
And these phase transitions are precisely,
transitions are preciselypoints in the material where
everything is happening at the same time
and at all possible scales.
And everything is very important there.
All these quantum effectsare very important.

(18:52):
And so taming this strongly coupled effect
are important both inthis real world situation
but also they will be needed to understand
what's the ultimatetheory of quantum gravity?
What's the ultimate theorythat describes our universe?
And that puts togetherall the rules of physics
that we know into a unified rule.
- And we can't go to a black hole

(19:13):
or the beginning of the universe.
So it has to happen largelyon blackboards at first.
- How could we do progress in such field
where things are so abstract
and where you are trying to even develop
the rules of the game?
So what do you use?
So you use lots of thoughtexperiments, like you said,

(19:33):
you cannot jump into a black hole,
but you can do a thought experiment.
Suppose I jump,
I throw Alice into a blackhole and Bob stays outside
and Bob sends a signal to Alice
and as Alice is fallinginto the black hole,
she keeps sending the signal back to Bob
at the rate of three photonsper second, et cetera.

(19:54):
And you do these thought experiments
and you start imagining
what would happen if you do this kind,
if you go to these extreme situations
and often these thoughtexperiments allow you to deduce,
to come up with new rulesfor how physics work.
So that's how Einsteindeveloped many of his ideas was

(20:15):
by imagining he had these experiments
where he would jump and if I'm falling
and something is falling nearby me,
how can I tell that I'm falling?
I just, I look at this red ball
that is just falling with me.
How do I know that we are bothfalling and we are not both
just standing in space?
And indeed you cannot,
if all you see is the red ball
that is close to you andyou are both falling,

(20:35):
you'd see the red ball and you are falling
or you are in the middle of empty space,
it's the same thing.
And so he said, oh,
basically then gravityshould just be like falling,
should just be like goingfreely in empty space
and then maybe gravity can be geometrize
and maybe gravity is just
the formation of space-time and so on.
And eventually it led himto the theory of relativity.
So by thinking of thethought experiments, right?

(20:56):
So he was just thinking,
I fall and I have thisred ball nearby and boom,
gravity came about.
So thought experimentsis one of the key thing.
The other, like I said beforeis computers, often we say,
I have this crazy stuff,everything goes on and it's,
I put it in a computer and I ask, okay,
I cannot compute all these things.

(21:16):
I'll ask the computer to compute
and the computer will crunch numbers.
And a few days later tells me, okay,
the result is 7.3 and then I have to go
and develop totally different tools
that could run somecomputation in pen and paper
and give me the 7.3.
And now I have some hints from computers.
So computers are like a wayof creating your own universe

(21:37):
are like thought experiments,but with numbers,
I run my computer computation
and I have this predictionfor what it could be.
And recently in physicsthere are other ideas
that are now emerging as alternatives
for studying these theoriesat a very strong coupling.
And we might at some point discuss some of
these ideas that go bythe name of holography

(22:00):
and ADSCFT and that arenew descriptions of physics
that sometimes give you a totally
different perspective on a problem.
You're stuck on trying tounderstand this problem
and then a new idea comes that says,
well actually this problem is equivalent
to this other problemthat's totally different.
And now suddenly youare attacking a problem
and you have two differentdescriptions of the same thing.
You have two differentapproaches that you can use.

(22:22):
And so that's another concept that we use,
which is this concept ofdualities or correspondences,
which are often in physics,there are more than,
there is more than one wayof describing the same thing,
like a fluid in a swimmingpool like we said before.
One way of describing is justdescribe where is the water,
how is it moving and at what velocity,

(22:43):
what's the temperature?
Is it too cold?
Is there too much salt in it?
And you describe theproperties of the water
and the fluid that'smoving the swimming pool.
Another description would be you go,
you zoom in and you see oh,
it's just a bunch ofatoms and you describe
the position of all the atoms
and where they are and whatthey're trying to do, et cetera.
And of course it's the same thing.
- But that sounds much harder.

(23:04):
- But the atom one sounds much harder
in this particular case.
It's true.
In fact, what happens is that
the atom one is much harder because
there are many, many more atoms and so on.
But it's also more fundamentalbecause it's the same atoms
that describe the movement inthe swimming pool that will
describe water vapor thatis totally different, right?
So if you have water vapor,

(23:25):
it's the same molecules ofwater that describe water
in the swimming pool andthat describe a tsunami.
And so tsunami, the swimmingpool and water vapor,
it's more or less the same thing.
Ice is also the same thing, right?
So it's the same molecules of water.
And so what happens is that sometimes
the rules at the microscopic level,
the rules for this atomsthat will be the atoms

(23:46):
of water are very, very simpleat the microscopic level.
But then because you put so many of them
and even with a very simple rule,
complicated emergent phenomena appear,
and you can get ice, you can get vapor,
you can get liquids,
you can get all these different things out
of very simple rules,
it's like in a game you can have a game

(24:07):
with very simple rules like chess.
And then you have thesebeautiful games that people say,
oh wow, this was a masterpiece,how amazing and so on.
And the rules of chess are the same.
But then some games are amazingand some games are boring.
And similarly with water,
some phases of water are very boring
and the most exciting phases of water
are in the transition between liquid
and vapor and when it'sreally transitioning

(24:30):
and then it's where this quantum effects
become more important andwhere everything matters
and that's where eventhough the fundamental rules
are the same, the emergent phenomena,
the emergent effects can be much richer
than the fundamental rules.
Now, it's true that thefundamental rules can be simple,

(24:53):
but indeed predicting what'shappening at an emergent level,
it's often very complicated.
So in that sense,
it's easier to use theequations that describe
the water in the swimming pool of course,
than describing all theatoms in the swimming pool.
- You said that the hardproblems that you're working
on in quantum fieldtheory require new tools.
Can you tell us whatsome of these tools are
that you use to tackle thesevery difficult problems?

(25:15):
- Like we said in quantum mechanics,
many things happen at onceand you cannot really say
for sure what's going to happen
because everything is happen at once.
When I told you that particlestravel from a flashlight from
a point to another, theyactually do many things at once.
And in particular,
even to say that particlegoes from A to B,
you cannot know for surethat it goes from A to B.
You can only compute probabilities.

(25:37):
And so physics is all aboutcomputing probabilities.
There is some probabilitythat it goes from A to B,
but it can go from A to C,
it can go from A to D,
it can go from A to any other point.
And so at the end of the day,
what you are studying arewhat are the probabilities
of something to happen in physics,
and sometimes to do thesecomputations in physics

(25:58):
and to compute all this,
what's the probabilityfor something to happen,
you have to do these long computations,
you have to develop these new tools.
But you could flip it around and say,
well, if it's a probability,
it's a number between zero and one,
you can ask,
instead of doing thecomputation, let me think,
what could be the possible results?
It must respect the rulesof causality and relativity.

(26:21):
So if I'm very far away,
I cannot influence what'shappening here right away.
And you start thinking insteadof doing the computation,
is there a way of tryingto constrain to fix,
we call it to bootstrap whatcould happen just by trying
to impose very fundamental
principles on the result directly.
So instead of trying to describewhat is really going on,

(26:42):
can we think of a question,
a physics question likewhat's the probability
of a photon reaching my handcoming from Lawrence's hand?
And then instead of trying todo this honest computation,
let's try to fix the result to ask
what are the possibleoutcomes of this result?
In any possible theory we might not
even know the rules of the game.

(27:02):
We might not even knowthe fundamental theory
that we could be studying quantum gravity.
And this is a new perspective,it's called the bootstrap.
And it's the idea of trying
to use very fundamentalphysics principles,
quantum mechanics, relativity,
some very simple mathematicalprinciples as well,
and trying to use thesefundamental physics principles

(27:22):
that we believe are sacred to try
to carve out the space of
what is possible and what's impossible
in a given experiment, ina given physical quantity.
So this is a verydifferent way of thinking.
Instead of thinking I have one theory
and one computation I have to do
and I don't know howto do the computation.
And I try and I try and I try,

(27:43):
I say, no, no, no,
let me take a step back andsay there is some theory,
there is some computation.
I dunno what the computation is,
but I know that theresult must be compatible
with the fundamentalprinciples of physics.
So what could the result be?
And so this is a new approach.
Now, typically what you'dstudy in this approach
is then you ask thisvery general questions

(28:03):
of what could be the outcomes
of some probability of some experiment.
And of course, just by thinking of
what could be possibleand what is impossible,
you cannot get the 7.3that I mentioned before.
You cannot get a sharpnumber, but you can say,
well, it could be between five and eight.
And then you start inputtingmore physical principles.
You start saying, oh,

(28:24):
and I also want to impose
a little bit of Einstein'stheory of relativity and so on.
And now you run the thoughtexperiment of what could happen
and you get between 5.3 and 7.8
and you start squeezing the result.
You start squeezing the possible outcomes
of what's possible and impossible.
And the question we might ask is the space
of what's possible and impossible

(28:45):
that's not the one dimensional space
because there's not one experiment,
there are millions ofexperiments we could do.
So it's an infinite dimensional space.
So you should think ofit as like a sculpture
in infinite dimensions and the inside
of the sculpture is what's possible.
And the outside of thesculpture is what's impossible,
and how is this space,
can we study this metaphysic space,
this space of allpossible physics outcomes?

(29:06):
Can we study it?
Does it have nice featureslike a nice sculpture?
Does it have pointy edges, pointy corners?
So that's something weare trying to understand
that many people are trying to understand
is what is this possible space of theories
and can it be that some of the theories
that we struggle to solvebecause they are so strongly
couple and the quantumeffects are so strong,

(29:28):
could it be that theyoccupy special places
in this space of theories?
Could it be that there is special points
in this landscape of what'spossible and impossible?
And perhaps there are specialpoints and tips of some
corners of this space of theories
and perhaps there are some locations
that are privilegedand that could indicate

(29:51):
more exciting things going on.
So that's one approach.
- You said this is the bootstrap approach.
- This is the bootstrap approach.
- This seems like such a realworld nitty dirty bootstrap.
Can you explain what itmeans in this context?
- Bootstrap alludes toan impossible picture.
It's the picture thatyou hold your yourself

(30:11):
from your bootstraps and youpush and then you are flying.
You lift yourself out ofthe air by pushing off your,
by pulling off your bootstraps.
And why is it relatedto what I said before?
Because I'm trying to getthe result of a computation
without doing the computation,that really looks impossible.
I should not be ableto get away with that.

(30:32):
- That's like yanking yourself
into the air by your bootstraps.
- It's like, I want toknow this result 5.3
to 7.8 without doing the computation.
How come?
Why?
How could I do it?
And that looks counterintuitive.
It looks strange, and that'swhy we like this picture.
Now it turns out that whywould this be possible?
And it's possible becausephysics is such a beautiful,

(30:53):
but at the same time, rigid framework,
it's amazing that things can work,
because so many things need towork at the same time, right?
So you need with the samerules of electromagnetism
to explain radio waves and properties
of matter and electronicsand spectrum of the sun,

(31:15):
the same rules need todescribe so many things.
And so everything is sorigid that if you ask,
could I change thisparameter a little bit here,
I want to explain some physics experiment
where in some material I got
some blue line instead of some red lines,
so I'll change this law of physics,
but then everything else will fail, right?
So you cannot justchange things at random.
So everything's very, very rigid.

(31:37):
So even without doingcomputations sometimes
because things are so constrained,
just by thinking what could happen,
you can indeed nail things down.
And it brings us back to this power
of thought experiments thatthis is often built on thinking,
suppose I want to studythis probability needs to be

(31:57):
a number between zero and one.
But if this number was 0.7,
it might imply that theoutcome of another experiment,
another thought experiment will be 1.3,
but probabilities cannot be 1.3,
they need to be smaller than one and then
that's 0.7 needs to be excluded.
Okay, so let me try 0.1,
but 0.1 would then implythat this other experiment

(32:18):
would predict a signal arrivingthere faster than light.
Okay, so then 0.1 is also excluded.
And by just thinking aboutall these thought experiments,
now we are starting to squeezethe space of what's possible
and impossible and we are getting
to a smaller and smaller space.
- It's like detective work,
it's like eliminating the possibilities.
- It's very much like detective work.
- And I think this type of approach
is usually referred toas a bottom up approach,

(32:39):
whereas some of the other ones
are called a top down approach.
In general, what arethe types of situations
where you wanna use somekind of bottom up approach
versus a top down approach?
- Exactly. Yeah.
So there is two descriptions.
I confess that I always mixed them up,
so I will not try to usebottom up and top down

(32:59):
because I never know which one is which,
but I know such thing exists.
But basically=
- Your boots are on thebottom (indistinct).
- I never know which one istop up, top down, bottom up.
Yeah, for me, I never understoodthe logic between that,
but indeed there is this picture

(33:19):
that you can try to understandthe rules of the game,
the rules of the world by either trying
to get the very big picture
of what could be thepossible and impossible,
what can happen in themost general situations.
And the other way you couldmake progress is saying no,
let me pick one special example and learn

(33:40):
that special examplein great, great detail.
Those are two very extremeways of getting knowledge,
totally big picture.
I mean it'll be like saywhat do we have in common?
We all want things, we allmove from one place to another.
We all have anxiety, et cetera.
That's a very general way ofdescribing humanity, right?

(34:01):
Or you can just follow one person
and learn about all itsinner desires and so on.
And even though it is just one person,
if you really learn about everything
that person feels or thinks,
you really learn a greatdeal about humanity.
And so in physics it's the same thing.
You can either get the full picture
of what's happening inall possible generality,

(34:22):
but then you will not go as deep
in any particulardirection, or you can say,
let me focus on one exampleand let me go really down on
along that rabbit hole andtry to understand everything
from all possible points of view
about that particular problem
or that particular theory.
And in that way by,
based on that particular example,

(34:43):
try to then draw general lessons
that could be valid in muchmore general situations.
- You said we can think of thewhat it gives us as you know,
an ice sculpture or somecomplicated landscape with these
peninsulas or islands or different things.
So is the ultimate goalto try to figure out
where our reality, our world fits in?

(35:04):
And this is just some pointin one of these landscapes?
- Right, exactly.
So we could imagine we have this map
and there's this cross, you are here,
and now there are two possibilities.
Maybe we carve out this map
of what's possible andwhat's impossible, right?
And maybe this map is like Canada
and maybe we are at somepoint in the middle of Canada.

(35:27):
Well then it's hard to find us, right?
Canada's very big.
If you are in some random pointin the middle of of Canada,
no one will ever find you.
But if you are at the tip of the peninsula
or in the middle of a verysmall island or something,
those are special pointsyou could look at.
And it so happens,
and sometimes we understand why,

(35:47):
sometimes we don't understand why,
that often the mostinteresting theories are
lying in these most interestingspots, these corners,
these tips, these places whereyou cannot go any further.
It's like at the boundary
between what's possibleand what's impossible.
Now why would people live at the boundary?
That's where people live in Canada, right?
They live at the boundarybetween the US and Canada, right?

(36:09):
Why?
Because they were trying to go down
because it was warmerand then they stopped
where they could not go anymore.
So with physics itcould do the same thing.
The theory could tryto go in some direction
because it wants to maximize
some physical principle and he wants to
increase the entropy or something
and it's trying to move andthen boom, cannot move anymore.
So I got stuck here and then it's
the boundary which notpossible and impossible.

(36:30):
And so if if there'ssome underlying principle
that we might not know that is trying
to push theories in someparticular direction,
then it's natural that they stop
where they cannot go any further.
And that is the boundary
between what's possible and impossible.
And so that gives us hopethat if we could carve out
this space of what'spossible and impossible,
it's probably at the boundarythat the most interesting

(36:52):
theories are if indeed such principles
of wanting to go towards something.
Like again, in countries we want
to go towards the water or towards
the warmer climate typically, right?
And so there are these two principles
that push you towardswater or warmer climate.
If there is something similar in physics
that pushes you towards, I dunno,

(37:13):
some information theoretical principle
or some anthropic principle or something
that pushes you in someparticular direction,
then you would expect interesting theories
to lie at the boundary.
So far that seems tobe what we are finding
when we study this space ofthe interesting theories.
And then we try to put these crosses
of we are here, we are here.
Or interesting theories here.
Another interesting theories there,

(37:34):
these interesting theories
and this crosses of where we are seem
to indeed be very close to the boundary
as far as we can tell.
- One thing I really loveabout these explanations
that you give is you're helping
to have us develop these reallynice pictures in our head.
Just now you're tellingus about these landscapes
and peninsulas and making connections
to the Canadian border.
And earlier you were tellingus about quantum field theory,

(37:57):
you were talking aboutmembranes and bubbles,
these kinds of things.
Rather than just having to resort to math,
we can develop these nice pictures.
I also looked at some ofthe titles of your papers
and you had some other nice expressions,
which I don't understand,
but I can picture themlike spinning hexagons.
There was a paper aboutstampedes, non-zero bridges.
So I'm just curious aboutthese kinds of pictures

(38:18):
that you help us to create
when you're making these explanations.
Is this fundamental tohelping you to understand
these concepts or is this something
that you do to help communicate
the work to the public at the end?
- I think it's both.
I think the style of physics that I do,
I like to have a physical,
to have some kind ofpicture of what's going on.

(38:38):
- In your head or are you actually
sketching out pictures as well?
- Both, this stampedeexample, for example,
is really literally processeswhere particles are moving
in a tight space and thereforeit's really like a stampede,
they are moving and hitting each other
and trying to pass fromone point to the other.
And you could ask could

(38:59):
those type of stampede-like behavior
happen at the mostfundamental level of nature?
Could gluons sometimes try to move
from one point to the other and be hitting
another gluon and say, get away,
let me pass.
And pushing each other and trying
to move from point like a stampede.
And indeed we found somelimits where in some physics

(39:20):
situations where particles are trying
to move at a speed of lightfrom one point to another.
And because they are forcedto move at a speed of light,
if a bunch of particles are trying
to move at the same timeat a speed of light,
they will be on top of each other.
There's only one speed of light.
And then they will make the stampedes
and they will try tointeract with each other.
And that was cute becausethen we started, we looked,

(39:41):
and there are some techniquesfor studying this stampedes.
Actually people that study this stampedes,
they studied very different situations,
like boarding an airplane,like who boards first,
and maybe not in Canada,
Canada probably peoplehave board in a steady way,
but if you're trying to board an airplane
and you hit each other and so on,

(40:01):
and or in traffic jams and so on
when the cars need to slowdown and accelerate and so on.
And so there are techniquesdeveloped for counting
how many ways it's possible to board
an airplane or to move in traffic.
And those same type of countingways will be the same kind
of counting techniques thatwe use to count how many ways
the gluons can move when they have to move
at the speed of light togo from point A to point B.

(40:23):
- This is kind of going back
to earlier when you were telling us
about some of the tools that you make use
of in studying thesequantum field theories.
And I know another onethat you I think said,
but we didn't talk abouttoo much is holography,
which is making some of these connections
but in different dimensions.
And could you tell us a little bit more
about this tool of holography?
- So before mentioningholography, let me mention again,

(40:44):
a little bit about this emergence.
So this emergence is the idea that,
so something that emergesthat was not there again,
like the a beautiful chess game.
The chess game by itself is not beautiful.
Just the horse moves likean L and the pawn moves
by one step and then suddenlybeauty comes out of it, right?
When the game is amazing.

(41:05):
So beauty was not there.
And then it comes about, it'sthe same thing with a fluid.
Like we said, a fluidis just made of atoms.
So this notion of something being fluid
and smooth and so on, it's an illusion,
it's something emergent,
it emerges because we are notlooking very, very closely.
So we could say that a fluidemerges when we zoom out.

(41:25):
When we look from far away,then yes, a fluid exists,
A fluid makes sense, but whenwe go in, oh, it was fake.
Same with temperature.
What is temperature?
Temperature is nothing.
There's no such thing as temperature.
What exists are particles moving around.
If particles move very, very, very fast,
you put your hand there andthe particles moving very fast

(41:45):
will hit the particles in yourhand and now the particles
in your hand are moving veryfast and your hand is warmer.
And that's what touchinga hot thing means.
You touch a very cold thing,
the particles in the coldobject are not moving.
So the ones in your hand, they are moving.
So you touch them and nowthe ones in your hand,
they shake the ones in thecold stuff and therefore they
lose energy because theyhave to waste energy

(42:08):
to wake the other ones up,
and therefore, your hand cools down.
So what exists are particlesmoving and particles dancing.
But what emerges is thisnotion of temperature,
is this idea that there issuch thing as being hot,
being cold,
but again, that's emergent.
What's fundamental is particle moving.
Now in physics, it's nota shock if I tell you no,

(42:30):
it's not really,
temperature is not reallysomething fundamental.
What's fundamental is particles, no,
fluid is not something fundamental.
What's fundamental is particles.
But a more recent idea thatis pushing this idea of
emergence to an extrema issaying that perhaps even gravity,
even space-time is emergent,
perhaps even if you want reality.
Even us, we don't exist.

(42:52):
We are emergent.
And the idea is that we say in this room,
we are here in three dimensions, right?
We might be the imageof a hologram, right?
Right, like Princess Leia,right, in "Star Wars", right?
So we might be a bunch of holograms here
and maybe we don't exist,
we are just projected holograms
into this three dimensional space.
But we are actually just being generated

(43:14):
by a 2D hologram at theboundary of the universe, say,
now this seems like a crazy idea, right?
If I say we don't exist,gravity doesn't exist,
space-time doesn'texist, it's all emergent,
it's all an illusion, andwe are all a hologram.
So let me tell you a little bit,
where would such strange idea come about?
That there could besomething like a membrane,

(43:35):
a hologram that coulddescribe something inside.
Now the idea comes from the following,
by thinking about information.
So there is thisfundamental idea in physics,
which is that mass alwaysgrows, there's always more mass.
In physics, we call it entropy.
So entropy is always increasing.

(43:56):
You break a glass, you getpieces all over, right?
And the glass is not goingto reconstruct itself
into a beautiful glass, right?
So things always increase.
The entropy is always increasing.
So we dying is because our entropy
is growing, growing, growing, growing,
eventually we die.
When we clean up our room,
something that's very popular these days,

(44:17):
you have to clean up your room.
When you clean up,
when you clean up your room.
- You're a father, aren't you?
- When you clean up your room,
you are reducing theentropy in the room, right?
But the entropy, I said,always needs to increase.
So what's happening isthat to clean up the room
and to reduce the entropy of the room,
you are increasing your own inside entropy

(44:39):
and you are coming closer to being dead.
So-
- I've never thought of it that way.
- Yeah, so be careful.
You need to clean up your room.
So entropy always grows.
And so there is this notion of disorder,
and entropy also quantifiesthe amount of information.

(45:01):
If you have an emptyroom, it cannot be messy.
If you have a room full of books,
it can be very messy, right?
You can tear all the books apart,
throw pages around and so on.
So the more mess you have,
the more potential information you have.
Now, let's try to make a really, really,
really messy room by throwing more
and more stuff inside the room, right?
So we have this room andwe keep throwing books

(45:23):
like we said, we throw some ketchup,
we throw lots of stuff inside
the room to make it really, really messy.
So what happens?
Well, what happens that at somepoint the room is so heavy,
so full, so big, so full ofstuff, it forms a black hole.
- So this is a thought experiment.
- This is an example.
- You haven't made aroom this messy before.
- Well, you should say,

(45:44):
but no, not that messy.
- Not with ketchup.
- No, no, the ketchup was missing.
And so you have this ideathat things can be messier
and messier and messier and messier
and eventually they form a black hole.
But if the mass is always increasing
and if you eventually form a black hole,
it means the black hole isthe messier object there is,

(46:05):
because it's the endpoint of a messier room.
And so that means that the amount of mass,
the amount of informationis biggest in a ball.
If that ball is a black hole, as we said,
we put more and more stuff,
more and one information is there inside
and suddenly we have a black hole.
On the other hand,
a black hole because it's sucha simple, after all, object,

(46:27):
much simpler than a messy room.
It's just a black ball where light gets in
and cannot get out, thereare things we can compute,
we can study about blackholes and we can quantify
how much disorder, howmuch of this mess there is.
When people compute withpeople like Bekenstein
and Stephen Hawking andmany people studied,

(46:48):
asked what is the amount ofdisorder inside a black hole,
they found a very surprising thing.
The bigger the black hole,the bigger the disorder.
That's normal, right?
If a room is twice as as big,
the disorder can be twice as big inside,
but it was not proportional tothe volume of the black hole.
It was proportional to thearea of the black hole.
And that's very surprising, right?

(47:09):
If you see a huge buildingand you see a building
that the volume is twiceas big, you say inside,
that can be twice as much mess.
You don't say the mess is proportional
to the area of the building.
When would you say thatthe mess is proportional
to the area of the building?
If all the mess is in the wall,
that's the only scenario whereyou would say if a building,
if a room doubles inarea, the mass doubles,

(47:31):
if someone tells youthat, then you say, oh,
inside that room you just have
a bunch of papers on the wall, right?
Like the serial killerinvestigators, right?
With all these strings andnewspaper clips and so on,
everything is on the walls.
There's nothing in the middle, right?
Because then the wall surfacedoubles and the mass doubles.

(47:51):
And so what we are saying is
that we were throwinginformation in this room,
we form a black hole,
and now we can describe this black hole
and the amount of informationis only at the boundary.
It's only at the walls.
It's only at the end.
Well, but then if you take this seriously,
it means that you should be able,

(48:12):
if even in the most extreme situation
where you have the mostamount of information,
if it's possible to describeit just by looking at the wall,
when you have less, youshould also be able to.
And so the ultimate conclusionof this crazy thought
experiment is that youshould be able to describe
what's inside the universe by describing
the boundary of the universe.
Now, this could be a dinner chat, right?

(48:34):
I mean we are having some drinks
and we are having some fun,
and we come up with these crazy ideas.
But then in '97,
Maldacena said this isnot just a crazy idea.
Here is one theory of quantum gravity
that describes an exampleof what could be a universe.
And here is an hologram at the boundary

(48:54):
of this universe and theyshould be the same thing.
And this idea that you couldnot only speculatively,
but really write equationsthat says this reality is equal
to this description interms of an hologram
that is just at theboundary of the universe
is what's called holography,
also goes by the name of ADSCFT,

(49:16):
or gauge gravity dualities.
These are all names for the same thing.
And it's a concreterealization of what was before
a crazy idea that came mostly
from the thought experimentswith the black holes.
Because if the idea is that everything
can be described by the walls,
but we don't feel like weare stuck to the wall, right?
We feel like we are here.
So what's the way out?

(49:37):
Everything is described by the wall,
but we feel like we are here.
Well, then maybe we are ahologram projected from the wall
and maybe all theinformation is on the wall.
And if you really look at the wall,
you see all the rules of the game,
the analog of the atoms in the water,
and you see in the wall,
the electrons in the chipsand the quantum computer
that is at the boundary of the universe.
And do, do, do, do, do, do.

(49:58):
But then from far away youhave this princess Leia's,
which are us,
and this hologram is beingprojected in and we emerge.
And even the inside of thewall, the universe, the space,
the gravity would emerge.
We would all be emergent concepts
that would be producedby this quantum hologram.

(50:19):
This idea would've farreaching implications
because it would tell you that gravity,
for example, would be emergent.
And at some point we saidit's very hard to put gravity
and quantum mechanics togetherand what this idea would say,
yeah, throw away gravity.
Gravity doesn't exist.
Gravity is emergent.
All there exists is quantum mechanics

(50:40):
in this quantum computer.
That's the hologram.
And then gravity is fake news.
It's just you think there's gravity,
but it's like you have a hologram
of a colibri flying hereand it's not flying,
it's just a hologram.
That could be how the world works.
Maybe the world is holographic.
- Well, Pedro,
we'd like to share with you now a question
that was sent in by a student.

(51:01):
She'd like to ask you to say
a little bit more about ADSCFT.
- Hi, my name is Anna.
I'm currently a sci studentat Perimeter Institute,
and I have the following question.
Could you give the main gist
of the so-called ADSCFT correspondence
and explain why people

(51:21):
in your research communityare so interested in it,
even though we probablylive in a different type of
universe, not anti-DeSitter, but De Sitter space.
- Let me go one step backand say we have this thought
experiment of the messy roomthat led us to this idea

(51:42):
that there should be somehologram description of reality.
Someone tells you that it'slike one of those emails,
I have a theory about everything,
but okay fine.
- We do get a lot of those emails.
- We do get a lot of those.
Okay, what can I do?
You have to be a bit more specific,
and it's hard and I don't know.

(52:03):
And no one knows what's the hologram
that describes our universe.
Then we ask, is there a toy universe,
a toy theory that we can play with,
which would be an alternative universe,
A simpler one where youwould have, in that universe,
you would still have gravity,
you would still have particles,
but it would be a toy theory.
And in that toy theory,

(52:24):
you can make these ideasprecise and at least you have
a mental laboratory where you can exercise
and practice and test these ideas
and see if they make senseand push them forward.
And it's related to this bottomup and top down approach.
And I never know which onethat Lauren was referring to.
And that would be amazing.
And indeed we were able to make
these ideas precise in some toy examples.

(52:47):
And this question was referring
to that the examples we describe,
we manage to make this precise are toys.
They're not the real thing.
And so given that they are toys,
why do we like them so much, right?
Why don't we care about the real thing
and not about the toy?
And as usual, the answer is,
we start first trying tounderstand these toys.

(53:09):
And now there are two possibilities.
Some people will try to make
this toys more and more realistic.
Try to say, I will try to add more
and more ingredients to make this
more closer and closer to the real world.
Some people will stay longerwith the toys and say, no,
I want to play with this toy a bit longer.
I want to go deeper and deeper
and try to extract morelessons from this toy.

(53:30):
And it's a spectrum.
ADS is related to the name of this toy.
It turns out that it's betterto describe this holograms.
If there is a wall we have,
we need a wall to hang the hologram.
And if you have just a regular space-time,
imagine space-time that goes on forever.
Where's the wall?
There's no end.

(53:51):
You just go, go, go, go.
When you are waiting for a place
to hang the hologramand you don't find one.
So it would be better if yourspace-time was a very big box
because when your space-timeis a very big box,
you go to the boundary of the box
and put the hologram there.
And ADS, it's a space-time that's a box.
There is an end where youcan put this hologram.

(54:12):
Now I should say it's a fantastic box.
It's not a random box.
Let me tell you something special about,
let me give you an example.
Take a shoebox, right?
There is a midpoint.
There is a point whichis the middle, right?
And then there are the cornersand the walls and so on.
But there is a special pointwhich is the middle of the box.
This anti-De Sitter is abox, but there's no middle.

(54:33):
All points are the same.
There's no special point.
It's a strange box.
Why do I call it a box?
What is special about the box?
Because I take this red ballhere and I throw the red ball
and I'm talking to you and Iget hit by the red ball again.
So I say this is a box.
I throw the ball and the ball comes back
and I'm here and I throw thisred ball and I get it back,
I throw it and I get it back.I throw it and I get it back.

(54:55):
And doesn't matter which direction
I throw it and I get it back.
And doesn't matter whereI am in space-time,
When I throw the red ball, I get it back.
So in that sense, there is no center.
It's all the same,
whatever you are, you take a red ball,
you throw in the red ball,
you receive the red ball back.
So you feel like a box.
But if I feel like a box,
Colin feels like a box.
Everyone feels like beingat the center of the box.

(55:17):
It's very democratic box.
So it's the most perfect box there is.
It's called ADS.
It stands for anti-De Sitter,
which is the name of a geometerthat thought about this box.
And in this box, in this very big box,
we understand that what happens inside
the box can be described
by a hologram at the boundary of this box.
Now we don't live in a box,

(55:37):
at least we don't knowif we live in a box.
Maybe we do.
Maybe the boundary isvery, very far, far away.
But one thing you couldsay is that whether we live
in the box or not should notmatter if the box is huge.
Should the rules ofphysics here for us change?
If in the gazillion, gazillion,
gazillion parsecs there is a wall?

(55:59):
Probably not.
It's really, really super, super far away.
Who cares.
From that point of view, some people,
I would say that if youcan think of physics
with a fake box provided you say the box
is big enough and if with
that fake box you candescribe what's inside,
you can always pretendthe box is big enough

(56:20):
that it doesn't matter
that if we are inside the box or not.
So if you can learn somethingabout physics inside
the box from a big box,that's good enough.
But I say this because I don'tknow how to do holography
if I have no box, if Iknew I wouldn't say this,
I would just do holographywithout the box.
If I knew how to realizethis crazy holographic
ideas directly in our universe,which goes on forever,

(56:43):
I would prefer that.
And so some people are trying,
some even some people hereat Perimeter like Sabrina
and others are trying to study better
what happens at infinity in the universe.
And is it really impossibleto put an hologram there?
Do we really need a box?
It'll be very difficult.
So there are things tounderstand and things
get even more subtle whenyou think about cosmology.

(57:06):
When you think that theuniverse is expanding
and it's growing and thenit's even harder to imagine,
where do you put the hologram?
- And we have one more question
that was sent in from another colleague
of ours here at Perimeter.
- I'm Dao from Perimeter Institute.
A question is that I have heard

(57:27):
that you said you have solved
a quantum field theory a few times.
I wonder what that exactly mean
and when will we actuallysolve quantum field theory?
- So why do we say solving?
Solving means computing.
If I want to study a physical quantity,
we have to take our theory

(57:47):
and understand what arethe rules of the theory,
what's the outcome of the experiment
and how do I go from the rules
to the outcome of the experiment?
Sometimes we can bypass thatstep by doing this bootstrap
kind of ideas and studyingwhat's possible and impossible.
But then we have toytheories and real theories.
So again, it's like describing say

(58:08):
the trajectory of a tennis ball, right?
If I just say it's a parabola,
there's gravity and so on, it's easy.
If I say no, but there'swind now it's a bit harder.
Pieces of the ball are falling
as it's going now it's harder.
So the more realisticyou make it harder it is.
And you can never really do a perfect job.
You do better and better and better,
but there's always moreeffects to take into account.

(58:30):
So when are we going to solve
real world quantum field theory
and be able to wake up andwith a clean page of paper
and at the end of the pagecompute a mass of the proton?
I don't know.
That would be amazing if I could compute
a mass of the proton even withtwo digits in my lifetime,
I would be delighted.

(58:50):
We know the answer to this, right?
We can put it in computersor we can measure it.
We can take a scale and and figure it out.
But computing it from firstprinciples we don't know.
Now on the other hand,
solving quantum field theorymeans developing techniques,
new techniques that we can use
to do better and betterin quantum field theory.
And that requires solvingthese toy theories

(59:12):
and understanding how to develop
these techniques in simplified examples.
In the same way that ifyou want to solve chess,
you will solve checkers first.
It's easier, right?
You will develop computertechniques for counting all
possible checkers or for developing
artificial intelligence,for solving checkers.
And then you'll applyto chess and then to go
and eventually to givedating advices and so on.

(59:37):
So the more complicated it goes,
you will develop step by step, right?
And so similarly with physics,
what we want to do is be ableto tame these quantum effects
and in particular thesestrong quantum effects
in the analog of checkers, inthe simplest possible case,
let's have at least oneexample where we can do it.
And if we can reallynail one example down,

(59:59):
everyone will believe, okay,now it's a question of time.
We have to work harder,but we'll do the next,
we'll do chess and thenwe'll do go, et cetera.
But we need the first example.
And it was the case inother areas of physics
before like statistical mechanics,
we needed to solve onestatistical mechanics system.
And there was a beautifulsolution in '49 I believe,

(01:00:22):
of the so-called twodimensionalizing model,
which is a particularmodel in two dimensions
of statistical mechanics
of a particular two-dimensional material.
And it was the first example
that was possible to solve exactly.
And then it was like a Pandora box.
Once that one has solved manyothers followed afterwards
and we learned many general lessons
about phase transitionsand properties of matter,

(01:00:43):
and so on, the energylevels of the hydrogen atom
that we learn in school,
it was crucial to havethat one solution exactly.
And then we developedtechniques, sometimes exact,
sometimes approximate forstudying many other atoms.
And now we know chemistry.
And so it's often about
breaking this barrier ofsolving a quantum field theory.

(01:01:06):
Solving a quantum fieldtheory is like solving a game.
And there are easier gamesand more complicated games.
And even solving a game canmean many things like chess
is solved when you haveseven pieces on the board.
If you have more than seven,
it's not completely solved yet.
So chest with seven pieces done,
chest with nine pieces not done yet.

(01:01:28):
Similarly, in some quantum field theories,
we managed to understand for example
the analog of the spectrumof the hydrogen atom.
What are the energiesof that quantum theory?
What energies can the states have?
So that was something that we did
and that was probably greatly
why I am at Perimeter was
because we solved that problem.
That was a tough problem,

(01:01:48):
that was an open problem in the field.
How do we compute those energy levels?
But that's the zero.
The first thing we ask about
the nitrogen atom is whatare the energy levels?
Then we ask, okay,
now I take two hydrogen atoms
and I throw them against each other.
What happens?
Oh, that's much harder thanjust studying the energy level.
And then once we do that,
we ask the next question andthat's like solving chess

(01:02:09):
step by step and in more andmore complicated situation.
- So it's like you start with a toy,
you solve that toy model,
you make a more complicatedtoy, you solve that.
And maybe someday thistoy can be so complicated
that we can solve it andthen represent the universe.
- That's the hope, yeah.
It's also the hope that sometimesphysics tends to look more

(01:02:30):
and more like a toy inthe sense that it's often
the case that physics looks complicated.
And then we find this unifying principles,
this idea that there was a electricity
and magnetism and there wassome loss for electricity,
some loss for magnets,but it was complicated.
And then we understood, oh no,
they can be combined andactually things are simpler.
And it's not like we have the electricity

(01:02:52):
and the magnets and so no, no,
they really talk to each otherand there's a single thing,
and now it became closer tothe to than to the real world.
And so it's also the hope,
but that might be just a dream,
that the world can be closer to a toy
and that perhaps the fundamental rules
will link things together that right
now look very complicated and disparate.

(01:03:13):
And that doesn't seem to be a connection
between the expansion of the universe
and the mass of theelectron or whatever, right?
There are many things inphysics that look totally
independent and different from each other
that maybe once we will understand really
what are the rules of the game,
maybe they'll be connected,maybe things will be simpler.
It might be that thegoal is not create a toy
and make it more and more complicated,

(01:03:34):
but understand what arethe underlying rules
and perhaps the fundamentalrules will be that (indistinct).
- When we started this conversation,
Lauren admitted that shewas a little intimidated
because of all the terminology,
you know, quantum field theory.
And I can emphasize thatI was 10 times more,
a hundred times more
'cause I haven't studiedphysics in university.
Lauren's a quantum scientistand she was intimidated.

(01:03:56):
But I wanna say that just toreiterate that your ability
to draw pictures verbally andthen I create them in my head.
I don't know what your sculpture looks
like of what's possible versus impossible.
Mine is a very cool quartzcrystalline structure.
But the idea that you can conveythese ideas in a clear way,
I think it relates toyour teaching as well.
You've done a lot of teaching and outreach

(01:04:17):
and I know that I think you two have
worked together before on teaching.
Can you talk about what,
how you approach teaching these subjects
to younger people and you even do outreach
to non-scientists like myself.
- I like teaching very much.
It's one of the most excitingthings about what we do.
I mean especially the teaching that I do,

(01:04:38):
which is a huge privilege,is that we get to teach,
first of all amazing studentsthat are really super excited
about being here and no oneis studying some particular
physics subject because you have
to get some grades or some credits.
No, people are really excited
and they want to learn physics
because they really are passionate
about understanding hownature works the way it works.

(01:05:02):
Often to teach,
you have to really understand things
in a very deep way ifyou want to simplify it.
It's easier often to protectyourself in the math.
Writing equations is easy.
Solving equations is easy.
- Says you.
- No, no.
- But I'm an non-scientist.
But I see where you're coming from.
- But it's something mechanical,it's something you learn,

(01:05:24):
you have to learn how to do it.
- Or you learn to ask a computer to do it.
- Ask the computer to do it.
- You have to know how to ask the computer
or know what to ask the computer as well.
- It's a language, you learn it, right?
But teaching forces you
to have a clean pictureof the fundamentals,

(01:05:44):
to not to be lost in the technicalities
and details that sometimes don't matter,
but really focus on what is really
the problem we want to solve.
What's really simple, what's really hard,
and I think that's veryimportant for a physicist
to keep some mental sanity is to teach,

(01:06:04):
teaching too much is not good
and you don't have time to do research.
But teaching a good deal I think is very,
very powerful and useful.
In particular when you areteaching some of these subjects
that are not yet in textbooks
or that are a little bit more advanced,
you are really oftengoing into the unknown,
going into the world ofwhat is not yet known.

(01:06:26):
And as you try to understand things
and try to bring them to the students,
you are trying to cleaning it up,
purifying it, and really polishing it.
And it's really something precious
that you are allowingyourself to tell someone.
And when you explainthe way you understood
some high energycollision of two particles

(01:06:49):
and why when these twoparticles hit each other,
things can fly in all possible directions
with all equal probabilityand you understood
why was it all equal probability
in that particular case andyou managed to simplify.
It's really a magical moment
when you manage to get that across.
So yeah, it's something transcendental
that you go into thisplatonic world of ideas,

(01:07:10):
you drag them down and thenyou give them as a gift.
- Earlier in thisconversation you mentioned
how some people in Canada,
they amass along the borderso they can be far south
for the warm weather, butyou spend about, what,
half your year in Brazil
at the South American Institutefor Fundamental Research.
Can you tell us how that came to be
and what drew you there?

(01:07:30):
- Yeah, that's true.
So I spend a few months every year there.
So I go back and forth.
So it's convenient.
It's the same time zone more or less.
So I go with a cloud ofstudents typically moving up
and down in this strong coupled system.
So indeed I first got to know
this institute in South America,

(01:07:52):
the goal of this institute that SAIFR
that ICTP SAIFR that you mentioned
is to serve as a hubfor all of South America
for theoretical physics in South America.
And so what happens inpractice at this institute
is that you have schools and workshops
and conferences running all the time
with students from allover South America going

(01:08:13):
there for a week or twointeracting with excited students
that are really passionateabout a particular topic
and say strongly correlated electrons
and then going back to their
home institutions in Chile, Argentina,
Bolivia, et cetera.
And then a few months later coming back
to another event that happens there.
And at some point acommunity starts to emerge.

(01:08:34):
You start to know peoplefrom the various places,
people that were previouslytotally isolated,
now they get to meet each other at SAIFR.
Top scientists from all overthe world get to go to SAIFR.
And at the same time you have access
to this huge pool of 400million people in South America,
the best of the bestthat start to go there

(01:08:56):
and they have an opportunity to be exposed
to all these top people and then
eventually come here and join us
at the Perimeter Masters International,
or come here for PhD,
or become future postdocs, et cetera.
So it really serves as a hub not only
to connect everyone in South America,
but to connect South Americato the world more broadly.

(01:09:16):
It's a relatively recent institute,
it's like 15 years, atthe level of what it does,
which is organizing thisschools and workshops,
it's already one of the leadinginstitutes in the world.
I'm Portuguese, which thelanguage is the same as in Brazil,
more or less the same type of culture.
But everything ismultiplied by 10 in Brazil,

(01:09:38):
people are happy,
they are 10 times as happy as in Portugal
and people are sad, theyare 10 times as more
depressed as if they were in Portugal.
So everything in Portugalhappens, whatever.
I take that I know howit works in Portugal,
I multiply by 10 and I get a good feel
of what would happen in Brazil.
So I have a good intuitionabout the culture.

(01:09:59):
I thought this project,
trying to create this instituteand grow it was exciting.
I spoke with some people at PI
that encouraged me to try to do it.
We leveraged many of thethings that we knew at PI
to create things thatare sometimes similar,

(01:10:19):
sometimes different because you have
to adapt their differentway of doing things.
But for example,
we translated all of theoutreach material of PI
to Portuguese and now to Spanish as well.
People from outreach,
Greg and friends, went overto Brazil several times
to give workshops for highschool teachers and students.

(01:10:42):
I gave several lectures onrelativity and quantum mechanics
on Saturday mornings for high school kids
that wake up at 4:00AM to take these trains
to go to attend this lecturesand understand how space
and time can morph into each other.
And so it's lots of fun.
I think the impact ishuge and can be huge.

(01:11:04):
It's obviously superuseful for these students
that otherwise would not have
a contact with some researchers
that are really doing researchin these exciting topics.
But it's also fantasticfor us that we have access
to this amazing pool of talent.
When we'll finish this podcast,
I'm going to chat withAlessandro that came

(01:11:24):
from this program and we are going
to try to play a little bit more
with this time modelsthat I told you about.
- We did have one morequestion that's less technical.
It's from a young person here in Waterloo,
so maybe we can play that one for you.
- My name's Alice and I'm in grade two.
What would you consider tobe a good day at your job?

(01:11:45):
- That's a very good question.
So what would I consider to be a good day?
As you said at some point a lot
of the work we do inpractice is detective work.
You are trying to think of many,
many thought experimentsand try to see could it be
that this experiment result
has anything to say aboutthis other experiment result.

(01:12:08):
And you keep trying and 99%
of the time you are trying out things,
converting the thoughtexperiments into equations,
trying to solve the equations,simplifying equations,
not solving the equations you want,
but solving simpler equations
so that later you can solve
the equations you want to solve.
And then some days you crack one of them,

(01:12:29):
some days it works, you find,
oh this is the right question.
So those are amazing days.
And even better typicallyis when you do it
in the blackboard with someone else.
When sometimes you are,
you are in a blackboard,
then you are thinking we need to,
I dunno, understand the movement
of these gluons whenthey're trying to move

(01:12:50):
at a speed of light and thensomeone points out well,
but if they are moving all together,
they cannot pass by eachother, then someone says,
oh maybe that's about counting how things
go when they cannot pass by each other.
Could this be related
to this counting problems of stampedes,
and I think truth has thisattractor force to it.
It's like a basin,

(01:13:10):
like water swirling around,
so sometimes you feel lost,
but when you are close tosomething that makes sense,
close to something that is,
oh that's the right thing,
it pushes you towards it.
And so there are these moments
where you are on the blackboard,
and you have this feeling that you
are being pushed towardstruth and that's amazing.
That's an amazing feeling.

(01:13:31):
You just go with the flow and it's like
a dance and each of you are changing ideas
but you feel like, ohwe are going somewhere.
And that feeling of lettingyou flow and you don't,
you just let go and you willeventually get to something
awesome because you feellike you are moving closer
to something deep is fantastic,
but often you are just lost,

(01:13:51):
you are scattered, youare moving left, right,
left right and thensuddenly there's this click
and you feel like youfound one of these streams
that will swirl to something true.
- Amazing.
Well this has been so much fun, Pedro.
Thank you so much for sharing your time.
I think we're gonna be leavingwith a lot of new lessons
to ponder and I think we'reall gonna remember not to let

(01:14:12):
our rooms get too messy 'causewe might create a black hole.
- But if you clean them,you are closer to dying.
- So we'll just keepthe room sort of tidy.
- Yeah, that's good.
(upbeat music)
- Thanks for listening
to Conversations at the Perimeter.

(01:14:33):
If you like what you hear,
please help us spread the word,
rate, review and subscribe
to Conversations at Perimeter
wherever you get your podcasts.
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Thanks for being part of the equation.

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.css-14f5ked{margin:0;word-break:break-word;display:-webkit-box;-webkit-box-orient:vertical;box-orient:vertical;-webkit-line-clamp:2;overflow:hidden;}Pedro Vieira on a theory of all quantum field theories