Can quantum gravity save physics?

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Speaker 1: Hey, Daniel, do you think physics is in a rut? A rut like we've been solving deep problems and delivering amazing technology for too long. You're getting tired of it? Yeah, I mean it's getting a little repetitive, all these discoveries. No, No, I mean, like, is it on the right track? You know? Do you think it will it get to the final deep questions about the universe? I don't know. Sometimes I wonder if we need like a revolution in physics. Does physics need to be disrupted? Somebody out there should start uber for physics. I'm more of a lift person, to be honest, I think they have a better outlook in the world. But yes, that's what I mean. You know, I want to be able to call a physicist anytime in my with my phone. Um, well you already have that. That's this podcast. Nice, I'll give you four stars. Daniel here. Hi am Rhema cartoonist and the creator of PhD comics. Hi. I'm Daniel Whitson. I'm a particle physicist, and I'm here to answer any question of physics on your app anytime. Welcome to our podcast. Daniel and Jorge invent new physics apps. For people who need physics on demand. Daniel and Jorge make up terrible business ideas and pretending they're awesome. Now it's Daniel and Jorge Explained the Universe, a production of Our Heart Radio, in which we talk about all the things amazing and beautiful but crazy and bonkers about our universe because the more we learn, the more it makes sense, the more it seems surprising and amazing. You know, within we joke about it. But I think it would be kind of a useful app. Don't you think, like anytime anyone has a question about anything, you could just request a physicist on your phone own and the nearest or the most available physicists would answer and then answer your question. Do you think that would be used a lot? Like what kind of questions would people ask? How do we reconcile the different theories about quantum physics? Sure? I think it's more prosaic. It's like can I build a force field to keep my teenager in his bedroom all night long? Also? How does my toaster work? Why doesn't my toaster work? That's probably be the number one question again? Can you come fix my toaster? No, we're not engineers. You need a different Uber for that. Yeah, but think about all those grad students and post talcs and physics who are looking for a little extra money. You know, they would, I'm sure they would sign up to answer people's questions. Yeah, you know, there actually is a physics consulting service where if you think you have the great next theory of physics but you're not being taken seriously by the mainstream establishment, there is a service where expert physicists will read your theory and give you like top level criticism of it. Why it's probably not the winner of the next year's Nobel Prize. Yeah, it's called Daniel at Daniel and Jorge dot com. It's true. There are people who send me their theories two questions at Daniel and Jorge dot com, and I try to take a look at them. But there is a service which will spend you know, half an hour or an hour actually digging into your theory. Yeah, but they charge a lot, don't they don't They charge you like fifty or something. You think that's a lot. You think, like, you know, deep deep expertise of the universe should coming like seven. Sure, I mean, you know, I want to know how my toaster works. But not for fifty dollars, buy any toaster Roman for fifty dollars. Well, then I guess you don't really want to know how it works. You just want to buy a new one. I want to buy a transparent toaster so I can see how it works. Well, that's the difference between the scientists and the engineer. The engineer just wants to make it work, and the scientist wants to know why it doesn't work. Yeah, we we just want to eat toast. That's the basic difference. But sometimes scientists also want a theory that just works. And in physics we were making progress towards sort of like answering those deep questions of the universe, but we're not quite there yet. Yeah, you guys have been pretty successful. You have a standard model of the universe and some pretty good theories. You know, you have the standard model, not the not the non standard model. We should have called it something else, right, Standard models just so boring. It's like the Beije model of the universe. We should have called it the amazing model. Yeah, but you have pretty much everything answered right, kind of, except for some basic things about the universe. Yeah, we've made a lot of progress, which is incredible. We've explained electromagnetism, we've explained the weak foce, we've made a lot of progress and understanding the strong nuclear force. And then, in a totally different camp, with totally different people, using completely different mathematics and ways of thinking, we've made some in rows into understanding gravity. Yeah, but there are some You have all these great theories, but there's something not wide right right, Like it works, it's pretty successful for a lot of things, but there's something kind of fundamentally almost wrong with all of your theories. What did you say, Yeah, the problem is we have no theory that describes sort of all of it. We have people who started from the quantum mechanical side of things understanding that building an incredible mind bending theory of the universe that seems to be like an accurate description of the way things actually are. And then we have people working from the other side, starting from gravity and saying like, let's understand gravity is a bending of space man, and they're actually making progress. Also, the problem is these two theories don't agree with each other. They don't play well, they don't have completely different views of how the universe actually is. They're sort of incompatible. It's kind of like if you're building a bridge across the Atlantic, and you know, you build one side starts building a bridge, and the other side starts building a bridge from their side, and you find out that you're nowhere near each other in the middle. Yeah, exactly. Or you're using technologies which just cannot fit to other and that's the problem. Right. You're hoping that there is one truth that you can start in different places in your investigation and sort of work together to find the central truth. But sometimes we wonder if these models just depend on your perspective. There's different ways to look at the universe, and some are successful and some are limited, and so we don't know like which is the right way to look at the universe, if if either of them are even right, or we need something totally new, right, if either of them right, because it almost sounds like maybe both of them are not looking at the world, at the universe and quite the right way, you know, if you can't make them work together. Yeah, And there's a lot of examples in history where physics has been like very confidently almost wrapped everything up except for a couple of little details which turn out when you pull on them, unravel everything and reveal like a completely different view of the universe that was quantum mechanics. You know, in the late eighteen hundreds, people felt like we've almost got this thing wrapped up except for like, you know, the photoelectric effect and the black body radiation, which revealed, of course that the universe is quantized on a fundamental scale, which blew everybody's minds. Kind of makes you wonder if maybe physics needs to be shaken up a little bit, you know, kind of reset or you know, disrupted, or you know, start from a totally needed perspective. Yeah, I think it's possible physics today has all those elements of needing a new idea, of needing a new perspective, of discovering, of stumbling over something which shows us that we've just been asking the wrong questions or starting our solutions from the wrong point of view. And that's not embarrassing, you know. That's the way we explore the world. We start from our current understanding. We see how long it works, and when it breaks, we think, can we just add another little widget to it, or do we have to toss everything out and build it from a different kind of bit In today's episode, we'll be talking about one such idea that might help bridge this connector gap between quantum mechanics and gravity. And it's a pretty new theory when you say, it's sort of it's come out in the last maybe what ten years, twenty years. I think it has its history in the eighties, but it's been picking up steam in the last ten or fifteen years, and it's sort of emerging as a dark horse. It was like first was sort of laughed off as a fringe theory and you know, not nearly this popular other theories of everything in quantum gravity, and so I think it's recently gaining some sort of respectability. It's like millennials, you know, born in the eighties but kind of easily discounted, but now they're taking over the world. Are millennials getting respectable? When did that happen? When we died? There? I'm working on my theory of quantum respectability and currently I have zero there you go. So today we'll be talking about what about that theory. And the theory is loop quantum gravity. What is it? Why is it in a loop? And will it solve all the problems in physics? And here I have to make a shout out to one of our listeners, Jim Milco. Jim has been sending me questions for about as long as we've been doing this podcast, and he has been consistently asking for a podcast episode about loop quantum gravity. So thanks for your patients, Jim, here's your episode. Awesome as he sent you, any idea is worthy of a Nobel price yet, Jim has sent a lot of ideas and they're always fun to read. So if you have ideas about the universe, or just questions, or there's something you really want to understand more deeply and you think we might be able to break it down, please send your questions, your suggestions, your fundamental new theories of the universe to questions at Daniel and Jore dot com. Yeah, thanks to Jim for listening and to everyone out there listening. And so as usual, we were wondering how many people out there know or have heard of loop quantum gravity. It's sort of a it's sort of a it has two sort of technical familiar words, but then a pretty common word, which is loop. I was very curious in these interviews to see if anybody had heard of this thing and had any ideas. So I went in with a completely open mind. So before you hear these answers, think to yourself, do you know what Luke quantum gravity is? Could you describe it? What would you answer? Here's what people had to say. I mean, I would assume it has something to do with gravity, nothing at all, nothing at all. So I heard of quantum mechanics and read like brief history of time. Okay, but I don't don't know what loup quantum gravity could be. What came to mind first was entanglement. But I don't think that necessarily has anything to do with that. So it's just as opposed to just making up something that I think it might be. I don't know. I have not no no idea that I'm not No, I have it nothing. I don't know, all right, not a lot of penetration in the public market. No, these folks have to work on their pr for sure. String theory has them beat, for sure. Maybe they need to get it mentioned in the next Big Bang Theory episode. Oh maybe we should create a setim called loop quantum gravity. I want to see you pitching that, like, Hey, it worked for the Big Bang theory. We just need another physics theory named titled sitcom that makes fun of nerds. Yeah, well I'm in as long as we have characters that both are smart and can talk to human beings. Oh really yeah? Any Oh that hurts, man, That hurts. So not. Not only a lot of people seem to have heard of it. I mean I imagine they most of them have heard of quantum mechanics or have heard the word quantum if anything, from the Marvel movies, and they surely have heard of gravity, but loop quantum gravity is pretty pretty unknown. Yeah, it's a little bit esoteric, but it's fascinating and it has really interesting ideas, and those ideas could have really deep implications. It's the kind of theory which, if it's correct, really changes the way we think about the entire universe and the fundamental nature of our relationship with it. So it's super fun to think about. And it's also really complicated and very technical, and there's a humongous amount of complicated math involved, none of which none of which you will have to understand to get the basic ideas today, which is why Daniel, you went out there and found an expert to talk to about this topic. Right, that's right, because I'm a particle physicist and my expertise doesn't extend to theories of quantum gravity. So I went and I talked to Bianca Dittrich. She's a professor at the Perimeter Institute in Canada, and she's an active researcher on the forefront of quantum gravity. So here is Bianca. So my name is Bianca Ditwich, originately from Germany. Also studied in Germany and then oscillated between the Canada and Europe a number of times and ended up at in Canada. It's a parameter institute, a faculty. All right, cool, So Bianca explained to you what loop quantum gravity is. Yeah. I had a long conversation with Bianca about why we need loop quantum gravity, how it solves the problems in physics, where the current criticisms of it are, how we could test it, and of course then I had to ask her at the end about black holes. Of course, because every good conversation ends in a black hole literally, right, maybe at the end of the universe, all that every conversation, everyone will ever have, all that information will end up maybe in a black hole. Well, what happens if you take a black hole and you put it in a toaster oven. There's a question nobody's ever asked, if I need to ask that a crunchy outside of the inside will still be cold black hole temperature. Yes, So what if I put a black hole in a microwave? Then do I heat up the inside of the black hole. That's a question I've never thought of. That's really a fun question. All right, Well, that step us through what Bianca explained to you, Daniel was so first of all, I guess what is the big problem in physics? Did that needs to be fixing? The big problem is that, as we said earlier, we have two different basic ideas about sort of how the universe works at the smallest scale, and they start from very different places, and as you said that they when they come together across the Atlantic, they just don't meet. And those two ideas are quantum mechanics and general relativity of theory Einstein's theory of gravity. It's weird to me that there would be too so many so different theories about the universe. You know, wouldn't they all sort of meet at the at the you know, plus and minus mathematical level. Well, you'd hope so. And this is sort of the way physics starts. You know, if your caveman cave woman physicist, you begin building your model of the universe by sort of looking at all this stuff around you, cataloging what it can do, and then saying, can I understand all of this stuff in as simple as possible terms? And so you're like, all right, there's the thing where things fall down, there's the thing where lightning comes from the sky. You know, there's a thing where there's wind. And then you have a very long list of stuff and try to boil it down and say, oh, you know, this lightning thing is the same as this other thing where I get zapped, and things falling down is actually the same thing as stars moving through the sky. There's all these moments when we sort of shrunk the list of ideas we need to explain the universe. And so you hope that as this list gets shorter and shorter, they fit together nicely. And that's happened a lot of time so far. Electricity is the same thing as magnetism, and it's all part of this other thing called the electroweak force that has happened. But sometimes you get, you know, you get two puzzle pieces and they just don't fit together, all right. So maybe step remind us Daniel, what each of these theories are, Like, what's the what's the easy way to describe what general relativity is, And what's the easy way to describe what quantum mechanics is. So general relativity is a way to try to understand what is gravity. And remember that Newton said gravity is things pulling on each other, things that have mass pull on each other. So take a force, yeah, Newton, that was the old thinking. That was the old thinking. Newton thought gravity is a force, and it seems kind of like a force. It acts like a force, like electricity and magnetism, and he was able to write down an equation that described how the Earth moved around the Sun and how to move the Moon moved around the Earth, and it works pretty well. Now, Einstein came along and he gave us general relativity, which is a complete reconception of how gravity works. He said, no, no, gravity is not a force. Gravity is a change in the shape of space and time itself. Like you have a blob of mass or blob of any kind of energy density, it changes the shape of space and that's what makes things move in curves. Rather than moving and what feels like a straight line to us, things moved through bent space. It's not like a mysterious invisible force that pulls things together. It's more like when things exist, they distort, they bend the space around them, and that's why they come together. Yeah. And it's a beautiful idea and it's been tested exilient ways and it works. It describes very tiny deviations in Mercury's orbit. It describes how light bends around the Moon during an eclipse. Big stuff, big stuff, Yeah, really thoroughly extensively tested. It predicted gravitational waves, which we've actually seen black holes to right and black holes. Yeah. And and the thing to understand the things that that's going to be make it in conflict with quantum mechanics is that it's a classical theory meeting that it assumes that, like you can describe everything perfectly, that everything has a position and a direction and a location, right, and that the space is operating in is smooth, Like you can subdivide space as many times as you want between me and you. There's an infinite possible locations for like a ball that we're throwing back and forth. That's Einstein's vision of space. Right, it assumes that everything is kind of smooth and continuous, and there there are no bumps in the universe at the microscopic level. That's right. You can zoom in forever according to Einstein and general relativity, and things still stay smooth and and everything has a fixed location. Right, the ball is somewhere and space and time makes some sense. There's no uncertainty or or weird or weirdness. That's right. The world can be known, everything can be determined according to general relativity. But then you have quantum mechanics, that's right. And then on the other hand, building from the other side of the ocean, you know, the toaster side of the ocean instead of the microwave side of the ocean. Um completely mixing metaphors, there is quantum mechanics. And quantum mechanics, of course developed in the early part of this century to explain some things that didn't make sense, some experiments we saw which just could not be described using a classical understanding of electromagnetism. It was really what was light that gave us the first clues. We did a whole podcast episode about the photoelectric effect and how we know, the photon is a thing, and they did these experiments that just didn't make sense unless you thought of light as being made of tiny little packets of energy rather than a smooth, continuous beam. You had to think about it as these tiny packets. When you zoom in, things don't behave nice and smooth and continuous there are sort of lumpy and clumpy. Yeah, And there were also just some problems with the theory, like there's a kind of radiation in the universe called black body radiation, which is just how things glow, Like everything in the universe has a temperature and it glows with that temperature, and quantum mechanics predicted that well. Classical theory electromagnetism predicted a certain spectrum, like if you're this temperature, you should glow with this color. If you're that temperature, you should glow with that other color. The problem was that it predicted that for certain temperatures you should glow at a crazy nonsensical color, like if you get an infinite amount of radiation. The original theory, the one before quantum mechanics, said that things that certain temperatures should have an infinite amount of light at very very low wavelengths, right, it really really sort of it's called ultra violet light, and that they should have like a ridiculous amount of infinite amount of ultra violet light, which is nonsensical. So you can do an ex perament basically that proves that general relativity doesn't work for all cases. Well, you can do an experiment that proves that the old theory of electricity and magnetism doesn't work. And we knew that didn't work. We're like, okay, this theory is making a prediction that just doesn't make sense. But if you add quantum mechanics to it, quantum mechanics says, oh, that's just because light is not continuous. If you make light into chunks, then if you predict that, you don't see this crazy behavior. And that's in fact what we saw in nature. And so we needed to sort of change the theory. The theory gave nonsensical results. I mean, I have to add this bit to it to say bit that says, okay, well there's a minimum size to a photon, and that solved that problem. It said, okay, now you can make realistic predictions. And so but the thing to understand is that like quantum mechanics changes also the way we think about the universe, right, it says things are uncertain, things are fluctuating. This is like crazy randomness the heart of the universe. All right. So those are the two two big titans in physics theories, right, general activity and quantum mechanics to try to explain the universe. And the problem is that they don't play well together. So let's get into a little bit of why they don't play well together and whether this idea of loop quantum gravity can help solve that. But first let's take a quick break, all right, Daniel, So general relativity and quantum mechanics don't play well together. Is it fair to say? Is it? Is it kind of like I'm trying to think of an analogy. Is it kind of like microeconomics and macro economics, Like, you know, macro economics is good for you know, big economies and countries and and micro economics is good for like how people make decisions, but like somehow they don't always. It's weird to think about where they meet in the middle. Perhaps, but in those two cases, one should be an extreme case of the other. Here it's really you have a fundamental different view of how the universe works. You know, it's it's more like totally different kinds of art, you know, like figurative art versus symbolic art. Like what's a better way to describe the human experience? You know? Is that through abstract splashes of color or is it by you know, depicting the things we see in interesting juxtapositions or something. I don't know, it's right, right, it's like a totally lens or you know, totally set of glasses. Yes, it's a completely different way of thinking about the world. And people have been trying for a long time to bring them together to say, all right, are these things in conflict or can we make a consistent sort of quantum theory of gravity that brings it all together? Right? And so here is Bianca talking about that question. It's one of the really big outstanding questions and physics and and my view it really means quantum gravity should include a new notion of space time. We have to replace it, however, with something completely new we expected that which change quite dramatically as a foundation, so physics um and get very interesting insights into you know, the nature of space and time. All right, So it seems like we need something totally new here um, And so Daniel, can you explain to me kind of what the problem is, like why can't they play well together? The ones that we have. Well, people have been trying to bring these two things together since basically they've existed. You know, people have been working on making gravity into a quantum theory for a very very long time. But you know, quantum theories traditionally thinking about forces very differently than gravity does. They think about forces is like ripples in a quantum field. You know, you have like the electromagnetic field, and you think about how do photons communicate the electromagnetic field. There are ripples in this electromagnetic field, and you quantize those ripples. That's how quantum mechanics thinks about forces. But in general, relativity of force is really more like a bending out space. That's right, And mostly you can do this. You can say, like, all right, well, can I make a quantum theory of gravity? Can I build a theory where I exchange where gravity is a field and it has ripples and those ripples are quantized, just like I can for electromagnetism, And mostly you can, actually you can. It's mostly it works, but it fails in some moments. Couldn't you just have quantum fields in a general relativity you know world? You know, like, couldn't you have quantum particles that bend space as well? Like why can't I a quantum particle also bend space like a planet? Well, but then gravity is coming from something that's classical, that's smooth and continuous, and general relativity is just not a quantum theory, right, Like it assumes that you know everything about the location and the time and the position. What couldn't you just make the bending also kind of uncertain? I'm just trying to figure out why why you can't marry the two, you know, why can't these particles and these fields exist in a world that is as Einstein envisioned with the bending and the gravity that's exactly what Looke gravity is? Or hey, you just basically invented the idea prize right now, Thank you very much. That's right, And you know this other idea that like, let's take the gravitational field and think about as a quantum field that mostly works. The important thing to understand is that it only fails when gravity gets really really strong, right, things like inside a black hole, and what happens is that it makes nonsense predictions like it just like we saw for the black body radiation. It predicts like infinite amount of energy will be released, but we know that's not true. We know it doesn't happen. And so the theories work except in extreme circumstances. And that's what makes it really hard, is like it's easy to fix the theory when you can see it breaking. But these theories general relativity and quantum mechanics, they only disagree about like what happens in the inside of a black hole, and we can't see that. Why don't we just like nor black hole? If we just like nor black holes in the universe, wouldn't that make our there's a lot wouldn't that make physics a little bit easier, you know, because it's all it seems like it seems like everything breaks in a black hole, and so why don't It's like, you know, the mess in my closet. Let's just ignore it's the engineering, you man, that's the engineering. The scientist in me is desperate to know. You have to know inside a black hole. Yes, if you have, you created a black hole in your closet, because if so I'm coming over and I want to check it out. I think we're close to the singularity. To be honest, in there, you don't want to go in there that you may never come out like you have all these rejected toaster microwave prototypes in there, that's right, all these rejected app ideas as well. And so these two things aren't working together. Well. People have tried, but they just give nonsense predictions like the calculations you get suggested, you know, infinite amount of energy in a radiated inside a black hole when gravity gets really strong, And we can't easily test that because we don't have a black hole and and I don't have a galaxy sized particle collider. I would need to create the sort of energy density in order to break quantum gravity. And so what we need is we need to see inside a black hole, or we need a new idea, and so we need Jorge's vision of quantum gravity. I have an idea. How about something called loop quantum gravity. That's such a great name too. All right, So we have these two fourth theories, and you know, one of them has an h thunderbolt cable connector and the other side is a mini USB cable connector, and so they don't fit. And so that's what this idea of loop quantum gravity is. It's like, maybe it's like an adapter. That is it like an adapter that makes it to work or is it a totally new cabling system. Yeah, it says instead of trying to think of gravity is like a Newtonian gravity as a force um and and and quantizing that gravitational field directly. Instead take Jorges idea, which is just takes space itself and quantize that and say, well, maybe general relativity is basically right, except that instead of working on a smooth continue in US space, we work on a space that is quantized, like there are pixels of space instead of space being infinite and smooth, they are like little chunks of space. It would explain how quantum particles move, or it would explain how gravity can be quantized. It would allow us to build a theory of quantum gravity that doesn't make nonsense predictions, because it basically makes the places where those predictions get nonsense impossible, like the predictions of quantum gravity breakdown really high energies and really small distances. So the idea is basically, well, what if really small distances aren't allowed, like there's just nothing smaller than ten of the minus thirty five where things break down, then there isn't the problem. Let's just ignore the closet and assume that the closet is QUANTIEDA it's just one thing, I see. Yeah, it's it's not ignoring the closet. It's saying the closet doesn't exist. You were so worried about the closet. It turns out it's impossible. There is no clause it. But it sounds like you're trying to make gravity quantized. You're not trying to make quantum things the bendy like space. Well, if space is bendy and quantized, then you quantum things move through quantizes bend bendy space. So this would allow you to bend space in a quantized way. Yeah, it would allow you to bend space in a quantized way. And it's it's kind of a beautiful idea because it's very similar to the origin of quantum mechanics itself. Right, we solve the problem of black body radiation, of this thermal emission giving crazy numbers by saying, oh, maybe we can just um quantize photons and that mathematically solves that problem. Here, we're solving the problem of infinite emissions inside black holes and really high energies and small distances by saying, oh, maybe we quantize space, and so those small distances are impossible. So there's there's a beautiful sort of analogy there to previous structures of solutions that suggests like tensively like, oh, maybe this is the right track because it feels like nineteen fifteen a little bit. That's right. We're gonna party like it's people are growing mustaches and wearing top hats, and you're like, I like it. I like it, except for the fact that like, no women are allowed to do physics. That's the bad part, all right. So the idea is that maybe space and time itself like space and time like there are no there's no there's no infinite as at least small second. That's right, And that's a question people have been sending me over email many times. If you quantize space, you also quantize time. And you know, space time is a fabric in this theory and they're deeply connected, and so if you're quantizing space, then yes, you're also probably quantizing time. Well, it's like you're there's actually a ticking clock to the universe. Maybe, yeah, there's a minimum time that makes sense. You know, you can't infinitely divide a second into arbitrarily tiny slices. There's a minimum of time below which makes no sense. Okay, And so what does that mean? I guess space is not really like space. Space is more like a clump of little spaces. Well, the reason it's called loop quantum gravity is that the way to think about it is that space is a bunch of these loops and they're sort of woven together. And so think about spaces like chain mail. If you're into D and D. You know, it's like a bunch of links that are all connected to each other. And space comes out of that, right spaces, There are these The fundamental thing in the universe are these little loops, and the loops are woven together. And the way that things move through space, and the way that gravity happens is how these little loops interact with each other and pull on each other and are built together. To what are these little loops? They're like little bits of the universe. That's the next Nobel prize, man, you want to win two Nobel prizes in a single podcast, in a single let's do you have enough for this to be a physicist? For the what do you call the hat the hat trick? A hat trick? You're gonna go for three Nobel prizes. That's when you get fifteen minutes off, Daniel, Let's do it when you invent the perfect toaster microwave combination, that definitely is worth a novel prize. Well, nobody knows what these loops are, and you know that's sort of the next question, Like if you can start from these loops and build a theory that describes everything and predicts the universe and it is consistent with everything we know, then the next question is you're like, well, why these little loops? What do they mean? How do we get those little loops right? Where do they come from? Do they emerge from something deeper? You know, physicists are great at making answers that create more questions. But I guess why call them loops? Are they actually like little rings that are tied together? What does that? Why not call them I don't know, bubbles or uh pixels or you know what I mean? Like, what is it about the idea of a loop that gives it the name loop quantum gravity. Well, I guess bubbles would work also, But I think it's just a visualization idea. You know that these things are linked together um and it's sort of a mesh, and you know there are also loops, like we're doing calculations around in a circle. You have to avoid a sort of trap, Like you can't think about these as loops in space. They're not like um circles on some axis, right, because when you have what is that access, we're talking about the very nature of space itself. It's not like these loops are somewhere and like this loop is at this point in that loop is at that point. The loops are the points like space is these this thing we're talking about like the nature of space itself, which is really hard to sort of wrap your mind around. Oh, I see you. You were just looking at you guys were just looking for like a word that implies that space is made up of little things that are kind of interconnected or overlapping, right, or like hooked together. M m okay. And so you went with them loops. I was like loops. They went with loops. I think it's not a terrible idea, you know, they could have called it the bubble quantum gravity, but hear it. But you know, we talked about this another time in the podcast. You can think of these things as little bubbles, and then you get to talk about things like space foam and quantum foam because these loops aren't fixed, right, they can like fluctuate in and out of existence. I see. So these are all related ideas loop quantum gravity, space foam, quantum foam. They're all sort of coming at this idea of a bubbly space and thinking about space not as continuous and smooth but made up of little quantized bits, like let's quantize space itself because general relativity is an attempt to understand gravity in terms of the nature of space. And so if we're gonna get a quantum theory of gravity, the idea is, let's let's just quantize space itself, all right. So that's quantum loop or loop quantum gravity, and so we'll get into what are some of the open questions and why people think it may or may not be the ultimate answer to how the universe works. But first let's take a quick break. Okay, Daniel, what does it all mean? What does loop quantum gravity mean? If it's true or if it's not true, and why do we think it might not be true? All right, well here's the big letdown. Right, So, so far it seems like you're gonna break the loop. I'm gonna break the loop. Yeah. It seems like a beautiful idea. It seems really promising. It seems like dot dot dot the next obvious step in physics. Right. The problem is there's a lot of problems with it, and there's a lot of open questions. And that doesn't mean it's not gonna work. It just means it's far from clear that it is going to work. Well, I see, there's a room for us skepticism. Yeah, and even among physicists there's a lot of skepticism about loop quantum gravity. It's a bit of a fringe theory. There are a lot more people working, for example, on string theory, which is a completely different attempt to unify gravity and quantum mechanics and all of the other forces all at once. String theories like a theory of everything, where loop quantum gravity is like, let's just focus on make in gravity of quantum theory. If you can do that, then that's another way to unify these two big theories. Yeah, the string theory is sort of a The other idea is that the opponent of Looke quantum gravity, let's build a tunnel under the Atlantic instead of trying to make these two bridges work. And the string theory is a much bigger community, has a lot more people working on it, people are sort of more excited about it, whereas Looke quantum gravity is like, you know, the younger sibling in terms of theories of quantum the underdog exactly making me like it more now, And one reason is that it sort of hasn't yet delivered on its promise. Like, what you want to do is start from this concept of loops and from it should emerge Einstein's theory of relativity. You should be like, I'm gonna have a completely different picture of the world, but it's going to give the same predictions sort of at my scale. I should still describe how a ball flies over my neighbor's wall, and how and gravitational waves and all of that stuff. You should be able to sort of are from there and build up to general relativity. You start off imagining the world the universe is a mesh of of loops, but it doesn't give you baseballs and fences. Yeah, not yet. And it's not like they can't do it. It's just like, well they haven't quite figured it out yet, and it's hard, right this is are difficult things, and people are hacking with their machetes through the jungle of mathematics that's involved, and it's not always clear that we have the right tools, and sometimes progress is paused for like a decade or thirty years until somebody's like, hey, we've heard this new idea from mathematics. Turns out that's exactly what we need over here. That kind of thing has happened in physics at a lot of times. And there are other sort of really interesting problems with loop quantum gravity, which actually conflict with relativity. Okay, so there's maybe something fundamentally wrong here, you know, or not necessarily wrong, but it would again change the way we think about the universe. Like, here's the problem with quantizing space. Says there are space pixels, right, that means that in some sense there's a minimum distance to the universe. But what is that distance? Because we know that distances depend on velocities, right, Like as you move faster, things get shrunk, so do things that are moving see smaller space pixels than things that are stationary, And if so, it would mean that there is some sort of absolute velocity to the universe. Right, if you can measure your space pixel the size of your space pixels, that tells you sort of what your absolute speed is. But special relativity has always told us velocity is relative. There is no absolute frame, and so this is sort of conflict with a really really core concept and special relativity. Couldn't it be like the speed of light, which like it always looks the same no matter how fast you're going. Couldn't these loops always look the same no matter how fast you're going. Yeah, So there's a lot of ways to try to solve this problem. One is to say, you know, we have these weird space pixels which to form under certain circumstances, and there's some very complicated mathematics involved to do essentially what you just described. You're really hidden out of the park today with like going for the hat track here, wait wait till you see why what I pull out at the last five minutes. And the other is to accept it and to say, you know what, maybe there is maybe there is an absolute reference frame. Maybe special relativity is slightly wrong, But wouldn't that throw the whole light speed thing off. Well, not all of special relativity has to be wrong. The consequence would be that light would travel at a slightly different speed as a function of its frequency. So like high frequency light would travel that is slightly different speed from lower frequency light like X rays and UV light would move faster than radio waves. But it'd be a really small difference to be really hard to tell, all right, So there's some skepticism or I guess some open questions about it, because it's not quite hitting it out of the ballpark, over your fence or not into your neighbors house or not um And so I guess the question is what does it all mean? I mean, what if it happens to be true? And how are we even going to know of something like this is true? Yeah, so of course the deep question I have is what does this mean for the nature of the universe? And how could we see it? Right? Where would it manifest itself? And as we talked about at the beginning of the episode, the place this would really be important, the place you would notice at difference, the place where it matters is inside a black hole of course, of course, right, so let's go there, Let's go into the closet there. That's right, we're opening the closet and all who knows what's in there and who knows what's been growing? And so I asked Bianca the professor, because she thinks about this stuff, right, she thinks about what happens inside black holes when gravity gets really strong and space gets twisty. And I asked her, like, what is your mental image of inside a black hole? Because I want to know, right, I mean, there's a black hole in her mind at least, and I want to know what does it look like inside her mind? Effects will be only important very near the singularity. There, the space and coviatures really getting very large. And maybe I'd like to think that there's a new universe which opens up with each singularity, which gives us much more universes than we know of. Yeah, she went there, she went for there are new universes inside black holes. I love it. There's a new universe in my closet. I would believe that. And inside that closet there are other closets with other closets inside them, I meaning meaning that's kind of a consequence of quantum blue gravity is that when you get to these extreme singularities, these loops kind of what like open up or you know, become their own little universes. Is that the idea, I will not pretend to even understand what that means, because there's some crazy consequences that like, if there's a universe inside every black hole, is our universe inside a black hole? You know, maybe who knows? It's crazy stuff? Yes, is your answer. I go with the s and that's my third Nobel Prize claim. Or is that the conclusion to your pitch or your loop quantum gravity sitcom episode there Echo at the end of season twenty, it turns out they're all in a black hole. That's right, And you know, we might be able to figure this out in ways other than going to a black hole. We actually do have ideas for how you could test this, and they revolve around seeing whether we can tell the difference in the speed of light from high frequency and low frequency light. The speed of light changes, which we didn't think it did, but if it changes with frequency, then that's an indication maybe there's something loopy going on. Yeah, maybe there's something loopy going on, And this would be a really small effect. So what you need to do is like have a race between photons of different frequencies high frequency versus low frequency, and the race would have to be super long because light goes super past of course, and it's different is really small. And so what they do is they look for light sources that are really really far away and then try to measure see if there's a difference in like the arrival time of photons. Would you would I need to know where they're coming from, right, Yeah, So we have these sources which are variable, like quasars and pulsars that are really far away. You know, they pulse, and so you can sort of line the two up and say, well, we know these two were emitted at the same moment because they are the top of the pulse at the same time or something. And then you can say, well, did they arrive in sync or are they sort of out of sync? And so there's a satellite which looked out and measured this and looked for this. What did they find, Well, this is the integral satellite. And that's some tortured acronym for I don't even know what is it. Really, It's called integral up and what they found they didn't see anything, right, They didn't see any difference in the arrival times. I didn't see any loops. And they were able to say that if there are loops, they're smaller than ten to the minus forty eight meters. What they can do that, that's by looking at two photons. Yeah, by looking at these photons, because the photons come from really really far away, and so the differences would be added up across the universe. And they claim they should have seen them. They saw that they were in sync. They were in sync. Yeah, and this is thirteen orders of magnitudes smaller than what we think the size of the loops are. So like that seems pretty definitive. Yeah, but what did the loop quantum gravity folks say, Well, you know, there's only one branch of loop quantum gravity. The people who say that maybe that's that special relativity needs to be tweaked. Other people have solutions, as you say, to make light travel the same speed no matter what, by having you know, crazy deformations these things, and all sorts of complicated interactions between light and space, and so it just sort of rules out one branch of loop quantum gravity. You can never kill these theories. Man, They're like weeds. They always there's another version that crops up there, like exide one head of But they're fascinating, you know. Each one is a wonderful exploration and sort of the intellectual space of like how could the universe work? What possible universes might we live in? And the amazing thing is that there is an answer, right, there is one way that the universe actually works. And what's kind of interesting I think is that it wouldn't affect your everyday life, like things would still work the same way. But who knows what's going on down at the fundamental level? Right, it could be one of these crazy theoretical ideas. Yeah, and it's important now to scientists, but it might eventually be important to engineers. You know, knowledge that seems impractical and useless but reveals the fundamental nature of reality could eventually be useful and hate to me, that's fun anyway. I don't really care if it's useful. I just gotta know, right, Yeah, I mean I think that's how micro rays were invented, actually by accident, by looking at something else. And where would we be? Where would burritas be these days? That's right. And you know, there have been actually fascinating advances in microwave and the technology the engineer ring of microwaves. And I guess, you know, if you can unlock kind of the magic and the power that's happening at that microscopic level, who knows what you could do, right, what kind of energies you can get, or what kinds of warp drives or microwave ovens. Yeah, microwave toaster opens powered by artificial intelligence. Yeah, And so I have to give a shout out to Nico. He's one of our listeners and he heard our episode about microwave ovens actually, and he wrote to me and told me that they have a new fancier AI powered microwave oven um which can heat stuff up at different temperatures and monitor to make sure the heating is all smooth and fancy, and actually dropped by my house and delivered one of these prototypes, which has been a lot of fun. That's amazing that you let one of your listeners into your house. Hey, he was offering a brand new appliance, so I thought. And while he was there, I asked him about Luke quantum gravity, but he didn't have any ideas. Oh man, there's no setting on that in the microwave, not yet. Um. The next in the prototype will be quantum gravity powered, I'm sure. All right, Well, it sounds like the answer is stay tuned. You know, that's what we explain, what Luke. Quantum gravity is and who knows, and maybe it is the way that the universe works at that level, and we may find out in the near future. That's right, And these are the kinds of mysteries that are going to help us figure out the deep nature of the universe. You know. It's when things break that we have an opportunity to figure out how things should work. And so it's exciting to have these kind of problems. It's exciting to know sort of where to work, even if we don't know what the answer should look like. All right, well, we hope you enjoyed that. Daniel. How much this podcast costs me in terms of your physics time? One loop one? Let's tend to the minus thirty five dollars, My tend to the minus thirty one plank dollar? Yeah, pick up plank dollar. Oh good, I gotta I got a couple here in my pocket. Okay. Quantum of the mover to me all right. We hope you enjoyed that. Thanks for joining us, and as usual, thank you to everybody who wrote in with your questions. See you next time. Before you still have a question after listening to all these explanations, please drop us the line. We'd love to hear from you. You can find us on Facebook, Twitter, and Instagram at Daniel and Jorge That's one Word, or email us at Feedback at Daniel and Jorge dot com. Thanks for listening and remember that Daniel and Jorge Explain the Universe is a production of I Heart Radio. For more podcast from my Heart Radio, visit the i Heart Radio, a Apple Podcasts, or wherever you listen to your favorite shows.