Why do we care about loop quantum gravity? Is it a viable path to uniting quantum mechanics and general relativity? What does it mean for spacetime to be discrete? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

Imagine you're watching a play or, or better yet a musical. Yeah. Yeah. Yeah. Musical, let's call it Hamiltonian. And yes, that is a terrible physics pun because the Hamiltonian is a way to account for the total energy of a system is especially used in quantum, you know, never mind. It's just if I have to explain the joke, it's a terrible joke, but it's my joke nonetheless. So Hamiltonian, you're gonna watch a musical called Hamiltonian and there's a bunch of actors and these are all quantum actors in this play. You got your photos, you got your top quarks, you've got your electrons, your pretty sure neutrinos are somewhere but you, you can't really see them. It's just all the, the, the particles and the forces that, that make up existence are actors in this play. And they've got a great musical number that opens the second act. It's just amazing. And where are they? They're on a stage, right? And it's a fixed immovable stage.

They walk and run and dance and sing and do all the other musical stuff on, on a stage. This part is pretty obvious, but it's gonna become less obvious as we go on, you know, all the actors have their marks where important things happen. These important things are events in there, special locations in time and space where important things happen at this point in the play, at this time, at this location on this stage, maybe there's an interaction or moving solo or, or some pyrotechnics happening and the actors move from event to event and they hit their marks in space and time on the stage. And, and we all have a great time. The forces and particles that make up our everyday existence do the exact same thing. They, they live on a stage and they do their thing. They go from event to event, they interact with each other and electron bounces off of a proton. No, that wouldn't happen. An electron bounces off of another electron or an atom absorbs a photon or just just stuff happens and all the stuff of our everyday life happens on a background.

It happens on a stage. The stage is space time, it's the floor, it's the thing that supports the action of the actors. And in fact, the action of the actors depends on the space time stage. If, if you change the way the stage works or looks, the play will be different. You can have the same play out in the vacuum of space in zero gravity or on Mars or inside the giant bouncy bounce house the way the actors move, depend on what's underneath them. The way the physics operates depends on the background, depends on the nature of space time itself. Physics, as we know, it is dependent on a particular space time, you change the space time, you change how it's bending or warping and your physics changes in Newtonian physics. The Newtonian view of spacetime is that space time is fixed and eternal. This, this is a low budget production, it is off, off, off, off off Broadway.

It's just a few folks in like a chair and a lamp for props and the interesting action happens on the stage. But otherwise the stage doesn't do anything else. It's just fixed immutable and it's very boring in general relativity. Space time, it's a little bit more on Broadway. The stage itself becomes dynamic and interesting and alive. Maybe there are trap doors or the setting changes or there are different heights available and gr the stage of space time, it's almost becomes a character in its own right. It changes in response to the motion of the actors and, and, and changes in the stage bending and warping and how the stage looks influences how the actors hit their marks, how they do their action. There's almost a a dialogue and a stretch of this metaphor as far as it can possibly go a dialogue between the stage and the actors, there's a communication there, but still the stage is just the background and the real stuff happens on top of it.

But what if we went all the way? What if we made the stage? I don't know, robotic, artificially intelligent attach speakers to it and let it talk. What if we promoted the stage to its fullest possible realization? What would that mean? Well, what if we were to promote space time to its fullest possible realization spacetime is the background. Spacetime is the stage on which all other interesting physics happen. And yet, gr general relativity tells us that that stage wiggles around and moves around, but it's still not quite at the same level as any of the other actors. Well, if we're gonna take the stage to its ultimate level, we're gonna take space time to its ultimate level, which means we're going to take gravity to its ultimate level. We're gonna make a quantum theory of gravity because that's the big difference. All the actors are quantum actors, photons, electrons top quarks neutrinos doesn't matter.

W and Z bosons, they're all quantum actors. The stage isn't a quantum actor, it's just a background, a flexing moving interesting background, but it's a background nonetheless, that supports the action of the quantum actors. So if we want to explore what it would be like to have the stage itself be one of the characters, it means we have to explore what gravity looks like in a quantum reality. So when we ask just how crazy we can make this stage, we're asking just how strong and crazy can we make gravity? Because quantum gravity only matters at small scales and where gravity is strong on big scales, microscopic scales. General relativity is great past every single experimental test we can throw at it at small scales but where gravity is weak, it's fine. No one cares. There's nothing interesting happening. I might as well not even have a stage but we wanna know what happens when the stage goes nuts when the stage wants to be its own character.

And when that happens, when the stage itself is moving and vibrating acting, is it an actor in its own? Right? Is it just another character? Is it something else entirely? If the stage is its own thing, how are we supposed to interpret things like the passage of time or the distances between points? If if that's an entity in its own right? Not just a background that we ascribe events to and actors hitting their marks. What does it all mean? I'm not just talking in circles. These are some of the deepest questions we can possibly ask. What is the nature of time? What does motion really mean? And the question of strong gravity or quantum gravity forces us to address these questions? Because understanding the ultimate nature of gravity means understanding the ultimate nature of space and time. Why? Because gravity in spacetime are interlinked. This is the ultimate lesson of general relativity. Talking about gravity is the exact same thing as talking about the nature of space time and vice versa gravity, what we call the force of gravity is changes in the underlying space time.

So if we want a quantum picture of gravity, this will force us to reinterpret the nature of space time itself. You can't get away from it. And uh well, we have no idea what's going on. We have no answer. We have no theory of quantum gravity. We have no understanding of what happens to gravity when it gets strong at the very smallest scales. All attempts at reconciling general relativity and quantum mechanics have either failed outright by giving incorrect results or have failed to produce answers. They lead to mathematics that we don't know how to solve. But you know, that doesn't stop people from trying when, when there is string theory. And I did a whole series on string theory. Was it like eight episodes long unpacking string theory? And string theory gets tons of attention not just in this podcast with eight episodes devoted to it, but also in popular culture and also physics culture, people have been working on string theory in some form or another for over half a century. A lot of theoretical work has gone into string theory uh because it's not just a theory of quantum gravity, it's also a theory of everything.

It's also trying to understand the unification of the forces, the origins of fundamental constants. It's just trying to do it all and so of course, that looks really, really appealing and gets lots of attention. But string theory has one little like, you know, has some, some issues which we addressed in the series on string theory. But there's also one part of string theory that I didn't bring up in that series and, and makes you kind of scratch your head when we're exploring this metaphor of what happens when the stage itself becomes an actor, when it gets promoted, when it becomes a quantum entity in its own right. The string theory answer is to fire it, it demotes spacetime back into a fixed background. That's right. General relativity takes a flat universal spacetime of Newton and and promotes it to be this dynamic entity that gets to talk to the actors on the stage but doesn't get to be one of the actors on the stage. And then if we try to take it to a new level, string theory says, no, no, no, no, you're back to being a flat boring background.

But we're gonna add a new character. We hired someone, we call this new character. The Graviton, the Graviton walks on top of the fixed background and can make little changes like like think of it like a stage hand that comes out and moves around parts of the set in string theory. Everything is a string or made up of tiny little vibrating strings, an electron string photon string. Graviton, which is what we experience as the force of gravity is, is a string itself. String theory says no, no, no, no. OK. Einstein, you were smart. You connected gravity with geometry. You told us that what we experience is the force of gravity is really the mending and warping of space time. Uh We're gonna, we're gonna, we're gonna walk that back pal, we're gonna just, just take it a few steps back here. We got a little do wild and crazy. And so we're gonna say gravity is a force just like the electromagnetic force, just like the weak nuclear force. You know, electromagnetism gets its photons, uh gravity gets its gravitons. Oh And that whole space time thing.

Well, we're just gonna assume it's flat and infinite, you know, kind of like what Newton did and we're gonna play all of our physics games on top of it. So string theory, everything in string theory is still dependent on the background. It doesn't explain the existence of the stage itself, which seems a little messed up. String theory wants to be a theory of everything but assumes from the beginning that there is a fixed flat space time to work off of and then builds from that. So it's not quite a theory of everything because it doesn't explain the existence of the stage on which the actors do their thing. Yeah, it just adds a new actor called the graviton to mediate the force of gravity and then leaves the stage alone, that's at least in the present formulations of string theory that might change. People are trying to work on a background, independent version of string theory. Um But there's another approach because we would like our physics to be independent of the background. We would like to not have to assume the existence of a stage and the nature of the stage and then build our physics on top of it, we want to promote the stage into being its own actor where it's still a stage.

But it gets like top billing, you know, come see Hamiltonian starring a wooden stage like that, that can be a thing. It's just one of the actors in its own right where uh the geometry that we associate with gravity is is manifest, it comes from something else. We don't have to assume its existence ahead of time. And this other approach, it's been following the shadow parallel line of development. The string theory has gotten all the attention in the press and the books and the TV shows. And while this other theory called loop quantum gravity like hangs out doing a little cable access show at 3 a.m. on Wednesdays, it's not nearly as studied, not nearly as worked on, not nearly as successful as string theory is. And string theory has produced a lot more papers than scientists and hasn't exactly been successful in explaining the natural universe that we live in. But you know, the different definitions of success. But loop quantum gravity is still there a viable candidate for quantum gravity.

It is not loop quantum gravity is not a theory of everything is not concerned with unification. It's not concerned with the origins of the universe. It's not concerned with trying to understand the origins of the fundamental constant. It's only focused on giving us a quantum description of gravity which which in some ways makes it more powerful because it's much more narrowly focused. You're not out there trying to solve every problem in humanity like string theory probably wants to bring about world peace. If if you add it enough to the theory, loop quantum gravity isn't that it loop quantum gravity asks a completely different question of our stage of our background of gravity in our play when that background gets so strong and dynamic. What if we go all the way? What if we take the lessons of relativity? That gravity is space time and make the stage itself one of the actors, if all the other actors are quantum beings? And what if the stage itself is a quantum being? If we're going to make a quantum theory of gravity? Then why don't we do what Einstein tells us to do and make a quantum theory of space time?

And the hope is the hope is that when you do this, when you say you know what gravity is space time. And if I want a quantum theory of gravity, I need a quantum theory of space time. The stage, hopefully when you construct the quantum mechanical entity and we'll get to that in a second that becomes the stage. You don't have to refer to anything else. Like if I want to build a string theory, I need to refer to an existence of his age. I I need a theater in order to build a string theory play, I don't need a theater to build a loop quantum gravity play because the loop quantum gravity play, one of the actors is the stage itself. So when the crew comes in and the cast comes in, they start doing rehearsals. You've got your photos, you've got your electrons, you've got your top courts, you've got your zebo ons and you've also got a character called the stage that provides the background for everybody else. It sounds weird. So let's stop with the metaphors and talk about, well, some weird physics before I continue, I want to let you know that this show is brought to you by the wonderful folks at better help.

That's better help dot com. I, I know a lot of you listen to this show as a form of therapy, a as a way of, of escaping the world and, and just going among the stars on this wonderful journey. Uh I am a big advocate for therapy. I personally see a therapist and you would be surprised if you don't currently see a therapist, how much they can really help you just navigate a, a difficult life, just like you see a doctor to help you with physical conditions. You should see a therapist. Better help dot com is a way to do that. That's convenient. It's affordable. Uh These are professional counselors that you can connect to online, a range of expertise worldwide. It really is an invaluable resource. Uh As a listener, you can get 10% off your first month by visiting our sponsor at better help dot com slash spaceman. You can join 1 million people who have taken charge of their mental health. Again, that's better help he LP dot com slash spaceman.

If literally everything in the universe is quantized energy angular momentum particles fields, strong nuclear, weak nuclear electromagnetic atomic orbitals. The list goes on and on and on. There's a we are built on a quantum world. You are a quantum mechanical entity, maybe space and time are two. If we want gravity to be a of a quantum description, then we need a quantum description of spacetime. I mean, why not? Right kind of makes sense. It follows from general relativity. It takes that lesson Weinstein doesn't try to reject him but says, you know what? You're right. If we're gonna make a quantum theory of gravity, we're gonna make a quantum theory of space time. A consequence of this is that there are fundamental units of time and space, there are fundamental units of energy. There are fundamental units of angular momentum. There are fundamental units of, of electromagnetic ness. We call those the photons. You can't have half a photon or a third of a photon. You can't slice up the one photon pie and serve it for dessert.

It's you only get one photon. That's the smallest thing you can possibly have. Maybe there's the smallest possible thing you can have in time and space. It means that very, very small scales. How small we we're not sure somewhere around the plank length, but that's not really well motivated. It could be anything. I mean, it's probably smaller than say a meter across. Otherwise we would have noticed but some super microscopic scale like 10 to the minus whatever meters you can't have anything smaller than that. The very notion of space breaks down, the very notion of length scales breaks down it. It's like like pixels on the screen. If you, if you hold a magnifying glass over your phone or your computer, everything looks smooth when you're away out here. But then you zoom in and you see it's just made up of a bunch of little chunks. Your screen is quantized in space, there's nothing smaller than a pixel on your screen because that's the fundamental unit, you can't have half a pixel. And so if you were to zoom in enough, you'd see this discrete nature, this quantum nature of space and time, there's some unit of time, which is the smallest possible unit of time.

So that, oh up here in the macroscopic world, everything seems smooth. I move my arm around and it looks all smooth. You know, I look at the clock and I wait for my TV show to come on and it feels like the passage of time is all continuous. But really it's going to, to, to, to, to, to forward. And as my move, I move my arm around, it's really going tu tu tu tu tu. I'm, I'm making up these sound effects as I go tu tu tu tu. OK. That's cool. So how do we go from Chunky space time to the loops and loop quantum gravity mostly through Patreon to support the writing of my stage play Hamiltonian coming to off, off, off, really, really, really off broadway. And in fact, it might be in another state. We don't, we don't know, we're not gonna get into the details, but I'm writing Hamiltonian. I need your support. That's Patreon dot com slash PM Sutter, P as in Paul, M as in Matthew, Sutter. It's like butter. But with an s now how do you get loop quantum gravity in, in playing with the math of chunky spacetime and figuring out how to represent?

Because it's one thing to have an idea like maybe spacetime is July and quantized. But how do we actually write down the mathematics that support that and describe that and give us a working quantum theory of of space time and gravity physicists in the late 19 eighties, early 19 nineties realized that they could use this random math representation called. Are you ready for this spin networks, spin networks? Listen, I waffled a lot when jotting down my notes for this episode of whether to include this concept or not because it's well complicated and Mathy and weird uh which hasn't stopped me before honestly, but, but it almost stopped me this time. So if you want to skip ahead a few minutes, I'm not gonna mind. Listen. Math is a bunch of tools. Well, OK. Math is a very powerful exploration of logic and the universe. Uh from a physicist perspective. Math is a bunch of tools. It's a toolbox like uh sometimes you invent the tools themselves.

Sometimes you go down to the math department and knock on a few doors and you just see what all the math folks have been working on. And if you see one of these tools that happens to help you understand a model something you see in the natural world or provide some explanatory framework, uh You think then you, you maybe you buy them coffee some time and then you use your new tool to explain the world, the tool you have at your disposal at any one time may not be the best ideal tool. But if it seems to work, you're gonna go with it, you may end up using a hammer to get a screw into a wall. But hey, mission accomplished, right? Besides, we don't always know what the best tool is or the better tool may not even exist. So we have to go with the tools actually in the toolbox and not some hypothetical tool that may exist at the hardware store in the late 18 19 eighties. When we were trying to understand how to actually quantize spacetime. How does this actually work? What is the mathematical tool, the mathematical language that we're going to use to describe?

This? Physicists realized one that they could rewrite Einstein's equations from being in terms of points to in terms of line. So when we think of space time, we, you think of a grid right and at the intersection of that grid, you have a bunch of points. And Einstein's math, the tool he used for general relativity describes a little relationships amongst all these points. That's how we build. Gr well, it turns out you can also use a bunch of lines just thrown around and talk about the relationships between the lines. It's no big deal and lines are kind of loopy sometimes when they feel like it. And so that's how you get the loop in loop quantum gravity. Uh The next step is that we realize that we could represent these lines or loops of space time as a special kind of diagram where a bunch of lines connect together in a bunch of interesting ways this diagram has special rules that it must follow and those rules look like the rules of quantum spin. So spin network I'm being deliberately vague here because at the end of the day, what matters is that there is a certain mathematical representation of quantized space time.

That's really all we care about. There's a mathematical representation for curved space time, Rieman and geometry. If any one of you are interested, there's a mathematical representation for the quantum fields in field theory for the strong nuclear force for the thermodynamics of your refrigerator. There's all these math tools backing it up physics is a mathematical description of our universe after all. And loop quantum gravity has a particular mathematical description of quantized spacetime called spin networks. That's basically it. But the real question is what does all that math mean? How do we describe it? How do we understand it? Like? OK, great. We have a tool that we can use to describe quantized gravity quantized space time. What does it mean? How are we supposed to interpret it? I'm sitting here in front of a microphone and it's now my job to explain what does this mean? Well, as usual in the quantum world math leads the way and interpretations are a little well lagging behind.

I have yet to do my series on quantum mechanics. But one of the big discussions in quantum mechanics is all about the interpretations like we have all this math fancy mathematical structures for figuring it out. Uh But we don't exactly know how to put it into words. Some people are saying maybe we don't need to but that, that's a discussion for a different day, feel free to ask. And it's the same with loop quantum gravity like we have the math this spin network also a spin foam. If you're feeling fancy, what does it mean? How do we go from there to AAA cocktail party description of what the universe looks like and what gravity acts like? Well, one view is that space timing this you've got your normal usual fabric of space time but then you can zoom in and you see all these little loops and networks at the quantum level like you can look at a blanket from far away and it looks smooth and harmonious and maybe there are wrinkles and divots and oh this is this is gravity this I see it but then you zoom way down in and you see all the little threads you see all the way every piece of this fabric I can see, I can see it's quantized.

There's parts of this blanket that once you get down to such a small scale, there's no such thing as a smaller part of the blanket because you're at the fundamental unit of blanket ness. This blanket is quantized folks. And then you look down and zoom into space time and you see a bunch of loops and networks of little, little space time elements, little pixels, little Vaux sos little gremlins, I don't know and they're all interacting with each other in very complicated quantum ways. Another view on the opposite end of the spectrum is that space time doesn't exist. You heard me? Right. Yeah. The, the the there's just the spin network that there's just the math that the fundamental object of reality is the spin network is this mathematical representation of loops and lines and graphs. And it's through that quantum interaction of that graph, it's through that quantum evolution of that spin network that space time arises from it that it's incorrect to think of space time as an entity.

Instead you should think of the loop network, the spin network, the spin foam. And then what we call space time is a macroscopic low energy uh approximation that emerges out of all that quantum shenanigans. I almost said chaos but it's not quite chaotic. You can go either way because really all we have is the math this is probably deserving of an episode in its own right. Sometimes physicists take a mathematical model and say, yeah, this is this is reality like we take general relativity and we say yep, space time is curved get over it. And that's your experience of gravity. And other times we just like in quantum mechanics, we just take the math and we say, well, we don't really know what's going on at a quantum level, we just have this mathematical description. But you can't really picture it or put it into words uh just there's something funky going on. And then we've got all this math so we can make predictions, feel free to ask about just how do we interpret the equations in physics of what is real?

I? And so there are these two camps in loop quantum gravity that either space time does exist and is the thing, it just has this very interesting rich quantum structure. And then we use the spin network mathematics to describe that quantum structure and quantum evolution and be able to make predictions. And then there's the other end of the spectrum which is to say that spacetime doesn't exist. The fundamental entity of reality is the spin network. And that space time emerges from that uh about that, what we see on these microscopic low energy scales is is a flawed perspective of the underlying reality. You can go either way well. So we'll leave that aside. That's a a homework exercise for all of you is to debate the relative merits of those two approaches. But we can ask like OK great loop quantum gravity quantize theory of spacetime quantize theory of gravity. Got it. What does it mean? Well, number one, it means no more singularities, the singularities appear in general relativity in two places and the very centers of black holes.

And it's the very beginning of the universe, these are points of infinite density, which is really just saying these are places where the mathematics of gr breaks down. This is this, this is strong gravity at small scales as strong as you can get. And as small as you can get um in loop quantum gravity because you can't have a singularity, you can't have an infinitely tiny point, there is a no smaller than this kind of scale. And so when you try to compress things down to that very, very small scale, a repulsive quantum force uh emerges. And now it gets to the same like quantum degeneracy pressure that keeps up like neutron stars and white dwarves but just happening at a very, very tiny scale. And so what you would see at the center of a black hole is not a singularity but like a little, a little nugget, very tiny, very compact, very angry but but a nugget not a singularity. Another thing, another consequence of loop quantum gravity is uh no more big bang. Yeah. Yeah. Because of the same thing, the same repulsive quantum force, there's no singularity at the beginning of the universe.

Uh This is coming from a branch of loop quantum gravity called loop quantum cosmology. That takes the main mathematics of loop quantum gravity that people are using to understand fundamental quantum gravity and and chopping off a lot and making a lot of assumptions and trying to directly apply this to the nature of the early universe in loop quantum gravity or Luke quantum cosmology. There is no uh Big Bang when you instead you have big crunches and then rebounds and, and, and this is a little bit difficult to reconcile with the existence of dark energy and accelerated expansion. But I'll leave that to the side for now. What it does do is solve the problem of what's at the beginning of the universe. What's at the beginning of the universe is the smallest possible thing and then bounces off of that. It's possible in Luke quantum gravity to derive uh the entropy of a black hole. You know, Stephen Hawking with Hawking radiation, figure out that black holes glow, they have a little bit of radiation that radiation as a spectrum and anything with the temperature you can assign an entropy to it.

Um But that formula for the entropy that that Hawking and others have have come up with are more by analogy not through here is a fundamental understanding of space time gravity, black holes, etcetera. Um But you can get there from loop quantum gravity, you can recover that formula which is pretty cool. Another consequence of loop quantum gravity is that timing is a, is an illusion maybe that's kind of cool. Um Here's the thing. Loop quantum gravity is a theory of gravity which means it's a theory of space time, space and time. Some of the equations in loop quantum gravity uh don't reference time at all, there is no time parameter in those equations. Instead the equations just work. They just do their thing, different parts of the spin network interact with each other in a quantum way. And then, and then there is no time. They're just the interactions. I know that's weird to think about because you're like, how can there be interactions without the passage of time?

But you know, that's, that's what you get for dropping uh time out of your equation. There are still other parameters that have to evolve. And so my own personal opinion is maybe there's some mapping to time and we're just kind of masking things. But you know, that's neither here nor there. So it could be possible that loop quantum gravity is really just a theory of quantized space and that the whole time thing is an illusion and it's an illusion that's brought about through entropy, through consciousness through whatever. Uh but, but time is not a fundamental dimension or does it have to be a fundamental dimension is in this theory, you can build all of physics with your fermions and your photons and your top quarks and your neutrinos and your gravity with just three dimensions of space. And then the whole concept of time just goes away that's cool. But is any of this right? Is loop quantum gravity correct? Is it a correct description of quantized gravity? I mean, it sounds pretty straightforward quantized gravity quantized space time we got this mathematical tool set that does the job and we can make predictions some problems.

One, no, we have no clue what the chunkiness level is. We have no clue what, what is the fundamental unit of space and maybe time we don't know. And so you would hope that a theory of quantized gravity would actually tell you what it is. Uh But it doesn't, that's, that's, that's a bit of a bummer. There's also this major issue that there's no known way to go from the spin networks describing the quantum gravity to a smooth space time that looks like general relativity at microscopic scales. Right? Remember quantum gravity, loop quantum gravity is a theory of gravity of strong gravity at small scales. So you should, in fact, you have to take this theory that applies at high energies and small scales. And then if you dial things back down to big scales and low energies, you need to recover the physics, you already know general relativity should exist as as a subset of loop quantum gravity in situations where gravity isn't very strong or it's at very large scales.

You should see that you should recover if I zoom into my blanket and I see all the little loops and weaves and everything. And I've got my mathematical description of the loops and weaves when I zoom out that math needs to recover the smooth fabric again. And loop quantum gravity doesn't, we don't know how to connect Luan gravity in the high energy regime. When we take the low energy macroscopic versions of loop quantum gravity, it looks nothing like gr and so right there, you might say, well, we we can test gravity at big scales and if you make a low energy version and it disagrees with the experiments then isn't, doesn't mean your theories wrong. Well, you know, valid argument and lastly, Luke quantum gravity has a big problem with special relativity. You see in special relativity, uh when you move really quickly, um your perception of distances changes, you get time dilation, you get length contraction, uh measurements of distances and intervals in time change, the faster you go.

So if there's some fundamental chunk of space time, there's some quantum block pixel of space time, different observers traveling at different speeds are gonna have different views of what that fundamental chunk is. As someone staying still will say, well, it's this big by this big by this big. Well, someone moving will say no, no, no, no, it's this big by this big, by that big, it's totally different. And then you change that quantum nature that is the fundamental quantum of space time itself. So you change how big it is, you change how it looks and you change all the physics. So in loop quantum gravity, different observers have different views of fundamental physics which breaks the fundamental assumption of special relativity, which is that physics is universal. That's how we got to special relativity by saying, oh hey. Yeah. Yeah. Yeah. Different observers will have different measurements of length of time duration. OK. But what they do agree on is physics and me sitting here on earth, I experienced the exact same physics as someone in a rocket ship going 90% the speed of light.

The physics are universal and special relative. He has been tested. Well, what? And so it's kind of a problem for Luke quantum gravity to say, well, actually it turns out your perception of gravity of how gravity works changes depending on how fast you go. That's no bueno folks. So there are some big problems just like with string theory, there are big problems with string theory and and at the end of the day, the math here of spin networks is not exactly easy. There are problems in loop quantum gravity that have not been solved where we can write down equations and we don't know how to proceed. We don't know how, how to solve them. We don't know how to move on. People are still work on loop quantum gravity hoping to solve these and some other more technical issues with the theory just like string theory loke quantum gravity seems to end up in roadblocks of unsolvable math. And what can I say when it comes to quantum gravity? It's like we're just going in loops. I'm very proud of that one. Thank you to Ty Z on email mark Z on Patreon and Paul W on Patreon for the questions that led to today's episode.

Thank you to all my top Patreon contributors. Actually, all my Patreon contributors, that's patreon dot com slash PM Sutter. Thank you to Justin ZG Chris L. Barbara K Duncan, M Coy D, Justin Z Nate, H and F NAA Aaron Scott M Rob H Pot, Justin Lewis, M Paul G, John W and Alexis. Those are the top ones, but there are many more patreon dot com slash PM. Sutter. Don't forget to go ask a spaceman dot com. Hashtag Ask a spaceman, ask us spaceman at gmail dot com. Get your questions to me. You can keep up with me on all social media at Paul Matt Sutter. Go on my website PM Sutter dot com. There's links to my book and stuff, all the shows I'm doing and I'll see you next time for more complete knowledge of time and space.

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