Speaker 1: People might disagree about what kind of art they like.
Speaker 1: In fact, pretty much everybody does. But we all know
Speaker 1: what it means when we say that something is beautiful.
Speaker 1: It means that we appreciate it, that it moves us,
Speaker 1: that it strikes us, that we see an elegance in it.
Speaker 1: But what does it mean if you say a theory
Speaker 1: of physics or a little bit of math is beautiful?
Speaker 1: How can math be gorgeous? How can physics be elegant?
Speaker 1: What does that really mean? Well, string theory is the
Speaker 1: theory of physics that's most often described as a bit
Speaker 1: of twenty first century gorgeous physics that fell into our laps.
Speaker 1: What does that really mean? What is so beautiful about
Speaker 1: string theory? And just because something is beautiful, does that
Speaker 1: tell us whether it's more likely to describe our universe
Speaker 1: to actually be right? That's the question we're going to
Speaker 1: be asking today on the podcast What's so beautiful about
Speaker 1: string theory? Welcome to Daniel and Kelly's Extraordinary Universe.
Speaker 2: Hello, I'm Kelly Wiener Smith, and I know nothing about
Speaker 2: string theory. In fact, sometimes my eyes crossed and I
Speaker 2: just blankly stare at the wall. When the conversation comes up,
Speaker 2: But today I'm gonna understand it.
Speaker 1: Hi. I'm Daniel Whitson. I'm a particle physicist, which might
Speaker 1: make it sound like I should know string theory, but
Speaker 1: actually it means I just smashed particles together without understanding
Speaker 1: the nature of the universe.
Speaker 2: Oh all right, Well, so I've got a question for you.
Speaker 2: I listened to your beautiful opening about what does it
Speaker 2: mean to have a gorgeous equation? So my question for
Speaker 2: you is, what is the most beautiful equation?
Speaker 1: The most beautiful equation? Oh my gosh. To me, the
Speaker 1: most beautiful equation is actually not in physics. It's in math.
Speaker 1: Oiler's identity. It says E to the IPI plus one
Speaker 1: equals zero, And I just think it's incredible because it
Speaker 1: combines like a bunch of different stuff. You have E,
Speaker 1: pie zero, one and I all together, and it's so compact,
Speaker 1: and it's just so much encoded into it. It's like
Speaker 1: so dense with useful information. It tells you about how
Speaker 1: you can think about signs and cosigns in terms of
Speaker 1: complex numbers. To me, it's just fascinating to have so
Speaker 1: much information packed so tightly and so beautifully into a
Speaker 1: single equation.
Speaker 3: Awesome, A fine choice.
Speaker 1: But I also have to say that in grad school,
Speaker 1: the moment I discovered I was not going to be
Speaker 1: a theoretical physicist was when I was sitting next to
Speaker 1: my office mate and I realized that he did his
Speaker 1: homework just like I did, but he did it two
Speaker 1: or three times in different fonts because he got really
Speaker 1: excited about like writing these equations. He's like, Oh, I'm
Speaker 1: all writing in italics, or I can write these symbols
Speaker 1: another way. And I realize, like, wow, this kid really
Speaker 1: jams out about like writing down the equations. It's something
Speaker 1: about being a theoretical physicist that I just didn't have.
Speaker 1: I was like happy to be done with it once.
Speaker 2: Yeah, I gotta be honest, that doesn't strike me as
Speaker 2: super efficient. I'm going to do the same thing three times,
Speaker 2: but I'm glad that he's super into it.
Speaker 1: Yeah, but there's something about the equations and the formalisms
Speaker 1: and the expressions and even the fonts. The way you're
Speaker 1: writing these mathematical symbols, then you've got to be excited
Speaker 1: about if you're going to work in the nitty gritty
Speaker 1: of figuring these things out. Because being a theoretical physicist
Speaker 1: is a lot about writing equations on paper, So if
Speaker 1: you don't like that, then probably shouldn't be one.
Speaker 3: Yeah, fair enough.
Speaker 2: Well, today we're talking about a theory that is regularly
Speaker 2: described as beautiful, and I'll tell you by the end
Speaker 2: of the episode, I'm moderately convinced that string theory is beautiful,
Speaker 2: maybe even more than moderately convinced. But I think we
Speaker 2: should see what our audience thinks about out what's so
Speaker 2: beautiful about string theory? Is this something that people know already?
Speaker 1: That's right. I reached out to our listeners to ask
Speaker 1: them what do they think is beautiful about string theory.
Speaker 1: If you'd like to contribute your voice for future episodes,
Speaker 1: please write to us two questions at Danielankelly dot org.
Speaker 1: We will sign you up. Also send us questions about anything.
Speaker 1: I got a recent question about somebody's dating life which
Speaker 1: I totally couldn't answer, but I enjoyed reading anyway, so
Speaker 1: feel free to write to us.
Speaker 3: You should have sent that to me.
Speaker 2: I was on Dan Savage's podcast, and I feel like
Speaker 2: that makes me a relationship expert, so you can just
Speaker 2: send those to me.
Speaker 4: I've got it covered.
Speaker 1: Okay, there, you go, folks, We are self proclaimed experts
Speaker 1: in anything, So think about it for a minute. What
Speaker 1: do you think is beautiful about string theory? Here's what
Speaker 1: some listeners had to say. I don't think the universe
Speaker 1: would be so elegant to be a dangled, naughty strandfield.
Speaker 4: Mess unifies general relativeivity and quantum mechanics. Physicists love elegance
Speaker 4: and symmetry. It may also provide a new baseline of
Speaker 4: what's the tiniest thing, And then we.
Speaker 1: Get to ask the question is that it? Or is
Speaker 1: there something beyond that? The amount of money that Brian
Speaker 1: Green was able to make by taking advantage of popularizing it,
Speaker 1: we're able to mathematically explain why gravity is so weak
Speaker 1: compared to the other forces.
Speaker 5: So nusks, Dodd, what is string theory? The dad says,
Speaker 5: why you ask such difficult questions? Ask me something easier.
Speaker 5: So the sun says, okay, why does mum get so angry?
Speaker 4: Ah?
Speaker 5: Well, string theory is a theoretical framework.
Speaker 6: It's a kind of symmetrical beauty.
Speaker 1: The beauty to me is that we keep searching for
Speaker 1: the boundaries that would have to be the g string.
Speaker 7: Nothing's beautiful best string theory except that confusion is beautiful.
Speaker 7: People want to try to bring everything thing all together
Speaker 7: into one unified theory that explains everything. I think that's
Speaker 7: what makes it beautiful.
Speaker 6: And they look like worms string theory in all the theories
Speaker 6: that try to bridge this gap really show the spirit
Speaker 6: of scientists and researchers and physicists everywhere to keep on trucking.
Speaker 3: The totality of its failure in the same way that
Speaker 3: a lot of other unfalsifiable and self sealing things are
Speaker 3: in life that we.
Speaker 1: Love that the maths of it all is quite elegant.
Speaker 8: What I like about the idea of it is that
Speaker 8: rather than trying to think of the universe in discrete particles,
Speaker 8: kind of just thinking about it in like pulses of energy.
Speaker 4: I cool it confusing.
Speaker 2: Well, Daniel, it looks like we're teaching the controversy today
Speaker 2: because our.
Speaker 3: Answers range from I don't think there's anything.
Speaker 2: Beautiful about it to you know, it's pleasing esthetically, And
Speaker 2: there was a good range of answers, what do you
Speaker 2: think is it beautiful?
Speaker 1: I think it's fascinating to apply this subjective standard to
Speaker 1: something which is supposed to be objective. Right, we're talking
Speaker 1: about like the answer to the question of how the
Speaker 1: universe runs itself, the machinery of the cosmos. Why do
Speaker 1: we care about whether it's beautiful? Why should beauty be
Speaker 1: a guide? Like if we have two theories, should we
Speaker 1: pick the one that's more beautiful and follow that because
Speaker 1: we think it's more likely. I think it's this sort
Speaker 1: of like bias we have that we think nature should
Speaker 1: be beautiful because we mostly look around and we're like, oh, yeah,
Speaker 1: the world is pretty. I wonder if aliens evolving on
Speaker 1: an ugly planet, one that they find like if kind
Speaker 1: of yucky, would tend to be biased towards yucky theories
Speaker 1: of physics because their life is pretty yucky. Or maybe
Speaker 1: everybody of alves to think that their planet is beautiful
Speaker 1: and everybody tends towards beauty. I don't know. To me,
Speaker 1: it's a deep sort of philosophical question of what is
Speaker 1: beauty anyway, and why do we appreciate it in our
Speaker 1: world and why do we look for it in our physics?
Speaker 2: All right, So first an observation in my experience, it
Speaker 2: seems to me that when people say, oh, this equation
Speaker 2: is beautiful, what it usually means is it makes their
Speaker 2: life easier. It explains a lot of things. And maybe
Speaker 2: this is human laziness is the wrong answer, because most
Speaker 2: of the people working on these equations or anything but lazy.
Speaker 2: But like, oh, it's nice. It explains a lot of things.
Speaker 2: I don't have to worry about that stuff. So you
Speaker 2: did seem earlier to think that Euler's equation was beautiful,
Speaker 2: but now you seem to be a little bit more
Speaker 2: critical of people saying talking about equations that describe the
Speaker 2: universe as beautiful.
Speaker 3: Do you feel like there's some difference there.
Speaker 1: No, I can see beauty and I can appreciate it. It's
Speaker 1: like when you see a piece of machinery and there's
Speaker 1: only a few moving parts, but it can do something
Speaker 1: really complex, or you look at a piece of code
Speaker 1: and you're like, wow, that is so simple and yet powerful.
Speaker 1: You can appreciate the beauty of that. I just don't
Speaker 1: know why the universe has to work that way, Like
Speaker 1: the universe could be a total mess. We could discover
Speaker 1: the way it works, be like, actually, I have some
Speaker 1: notes this could have been done better, you know, like
Speaker 1: more documentation please. So I can definitely appreciate beauty when
Speaker 1: I see it, and I think I can even capture what
Speaker 1: he's beautiful about something. I just don't know why we
Speaker 1: expect the universe to be beautiful. I mean, I hope
Speaker 1: that it is, but we'll see.
Speaker 2: I mean, I think the universe is beautiful, like the
Speaker 2: sunsets are beautiful, the biodiversity is beautiful. But it certainly
Speaker 2: seems to me that anytime we try to explain what's happening,
Speaker 2: there's nothing beautiful in this.
Speaker 3: But maybe that's just the.
Speaker 2: Biologist working on ecological models, where we're like, this is
Speaker 2: a mess, and cells are a mess, and everything's a mess,
Speaker 2: but we're all just muddling forward.
Speaker 1: And let's not even get into chemistry because that's a disaster.
Speaker 3: Now, we weren't going to get in a chemistry Daniel,
Speaker 3: that's not what we do.
Speaker 1: That's right, exactly. But neither of us are also string theorists.
Speaker 1: And so I reached out to somebody I know online,
Speaker 1: Thomas Van Reed, who is a string theorist and writes
Speaker 1: about this stuff. And I've seen him on social media
Speaker 1: venting gently about how string theory is not well understood
Speaker 1: and mis explained and misunderstood by the general public. So
Speaker 1: I invited him to come on the podcast and tell
Speaker 1: us all about it.
Speaker 3: Let's jump right in.
Speaker 1: So then is my great pleasure to welcome to the
Speaker 1: podcast Professor Thomas van Reid. He's a theoretical physicist at
Speaker 1: the Institute for Theoretical Physics at ku LEEUFN. Some of
Speaker 1: his recent work include papers called the Stability of axion,
Speaker 1: saxyon wormholes, and the quantum theory of gravitation, Effective field theories,
Speaker 1: and strings yesterday and today. So I thought he'd be
Speaker 1: a good person to talk to about string theory and
Speaker 1: its alleged elegance. Thomas, thank you very much for joining us.
Speaker 4: It's my pleasure to be here.
Speaker 1: So my first question for you is, what is this
Speaker 1: big problem that everybody's trying to solve. We hear a
Speaker 1: lot in popular science about how we have general relativity
Speaker 1: and we have quantum mechanics and these two theories don't
Speaker 1: work well together, and we need some theory of quantum gravity.
Speaker 1: Why do we need a theory of quantum gravity? What
Speaker 1: is this big issue? Why can't we just have gr
Speaker 1: and quantum mechanics and be happy with those.
Speaker 4: So in everyday life, gravity is a classical force and
Speaker 4: there's no problem in understanding gravity. Sometimes it's a bit complicated,
Speaker 4: especially you know when you're looking at say, black hole mergers,
Speaker 4: you need full blown relativity. But it's still a classical theory.
Speaker 4: You can put it on a computer, you can do
Speaker 4: advanced calculations and you can understand what's going on.
Speaker 1: And what do you mean when you say a classical theory?
Speaker 1: What does classical mean? It sounds like a technical word
Speaker 1: you're using.
Speaker 4: Classical can mean two things. In physics, it's very confusing.
Speaker 4: So classical can mean that you do newtont mechanics and
Speaker 4: you don't do relativity. Relativity is a correction to Newton mechanics,
Speaker 4: and the correction takes into account that the speed of
Speaker 4: light is finite. So physics theories are always corrected by
Speaker 4: some numbers. And you can think of the difference between
Speaker 4: relativity and Newton mechanics to be that one theory is
Speaker 4: corrected by the other by numbers which go like one
Speaker 4: added by the speed of light, which is a very
Speaker 4: tiny number. So that's the first sense of classical. I
Speaker 4: actually meant the second sense of classical, and that's where
Speaker 4: you say I take say it doesn't matter whether it's
Speaker 4: a Newtonian or a relativistic theory, but I add quantum mechanics.
Speaker 4: They're in quantum mechanics. We also have a small number
Speaker 4: called Plank's constant, right, So very informally speaking, you could
Speaker 4: say that quantum mechanics correct classical mechanics by terms in
Speaker 4: equations that are powers of this small number.
Speaker 1: So classical is a fuzzy word that basically means old fashioned,
Speaker 1: the same way like you might call classical music Mozart,
Speaker 1: but the kind of music I like to listen to
Speaker 1: is called classic rock on the radio, even though it's
Speaker 1: not that old. And so you're talking about two different
Speaker 1: senses in which physics has evolved from Newton to Einstein,
Speaker 1: and then from Einstein to like Schrodinger and Heisenberg and stuff.
Speaker 1: And so in this sense, when you say a classical
Speaker 1: theory physics, you mean without quantum mechanics.
Speaker 3: And you know, some of that must is pretty old.
Speaker 3: By that man, we are getting a little bit old,
Speaker 3: I'm sorry.
Speaker 1: And for the record, notes are totally rocks.
Speaker 2: Ok Okay, no, I'm not disagreeing there. So the biologist
Speaker 2: who's trying to keep up with the physicists here, all right,
Speaker 2: So it sounded to me like you were saying, quantum
Speaker 2: mechanics and general relativity can be reconciled if you just.
Speaker 3: Divide by the right terms.
Speaker 2: I was under the impression that they describe completely different
Speaker 2: phenomena and kind of don't work together at all.
Speaker 4: It's too quick to say that they can easily be combined,
Speaker 4: but it's also too quick to say that they cannot
Speaker 4: be combined. So, first of all, indeed, are there regimes
Speaker 4: of interest where the two theories should be combined. Because
Speaker 4: usually gravity we think of very large things. Gravity is
Speaker 4: so weak that in order to see it you need
Speaker 4: to have large objects, massive objects. You know, the Earth
Speaker 4: is pretty big, and I can still lift my glass
Speaker 4: of water from my table, meaning that, you know, the
Speaker 4: electromagnetic forces in my body are stronger than the gravity
Speaker 4: of the full Earth. So gravity is weak and things
Speaker 4: need to be big to be able to see it.
Speaker 4: There's another option, if things are dense enough, you know,
Speaker 4: imagine taking the Earth and compressing it into the size
Speaker 4: of my water cup, Okay, and then even compressing it
Speaker 4: more so, then of course I will get into a
Speaker 4: regime where you say, well, you know, it becomes very small,
Speaker 4: and then the theory of econom mechanics becomes important. Yet
Speaker 4: also gravity becomes strong. And you can ask do we
Speaker 4: know of such regimes? And we do. I would say
Speaker 4: it's the most important regime for all of physics. It's
Speaker 4: the early universe. So if we go back in time
Speaker 4: and we look with our telescopes, so looking with the
Speaker 4: telescope means that you look back into the you look
Speaker 4: into the past, you see that the universe was denser.
Speaker 4: And if we just follow our classical equations, it actually
Speaker 4: tells us that the density will go to infinity, which
Speaker 4: is of course not true, but it isn't in the
Speaker 4: cation that in the very early universe, you know, everything
Speaker 4: was very tiny, so quantum mechanics was absolutely important and
Speaker 4: gravity was huge, so we need a theory of quantum gravity.
Speaker 4: One other example that we have already measured are black holes.
Speaker 4: Some black holes have always been a sort of a
Speaker 4: theoretical invention, but they're not anymore. We have seen them.
Speaker 4: They're out there, okay. And what you sometimes wrongly hear
Speaker 4: when people you know, talk about signs in a for
Speaker 4: the for the bigger public, they would tell you that
Speaker 4: black holes, for sure are objects were Quantum gravity is
Speaker 4: important because gravity is strong near a black hole. That's
Speaker 4: actually not entirely correct. If you look at a big
Speaker 4: black hole, for instance, a black hole in the in
Speaker 4: the middle of our galaxy, the gravitational parts that the
Speaker 4: horizon of that black hole is big, but it's not
Speaker 4: ridiculously big, okay, And the bigger a black hole, the
Speaker 4: weaker gravity is at the horizon of a black holes.
Speaker 1: It's kind of entry intuitive a bit is that because
Speaker 1: you're further from the.
Speaker 4: Center exactly, and it's also because the density of the
Speaker 4: black hole goes down as the black hole grows like
Speaker 4: the black hole. I think, if I'm not mistaken, you
Speaker 4: can always fact check this. But I think the black
Speaker 4: hole in the center of our galaxy as a density
Speaker 4: compared to water. So it's wrong to say that for
Speaker 4: sure quantum mechanics will be important near the horizon of
Speaker 4: a black hole. But what we are pretty convinced of
Speaker 4: is that if you would jump in a black hole,
Speaker 4: we don't know what's there. But it cannot be classical
Speaker 4: physics anymore because at some point the classical equations tell
Speaker 4: you rubbish. They tell you things which are impossible, so
Speaker 4: we know the classical theory has to break down. So
Speaker 4: the assumption is that just as in the early universe,
Speaker 4: in the very center of a black hole, there's also
Speaker 4: quantum mechanics and gravity at play at the same time.
Speaker 1: So I want to get back to what you said
Speaker 1: about things breaking down, but in a minute. First, I
Speaker 1: want to focus on this question of quantum mechanics and
Speaker 1: gravity at play at the same time. So you told
Speaker 1: us earlier that general relativity, or we can call it gravity,
Speaker 1: describes usually big things, and quantum mechanics usually describe small things.
Speaker 1: And now you're saying that at the beginning of the
Speaker 1: universe and inside black holes, we think both of those
Speaker 1: are relevant. And that's why we need a unified theory,
Speaker 1: because we need some way to describe that and to
Speaker 1: disagree to conflict. Why is there a conflict and their predictions.
Speaker 1: Couldn't it just be that they make the same prediction
Speaker 1: for what happens in that scenario. Couldn't it just be beautiful,
Speaker 1: a fortunate harmony among the theories.
Speaker 4: How what's the quickest way to explain? So let me
Speaker 4: give you an example that I hope more people know,
Speaker 4: maybe even from high school or first year of university. Say, okay,
Speaker 4: think of an electric field. So you have a charge particle,
Speaker 4: and a charge particle is surrounded by an electric field
Speaker 4: that it sources itself. So when you look at it classically,
Speaker 4: and when you think of a particle classically, it means
Speaker 4: that a particle is a point. And maybe if people
Speaker 4: remember this still, there was this formula that it said
Speaker 4: that the strength of this electric field went like a
Speaker 4: negative power of the distance from the particle. Say, you know,
Speaker 4: take it's one over our squared. That's actually that the force.
Speaker 4: Then you find that this force becomes infinitely big as
Speaker 4: you approach the particle. Okay, it's the same for the energy.
Speaker 4: The total energy carried by that particle would be infinite.
Speaker 4: And we know that can that cannot be correct. And
Speaker 4: then we have learned later on one hundred years ago,
Speaker 4: when people understood the quantum theory of charged particles, there
Speaker 4: was nothing going infinite. Things were just super well behaved,
Speaker 4: numbers were finite. Nothing weird was happening. And gravity is
Speaker 4: that sounds completely analogous, right. Even the formula for the
Speaker 4: force of gravity is almost the same as a formula
Speaker 4: for the Kulan force. Both can give you infinities. And
Speaker 4: for the cool on force, we learned that that infinity
Speaker 4: is gone when you treat it quantum mechanically.
Speaker 2: Both general relativity and quantum mechanics at some point start
Speaker 2: giving you infinities that make no sense when you push
Speaker 2: them to their extremes.
Speaker 4: So quantum mechanics doesn't give you infinities, but the classical
Speaker 4: theory does, so relativity does. Yeah. So then the question is,
Speaker 4: if you treat relativity in a quantum mechanical way, would
Speaker 4: you get sensible numbers? And do you Well, that's a
Speaker 4: good question, so yeah, to be able to answer it,
Speaker 4: I should I should tell you this is the theory
Speaker 4: that of quantum gravity. And to say that something is
Speaker 4: the theory of quantum gravity, I mean you can write
Speaker 4: down a theory which is extremely hard, but imagine you
Speaker 4: succeed and people succeeded. You don't know whether it's the
Speaker 4: only option, right, so you need to Normally you test
Speaker 4: the theory. And the problem is that to go out
Speaker 4: and test it requires you to look for these, you know,
Speaker 4: places where gravity is so strong, and I guess none
Speaker 4: of us wants to jump in a black hole. You could,
Speaker 4: but you know, you could test it, but unfortunately you
Speaker 4: would not be able to tell anymore to anybody else
Speaker 4: because you cannot escape from the black hole. Right, So
Speaker 4: nature is playing a very mean trick on us. It
Speaker 4: seems that humanity, in order to test the theory of
Speaker 4: quantum bravity, it is forced to do something that kills you.
Speaker 4: You know, this actually goes under the name of censorship,
Speaker 4: cosmic censorship. It's actually something quite serious in the physics community.
Speaker 4: Gravity works such that you could actually just see quantum bravity. Unfortunately,
Speaker 4: there's always what we call a cosmic horizon preventing you
Speaker 4: to see it. Either it's the horizon of a black hole,
Speaker 4: or you have to go back in time. But if
Speaker 4: you take your telescope, it's actually impossible to directly look
Speaker 4: at the Big Bang. So that's kind of mean. Otherwise
Speaker 4: you could just observe it.
Speaker 3: Yeah, it's frustrating.
Speaker 4: Religious, I would say God is playing you know, is
Speaker 4: an evil person, so to speak.
Speaker 1: Yeah, I also believe in cause free speech. I think
Speaker 1: the universe should be free to tell us how it works.
Speaker 1: And I'm bummed about all this censorship. Okay, I have
Speaker 1: a lot more questions, but first let's take a quick
Speaker 1: break and let our brains rest a moment. Okay, we're
Speaker 1: back and we are talking to Thomas Van Riet, a
Speaker 1: self proclaimed string theorist, about how strength theory works and
Speaker 1: why it's so pretty. So let's go back to this
Speaker 1: question of infinities. You said just a moment ago that
Speaker 1: it's hard, and we hear this a lot. Quantum gravity
Speaker 1: is hard. It's the hardest problem. And you're telling us
Speaker 1: that we have general relativity, which works beautifully outside of
Speaker 1: event horizons, and after some critical density in the universe.
Speaker 1: We have quantum mechanics, which works beautifully and very effectively
Speaker 1: for very small things and very high energies. Why is
Speaker 1: it hard to bring these two things together? What is
Speaker 1: the challenge? I mean, We've made a quantum version of electromagnetism,
Speaker 1: We've made a quantum version of the nuclear forces. Like,
Speaker 1: why is it so hard to take gravity and make
Speaker 1: it quantum?
Speaker 4: It is hard for two reasons. Maybe they're more, but
Speaker 4: there are two that are very sort of prominent. So
Speaker 4: one of them technically goes under the name that the
Speaker 4: theory of gravity is non renormalizable. We can come back
Speaker 4: to that it has to do with infinities of sorts.
Speaker 4: But actually we have some experience with non renormalizable theories
Speaker 4: in the past and resolve them. But still it usually
Speaker 4: means bad.
Speaker 1: It's tough.
Speaker 4: That's already what it tells you. The second thing is
Speaker 4: that gravity, at the same time is a theory or
Speaker 4: a classical theory of space and time, a theory of
Speaker 4: the background. Okay, so in quantum mechanics, the way the
Speaker 4: loss of quantum mechanics are formulated is that you assume
Speaker 4: that this background is fixed. What does it mean in
Speaker 4: practice is that you have two For instance, in let
Speaker 4: me try to give a good example that doesn't sound
Speaker 4: too abstract. In quantum mechanics, you use the notion of
Speaker 4: two points in space time, whether they're what we call
Speaker 4: costly connected, whether they can talk to each other, whether
Speaker 4: you know you can send a lightweight from one point
Speaker 4: to the other.
Speaker 1: You're talking about light counes.
Speaker 4: Exactly, but somewhere maybe less technical. Imagine that right now
Speaker 4: there are people I don't know, ten kilometers away from us.
Speaker 4: If we want to communicate with them, we cannot do
Speaker 4: it right now. We can do it in a you know,
Speaker 4: a little bit of time, the little time it takes
Speaker 4: to send the light way. But so the two points
Speaker 4: that are us here right now, and then people a
Speaker 4: little bit away, we are what you call disconnected. But
Speaker 4: imagine now that you have a theory of space and
Speaker 4: time where space and time are globally that's what relativity
Speaker 4: tells you. Then maybe that that whole notion changes, right
Speaker 4: as a quantum mechanics is telling you that things fluck
Speaker 4: to it. Things are very wobbly at a small scale.
Speaker 4: They're so lobbly that maybe you know in your equations
Speaker 4: what you thought are disconnected points, maybe they're not disconnected
Speaker 4: anymore because you're wobbling your background that is telling you
Speaker 4: that it's connected or not, And that's what makes it
Speaker 4: kind of annoying.
Speaker 2: Daniel, is this going back to our map analogy that
Speaker 2: we keep bringing up on the show or is this
Speaker 2: something different?
Speaker 1: This is the same, Yeah, that's exactly right. In GR,
Speaker 1: we have the concept of distances which are not fixed, right,
Speaker 1: which can change. Whereas you're saying, in quantum mechanics, this
Speaker 1: is essentially the background. So quantum mechanics is assuming that
Speaker 1: there's a stage on which everything is happening, and GR
Speaker 1: is like the theory of that stage, and it's changing
Speaker 1: underneath it. But what I don't understand is why that
Speaker 1: makes it hard, Like can't we do quantum mechanics on
Speaker 1: curved space? You know, you can think about your fields
Speaker 1: and my quantum mechanical view of space time is like, yeah,
Speaker 1: you have this backdrop and you put fields on top
Speaker 1: of it, and then you do the physics of the
Speaker 1: fields and stuff is propagating. Is it hard to do
Speaker 1: that quantum mechanical field theory in a curved space time?
Speaker 1: Having people been able to do that? What's so hard
Speaker 1: about that?
Speaker 4: Very good? So indeed, relativity can curve space time, and
Speaker 4: then you need to formulate quantum mechanics on curved space.
Speaker 4: That is not easy, but that people have done for sure.
Speaker 4: One example why that is not so easy is that
Speaker 4: quantum mechanics tells you there has to be a global
Speaker 4: time direction. Things move forward or backward in time. On
Speaker 4: a curved space time, what you thought was time can
Speaker 4: actually at some point become space or vice versa. And
Speaker 4: a famous example is a black hole. Imagine you approach
Speaker 4: a black hole, you jump through the horizon. What do
Speaker 4: you know is that you have to move forward to
Speaker 4: the singularity. But you see that means that that spatial
Speaker 4: that action became time because what is time for us?
Speaker 4: Time is the only dimension in which we cannot stop
Speaker 4: moving forward. I can decide now to sit on my chair,
Speaker 4: but I cannot decide to move backward in time or
Speaker 4: be still in time. I always have to move forward.
Speaker 4: So the fact that once you pass the black hole horizon,
Speaker 4: you're moving forward in time, forward towards similarity me is
Speaker 4: that that forward direction became time, and time became actually
Speaker 4: a spatial direction. So for quantum mechanics, for shredding equation,
Speaker 4: that's pretty annoying, but we learned how to deal with it,
Speaker 4: and dealing with it gives you this amazing phenomenon, like
Speaker 4: talking evaporation of black hoves. But what I said about
Speaker 4: quantum gravity is still something different because in quantum gravity,
Speaker 4: it's that curve background that is it self fluctuating under
Speaker 4: the confluctuations. Right, So there's no problem doing quantum mechanics
Speaker 4: on a curve background. It's just a bit more complicated
Speaker 4: because of the problem I told you. But now the
Speaker 4: background itself should become dynamically in equalum theory, so that
Speaker 4: your standard sharting equation is not well formulated to deal
Speaker 4: with the fact that the background itself is the thing
Speaker 4: that is part of the theory.
Speaker 1: I see. So you can do quantum mechanics on flat space,
Speaker 1: that's easy. You can do quantum mechanics when space gets curved.
Speaker 1: That's a little bit more technical, but people with big
Speaker 1: brains to figure that out. But having the space itself
Speaker 1: respond to the quantum mechanics, to have it all be
Speaker 1: dynamical and link together and be harmonious, to have this
Speaker 1: back and forth where energy is telling space how to
Speaker 1: bend and space is telling matter how to move, that
Speaker 1: is too technical for people to have figured out, or
Speaker 1: that's the challenge.
Speaker 4: That's a challenge ship absolutely.
Speaker 1: And this I think also connects to the other comments
Speaker 1: you made about renormalizable theories, which I think is worth
Speaker 1: digging into for a minute because it connects to the
Speaker 1: example you talked about a moment ago about an electron
Speaker 1: having apparently infinite charge or apparently infinite energy. Right. If
Speaker 1: you take an electrons charge and you look at it
Speaker 1: from a distance, it appears to have charge of negative one. Right,
Speaker 1: But as you say, an electron is surrounded by its
Speaker 1: feel and that field you can think of as a
Speaker 1: cloud of potential particles. And so if you actually think
Speaker 1: about what the charge of the electron is that we measure.
Speaker 1: It's the charge of the electrons surrounded by the cloud, right,
Speaker 1: And as you penetrate deeper into the cloud, you measure
Speaker 1: a more and more negative charge. And then that charge
Speaker 1: if you get all the way through the cloud to
Speaker 1: the electron, the charge apparently becomes negative infinity, which is
Speaker 1: crazy and bonkers and unphysical. And so you were talking
Speaker 1: earlier about renormalizable theories and how we've managed to patch
Speaker 1: this up with quantum mechanics. Can you say a few
Speaker 1: words about what it means to renormalize the theory? How
Speaker 1: do you get rid of an infinity in the theory?
Speaker 1: How do you solve that kind of problem where you're like,
Speaker 1: hold on a second, electrons can't have negative infinity charge?
Speaker 1: How do you solve that? What is renormalizablainy I just.
Speaker 2: Clarify real quick so that the biologist and me wants
Speaker 2: to confirm. So when you guys say you're getting infinities,
Speaker 2: that's just a fancy way of saying we're wrong.
Speaker 3: Like it's just this is not working, right, Okay, got
Speaker 3: it all right?
Speaker 1: Yes?
Speaker 4: Absolutely absolutely. To be honest, I think to explain to
Speaker 4: normalizability that the story of the infiniti This is what
Speaker 4: usually is told. Is I think leading people astray. It's
Speaker 4: not the right way of explaining it. May I try
Speaker 4: to explain it differently, but please go ahead of It's okay. Yeah,
Speaker 4: maybe it's good to go back to. You know what
Speaker 4: Newton got famous for, you know his theory, and what
Speaker 4: is his theory saying? Maybe people remember that. You know,
Speaker 4: there's an equation that he got famous for, which is
Speaker 4: called ethical en times a, which is essentially telling you
Speaker 4: that a force on a particle equals the mass of
Speaker 4: the particle times the acceleration that the particle is undergoing
Speaker 4: because of the force. So you can ask yourself, is
Speaker 4: this really a lot? A lot of physics? Means that
Speaker 4: you suddenly there are three things you know and you
Speaker 4: actually found a connection between them. I would say, isn't
Speaker 4: this a definition? The Newton just defined the workforce by
Speaker 4: saying it's en times a. It looks like that. But
Speaker 4: there's another famous example that you know in high school
Speaker 4: you learned Olmslow and usually you call it you say
Speaker 4: that resistance is voltage divided by current. See, that's not
Speaker 4: a lot. That's the definition of resistance. There's no information
Speaker 4: in that equation, but a definition on was successful because
Speaker 4: he said this R is a constant. That's that's the law.
Speaker 4: R is a concept, and then it becomes something with
Speaker 4: predictive power. Namely, I measure the voltage over a resistant
Speaker 4: over a resistance, and then I know the current that
Speaker 4: goes through it. But that's only because my real equation
Speaker 4: is art equal the concept. So what's the equation of newtant?
Speaker 4: Newton's idea was only successful because he essentially wanted to
Speaker 4: tell us this F is universal. Imagine the way an
Speaker 4: apple falls under this F whether how it falls in
Speaker 4: Cambridge or in China, it will be the same formula.
Speaker 4: So that means that once you have the formula for
Speaker 4: the F and you say it's true all over the universe,
Speaker 4: it becomes very strong the prediction. So Newton and his
Speaker 4: program was having a lot of predictive power by you know,
Speaker 4: moving around in the universe. He found something true all
Speaker 4: over the universe. Okay, so this is the same with normalizability.
Speaker 4: So instead of moving around from the left to the right, up, down,
Speaker 4: whatever future past, renormalizability has to do with zooming in
Speaker 4: and zooming out. If I have an equation and I
Speaker 4: want you to have predictive power. It has to tell
Speaker 4: me what also happens when I zoom in or I
Speaker 4: zoom out. Okay, zoom, let us think of zooming in.
Speaker 4: That's what really the problem lies. It means that I
Speaker 4: go to very small length skills. I want a theory
Speaker 4: which gives me a single equation with all the constants
Speaker 4: known and measured, so I can tell you what happens
Speaker 4: at small distance skills when I zoom in. A non
Speaker 4: renormalizable theory, it doesn't give you infinities. People should stop
Speaker 4: saying that they give you completely finite numbers after some
Speaker 4: mathematical trickery. But what it does is that the more
Speaker 4: you zoom in each time, the equation gains another constant
Speaker 4: that we know the value, and we have to go
Speaker 4: out in nature nature and measure it. Right. So imagine
Speaker 4: you want to somebody asking me, okay, Thomas, what happens
Speaker 4: in a gravitational field at you know, a micrometer. I say,
Speaker 4: oh my god, I have to you know already maybe
Speaker 4: correct Neutant's law for quantum gravity. And I say, yeah,
Speaker 4: there's an extra constant in the equation. It's not you know,
Speaker 4: the force is not one over our square. There's maybe
Speaker 4: one over our cube, but it's not one. It will
Speaker 4: be some number multiplying the equation. Okay, I go out
Speaker 4: in nature and measure it. Good. I have the number,
Speaker 4: and as somebody wants to go ten times smaller or
Speaker 4: twenty times smaller, suddenly a term which goes like one
Speaker 4: R to the power for becomes important. You need to
Speaker 4: know the coefficient of the term. I again have to
Speaker 4: do a measure. So you see I don't have predictive power.
Speaker 4: That's the definition of a non revisable theory. It means
Speaker 4: that you know, the smaller you get, the more constant
Speaker 4: theory has to be more precise, but it cannot predict
Speaker 4: what the constants are, whereas renormalizable theory it says, hey,
Speaker 4: I don't need any new constant guys. I mean, I
Speaker 4: can tell you with the computation on what you know,
Speaker 4: how the theory behaves at the smallest length skills. So unfortunately,
Speaker 4: the way historically this came about, and that's where the
Speaker 4: word nenormalizable comes from, is that you know, we were
Speaker 4: getting infinities and then we found we always say a
Speaker 4: mathematical treat but in fact it's a physics treat to
Speaker 4: get rid of them. But you see, that's that's not
Speaker 4: the essential part. The essential part is whether the theory
Speaker 4: is predictive, whether there's only a few constants that you
Speaker 4: can get out, go out and measure, or whether you
Speaker 4: need an infinite amount of consonts if I want to
Speaker 4: get infinitely small. And so if we take Einstein's theory
Speaker 4: classical theory of gravity, we apply our usual techniques of
Speaker 4: quantum theory, we find that it's non renormalizable, not meaning
Speaker 4: that it gives you infinities. This is actually, I think
Speaker 4: a bad explanation. It gives you too many consonants that
Speaker 4: we don't know what they are, and we will have
Speaker 4: to measure.
Speaker 1: I see. So a renormalizable theory, you can say I
Speaker 1: don't really know what's going on inside the electron. Maybe
Speaker 1: there's other particles, maybe not, But I can measure the
Speaker 1: charge and I can move on, and I can say
Speaker 1: it's all wrapped up in this number. They'll charge it
Speaker 1: the electron. And as long as I can make a
Speaker 1: finite number of measurements, like I don't have to measure
Speaker 1: an infant number of properties in the electron, then I
Speaker 1: have a theory. I can use because that can make
Speaker 1: a finite number of measurements in a finite amount of time.
Speaker 1: So a non renormalizable theory, you're saying, is one where
Speaker 1: you can't ever capture all those details in a single
Speaker 1: number or two numbers, or even a finite number of numbers.
Speaker 1: You'd need to measure an infinite number of parameters to
Speaker 1: have a theory that you can actually use to make calculations.
Speaker 1: But you said a minute ago that we have other
Speaker 1: non renormalizable theories. I think, for example, quantum chromodynamics, it's
Speaker 1: non renormalizable, and we've made that work. I mean, I
Speaker 1: know it's a headache, but we've made it work. What
Speaker 1: is it about gravity that's so special that we can't
Speaker 1: use our non renormalizable fancy clever tricks to get quantum
Speaker 1: gravity to work?
Speaker 4: Right, So you're saying humanity has dealt normalizable theories, made
Speaker 4: them to work, and I can tell you what the
Speaker 4: problem is. So I guess don't forget that before we
Speaker 4: already said that gravity had two problems for it to
Speaker 4: be hard to quantize. Non rymalizability was one of them.
Speaker 4: So there's still the other one. So that's part of
Speaker 4: my answer, But it's still I believe it's very different.
Speaker 4: Like the other part of my answer will be the following.
Speaker 4: Usually what happens in physics. Actually, every instance we have
Speaker 4: seen so far where the theory was non renormalizable, we
Speaker 4: actually cured it by realizing that we didn't have all
Speaker 4: what we usually call degrees of freedom. Okay, so what
Speaker 4: does that mean? So I assume it I go to
Speaker 4: small distances, and I always assume that. You know, if
Speaker 4: I have the theory of the electron, there's only the electron.
Speaker 4: Say well, maybe they're very massive particles out there which
Speaker 4: require a lot of energy to be created. In physics,
Speaker 4: having a lot of energy is the same as going
Speaker 4: to very small distances. So all non renormalizable theories we
Speaker 4: have encountered were always made renormalizable by realizing that we
Speaker 4: didn't take into account fluctuation fields particles that were just
Speaker 4: very massive so that we didn't measure them yet.
Speaker 3: That's real quick.
Speaker 2: Saying you needed more degrees of freedom means there was
Speaker 2: something else that wasn't included in the equation that needed
Speaker 2: to be.
Speaker 4: The exactly absolutely absolutely, And then you see, oh this
Speaker 4: is nice, my theory becomes mathematically normalizable. But then actually
Speaker 4: we went out in nichere we found technologies to increase
Speaker 4: our energy in our experiment, and then we saw those
Speaker 4: particles that we predicted mathematically because we wanted the theory
Speaker 4: to prenormalizable. Okay, I think that's extremely beautiful. Like you
Speaker 4: you do something on mathematical clouds, it predicts new particles
Speaker 4: for it to work out, and there you measure them. Okay.
Speaker 4: And here's the funny thing with gravity. What string theories
Speaker 4: will typically tell you. What I think more and more
Speaker 4: people are leaning towards it, is that if you want
Speaker 4: to make gravity normalizable, it looks like you the infinite
Speaker 4: amount of particles with ever increasing energy. And that sounds
Speaker 4: super bad when you say that first, because you're like,
Speaker 4: oh my god, infinite amount of particles to solve your problem.
Speaker 4: It's like measuring an infinite number of concepts. You're not
Speaker 4: better off. Okay, So of course now I'm going to
Speaker 4: sell string theory here. No, what is so beautiful is
Speaker 4: that it's infinite tower of particles groups together in the
Speaker 4: motion of a string. It just meant that what we
Speaker 4: thought were particles, no, it was just a single object.
Speaker 4: There's no tower. It's just a string that can vibrate
Speaker 4: in different ways. So there's a lot of structure in
Speaker 4: that infant amount of particles that you need to invoke
Speaker 4: together innormalizable theory. Yeah, otherwise it looks very bad, like
Speaker 4: every time you take in a new particle you find
Speaker 4: that renormalizability still requires a new one, and you think,
Speaker 4: oh my god, you guys are just you know, in
Speaker 4: an never ending street of problems. No, we see that
Speaker 4: every single particle we have to add as exactly properties
Speaker 4: that we could have predicted from the previous one. So
Speaker 4: there's a beautiful structure, and what looks like an infinite
Speaker 4: tower of particles just becomes a single stringing object with
Speaker 4: almost no constant associated to it.
Speaker 1: So there, you just said the word beautiful. What is
Speaker 1: beautiful about that? Is it? Because wow, this is a
Speaker 1: hard problem, and now have a solution. Is it like
Speaker 1: my headache is gone? Or is there something objectively beautiful
Speaker 1: about this particular solution?
Speaker 2: Can I go back to a real quick question and
Speaker 2: then can we move to beauty because I don't want
Speaker 2: to miss my chance to understand this because I'm actually
Speaker 2: really following everything.
Speaker 3: I'm excited.
Speaker 2: Okay, So instead of needing to measure an infinite number
Speaker 2: of constants? Can we measure that string? Do we know
Speaker 2: how to measure the string? Does that make our situation
Speaker 2: any better?
Speaker 4: I'm going to be honest in practice. No, but this
Speaker 4: week we could have predicted you in advance. Okay, So
Speaker 4: this has nothing to do with string theory. I just
Speaker 4: told you that the regime ware gravity and quantum mechanics
Speaker 4: are relevant. Is either we have to jump through a
Speaker 4: black hole, which is not nice as an experience, or
Speaker 4: we somehow have to be able to move back in
Speaker 4: time to the Big Bang, or people are able to
Speaker 4: build you know, galaxy sized accelerated. That is a true
Speaker 4: statement independent of what the theory of quantum gravity is.
Speaker 4: It just you know, you predict what is the energy
Speaker 4: density needed to see those effects. Of course, one can
Speaker 4: be lucky and some effects of the highest energy densities
Speaker 4: or the smallest lendsciales can trickle you know how they
Speaker 4: say trickle down? Is it correct English? I don't know,
Speaker 4: but can leave an imprint on larger distances and smaller energies.
Speaker 4: Is if it's something that we're looking into, we are hoping,
Speaker 4: you know, I'm praying, but we don't know for sure,
Speaker 4: So I hope that explains a bit, yeah, and not
Speaker 4: to the beauty. It's I like the questions that I'm
Speaker 4: trying to find an analogy. Okay, so imagine I don't
Speaker 4: know whether this reminds you of the word beauty, but
Speaker 4: imagine you have a super complicated puzzive in front of you.
Speaker 4: I don't know, one billion pieces and you just don't
Speaker 4: know how to put them together, and suddenly you find
Speaker 4: two connecting and because you see two pieces that connect,
Speaker 4: you suddenly see the third piece lying there, and the
Speaker 4: more you put them together, suddenly it's just one structure
Speaker 4: that is like extremely simple. Okay. It's like imagine if
Speaker 4: a blackboard full of equations and you cannot solve them,
Speaker 4: and suddenly you realize that your equation was too complicate,
Speaker 4: that terms are dropping against each other, and you keep
Speaker 4: on canceling terms, and suddenly you have an equation left
Speaker 4: which is just one centimeter insights. You're like, oh my god,
Speaker 4: this is you know, this is amazing. That's the kind
Speaker 4: of beauty we're talking about that we think that renormalizing
Speaker 4: gravity is a nightmare. It gives you ugly theories. And
Speaker 4: then the first thing we try, which you know, just
Speaker 4: on mathematical grounds, and we get something that is in
Speaker 4: terms of the length of equations, is even smaller than
Speaker 4: any equation that we have had in the past. And
Speaker 4: that is what I think why so many people like it.
Speaker 4: And then the confusing part is that it's not because
Speaker 4: the size of the equation is small that it's easy
Speaker 4: to solve. It just means that it's very elegant, okay,
Speaker 4: in the sense that, for instance, there maybe elegance is
Speaker 4: better than beauty. The elegant thing of string theory is
Speaker 4: that they're no constants in the.
Speaker 1: Theory, no numbers at all.
Speaker 4: No numbers at all exactly. Any other theory non physics
Speaker 4: has to have a lot of numbers that you go
Speaker 4: out and measure. String theory doesn't have a number. Actually
Speaker 4: it only is one. It's the size of the string
Speaker 4: as variables.
Speaker 3: But no constants.
Speaker 2: Is that I'm having trouble imagining an equation with no
Speaker 2: numbers that's exactly correct.
Speaker 1: So it's like ex equals why not ex equals two
Speaker 1: point seven four times?
Speaker 4: Why right? Where I didn't know the two point seven
Speaker 4: I had to go out and measure it, all.
Speaker 2: Right, So I'm excited because this is the most that
Speaker 2: I've understood string theory in my.
Speaker 3: Life so far, but I could still use the brake.
Speaker 2: So let's go ahead, get some more coffee, a little
Speaker 2: bit more brain fuel, and we will be right back
Speaker 2: to talk more about string theory. All right, we are
Speaker 2: back with Thomas Van Reed. Let's jump back into string theory.
Speaker 2: So string theory we've discussed that it can help when
Speaker 2: you're in those really tiny little situations where you'd usually
Speaker 2: have to get a lot.
Speaker 3: More calculate, a lot more constant.
Speaker 2: Does it also work if you zoom out or is
Speaker 2: it just a theory for when you're super zoomed in?
Speaker 4: This is an excellent question. So that's where it gets hard.
Speaker 4: Surprisingly Okay, So when you zoom out in physics, it
Speaker 4: means okay, large distance also means low energy. And what
Speaker 4: is the hardest part of working with high energy theories
Speaker 4: like string theory, is to understand if I take the
Speaker 4: theory and I run into low energies like the energy
Speaker 4: densities that we like, have you know in your office, okay,
Speaker 4: then it's not you, and so it becomes very difficult
Speaker 4: to understand. So how would the world look like on
Speaker 4: this low density or large distances? I mean, string theory
Speaker 4: predicts a completely unique world. At small distances you see little,
Speaker 4: you know, vibrating strings behaving in a certain way. But
Speaker 4: then if you do mine it, it's not obvious what's
Speaker 4: going to happen. Okay, this is why we always say
Speaker 4: we have trouble or we are not sure whether we
Speaker 4: can reproduce the large the universe as we typically know.
Speaker 4: But this is not a problem of string theory. This
Speaker 4: is effect of all high energy theories. And maybe I
Speaker 4: can give an analogy. Okay, so imagine that you have
Speaker 4: a rocky landscape, hills, mountains, whatever, very very complicated, many valleys,
Speaker 4: and you have a football. But the football has a
Speaker 4: lot of energy, you know, so then it's like up
Speaker 4: there up the tops of the mountains, right because it
Speaker 4: just says lots of alosity. It's moving through those valleys
Speaker 4: and it's just all the way up. But then you
Speaker 4: know the restriction and the velocity is going down. Well,
Speaker 4: if I have many values, I don't know where the
Speaker 4: ball is going to roll down and where the value
Speaker 4: is that it's going to end. That's the problem we have. Well,
Speaker 4: I don't think it's a problem with theory, it's just
Speaker 4: it's typical. Actually, even the standard model has this property
Speaker 4: that this difficulty.
Speaker 1: That's a great explanation. Thank you. Can you circle back
Speaker 1: and help us understand more specifically how string theory solves
Speaker 1: these problems of quantum gravity. You talked about how howing
Speaker 1: string replaces the infinite number of parameters you might have
Speaker 1: to measure how does it solve the problem of quantum
Speaker 1: mechanics and general relativity working together on this dynamic space time.
Speaker 4: So first, I think it's very important to have a disclaimer.
Speaker 4: We don't have the full theory, right, so whether it
Speaker 4: solves all problems that we know quantum gravity we do
Speaker 4: not know. I have to be honest on this. I
Speaker 4: would say that from a mathematical point of view, the
Speaker 4: way it solves this is by not quantizing gravity. That's
Speaker 4: very strange to say what it is. That's why string
Speaker 4: theory did so quantizing. When we use the word quantizing,
Speaker 4: it means that we take our classical theory it has
Speaker 4: a certain amount of variables like electromagnetism has the electric
Speaker 4: field and has the electron field, and always said that's quantizing.
Speaker 4: Let's make it quantum mechanical. So you could say, well,
Speaker 4: relativity is what we call the metric field. That's a
Speaker 4: field that describes how space and time curve. Okay, and
Speaker 4: that's what people usually do. They say, Oh, we learned
Speaker 4: in history of quantized series. We take that classical what
Speaker 4: we call field and we turn it into a quantom
Speaker 4: mechanical field.
Speaker 1: But how do you do that? How do you quantize
Speaker 1: the theory? You don't just like tap your magic wand
Speaker 1: down and say and now you're quantum mechanical.
Speaker 4: Oh my god, this is tough. So mathematically you would
Speaker 4: say it turned a field into an operator, but that
Speaker 4: probably means a little two people listening, here's an attempt.
Speaker 4: I'm not sure it's even good. Quantum mechanics tells you
Speaker 4: that things there are only probabilities, right, what you thought
Speaker 4: as a particle. We sometly say it so a way,
Speaker 4: it's not a very good word. What we have in
Speaker 4: said is a probability distribution of where the party consulute.
Speaker 4: So objects are turned into probability distributions. That's quantizing a theory.
Speaker 4: And then there's a certain equation for the probability distribution.
Speaker 4: But more mathematically, it means that you take a field
Speaker 4: and you make it into an operator.
Speaker 1: No, that's a great way to think about it. A
Speaker 1: classical theory says that everything is specified and there is
Speaker 1: infinite information even if you don't have it, whereas a
Speaker 1: quantum theory leaves some uncertainty and says, well, this isn't determined.
Speaker 1: Maybe the electron is here, maybe the electron is there.
Speaker 1: Maybe the field is this value, maybe it has that value.
Speaker 1: And for those of you playing along at home, an
Speaker 1: operator here is making a measurement. It's like applying something
Speaker 1: to it and getting a result out. And so that's
Speaker 1: a crucial element of quant mechanics. Okay, so now we're
Speaker 1: going to try to quantize space time and you say,
Speaker 1: we can think of the metric as a field. The
Speaker 1: metric is like how much curvature there is at every
Speaker 1: point in space. So if we think of like the
Speaker 1: curvature and space as a field, why is it hard
Speaker 1: to quantize that. Why can't we think of that as like, well,
Speaker 1: maybe the curvature is this value, maybe the curvature is
Speaker 1: that value. Why can't we just think of that probabilistically.
Speaker 4: That was the problem of the problems we talked about before.
Speaker 4: Then you run into the problem of non renormalizability. If
Speaker 4: you do it that way, or you're run into the
Speaker 4: problem that you know, it's a background itself that has
Speaker 4: to become a probability. So the formalism of Qunlem mechanics
Speaker 4: gets very confusing at that point. And as I said,
Speaker 4: we already knew that a theory that is non renormalizable
Speaker 4: means that you're not having the right degrees of freedom,
Speaker 4: you're missing information. So the way string theory went about
Speaker 4: is people that discovered it. We're not trying to quantize gravity.
Speaker 4: Let's let's be clear on this. Okay. They wanted to
Speaker 4: solve another puzzle, and for some reason, which is a
Speaker 4: long story by itself, they were interested into string like
Speaker 4: objects and how they move and how strings move quantum mechanically.
Speaker 4: So they do their computation and they suddenly see that
Speaker 4: the string can fluctuate in what we mathematically call a
Speaker 4: spin to field. If you don't know what a spin
Speaker 4: to field is, it's just a fancy way of saying
Speaker 4: it describes what I call the metric field. They just says,
Speaker 4: but that's strange. There's no space and time, and yet
Speaker 4: they found the structure, which is what they knew from relativity.
Speaker 4: And then they started to look into it deeper, and
Speaker 4: they wanted to understand the equations that that metric field obeyed,
Speaker 4: and they were completely surprised that, you know, they didn't
Speaker 4: ask for it. They found Insten's equations. So this is
Speaker 4: also what I call absolute beauty.
Speaker 1: Okay.
Speaker 4: Other approaches to quantographty they said, let me take insent
Speaker 4: equations for true, just have them. I said, also just
Speaker 4: dropped them down. I said, I didn't know why these are
Speaker 4: my equations. Okay, he didn't derive them. And so the
Speaker 4: other approaches to quanto gravity say, let me take those
Speaker 4: equations that you know, quantitize them. String theories did something else.
Speaker 4: They were looking at strings vibrating for a completely different reason.
Speaker 4: They not only recover Einstein's equations, the classical ones they predict,
Speaker 4: they literally predict them, but they immediately have them quanta mechanically,
Speaker 4: and it meant that they needed they have all these
Speaker 4: possible vibrations of the string. It's just one vibration that
Speaker 4: gives you this metric field, but the string can vibrate
Speaker 4: in so many other ways. And then suddenly it gave
Speaker 4: them other things. They knew, for instance, all the forces
Speaker 4: in nature. They come in two kinds. Okay, there's gravity.
Speaker 4: This is a separate guy. It's is described by this
Speaker 4: metric field. And the other forces are with a manematical
Speaker 4: term are gauge forces, young meals forces. Electromagnetism is an
Speaker 4: example of it, okay. And the two other are the
Speaker 4: nuclear forces, and they are all described by one equation
Speaker 4: which is called the Young Mules equation, and a special
Speaker 4: kind of young music equations that maybe people listening who
Speaker 4: had a little bit of a scientific, you know, education
Speaker 4: remember are what we call the maximal equations, which are
Speaker 4: the equations of electric and magnetic fields. But this is
Speaker 4: part of a general mathematical equation which is called the
Speaker 4: young music question, which also mathematicians study for completely different reasons.
Speaker 4: But guess what they were looking at the other modes
Speaker 4: of vibration of the street, and without asking for it,
Speaker 4: the young music quations appeared, and at that point people
Speaker 4: were like, my god, this is insane. Okay, So this
Speaker 4: is where all the hype came from, all right, from
Speaker 4: string theory, like the excitement of people all came from this.
Speaker 4: Not only do we you know, we get the classical
Speaker 4: theories we didn't ask for that we've gotten. So you
Speaker 4: get all advance, so to speak.
Speaker 1: So I'm getting a sense from you that the elegance
Speaker 1: of string theory comes from the sort of discovery that
Speaker 1: it answers questions simply, and sometimes it answers questions we
Speaker 1: weren't trying to answer. In that sense, it feels more
Speaker 1: like you're accidentally revealing a big chunk of truth rather
Speaker 1: than you're like laboriously putting together an over complicated answer
Speaker 1: that's just an invention in your mind. Is that the
Speaker 1: feeling here that we've like uncovered a vein of reality.
Speaker 4: Absolutely, And I think that's for all of science. Like
Speaker 4: I can imagine in biology, when you understood like gene
Speaker 4: structure and I suddenly realize how things work. It becomes
Speaker 4: so simple, Like I think the same evolution happened in biology.
Speaker 4: You have all this phenomena. At some point we learned
Speaker 4: about to sell and the smaller organisms, and all of
Speaker 4: these phenomena suddenly can be explained in a more microscopic way.
Speaker 4: So the theory becomes simpler. It became maybe I'm simplifying.
Speaker 4: I mean, I'm not the expert here, but I would
Speaker 4: say biology at some point became the theory of the cell,
Speaker 4: which is so much smaller and so much in a
Speaker 4: way simpler.
Speaker 1: So is it then about the theory itself or is
Speaker 1: it about the insights the theory gives you about how
Speaker 1: the universe works, or just the place where we are
Speaker 1: where we're like wollw We're frustrated by these problems for decades,
Speaker 1: and now finally the headache has gone away. I mean,
Speaker 1: can you look at the theory itself and say, this
Speaker 1: theory is beautiful? Is there a chance that we could
Speaker 1: have revealed the theory which feels truthful? But then you're like, actually,
Speaker 1: I don't like it. It's kind of ugly.
Speaker 4: Right. I guess different versions of beauty were felt at
Speaker 4: different stages, right, So the people first making these discoveries
Speaker 4: of seeing einscience equations and so on, when you read
Speaker 4: their biographies, they really are like they talk about it
Speaker 4: extremely emotional, like they could be crying because they saw
Speaker 4: that part of beauty. But my generation came later, so
Speaker 4: we kind of you say, you got used to it,
Speaker 4: you don't feel that beauty on it. The thing that
Speaker 4: strikes me is that when you first learn about Newton's
Speaker 4: second law, I don't think none of us feels what
Speaker 4: Newton felt at the time, and I think we are
Speaker 4: underestimating the emotions because one other things that I didn't
Speaker 4: tell you what Newton's second ploy is. It tells you
Speaker 4: about what we call the deterministic view of nature. Newton
Speaker 4: realized that this equation was telling you that, you know,
Speaker 4: I'm sitting on my chair and later on I will
Speaker 4: walk away. But according to Newton's equation, I don't have
Speaker 4: the choice. I have no freedom. There's no freedom of
Speaker 4: you know, there's no free will.
Speaker 3: Do the philosophers know you all have solved that problem.
Speaker 4: No, it's the classical theory, right, So, but physics is deterministic,
Speaker 4: so the according to physics. I think we should emphasize this.
Speaker 4: There is no free will in physics. Anybody that tells
Speaker 4: you that there is is wrong physics. As I don't
Speaker 4: say we have solved it. But there is no free
Speaker 4: will in physics, absolutely not. Free will is completely in
Speaker 4: contradiction with physics. Not the illusion of free will, but
Speaker 4: free will. But it's no way, there's no room in
Speaker 4: physics for free will, not in the usual notion of
Speaker 4: the word free will. Any I want to say that
Speaker 4: the beauty that we feel or now the students you know,
Speaker 4: which are younger than me. There are different versions of beauty.
Speaker 4: It's more where you start applying the theory. I can
Speaker 4: give you one example of a thing that I found
Speaker 4: beautiful is like you have equations with singularities, like these
Speaker 4: infinities we talked about, and string theory can do calculations
Speaker 4: and you see there's no infinity, And then you learn
Speaker 4: how string theory tells you that there's no infinity, and
Speaker 4: it does it in a very creative and funny way.
Speaker 4: There's often a picture like you can even see it literally,
Speaker 4: it's not an equation, it's not formal us. You can
Speaker 4: just see it, and that's kind of beautiful.
Speaker 2: Too bad the listeners can't see your face because you
Speaker 2: have the biggest grin on that You've had the whole
Speaker 2: interview explaining how beautiful it is.
Speaker 3: Like you're clearly getting a total kick out of this.
Speaker 3: It's it's awesome.
Speaker 1: I think it is really hard to put yourself in
Speaker 1: the minds of earlier generations to appreciate how big some
Speaker 1: of those steps forward were. Like, it seems pretty basic
Speaker 1: what Aristotle accomplished. You like, things fall down. I could
Speaker 1: have said that, but you know, to systematize the world
Speaker 1: at all. What was a big step forward. I think
Speaker 1: you're right that it's underappreciated. So what is it that
Speaker 1: we're underappreciating now in terms of strength theory. I mean,
Speaker 1: there's a lot of popular writing about strength theory, a
Speaker 1: lot of popular conceptions about it. But from somebody on
Speaker 1: the inside, what do you feel like is most often
Speaker 1: misunderstood or misrepresented about the nature of strength theory.
Speaker 4: I have to be careful, careful not to become too
Speaker 4: sort of drawn into the sociological discussion, but I feel
Speaker 4: I cannot not say it so string theory, say, thirty
Speaker 4: twenty years ago, when it's people discussed it in science outreach,
Speaker 4: it was only the one, okay, And now it's the opposite,
Speaker 4: And I think that the opposite went so far that
Speaker 4: it's completely misrepresenting the field.
Speaker 1: When you say the opposite, you mean like people being
Speaker 1: critical of string theory because it hasn't yet predicted some
Speaker 1: experiment and been proven.
Speaker 4: Writers is exactly, this is an example exactly. So they
Speaker 4: will tell you this, and then of course you have
Speaker 4: to tell them, because they don't tell you this, that
Speaker 4: this is true for any theory of quantogravity and We
Speaker 4: knew this in advance. We knew this before we started
Speaker 4: working on string theory with absolute certainty that if you
Speaker 4: have access to the smallest Lend skills, you can falcify
Speaker 4: a theory one from the other. Okay, string theory from
Speaker 4: the other examples. What is completely not obvious is that
Speaker 4: some of these high energy small distance effects have an
Speaker 4: imprint at larger distances. At a moment, we don't know,
Speaker 4: and we're actually looking into it. And this program has
Speaker 4: a name. I think it's super exciting. It's called the
Speaker 4: Swamp program, and it's where we try to look into
Speaker 4: that question. But at the moment we do not know.
Speaker 4: But as any other supposed alternative to string hearing is
Speaker 4: not even there at that stage where they can even
Speaker 4: ask this question, does my theory predict something at a
Speaker 4: bigger length scale, because normally you don't expect it to
Speaker 4: be the case? Right, So can I get away of
Speaker 4: not sending a student into a black hole to learn
Speaker 4: about pornography? We don't know.
Speaker 2: I mean, master students are expendable. You could send like
Speaker 2: four or five of them.
Speaker 4: Unfortunately they can't explains what they're what they're seeing, right,
Speaker 4: so they couldn't explain it. That's that's a Otherwise I would,
Speaker 4: you know, be interested in maybe jumping into a black
Speaker 4: hole just to see because if you jump into a
Speaker 4: big black hole, actually it doesn't need to be a
Speaker 4: painful experience. You can pass the horizon without you know,
Speaker 4: feeling it too much, and then you could actually see
Speaker 4: a singularity.
Speaker 1: You know.
Speaker 4: Interstellary is a little bit about this, right, you jump
Speaker 4: into a black hole. There's a movie about it. But yeah,
Speaker 4: so I think is my frustration that there is a
Speaker 4: back correction to the original hype. But the back correction
Speaker 4: especially you know now on social media, but also i'd
Speaker 4: say conventional science outreach. To give an example, I saw
Speaker 4: my children that at an age where they get interested
Speaker 4: in science and they start googling things. So I see
Speaker 4: their first hit on Google when they ask a question
Speaker 4: which is about fundamental physics, and the first hit that
Speaker 4: they have is criticism on stream. There it became so
Speaker 4: mainstream that this is the first thing you see, and
Speaker 4: that's not healthy anymore.
Speaker 1: Okay, So just to make sure I understand, you're saying
Speaker 1: it's fair to criticize string theory and say you haven't
Speaker 1: made a prediction, which can be verified, But all the
Speaker 1: theories of quantum gravity also had that issue that we
Speaker 1: can't go inside a black hole, And many theories of
Speaker 1: quantum gravity haven't even come together and coalesced and enough
Speaker 1: detail to make any predictions, not to mention ones that
Speaker 1: can be tested. So then let me wrap up by
Speaker 1: asking you a last question, which is about the truth
Speaker 1: of strength theory. I mean, you're excited about string theory
Speaker 1: because you think it's simple and it feels like a
Speaker 1: compelling potential answer to the question of like what really
Speaker 1: happening in the universe. So do you think, for example,
Speaker 1: in some hypothetical scenario where aliens arrive on Earth and
Speaker 1: they're very advanced scientifically, and we can figure out how
Speaker 1: to communicate with them, et cetera, et cetera, what do
Speaker 1: you think are the chances that alien physicists are doing
Speaker 1: string theory that they have also stumbled upon this explanation.
Speaker 2: Daniel always has to get aliens into the show at
Speaker 2: least once, and so here we go.
Speaker 4: We all owe aliens. Actually, Okay, I don't know whether
Speaker 4: my answer is of any meaning, but I would say
Speaker 4: they will discover string theory. I actually don't even doubt it.
Speaker 4: I'm one hundred percent convinced. And as to the question before,
Speaker 4: people tell you that a theory without predictions is not science,
Speaker 4: and I think we have to really step away from this.
Speaker 4: So in science are two things. They're observables and they're computables,
Speaker 4: especially in theoretical sciences, and what a theory has to
Speaker 4: get right are the computables. For instance, if I have
Speaker 4: a theory that can explain phenomena at large distances, but
Speaker 4: I look at small distances and the theory tells me
Speaker 4: that I can go back in time and kill my
Speaker 4: mother before I was born. I know that theory is nonsense,
Speaker 4: but I cannot make an experimental verification. But the theory
Speaker 4: is just nonsense. It's not logical. And the thing that
Speaker 4: people that the audience and you know, the greater public
Speaker 4: needs to understand, quantum gravity is so extremely constraining in
Speaker 4: terms of just logical consistency that you almost uniquely arrive
Speaker 4: at an answer. And that's where this is true science. Okay,
Speaker 4: despite not having access at a moment to an experiment
Speaker 4: to test it, you almost uniquely are pushed into a
Speaker 4: direction to solve this problem of non remortalizing. Now I'm
Speaker 4: selling it too much, But I hope you're understanding what
Speaker 4: I'm trying to say.
Speaker 2: I've read a couple of books on strength theory and
Speaker 2: never understood them, but I've totally understood our conversation today.
Speaker 3: So I'm this has been awesome.
Speaker 4: Happy to hear that it's awesome.
Speaker 1: And if aliens arrive and they don't do strength theory,
Speaker 1: maybe they can listen to this episode to get a
Speaker 1: primer on how strength theory works. Exactly, exactly, wonderful. Well,
Speaker 1: thank you very much for coming on the show and
Speaker 1: talking to us about the hard problem of quantum gravity
Speaker 1: and how string theory might be the solution. Thanks very much.
Speaker 4: It us a lot of fun. Thank you so much.
Speaker 2: Daniel and Kelly's Extraordinary Universe is produced by iHeartRadio.
Speaker 3: We would love to hear from you, We really would.
Speaker 1: We want to know what questions you have about this
Speaker 1: Extraordinary Universe.
Speaker 2: We want to know your thoughts on recent shows, suggestions
Speaker 2: for future shows. If you contact us, we will get back.
Speaker 1: To you really mean it. We answer every message. Email
Speaker 1: us at Questions at Danielankelly dot.
Speaker 3: Org, or you can find us on social media.
Speaker 2: We have accounts on x, Instagram, Blue Sky, and on
Speaker 2: all of those platforms.
Speaker 3: You can find us at D and K Universe.
Speaker 1: Don't be shy, write to us,