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0:00:01 Sean Carroll: Hello, everybody, and welcome to the Mindscape podcast. I’m your host, Sean Carroll. A few years ago, I was being interviewed on camera for a TV show, a documentary about physics, and we were talking about black holes and crazy things about quantum mechanics and some speculative ideas. And during a break, the director said to me, “I love doing shows on this kind of topic, but my brain always hurts. It’s just so hard to think about this stuff.” And I said immediately, “Now you know how I feel every day, that’s what I do for a living. When I’m not podcasting I’m thinking about theoretical physics and crazy ideas.”

0:00:37 SC: And physics works at the boundaries of what we know. Physics is driven by puzzles and mysteries, by things that we don’t yet understand. If we’re lucky, those puzzles come from experiments, we get some data that we can’t fit together that doesn’t quite make sense in our current conception, so we try to improve our theories. But sometimes we have two theories that individually make perfect sense, but don’t fit together. That is the case with perhaps the single biggest looming question in modern theoretical physics: How we reconcile quantum mechanics, our best theory of how the world works at a fundamental level, especially at the level of tiny particles in the subatomic realm, and gravity, the theory that is explained by Albert Einstein’s theory of general relativity, the idea that space and time are curved. What we want is to fit them together, to have a theory of quantum gravity, that somehow the world is fundamentally quantum mechanical and it gives rise to curved spacetime that we notice as gravity.

0:01:40 SC: The problem is, you try to do that using the techniques that work for other theories of physics, you quantize general relativity, and it doesn’t seem to work. So for decades now, physicists have been trying to ask how do we find a quantum theory of gravity. One of the world’s biggest experts in this field is today’s guest, Carlo Rovelli. Dr. Rovelli is a professor at, if I can say this correctly, the Centre de Physique Théorique de Luminy of Aix-Marseille University in France, although he’s originally Italian, as he will explain. Carlo was a bit of a rabble-rouser as a youth. He has that fiery Italian temperament, but these days he works on reconciling Albert Einstein’s general relativity with the principles of quantum mechanics. Along the way, Dr. Rovelli has written several books. His 2014 book, Seven Brief Lessons in Physics, was a runaway New York Times bestseller. He just has a book that came out called The Order of Time, on the nature of time, and therefore of course, spacetime, which is important for understanding quantum gravity. The latest news I heard was that the audio book version of The Order of Time is going to be read by the actor Benedict Cumberbatch. As a fellow book author who has to read his own books, this makes me very jealous.

0:02:58 SC: But what we’ll talk about with Dr. Rovelli is the subject of quantum gravity. As I alluded to, given some thing that we think we understand, electromagnetism or the nuclear forces, there’s a cookbook procedure for taking that classical theory and quantizing it. It breaks down when you try to do this for gravity. So what do you do? This is a big topic in theoretical physics today. The most popular answer, as you may have heard, is something called string theory. It grew directly out of particle physics, just by saying that we should replace the idea of little point particles, being the most fundamental ingredients of nature, with little pieces of vibrating string. That sounds like a crazy, speculative idea, but it leads you directly to predict the existence of gravity. So who knows? It seems to be very promising, but it hasn’t exactly, let us say, to be very modest about it, explained all the things you want to explain about the universe.

0:03:55 SC: So Dr. Rovelli is a champion of a different perspective called loop quantum gravity. He was one of the originators of this idea. It’s relatively straightforward in its approach. It says take general relativity, we know what that is, Einstein gave it to us. And maybe the reason why it fails when we try to straightforwardly quantize it, is just that we’re not quite clever enough. Maybe there is a not quite straightforward way of taking general relativity and plugging it into the rules of quantum mechanics.

0:04:23 SC: So loop quantum gravity right now is sort of a plucky minority point of view, it’s not nearly as popular as string theory is, but to be very fair, we don’t know what is right; it’s certainly a respectable thing. And given that we don’t know the answer, it’s important that we pursue lots of different possibilities. Maybe it’s going to be the case that some hybrid version of both string theory and loop quantum gravity will eventually turn out to be right, maybe the techniques from one will be very useful in the other one. I asked Carlo about this in the podcast interview, and he immediately slapped me down. He was not in favor of the idea that string theory and loop quantum gravity will be some day reconciled, but he’s also a fair guy, he knows that we have to keep an open mind about these things.

0:05:07 SC: Since we don’t know how to quantize gravity, perhaps it comes down to taste, in the sense of there’s different important principles of quantum mechanics, of gravity, that somehow have to be reconciled. Some of them are gonna go, some of them are gonna be preserved. Which ones are most important? That’s why it’s important that we have different people working on different approaches, because we don’t yet know which one is going to be correct. So solving the biggest outstanding puzzle in theoretical physics today, that’s our podcast topic. Let’s go.

[music]

0:05:56 SC: Carlo Rovelli, welcome to the Mindscape podcast.

0:05:58 Carlo Rovelli: Thank you, Sean.

0:05:58 SC: It’s very good… Yeah, very great to have you here. We’re both at a conference on the nature of time and the observer, so I’m taking the opportunity to sit down with one of the world’s experts in quantum gravity and have a chat.

0:06:10 CR: Fantastic.

0:06:11 SC: So why don’t we start just with you explaining to the audience, ’cause it’s hopefully a broad audience and they might not be physics aficionados, just who you are, how you got to be where you are, what your interests are? Very briefly.

0:06:23 CR: I am Italian. I started my life and career in Italy, in physics, sort of late, because I was not a nerd at school focused on science, I was interested in everything. I moved to the United States, I was a faculty in America for about 10 years and I moved back to France. I’m now living in France and quantum gravity has been my interest, my focus, my passion and my obsession all through my life.

0:06:55 SC: All through your life? All through your life at some…

0:06:57 CR: Well, from the moment I got interested in this.

0:07:01 SC: Was there a thing that got you interested?

0:07:03 CR: Yes, I first… First, I sort of got in love with quantum mechanics and general relativity when I studied these things. I said, “Wow, this is incredible, this is… The world is different than what we think and it’s marvelous.” And then I was a student and I stumbled upon a review paper by Chris Isham, who’s a London physicist. At the time, there were very few people who were interested in quantum gravity.

0:07:32 SC: So what year are we talking?

0:07:33 CR: ’76, ’78, something like that. And Chris Isham was a little bit of the guru of quantum gravity at the time and he had this fantastic review paper, which he was explaining the problem. And I read it, I didn’t understand much, of course, but it said space and time are not what you think they are, even after Einstein, even after quantum mechanics. And more than that, it said, space and time, we don’t yet know what they are, we’re not sure what they are. And here is a fantastic open problem, the core of modern physics, we don’t understand the basic grammar of the universe, and I said, “Wow, this is what I wanna do with the rest of my life.” And actually, I did.

0:08:21 SC: Very good. I think that we can all agree the reconciliation of quantum mechanics and gravity is one of the great challenges of modern physics right now, it has been for quite a while. Let’s get there sort of by starting with explaining quantum mechanics, and explaining gravity. Maybe gravity is easier to do first, so what would be the hardest question…

0:08:42 CR: Yes, ’cause quantum mechanics is hard.

0:08:45 SC: Well, we will have a discussion about quantum mechanics…

0:08:47 CR: We’ll get to that, yes.

0:08:47 SC: We’ll agree on gravity.

0:08:48 CR: Gravity is sort of easier. Gravity is… What we understand about gravity is a stroke of a genius by Einstein that he got between 1910 and 1915. And what happened is this, that Newton had introduced this idea of a space in which we live, and an absolute time, which passes independently of everything, which was a Newtonian idea, it’s not an older idea. Because before him, space was just sort of how things are ordered, who is next to whom. And time was just sort of counting things that happened.

0:09:34 SC: A label, yeah.

0:09:35 CR: Right, so, day, night, day, night and you count them one, two, three and that’s time, which means that if nothing happens there’s no time, right? But Newton introduced this idea that time is time by itself and space is space by itself, is there even if there is nothing else. And Einstein, 10 years earlier, had figured out with space relativity that space and time, you can view them pretty much in a single thing that today we call Minkowski spacetime, which is a sort of background of the world on which things, things have all happened. And Einstein was fascinated by Maxwell’s theory, by electromagnetism, which was not very old at his time, because it was, what, 30, 40 years old, something like that. So much less old than what general relativity and quantum mechanics are for today.

0:10:30 SC: Yeah, so Maxwell put together electricity and magnetism into electromagnetism, roughly 1860s?

0:10:36 CR: 1860s, right.

0:10:38 SC: Yeah. And so Einstein is thinking turn of the century.

0:10:41 CR: Right, at turn of the century, 40 years later. And however, the success of Maxwell’s theory was immense, right? Because immediately it was used for technology, there were electric engine. In fact, the father of Einstein was building electric plants in northern Italy, he was working with the Maxwell equation.

0:10:58 SC: Well, there would be no podcasts without electromagnetism, so that’s…

0:11:01 CR: There would be no podcasts without electromagnetism, right, without an understanding of electromagnetism. And the idea of Maxwell and Faraday was that there is this field, so electromagnetism is a field, it’s something that is all over, it can move according to the equation that Maxwell wrote. And Faraday was visualizing the field as a sort of web of lines going all over, a very, very fine, infinitely fine sort of, and we’re immersed in this field. And Einstein realized that gravity also should be describe by a field, showed gravitation as a field, somehow. And he asked himself how to describe this field. And then he got the stroke of genius. The stroke of genius is that Einstein, is that… Newton’s spacetime and the gravitational field are actually the same thing, that’s a stroke of… That’s general relativity as I understand it.

0:11:56 CR: And in fact, generally, it is presented in books by saying, “Well, gravitational field is nothing else than spacetime.” But I like better to think of it the other way around. Spacetime, nothing else than gravitational field. It’s the same thing, right? When you understand that those things are the same, you can say it two different ways.

0:12:15 SC: So Einstein is saying that gravity is not a thing that lives in spacetime, it’s a manifestation of the nature of spacetime itself.

0:12:22 CR: Exactly, it’s not one additional thing that lives in spacetime, it’s a manifestation of Newtonian space and time. This clear background that Newton told us is there, is actually there, but is the gravitational field. It’s same thing as the gravitational field. Which means it can move, it can stretch, it can bend. And Einstein wrote the equation for that. And this is a gravitational wave, the black hole sort of things. So that’s one ingredient.

0:12:48 SC: Yeah, I’m onboard. I’m with you… So that’s the easy part, right?

0:12:52 CR: That’s the easy part.

[laughter]

0:12:52 SC: We give this great credit to Einstein, in around 1915 or so, he finally put the finishing touches on general relativity, one of the great intellectual accomplishments of history. And part of that is, that it was almost a singular genius, right? Einstein didn’t have a lot of competition while he was doing it. But at the same time, as Richard Feynman famously said, the day after he did it lots of people understood what he had done. It was a clear thing. In some sense, even though it’s a different notion of spacetime, general relativity is still within the classical Newtonian paradigm of physics. There’s something, spacetime, it has properties, it has curvature, and you can measure it as precisely as you want. And then we come to quantum mechanics, which is a wholly different thing. And why don’t you tell us what quantum mechanics is?

0:13:42 CR: Quantum mechanics is a thing that, Feynman said, nobody understands it.

0:13:50 SC: That was the follow-up to that quote, right?

0:13:54 CR: Right. Which means that everybody understands it, but everybody in a different manner, so to say. Quantum mechanics, which also owes a lot to Einstein, Einstein was the first who really understood that it was needed and he made the first steps. In my understanding, quantum mechanics, it tell us three things about the world which we didn’t know before. And it’s sort of the mathematical representation of these three things. The first thing, in my opinion, I’m not sure I’m in the full majority from this perspective, but I think is crucial, is that a lot of things are discrete, it’s quantum, they’re quantum. So the first thing that it is telling us, for instance, is this electromagnetic field that Maxwell and Faraday described so well, and that makes the engine turn, and that makes a radio work, it’s actually, in some sense, made by little packets, which are the photons. And Einstein was the first one to introduce the notion of photons. And now we know that when light falls on me, it actually falls not as a continuous things, but like little rain. It’s a granular “tick, tick tick, tick,” many little photons that are…

0:15:14 SC: So, by the way, already I disagree.

[laughter]

0:15:17 CR: I know.

0:15:19 SC: With what you say. I think this is great, though. Certainly, it was one of the things that inspired people to get quantum mechanics, the word quantum, right? The fact that classically you have an electromagnetic wave coming from a light bulb and you see that as light. But experimentally, if you just look at it carefully enough, really, really dim light, it comes in packets.

0:15:37 CR: It comes in packets.

0:15:39 SC: You see these little blobs. You don’t see something smooth. And then the interpretive question is, which is real? Is it the wave or the particle? And then how do we think about that. But let’s not get bogged down in that quite yet.

0:15:53 CR: Right, right. Yes, in fact I started by saying this is, in my opinion, a central aspect of quantum mechanics, but I’m not sure I’m in the majority here. Another way of saying this, is that up to quantum mechanics, we thought that we could describe systems arbitrarily well. The state of a system with arbitrary precision. We could measure position and velocity of a particle, or measure anything arbitrarily well. And the possible state of the system, the possible outcome of the measurement are described as something we call the phase space, which is the space of all possible results. And the dimension of the phase space is action, and the core of quantum mechanics is a constant, which is the Planck constant, which has the dimension of an action, which is a little volume in phase space. If I have one degree of freedom, the space of possible state, I can think after quantum mechanics that there is a sort of minimal volume into it. So it forbids us to say things more precisely about the state of something.

0:17:11 SC: And this is compatible, this is of a sense with your idea that the essence of quantum mechanics is this chunkiness, this discreteness, yes.

0:17:19 CR: The chunkiness, right. So the h-bar, the constant of Planck, which is a constant that characterizes the quantum phenomena, it’ll ask when we’re in a quantum regime or when we’re away, it’s a measure of the chunkiness of reality in some sense, another way of viewing things.

0:17:34 SC: So reality transforms from the smooth fields of Faraday to a pixelated timeframe, view of things.

0:17:41 CR: To the pixelated things, right. If it was only that, it would be comprehensible, but it is not only that.

0:17:48 CR: So there are two more steps. And the second step also is comprehensible. And, Sean, I believe you might disagree with this as well.

0:17:56 SC: I’m ready. I’m ready to disagree.

0:17:58 CR: Alright. So the second step, let me say in a way that you probably don’t disagree. Is that, we cannot make predictions about the future, quite fundamentally.

0:18:10 SC: Right.

0:18:10 CR: Not just because we haven’t measured precisely enough, not because we don’t know the calculations, but if we make everything we can on a physical system and we control it perfectly, and we know its physics as best as we can today, nevertheless, what we’re gonna see tomorrow has a margin of indeterminacy.

0:18:36 SC: We can make predictions, but they’re at best approximate, some probability.

0:18:38 CR: We can predictions, in fact, we do make predictions, quantum mechanics allow us to make prediction, it’s not that there is no prediction possible the other way around. But there is always some uncertainty that remains, which we can make small, but not zero. This is the famous nondeterminism of quantum mechanics, and that’s a second discovery of the world, I would say. And I think I’ve phrased it in a way that you may agree.

0:19:08 SC: I think if you phrase it in the terms of predictions, everyone had better agree. Yeah, that’s right.

0:19:11 CR: Exactly.

0:19:12 SC: And then again, what’s going on underneath.

0:19:14 CR: What’s going on underneath is a different story. And the third, and here we come to the hardest, because so far, alright, so a world pixeled and unpredictable is still a comprehensible world, which wouldn’t shock us too much, but the third is the most shocking one, and in my opinion, the third discovery of quantum mechanics is that it describes how properties, how systems have properties, but these properties, in the language of the textbook of quantum mechanics, these properties become actual, become real, when you measure them. Now, measuring it’s a nonsense world, because what does nature know about me measuring something…

0:20:05 SC: Certainly doesn’t sound like some concept you want as part of your fundamental description of the world.

0:20:12 CR: No, no. So measuring that is… And in my opinion, what the theory is telling us, is that property become concrete when two systems interact. And this, so far so good…

0:20:25 SC: So you’re replacing measurement with interaction?

0:20:28 CR: You replace measurement with interaction. But to make the machine work, you have to assume that when two systems interact, the property of one system becomes actual, with respect to the other system, and not with respect to everything else. And that point is really shocking.

[laughter]

0:20:50 SC: That’s right.

0:20:50 CR: And that point is really shocking. Now, I think if one accepts this idea, which is my way of viewing quantum mechanics, which I know is not the universal way, it’s not yours, in particular. Then that’s it, that’s what quantum mechanics is telling us.

0:21:03 SC: But it’s not so far from mine in that part in the sense that I am an Everettian, a believer in the many worlds interpretation, at least I think that’s my favorite. I’m happy to change my mind some day down the road, but Everett himself, when he invented it in the 1950s, when he published his paper, he called it the relative state interpretation. And the point that he was trying to emphasize was that what you see is only relative to the thing seeing it. This interaction creates an impression of the measurement outcome, that is intrinsically part of the relationship between the thing that is doing the measuring and the thing that is being observed. So I think that there’s some similarity of words there.

0:21:46 CR: Some similarities. I shouldn’t talk for you, but let me try it for one second. I think what you would add to what I say, is that you want to take a realist position where beyond that there is nevertheless a deterministic non-relational overall quantum state of the system, that we may take as the proper description of reality.

0:22:19 SC: I would like to say that, yeah…

0:22:20 CR: Would that be so?

0:22:20 SC: Yeah, I would. I’m a realist. I can’t even imagine what it would be like to be a scientist and not be a realist about reality. [laughter] And yet I know…

0:22:32 CR: Right, so I would like to be a realist, but less radically so. Let me say two things. There are a lot of quantities in physics which are relational, we know that they are. The velocity of an object is not a property of the object. There’s no sense in which the velocity of an object is a property of the object…

0:23:01 SC: Depends on who is measuring.

0:23:02 CR: Right, and in fact this is a good example, because here we use measuring, but of course it has nothing to do with measuring. The velocity of an object is a property of an object with respect to another object.

0:23:11 SC: Yes.

0:23:12 CR: So I can say that the Earth is set to velocity with respect to the Sun, but the Sun is not measuring, it’s not going out with meters and kilometers. So we use this language, measuring, and we interpret this language to say that some properties are relational. I want to be a realist about relational properties. And I want to think of reality in a realistic way, perhaps a little bit closer than you, to our direct experience of reality. I see the photons coming in, I see the table being the table…

0:23:47 SC: I see our direct experience of reality as tremendously overrated, frankly.

[laughter]

0:23:54 CR: And so I find that if we want to be a realist about the wave function, we have… Actually, let me step back for a moment. I think, given the strangeness of quantum mechanics, any way we think it is, comes with a price. So we have options. We have a number of options, and we have been talking about two options, okay, the sort of many world interpretation, in which you have this wave function of everything, which come with a price, or the sort of strictly relational view which I described, which also come with a price. And there are others and they have their own price.

0:24:32 SC: Right, that’s right.

0:24:34 CR: And many discussions about quantum mechanics are, “Well, which price are we ready to pay?” And the price of the relational interpretation I described is this weakening of realism. So to say, what the state of the system… Well, I don’t know, it doesn’t matter, it doesn’t have a state, it only has a state with respect to A, and it is the same with respect B, and it is the same with respect to C, and there is no solid objective, overall picture. The price of the many world interpretation, is that I have to accept…

0:25:07 SC: Many worlds.

0:25:08 CR: In some sense all these many worlds.

0:25:10 SC: Yes, absolutely, it’s a big price, it’s a big price.

0:25:11 CR: So it comes with an enormous ontology, a very heavy ontology, and which price do we wanna pay. In one way or the other, it contradicts our previous way of thinking of reality.

0:25:24 SC: Absolutely, and I… So I couldn’t agree more about the idea that there is a price to be paid. I could disagree a little bit about the heaviness of the ontology that is associated with many worlds, but I think that this is a good lesson, because all we need for today’s conversation is the clear fact that we have at the same time, a way to do quantum mechanics, we use quantum mechanics as physicists every day, it’s been spectacularly verified experimentally. We know how to use it, we have a recipe, we have a black box.

0:25:58 CR: Yeah, and it works, it works fantastically well.

0:26:00 SC: And we don’t have anything close to an agreement on what it is, what the fundamentals of it really say. So that will be a wonderful thing to figure out once and for all, but maybe we don’t need to do that for the question of quantizing gravity. So just to put the puzzle, the task, in perspective, since we grew up, we human beings, as classical people, classical mechanics was invented first. It is very intuitive to us, even if pendulums and inclined planes tortured us as undergraduates, we kind of get it in a fundamental, visceral way. So we always start by thinking about systems from this classical point of view and then we quantize them, we elevate the classical description to some quantum mechanical theory. Now, presumably nature or God doesn’t work that way.

0:26:52 CR: Doesn’t work that way.

0:26:52 SC: Right, nature just is quantum and we can look for classical limits, but nevertheless we poor benighted human beings, given the classical system, we want to quantize it. And we have ways of doing that. There are, again, cookbooks, recipes that work and then you apply this way to this wonderful thing called general relativity, Einstein’s theory of gravity, and we hit road blocks. So this is why quantum gravity is hard. And I guess roughly around the 1950s would be when people first started seriously thinking about it and not ’til decades later was any kind of progress really made. Would you agree with that?

0:27:29 CR: Yes, that’s when people started thinking. The first who realized there was a problem, not surprisingly, is Albert Einstein himself, who in 1915, wrote what we call today the basic equation of general relativity. And one year later in 1916, he wrote a paper saying… Well, of course, this is an approximation, because gravity should have quantum properties. It’s quite remarkable because quantum mechanics within textbook came out 10 years later, but…

0:28:00 SC: Well, he had invented photons, so inventing gravitons shouldn’t be that hard, right.

0:28:03 CR: He had already invented photons and there was already the idea that things were quantized at small scale in some sense. And I… It’s a beautiful page by Einstein in which he says in incredible clarity that in the future, his own theory has to be corrected to take into account of quantum mechanics. And here we are.

0:28:27 SC: 100 years.

0:28:29 CR: Here we are, 100 years later. But you’re right, this remains an isolated thing, though there was some people thinking about that in the ’30s, both in Russia and in the West. Rosen had a paper which started studying it, but especially Bronstein, Matvei Bronstein…

0:28:48 SC: Okay, in Russia, yes.

0:28:49 CR: Russian, yes, he was a friend of Landau, and he’s the first one who figured out that quantum gravity, namely finding the quantum property of the gravitational field, had to imply some radical revision of space and time. And he said it openly. It’s a nice story, because it comes from a mistake by Landau.

0:29:18 SC: For the listeners, Lev Landau, a Russian physicist, one of the great minds of 20th century physics. And didn’t make a lot of mistakes.

0:29:26 CR: No.

0:29:26 SC: So a mistake by Landau was a big deal.

0:29:28 CR: Right, it’s a big deal, in fact, in fact. But you know, good physicists do make mistake.

0:29:33 SC: We all do.

0:29:33 CR: Einstein made all sort of mistakes…

0:29:34 SC: And he was the best, yeah.

0:29:36 CR: Right, he was the best in doing mistakes and getting results. Maybe the things go together. When quantum mechanics started to be understood, the formulas and the mathematics, Landau assumed that because of quantum mechanics it should have been impossible to measure the value of a field, the electric field in a point. And Bohr, who was a big father of quantum mechanics, immediately understood that this was wrong and it was a long discussion, and Bohr and Rosenfeld wrote a paper showing that that Landau was wrong and Landau recognized that. But Bronstein, who was a friend of Landau, immediately said, “Well, let’s do the same for gravity,” and there, Landau is right. There is a sense in which you cannot measure the gravitational field in a point, because points lose their meaning in quantum gravity.

0:30:42 SC: Where is the point, that’s right.

0:30:44 CR: And why points lose their meaning in quantum gravity is because the gravitational field is also space. The gravitational field don’t live in space, but is space itself. So in the moment in which you look at the quantum properties of the gravitational field, you’re also looking at the quantum property of spacetime, of space. And one, a bit modern way of viewing what he was doing, is the following. If you try to pinpoint the position of a particle, in arbitrary small precision, you can, but because of quantum mechanics, the velocity of the particle becomes very indeterminate. But the velocity is also the momentum and the momentum is also the energy, so you have a very large energy, so to say, and a very large energy, because of gravity, energy is also mass, is MC squared, so it’s like having a large mass and if you have a large mass in a small area, you create a black hole. So when you try to zoom in a point very, too small, automatically you create a little black hole. And if you work out the numbers, you look at a scale and that’s a minimal scale you can look at, right?

0:32:06 SC: Yes, so quantum gravity sort of asserts itself when you try to localize things too carefully in spacetime. The idea of having a location seems something that is so intrinsic to how the world works that I would say, and probably you’ll agree, that even today, modern researchers who talk about quantum gravity very often secretly have in mind some spacetime manifold that has much more reality than it should, according to the rules of quantum gravity.

0:32:36 CR: I agree, and I would say that… How would I put this way? A lot of my career has been fighting against this temptation.

0:32:44 SC: Although, to be fair, Einstein was on the other side, right?

0:32:46 CR: Yes, Einstein was on the other side.

0:32:48 SC: Einstein himself was heartbroken at the prospect that if there were a quantum theory of gravity, spacetime would lose its primacy, he really felt… And he had a right, because he was the boss of spacetime, to think that without a notion of where you are in space, if that were not fundamental, he couldn’t quite see how even to do physics. But certainly at the intuitive level, it seems that that is how quantum gravity should work.

0:33:15 CR: Right, right.

0:33:16 SC: So do you think you know how to quantize gravity?

0:33:19 CR: Do I think I know how to quantize gravity… So let me put it this way. Me and my friends, a good group of friends, in a few decades, we have put together a tentative set of equations that can be written on a single piece of paper, which are a tentative solution of the problem. Now, do I hope this to be a quantum theory of gravity? Yes, I definitely do. Do I think it might be a quantum theory of gravity? Yes, I do. Am I sure this is a good quantum theory of gravity? No, not at all.

0:34:03 SC: Don’t be coy, you should say what the name of your theory is.

0:34:05 CR: It’s loop quantum gravity.

0:34:07 SC: Loop quantum gravity.

0:34:09 CR: It’s loop quantum gravity, right. So loop quantum gravity has been born from the early attempt in the ’50s and the ’60s that you mentioned from the work of William DeWitt and many other people, but then has evolved into a more precise mathematical structure to describe the quanta of gravity, this granular structure of spacetime, because in the… Well, what photons are for the electromagnetic field, we think there should be analogous quanta for the gravitational field, but that’s a specific characteristic of loop quantum gravity. Since the gravitational field does not live in spacetime, it’s not a field in spacetime, but it’s spacetime itself, the quanta do not live in space or in spacetime, they are themselves spacetime, so they should be not confused with the gravitons, which are sort of a first approximation to quantum gravity, which are little particles like photons that move in space, they are very, very, very much brothers of photons. The quanta of gravity in loop quantum gravity are the grains out of which space or the gravitational field is built, and they are discrete, so that they cannot be further cut in smaller pieces.

0:35:44 SC: So, that pixel granular structure is right there…

0:35:47 CR: The pixel granular structure is right there at the beginning. And we have a state in a Hilbert space. The mathematics of quantum mechanics that describe this pixelated space. And the space of our experience is like the many pixels seen from a large distance when you don’t see any more the discreteness.

0:36:12 SC: Like a good high resolution TV screen.

0:36:15 CR: Like a high resolution TV screen which looks nice and smooth and continuous.

0:36:18 SC: That’s right.

0:36:20 CR: But if you go very, very close you see the individual pixels.

0:36:22 SC: And is there a way to concisely explain the word “loop” in the phrase loop quantum gravity?

0:36:28 CR: Yes, yes. First of all, it has remained a little bit historically. Now, it’s less loopy than in the beginning. But…

0:36:37 SC: In some sense, yes.

0:36:38 CR: In some sense, right. Or it’s more loopy, I don’t know. The loop comes from this. The pixels themselves are, how would I say. They know who is next to who. They know who is next to who, so one can represent that with the links from loop to loop. So you have all these dots linked to one another, it forms a net. In fact, we call them network or more precisely, spin network, spin because there’s something of the mathematical spin in describing it. So imagine this network, and if you start from one of the pixel and go around link, link, link, link and loop back, you make a loop. So these are the loops on loop quantum gravity, and historically, the first mathematical description of these things that were found were only single sort of pixels along a loop, individual loops. This is when Ted Jacobson and Lee Smolin stumbled upon a solution of what was considered at the time the main equation of quantum gravity, which is called the Wheeler-DeWitt equation.

0:37:47 CR: And these solutions depended on loops and so we started wondering, what are these loops? And slowly, we realized that these are the quanta. These can be thought as the individual quanta of space, and in fact, they don’t just loop or not so link, but they can cross, so it’s better to think as a network than a set of loops.

0:38:12 SC: And I know that at least at some point in the development of the theory, there was an image of almost like chain mail, in the sense that there were little loops that sort of tied around each other and that gave space its effective geometry and reality. Is that still how things are thought of now, or has that kind of evaporated?

0:38:34 CR: Yes, yes, it’s a little less chain maily, chain mails as before, because at the beginning we were working really with still loops and we were thinking of loops, sort of linked to one another, so… I remember some point I was in Verona and I decided to buy a large number of key rings, and attached one to the other to make a sort of three-dimensional chain mail.

0:39:00 SC: This is what I’m remembering, good.

0:39:01 CR: Yeah. And then I was going around… I gave a talk in Princeton, going with my chain mail, and John Wheeler, who was a big grandfather of thinking about, when he saw it he was very excited, because he said, “Yes, yes, that’s how I think about space, at the very, very small… ” And he ran away and came with a copy of his book, I didn’t know that, but in his book, there is a picture that he drew decades before, which is a sort of chain mail.

0:39:31 SC: Alright. So, it’s inevitable to mention that once you have these little one-dimensional loops, it almost begins to sound like string theory.

0:39:43 CR: It begins to sound like string theory, right, because these loops are made by lines and it’s like strings. The first difference, with respect to, at least to old string theory, to how people started thinking of string theory at the beginning, is that a string is… A closed string is a little loop, but it’s a little loop that moves in space.

0:40:11 SC: That’s right.

0:40:12 CR: While these do not move in space, they make space themselves.

0:40:16 SC: Yeah. No, I think that’s exactly right.

0:40:18 CR: That’s a core difference, the original difference. You see, string theory did not grow as an attempt to solve quantum gravity. It came from another way, sort of stumbled on a possible solution of quantum gravity, which is a reason for taking it seriously, right, it’s the fact that… But it originated from the effort of making a unified theory of the different forces and the different pieces of the standard model and gravity. So people who worked in string theory, especially at the beginning, were not, how would I say, were mostly thinking in a sort of pre-general relativistic way. This is spacetime and over spacetime there are things that move. And while loop quantum gravity emerged more from the core general relativistic world where there is no fixed spacetime on which things happen, spacetime itself is something that we are…

0:41:23 SC: Let me give my version, ’cause I think it’s interesting to explore this rivalry, as it were. Loop quantum gravity and string theory, it’s safe to say, are the two leading candidates, just in terms of voting by people doing these in terms of quantum gravity.

0:41:38 CR: Yeah, in terms of number of people…

0:41:39 SC: And like you say, loop quantum gravity arose out of a fairly direct approach to the question of how to reconcile gravity in quantum mechanics. We knew gravity. Einstein had this theory of general relativity. We have rules for quantizing things. They don’t instantly work when you apply those rules to general relativity, but then you be clever, and people like Lee Smolin and Ted Jacobson and Abhay Ashtekar get lots of credit, and yourself, as well, and you sift through the subtleties here and you find ways to get a quantum mechanical theory that really respects the quantum nature of spacetime itself, and this is the goal, and you’re not trying to do a theory of everything.

0:42:20 CR: No, not at all, not at all, not at all.

0:42:21 SC: You’re not trying to also explain quarks and leptons, right? You’re not even trying to explain the possibility of extra dimensions. You think, “Look, I see the three dimensions of space and the one of time, let’s work there,” and that was the attitude that was taken, whereas string theory, as you said, started in the ’60s and ’70s with this idea that was originally based on data, data from the strong interactions of particle physics, the interactions that keep the quarks inside protons and neutrons, and they saw that certain particles, certain collections of quarks, as we now know them, had a relationship between how fast they were spinning and how heavy they were. And someone said, Veneziano, and then Susskind later, would say, “This looks like a relationship that you would get if they were little pieces of string that were spinning,” and so they developed this idea called string theory and they were frustrated because it kept predicting gravity. They didn’t want gravity, that wasn’t their goal, and it was people like John Schwartz, my colleague at Cal Tech, who eventually said, “Look, gravity exists. Maybe if we get a theory of gravity, that’s a good thing.”

0:43:28 SC: Everyone laughed. No one was really convinced by this, and it wasn’t until the 1980s, when Schwartz and Michael Green showed that string theory was actually a mathematically consistent theory, that they started saying, “Oh, okay, maybe this is real and not only potentially a theory of quantum gravity, but also a theory of everything at the same time.” Many… Despite the fact that string theory is by votes the leading theory of quantum gravity right now, there are many obvious problems with it. One is, as I think I’m completely on-board with what you say, it’s not really taking the quantum nature of spacetime itself seriously. It starts with a string moving through spacetime, the spacetime is there already and you can try to fix that in post-production, as it were…

0:44:17 CR: In post-production.

0:44:19 SC: But that’s a challenge, just as every theory is gonna have its challenges. So that’s one challenge. Another challenge is that when you ask, okay, what does it say about spacetime that a quantum mechanical string is vibrating in it, you find out that one thing it says is that spacetime is 10-dimensional. That is not a feature of the real world. So you say, “Well, okay, that’s a known problem that we’ve had in other theories, we can hide these extra dimensions, we can curl them up so they’re invisible, but guess what, there’s more than one way to do that. There’s probably an infinite number of ways to do that, at least numbers are thrown around 10 to the 500 different ways of doing that.” So your aspiration of getting a theory that was not only gravity but all the other forces of nature, gets mired in this ambiguity, this fact that we don’t know how to go from the ur theory in 10 dimensions down to the four-dimensional real world.

0:45:12 SC: So a string theorist would say, “Look, we have this wonderful thing, we have a model where we’ve been struggling to quantize gravity and here gravity is forced upon us from the rules of quantum mechanics itself, we should take advantage of that. Of course, there’s these issues with 10 dimensions and so forth, but hopefully those will be resolved.” Whereas someone in loop quantum gravity says, “Look, I have spacetime, I have general relativity. I should just take that seriously at face value, try to quantize it.” And there are other problems that come up, and you can probably speak to them as well as I can, but then you would also say, “And hopefully those problems will someday be resolved,” and in a field where there’s not a lot of direct helpful experimental data, because gravity is a weak force and the world looks pretty classical to us, it’s hard to know which approach is better a priori and we have to invest a little bit of judgment and taste to see which we like better.

0:46:10 CR: Yes. We have to use intuition, judgment, as you say, and bet.

0:46:16 SC: Yeah.

0:46:18 CR: But I think things are moving, things are moving, especially with respect to the last thing you said. I agree with everything you said, by the way. There is, perhaps, today, less a sense that we are moving in a realm completely detached from measurement, from experiments. Not that there are clear-cut quantum gravity experiments that choose between theories, certainly not.

0:46:52 SC: Not yet.

0:46:53 CR: Not yet, but I would say it’s not so far and there’s a number of things that have happened recently. First of all, remember that we have experiments that have ruled out SU(5), which are at a scale which is not very different from the quantum gravity scale, maybe two or three orders of magnitude.

0:47:14 SC: That’s right. So SU(5) was this idea from the 1970s, before we were worrying about quantum gravity. It was a particular take on how to unify all the other forces of nature.

0:47:24 CR: The unification without gravity.

0:47:25 SC: Without gravity. So they called it grand unification; it wasn’t that grand, also it didn’t work. Like you said, the good news was it made explicit experimental predictions, the bad news is we did the experiment. The proton should decay into lighter particles, positrons and neutrinos and so forth, and it hasn’t. We’ve been looking for that for a long time.

0:47:45 CR: It hasn’t. It hasn’t. And that’s an example in which there was a measurement at a scale not so far from the scale of quantum gravity, but more recently, there have been a number of empirical results which tell us something about quantum gravity, I believe. One which have not touched directly, I would say, neither string theory nor loop quantum gravity, but has touched pretty strongly a number of other attempts, which that predicted violations of Lorentz invariance.

0:48:21 SC: So explain what Lorentz invariance is first.

[laughter]

0:48:24 CR: No, you explain what Lorentz invariance is.

0:48:25 SC: I can explain what Lorentz invariance is. You know, again, Einstein, when he was inventing special relativity, so he was, before putting gravity into the mix, he was just trying to explain the symmetries of Maxwell’s very successful electromagnetism, and he realized that what you called space, what you called time, was in some sense a choice of an observer to split four-dimensional spacetime into the three of space and the one of time. And it didn’t matter if you were traveling near the speed of light, your clock moves differently with respect to the clock you left behind on Earth. But either choice was fine. There’s nothing that you would be able to know if you were in a sealed room. You don’t know some sort of absolute velocity that you have with respect to the universe. It’s only with respect to some other part of the universe. So that’s Lorentz invariance. Lorentz invariance is different things moving at different velocities don’t have any way of telling how fast they’re moving in any absolute sense, only with respect to other things.

0:49:28 CR: Yes. Which is what we were saying at the beginning, about velocity being…

0:49:32 SC: Exactly. That’s right.

0:49:33 CR: A relational notion.

0:49:34 SC: And the fact that it was a relational notion was true even before Lorentz invariance. There’s a Galilean invariance, which is still true for Newtonian physics. Einstein adds this ingredient that the way that you divide up space and time does depend on your motion through the universe. And you can test this idea, you can test it experimentally, right?

0:49:54 CR: Right. And so far it seems to be true.

0:49:57 SC: Yes, that’s right.

0:49:58 CR: Quite spectacularly correct, this invariance.

0:50:01 SC: Well, just to… Just to show how spectacularly correct, one of the predictions of Lorentz invariance is that if something is moving close to the speed of light, as we said, its internal clock seems to move at a different rate than a clock that is stationary with respect to us. So we see here on Earth, raining down from the sky, these elementary particles called muons. These are part of the standard model of particle physics. A muon is basically a heavier version of an electron, and you look at how fast it’s moving, coming down, these are not created out there in the sky, because they decay. Muons are not forever. They decay really quickly and, in fact, they don’t have enough time by our clock to go from the top of the atmosphere where they’re created in some cosmic ray collision to reach us down here. They should’ve decayed away. How is it possible that they don’t? And the answer is, they’re moving so close to the speed of light that their clock is ticking at a different rate. And this is kind of a crude but vivid demonstration. These days, with high precision atomic clocks and other kinds of things, we can test this ideal Lorentz invariance. And so far it’s one of the best symmetries that we’ve experimentally probed.

0:51:13 CR: Right. Exactly. Thank you. And some attempts to write a quantum theory of gravity, others than strings and loops, constructed some tentative theories of gravity where Lorentz invariance is broken, is violated. And there was a moment in which they received a lot of attention, these other attempts. One was called Hořava gravity, for instance. So the people were excited, “Oh, maybe we know how to do quantum gravity,” but then and we could even test it by looking that Lorentz invariance is violated.

0:51:47 SC: Sorry, it’s an important point to emphasize that scientists get thrilled when they can say that in some subtle way, everything we knew before was wrong, right?

0:51:58 CR: Yes. Very good, very good. I think they get too thrilled. [laughter] That’s what I think. And in fact, this is what happened in that case, right? Because a lot of people worked on this idea and a lot of experimenters and astronomers started to look at indications. And a program of observations of astrophysical phenomena that would violate Lorentz invariance was launched. And this was about 10 years ago. And I think that now with quite strong evidence that Lorentz invariance is not violated at the scale required by those theories.

0:52:39 SC: Right. Not where you might expect it to be.

0:52:41 CR: So a number of attempts to do quantum gravity has, I wouldn’t say ruled out, because ruled out is always a little bit too strong, but made less convincing, less credible by the fact that what was predicted was not there. So that’s one example in which a measurement told something relevant to quantum gravity. The second example, I have three, the second example, it’s very recent and it’s with gravitational waves, and in particular the last observation, which is two neutron star falling into one another, spiraling into one another and merging, that was a few months ago, ’17, if I remember correctly. Because, that was particular, because that was seen by the gravitational wave detectors but also by telescopes, radio telescopes, all sort of gamma ray…

0:53:35 SC: So light waves as well as gravitational waves.

0:53:36 CR: Light waves. So everybody saw these things, which makes observation credible. First of all you see it with a wide variety of instrument, it has to be there, but the fantastic thing is that this is a merger of two neutron stars that happened very far away. So the light and the gravitational wave have traveled from there to now for a very long time, and they arrived at the same moment within a very small…

0:54:08 SC: As far as we can tell.

0:54:09 CR: As far as we can tell. So because of that we have learned that gravitational waves and electromagnetic waves travel at the same speed, and in fact, if you put the numbers, they travel at the same speed in 1 part in 10 to the 14 or 15, so a very great precision. Before that, so one year ago, the experimental information, the observational information, the empirical information on the ratio of the speed of light to the speed of gravitational wave was 1 part in 10. So in one single stroke, we ameliorated our knowledge of one fundamental parameter of nature by a hundred billion times. Now, why this is great… So, why this is great, because again, a lot of people were studying modifications of general relativity that predicted that gravitational waves go a different, give different speed than electromagnetic waves. So a lot of work of theoreticians, like you and me, have been thrown in the waste basket, just in one stroke or, again, maybe made less credible, somehow, less interesting.

0:55:25 CR: And the third one, and I’m here perhaps touching on a more controversial thing here, is the non-discovery of supersymmetry at the LHC, which was a no news because it was a non-discovery, but I think was the great news at LHC, as important as the discovery of the Higgs that made the front page of all the newspapers. So when the LHC, the big machine, the accelerator of particle, turned on… Quite quickly saw the Higgs, fantastic, that was predicted by the standard model, but there was a very large part of the community of theoretical physicists that expected, very strongly, was almost convinced that supersymmetric particle had to come out and be seen.

0:56:25 SC: Supersymmetry being this hypothetical symmetry that says that particles of different spin should have friendly partner particles with different spin but the same exact property. So there should be, if there’s an electron, which we know and love, there’s a supersymmetric partner, then, called the selectron. If there is a quark, there are squarks. If there is neutrinos, there’s sneutrinos, etcetera. You should double the number of particles if supersymmetry is right. And like you say, many people thought that it was right around the corner, we would turn on this machine and squarks and gluinos would come popping out at us, and we haven’t seen anything yet.

0:57:04 CR: We have not seen anything yet. And string theory requires supersymmetry to work. And many people expected that this supersymmetry had to be visible at the energy of LHC. Now, when it was not seen, definitely this does not rule out string theory at all, because string theory could be, the supersymmetry could be still there, but somehow at a higher energy where we haven’t seen it. But nevertheless, a lot of people were expecting it, and I think if it had been observed, I, who am not a string guy, would have thought, “Well, these people had the right intuition about nature after all. So maybe these people have a point for them.” It wouldn’t prove this to make the theory right, but it would be a plus in their direction.

0:58:02 SC: I think that’s very fair, it’s not… There was no absolute rock hard prediction either way. But, had you seen supersymmetry, your credence in the string theory would have crept up a little bit.

0:58:11 CR: Exactly.

0:58:13 SC: The fact that you didn’t see it therefore means it had better go down a little bit.

0:58:15 CR: Exactly, exactly. I think this is a point about how to think about science. We often say that we propose theory and then they are falsified by an experiment. And so, if a theory is not falsified, it may still be right, but I think things are a little bit more complicated, because we put credibility in theory, and the more sort of good indirect things happen, the more our credibility goes up and the more things don’t work the credibility goes down, so this is…

0:58:51 SC: Or it should.

0:58:52 CR: Yes, exactly. So science also work this way, through hints, through indirect thing. And so what I’m saying is that violation of Lorentz symmetry is one; the speed of the gravitational/electromagnetic waves is another one. Supersymmetry is another one. We’re learning, we’re learning things which are all relevant toward quantum gravity. And there’s also a lot of attention now in doing, toward doing experiments toward quantum gravity.

0:59:29 SC: There’s always healthy…

0:59:32 CR: There is one experiment which is being proposed, which I find very exciting, which would not distinguish between quantum gravity theories, but would prove that space is quantized, so that gravity is quantized, which has been proposed by two independent groups last year, or two years ago. And I’m very excited about it, so I hope that people would do that soon. And so let me explain it because… You take one particle, a sort of nanoparticle, small things that people now know how to work with, and you quantum split, you put it in a superposition, or two positions. And you take another particle and you also quantum split. And these two particles, both split, are next to one another. So now there is the gravitational interaction between the two, so each one is in the Newtonian field of the other.

1:00:36 SC: So when we say nanoparticles, this is not an elementary particle like an electron, it’s like a little mote of dust.

1:00:41 CR: It’s a small dust, it’s a small grain of dust, right. And these… We… Not we, them – the people in the laboratories are able to put in the quantum superposition of two different positions. So you have two particles, both of them in a superposition next to one another and so each one of them feels the gravitational force of the two branches in which the other one is. And because of that, the state of the two particles gets entangled. And now there are very clean ways to see if two things are entangled.

1:01:25 SC: Yes.

1:01:27 CR: And if we find that they’re entangled…

1:01:28 SC: And this is true quantum entanglement.

1:01:32 CR: It’s quantum entanglement. This is true quantum entanglement.

1:01:34 SC: It’s not just they’re related, but they’re connected at this deep quantum mechanical level. Yeah.

1:01:40 CR: No, it’s true quantum, deep quantum mechanical level. If this happened, it means that the gravitational field itself was in a superposition of two different… Was in two branches, so to say. And since the gravitational field we know is spacetime, it means that spacetime can be put in a superposition.

1:02:00 SC: And this is a proposed experiment, it hasn’t been done yet.

1:02:00 CR: This is a proposed experiment, but if you work out the numbers, the people who manipulate these small particles, they say that they think they should be able to do it in a few years. And I think it would be wonderful, because there are still people around, even if in a minority, who says, “I don’t believe that spacetime has quantum properties.”

1:02:22 SC: I’ve met them, yes.

1:02:23 CR: And because, come on, spacetime is spacetime. It’s a heavy thing. And this experiment, if it can be done, would be splash. A clear indication that you can put… That there is this indetermination, this profound quantum nature also of space itself.

1:02:49 SC: So I’m about halfway through the number of things I wanted to talk about, but we need to go and be good conference participants. But let me close, I have two questions that I would like to at least throw out there. And the first one, there’ll be a temptation to talk about it for half an hour, but let’s not, there’s loop quantum gravity, there’s string theory, there are other approaches, causal set theory and Euclidean quantum gravity and so forth. Quantum gravity, we don’t have a consensus. We’re trying different things, but nevertheless, we live in a world where there are finite resources to do science; academic positions, grant money, prizes and so forth, glory. There’s a feeling that’s certainly been put forward that there’s been unfairness in how the representation of these different approaches to quantum gravity has been distributed throughout academia, in particular that the string theorists have kind of a dominant position that maybe they don’t deserve, given that string theory hasn’t really predicted anything definite that we’ve tested.

1:03:50 SC: It’s mathematically very beautiful, but there’s obstacles to it, like there are to other theories. I personally think that string theory has a good reason to be the leading candidate, but I also think that it would be very healthy for the field to be doing various other things. So what are your feelings about the fairness? Is the free market of ideas working well here or is there… Should we… Are we not being quite fair enough to this diversity of different approaches?

1:04:19 CR: Yes, you invite me to… Yes, I think that if I look back a couple of decades, the distribution of resources has not been fair with, let me say it clearly, with all respect and appreciation for string theory, this is not… Far from me for saying that string theory should not be funded, should be thrown out of university or anything like that. It’s a wonderful research direction, but it’s a research direction. It’s not an established result that we can take for granted. And as a research direction, it might be right, it might be wrong, it might be partially right, partially wrong and so on. And I think that other approaches, the loop quantum gravity in particular, has suffered because of the dominance of string theory. There is a non-linearity, there’s a band-wagon effect. Powerful people are in powerful positions and do one thing, and it would not be the first time in the history of science in which a group of people, a large group of people, dominate and push the field in a direction which is not useful. It could be. So I think a little bit more fairness and being more free market.

1:05:53 SC: Right. So to be fair, and maybe we alluded to this but it didn’t become clear, any approach to quantum gravity or any other unsolved puzzle in physics has this element of taste around it, in the sense that you will be making some sacrifices, right? Like some things in a certain approach work and are very nice and natural and other things seem problematic. And different people have different judgments as to say, “Well, this is a problem, but surely we will overcome that… ”

1:06:21 CR: Yes.

1:06:22 SC: With a few more years of work. And so, string theory clearly has a problem connecting from this purported 10-dimensional reality down to our four-dimensional reality. Do you wanna mention some of the problems that loop quantum gravity has?

1:06:36 CR: Well, yes, I think that in the version in which we’re working today… I work in the covariant version of loop quantum gravity. At least there is, and this is a plus, a well-defined set of equations. Okay? So, right now, let’s say this is the theory. Now, we don’t know really if this is consistent in the precise sense. We can use it order by order to give… To compute, but we don’t know if this converges in any sense. So even in the weakest possible sense, we don’t know if going to next order… The theory doesn’t go fully.

1:07:26 SC: This is one of those famous things in quantum theories, where you might calculate a good approximation but then you try to make your approximation better and everything blows up.

1:07:35 CR: And everything blows up. Right, right. So, since this has not been proved, this has not been checked, there aren’t even good indications for believing so. This is definitely a weakness of the theory. The theory could blow up still and be meaningless. However, I think that what we need is not more mathematics. We need to apply this theory to reality. I’m working on the application to black holes. A lot of people like me is trying to connect loop quantum gravity to astrophysics, cosmology… See if it can say something for, I don’t know, dark matter, fast radio bursts, gamma ray signals, signatures in the cosmic background radiation, because at the end of the day, what will convince us that one theory is good or not, is not in my opinion the mathematical cleanness. We don’t get it.

1:08:29 SC: No. And some day, that day might be a while before it comes, but you have to explain the data, you have to…

1:08:34 CR: You have to explain the data. So in the moment in which we say, look I have a prediction or explanation of the data and it works well, then the attention of the community will focus on that. Let me say about the string theory that things have changed with respect to some time ago. I think there have been… There is much more reciprocal respect than 10 years ago and I think… There was a moment in which string theory had very high hopes, and was sort of saying we had it. It has backtracked a little bit from that.

1:09:11 SC: Well, the hopes were always high. There were hopes for a quick resolution.

1:09:14 CR: Right. I would say there was a moment in which people thought, okay, now we’re gonna compute all the parameters of the standard model. Now we’re gonna do this and this and that, we’re gonna see supersymmetry, and then we’re gonna understand dark matter because dark matter is just a neutralino, or whatever, all thing will come together, and things are more difficult.

1:09:33 SC: Nature is subtle and occasionally malicious.

[laughter]

1:09:35 CR: Nature is subtle. So, the idea of, oh, we are there. It’s a bit more far away. People step back, which means also there is more respect. People are… I talk to string theorists more than before and there is exchange. Obviously, we can learn from one another, right, because that’s the way physics work.

1:09:56 SC: Well, and maybe this feeds into the answer to my final question, which is, predict the future, what do you see coming in the next 10 or 20 years? And among those things, do you see convergence between different attempts to think about quantum gravity, or do you see more diversity with more things coming or do you think that someone’s just gonna get the right answer and everyone will agree?

1:10:21 CR: I don’t know. Making prediction is always difficult, especially about the future.

1:10:27 CR: I don’t know, I think that it would be great to think that there is convergence, but in the history of science there have been huge debates, huge disagreements. We’re not in a particularly special situation, right? If you look back at the history of science there have always been very opposite theories, phlogiston or is it… And so on and so forth… Many, it’s Copernicus or…

1:10:50 SC: Computing model of the atom, yes.

1:10:52 CR: Right. Exactly. And usually one of the two sides turned out to be right, and the other turned out to be wrong. And often it took long. It took long to get clarity. What I hope and I would like to see that this is going to happen, is that, I don’t… We can compute a black hole tunnel into a white hole and recognize that some signals are exactly that, and then we have a clear grasp on something which is happening and so, we will see that this is a right direction to go.

1:11:31 SC: Well, gravity is out there in the universe, quantum mechanics is out there in the universe. It’s not completely unrealistic to imagine that somehow it will show up.

1:11:39 CR: Yes, I do expect it will show up, somehow I have a much… It seems much closer today than it seemed 20 years ago.

1:11:48 SC: Alright, that’s a great place to stop. Carlo, thank you so much for coming by.

1:11:50 CR: Thank you, Sean. Thanks a lot.

[music]