Why Space Itself May Be Quantum in Nature - with Jim Baggott

[Music] now phil has set me up rather nicely as I would have expected yeah this does seem a little bit like I'm reneging on a position I adopted when I wrote this book called farewell to reality which is highly critical of string theory and in particular an extension or addendum to string theory which has become obsessed with the notion that we might live in a multiverse and dismissed it as physicists theoretical physicists in particular going off on one speculating when they had no real grounds for this speculation and certainly no empirical evidence to back up anything they were saying and it seems like I'm going against that position because I'm about to talk to you now this evening about another theory which by the way has no empirical evidence to support it which is entirely contrived from the imaginations of theoretical physicists and you might say well Jim that's just a little bit wrong now my excuse is that one of the modes of belief adopted by many theoretical physicists working in the string community is that there is no alternative well we keep plugging away Jim with string theory because there is no alternative if you want to do something that transcends what's known as the standard model of particle physics or the standard Big Bang model of cosmology then we need string theory and I've taken umbrage at that I happened to have got to know two of the key theorists behind what I'm about to tell you this evening I don't know them very well and I trust them and certainly the claims that they have made for this theory that I'm going to talk to you about tonight does not for me cross the boundary between misleading for ah fiying mis-selling claims now I have nevertheless felt obliged to change slightly the title for this evenings talk and again I hope you won't be too disappointed the talk was advertised as Y space is quantum in nature and given that I've just said this is a theory for which there is no empirical support I thought I'd better change that 2y space might be quantum in nature and this is very much not a theory of everything a theory of everything would somehow be one set of mathematical equations that somehow over here gives us all the particles we know that exist over here would give us the universe that we know that exists all in one consistent set of theoretical structures that theory still eludes us today we have theories of Big Bang cosmology based on Einstein's general theory of relativity which I'll talk about and we have quantum field theories which we use to build something called the standard model of particle physics and so far the two have never really come together and here's one of the reasons the Einstein's general theory of relativity is about gravity or at least it provides an explanation for what we experience as gravity and it explains us in terms of interactions between mass and mass energy and space-time itself in the general theory of relativity space-time is dynamic there's an equation that says space equals time equals whereas most theories of physics say this equals and space and time are variables in that equation so this is very very different as far as theories of physics is concerned and it's said to be background independent so we don't presume the pre existence of space and time when we use the general theory of relativity and you'll have an understanding I hope in about 15 or 20 minutes or so as to why that might be it applies to big stuff and the universe as a whole planets stars black holes all of these things that we've come to terms with and make life quite exciting from a scientific perspective are all described in terms of Einstein's general theory of relativity it's not a quantum theory and so the other piece of the the other ingredient that somehow needs to be folded in we're thinking of a recipe in the kitchen we need to fold in this ingredient is quantum mechanics problem is that quantum mechanics describes the composition and behavior of matter in terms of its elementary constituents it means that these are equations in which stuff equals something in which space and time but they're in the background they are assumed to be a spacetime is passive it's assumed to be a continuous form against which physical events happen stuff happens and it applies to tiny bits and pieces like electrons quarks molecules even biological molecules but nevertheless small things that you wouldn't find under a microscope even and so the idea of bringing together the general theory valency which describes big stuff in the universe and quantum mechanics which applies to very very small stuff you can already get the sense as to why this might be tricky by the time you're down at the level of tiny bits and pieces gravity is irrelevant down here gravity is a very weak force it has no impact on electrons or quarks or protons or neutrons by the time you're up at the level of a black hole or a planet or a star do you really need to think about the behave of individual electrons probably no so the situation is that these two great theories for which there's lots of evidence to support that they reflect or represent the real state of affairs that we find in the physical world can't be bought together now what I'm going to describe is a journey that a few theorists have taken over the course of about the last 30 years or so attempting to bring them together now this is a theory for which as I've said there's no empirical evidence to support it and I think by the end you'll understand why that is least I hope you will and I also want to be clear that it's one of many different approaches that can be taken and I want to give you a flavor for why I think it's actually very instructive way to go so I'm going to start with a question now you'll get used to this from me these questions are always rhetorical so you can put your hand up and volunteer an answer if you wish I'm not expecting you to so I'm going to ask where are you in space that seems like an easy question Phil's already jumping up and down ready to what is arm in the air well what would you do if you're in a school science class you know what you do you'd start with a coordinate system an x coordinate a y-coordinate a Zed coordinate because we're in three-dimensional space you'll know from your physics class that Newtonian mechanics Newton's theories of motion rely on the notion that space and time are absolutes again things like force equals mass equals energy equals and space and time are variables in the equation and here we are in this wonderful lecture theater so that's where you are in space well except that we know this lecture theatre is in the Royal Institution that's okay we just make our coordinate system a bit bigger so it wraps around the entire building but then you say what hang on Jim the Royal Institution is in London okay we'll make our coordinate system a bit bigger and we wrap the entire of London in it over word with Sadiq Khan I'm sure he won't mind okay but London is in the UK which is part of Europe okay but that continent of Europe is you know it's on the surface of the earth we just make our coordinate system just that little bit bigger put it around the entire planet we don't really need a grant for this so that's okay we don't need funding but hang on the earth is just the third planet from the Sun so we put the coordinate framework around the entire solar system you know where this is going by now solar systems in the Milky Way the Sun is simply one star in probably two hundred billion stars in the Milky Way galaxy although you can't see it there's a little point on this picture here that says earth ok we've drawn a coordinate system now around the entire Milky Way galaxy but the Milky Way is actually part of a cluster of galaxies known as the local group ok we'll draw the box around that well ok we might as well admit now where we're going with this and say ok let's put the whole universe you are here you see what we've done by assuming absolute space in time we've ended up with a coordinate system that goes around the entire universe I I think of it as a kind of God's eye view almost as if you could step out of the universe and look down on all creation we also need to think about time now I can't read cosmic metronome without thinking of some 1970s progressive rock band have you heard the new album from cosmic metronome Jim it's really good but yeah we need something to orderly keep orderly time otherwise again there's no absolute measure that we can you know make it work and I think you'll agree that this is now starting to look a little bit philosophical Newton who was quite comfortable with the notion of absolute space and time because his equations of motion worked quite happily with that assumption was under fire his archrival Leibnitz German philosopher criticized him for this accused him of introducing occult forces that was by the way a fancy way of arguing in the time of Newton if you wanted to dismiss some mathematical philosophers notion you'd just accuse that person of introducing occult forces into their model of physics Einstein rejected 200 years later rejected this notion there were other reasons other experiments have been done that suggested that the notion of absolute space and time really wasn't a very desirable notion Newton believed that everything there is should be in the universe and that meant that this idea of having a coordinate framework that sits outside the universe couldn't be right he introduced two principles in a paper published in 1905 the first is that the laws of physics should be the same for everyone that's a wonderful slice of democracy for you right there the laws of physics should be the same for everyone in other words it doesn't matter where you are or how fast you're traveling if you're traveling with a uniform speed the laws of physics that you measure should be the same as the laws of physics that someone who remained sitting in this room can measure who's familiar with the Lord of the Rings book or film I don't mind there's a great passage you weren't thinking of special relativity when you read it I'm sure or when you saw it on the screen but it's when Pippen rides on Shadowfax in front of Gandalf in haste from Edda ass to Minas Tirith and there's a passage that reads as he fell slowly into sleep pipian had a strange feeling he and Gandalf was still a stone seated upon the statue of a running horse while the world rolled away beneath his feet with a great noise of wind and the point about the laws of physics being the same for everyone is that if you're moving or if you're still you can't use the laws of physics you can't use physical measurements to tell you which is moving and which is still speed is relative now you see that if you're riding a horse that's an experience that you can you know judge the horse needs hey it needs feeding at the end of its long journey and there's evidence that it was the horse that was moving all the time but abstract it down to inanimate particles objects moving through space and time and you can begin to see well when this electron goes from here to here is it really going from here to here or is the thing observing it moving from here to here and the electron is stationary this is what you have to start torturing your mind with when you start to want to make space and time relative not absolute the other thing that Einstein adopted the other principle was that the speed of light is finite and constant there were good reasons to believe that this was the case something called a mitchellson Morley experiment up on a mountain at the end of the 19th century had demonstrated fairly unequivocally that the speed of light is a constant independent of the speed of the source of that light now if the speed of light is finite in constant imagine you witness a remarkable occurrence you're out in some remote field in a thunderstorm and you witness two bolts of lightning strikes you have no hesitation those bolts strike the ground at the same time the light traveling giving you that information travels very very quickly you don't in fact even sense a delay between the bolt striking and you seeing the bolt strike I on the other hand at a very very fast speed a substantial fraction of the speed of light don't ask me how I'm doing that I see something quite different in this situation with the twin bolts striking the ground because I'm moving from left to right I see the right-hand bolt strike first because the light and remember I'm traveling now it's a substantial proportion of the speed of light the light takes a finite time to reach me and it gets to me sooner because I've moved to the right then the left hand bolt that takes actually a bit longer because it has a bit more ground to cover no big deal you see the lightning bolt strikes simultaneously I don't I see the right hand bolt strike first and then the left hand bolt who's right the laws of physics are the same for everyone if you accept that first principle and you accept that the speed of light is finite and constant then you have no choice but to say there can be no such thing as absolute simultaneity I might witness to events that are simultaneous in one frame of reference as it's called but there might be another frame of reference which is moving relative to the other where things aren't simultaneous and if I cannot get a sense of there being absolute simultaneous simultaneity it means basically there is no such thing as absolute time now we all know that this out of this this little set of mind games that Einstein played with himself as he was actually working as a patent clerk in the Patent Office in Bern in Salon resulted in Einstein's special theory of relativity published in 1905 as I've said and one of the consequences that we know that time dilates based on the speed with which a frame of reference adopts space contracts and of course we do know about energy and mass being equivalent but he wasn't quite done special relativity is called special when it was published it was the theory of relativity but it was realized that it work wasn't quite complete like a portrait it hasn't quite been finished it's special because acceleration in special relativity is problematic I've put it in inverted commas it doesn't mean that you can't do acceleration in special relativity it's just that acceleration doesn't have the same status it had in Newton's theories of motion and the other thing it can't account for is gravity now gravity was still a mystery what keeps the moon locked in Earth's gravitational embrace when I do this what pulls the book to the ground something mysterious reaching up from the ground reaching up from the core of the earth pulling it down this was the occult forces that Leibnitz referred to criticizing Newton's universal law of gravitation I will come and pick that up and then in November in 1907 two years after publishing his breakthrough paper Einstein had his happiest thought still working in the paper oh by the way by the way he'd had a promotion so that was okay he had his happiest thought he realized he was sitting in a chair in his patent office and suddenly a thought struck me if a man falls freely he would not feel his weight I was taken aback this simple thought experiment made a deep impression on me now okay Einstein was a genius I could have sat for hours in my office thinking that a man Falls freely not feeling his own weight and I'm afraid I would have never have made the connection you don't see that again okay and what Einstein was led to was something called the equivalence principle basically gravity and acceleration of the same thing gravitational mass the mass of the moon and the earth and the way that in Newton's universal law of gravitation they're drawn together by the force of gravity is the same as acceleration so if we sit on board a spaceship accelerating at high velocity through the the atmosphere or into outer space and we have the unfortunate experience of being left stranded at the mercy of Earth's gravitational pull then in fact there's nothing to choose between these circumstances there's an equivalence between what we experience is gravity what that book experienced and acceleration and if you have a mind like Einsteins you'll work out that that is actually because space-time itself can be curved now at this stage our brain start to melt we start to get Bob because how can that possibly be well bear in mind you can't see space I can't see space you can't see we know we're in it but we can't see it we can't see time I can measure it I've got a watch but I can't see it how do I know what shape it is by definition and so when a book falls from a height and hits the floor it's not the force of gravity reaching up and pulling it down towards the core of the earth it can't hang on against the local curvature of space-time in this room don't look for it you won't find it but it's a way that einstein reconciled the fact that occult forces can't possibly be right this is science we're talking about not magic and therefore one explanation for what happens with gravitating objects objects exposed to the force of gravity I'm going to keep using scare quotes when I say force is that they're sliding sliding down curved space-time and if we got a planet the size of the earth it distorts space-time around it this is the typical picture but it's a little bit misleading because you can't see space and time all you can do is make measurements and and there are measurements that have been exquisite satellite measurements of things that are known as frame dragging and it's literally that the satellite gyroscopes in the satellite being dragged by space and time itself as as the Earth turns it drags space-time around with it beautiful but don't over imagine it now John Wheeler summed this up very very neatly an American physicist in the 1950s he wrote a book about gravitation that says space-time tells matter how to move whilst matter tells space-time how to curve and the two are it's like a symbiosis okay I've got a heavy mass a curved space-time and that space-time tells me how that that matter is going to move and what we get is we get something called gravitational time dilation it's different from the time dilation from special relativity that was because we were going fast now we're just going up fighting against a gravitational field which is the same thing as space-time we get something called gravitational redshift we get black holes we like those and we get gravitational waves all right all good stuff all coming from what was the general what is the general theory of relativity which Einstein finally figured out how to write down in 1950 now a question you might have is fantastic what good is it it's those kinds of those are the kinds of times we live in and I just wondered did anyone here use GPS on their phone maybe Google Maps to find the Royal Institution this evening anybody show hands a few a few fact it's quite interesting the pioneers of GPS for American engineers were actually awarded the Queen Elizabeth engineering prize today yay and what I can tell you is if the tiny atomic clocks on board the 24 satellites that girdle the earth and create the GPS system if those clocks weren't corrected using special and general relativity we would run up clock errors which would give rise to distance errors of a rate of about 11 kilometers per day now I can guarantee you that if you were relying on GPS to find your way here tonight and you were working within an eleven kilometer radius circle good luck with that but that's per day after a week it's useless you can't use it at all so if you want to know why it's important next time you pull out your phone use GPS will you sail a boat use GPS just remember without relativity you'd be lost now that's relativity I hope you've got the sense for what relativity is about relativity is not a quantum theory it assumes that space-time although it curves is continuous it can be stretched like the surface of a trampoline which is the analogy that we often use when we see the earth as a big heavy ball sitting on a trampoline creating a dent and that's that's gravity basically but Einstein was also at the root of another incredible innovation you want to know why physicists get so excited about Einstein is because he did all these incredible things he really did yeah his latter years weren't quite so productive but boy he'd earned his badge of honor already by 1915 1930s now Einstein and debris together conspired to change completely our understanding of the little bits and pieces in 1905 he published a paper separate from relativity saying monochromatic light basically monochromatic means the same color so blue light if you want green light behaves as though the radiation were a discontinuous medium consisting of energy quanta light waves can be particles well we know what these particles are called today don't we photons next time you see a photon torpedo fired in an episode of Star Trek blame Einstein for explaining that light waves can be particles Prince Louis 5th Duke de Blois then a few years later in the early 1920s had an idea if light can be waves and particles then can particles be waves and he suggested that the idea could be generalized by applying it to all material particles and notably to electrons electrons can be waves now that notion of wave particle duality debate still rages the root of everything that came afterwards that we know today is quantum mechanics and quantum mechanics those waves now extended 3-dimensional extended objects if you want to think about them like that and now for torus quantum fields field just being a fancy term for something that stretches out and they underpin this thing called the standard model of particle physics and here's the physicists equivalent of the chemists periodic table it's a list a ingria list of ingredients if you like about which all material substance is composed you want to know how to build a proton you take two up quarks you on the left there and a down quark put them together bind them together with force carrying particles called gluons that's the little G and you've got a proton you want to build a neutron take two down quarks and an up quark bind them together with gluons and you've got yourself a neutron you want to build an atom you take your protons and your neutrons and you wrap electrons in orbit around the nucleus now you've got atoms you want molecules get the electrons outermost in atoms to hang to hook together and you've got a chemical bond between atoms and so on and so on next thing you know you've got DNA and you've got biology now there are other things in this list I don't want to go into too many details because I don't need everything I want though to leave you with one thought you know that experience that you had as kids right when you took to small bar magnets you know what I'm about to say and you got the north poles or the south poles it doesn't matter and you tried to push them together you know what you experienced some mysterious resist there's nothing there to see and yet the magnetic fields resisted flip them the other way and they attracted and they you know snap together well how does a force like that work that's the electromagnetic force and it's carried by photons so photons that are ubiquitous photons in this room they're responsible for carrying electromagnetism from one particle to the next and in this idea let's say we have two electrons and then we're going to run them into each other so let's see what happens when we do that they come in and they collide which doesn't sound like a lightsaber in real life and they go off in different directions so that's the way we understand how forces work in the standard model of particle physics this is a force of repulsion two like charges repel each other but what happens is that force of repulsion is felt through the exchange of a photon and that exchange pushes them apart okay if now more or less got all the ingredients you know I promised you a lecture on loop quantum gravity and I've said nothing it's all about quantum gravity yet and you're looking at your watch saying hang on Jim get on with it okay but before I finish let's just have a quick quick look at what we understand about the universe just that will come in handy as well so you might be familiar with a picture like this we begin there's a period a very snappy period of something called cosmic inflation very controversial but again broadly accepted for the time being as something that that must have actually happened that kind of blows up the from something that's the size of a quantum dot to something probably I don't know the size of a grapefruit or something the entire universe after 380,000 years we've got particles that are formed including protons and electrons and for the first time after 380,000 years these particles combine it's called recombination but forget the re they're combining for the first time in the history of the universe it should be called combination and when that happens a whole load of light that was bouncing back and forth these electrically charged particles remember virtual photons is released and we know this these photons today this light today is the cosmic microwave background radiation at the temperature of the universe then about 3000 degrees some of this light would have been visible light so the universe litter literally will let there be light but as the universe expanded further and cooled that light faded that means the energy of the photons was reduced and we have a period called the dark ages over time maybe about a hundred million years after the Big Bang we begin to get the first stars and galaxies so matter was beginning to be condensed we get the first stars form as more and more gas condenses into stars we start to get galaxies form we have to wait about nine billion years for our solar system to be formed along with earth of course and right at the death we have homo sapiens and judging from what we're doing with the planet I guess there's a sense in which we might merely be a blip in the history of the universe now that spans 13.8 billion years from start to finish from start to present day and that's the universe now all of that knowledge all of that structure is based on a combination of general relativity and particle physics the standard model of particle physics in terms of our understanding of how the universe began and evolved it's not based on bringing the two theories together we use quantum mechanics over here quantum field theory over here and we use general relativity over here but we don't bring them together to create a picture like this right quantum gravity you knew I'd get to it in the end now physicist Lee Smolin nice guy he published a book about 2000 so about 19 years ago three roads to quantum gravity so there are three different ways we can try and bring general relativity and quantum mechanics together under one roof as it were you can start with these two structures and you can try starting with quantum field theory and trying to find a way to make space and time emerge in it make it background independent Richard Feynman actually had a go at that but then got hopelessly bogged down and it didn't work but he wasn't disarmed that was about the 1960s you can say well forget general relativity and quantum mechanics I'm going to start over and a few brave souls have actually taken that road I'll talk about one of them later on or you can do what I'm going to talk about which is start with general relativity and find a way to as it were quantized this introduced a quantum element to it okay so the first thing you need to do is to start by reformulating general relativity so that it looks like a quantum field theory now if you're not familiar with the mathematics you might say hark Jim that's easy I could do that over breakfast let me tell you it's not easy here's the problem when a particle moves about on a flat surface we don't have to worry about which weight points now why should I be worried about which weigh anything points well physics is full of things called vectors an electron has a spin and it points in a magnetic field up or down so vectors are important and the way things point is fundamental to the way physics works so if I have a particle pointing upwards let's say and I move it about on a flat surface that's okay I can happily do that the way I move it about won't affect the way it's pointing but in general relativity space-time can be curved well here's the ultimate curvature a sphere let's see what happens to a vector as we move it around a sphere now I'm gonna use a wickedly ingenious invention here called a south-pointing chariot the Chinese invented these in the third century this is before the magnetic compass and if you wanted to have any idea of where you were going you needed something so they came up with this south-pointing chariot which has a carving on the top that points and this ingenious gear mechanism underneath that means it keeps pointing in that direction even though the cart may turn in that clever so let's take our south-pointing chariot and wheel it all the way to the equator starting from the North Pole we get it to the equator and then we turn to the east but remember that south-pointing chariot keeps pointing self okay so we'll go a quarter of the way around the equator and then we'll make our way back home and you can see that by the time we get back to the North Pole that chariot is now pointing at right angles to where it was pointing when it's set off moving although we haven't done anything specifically to the way it points moving it around the surface the sphere has changed the orientation of the vector now any theory based on general relativity which allows for space-time curvature has to accept it's known as the parallel transport of a vector fortunately in the early 1980s to Indian theorists Amitabh our sin and Abhay ashtekar came up with a connection theory which allowed general relativity to be reformulated and when it was reformulated it looked exactly like a quantum field theory now again I don't want to take credit away from sin and ashtekar who worked on this but Einstein and in fact Austrian physicist Erwin Schrodinger were there before but they struggled with the mathematics of these connection type theories in fact at one point Einstein wrote to Schrodinger and said it looks like a gift from the devil's grandmother suggesting he wasn't too enamored of where the maths was taking him but if it looks just like a quantum field theory your next question is will a quantum field theory of what exactly we're gonna create a quantum field theory of gravity we need objects for it to be a theory off don't we well the inspiration for what this might be a theory of came from something called lattice quantum chromodynamics now quantum chromodynamics is the field theory for what's known as the color force and the color force is what binds quarks together inside protons and neutrons what I didn't tell you when I put up the little equivalent of the periodic table was that quarks are not only up and down charm and strange top and bottom although I do wish we'd stuck with the original names for those last two quarks which was truth and beauty in addition to flavor up down charm strange and so on quarks also possess color now physicist running out of ideas at this time so they just call them red green and blue they're not literally red green or blue okay but they have quantum properties that we characterize as red a red quark a red up quark a green down quark and a blue up quark together make a proton and what you can see in these little threads here with the arrows is the gluons that bind them together and the way that this force works is in fact like they're held in a net if I try to pull and separate the quarks I'm gonna actually going to hit some resistance that they're really held together very very strongly these force lines are loose when the quarks are close together but if I try to pull them apart they kind of snap and prevent me from getting the quarks out now the problem is the equations of quantum chromodynamics are really quite complex and they're impossible to solve analytically you won't find a book where you've got an answer at the end that says you know well you know quark energy is QED you need to solve these equations on a computer and one of the techniques used to solve these equation on a computer is called lattice quantum chromodynamics it's a technique now again I couldn't run calculations of lattice QCD on my laptop I need a supercomputer and a lot of supercomputer time in other words I need a grant and you construct a lattice this is space and for that matter time entirely artificial I assume that I can organize my quarks and my gluons on this lattice I put the quarks at the intersection points of the lattice and I allow the gluons to run between them in all the different ways that gluons can interact and the way that lattice QCD works is that I have a certain distance between the lattice points and I do a calculation I then shrink a little bit that distance and I do another calculation and I shrink it a bit more and I do another calculation and so on and so on and then I extrapolate all the way to zero lattice separation that is zero lattice link and that allows me to get to something that looks like a continuum of space and time without the need to actually do the calculation at zero but look at this picture we've got of course we've got the quarks we've got gluons running around the lattice the links between the lattice points but over on the left there we've actually got quarks running around in a circle without any we've got blue Ansari running around in the circle without any quarks oh one of the reasons that's possible is that unlike a photon a photon is not electrically charged so when I bring the two electrons together that you saw in an earlier slide and a virtual photon passes between them and they move off in a different direction that photon is not charged but the gluons have color charge as it's said so they not only interact with quarks they interact with themselves which is why quantum chromodynamics is a bit of a beast everything interacts with everything else it's a real mess but it means that gluons can run round in circles and so here's a thought the physicist Kenneth Wilson had the idea he was interested in trying to create an analytical structure for quantum chromodynamics and he wondered if it might be possible to create a situation whereby we do without the quarks and the lattice and all we've got left then is the loops the loops of force running around in a circle and this was the inspiration for loop quantum gravity except the loops are not gluons there now loops of gravitational force in inverted commas okay so loop quantum gravity kicked off in about the mid-80s and initially it was all about the loops and the way that these intersect then it became well maybe they not and so the physicist theorists involved reached for the theory of knots I'm not going to say that's a knotty problem so you've got some characteristic knots here that's a trefoil knot on the top left there underneath you've got a trefoil knot but he's just going round and round so you can get a perspective on it and next to it is something called a whitehead link two loops but twisted together and and and knotted together so they can't be separated and the final one there is called the boramy and rings fans of the Marvel Cinematic Universe in the audience knots feature very heavily in Norse mythology so next time you look at an older version of an Avengers movie or a Thor movie look for the not on Thor's hammer you'll find in fact they've used a tree for not it's something you can do in an idle moment okay then okay maybe what's important is the way that we weave these loops together now this is a we've created by taking a whole bunch of key rings and linking them together in fact the Italian theorist Carlo Rovelli joked that he used all the available key rings in Verona to build this and then okay in the final step these were replaced by something called Penrose spin networks now Roger Penrose was Stephen Hawking's PhD advisor at Cambridge he's a smart guy but he also likes he's that kind of theorist that likes to plow very much his own furrow and he invented this structure primarily as a way of satisfying what he thought space ought to be like which is quantum in nature by coming up with a network that would do just that he didn't have any physical significance attached to these networks and so what happened is that the theorist developing loop quantum gravity found the networks that Penrose had invented some years before entirely at a whim and that kind of thing happens in science it's a happy happy set of circumstances so I want to be clear we will look at these pictures and imagine these loops existing in space it's the way I've drawn them how else can I draw them but in fact in loop quantum gravity these are space space is these they don't exist in space they make space I know it's difficult to get your head around and the two principal architects there were other theorists involved that certainly helped along the way two principal architects was you already know Lee Smolin American theorist who now works in perimeter Institute in Canada and you're I'm sure have heard of carlo rovelli italian theorist who's worked quite closely with lee smolin over the years and this was all work that the two forged together in the mid 90s to the mid towards the mid nineties now as I've said loop quantum gravity implies that space itself is quantum in nature what does that mean well here's a spin Network it's got nodes and it's got links and what Smolin and Ravelli discovered was the maths came out to tell them and i'm gonna give you some maths in a minute be ready they realized that what was actually happening was that at the nodes the points in this diagram is where you find quanta of all you the volume of space no I don't know either and along the links you'll find the quanta of area of space the inevitable consequence of quantizing Einstein's general theory of relativity you make space quantum in nature okay so these are the quantum states of space now I'm waiting for a cry from the audience I'll be Jim but what about time I will you see in this road to quantum gravity you lose time and I don't mean that in a sense you wake up one morning and you say what happened to you know the last year although I'm sure that does happen I mean time disappears from the equations it's known as the problem of frozen time so these quantum states of space that the quantum states and so there's a sense in which they fluctuate that they get the jitters something called Heisenberg's uncertainty principle means they don't stay still they move move about however we need to find a way to put time back into this picture and what smullin in Ravelli and others did was was to imagine that fluctuations in the quantum states of space create the appearance of time what happens is that as we change the number of nodes nevermind the number of links the clock ticks and that represents an advancement in time so we see something like this going on as these quantum states of space bubble and froth we get the illusion of time emerging I'm going to come back to this all right now again I want to remind you there's an important question of scale here what I don't want you to do is to say look I went to the Royal Institution this evening oh I had a great talk from from Jim he told us all about the quantum states of space and I'm gonna look for them I'm gonna look for them you won't find them for a very simple reason now ignore the maths that's area this little guy here is called the Planck length and it appears squared in this equation and J into J plus one square root of is just the quantum number of space area of space actually it's the Planck length that I'm interested in the Planck length it's called that because in fact it was Max Planck back in the early 20th century who realized in fact that with the discovery of his own constant which is h-bar here is H divided by 2 pi Planck's constant divided by 2 pi G is Newton's gravitational constant and C is the speed of light he realized that by combining these fundamental physical constants he could come up with some fundamental units of length time energy mass now you can see the Planck length appears squared in this equation for area don't worry about the equation it's incredibly small the Planck length is one point six times ten to the minus 33 centimeters so that's naught point 33 noughts one sings now don't even try and think about it the loops or networks are presumed to exist on this scale you won't find them don't look for them the Planck length is to a height I did some jiggling around with scales the Planck length is to a hydrogen atom what a large amoeba is to the Milky Way galaxy you won't find them a single proton contains about 10 to the power 65 quanta of volume to all intents and purposes a proton doesn't care an amoeba doesn't care the Milky Way galaxy certainly doesn't care that space is quantized in this level so you might then have a perfectly realistic and legitimate question so Jim honestly why why well let's take the theory at face value and see what it tells us particle physicists love particles okay that's what makes them tick that's why they like to build ever larger colliders so they can see more particles and it's long been thought that the force carrying particle for gravity is this thing called the graviton just like the photon carries the electromagnetic force when two electrons come together when two masses when two objects come together they they transfer gravitons between them and that carries the force of gravity that's what particle physicists tend to think but Luke quantum gravity says that gravity gravitons are actually so-called pseudo particles they're not force particles and to get some sense for what that means I know that you saw in the media a few years ago these fantastic reports of gravitational waves remember that yeah and you couldn't barely believe what the scientists were telling you because they said these are gravitational waves produced by two black holes merging my god really but yes so here are two black holes spinning around each other and merging and as they do so remember we can detect frame dragging around the earth so when you're talking about massive objects like black holes they really do mess about with the fabric of space/time creating waves ripples now Einstein Dubrow remember what they said if we can have waves then there are associated particles in that and those associated particles will be gravitons so loop quantum gravity has quite a nice way of treating gravitons as in effect pseudo particles not force particles particles like electrons can be modelled as open loops which puncture the spin network and move around on it so this isn't just some kind of abstract that theory sits over there and meanwhile we get on with the good stuff it is possible to start to bring particles from the standard model and put them onto this model of space-time from loop quantum gravity and that's a great step forward the theory can be used to calculate the entropy of a black hole don't worry about what entropy is anybody read Hawking's a brief history of time did he get through all the way to the end you might remember a chapter it was titled black holes ain't so black it bugged him but he realized in fact that black holes have an entropy and things with an entropy have a temperature which means they glow very upsetting cuz how can a black hole glow well it's very subtle it's called Hawking radiation and it's so weak that you can barely detect it above the cosmic background radiation that pervades the whole universe but nevertheless he was reconciled this little equation on the right hand side here is called the bekenstein Hawking formula for the entropy of a black hole and it's really simple entropy is equal to area the surface area of the black hole divided by four and here's this Planck length squared again okay so loop quantum gravity remember we have Network spin networks the lines because this pervades the entire universe remember this is space and time where the lines puncture the surface of a black hole they end down the surface with Samaria remember the links were where we find the quanta of area now you calculate the entropy not by working out how many links puncture the surface of a black hole but how many different ways they can puncture the surface of a black hole and that gives you a handle on the entropy and what you get is the bekenstein Hawking formula yay the theory eliminates singularities now I haven't mentioned these yet but the thing about general relativity is it's a continuous theory it assumes a continuum of space and time and theories like that they can give rise to infinities things can become infinitesimally small or infinitely large it's just in the nature of the maths it's the nature of the beast Penrose and Hawking in fact together produce various theorems in the 1970s to say that there was no way you could avoid these if you look at the center of a black hole or at the origin point of the Big Bang origin of the universe you'd expect to find a singularity things that have gone infinite now there's actually no such thing as infinity in nature so something has gone wrong but loop quantum gravity says that space-time is granular it's got bits I can have an ultimate unit of space volume or area I can't squeeze that any further it's a bit that it's like an electron a single electron or a single photon I can't get smaller than that I can't divide that any smaller and it means there can be no such thing as singularities there has to be if the universe collapses down or begins in in a in a very tight highly dense object it cannot be a singularity because quantum nature of space won't allow that an actual fact what Abhay ashtekar discovered was an intriguing possibility we talk about the Big Bang origin of the universe and we've got lots of evidence that suggests that that's indeed although that evidence comes from moments after the Big Bang by the way not the Big Bang itself that remains mysterious that's when space and time was supposed to have begun but loop quantum cosmology the universe equivalent based on loop quantum gravity says that there could be no singularities and maybe therefore the universe began not with a bang but a bounce in other words there may have been a universe before that condensed contracted compressed and and once it ran up against the ultimate indivisible quantum of space it bounced now make of that what you will however loop quantum cosmology does predict some interesting things I don't think this will be by any means a definitive and if you want to know why I think that I can maybe tell you later this by the way at the bottom is a picture probably not a familiar picture of the cosmic background radiation but what we've got plotted here is the square of the temperature variation with something called angular scale so imagine the universe is a sphere and we're looking at different angles along that sphere now this these Peaks that you can see three of them one tall one and then to of equal size equal height they're actually quite characteristic of the equations of hydrodynamics and because the cosmic background radiation was formed 380,000 years after the Big Bang it's left an imprint of what was going on in the universe at that time and what these are are sound Bebe's bouncing back and forth in the early universe acoustic waves I like to think the universe was singing or maybe screaming but notice in this picture a difference the standard model of Big Bang cosmology based on general relativity predicts a slightly different curve at high angular scale loop quantum cosmology predicts the lower curve and as you can see the arrow bars on the data most recent data from the Planck satellite don't allow us to be able to make a choice at this stage but when with future satellite missions we have the equivalents of DNA fingerprinting at this high angular scale who knows my feeling is it won't be definitive because it will probably be easy to come up with a theory that will reproduce whatever experiment says but hey finally I've maybe painted a picture that Smolin and Ravelli they're not only great scientific collaborators they're also good friends but don't take that to mean they agree on everything in fact I picked up a beautiful phrase I think from Lise Mullins wife I think Deana if you both agreed about everything then one of you would be redundant which i think is beautiful way maybe it's the way to understand their marriage I've said this approach to quantum gravity means we lose time it disappears from the equations now carlo rovelli again you might be familiar already with his book the order of time has no problem with that Einstein had no problem with that by the way one after another the characteristic features of time have proved to be approximations mistakes determined by our perspective just like the flatness of the earth or the revolving of the Sun the growth of our knowledge has led to the slow disintegration of our notion of time Herbert's Marlin doesn't agree he published a book a little while ago called time reborn I no longer believe that time is unreal in fact I have swung to the opposite view not only is time real but nothing we know or experience gets closer to the heart of nature than the reality of time I believe that to make sense of the universe we must embrace the reality of time in a new way that comes with a fairly hefty trade-off though by the way because if time is real in space is an illusion so just so you know I'll leave you with a couple of quotes you've probably got the idea that I'm a big fan of Einstein I still one of my favorite Einstein quotes is this one reality is an illusion albeit a very persistent but maybe just a little better than that is the great science fiction writer American science fiction writer philip k dick who once said reality is that which when you stop believing in it doesn't go away now if you wouldn't mind exiting through the gift shop where I believe copies of this book are available I do want to take this last moment to say a big big thanks to both Li and Carlo who were reading over my shoulder while I wrote the manuscript for this book we finished off with a Skype call where we chatted together look back over their history of collaboration and looked a little bit forward to the future and Lee quoted a guy called is George Brack who worked with Pablo Picasso and pioneered cubism and Brack said it was like being roped together on a mountain and Lee thinks that's a good day for ISM for his own collaboration with carlo rovelli like being roped together on a mountain i also have to thank these good folks busy busy scientists who gave of their valuable time to read the manuscript and made sure I hadn't committed to many howlers the folks at Oxford University Press who helped make the book a reality and you can find me on my web page or follow me on Twitter thank you very much [Applause]

Channel: The Royal Institution

Views: 850,052

Rating: 4.6111565 out of 5

Keywords: Ri, Royal Institution, loop quantum gravity, jim baggott, quantum mechanics, general relativity, quantum physics, theoretical physics, particle physics, space

Id: dW7J49UTns8

Length: 68min 56sec (4136 seconds)

Published: Wed May 15 2019