Will Emergent Gravity Rewrite Physics?

[Music] it feels sometimes like there are layers layers to the universe you know imagine a scene like you're eating some breakfast at the topmost layer is your conscious awareness and the decisions you are making you do you reach for the eggs do you reach for the bacon do you reach for the cereal these are decisions you're making beneath that layer there's the macroscopic physics the Motions of your hands and fing fingers the the momentum of the cereal as it pours into your mouth you the liquid slushing of the milk uh you reach the for your glass of orange juice or your cup of coffee and there's the potential energy change you know there's all the macroscopic physics beneath that there is biology you the crunching of the food between your teeth your your salivary glands your your stomach muscles Contracting and and doing stuff I'm not a biologist the peristalsis of your esophagus beneath that is biochemistry they proteins doing their job of chopping up fats and sugars they're the acids in your stomach breaking down Foods there are cell membranes emitting some elements and molecules and rejecting others you beneath the biochemistry is the chemistry the atomic bonds The Cove valent electrons beneath that is physics electrons orbitals electromagnetism strong nuclear forces beneath that Atomic physics is the Quantum field Theory the vibration of these esoteric Quantum fields that subsume the entire Cosmos and interact with each other in complicated ways beneath that is well we don't really know but that's not the point of today's episode the point is that there are layers and we feel like all of these layers are connected that you can start on any layer and use your understanding of that layer to work your way up for example if you understand biochemistry how molecules and elements interact with each other with your proteins and your DNA and all that then you can figure out how biology works and if you understand how biology works you can use that to understand the mechanics of the choices and actions of of eating breakfast and if you start at an even deeper level if you go all the way down to the basement to Quantum field Theory and if you worked really really really stupendously hard then you could use quantum field Theory to describe the action of eating breakfast in terms of interlocking overlapping interacting Quantum Fields but you can't I mean sometimes you can in limited ways I'll give an example in a little bit most of the time these layers are relatively separate from each other think about it quantum field Theory may help us understand how subatomic particles interact with each other and in high energy experiments it gives us some understanding or a refined understanding of how atoms behave but it's totally useless in providing an explanation for how acids in your stomach are breaking down your eggs and bacon use different tools like chemistry to figure that out and the key thing is that you can figure out most if not all of the rules of chemistry without ever knowing about Quantum field Theory imagine a universe where we never figured out quantum field Theory we would still know chemistry we would still know how the stomach breaks down food we would still be able to figure out biology and biological mechanisms how energy is transported inside the body all that without knowing Quantum field Theory even though there are layers they don't necessarily connect to each other in obvious ways and there's a word for this kind of layering how one layer even though we know at the base level yeah we're all interlocking Quantum Fields but we don't know how to use that language of interlocking Quantum fields to describe say the digestion process of bacon we can't jump all those levels and use quantum field Theory to describe that process there's a word for that kind of laring and that that word it's our word of the day it's emergence the cool idea behind emergence is that you can have layers to physical systems and discovering the work of one layer say a nice deep fundamental layer like Quantum field Theory or physics or even biochemistry doesn't always give you better information or any information at all about how the higher levels work that's because the properties of the higher level system emerge from lower level interactions in nonobvious ways presumably there's some way to make the connection but it's not always guaranteed I could go to the world's most renowned Quantum theorist the smartest person on the planet most e the the deepest expert in Quantum field Theory and say hey I'm going to give you a physical system and I'd like you to describe it with Quantum field Theory and they say sure it's my job and if I say uh it's two electrons interacting with a photon they say got it all right they write down some equations they're good I say okay now I've got a different physical system for you the digestion of bacon in a human stomach and they'll say get out how did you find me we can even though it's all really deep down based on Quantum field interactions we don't know how to express that as another example we can know all we want about biochemistry and neurology and how the brain works and synapses and sodium ions and all that but we can't explain the emergent property of Consciousness can we it may not even be an issue of practicalities it may not even be an issue of oh we just don't have this sophisticated mathematics or computational expertise or what have you it may be literally impossible to do this because what emerges out of deeper systems isn't always connected to the deeper system let me give you three examples to show how sometimes emergence can be understood and obvious and sometimes it can't and we we don't exactly know how to get out of it for example here's an easy example we have various rules of thermodynamics like temperature pressure pressure the ideal gas law you know PV equals nrt pressure and volume are proportional to temperature of a gas this is all thermodynamics these are emergent properties of something else you can understand the physics of an ideal gas with pressure and volume and temperature and the relations between them and all that without ever knowing what the gas is made of or that's made of atoms and molecules interacting with each other at a microscopic scale it turns out we can and we can figure it out we know that say the property of temperature arises from the kinetic emotions of the gas particles if if there's a bunch of gas particles and they're hitting you they are delivering a bunch of kinetic energy and you add all this together and you get a sense of the temperature there's an entire branch of physics known as statistical mechanics that's really cool and doesn't get nearly enough love in the popular press that connects the fundamental physics of say gas particles interacting with each other at microscopic scales to an emergent property of temperature or pressure or volume here's another way of saying this if I if if you're probably in a room filled with air I hope you are right now there's countless air molecules bouncing around you feel the temperature of the room right you can feel that that is a quantity you can measure and experience but if you were to ask any individual air molecule what its temperature is it would say I don't know what you're talking about I'm an air molecule all I have is momentum and kinetic energy and velocity and position maybe electric charge if you're lucky that's all I got that's what I know about myself I don't understand what you're talking about but if you get that one air molecule together with a quadrillion of its friends there is an emergent property called temperature that arises from all of their countless individual interactions and we know how to make that link and that's super cool here's another example if you take a material like Silly Putty materials have properties like viscosity and elasticity we know that these properties of viscosity elasticity stretchiness bounciness all arise from a complex set of molecular interactions we know that certain substances are stickier or squishier or bendier than others because of what they're made of and how those particles in or those elements interact with each other sometimes we can describe the these properties of squishiness or bendiness or stickiness from first principles where we can start with the very basic molecules and then we can look at how they interact with each other and we can build up a measure of how squishy the overall object is sometimes we can't sometimes we don't know why a certain material is sticky when it's not when when it shouldn't be or or stickier than other materials we just don't know it's an emergent property heck explaining why glass is transparent is possible with Quantum field Theory but it ain't easy feel free to ask if you want the Deep dive on that it's possible a hard example is super conductivity all the quantum physics tools in the world can't help us explain why superconductivity especially at high temperatures actually happens it just does some materials are superconductors at high temperatures others aren't we can't really explain why so what does this have to do with gravity back in the funky '70s stepen Hawking discovered that black holes aren't entirely black they actually leak a little bit of radiation it's very slow very SL light but it's there it's through various complex Quantum interactions at the Event Horizon black holes aren't totally black okay don't need to worry about how that happens just that it does nearly simultaneously another theorist by the name of Jacob beckenstein discovered that because black holes are emitting radiation that you could take our laws of thermodynamics things like language like pressure temperature entropy volume all that all the all the normal properties that you would ascribe to a room full of air you could actually use those that exact same language is the weirdest thing and we do not understand this connection at all that you can take those exact same properties and use them to describe black holes and especially the radiation given by these black holes and how black holes evolve and when they merge together what happen that you could connect make this connection between thermodynamics and black hole properties I will not get into the details of that connection feel free to ask if you'd like I would love to do an episode on black hole thermodynamics but just the key Point here is that he was Jacob backin scene discovered that once Hawking radiation exists by the way backin scene almost discovered it on his own before Hawking so we almost called it backin scene radiation but we didn't that you can take all this language you could you could describe a black hole in terms of its temperature in terms of its volume in terms of its pressure in terms of its entropy and that you could take all of our normal laws of thermodynamics and apply them to black holes so this is where things get a little weird because thermodynamics pressure temperature volume entropy all that are emergent properties remember a a box of gas each individual particle doesn't know what temperature is but the whole box you can measure this property of temperature because it emerges from the countless interactions of all the the subatomic particles or just atomic particles if you're dealing with a like air it's an emergent set of laws they're not intrinsic to the base the lower level of the physics they're they're sitting up at a higher level they emerge through through you know physics and here we are taking this emergent property this emergent language of thermodynamics and pasting it onto black holes huh so here's the line of thinking thermodynamics are all emergent they emerge from some deeper physics we can write down those emergent laws like the ideal gas law that you might have learned in high school or you learned and then prly forgot we can write down similar laws for the behavior of black holes black holes are made of gravity so maybe this is nature telling us that gravity is emerging that there's a more fundamental thing happening something we we haven't discovered yet that's creating Gravity the same way that countless particle interactions create temperature pressure the ideal gas law that what we see as gravity is an emergent property of the universe that there's some deeper interaction some deeper physics some some lower level you we thought gravity was on the the ground floor turns out maybe there's a basement to it and there's deeper physics happening and that that deeper physics gives rise it emerges into our experience of gravity this is the thinking behind a 2009 paper by theorist Eric ferland he asked okay if gravity is emergent where is it emerging from it turns out with black hole thermodynamics the most important property the part of the black hole you actually care about is its surface which is the weirdest thing it's it's it's bizarre and no one expected this like if you add information to a black hole uh yes the volume of the black hole goes up like if you dump stuff into a black hole the volume goes up you add information the volume goes up but uh it's disproportionate to the amount of information you add instead what goes up directly proportional to the amount of information is the surface area like the surface area of a black hole cares more about what it ingests than the volume does which is weird this is potentially another clue that gravity cares more about surfaces than than volumes and so to put a bow on verin's idea that maybe there are some very exotic very strange very unknown interactions happening at the edge of our universe that give rise to gravity we don't know what these interactions are we don't know what the deeper level of physics is but that's not important we actually don't need to know that part because we right now we only care about the emergent part just like you can develop an entire theory of thermodynamics the relations between pressures and volumes and temperatures you can build gas engines you can do all sorts of cool things you can do hot air balloons you can do whatever you feel like with the laws of thermodynamics without ever knowing what air is made of without ever discovering the deeper physics it's cool that we did do that and we can draw those connections but we don't need to you can have an alternate universe where we never figured out that air is made of molecules interacting and we can still have thermodynamics and all of the technology that flows from that this is similarly we don't know at least right away what the fundamental the the real interaction is happening at the surface of the universe what's actually happening all we know according to verin is that gravity our experience of gravity emerges from it and it's bigger than that it gets really wild because gravity is more than gravity we know this from Einstein gravity is the bending of SpaceTime it's inertia it's motion gravity is intimately linked to our fundamental concepts of like existence in the universe and so verin's idea is that something is happening on the surface of our universe gravity is emerging from it and it's not just gravity it's also space it's also time it's also inertia it's also motion that everything we associate with our three-dimensional universe is actually happening at the two-dimensional boundary if this sounds like holographic theory if you remember the holographic Theory episodes from way back when it's very similar it is inspired by that but adds this twist that gravity is an emergent property of interactions happening at the boundary of our universe in fact in this view our universe is entirely two-dimensional it is just the surface and there's some really cool probably Quantum interactions happening on that surface and out of that a three-dimensional Cosmos with space and time and gravity and inertia and motion emerges out of that so the goal of this emergent theory of gravity is to figure out how to express all of our knowledge of gravity and inertia and space and time and motion in terms of languages like thermodynamics because we know thermodynamics is an emergent property of the universe we know that thermodynamics is somehow strangely connected to the properties of black holes so if we use similar laws if we take our laws of thermodynamics and recast them I'm skipping over a bunch of mathy math here if we recast them in terms of gravity then we can write down emergent theory of gravity without knowing what the the deeper physics is and and we can do that Verlin did that in 2009 first glance is so what congrats you've Rewritten gravity that's kind of cool but also not super useful you you've just taken what we know about Einstein's gravity general relativity and wrote it in a different language okay so you translated it good job but here's where the magic happens and makes emerging gravity even more interesting after some massaging of the math which is valid or not we'll get to that and then you also introduce the reality of dark energy some stuff pops out some discrepancies where emergent gravity starts to look different than Einstein's general relativity where the the physics of our universe starts to diverge from what we' expect from gr and this actually only happens and this is what makes it super interesting is that this only happens this discrepancy from Einstein's relativity only appears when gravity is super weak and that's different than the normal approaches to modifying gravity in all of our searches for quantum gravity we care about when gravity is super strong like at the center of a black hole now with emergent gravity if you recast gravity in terms of this emergent language and you introduce the realityy of dark energy you start to get differences in gravity when gravity is very weak you get different gravitational interactions between objects like I said I'm I'm skipping over a bunch of mathy math it gets very complicated I hope I'm not insulting your intelligence by skipping over it how this actually works in the mathematics I believe that all of you are wise in learned and contributors to patreon that's patreon.com pmut your patreon contributions are an emergent property of all of your individual actions and I greatly I truly do appreciate it the key idea that allows verland and others to make progress this emerging gravity idea to make progress is that we don't need to know the detailed microscopic processes all we need to know is that gravity is emergent and it kind of sort of looks like thermodynamics then once you add in Dark Energy you start to get differences between emerging gravity and general relativity and that means you can start testing it one thing that you can imagine that this emergent gravity does is that it makes space itself have properties more like matter it can have properties like elasticity or compressibility it can have a little bit of squishiness or a little bit of stretchiness and this allows space to be able to squeeze in on itself in certain conditions which gives the appearance of extra m Mass when there is none so if you go out in the middle of nowhere like if you're in the solar system there's so much mass there's so much normal gravity you're never going to see this difference but if you go out in the middle of nowhere say between galaxies when there ain't a lot of stuff all your left is is space just sitting around doing its thing and in this view of emerging gravity space picks up properties like compressibility and it can squeeze in on itself and so it looks like there's something there because matter is what usually makes space squeeze on and on itself but here you've got nothing space is squeezing in all on its own and that kind of looks like dark matter right if I look at a Galaxy it looks like there's extra gravity inside of the Galaxy that can't be explained by the visible component of matter in this view of emerging gravity what's happening is that once you get up to Galaxy scales space itself is squeezing in on itself which gives some the appearance of extra gravity there is extra gravitational force inside of those galaxies but it's not due to an invisible component like dark matter it's due to the fact that gravity can just squeeze in on itself all on its Lonesome in this emergent picture we have to be careful that this is not modified Newtonian Dynamics you know that M Theory where we're going to take Newton's law of motion and start tweaking with it at Galactic scales this is not this is a completely different approach this is not trying to modify gravity this is coming up with a brand new theory of gravity which is cool we like cool ideas we like random stuff we like getting a little crazy here let's run with it this is the game and the game has two questions two sides to it one how well do we trust the theoretical calculations when they make a prediction say oh this is what's going to happen once you get out to Galactic scales and emerging gravity you can say well do we trust that result and then you could also ask well once you make a prediction and say you trust the prediction you trust the math does it survive experimental observation does it survive rigger and the answer is a little mixed on the theory side there are limitations to the math there are a lot of assumptions that go into a theory of emerging gravity you have to assume that black hole thermodynamics is actually valid you have to assume that step Hawkings radiation is valid you have to assume that the holographic principle holds true that that surfaces are more important than interior contents uh you have to assume that you can import this language of thermodynamics and apply it to gravity and it's all cool you have to assume that dark energy exists and that you know how to fold that reliably into your calculations because last time I checked there's no Dark Energy in uh like a gas engine so normal thermodynamic mathematical equations don't usually incorporate Dark Energy but now we have to so it's not super clear how these ideas connect to the real Universe which I know sounds Beyond ridiculous cuz like isn't this the entire point of physics but with really complicated really theoretical ideas like emerging gravity they these rest on a lot of assumptions and it's actually really hard to put it in a physical context in a way that we could observe it or test it it's one thing to say okay gravity may be emerging cool idea it's another to say this is how Stars will behave in a galaxy or this is how the cosmic microwave background will look taking these super theoretical far out ideas which are super cool and we need them and I love them to actually test them requires a lot of work and a lot a lot of guidance and a lot of assumptions and you can challenge these assumptions you can say well okay in a super simplified toy model uh this is how emerging gravity works but I I don't know if if you can really make reliable predictions about how stars are going to orbit inside of a galaxy because that's like a kind of complicated situation and we don't know we don't since this is a brand new idea we don't have decades and Decades of experience of dealing with real World situation so we can figure out how the math applies like general relativity oh my gosh it took we got some test cases out early on but then it took decades to really work out how this Theory all the mathematical tools really interacted with the universe and gave us robust predictions that we could trust we could say yes general relativity definitely says this about what happens when two black holes Collide it takes a lot to get there emerging gravity isn't quite up there but you know there are assumptions there are some say oh you know there are predictions like okay under this laundry list of assumptions here is how Stars should behave in a galaxy and under those somewhat plausible I'll give it that set of assumptions you get a an effect that looks a lot like dark matter at least at Galactic scales and so most of the tests of emerging gravity have taken place in studies of Galaxy rotation curves of how stars or orbit around galaxies and the conclusion here after dozens of observations is uh maybe I know that's what I got I'm I'm just I'm don't shoot the messenger here some studies of some galaxies say that emerging gravity does a better job of describing the behavior of the Motions of stars than just a simple Dark Matter model does other surveys other studies with other samples of galaxies say no actually dark matter is better than emerging gravity it actually doesn't do even a better job at explaining the data and so there's a little bit of back and forth there have been some tentative tests Beyond Galactic scales there was a study done on Galaxy clusters Dark Matter also emerges at Galaxy cluster scales where the gas inside of the Galaxy cluster is way too hot to be explained by the meager gravity provided by the visible content and when you get up to Cluster scale um emerging gravity just just doesn't cut the mustard it just doesn't do it um dark matter is a better hypothesis it's able to better explain the observations you know Dark Matter honestly it's it's not the greatest hypothesis uh but it's the best one we got cuz it it can explain so much it can explain Galaxy rotation curves it can explain the hot gas inside of Galaxy clusters it can explain how Galaxy clusters merge together it can explain the growth of of the cosmic web it can explain the patterns in the cosmic microwave background like it can do a lot of stuff and a fullon replacement of dark matter like a theory of emerging gravity its ultimate test is to explain all the same things that dark matter can and find little differences here and there so we can tell the difference can it do a better job at explaining the universe that we see so far it's not looking so hot that's not necessarily a knock you know it's been 15 years since verin's original idea it hasn't exactly fixed a lot of things or improved on dark matter as theories of the universe go it's it's honestly not in the hottest shape but it's not it's not dead yet it's it's still there it hasn't been ruled out by observations and so that it can it can maintain as a contender for explaining our universe and it can also fall back on well hey we don't fully understand this this is a brand new idea here cut us some slack we're still trying to shake out all the bugs find potential mistakes make sure our equations sit on good a solid firm footing so maybe our predictions aren't that reliable and that we need to go back to the chalkboard literally uh to figure out some more reliable predictions so so there's room here there's room to talk it's a new idea why does it still hang around though um because it's interesting honestly and we like interesting things we like new things you know physicists are like cats when there's like a shiny new toy where we're attracted to it like I said emerging gravity doesn't just change uh like mod you know it doesn't just modify general relativity it changes are Notions of space of time of motion of inertia of fals ma all those properties that rely on SpaceTime If gravity is emergent then so is spacetime then so is motion then what we call inertia is an emergent property of some deep Quantum interaction what we call three dimensional volume is an emergent property of some deep Quantum interaction it's fair to say like what we call reality is an emergent property of some deep Quantum interaction that's interesting like I always say if it's interesting it's probably wrong but that doesn't mean we can't stop thinking about it we don't know if this idea of emerging gravity is correct it relies on a lot of assumptions we don't know if those assumptions are correct we're not super confident about the experimental tests it's a new idea it's an intriguing idea it seems to tie together lots of interesting coincidences and it shows that maybe there's yet another deeper layer to the universe which is what makes physics so much fun thank you to Rebecca H Tock Raj Thomas laav B Francisco J Steven H and Kirk B for the questions that led to today's episode and of course thank you to all my patreon contributor I can't thank you enough you have shown so much Support over the years I am eternally grateful I'd like to thank my top contributors this month that's patreon.com PM my top ones this month Justin G Chris l barbar k Alberto M Duncan M Cory D tomg Nyla John S Joshua Scott M Rob H leis M John W Alexis Gilbert M David L Rob W Valerie H Demus J juules R Mike G Jim L Scott J Lewis I Peter E David S L John Boy and Scott R thank you for everyone and keep those questions coming uh ask a Spaceman gmail.com or go to the webbsite askas spacem man.com keep going with the uh the positive reviews on iTunes and Spotify really helps the show gain more visibility and I can't wait to get more of your questions and answer more of your questions on the show it really is a joy to share this with you I will see you next time for more complete knowledge of time and space

Channel: Dr. Paul M. Sutter

Views: 100,023

Keywords: space, cosmos, universe, astronomy, physics, astrophysics, cosmology, science, gravity, emergent gravity

Id: WFqID8NEVUU

Length: 33min 4sec (1984 seconds)

Published: Thu Apr 11 2024