What is so beautiful about string theory?

Daniel and Kelly talk to Thomas Van Riet about how string theory unravels the puzzle of quantum gravity and whether math can be beautiful.

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Episode Transcript

Speaker 1 (00:07):
People might disagree about what kind of art they like.
In fact, pretty much everybody does. But we all know
what it means when we say that something is beautiful.
It means that we appreciate it, that it moves us,
that it strikes us, that we see an elegance in it.
But what does it mean if you say a theory
of physics or a little bit of math is beautiful?

(00:28):
How can math be gorgeous? How can physics be elegant?
What does that really mean? Well, string theory is the
theory of physics that's most often described as a bit
of twenty first century gorgeous physics that fell into our laps.
What does that really mean? What is so beautiful about

(00:49):
string theory? And just because something is beautiful, does that
tell us whether it's more likely to describe our universe
to actually be right? That's the question we're going to
be asking today on the podcast What's so beautiful about
string theory? Welcome to Daniel and Kelly's Extraordinary Universe.

Speaker 2 (01:24):
Hello, I'm Kelly Wiener Smith, and I know nothing about
string theory. In fact, sometimes my eyes crossed and I
just blankly stare at the wall. When the conversation comes up,
But today I'm gonna understand it.

Speaker 1 (01:35):
Hi. I'm Daniel Whitson. I'm a particle physicist, which might
make it sound like I should know string theory, but
actually it means I just smashed particles together without understanding
the nature of the universe.

Speaker 2 (01:45):
Oh all right, Well, so I've got a question for you.
I listened to your beautiful opening about what does it
mean to have a gorgeous equation? So my question for
you is, what is the most beautiful equation?

Speaker 1 (01:59):
The most beautiful equation? Oh my gosh. To me, the
most beautiful equation is actually not in physics. It's in math.
Oiler's identity. It says E to the IPI plus one
equals zero, And I just think it's incredible because it
combines like a bunch of different stuff. You have E,

(02:20):
pie zero, one and I all together, and it's so compact,
and it's just so much encoded into it. It's like
so dense with useful information. It tells you about how
you can think about signs and cosigns in terms of
complex numbers. To me, it's just fascinating to have so
much information packed so tightly and so beautifully into a

(02:40):
single equation.

Speaker 3 (02:41):
Awesome, A fine choice.

Speaker 1 (02:43):
But I also have to say that in grad school,
the moment I discovered I was not going to be
a theoretical physicist was when I was sitting next to
my office mate and I realized that he did his
homework just like I did, but he did it two
or three times in different fonts because he got really
excited about like writing these equations. He's like, Oh, I'm
all writing in italics, or I can write these symbols

(03:04):
another way. And I realize, like, wow, this kid really
jams out about like writing down the equations. It's something
about being a theoretical physicist that I just didn't have.
I was like happy to be done with it once.

Speaker 2 (03:17):
Yeah, I gotta be honest, that doesn't strike me as
super efficient. I'm going to do the same thing three times,
but I'm glad that he's super into it.

Speaker 1 (03:24):
Yeah, but there's something about the equations and the formalisms
and the expressions and even the fonts. The way you're
writing these mathematical symbols, then you've got to be excited
about if you're going to work in the nitty gritty
of figuring these things out. Because being a theoretical physicist
is a lot about writing equations on paper, So if
you don't like that, then probably shouldn't be one.

Speaker 3 (03:42):
Yeah, fair enough.

Speaker 2 (03:44):
Well, today we're talking about a theory that is regularly
described as beautiful, and I'll tell you by the end
of the episode, I'm moderately convinced that string theory is beautiful,
maybe even more than moderately convinced. But I think we
should see what our audience thinks about out what's so
beautiful about string theory? Is this something that people know already?

Speaker 1 (04:04):
That's right. I reached out to our listeners to ask
them what do they think is beautiful about string theory.
If you'd like to contribute your voice for future episodes,
please write to us two questions at Danielankelly dot org.
We will sign you up. Also send us questions about anything.
I got a recent question about somebody's dating life which
I totally couldn't answer, but I enjoyed reading anyway, so
feel free to write to us.

Speaker 3 (04:25):
You should have sent that to me.

Speaker 2 (04:26):
I was on Dan Savage's podcast, and I feel like
that makes me a relationship expert, so you can just
send those to me.

Speaker 4 (04:33):
I've got it covered.

Speaker 1 (04:33):
Okay, there, you go, folks, We are self proclaimed experts
in anything, So think about it for a minute. What
do you think is beautiful about string theory? Here's what
some listeners had to say. I don't think the universe
would be so elegant to be a dangled, naughty strandfield.

Speaker 4 (04:55):
Mess unifies general relativeivity and quantum mechanics. Physicists love elegance
and symmetry. It may also provide a new baseline of
what's the tiniest thing, And then we.

Speaker 1 (05:09):
Get to ask the question is that it? Or is
there something beyond that? The amount of money that Brian
Green was able to make by taking advantage of popularizing it,
we're able to mathematically explain why gravity is so weak
compared to the other forces.

Speaker 5 (05:28):
So nusks, Dodd, what is string theory? The dad says,
why you ask such difficult questions? Ask me something easier.
So the sun says, okay, why does mum get so angry?

Speaker 4 (05:40):
Ah?

Speaker 5 (05:40):
Well, string theory is a theoretical framework.

Speaker 6 (05:43):
It's a kind of symmetrical beauty.

Speaker 1 (05:45):
The beauty to me is that we keep searching for
the boundaries that would have to be the g string.

Speaker 7 (05:53):
Nothing's beautiful best string theory except that confusion is beautiful.
People want to try to bring everything thing all together
into one unified theory that explains everything. I think that's
what makes it beautiful.

Speaker 6 (06:08):
And they look like worms string theory in all the theories
that try to bridge this gap really show the spirit
of scientists and researchers and physicists everywhere to keep on trucking.

Speaker 3 (06:21):
The totality of its failure in the same way that
a lot of other unfalsifiable and self sealing things are
in life that we.

Speaker 1 (06:30):
Love that the maths of it all is quite elegant.

Speaker 8 (06:34):
What I like about the idea of it is that
rather than trying to think of the universe in discrete particles,
kind of just thinking about it in like pulses of energy.

Speaker 4 (06:46):
I cool it confusing.

Speaker 2 (06:48):
Well, Daniel, it looks like we're teaching the controversy today
because our.

Speaker 3 (06:53):
Answers range from I don't think there's anything.

Speaker 2 (06:55):
Beautiful about it to you know, it's pleasing esthetically, And
there was a good range of answers, what do you
think is it beautiful?

Speaker 1 (07:03):
I think it's fascinating to apply this subjective standard to
something which is supposed to be objective. Right, we're talking
about like the answer to the question of how the
universe runs itself, the machinery of the cosmos. Why do
we care about whether it's beautiful? Why should beauty be
a guide? Like if we have two theories, should we
pick the one that's more beautiful and follow that because

(07:24):
we think it's more likely. I think it's this sort
of like bias we have that we think nature should
be beautiful because we mostly look around and we're like, oh, yeah,
the world is pretty. I wonder if aliens evolving on
an ugly planet, one that they find like if kind
of yucky, would tend to be biased towards yucky theories
of physics because their life is pretty yucky. Or maybe

(07:47):
everybody of alves to think that their planet is beautiful
and everybody tends towards beauty. I don't know. To me,
it's a deep sort of philosophical question of what is
beauty anyway, and why do we appreciate it in our
world and why do we look for it in our physics?

Speaker 2 (08:00):
All right, So first an observation in my experience, it
seems to me that when people say, oh, this equation
is beautiful, what it usually means is it makes their
life easier. It explains a lot of things. And maybe
this is human laziness is the wrong answer, because most
of the people working on these equations or anything but lazy.
But like, oh, it's nice. It explains a lot of things.
I don't have to worry about that stuff. So you

(08:22):
did seem earlier to think that Euler's equation was beautiful,
but now you seem to be a little bit more
critical of people saying talking about equations that describe the
universe as beautiful.

Speaker 3 (08:34):
Do you feel like there's some difference there.

Speaker 1 (08:35):
No, I can see beauty and I can appreciate it. It's
like when you see a piece of machinery and there's
only a few moving parts, but it can do something
really complex, or you look at a piece of code
and you're like, wow, that is so simple and yet powerful.
You can appreciate the beauty of that. I just don't
know why the universe has to work that way, Like
the universe could be a total mess. We could discover

(08:56):
the way it works, be like, actually, I have some
notes this could have been done better, you know, like
more documentation please. So I can definitely appreciate beauty when
I see it, and I think I can even capture what
he's beautiful about something. I just don't know why we
expect the universe to be beautiful. I mean, I hope
that it is, but we'll see.

Speaker 2 (09:15):
I mean, I think the universe is beautiful, like the
sunsets are beautiful, the biodiversity is beautiful. But it certainly
seems to me that anytime we try to explain what's happening,
there's nothing beautiful in this.

Speaker 3 (09:25):
But maybe that's just the.

Speaker 2 (09:26):
Biologist working on ecological models, where we're like, this is
a mess, and cells are a mess, and everything's a mess,
but we're all just muddling forward.

Speaker 1 (09:34):
And let's not even get into chemistry because that's a disaster.

Speaker 3 (09:37):
Now, we weren't going to get in a chemistry Daniel,
that's not what we do.

Speaker 1 (09:40):
That's right, exactly. But neither of us are also string theorists.
And so I reached out to somebody I know online,
Thomas Van Reed, who is a string theorist and writes
about this stuff. And I've seen him on social media
venting gently about how string theory is not well understood
and mis explained and misunderstood by the general public. So
I invited him to come on the podcast and tell

(10:01):
us all about it.

Speaker 3 (10:02):
Let's jump right in.

Speaker 1 (10:07):
So then is my great pleasure to welcome to the
podcast Professor Thomas van Reid. He's a theoretical physicist at
the Institute for Theoretical Physics at ku LEEUFN. Some of
his recent work include papers called the Stability of axion,
saxyon wormholes, and the quantum theory of gravitation, Effective field theories,
and strings yesterday and today. So I thought he'd be
a good person to talk to about string theory and

(10:29):
its alleged elegance. Thomas, thank you very much for joining us.

Speaker 4 (10:32):
It's my pleasure to be here.

Speaker 1 (10:34):
So my first question for you is, what is this
big problem that everybody's trying to solve. We hear a
lot in popular science about how we have general relativity
and we have quantum mechanics and these two theories don't
work well together, and we need some theory of quantum gravity.
Why do we need a theory of quantum gravity? What
is this big issue? Why can't we just have gr

(10:56):
and quantum mechanics and be happy with those.

Speaker 4 (10:59):
So in everyday life, gravity is a classical force and
there's no problem in understanding gravity. Sometimes it's a bit complicated,
especially you know when you're looking at say, black hole mergers,
you need full blown relativity. But it's still a classical theory.
You can put it on a computer, you can do
advanced calculations and you can understand what's going on.

Speaker 1 (11:21):
And what do you mean when you say a classical theory?
What does classical mean? It sounds like a technical word
you're using.

Speaker 4 (11:26):
Classical can mean two things. In physics, it's very confusing.
So classical can mean that you do newtont mechanics and
you don't do relativity. Relativity is a correction to Newton mechanics,
and the correction takes into account that the speed of
light is finite. So physics theories are always corrected by

(11:48):
some numbers. And you can think of the difference between
relativity and Newton mechanics to be that one theory is
corrected by the other by numbers which go like one
added by the speed of light, which is a very
tiny number. So that's the first sense of classical. I
actually meant the second sense of classical, and that's where
you say I take say it doesn't matter whether it's

(12:11):
a Newtonian or a relativistic theory, but I add quantum mechanics.
They're in quantum mechanics. We also have a small number
called Plank's constant, right, So very informally speaking, you could
say that quantum mechanics correct classical mechanics by terms in

(12:32):
equations that are powers of this small number.

Speaker 1 (12:35):
So classical is a fuzzy word that basically means old fashioned,
the same way like you might call classical music Mozart,
but the kind of music I like to listen to
is called classic rock on the radio, even though it's
not that old. And so you're talking about two different
senses in which physics has evolved from Newton to Einstein,
and then from Einstein to like Schrodinger and Heisenberg and stuff.

(12:55):
And so in this sense, when you say a classical
theory physics, you mean without quantum mechanics.

Speaker 3 (12:59):
And you know, some of that must is pretty old.
By that man, we are getting a little bit old,
I'm sorry.

Speaker 1 (13:04):
And for the record, notes are totally rocks.

Speaker 2 (13:06):
Ok Okay, no, I'm not disagreeing there. So the biologist
who's trying to keep up with the physicists here, all right,
So it sounded to me like you were saying, quantum
mechanics and general relativity can be reconciled if you just.

Speaker 3 (13:20):
Divide by the right terms.

Speaker 2 (13:21):
I was under the impression that they describe completely different
phenomena and kind of don't work together at all.

Speaker 4 (13:27):
It's too quick to say that they can easily be combined,
but it's also too quick to say that they cannot
be combined. So, first of all, indeed, are there regimes
of interest where the two theories should be combined. Because
usually gravity we think of very large things. Gravity is
so weak that in order to see it you need
to have large objects, massive objects. You know, the Earth

(13:51):
is pretty big, and I can still lift my glass
of water from my table, meaning that, you know, the
electromagnetic forces in my body are stronger than the gravity
of the full Earth. So gravity is weak and things
need to be big to be able to see it.
There's another option, if things are dense enough, you know,
imagine taking the Earth and compressing it into the size

(14:12):
of my water cup, Okay, and then even compressing it
more so, then of course I will get into a
regime where you say, well, you know, it becomes very small,
and then the theory of econom mechanics becomes important. Yet
also gravity becomes strong. And you can ask do we
know of such regimes? And we do. I would say

(14:34):
it's the most important regime for all of physics. It's
the early universe. So if we go back in time
and we look with our telescopes, so looking with the
telescope means that you look back into the you look
into the past, you see that the universe was denser.
And if we just follow our classical equations, it actually
tells us that the density will go to infinity, which

(14:57):
is of course not true, but it isn't in the
cation that in the very early universe, you know, everything
was very tiny, so quantum mechanics was absolutely important and
gravity was huge, so we need a theory of quantum gravity.
One other example that we have already measured are black holes.
Some black holes have always been a sort of a

(15:18):
theoretical invention, but they're not anymore. We have seen them.
They're out there, okay. And what you sometimes wrongly hear
when people you know, talk about signs in a for
the for the bigger public, they would tell you that
black holes, for sure are objects were Quantum gravity is
important because gravity is strong near a black hole. That's

(15:41):
actually not entirely correct. If you look at a big
black hole, for instance, a black hole in the in
the middle of our galaxy, the gravitational parts that the
horizon of that black hole is big, but it's not
ridiculously big, okay, And the bigger a black hole, the
weaker gravity is at the horizon of a black holes.

Speaker 1 (15:59):
It's kind of entry intuitive a bit is that because
you're further from the.

Speaker 4 (16:02):
Center exactly, and it's also because the density of the
black hole goes down as the black hole grows like
the black hole. I think, if I'm not mistaken, you
can always fact check this. But I think the black
hole in the center of our galaxy as a density
compared to water. So it's wrong to say that for
sure quantum mechanics will be important near the horizon of

(16:23):
a black hole. But what we are pretty convinced of
is that if you would jump in a black hole,
we don't know what's there. But it cannot be classical
physics anymore because at some point the classical equations tell
you rubbish. They tell you things which are impossible, so
we know the classical theory has to break down. So
the assumption is that just as in the early universe,

(16:45):
in the very center of a black hole, there's also
quantum mechanics and gravity at play at the same time.

Speaker 1 (16:52):
So I want to get back to what you said
about things breaking down, but in a minute. First, I
want to focus on this question of quantum mechanics and
gravity at play at the same time. So you told
us earlier that general relativity, or we can call it gravity,
describes usually big things, and quantum mechanics usually describe small things.
And now you're saying that at the beginning of the
universe and inside black holes, we think both of those

(17:14):
are relevant. And that's why we need a unified theory,
because we need some way to describe that and to
disagree to conflict. Why is there a conflict and their predictions.
Couldn't it just be that they make the same prediction
for what happens in that scenario. Couldn't it just be beautiful,
a fortunate harmony among the theories.

Speaker 4 (17:32):
How what's the quickest way to explain? So let me
give you an example that I hope more people know,
maybe even from high school or first year of university. Say, okay,
think of an electric field. So you have a charge particle,
and a charge particle is surrounded by an electric field

(17:52):
that it sources itself. So when you look at it classically,
and when you think of a particle classically, it means
that a particle is a point. And maybe if people
remember this still, there was this formula that it said
that the strength of this electric field went like a
negative power of the distance from the particle. Say, you know,

(18:13):
take it's one over our squared. That's actually that the force.
Then you find that this force becomes infinitely big as
you approach the particle. Okay, it's the same for the energy.
The total energy carried by that particle would be infinite.
And we know that can that cannot be correct. And

(18:34):
then we have learned later on one hundred years ago,
when people understood the quantum theory of charged particles, there
was nothing going infinite. Things were just super well behaved,
numbers were finite. Nothing weird was happening. And gravity is
that sounds completely analogous, right. Even the formula for the

(18:55):
force of gravity is almost the same as a formula
for the Kulan force. Both can give you infinities. And
for the cool on force, we learned that that infinity
is gone when you treat it quantum mechanically.

Speaker 2 (19:07):
Both general relativity and quantum mechanics at some point start
giving you infinities that make no sense when you push
them to their extremes.

Speaker 4 (19:15):
So quantum mechanics doesn't give you infinities, but the classical
theory does, so relativity does. Yeah. So then the question is,
if you treat relativity in a quantum mechanical way, would
you get sensible numbers? And do you Well, that's a
good question, so yeah, to be able to answer it,
I should I should tell you this is the theory

(19:36):
that of quantum gravity. And to say that something is
the theory of quantum gravity, I mean you can write
down a theory which is extremely hard, but imagine you
succeed and people succeeded. You don't know whether it's the
only option, right, so you need to Normally you test
the theory. And the problem is that to go out
and test it requires you to look for these, you know,

(19:57):
places where gravity is so strong, and I guess none
of us wants to jump in a black hole. You could,
but you know, you could test it, but unfortunately you
would not be able to tell anymore to anybody else
because you cannot escape from the black hole. Right, So
nature is playing a very mean trick on us. It
seems that humanity, in order to test the theory of

(20:19):
quantum bravity, it is forced to do something that kills you.
You know, this actually goes under the name of censorship,
cosmic censorship. It's actually something quite serious in the physics community.
Gravity works such that you could actually just see quantum bravity. Unfortunately,
there's always what we call a cosmic horizon preventing you
to see it. Either it's the horizon of a black hole,

(20:41):
or you have to go back in time. But if
you take your telescope, it's actually impossible to directly look
at the Big Bang. So that's kind of mean. Otherwise
you could just observe it.

Speaker 3 (20:51):
Yeah, it's frustrating.

Speaker 4 (20:53):
Religious, I would say God is playing you know, is
an evil person, so to speak.

Speaker 1 (20:58):
Yeah, I also believe in cause free speech. I think
the universe should be free to tell us how it works.
And I'm bummed about all this censorship. Okay, I have
a lot more questions, but first let's take a quick
break and let our brains rest a moment. Okay, we're

(21:29):
back and we are talking to Thomas Van Riet, a
self proclaimed string theorist, about how strength theory works and
why it's so pretty. So let's go back to this
question of infinities. You said just a moment ago that
it's hard, and we hear this a lot. Quantum gravity
is hard. It's the hardest problem. And you're telling us
that we have general relativity, which works beautifully outside of
event horizons, and after some critical density in the universe.

(21:52):
We have quantum mechanics, which works beautifully and very effectively
for very small things and very high energies. Why is
it hard to bring these two things together? What is
the challenge? I mean, We've made a quantum version of electromagnetism,
We've made a quantum version of the nuclear forces. Like,
why is it so hard to take gravity and make
it quantum?

Speaker 4 (22:12):
It is hard for two reasons. Maybe they're more, but
there are two that are very sort of prominent. So
one of them technically goes under the name that the
theory of gravity is non renormalizable. We can come back
to that it has to do with infinities of sorts.
But actually we have some experience with non renormalizable theories

(22:34):
in the past and resolve them. But still it usually
means bad.

Speaker 1 (22:38):
It's tough.

Speaker 4 (22:39):
That's already what it tells you. The second thing is
that gravity, at the same time is a theory or
a classical theory of space and time, a theory of
the background. Okay, so in quantum mechanics, the way the
loss of quantum mechanics are formulated is that you assume
that this background is fixed. What does it mean in
practice is that you have two For instance, in let

(23:02):
me try to give a good example that doesn't sound
too abstract. In quantum mechanics, you use the notion of
two points in space time, whether they're what we call
costly connected, whether they can talk to each other, whether
you know you can send a lightweight from one point

(23:23):
to the other.

Speaker 1 (23:24):
You're talking about light counes.

Speaker 4 (23:25):
Exactly, but somewhere maybe less technical. Imagine that right now
there are people I don't know, ten kilometers away from us.
If we want to communicate with them, we cannot do
it right now. We can do it in a you know,
a little bit of time, the little time it takes
to send the light way. But so the two points
that are us here right now, and then people a

(23:47):
little bit away, we are what you call disconnected. But
imagine now that you have a theory of space and
time where space and time are globally that's what relativity
tells you. Then maybe that that whole notion changes, right
as a quantum mechanics is telling you that things fluck
to it. Things are very wobbly at a small scale.
They're so lobbly that maybe you know in your equations

(24:09):
what you thought are disconnected points, maybe they're not disconnected
anymore because you're wobbling your background that is telling you
that it's connected or not, And that's what makes it
kind of annoying.

Speaker 2 (24:21):
Daniel, is this going back to our map analogy that
we keep bringing up on the show or is this
something different?

Speaker 1 (24:26):
This is the same, Yeah, that's exactly right. In GR,
we have the concept of distances which are not fixed, right,
which can change. Whereas you're saying, in quantum mechanics, this
is essentially the background. So quantum mechanics is assuming that
there's a stage on which everything is happening, and GR
is like the theory of that stage, and it's changing
underneath it. But what I don't understand is why that

(24:48):
makes it hard, Like can't we do quantum mechanics on
curved space? You know, you can think about your fields
and my quantum mechanical view of space time is like, yeah,
you have this backdrop and you put fields on top
of it, and then you do the physics of the
fields and stuff is propagating. Is it hard to do
that quantum mechanical field theory in a curved space time?
Having people been able to do that? What's so hard

(25:10):
about that?

Speaker 4 (25:10):
Very good? So indeed, relativity can curve space time, and
then you need to formulate quantum mechanics on curved space.
That is not easy, but that people have done for sure.
One example why that is not so easy is that
quantum mechanics tells you there has to be a global
time direction. Things move forward or backward in time. On

(25:31):
a curved space time, what you thought was time can
actually at some point become space or vice versa. And
a famous example is a black hole. Imagine you approach
a black hole, you jump through the horizon. What do
you know is that you have to move forward to
the singularity. But you see that means that that spatial

(25:52):
that action became time because what is time for us?
Time is the only dimension in which we cannot stop
moving forward. I can decide now to sit on my chair,
but I cannot decide to move backward in time or
be still in time. I always have to move forward.
So the fact that once you pass the black hole horizon,

(26:12):
you're moving forward in time, forward towards similarity me is
that that forward direction became time, and time became actually
a spatial direction. So for quantum mechanics, for shredding equation,
that's pretty annoying, but we learned how to deal with it,
and dealing with it gives you this amazing phenomenon, like
talking evaporation of black hoves. But what I said about

(26:36):
quantum gravity is still something different because in quantum gravity,
it's that curve background that is it self fluctuating under
the confluctuations. Right, So there's no problem doing quantum mechanics
on a curve background. It's just a bit more complicated
because of the problem I told you. But now the
background itself should become dynamically in equalum theory, so that

(26:58):
your standard sharting equation is not well formulated to deal
with the fact that the background itself is the thing
that is part of the theory.

Speaker 1 (27:08):
I see. So you can do quantum mechanics on flat space,
that's easy. You can do quantum mechanics when space gets curved.
That's a little bit more technical, but people with big
brains to figure that out. But having the space itself
respond to the quantum mechanics, to have it all be
dynamical and link together and be harmonious, to have this
back and forth where energy is telling space how to

(27:29):
bend and space is telling matter how to move, that
is too technical for people to have figured out, or
that's the challenge.

Speaker 4 (27:35):
That's a challenge ship absolutely.

Speaker 1 (27:38):
And this I think also connects to the other comments
you made about renormalizable theories, which I think is worth
digging into for a minute because it connects to the
example you talked about a moment ago about an electron
having apparently infinite charge or apparently infinite energy. Right. If
you take an electrons charge and you look at it
from a distance, it appears to have charge of negative one. Right,
But as you say, an electron is surrounded by its

(27:59):
feel and that field you can think of as a
cloud of potential particles. And so if you actually think
about what the charge of the electron is that we measure.
It's the charge of the electrons surrounded by the cloud, right,
And as you penetrate deeper into the cloud, you measure
a more and more negative charge. And then that charge
if you get all the way through the cloud to

(28:20):
the electron, the charge apparently becomes negative infinity, which is
crazy and bonkers and unphysical. And so you were talking
earlier about renormalizable theories and how we've managed to patch
this up with quantum mechanics. Can you say a few
words about what it means to renormalize the theory? How
do you get rid of an infinity in the theory?
How do you solve that kind of problem where you're like,
hold on a second, electrons can't have negative infinity charge?

(28:41):
How do you solve that? What is renormalizablainy I just.

Speaker 2 (28:43):
Clarify real quick so that the biologist and me wants
to confirm. So when you guys say you're getting infinities,
that's just a fancy way of saying we're wrong.

Speaker 3 (28:51):
Like it's just this is not working, right, Okay, got
it all right?

Speaker 1 (28:54):
Yes?

Speaker 4 (28:55):
Absolutely absolutely. To be honest, I think to explain to
normalizability that the story of the infiniti This is what
usually is told. Is I think leading people astray. It's
not the right way of explaining it. May I try
to explain it differently, but please go ahead of It's okay. Yeah,
maybe it's good to go back to. You know what
Newton got famous for, you know his theory, and what

(29:17):
is his theory saying? Maybe people remember that. You know,
there's an equation that he got famous for, which is
called ethical en times a, which is essentially telling you
that a force on a particle equals the mass of
the particle times the acceleration that the particle is undergoing
because of the force. So you can ask yourself, is

(29:40):
this really a lot? A lot of physics? Means that
you suddenly there are three things you know and you
actually found a connection between them. I would say, isn't
this a definition? The Newton just defined the workforce by
saying it's en times a. It looks like that. But
there's another famous example that you know in high school
you learned Olmslow and usually you call it you say

(30:02):
that resistance is voltage divided by current. See, that's not
a lot. That's the definition of resistance. There's no information
in that equation, but a definition on was successful because
he said this R is a constant. That's that's the law.
R is a concept, and then it becomes something with

(30:24):
predictive power. Namely, I measure the voltage over a resistant
over a resistance, and then I know the current that
goes through it. But that's only because my real equation
is art equal the concept. So what's the equation of newtant?
Newton's idea was only successful because he essentially wanted to
tell us this F is universal. Imagine the way an

(30:44):
apple falls under this F whether how it falls in
Cambridge or in China, it will be the same formula.
So that means that once you have the formula for
the F and you say it's true all over the universe,
it becomes very strong the prediction. So Newton and his
program was having a lot of predictive power by you know,

(31:06):
moving around in the universe. He found something true all
over the universe. Okay, so this is the same with normalizability.
So instead of moving around from the left to the right, up, down,
whatever future past, renormalizability has to do with zooming in
and zooming out. If I have an equation and I
want you to have predictive power. It has to tell

(31:27):
me what also happens when I zoom in or I
zoom out. Okay, zoom, let us think of zooming in.
That's what really the problem lies. It means that I
go to very small length skills. I want a theory
which gives me a single equation with all the constants
known and measured, so I can tell you what happens
at small distance skills when I zoom in. A non
renormalizable theory, it doesn't give you infinities. People should stop

(31:49):
saying that they give you completely finite numbers after some
mathematical trickery. But what it does is that the more
you zoom in each time, the equation gains another constant
that we know the value, and we have to go
out in nature nature and measure it. Right. So imagine
you want to somebody asking me, okay, Thomas, what happens
in a gravitational field at you know, a micrometer. I say,

(32:11):
oh my god, I have to you know already maybe
correct Neutant's law for quantum gravity. And I say, yeah,
there's an extra constant in the equation. It's not you know,
the force is not one over our square. There's maybe
one over our cube, but it's not one. It will
be some number multiplying the equation. Okay, I go out
in nature and measure it. Good. I have the number,
and as somebody wants to go ten times smaller or

(32:34):
twenty times smaller, suddenly a term which goes like one
R to the power for becomes important. You need to
know the coefficient of the term. I again have to
do a measure. So you see I don't have predictive power.
That's the definition of a non revisable theory. It means
that you know, the smaller you get, the more constant
theory has to be more precise, but it cannot predict

(32:56):
what the constants are, whereas renormalizable theory it says, hey,
I don't need any new constant guys. I mean, I
can tell you with the computation on what you know,
how the theory behaves at the smallest length skills. So unfortunately,
the way historically this came about, and that's where the
word nenormalizable comes from, is that you know, we were
getting infinities and then we found we always say a

(33:19):
mathematical treat but in fact it's a physics treat to
get rid of them. But you see, that's that's not
the essential part. The essential part is whether the theory
is predictive, whether there's only a few constants that you
can get out, go out and measure, or whether you
need an infinite amount of consonts if I want to
get infinitely small. And so if we take Einstein's theory
classical theory of gravity, we apply our usual techniques of

(33:43):
quantum theory, we find that it's non renormalizable, not meaning
that it gives you infinities. This is actually, I think
a bad explanation. It gives you too many consonants that
we don't know what they are, and we will have
to measure.

Speaker 1 (33:58):
I see. So a renormalizable theory, you can say I
don't really know what's going on inside the electron. Maybe
there's other particles, maybe not, But I can measure the
charge and I can move on, and I can say
it's all wrapped up in this number. They'll charge it
the electron. And as long as I can make a
finite number of measurements, like I don't have to measure
an infant number of properties in the electron, then I
have a theory. I can use because that can make

(34:19):
a finite number of measurements in a finite amount of time.
So a non renormalizable theory, you're saying, is one where
you can't ever capture all those details in a single
number or two numbers, or even a finite number of numbers.
You'd need to measure an infinite number of parameters to
have a theory that you can actually use to make calculations.
But you said a minute ago that we have other

(34:40):
non renormalizable theories. I think, for example, quantum chromodynamics, it's
non renormalizable, and we've made that work. I mean, I
know it's a headache, but we've made it work. What
is it about gravity that's so special that we can't
use our non renormalizable fancy clever tricks to get quantum
gravity to work?

Speaker 4 (34:57):
Right, So you're saying humanity has dealt normalizable theories, made
them to work, and I can tell you what the
problem is. So I guess don't forget that before we
already said that gravity had two problems for it to
be hard to quantize. Non rymalizability was one of them.
So there's still the other one. So that's part of

(35:17):
my answer, But it's still I believe it's very different.
Like the other part of my answer will be the following.
Usually what happens in physics. Actually, every instance we have
seen so far where the theory was non renormalizable, we
actually cured it by realizing that we didn't have all
what we usually call degrees of freedom. Okay, so what

(35:39):
does that mean? So I assume it I go to
small distances, and I always assume that. You know, if
I have the theory of the electron, there's only the electron.
Say well, maybe they're very massive particles out there which
require a lot of energy to be created. In physics,
having a lot of energy is the same as going

(36:01):
to very small distances. So all non renormalizable theories we
have encountered were always made renormalizable by realizing that we
didn't take into account fluctuation fields particles that were just
very massive so that we didn't measure them yet.

Speaker 3 (36:17):
That's real quick.

Speaker 2 (36:18):
Saying you needed more degrees of freedom means there was
something else that wasn't included in the equation that needed
to be.

Speaker 4 (36:22):
The exactly absolutely absolutely, And then you see, oh this
is nice, my theory becomes mathematically normalizable. But then actually
we went out in nichere we found technologies to increase
our energy in our experiment, and then we saw those
particles that we predicted mathematically because we wanted the theory
to prenormalizable. Okay, I think that's extremely beautiful. Like you

(36:43):
you do something on mathematical clouds, it predicts new particles
for it to work out, and there you measure them. Okay.
And here's the funny thing with gravity. What string theories
will typically tell you. What I think more and more
people are leaning towards it, is that if you want
to make gravity normalizable, it looks like you the infinite
amount of particles with ever increasing energy. And that sounds

(37:04):
super bad when you say that first, because you're like,
oh my god, infinite amount of particles to solve your problem.
It's like measuring an infinite number of concepts. You're not
better off. Okay, So of course now I'm going to
sell string theory here. No, what is so beautiful is
that it's infinite tower of particles groups together in the
motion of a string. It just meant that what we
thought were particles, no, it was just a single object.

(37:26):
There's no tower. It's just a string that can vibrate
in different ways. So there's a lot of structure in
that infant amount of particles that you need to invoke
together innormalizable theory. Yeah, otherwise it looks very bad, like
every time you take in a new particle you find
that renormalizability still requires a new one, and you think,
oh my god, you guys are just you know, in
an never ending street of problems. No, we see that

(37:49):
every single particle we have to add as exactly properties
that we could have predicted from the previous one. So
there's a beautiful structure, and what looks like an infinite
tower of particles just becomes a single stringing object with
almost no constant associated to it.

Speaker 1 (38:04):
So there, you just said the word beautiful. What is
beautiful about that? Is it? Because wow, this is a
hard problem, and now have a solution. Is it like
my headache is gone? Or is there something objectively beautiful
about this particular solution?

Speaker 2 (38:17):
Can I go back to a real quick question and
then can we move to beauty because I don't want
to miss my chance to understand this because I'm actually
really following everything.

Speaker 3 (38:24):
I'm excited.

Speaker 2 (38:25):
Okay, So instead of needing to measure an infinite number
of constants? Can we measure that string? Do we know
how to measure the string? Does that make our situation
any better?

Speaker 4 (38:35):
I'm going to be honest in practice. No, but this
week we could have predicted you in advance. Okay, So
this has nothing to do with string theory. I just
told you that the regime ware gravity and quantum mechanics
are relevant. Is either we have to jump through a
black hole, which is not nice as an experience, or
we somehow have to be able to move back in

(38:56):
time to the Big Bang, or people are able to
build you know, galaxy sized accelerated. That is a true
statement independent of what the theory of quantum gravity is.
It just you know, you predict what is the energy
density needed to see those effects. Of course, one can
be lucky and some effects of the highest energy densities

(39:20):
or the smallest lendsciales can trickle you know how they
say trickle down? Is it correct English? I don't know,
but can leave an imprint on larger distances and smaller energies.
Is if it's something that we're looking into, we are hoping,
you know, I'm praying, but we don't know for sure,
So I hope that explains a bit, yeah, and not
to the beauty. It's I like the questions that I'm
trying to find an analogy. Okay, so imagine I don't

(39:42):
know whether this reminds you of the word beauty, but
imagine you have a super complicated puzzive in front of you.
I don't know, one billion pieces and you just don't
know how to put them together, and suddenly you find
two connecting and because you see two pieces that connect,
you suddenly see the third piece lying there, and the

(40:03):
more you put them together, suddenly it's just one structure
that is like extremely simple. Okay. It's like imagine if
a blackboard full of equations and you cannot solve them,
and suddenly you realize that your equation was too complicate,
that terms are dropping against each other, and you keep
on canceling terms, and suddenly you have an equation left
which is just one centimeter insights. You're like, oh my god,

(40:25):
this is you know, this is amazing. That's the kind
of beauty we're talking about that we think that renormalizing
gravity is a nightmare. It gives you ugly theories. And
then the first thing we try, which you know, just
on mathematical grounds, and we get something that is in
terms of the length of equations, is even smaller than

(40:46):
any equation that we have had in the past. And
that is what I think why so many people like it.
And then the confusing part is that it's not because
the size of the equation is small that it's easy
to solve. It just means that it's very elegant, okay,
in the sense that, for instance, there maybe elegance is
better than beauty. The elegant thing of string theory is

(41:10):
that they're no constants in the.

Speaker 1 (41:12):
Theory, no numbers at all.

Speaker 4 (41:14):
No numbers at all exactly. Any other theory non physics
has to have a lot of numbers that you go
out and measure. String theory doesn't have a number. Actually
it only is one. It's the size of the string
as variables.

Speaker 3 (41:30):
But no constants.

Speaker 2 (41:31):
Is that I'm having trouble imagining an equation with no
numbers that's exactly correct.

Speaker 1 (41:36):
So it's like ex equals why not ex equals two
point seven four times?

Speaker 4 (41:40):
Why right? Where I didn't know the two point seven
I had to go out and measure it, all.

Speaker 2 (41:44):
Right, So I'm excited because this is the most that
I've understood string theory in my.

Speaker 3 (41:47):
Life so far, but I could still use the brake.

Speaker 2 (41:50):
So let's go ahead, get some more coffee, a little
bit more brain fuel, and we will be right back
to talk more about string theory. All right, we are

(42:16):
back with Thomas Van Reed. Let's jump back into string theory.
So string theory we've discussed that it can help when
you're in those really tiny little situations where you'd usually
have to get a lot.

Speaker 3 (42:27):
More calculate, a lot more constant.

Speaker 2 (42:29):
Does it also work if you zoom out or is
it just a theory for when you're super zoomed in?

Speaker 4 (42:34):
This is an excellent question. So that's where it gets hard.
Surprisingly Okay, So when you zoom out in physics, it
means okay, large distance also means low energy. And what
is the hardest part of working with high energy theories
like string theory, is to understand if I take the
theory and I run into low energies like the energy

(42:55):
densities that we like, have you know in your office, okay,
then it's not you, and so it becomes very difficult
to understand. So how would the world look like on
this low density or large distances? I mean, string theory
predicts a completely unique world. At small distances you see little,
you know, vibrating strings behaving in a certain way. But

(43:16):
then if you do mine it, it's not obvious what's
going to happen. Okay, this is why we always say
we have trouble or we are not sure whether we
can reproduce the large the universe as we typically know.
But this is not a problem of string theory. This
is effect of all high energy theories. And maybe I
can give an analogy. Okay, so imagine that you have

(43:37):
a rocky landscape, hills, mountains, whatever, very very complicated, many valleys,
and you have a football. But the football has a
lot of energy, you know, so then it's like up
there up the tops of the mountains, right because it
just says lots of alosity. It's moving through those valleys
and it's just all the way up. But then you

(44:01):
know the restriction and the velocity is going down. Well,
if I have many values, I don't know where the
ball is going to roll down and where the value
is that it's going to end. That's the problem we have. Well,
I don't think it's a problem with theory, it's just
it's typical. Actually, even the standard model has this property
that this difficulty.

Speaker 1 (44:18):
That's a great explanation. Thank you. Can you circle back
and help us understand more specifically how string theory solves
these problems of quantum gravity. You talked about how howing
string replaces the infinite number of parameters you might have
to measure how does it solve the problem of quantum
mechanics and general relativity working together on this dynamic space time.

Speaker 4 (44:40):
So first, I think it's very important to have a disclaimer.
We don't have the full theory, right, so whether it
solves all problems that we know quantum gravity we do
not know. I have to be honest on this. I
would say that from a mathematical point of view, the
way it solves this is by not quantizing gravity. That's
very strange to say what it is. That's why string

(45:00):
theory did so quantizing. When we use the word quantizing,
it means that we take our classical theory it has
a certain amount of variables like electromagnetism has the electric
field and has the electron field, and always said that's quantizing.
Let's make it quantum mechanical. So you could say, well,
relativity is what we call the metric field. That's a

(45:23):
field that describes how space and time curve. Okay, and
that's what people usually do. They say, Oh, we learned
in history of quantized series. We take that classical what
we call field and we turn it into a quantom
mechanical field.

Speaker 1 (45:37):
But how do you do that? How do you quantize
the theory? You don't just like tap your magic wand
down and say and now you're quantum mechanical.

Speaker 4 (45:43):
Oh my god, this is tough. So mathematically you would
say it turned a field into an operator, but that
probably means a little two people listening, here's an attempt.
I'm not sure it's even good. Quantum mechanics tells you
that things there are only probabilities, right, what you thought
as a particle. We sometly say it so a way,
it's not a very good word. What we have in

(46:06):
said is a probability distribution of where the party consulute.
So objects are turned into probability distributions. That's quantizing a theory.
And then there's a certain equation for the probability distribution.
But more mathematically, it means that you take a field
and you make it into an operator.

Speaker 1 (46:26):
No, that's a great way to think about it. A
classical theory says that everything is specified and there is
infinite information even if you don't have it, whereas a
quantum theory leaves some uncertainty and says, well, this isn't determined.
Maybe the electron is here, maybe the electron is there.
Maybe the field is this value, maybe it has that value.
And for those of you playing along at home, an
operator here is making a measurement. It's like applying something

(46:49):
to it and getting a result out. And so that's
a crucial element of quant mechanics. Okay, so now we're
going to try to quantize space time and you say,
we can think of the metric as a field. The
metric is like how much curvature there is at every
point in space. So if we think of like the
curvature and space as a field, why is it hard
to quantize that. Why can't we think of that as like, well,

(47:10):
maybe the curvature is this value, maybe the curvature is
that value. Why can't we just think of that probabilistically.

Speaker 4 (47:15):
That was the problem of the problems we talked about before.
Then you run into the problem of non renormalizability. If
you do it that way, or you're run into the
problem that you know, it's a background itself that has
to become a probability. So the formalism of Qunlem mechanics
gets very confusing at that point. And as I said,
we already knew that a theory that is non renormalizable

(47:37):
means that you're not having the right degrees of freedom,
you're missing information. So the way string theory went about
is people that discovered it. We're not trying to quantize gravity.
Let's let's be clear on this. Okay. They wanted to
solve another puzzle, and for some reason, which is a
long story by itself, they were interested into string like

(47:58):
objects and how they move and how strings move quantum mechanically.
So they do their computation and they suddenly see that
the string can fluctuate in what we mathematically call a
spin to field. If you don't know what a spin
to field is, it's just a fancy way of saying
it describes what I call the metric field. They just says,

(48:20):
but that's strange. There's no space and time, and yet
they found the structure, which is what they knew from relativity.
And then they started to look into it deeper, and
they wanted to understand the equations that that metric field obeyed,
and they were completely surprised that, you know, they didn't
ask for it. They found Insten's equations. So this is

(48:40):
also what I call absolute beauty.

Speaker 1 (48:42):
Okay.

Speaker 4 (48:43):
Other approaches to quantographty they said, let me take insent
equations for true, just have them. I said, also just
dropped them down. I said, I didn't know why these are
my equations. Okay, he didn't derive them. And so the
other approaches to quanto gravity say, let me take those
equations that you know, quantitize them. String theories did something else.
They were looking at strings vibrating for a completely different reason.

(49:05):
They not only recover Einstein's equations, the classical ones they predict,
they literally predict them, but they immediately have them quanta mechanically,
and it meant that they needed they have all these
possible vibrations of the string. It's just one vibration that
gives you this metric field, but the string can vibrate
in so many other ways. And then suddenly it gave
them other things. They knew, for instance, all the forces

(49:27):
in nature. They come in two kinds. Okay, there's gravity.
This is a separate guy. It's is described by this
metric field. And the other forces are with a manematical
term are gauge forces, young meals forces. Electromagnetism is an
example of it, okay. And the two other are the
nuclear forces, and they are all described by one equation
which is called the Young Mules equation, and a special

(49:49):
kind of young music equations that maybe people listening who
had a little bit of a scientific, you know, education
remember are what we call the maximal equations, which are
the equations of electric and magnetic fields. But this is
part of a general mathematical equation which is called the
young music question, which also mathematicians study for completely different reasons.
But guess what they were looking at the other modes

(50:09):
of vibration of the street, and without asking for it,
the young music quations appeared, and at that point people
were like, my god, this is insane. Okay, So this
is where all the hype came from, all right, from
string theory, like the excitement of people all came from this.
Not only do we you know, we get the classical
theories we didn't ask for that we've gotten. So you

(50:29):
get all advance, so to speak.

Speaker 1 (50:31):
So I'm getting a sense from you that the elegance
of string theory comes from the sort of discovery that
it answers questions simply, and sometimes it answers questions we
weren't trying to answer. In that sense, it feels more
like you're accidentally revealing a big chunk of truth rather
than you're like laboriously putting together an over complicated answer

(50:53):
that's just an invention in your mind. Is that the
feeling here that we've like uncovered a vein of reality.

Speaker 4 (50:59):
Absolutely, And I think that's for all of science. Like
I can imagine in biology, when you understood like gene
structure and I suddenly realize how things work. It becomes
so simple, Like I think the same evolution happened in biology.
You have all this phenomena. At some point we learned
about to sell and the smaller organisms, and all of

(51:22):
these phenomena suddenly can be explained in a more microscopic way.
So the theory becomes simpler. It became maybe I'm simplifying.
I mean, I'm not the expert here, but I would
say biology at some point became the theory of the cell,
which is so much smaller and so much in a
way simpler.

Speaker 1 (51:38):
So is it then about the theory itself or is
it about the insights the theory gives you about how
the universe works, or just the place where we are
where we're like wollw We're frustrated by these problems for decades,
and now finally the headache has gone away. I mean,
can you look at the theory itself and say, this
theory is beautiful? Is there a chance that we could
have revealed the theory which feels truthful? But then you're like, actually,

(51:59):
I don't like it. It's kind of ugly.

Speaker 4 (52:01):
Right. I guess different versions of beauty were felt at
different stages, right, So the people first making these discoveries
of seeing einscience equations and so on, when you read
their biographies, they really are like they talk about it
extremely emotional, like they could be crying because they saw
that part of beauty. But my generation came later, so

(52:24):
we kind of you say, you got used to it,
you don't feel that beauty on it. The thing that
strikes me is that when you first learn about Newton's
second law, I don't think none of us feels what
Newton felt at the time, and I think we are
underestimating the emotions because one other things that I didn't
tell you what Newton's second ploy is. It tells you
about what we call the deterministic view of nature. Newton

(52:48):
realized that this equation was telling you that, you know,
I'm sitting on my chair and later on I will
walk away. But according to Newton's equation, I don't have
the choice. I have no freedom. There's no freedom of
you know, there's no free will.

Speaker 3 (53:01):
Do the philosophers know you all have solved that problem.

Speaker 4 (53:04):
No, it's the classical theory, right, So, but physics is deterministic,
so the according to physics. I think we should emphasize this.
There is no free will in physics. Anybody that tells
you that there is is wrong physics. As I don't
say we have solved it. But there is no free
will in physics, absolutely not. Free will is completely in
contradiction with physics. Not the illusion of free will, but

(53:25):
free will. But it's no way, there's no room in
physics for free will, not in the usual notion of
the word free will. Any I want to say that
the beauty that we feel or now the students you know,
which are younger than me. There are different versions of beauty.
It's more where you start applying the theory. I can
give you one example of a thing that I found
beautiful is like you have equations with singularities, like these

(53:46):
infinities we talked about, and string theory can do calculations
and you see there's no infinity, And then you learn
how string theory tells you that there's no infinity, and
it does it in a very creative and funny way.
There's often a picture like you can even see it literally,
it's not an equation, it's not formal us. You can
just see it, and that's kind of beautiful.

Speaker 2 (54:04):
Too bad the listeners can't see your face because you
have the biggest grin on that You've had the whole
interview explaining how beautiful it is.

Speaker 3 (54:09):
Like you're clearly getting a total kick out of this.
It's it's awesome.

Speaker 1 (54:13):
I think it is really hard to put yourself in
the minds of earlier generations to appreciate how big some
of those steps forward were. Like, it seems pretty basic
what Aristotle accomplished. You like, things fall down. I could
have said that, but you know, to systematize the world
at all. What was a big step forward. I think
you're right that it's underappreciated. So what is it that

(54:33):
we're underappreciating now in terms of strength theory. I mean,
there's a lot of popular writing about strength theory, a
lot of popular conceptions about it. But from somebody on
the inside, what do you feel like is most often
misunderstood or misrepresented about the nature of strength theory.

Speaker 4 (54:47):
I have to be careful, careful not to become too
sort of drawn into the sociological discussion, but I feel
I cannot not say it so string theory, say, thirty
twenty years ago, when it's people discussed it in science outreach,
it was only the one, okay, And now it's the opposite,

(55:08):
And I think that the opposite went so far that
it's completely misrepresenting the field.

Speaker 1 (55:14):
When you say the opposite, you mean like people being
critical of string theory because it hasn't yet predicted some
experiment and been proven.

Speaker 4 (55:21):
Writers is exactly, this is an example exactly. So they
will tell you this, and then of course you have
to tell them, because they don't tell you this, that
this is true for any theory of quantogravity and We
knew this in advance. We knew this before we started
working on string theory with absolute certainty that if you
have access to the smallest Lend skills, you can falcify
a theory one from the other. Okay, string theory from

(55:43):
the other examples. What is completely not obvious is that
some of these high energy small distance effects have an
imprint at larger distances. At a moment, we don't know,
and we're actually looking into it. And this program has
a name. I think it's super exciting. It's called the
Swamp program, and it's where we try to look into

(56:03):
that question. But at the moment we do not know.
But as any other supposed alternative to string hearing is
not even there at that stage where they can even
ask this question, does my theory predict something at a
bigger length scale, because normally you don't expect it to
be the case? Right, So can I get away of

(56:23):
not sending a student into a black hole to learn
about pornography? We don't know.

Speaker 2 (56:28):
I mean, master students are expendable. You could send like
four or five of them.

Speaker 4 (56:32):
Unfortunately they can't explains what they're what they're seeing, right,
so they couldn't explain it. That's that's a Otherwise I would,
you know, be interested in maybe jumping into a black
hole just to see because if you jump into a
big black hole, actually it doesn't need to be a
painful experience. You can pass the horizon without you know,
feeling it too much, and then you could actually see

(56:53):
a singularity.

Speaker 1 (56:54):
You know.

Speaker 4 (56:54):
Interstellary is a little bit about this, right, you jump
into a black hole. There's a movie about it. But yeah,
so I think is my frustration that there is a
back correction to the original hype. But the back correction
especially you know now on social media, but also i'd
say conventional science outreach. To give an example, I saw

(57:15):
my children that at an age where they get interested
in science and they start googling things. So I see
their first hit on Google when they ask a question
which is about fundamental physics, and the first hit that
they have is criticism on stream. There it became so
mainstream that this is the first thing you see, and
that's not healthy anymore.

Speaker 1 (57:35):
Okay, So just to make sure I understand, you're saying
it's fair to criticize string theory and say you haven't
made a prediction, which can be verified, But all the
theories of quantum gravity also had that issue that we
can't go inside a black hole, And many theories of
quantum gravity haven't even come together and coalesced and enough
detail to make any predictions, not to mention ones that

(57:56):
can be tested. So then let me wrap up by
asking you a last question, which is about the truth
of strength theory. I mean, you're excited about string theory
because you think it's simple and it feels like a
compelling potential answer to the question of like what really
happening in the universe. So do you think, for example,
in some hypothetical scenario where aliens arrive on Earth and

(58:16):
they're very advanced scientifically, and we can figure out how
to communicate with them, et cetera, et cetera, what do
you think are the chances that alien physicists are doing
string theory that they have also stumbled upon this explanation.

Speaker 2 (58:28):
Daniel always has to get aliens into the show at
least once, and so here we go.

Speaker 4 (58:34):
We all owe aliens. Actually, Okay, I don't know whether
my answer is of any meaning, but I would say
they will discover string theory. I actually don't even doubt it.
I'm one hundred percent convinced. And as to the question before,
people tell you that a theory without predictions is not science,

(58:55):
and I think we have to really step away from this.
So in science are two things. They're observables and they're computables,
especially in theoretical sciences, and what a theory has to
get right are the computables. For instance, if I have
a theory that can explain phenomena at large distances, but
I look at small distances and the theory tells me

(59:17):
that I can go back in time and kill my
mother before I was born. I know that theory is nonsense,
but I cannot make an experimental verification. But the theory
is just nonsense. It's not logical. And the thing that
people that the audience and you know, the greater public
needs to understand, quantum gravity is so extremely constraining in

(59:41):
terms of just logical consistency that you almost uniquely arrive
at an answer. And that's where this is true science. Okay,
despite not having access at a moment to an experiment
to test it, you almost uniquely are pushed into a
direction to solve this problem of non remortalizing. Now I'm
selling it too much, But I hope you're understanding what

(01:00:03):
I'm trying to say.

Speaker 2 (01:00:04):
I've read a couple of books on strength theory and
never understood them, but I've totally understood our conversation today.

Speaker 3 (01:00:10):
So I'm this has been awesome.

Speaker 4 (01:00:11):
Happy to hear that it's awesome.

Speaker 1 (01:00:14):
And if aliens arrive and they don't do strength theory,
maybe they can listen to this episode to get a
primer on how strength theory works. Exactly, exactly, wonderful. Well,
thank you very much for coming on the show and
talking to us about the hard problem of quantum gravity
and how string theory might be the solution. Thanks very much.

Speaker 4 (01:00:31):
It us a lot of fun. Thank you so much.

Speaker 2 (01:00:41):
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