Savas Dimopoulos on the universe’s biggest unsolved puzzles

Savas Dimopoulos is an influential particle physicist based at Stanford University, who also holds the Coril Holdings Archimedes Visiting Chair in Theoretical Physics at Perimeter Institute. He worked at CERN during the 1990s, and was featured in Particle Fever, a 2013 documentary about the hunt for the Higgs boson at the Large Hadron Collider. In this episode, he talks with Lauren and Colin about how he approaches the grand, open questions that keep him up at night – including two particularly fundamental questions about the universe that he feels still need explaining. Dimopoulos also discusses how particle physics and collider experiments have evolved throughout his career, what motivated him to become a physicist, and why he has never looked back. View the episode transcript here.

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Conversations at the Perimeter is co-hosted by Perimeter Teaching Faculty member Lauren Hayward and journalist-turned-science communicator Colin Hunter. In each episode, they chat with a guest scientist about their research, the challenges they encounter, and the drive that keeps them searching for answers.

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Episode Transcript

(00:00):
(gentle upbeat music begins)
- Hello everyone and welcome back
to Conversations at the Perimeter.
I'm Lauren and I'm hereas always with Colin.
- Hello.
- In this episode we'resharing our conversation
with Savas Dimopoulos.
Savas is a faculty member

(00:21):
at Stanford University in California,
and he's the Coril HoldingsArchimedes Visiting Chair,
here at Perimeter Institute.
He's a renowned particle physicist
whose career spans over four decades.
- So I've been wantingto have Savas as a guest
on this podcast ever sincewe first launched it.
So I was thrilled thatwe made this happen.
I first met Savas nearly 10 years ago

(00:42):
during one of his annualvisits to Perimeter,
and I was immediately struckby his kindness and his wisdom,
and really by his undiminishedpassion after all these years
for exploring the most puzzlingmysteries in the universe.
- In this conversation,
he shares his thoughts onfundamental, huge, open questions
like, why is gravity so weak?
- Why is the universe so big?

(01:03):
- And is there a multiverse?
And he also talks abouthow he remains motivated
to search for answersto such huge puzzles.
- Savas was also one of thescientists featured prominently
in the award-winning 2013documentary, Particle Fever,
about the hunt for theHiggs boson at the LHC,
CERN's Large Hadron Collider.

(01:24):
Savas tells us somehistory of collider physics
and he explains how a renaissancein small-scale experiments
could reshape how physics is done
in the generation between the LHC
and the next big super-collider.
We were fascinated by this conversation
and we're pretty surethat you will be too.
So let's step inside thePerimeter with Savas Demopolis.

(01:45):
(gentle upbeat music fades)
Savas, thank you so much for joining us.
I've been looking forwardto chatting with you
for a long time now.
- My pleasure.
- It's been a bit of a breakfor you coming to Perimeter
because of the pandemic, butwe're glad to have you back.
And I was looking at yourStanford webpage the other day,
and it says that your jobis to search for answers

(02:07):
to the biggest mysteries in the universe.
That's about the biggest job description.
Can you tell us what does that mean?
What do you do for a living?
- I assure you, thejob description is big,
but it is not matched bysalary. (both laughing)
- It would have to bean astronomical salary.
- It would have, but I'm happy,
because my main reward isthat I'm given the time

(02:31):
to just think about the universe,
and that's the reward enough for me.
- So what are the bigquestions about the universe
that are driving you these days?
- Yeah, so there are several,
but I want to give you some big principles
that guide the questionsthat we are asking.
One of the big principles iswhat's called, "Naturalness."

(02:53):
And the idea of naturalness,actually, is in all of science.
In the case of physics,
naturalness has to dowith trying to understand
very large numbers.
For example, if you takethe size of the universe
and you compare it with thesize of an atomic nucleus,

(03:13):
you get an enormous number, 10 to the 40,
which is 1 with 40 decimals next to it.
With such enormous numbersit's natural to ask,
"How come the fundamentalparticles of the theory
are so much smaller than the universe?"
Or conversion, "Why isthe universe so big?"

(03:34):
You can ask it in many different ways,
but one of the ways it's asked,
it's called a, "Cosmologicalconstant problem."
Another question is,"Why is gravity so weak?"
So for example, what I meanby the weakness of gravity,
when I lift this glass of water,
the electrical forces frommy fingers to the glass

(03:59):
are large enough tocompensate or to overcome
the gravitational attractionof the entire planet Earth.
And if you think about it,- Hmm.
this is amazing.
The entire planet Earth is enormous
compared to my fingers,- Mm-hmm (affirmative).
yet I'm able to overcome the gravity
of the earth with the electricalforces, or atomic forces,

(04:23):
that my fingers exert on on the glass.
So the only reason why this is possible
is because the intrinsicstrength of electrical forces,
or atomic forces, is far, far bigger
than the strength of gravity.
It's, again, it's about40 orders of magnitude,

(04:44):
1 with 40 zeros bigger.
That is called, "The hierarchy problem."
And these questions,
the enormity of the universeand the weakness of gravity
have been driving, in someways, theoretical thinking
for the last 40 some years.
And much of the theoreticalcommunity in my field,

(05:08):
which is called, "High energy physics,"
has been driven by these questions.
Now, one of these questions,
the so-called, 'hierarchy problem,'
has had some possible answers.
And much of what many people did,
including myself, over the last 40 years,

(05:28):
was to search foranswers to this question,
the weakness of gravity.
Why is gravity so muchweaker than electricity?
Or why is gravity so much weaker
than all the other forces of nature?
To answer these questions,
we came up with theoretical ideas.
There is three or four,depending on how you count,

(05:51):
but the simplest one to describein words and with pictures
is the idea of large extra dimensions,
which was proposed back in 1998
by myself and a couple of collaborators,
Nima Arkani-Hamed and Gia Dvali.
The basic idea of thatframework is that gravity,

(06:14):
in contrast to the other forces of nature,
lives in more than three dimensions.
As a result,
it spreads inside a spacebigger than three dimensions,
maybe four, maybe five, maybesix, et cetera, dimensions.
And in so doing, it dilutes its strength.
It spreads itself thin in a sense.

(06:36):
- So gravity's having aninfluence in the dimensions,
we might not experience ourselves?
- Exactly right, at least not directly.
The picture there canbe described as follow:
Imagine the surface of this table
that represents our universe.
By our universe,
I mean the three-dimensionalspace of our universe, okay?
So clearly this is not a precise...

(06:58):
The surface of the tablehas two dimensions.
Our universe has threedimensions, but nevertheless,
imagine the surfacerepresents our universe.
So all ordinary forces, whichis electricity, magnetism,
the so-called, 'strong interactions,'
which keep an atomic nucleus together,
or the 'weak interactions,'

(07:19):
which are responsible for radioactivity,
all of the other forces of nature
stay in this three-dimensional space,
and are confined to this table.
Whereas gravity can spreadalso perpendicular to the table
in these extra dimensions thatwe usually call, 'height.'
So because gravity spreadsin more dimensions,

(07:42):
it dilutes its intrinsic strength.
It's like when a river which moves,
let's say in one direction,in one dimension,
spreads itself into several tributaries,
it loses its strength.- Hmm (affirmative).
- So it is with gravity
that this extra dimensionalspace dilutes its strength.
And this idea receivedtremendous attention,

(08:06):
both theoretically and observationally.
The big experiment that we call,
"The Large HadronCollider," at CERN in Geneva
is looking for signatureof these theories.
And I can describe to you a couple of ways
you can look for this thatfollow from this picture

(08:29):
of the table representing ourthree-dimensional universe
and the vertical directions,- Mm-hmm (affirmative).
the extra dimension.
So one test is the followingof this hypothesis:
Imagine the surface, whichrepresents our universe,
is like a pool table.
The surface of the pool table
represents our three dimensions.

(08:51):
Billiard balls on the pool table
represent elementary particles,
like the proton or theelectron, et cetera.
Now, normally when weplay with billiard balls,
the billiard balls collideand when they collide,
of course, they stillstay in two dimensions,
they stay in ordinary space,

(09:12):
but the sound that the collision creates
propagate also in the third dimension,
inside the space of the extra dimensions.
So even if we were not lookingat the extra dimensions,
just by listening to the sound
that the collision of thebilliard balls produces,

(09:35):
we could infer about, well, what happened,
the collision and thefact that some sound or,
was emitted inside the third dimension.
So we could infer about the presence
of the extra dimensions.
So LHC is looking for the analog of that.
You collide to elementary particles,
which in that case is protons.

(09:56):
And if there are extra dimension,
some of the energy of thiscollision may manifest itself
by particles that comeinto the extra dimensions.
So some of the energy that wasin our universe, if you wish,
in our, what we thought wasthree-dimensional universe,
will be missing beforethe collision and after.

(10:17):
Some of the energy has been carried out
in a new space that weare normally not aware of.
This is called, "Themissing energy signature."
You collide two particlesor two billiard balls,
and there is some energy missing
because it went to newparticles or to the sound waves
in the case of the billiard ball.
And by looking verycarefully at energy imbalance

(10:40):
before the collisionand after the collision,
you can look for thespace of extra dimensions.
- Can you say a little bit more
about where the seed of thisidea comes from because,
as you're saying, there aresome experimental signatures
that you can look for,
but is that something that youcome up with after the fact?
Or is it these experimentalsignatures that inspired you

(11:02):
to look for a theory in higher dimensions
in the first place?
- Well, that's a very interesting question
because in some sense,
for the case of extra dimensions,both played their role.
Historically, I was made aware
by talking to some of myexperimental colleagues at Stanford
that gravity has been tested

(11:25):
to only distances of about,
back then it was about a centimeter.
This means, Newton's law of gravitation
that the force between two particles
was like the inverse square law.
Had only been tested down to a distance
of a little less than a centimeter,
and this was back in 1990.
So I was astonished to hear that

(11:46):
because when I was an undergraduate,
in my lab, we tested Newton's law
to a distance which wasmaybe 15, 20 centimeters,
not much larger thanthe 1 centimeter or so.
The original measurement wasdone 200 years ago. How come?
So that immediately plantedto me the seed of an idea

(12:09):
that I should be braveabout creating theories
where the law of gravity is different,
distances below a centimeter.
Newton's, what's called,"Inverse square law,"
is not obey that shorter distances.
So that sort of opened the door for me

(12:29):
that I could contemplate such possibility
without immediately being disproven
by non-experimental facts.
The other thing theory also played a role,
in the sense that I waslooking for an explanation
of the weakness of gravity.
However, for several years,
I didn't make the connectionbetween those two.

(12:51):
In fact, I wrote papersproposing new particles
that would cause deviationsfrom Newton's law of attraction,
but without any referenceto extra dimensions.
And then finally, after a few years,
my colleagues and I startedmaking the connection

(13:12):
and that's how
the theory of large extradimensions was proposed.
In fact, your question also is related
to the second test ofthese theories, namely,
you can study Newton's lawat very short distances.
So when I started talkingabout this possibility in 1990,

(13:33):
several of these inparticular colleagues of mine
at Stanford were inspired,experimental colleagues,
and we started talking aboutthem testing Newton's law.
We spoke for a long time,maybe a couple of years,
with a friend of mine, Aharon Kapitulnik.
And we have good friends,
we have dinners together andwe drink good wine together.

(13:54):
So it was at that settingthat we started talking about
these very wild and speculative ideas,
and he decided to test them.
And he and several otherpeople around the world
started looking and today,
the force of gravity thatNewton say inverse square law
has been tested,

(14:16):
done with distance ofabout a hundred microns.
So far smaller than a centimeter,which used to be the case.
And now there is enormous amount of effort
to test it at shorterand shorter distances.
Now, what does this have todo with extra dimensions?
Well, if there is extra dimensions,
the so-called, 'inversesquare law,' will be modified.

(14:37):
For example, if instead ofthree spatial dimensions
you have a fourth,
the inverse square law willbecome the inverse cube law.
And if it's two dimensions,
it'll be the inverse fourthpower law, et cetera.
So that's what theseexperimentalists are looking for.
A deviation from one over distance square
to one over distance cubeor fourth, et cetera.

(15:00):
And clearly, no suchdeviation has been seen,
but people are looking at shorterand shorter distances now.
And in fact there wasa very nice workshop,
or actually it was a school last week,
where many of these top experimentalists
were giving lectures tostudents from all over the world

(15:22):
and to each other.
Actually there were manyprofessors, experiment and theory,
about the new frontiers,
how to look for such new dimensions.
And this is a very nice story
because it shows youhow a theoretical idea
that can be describedwithout too much mathematics
can in fact connect with experiment.

(15:44):
Now, part of the reason forthat is 30, 40 years ago,
it would be incredible foranyone to propose looking for
such small forces at, let'ssay, below a hundred microns.
Such new forces has been looked for
down to distance of 40 microns.
To give you an idea,
a hundred microns is smallerthan the width of human hair.

(16:08):
So it's incredible that youcan even conduct an experiment,
let alone a precise experimentthat will measure the force
between two not visibleparticles to such a precision.
And so why was this possible?
Definitely it was impossible 50 years ago.

(16:28):
Microtechnology. In other words,
there has been a driving force in part
because of application
to manipulate things atextremely short distances.
And over the last several decades,
experimental physicists havebeen at the forefront of this
manipulation of the very small.
When they started doing that,

(16:49):
their objective was not to test gravity.
I don't think there would beenough money (Colin laughing)
funding such an effort from thephysics of 40, 50 years ago.
Usually, physicists like to emphasize
how physics makes our lives better.
We have all of technology, electricity,

(17:09):
and how useful quantum mechanics has been,
lasers, et cetera.
But there is also, of course, the converse
where technology allowsphysics to progress,
and these things go hand in hand.
So when I started tothink about this in 1990s
and started talking to mygood experimental friends,

(17:30):
partly motivator for social reasons
to have a good time onthe weekends, et cetera.
Then I realized, "Oh my god,these people are amazing!"
I couldn't believe it.
They can look at a hundred microns
smaller than the width of a human hair.
Yeah, just by all means do it.
So they went from a centimeter,which you can visualize,
to extremely small distances

(17:52):
and they'll be progressing further.
I actually think this paradigmsort of summarizes much of,
I mean this is sort ofat the highest level,
summarizes though,
the interplay betweentheoretical ideas and technology
and experimental progressand the back and forth.
- You mentioned a few minutes ago,

(18:12):
the term, 'naturalness.'- Yes.
- It's not one that I'vecome across very often.
Can you explain how that sortof fits into this picture?
- Yeah, so the way itfits into the picture,
I can explain in the contextof the hierarchy problem.
So let's back up.
So the hierarchy problem wasthe problem of understanding
why gravity is so weak.

(18:34):
So the connection is,
if there are extra dimensions of space
in which all elementaryparticles that we know of,
electrons, protons, all the forces,
the other forces we know,electricity, magnetism, et cetera,
are constrained to thisthree-dimensional space.
This three-dimensionalspace we call our universe.

(18:55):
Now if gravity is not constrained
to this three-dimensional place,
but it spreads into the extra dimensions,
then it'll dilute its strengthand it'll become weaker.
Now how weak? Well,
it depends on the sizeof the extra dimensions.
The bigger the size ofthe extra dimensions
or the more extra dimensions you have,
the more rapidly you dilutethe strength of gravity.

(19:18):
So in fact, you can infer
some relation between thesize of the extra dimensions
and the weakness of gravity.
So that's the connection.
The gravity is weak becausethere is a large amount of space
in extra dimensions
inside which gravity dilutes its strength.
- Okay.
- That's the connection.

(19:40):
So what used to be, and you know,
40 decimals now translates to
how many extra dimensions youhave and how big they are.
They cannot be ultra small,
but they can be evenas small as 10 microns,
a hundred microns,
and still explain the dilutionor the weakness of gravity.

(20:01):
So naturalness came becauseyou transcribe the problem,
which look like a 40decimal problem to some
geometric problem thatyou can imagine solving.
So that's an example of anapproach to the natural.
Now there are, I don't wantto get, because I'm not,
it's not my field, but inother fields, for example,

(20:21):
in biology, in some sense,
Darwin's theory made manyof the biological wonders.
So what seems unimaginablycomplicated, like a human being,
where millions of thingshave to work synchronously,
very precisely, can think, theheart, the mind, everything,

(20:42):
this become a natural consequence
of what's called, "Evolution."- Mm-hmm (affirmative).
- Now not everybody buysthat, but scientifically,
I think there is no questionthat that's a valid theory.
So that's another example
where you take an incredible mystery,
you look at it from adifferent perspective
where this mystery looks more natural.

(21:03):
- Mm-hmm (affirmative).- In physics,
it usually has to do withexplaining big numbers.
Numbers that are about arelike 1 or 10 or a 10th,
we feel, "Oh, okay,
well such and such is about asbig as such and such, okay."
But when you havedisparities of many, many,
many orders of magnitude,they beg for an explanation.

(21:25):
And the other example of this,
is the enormity of the universe,
or the so-called, 'cosmologicalconstant problem.'
- That's a question I've been dying to ask
a physicist is,- Yes, please.
why is the universe so big?
- So the universe, why it's so big...
First of all, how big it is,as we were saying before,
if you compare it to thesize of anatomic nucleus,

(21:48):
it's again, about 40 ordersof magnitude bigger than
the size of anatomic nucleus.- Mm-hmm (affirmative).
Again, it begs for a mystery.
You start, if you wish, witha theory that has nuclei
and electrons and atomsand all of a sudden,
you have this enormous universethat supposedly follows
from the same equationsthat have this tiny nuclei,

(22:12):
et cetera. How can this be?
This problem has many, many facets
and I cannot do justice to it.
I'll just tell you that
there is no solution to this problem.
At least there is no solutionwithin the usual framework
that science proceeds,
where you write down thelaws of nature which means,

(22:32):
some equations that dictatehow the universe works.
And then you can derive that,
"Oh, therefore, the universe is large."
There is no mathematical theory of this.
There is a very controversialapproach to this problem,
which was proposed back in1987 by more than one person,

(22:56):
but in particular, a verywell known physicist called,
"Steven Weinberg," who justpassed over a year ago.
The basic idea there isembedded in what's called,
"The idea of the multiverse."
But before I take you backto what's the multiverse,
I want to draw an analog.
And this goes back again tosome ancient Greek physicist

(23:19):
called, "Aristochos."
Aristochos was one of thefirst people that believed
there were many, many solar systems.
That was not a very popular idea,
either at the time ofAristochos or even in 1600,
when what we call,"Modern science," emerged.

(23:40):
Most people, even by 1600,
believe that there wasonly one solar system.
That was it.
So then, in the context ofthese many mysteries up here,
if you believe there is one solar system,
it looks amazing that that solar system,
in particular, theplanet Earth and the sun,
the distance betweenthe earth and the sun,

(24:03):
were made just perfectlyto allow the conditions
on earth to be friendly to our existence.
For example, if we werea few percent closer,
a few percent further than the sun,
the earth would either boil or freeze
and we wouldn't be around.
The chemical compoundsthat we see on earth
are just exactly what we need

(24:25):
to exist and to flourish, et cetera.
So it looks like, again,there is some, you know,
higher intelligence thatreally cares for us. Ah,
- Like turning a knob until they...
- Turning a knob.- Just right.
- Exactly right. Oh, okay.
Oh, we don't have Savas,so let me go back.
(all laughing)
So it looks like a miracle in many ways,

(24:49):
considering how muchit takes to have life.
And this point of view is very popular.
It was obviously alsopopular with the church.
There is some deity that really cares.
That's why everything was made perfectly
for our existence, et cetera.
Then in 1600, there was apriest called, "Giordano Bruno,"

(25:12):
from Italy, who really believedin our Aristochos's ideas,
and he started discussing them in public.
And eventually, he was burntat the stake for his beliefs.
He was burned at the stake in 600.
Galileo was almost burnedat the stake around 1630s.

(25:34):
Galileo died in 1642 andNewton was born in 1642.
So that was really the beginning
of the renaissance of science.
And so many ideas in many waysthey went back to Aristochos.
Aristochos who actually could argue
that the lights that we see in the sky
are actually solar systems,
and because they're so far

(25:55):
you can't tell that they're moving,
but they're moving, et cetera.
So they started going back,
and then Galileo, of course, invented,
or co-invented, the telescope.
And people started looking at planets,
which had moons around them.
And then they said, "Okay,
it looks like things like our solar system
actually are probably out there,"

(26:17):
and they started makingobservations, so modern science.
And now, of course, if you ask anybody yet
now, of course, thereis many solar systems.
In fact, if you takethe number of galaxies,
there is about a hundred billion galaxies,
and each one has abouta hundred billion stars,

(26:38):
10 to the 22 stars, again,
and 1 with 22 zeros,stars in the universe.
And most stars haveplanets. We are not unique.
So the chance is that whenyou have such a huge number
of stars that senses that in some of them
there are friendly conditions

(26:59):
that allow life like our own,
or maybe quite different than our own,
to exist is extremely likely.
It hasn't been proven
because we haven't made an observation.
It hasn't been proven yet,
but I think most scientistsbelieve that it's very likely
that conditions similar toour own or even different,

(27:20):
has allowed the evolution of intelligence
and life in other places.- Mm-hmm (affirmative).
- So notice what happenedthat what used to be unnatural
or required great care,
namely the occurrenceof life in the universe,
is by changing yourperspective and, of course,
encouraged by observations,

(27:41):
it became something not justpalatable but very likely.
So that's an example ofhow a change of perspective
converts something that looks miraculous
to something that looks natural.
- That's all within ourown known Universe, right?
- Exactly.- Okay.
- So now we are taking the next step.

(28:02):
- Okay.
- So we go back to, whyis our universe so large?
Now this is correlated with, as I said,
what's called a, "Cosmologicalconstant problem."
The cosmological constant is essentially
the energy density that is inthe vacuum of the universe.
This is an energy densitythat we are not aware of,

(28:27):
but in principle it's there.
And in fact, if it was there,
there are measurable consequences.
The energy of the vacuum...
If you ask any theorist,
what would you think theenergy of the vacuum is?
They would pull out penciland paper and say, "Oh,
it's probably this number."
And the number that they would get

(28:48):
is 120 orders of magnitude
larger than what it actually is.
And what it actually is, is not zero.
Has been measured back inthe 90s, very precisely,
by astrophysicists and cosmologists
because it has consequenceson how the universe expands
or if it expands orcontracts, how rapidly.

(29:11):
So with cosmological observation,
looking at how far awayobjects like supernovas
recede from us, how rapidlythey move away from us,
you can tell if there wascosmological constant or not.
And it's 120 orders of magnitude smaller
than it should have been

(29:32):
by just taking what you knowin your theory and computing.
So very much like, andvery closely related to,
the fact that the size of the universe
is 40 orders of magnitude bigger
than the size of an atomic nucleus.
So they're very closelyconnected problems.

(29:52):
And finally, a few physicists,
and together with Steven Weinberg said,
"There is many, many universes."
All of these universeshave different value
of the cosmological constant.
Some are big, some are small, et cetera.
When you have cosmological constant,
that affects how the universe expands.

(30:12):
So if you have too much,it expands very rapidly.
So if you have small enough,
then it expands slowly enough
to allow for galaxies to form.
Our planetary system belongs to a galaxy,
and stars and theirplanets are in galaxies.
So galaxies are very important

(30:34):
because they're relativelydense structures
that allow stars to form.
And stars are important
because there are planets around stars
and that's where life forms.
Life benefits from having
the heat of the stars provide energy,
so it's important for life.

(30:55):
Galaxies are important for lifebecause we live on planets.
Planets are near the sun.
They draw energy,
and stars like our ownsun belong to galaxies.
So if the cosmological constantwas any bigger than it is,
then galaxies wouldn't form.
So we wouldn't have starsand we wouldn't have planets,

(31:16):
we wouldn't have life.
So to do that,
Weinberg had to postulate the existence
of many, many, many, manyuniverses. (chuckling)
And again,
the number of these universesis enormous that you need,
because the cosmologicalconstant is so much smaller
than its natural value,

(31:37):
which would've been 120orders of magnitude bigger.
So this was the proposal in '87.
And in fact, using thisidea, he derived a prediction
for how big the cosmologicalconstant should be,
because if it's any bigger than that,
galaxies cannot form,

(31:59):
but there is no reasonwhy it should be smaller
than the maximum it could beto allow for our existence.
So he made the prediction in '87
and the prediction sureenough was confirmed
within a factor of an order of magnitude,
which is not consideringthe range of the prediction
that it predicts a quantity

(32:21):
that is off by 120 orders of magnitude.
But it did something within a factor of 10
and it turned out to be whatthe cosmological constant.
So it looks like our universe is tuned.
It doesn't have as big acosmological constant as it could
because it would be crazy.
The universe would beexpanding at an enormous speed.

(32:43):
We wouldn't, not even atoms would form,
let alone galaxies and stars, et cetera.
So it's not as big as it could be.
It's smaller and smallerand smaller, far smaller.
It's 120 orders of magnitude smaller,
but that's when you stop.
The moment it's 120orders magnitude smaller,
you form life and that's you stop.
So in fact, he proposed itas a way to test his theory

(33:06):
just about 10 years before it was tested,
because the idea wasexceedingly unpopular in 1987.
In fact, I remember'cause I was visiting him.
It was October 19th, 1987,
'cause the same day I was visiting him,
there was a big stock market crash.
And I was giving a talk(both laughing)

(33:27):
at the University of Texas where he was.
So he showed me his theory and I said,
of course, I was very polite,"Oh interesting," et cetera.
But I said, "Oh, the oldman has completely lost it."
(all chuckling)
My definition of old back then,
I think he was like 56 or 50.

(33:49):
Yeah, he was in fact, yeah, 55 back then.
Old by my then standards andI think he was ultra young.
But he tells me this thing,
"Many universe in my head is spinning."
And I say, "Oh, I understand."
He's about to die pretty soon.
He wants this big questionsanswered. (laughing)

(34:09):
And what can you do? Yes, sleep a lot.
And I wasn't alone, Ithink everybody thought,
"Hey, Weinberg has lost it."(chuckling)
He was viewed until theend of his life as a major,
if not thee major physicist of his time.
So he seems to have been right,

(34:30):
at least with the numerical prediction.
Whether the multiverse existsis exceedingly controversial
for several reasons.
One is, the number ofuniverses you need to explain
this cosmological constant is enormous.
Now we are talking about really enormous,
like 10 to the 120, 10 tothe hundred 30 universes,

(34:52):
you know, one with the hundred 20.
This is sort of theminimum number you need
to begin to explain the cosmology.
- This sounds like theopposite of naturalness.
- Exactly, so in a sense the complaint is,
"My God, you transcribe the problem
to a different large number,
and unless you have a sort of theory,

(35:12):
how are so many universes created,
you haven't made progress.It's a great point.
That's one of the reasons.
And then the controversy get even stronger
because there is a veryspeculative, again,
controversial theorycalled, "String theory,"
which turns out,
it can predict the existenceof so many universes.

(35:34):
However, it's alreadya controversial theory,
the fact that...
So it's very much an open question
and the question in the end in science
are not decided by conversationor writing down formula
or the prestige of theperson who made the proposal
and whether they havea Nobel Prize or not.

(35:56):
This don't count for anything.
It has to be experiment in the end.
The one piece of experimental evidence
for Weinberg's multiverse was,
of course, the fact thatthe cosmological constant
was measured to be what he had predicted.
But you need more than that in science,
especially with such big ideas.

(36:17):
So there are some proposalson how to test this idea.
One is called, "Split supersymmetry,"
that I was involved with.
However, even if yousee split supersymmetry,
I don't think it'll be enoughto prove the multiverse.
You need many more data.
And the problem is the idea,

(36:38):
it's not obvious what to go and measure.
For example, when Aristochosand Giordano Bruno, et cetera,
postulated the manysolar systems hypothesis,
multi solar systems,
eventually there was adiscovery of the telescope
which allowed you to beginthis path towards discovering

(37:02):
that there is much morein the universe out there.
Those sort of blinking lightsare not there for decor.
In fact, they're a worldlike us. (Colin chuckling)
Many of them are whole galaxiesso they have 10 to 11 stars.
But there was a way toprogress through experiment,
through observation.
And there is no clearpath through experiment,

(37:22):
through observation toprove the multiverse so far.
So I think it'll remain controversial for,
I would say maybe,
decades if I'm optimisticif not more than a century,
which is a very long time scale.
But maybe I'll be proven wrong.

(37:43):
There are other predictions.
There is another idea whichis called, "The axiverse."
I don't want to get into it.
There are other predictionsof having many universes.
- Mm-hmm (affirmative).- In particular,
the axiverse is the idea that
if there are many universes,
there's also manyparticles in our universe,
that again, the conferencethat we had last week here

(38:05):
touched upon how youcan go out to discover
this many particles.
So if you see many particles,you see split supersymmetry,
maybe people will start believing.
I'll be convert very rapidly
because I'm psychologicallyprepared for wild ideas.
And that's why I worked on trying to find,
I mean I was involved bothwith split supersymmetry

(38:26):
and the axiverse ideasbecause I still want to see
how you could test the existenceof many universes and...
- So I really love thispoint you've raised
a couple of times abouthow the types of questions
we can hope to answer in our theories
really depends on thetechnology that we have.

(38:46):
And when I read about your work online,
I've seen the line a few times
that your career in particlephysics spans four decades.
So I would assume thatthe types of questions
that you've been able to answer
have evolved a lot throughout your career.
So can you tell us a little bit about this
and how the types of questionsyou've been able to study
have changed with technology?

(39:07):
- Yes, yes.
I was trained in the late70s as a particle physicist.
Again, to give you a perspective,
I'll sort of zoom out to tell you what,
how particle physics started.
So a key day in the history of science,
a key year is 1945.

(39:29):
That's when the public andpoliticians and everybody
realized that actuallyscience has consequences.
It can be used in a bad or in good ways,
but knowledge allows you to do things.
So they started fundingthe science very heavily
and that led immediately towhat we call, "Big science."

(39:55):
Big science means, for example,
what are called, "Colliders."
Colliders are, essentially, youtake two beams of particles,
one from the left and one from the right,
and you collide them andyou see what comes out.
And the more the energy, the more,
the faster the particlesgo towards each other,

(40:18):
the more energy you haveto produce new particles.
By new, I mean, things thatwe are not familiar with,
like the electron or theproton are familiar particles.
We know them from...
Because we are made outof nuclei and electrons.
New particles, I mean,things that live for a very,

(40:39):
the briefest amount of time.
You create them and then theydecay into other particles,
familiar particles.
- Are these collisions thathappen in nature as well
or are you creating thingsthat only exist in the lab?
- They can happen bothin nature and the lab.
In nature, they happen very far away from,
you need very violent conditions.

(41:00):
Or they happen in whatare called, "Cosmic rays,"
very energetic particles
that have been acceleratedsomewhere in the universe.
And they come towards us,
not by intelligent life, butby astrophysical processes.
But we study them on earth
because you need a lot of collisions
to be able to study what you predict.

(41:21):
And in the universe,definitely in our location,
there is not a lot of collisions.
You have an occasionalcosmic ray come and hit,
but it'll hit something in the atmosphere.
You won't know it, but theycan happen also naturally.
So you have these collisions,you study the decay,
the product of these collisionsand that's how you find out,

(41:43):
in some sense, new particles.
Sometimes you find out whatsomething is made out of.
If you collide nuclei oran electron, a nucleus,
you find out what's inside the nucleus.
Sometimes you produce a newparticle that was not inside,
but the energy that you produced
allowed you to createnew particles, et cetera.

(42:04):
That's called, "The colliders."
Colliders are very big projects.
An example is a certain collider,
the most recent one in theLarge Hadron Collider at CERN.
And they involve hundreds ofpeople now working for decades.
It started out working for years now.

(42:27):
The colliders have beengetting bigger and bigger.
To give you an idea, theLarge Hadron Collider at CERN
has a circumference of26 or so kilometers.
It's about a couple ofhundred meters underground
and it involves magnets
going all around these 26 kilometers.

(42:47):
And these magnets are very important
because they navigate theprotons that are accelerated
to go on a very precise trajectories.
Again, within microns,
things have to be exactly where they are
within tiny, tiny distance
or else they will miss each other,
they won't collide.
So half the protons go clockwise,
the other half counterclockwise

(43:08):
and then magnets navigatethem and eventually,
they collide in four different spots
where you have detectors.
They are huge, like a 5-story building
that are instrumented withvery sophisticated machines,
versions of the human eye.
You can see what happens.

(43:29):
You can see what particles you produced.
And just like the eye has togo to connect to the brain,
so there is then cablesthat take these events,
they analyze them, computers.
And then they tell you,"Okay, you produced it."
It's beyond my imagination
that humans have been able todo such complicated things.

(43:49):
It all started with thewillingness to support science
that was started in 1945.
In the beginning,
colliders would take afew months to a few years
to be built, only a fractionof the cost that they are now.
Now they have reached the point,
for example, the Large Hadron Collider

(44:10):
is about a billion per year to run,
and so it was like 10 billion to build.
It's a big project and themoney is not the main problem.
The problem is that it takestime and expertise to build it,
to have 27 kilometers worth of magnets.
These are huge magnets,

(44:31):
where it is they have to beultra cold and they have...
It's a miracle that you canhave control to this level.
It's even, as a European, for me,
it's even more of a miracle.
It was created, in a sense,
as a result of the Second World War,
where European countrieswere fighting each other.

(44:52):
At least that's how it started.
And then the same European countries
collaborated at this spectacularlyprecise accomplishment.
One of the great accomplishmentsof, I think, humans
to create this machine.
Work so well and we'velearned so much from it
and all the predecessors.
LHC is only the last example,

(45:14):
and there have been tens of colliders,
you know, various sizes,et cetera, since then.
So we've been on this largescience road over 70 years.
Now we've reached the pointwhere the next collider,
the next upgrade that will takeus to even bigger energies,

(45:36):
may take, if we are lucky,20 to 30 years to build.
- And why is it that long?
Is that because of the technologyneeded or the investment?
- Or how big it has to be?
- I think all of the above.- Hmm (affirmative).
- Plus it takes time.
Even if Bezos gives youall his money saying,
"Okay go build it,"(chuckling)

(45:56):
the money would be plenty in his case.
However, it would still take a long time
to assemble the people.
And then the technology,even if the technology exists
because the technology does exist.
If you make it long enough,you can have enough magnets
and enough to accelerateparticles to very high energies,

(46:19):
the next energy frontier.
10 times bigger energy than the LHC.
So the technology exists,but the time it would take,
I would guess at least 15years, probably much more.
Even with all the money,
I think it would take couple of decades.
- Well this comes across inthe movie, Particle Fever,

(46:40):
the documentary that you're in,
which is largely set atthe Large Hadron Collider.
'Cause you personally had to wait
how many years of your career
for that to to be completedand be brought online?
That was a long wait for...
- That was a long wait. By the way,
I didn't think it was goingto be a long wait (chuckling)
when I started.
You know, humans tend tobe optimistic by nature.

(47:01):
That's why we've evolvedso well. (Colin laughing)
I can tell you, anecdotally, in 1983,
there was the first studygroup of what was then called,
"The Superconducting Super Collider,"
which was a very similar collider.
Actually would havehigher energy than the LHC
that was going to bebuilt in the US, the SSC,

(47:21):
Superconducting Super Collider.
And the date that was discussed
was well by 1990 we should be running.
This was the first study.
So it took much longer and it wasn't even,
the SSC was canceled in'93 for political reasons.
The moment a site was chosen,which was Texas, to build it,

(47:44):
then a support from the restof the states diminished.
And in the end it was not built,
which is really a shame
because it would bevery good for the world
to have two collidersin the same competition
and at any rate, so it took much longer.
So I didn't think it would take from '83

(48:05):
until 2008 when it first started.
So this time scale, it seemslike it was getting longer.
I anticipated this in the 90s.
That's why I started thinkingabout small-scale experiments.
I didn't anticipateexactly dates, but I said,

(48:26):
"Well there is a lot oftechnology happening,
so what can we do with it?"
Because I was learningthese things from my friends
that I have dinners andwine tasting, et cetera.
So I could see that therewas a whole other field
of experimentation.
So that inspired me tostart thinking about this.
And now it's a majorpart of what's happening.

(48:50):
Because the next colliderwill take so many decades,
many people have started doing it,
especially in the last five years.
There has been what is called,"The golden age of small,
doing fundamental physicswith small-scale experiments."
- So you don't have to wait three decades
for a collider to be built?- You don't have...

(49:10):
- I think colliders arestill very important.
You are not looking forexactly the same physics
if you do small-scale,high precision experiments
and collider experiments.
Collider experiments, eventuallyyou produce new particles.
When you produce them,
even though they live for very short time,
you can study them.

(49:32):
You can see what are their decay products
and from there you learn a lot.
You learn all there is to know
about their fundamentalproperties, their mask,
their electric charts, andwhat's called, 'their spin,'
and how they couple to other particles.
You learn a lot in detail.
And the moment you've produced a particle,
the signature of that is fairly clean.

(49:56):
With small-scale experiments,
the discoveries are more indirect.
You see a new effect,
and then you have toinfer from that effect
what it is that produced this effect.
And it could be the same particle
that you would havediscovered in a collider,
but you'll see it more indirectly.
So usually it takes more thanone small-scale experiment

(50:17):
to study, let's say the same particle
or the same phenomenon.
Nevertheless, I thinkthese are complimentary.
So there is a lot that can be done
thanks to the amazingtechnological developments
for what's can be called,"The high precision frontier."
So there is a lot that can be done

(50:38):
and now it is a golden erafor this many experimentalists
have turned their attention to this.
Many of these people,
what they were doing fortechnological purposes,
and now they're doing it tomake major new discoveries
about the laws of nature new.
So it's very exciting.

(50:58):
- I remember in thatdocumentary, Particle Fever,
which is largely about thesearch for and discovery
of the Higgs boson, sortof the most famous outcome
of the Large Hadron Collider.
I've always wanted to ask you, that movie,
it shows people packing an auditorium
for the big announcementof the Higgs boson
and you couldn't get pastsecurity, they locked you out.
What happened?

(51:19):
- I was late.(all laughing)
So what happened was,
I had several students andposts docs that went there early
and they kept a seat.
In fact, they showed in Particle Fever,
the empty seat for me.(all laughing)
But even though therewas a seat available,
I couldn't go in becausethere was a big backlog

(51:43):
and they didn't...
Anyway, so I had to watchit from a TV outside.
- Yeah. But you werethere at the LHC at CERN
when the discovery was announced.
How did that feel foryou for that milestone?
- Oh, it felt fanta...
You know, it's like whensomething amazing happens,

(52:03):
you feel that you live in a dream.
That's how it was.
That was, by the way, December of 2011.
That actual first announcement,
that was the incident that was shown.
July 4th, 2012 was theofficial announcement.
And at the time of theofficial announcement,
I was actually in Santorini on vacation

(52:26):
looking at the announcement
and some beautiful views of the sea.
- That sounds nice.
It's better than beinglocked out by security.
- Exactly, but the first...
I'm glad I was there thoughfor the first announcement.
- Mm-hmm (affirmative).- And it was amazing.
It was amazing.

(52:46):
Scientists are like humans.(both laughing)
So the moment you dreamof something, it happens.
You accomplish and say,"Okay, what's next?"
Very soon, you get used tonow we are looking forward
to seeing what may be beyondwhat's called, "New physics,"
beyond what we call, "Thestandard model," now.

(53:08):
With the discovery of the Higgs,
marks the end of what wecall the standard model
and we are now on a pathto discover new particles.
That's what we are looking forward to.
- We have a student question submitted
that's about the standardmodel, by Felicity.
And maybe we could play that for you?
- Yeah, sure.

(53:30):
- Hello Savas, I'mFelicity in grade eight.
What are the discrepancies
in the standard model for physics,
and what makes them as such?
- Okay, that's an interestingquestion bec... (chuckling)
the word 'discrepancies'
suggest that there is something wrong
with the standard model,that something doesn't work.

(53:52):
That 'by doesn't work,'I mean it's contradicted.
The standard model makes a prediction
that when you do experimentX, you'll find A,
but you don't find A, whenyou do it, you find B.
So there is no discrepancyof the standard model
in that sense. If there was,

(54:13):
it wouldn't be the standardmodel of particle physics.
It would be a theorythat has some problems.
So there is no real discrepancy.
What I described to you,the hierarchy problem,
the cosmological constant problem,
are not logical contradictionswith the standard model.
In a sense, they're a static criteria,

(54:35):
that in the same theory,
you have two numbers that differby 40 orders of magnitude.
There must be a reason for it.
The standard model isnot fundamental enough
to address these questions of,
why is the universe so muchbigger than an atomic nucleus,
or why is gravity so muchweaker than the other forces.

(54:57):
It is, essentially, thenaturalness criteria
and aesthetic criteria.
- I remember once, yousaying something like,
"The biggest mystery is that
the universe iscomprehensible to us at all."
- That is, in a sense, a meta question.
It's almost in the realm of philosophy.

(55:19):
Indeed, several bigphilosophers and physicists
have said the same thing in different ways
that the most, (I forgot,maybe it was Einstein),
who said that, "The mostincomprehensible thing
about the universe isthat it's comprehensible."
The fact that there is alanguage for the universe,
which is called, "Mathematics."

(55:41):
The fact that the universeobeys mathematical laws
is just astonishing, what's called,
"The unreasonableeffectiveness of mathematics."
In mathematics,
you can ask a question andno matter how hard it is,
if it's within the realmof mathematics and physics,
and it may involve millions of steps,

(56:04):
but you arrive at something that's true.
Now it's very rare you start,
you have a starting point, some question,
and then a million steps later
you arrive at a conclusionthat's still true.
Because a million steps is a lot of steps
and all it takes is few missteps
to be led to the wrong direction,
and mathematics that doesn't do that

(56:24):
if you ask the right question.
I think it was Pythagoras
who said that, "O Theós geometreí,"
which means in English,(chuckling)
that, "God geometrizes everything."
By geometry, he meant mathematics,
that God speaks thelanguage of mathematics,

(56:45):
if you want to paraphrase.- Mm-hmm (affirmative).
That's an incredible mystery.
And the fact that mathematicsis a precise language,
like one plus one equal two,
there is no if, but, approximate.
Well, it's a matter ofopinion, (Colin laughing)
and there is left wingers and right.
- Now that's fake news.
- Yeah, fake news. There is no...

(57:06):
And, of course, that's anexceedingly simple example,
but with math you can havevery complicated examples
that describe what happens
in a complicated situation in nature.
You know, how the sun worksand creates energy for us.
And there is trillions of steps and to do,
but before you figureout how the sun works,

(57:28):
how come it produces all this energy?
What will it do next?
Or the loss of gravity,you don't have to go...
Newton told us, gave us equations,
you can use these equations to predict
where any planet will beat any point in the future,
and where it has beenany point in the past,
10 billion years ago or10 billion years from now.

(57:51):
And you can tell exactly,
if you'll have an eclipseand what it'll be.
So this power of extrapolation
gives a new meaning tothe concept of truth
that, "Oh my God, this is real true.
There is no fake stuff."
It's amazing that such a thing exists,
and in fact, it's whatdrove me into physics,

(58:14):
what I told you about Newton's equations.
When I was, I think I was 13 years old,
one of my classmates backin Greece told me that,
"There is these equations thatdo exactly what I told you.
You can predict the positionand speed of a planet
any point in the future
if you know it today or any point."

(58:35):
I said, "Impossible. No way."
It's so complicated. Thereare all these other planets
and there is so muchhappening at the same time.
And that's when I said,"I want to do this.
What is it called?"
I know, I knew it was called, "Physics,"
because...- Mm-hmm (affirmative).
- And this comes up in the movie also.
I was interested in the concept of truth.

(58:57):
When I went to Greece forthe first time, I was age 12.
I was born in Constantinople,
but then my family was expelledbecause they were Greeks
to go to Greece and we went there.
And all of a sudden,it was a free country.
There was left and right,
and I would hear a speech bythe left leaning politicians.

(59:19):
"Well that makes perfect sense."
Then I would go to the same topic,
a speech from the right leaning.
I said, "Oh that makes sense too,
but they are opposite conclusions."
So I was confused.
What does it mean to be true?
And then I realized that withlanguage you can play games,
whereas with mathematics,it's such a precise language

(59:41):
that you don't play games.
If you ask a precise question,
you get a precise answer.
So I said, "I want to do that."
And then I was, for abouta year, I was wondering,
if I should do mathematics or physics.
And it was that commentby my classmate that,
because you can predict precisely
what will happen in the future.

(01:00:03):
And then I realized
that physics has anadvantage over mathematics.
That in physics it's not just the logic
and what two or threemathematicians think,
or a million mathematicians think.
It is nature that goesand tests your theory
to see if it's actuallyrealized in nature or not.
So that gives an additional foundation

(01:00:25):
to the concept of truth.
And I said, "Ah, no."
In math, there is truth.
In physics, it's super true
because even nature agrees with you.
The truth does not depend onthe eloquence of the speaker.
And in fact, nature can answerwhat the truth is in physics.
So those were veryattractive ideas for me.

(01:00:45):
So I decided to spend my life on it.
I'm glad I did.
- So you decided at that stageto spend your life on this
and you haven't looked back since?
- No, for sure, I haven't looked back.
It's very funny because manyof my relatives would tell me,
"You know, with your brainyou can make a lot of money."

(01:01:05):
Said, "I know. I don't want money.
I want time to do what I enjoy doing."
And they thought I was a bit strange.
True.(all laughing)
- But you're stillenjoying what you're doing?
- I'm still enjoying, yeah.
Yeah, there is this childlike curiosity
and joy that you discover.

(01:01:27):
You know how children,
they're excited becausethey discover new things.
And in science, there's somany interesting questions
that even now, there'sinteresting questions.
When you understand something,
you get the joy of understanding.
You see connections and...
- Well, Savas,
we're delighted that youstill enjoy your work,
and we're very excited thatyou stopped to chat with us.

(01:01:49):
This has just been fascinating.
- Thank you.(gentle upbeat music begins)
It has been a pleasure for me too.
- Thanks so much for listening.
Be sure to subscribe
so you don't miss anyof our conversations.
We've interviewed somany brilliant scientists
whose research spans fromthe quantum to the cosmos,
and we can't wait for you to hear more.
And if you like what you hear,

(01:02:10):
please rate and review our show
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Great science is for everyone,
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And thanks for being part of the equation.