The Story of Loop Quantum Gravity- From the Big Bounce to Black Holes

inside a black hole lies a mystery what happens at its center the same mystery applies to the Big Bang Einstein's theory of gravity seems to give way predicting the existence of a singularity a point of infinite density curvature and pressure when the entire observable universe is squeezed into a space far smaller than an atom new physics is needed taking account of both gravity and the strange microscopic world of the quantum we need a quantum theory of gravity one of the leading candidates is loop quantum gravity in this film will tell the story of this theory how it was developed and how it might finally be tested we'll meet some of the key players in the field from the younger generation of scientists to the founders of the field including a by ashtekar who even as a teenager found a mistake in a text book written by Nobel Prize winner in richard fineman well being a little bit brash at that time I decided to write the final and I didn't really expect that he would reply but I was surprised that he did and he said something like you know congratulations the book was wrong you're right but then he also ended basically by saying you can rely on yourself I think this was kind of a subtle nor saying I shouldn't keep quiet it's just a passion for astronomy was there since I was a child the first book I remember I pick it up myself from a book store was about astronomy and the other one was about cats so I was a cat and star person I count myself fortunate for having read and spent time in going intellectual in various directions then I got completely enamored in physics so forgotten in in love with physics I remember distinctively at some point I said this is so beautiful maybe I want to do that in life when I was very young I wanted to be your researchers in biology you I was really interested in animals plants and all the living world and then I switched to physics I don't really know why probably because it was even stranger to me it was a desire to go and to see what cannot be immediately seen I got interested in physics to architecture I got interested in architecture because apartments her father who is the great visionary and ecologist and engineer and architect I thought that I would design greenhouses and swimming pool covers and second homes and things like that energetic tones but I had this idea and I've written a code a program which could strategy let it go we basically make a triangulation out of any curved surface little did I know that that would be what I would be actually doing in science years later the realization that was a problem with quantum gravity is by Einstein immediately after having done his theory general relativity he wrote his the main equations of generality VT in 1915 and one year later in 1916 he write his paper saying well actually by the way this cannot be the end of the story because there is quantum theory not very promising a young physicist a friend of Lantau who my opinion is the first one who had full clarity about the deepness the death of this problem so the beginning people like particularly people like fireman and Bryce David he tried to incorporate try to generalize the matters that had been used in quantum electrodynamics to gravity it's quite usual in physics that we end up with infinite quantities so problems that nature doesn't like infinity so renormalization is a technique that was invented to get rid of those infinities and it works very well and it leads to miserable results but for gravity turned out that the theory is not renormalizable and therefore this idea ultimately had a real roadblock usual physics is done with space and time and the physics happens in space and in time so to say the space is there is give while in quantum gravity space itself that becomes a quantum object so the usual technique of quantum field theory don't work and I get I guess the breakthrough in the 60s was by John Wheeler and and Bryce DeWitt who wrote with this very famous equation which is a window DeWitt equation the wheeler DeWitt equation was an equation invented by to width by Bryce to it which is like the dynamical equation of quantum gravity analogous to the Schrodinger equation of quantum mechanics wheeler DeWitt equation is not well defined mathematically but full of ideas and people the time especially John Wheeler produced a lot of thing tuition that later on became the driving force for quantum gravity John Villa in particular was much more interested in also issues having to do with singularities of general relativity he often emphasized an analogy with hydrogen atom see from the hydrogen atom if you ask for what is the lowest energy available to it then the classical theory that lowest energy is minus infinity but that is an artifact of ignoring corner physics so vivir was convinced that singularities of general relativity are are really artifacts of ignoring quantum effects here this picture if you if you if you fly over an ocean at very high altitude you see a the ocean as a flat surface that's how we see the geometry our scale but if you come down you see the waves and then you see the foam over the waves and and that's how the short scale structure of space-time should look like I started thinking about working on gravity as a graduate student and in fact the very first paper I wrote was the review paper because I wanted to learn all about it and so it was a review paper I wrote my advisor Bob garage I chose to go to Harvard where there was no interest in quantum gravity but there was great mastery of the standard model and I had this idea that maybe the the mathematical and physical techniques that could guided to understand in QCD in the standard model would be useful for quantum gravity I decided that this was a a worthwhile problem to to get into you know whenever you your 20s you're all the courage of the waltz so I said okay that's what I would do to gauge means to measure and engage theory is a theory where the degrees of freedom the things that vary our fields that tell you how to bring systems to the observer in order to measure them I heard a talk by Ken Wilson and this was he introduced a revolutionary technique to understand the quantization of how these gates fields describe quantum mechanically and the important thing that he discovered is that they make lines or tubes of flux which can be quantized into carrying discrete amounts of flux of the charge first person I went to see is Chris I seen the person whose review papers had pushed me into into the fields I went to London I remember distinctively Chris telling me that a few days before I left he told me that there was in in in America this young Indian about a streaker who apparently had made the change of variables in general relativity who my simplify the problem somehow so I got interested I wrote to to wash the car and I went to see him in Syracuse and the work of a PI was based on a previous work I meet a person who had written general relativity classical generative it is on Stein theory in terms of some variable a connection instead of a metric what are metrics well they define distances two points are the different angles between two vectors they define light cones at every point so they give a meaning of causality and therefore they metric appear in every differential equation we write down in general relativity so they's where they always regarded as a fundamental object but it turns out that all other theories of all other theories of all of the basic forces of nature the electromagnetic force the weak force and the strong force they are really based on on connections other than metrics we know in physics we are interested in the way things change and it is encoded in the concept of a derivative but it's complicated to define a derivative in a curved space so basically the connection is here to define properly a derivative that is a way of comparing things in a curved space the key point as far as technical details are concerned is the fact that when return in terms of connections and specifically in terms of ashtekar connections general relativity looks like a gauge theory gauge theories are theories of particle physics and we know how to quantize them a barrister car showed up and he had called me and said that he had managed to make a complete formulation of general relativity extending amitaba son's work because I would have a sense work was wonderful but it was incomplete the equations of general relativity are of infinite order any power of the fields is involved and that makes them very hard to study even classically but it makes them basically impossible to quantize when our buyer was done his form of general relativity had only quadratic equations so it was the vast simplification and it was clear to me that that was the way to go the way to go was to take the toolkit of trying to reformulate the quantum theory in terms of loops of flux and apply it to a biased formulation of general relativity lien you know very a solid background coming from quantum field theory I was stood up Sidney : Coleman so but Lee like to make big jumps Carlo was very interested in these foundational questions they had many different personalities and in some ways it was a very extremely pleasant sort of kicking together that occurred at the workshop and then and then it continued it was continued how hot is he yes Karl and by became very good friends and in fact for some time the three of us were quite close it's kind of emotional Carlo and I have some similarities we were both came out of left-wing of politics we were the generation below the generation that had made the great resistance to the Vietnam War and begun the feminist movement and by is a mathematical relative ascent of mathematical physicist and he the at the minimum the best of his generation at that there is nobody I know better to take an idea in this realm and make it a precise mathematical idea I'm the opposite I was trained as an American elementary particle theorists by the particle physicists who emphasize intuition Harlow has some of both of our strengths he's at least as good me as a physicist he's a fabulous physicist and he also has an easier time than I do speaking the rigorous language of mathematical physics and working with her by Lisa morning in the antique jacob sona made a step which turned out to be crucial which is they found some solutions of the wheeler DeWitt equation get eaten by a by ashtekar with the new variables and these solutions were very mysterious because there was one solution for every loop in space so you start with space to draw a loop and then there is a mathematical technique from this loop to have a solution with a delete equation at that stage ideas being proposed in lieu quantum gravity still used a background structure known as a lattice one day I was talking with Ted and we wondered what would happen if we just took the lattice away and had these quantized flux lines of abides variables just moving around in space without being stuck on lattice and so we wrote that down and we we tried to apply the wheel of do it equation because since there's no time outside the universe the weird weight equation says you take a quantum state of the gravitational field you act on it with a certain operation you're supposed to get zero and we plugged in these lines of a quantized electric flux and discovered that as long as the lines didn't intersect each other if they were closed loops or knotted or braided we got a solution to the wheeler 2 equation just like that and we got an exact solution not an approximate solution which nobody had ever dreamed was possible but we had it was literally took 10 minutes so all of a sudden we had the quantum geometry in front of us that was that was a wonderful moment so we had solved the wheel of jewitt equation but there were three other equations that you had to solve to make a corner in theory of gravity they were called the different morphism equations and they express the idea that there's no preferred observer there's no preferred coordinate system in space we couldn't solve the jiffy morphism equations with the same variables that we used to solve the real dewitt equation when Carlos showed up I was convinced if the problem was impossible and we were stuck then I looked at Carlo and I said for some reason do you like to sail and he said he did and I was just learning to sail so he went sail and talked about his girlfriend troubles and my girlfriend troubles in we started to become friends with we really bonded then he disappeared for three days and the next day he came came into my office took off his new jackets as he was always freezing in America and said I've solved all the problems instead of starting from a space and a connection I the quantum theory directly in terms of linear combination of the solutions of loops and we immediately realized that by doing that we could solve the full set of formal equations of quantum gravity we were not describing excitations that move in space this loops those words excitation of space itself we were describing space-time as a collection of threads if you look at a two-dimensional surface by English like the shirt for example right now then from a distance it looks like a nice smooth continuum but if you look under a magnifying glass you will quickly see that it is really open by one-dimensional threads so similarly the idea is that the Roma tree in this room is really open by one dimensional content rates and what are these condom threats these condom threats are fundamental excitations of the gravitational field so there are loops of object but they're there that there are the loops from which geometry emerges John Wheeler got very excited with loop quantum gravity he wrote to me in his very emphatic style he always used with a lot of wow I still have his little letter of my office on a wall because of you know for me he was the great hero the great intellectual heroes so receiving a letter for him excited about my work for you know the young kid I was with more than I could have a dream I came to Princeton with a piece of little thing which I made with a lot of clearings attached to one another which was supposed to give the idea of the three-dimensional structure of our space-time and when John saw it he jumped and said yes that's what I was always been thinking and he came with his book and a picture in his book look it's the same thing it's a quantization of general relativity so unlike string theory it's not a different theory than general relativity which might have something to do with gravity of general relativity in it it is general relativity as given to us by Einstein as modified by a by a streetcar and then quantum mechanics using ideas that were providing the steady gait series by Polycarp and Wilson and other people but it is just the quantization of general relativity we realize that loops had some technically we're going to be inconvenient and therefore the loop representation is really not so convenient to use and to get the interesting properties from it and so on so we introduced instead of loops it's called as graphs so loops are sort of closed curves where as graphs are objects in three dimension space for example in which has nodes and their links and they can have many many configurations and it turned out that the same ideas that were underlying the loop representation can be carried over with the same integration theory and so on two graphs and then several of the technical problems disappear in this particular way and so in a certain sense it is really true that the loop representation is a misnomer for what is being done in look or no gravity or loop quantum gravity is a misnomer but it is very similar to what is string theory I mean string theory is a misnomer because originally there are strings but in previous then people realize that there also extend other external objects membranes and so on and strings are no more fundamental than other objects they're all on the same footing so to say but since it was called string theory they still call string theory as loop quantum gravity was being developed a student of Smolin's burned Bruggeman was given the task of loop quantum gravity to the theory of the strong nuclear force quantum chromodynamics but in doing so he discovered the work of Anthony Trias and Rudolfo Gambini and then one day he came to my office with a look of horror on his face and he said this has been done before I was shocked I was embarrassed I was pissed off it myself and faith and everything so it wasn't only our discovery and it was just a few years before Carlo and I had invented this technique I wrote him a letter they had an address in Barcelona and said that I was very sorry that we had developed this technique in quantum gravity and the astronaut variables and we had just had not known of their work I actually wrote a physical letter and Rodolfo says what are you doing tomorrow so he said we'll be there and he hung up and they went downstairs garden his car she's a little sports car with the two of them and I drove from there and that was the afternoon overnight from Barcelona across France across the Alps to Trento in north of Italy and they arrived the next morning Gambini very quickly after that began to work with an Argentine young Argentinian physicist Jorge Poole in and the byline of Gambini and Poland have been they've done many things together during the 1990s a key breakthrough in loop quantum gravity was made how to calculate the values of the area and the volume of the discrete chunks or atoms of space we did that in the middle of the 90s was another super exciting period and it was the other moment of great sort of collaboration and joined with Lee Smolin Lee was in Italy at the time Lyra by were often coming to Italy we would work together and when liyan about when Lee came one summer was 95 I think we just realized we could do that in fact this is an idea by Lee he came and said we can do this calculation and we plunged into it and of course we did it wrong at some first moment and then we corrected it and that's a moment in which we realize that the mathematical solution of the this spectral problem this diagonalization of the of the operator aryan volume to compute the discreteness had as a solution Roger Penrose P networks I think it was in the early 50s when I had been worrying about various things that maybe space and something the small scale would be discrete in some sense and also quantum mechanical and also I remember I was a good friend of Dennis Shamas who used to drive very fast Ron Carter his his car and when you got thrown to the side he said well that's the action of the fixed stars so the idea was that the inertial frame was determined by a distant thing so we used to talk about what happened if you removed all the stars then would you would you lose your feeling of inertia or something and with the car determine your frame and then suppose you took the car to pieces and took all the atoms away and I thought of the extreme case where there were just two particles left both of which had spin 1/2 I see in the normal description you'd have a quantum account description we give you you'd have a direction the wave function would determine a direction the spin would be about that direction so you'd have different cases like that but the idea was that you wouldn't have any concept of direction you just have the only thing you would know would be the total spin so if you bring these two spin 1/2 things together they could make a spin one thing or spin 0 thing depending on whether they're one way or the other and there were probabilities involved then I thought well suppose you bring more together and so on and then there's the question of how do you know whether the probability has come from the directions of their spin in a sense or does it come from something just because you don't know their probabilities is all you're going to get when you bring these together and take them apart and so on you only get the final thing is you'd have a great network of these think of them in in sort of time you have these particles coming together and then this one comes together with that and all you know is the total spin value so this was a way of trying to make a discrete model which is purely quantum mechanical purely combinatorial you didn't introduce any complex numbers or any differential equations or any things like that it was just counting loops counting loops and assigning values to the loops but I think there are a lot of things in spin networks which could have been explored more and I was glad to see that the loop quantum variable people did pick up on this or they use the theory in a rather different way from the way I'd originally intended but it's interesting to see these differences as well only took a plane you went to Oxford to talk to Roger because we're confusing whether we should put the plus or minus there and remember a phone call with Li and I asked me so what did Roger says is the plus or minus and Li was a sort of sample he said well Roger said it doesn't matter both are okay and clearly he was not understanding and then of course we come out of the technical difficulties we clarify that and this is a technically I think this is the major success of loop quantum gravity because there's a precise prediction of them the discrete values all Roger Penrose was not directly involved in the development of Lu quantum gravity it's no surprise that his ideas were influential not least because of AI ashtekar on the recommendation of the great physicist Subramanyam Chandrasekar was one of Sir Rogers postdocs sure he came in extremely good recommendations or else we wouldn't have taken him on and he obviously fulfilled these recommendations and I was very impressed with what he done Chandra felt that the place I should go to was Oxford and that was to work with Raja so Chandra actually apparently wrote Roy Roger told me this much later I let a letter completely unsolicited for me I also then after Tara told me this I actually applied to this position and then you know eventually I got a telegram from Raja saying offering you a postdoctoral position do you accept and I didn't know what it meant because I night I was poor like the student and in any case from India there was no way to get any money from anywhere so I called him was Roger of course didn't know what the salary was so I said is it enough to live on yes yes that's okay I accept Oxford in the 1970s Stephen Hawking in jacob bekenstein had shown how to calculate the entropy of black holes but this was based on the assumption that space is continuous two decades later loop quantum gravity researchers began to wonder if the formula could be recovered from the principles of the discrete quantum of space-time after some preliminary studies which revealed pieces of the puzzle the full computation of black hole entropy was presented in a landmark paper by ashtekar Baez quarry Chi and Krasnov and it is very simple a black hole has a surface which is a sphere and the the quantum state of the geometry aloo piece or a little lines that had punched the sphere and on any punch there is some degree of freedom when I say degrees of freedom I mean the independent ways in which something can move and you just count this degree of freedom and the number of them should tell how many degrees of freedom there are on the on the on the on the surface of black hole which correspond to the geometry of the horizon so the horizon is a sphere but it's not the exact yes here it's a sphere that can move fluctuate and these these fluctuations are degrees of freedom that sits on the surface and you can count them and in the classical theory they're infinite of course because geometry smooth so you can do arbitrary arbitrary small fluctuations but in loop quantum gravity there's no continuum geometry it's discrete the geometry so there is no excitation smaller than the the scale of the the minimal scale of geometry which is the Planck scale so you can count the number of fluctuations of the geometry and the counting of degrees of freedom correspond to a calculation of the entropy when we do look on gravity there is a certain ambiguity in conversation this is not uncommon there is a similar ambiguity in quantum chromodynamics the theory of strong interaction which is called the theta ambiguity and so theta is a free parameter in quantum chromodynamics it is no classical analog and its value is determined experimentally so similarly there is an ambiguity in the in the loop quantum gravity when you go from classical theory to quantum theory and it is called the barbero emerge-ii parameter if there was an experiment by which i could measure the smallest area again i will notice fair value and the theory will be determined and how it happy but this is far beyond any technology don't you know how to think of such experiments today since the contour map area depends on the Marberry merely parameter the number of microstates depend on this parameter and we just blindly put this parameter equal to the value that is needed to get agreement with the bekenstein Hawking formula this requirement fixes the Barbarian is a parameter but I did it just for one black hole and now one can ask the question about any other black hole and one finds that the leading order term is is again the same so in that sense there is a check on this barbarian it's a parameter in this area again more recent work in cosmology is also giving checks on this this area gap and therefore indirectly on the Barbarian music parameter we can leave this as a free parameter and determine it using one of the two methods and then check if it is true in the other loop quantum gravity is not the only theory to have success in the field of black hole entropy string theorists also claim to be able to infer this formula but one of the key differences highlighted by loop theorists is the background independent nature of the theory you do quantum gravity not by having a space and physics on this background but rather by doing the quantum physics of space itself that's background independence very simply string theory developed as a background dependent Theory so developed us on honor on a fixed space in recent years there have been a lot of effort to bring string theory into a background independent language in fact in some sense the ideology of string theory has come much closer to loop quantum gravity in recent years it's a very beautiful idea string theory it's a very courageous attempt I think it has not delivered a lot of beautiful things have come out of string theory but the project of making a theory of nature answering theory isn't my judgment basically stuck the dreams of eighties have not come true nobody's been able to compute the parameter of a standard model nobody has been able to derive the groups of the standard model and so forth in fact the few predictions so to say of string theory have been disappointing because one sort of prediction in which most string theories were very convinced of was that if there is suppose the cosmological constant it has to be negative then the cosmological constant was measured and turned out to be positive and the other prediction was that we were supposed to find supersymmetry at low energy at lap or HC and it did not come out this doesn't mean it's necessarily wrong but I think is a search program is is losing ground string theory comes in an infinite number of versions with because it has to have these extra dimensions within there being an infinite number of ways to tie up the extra dimensions and so there's a huge problem of empirical content or productivity string theory really at a fundamental level appears to require supersymmetry and in loop quantum gravity supersymmetry or higher dimensions or a negative smile constant are not at all necessary the current gravity is not claim to be a theory of the unification it doesn't offer a specific way to unify gravity with the other forces and unify them with the different elementary particles the way the string theory at least initially was thought to provide such an insight reunification Luke went to where it is far less ambitious I personally think that we are not at the point of writing the theory of everything we don't know probably a lot about the universe and maybe there are other fields on the particle size of whatever we don't know so we better do one problem at the time and the problem of the quantum aspect of gravity it's a well-defined problem we have a lot of information about it so we can write a theory just about that and do quantum theories is the theory about that so it's a quantum theory of gravity and I would say it has happened the opposite of string theory because in the eighties when we started there were a lot of unclear issues and things that seemed very hard to to go ahead with and it's it's it has progressed in the 90s and the at the turn of the millennium and and in this first twenty years of the new millennium more and more and more and each big problem has been solved a frequent criticism of loop quantum gravity is that the theory cannot specify the dynamics of how a system evolves one way of breaking the definition of a theory especially quantum theory is to separate the kinematics and dynamics the kinematics is everything you can measure the dynamics how things move it one still finds this these comments this is Luke one twenty one doesn't know how to ride the dynamics but this was 25 years ago this there is some criticism allowed to loop quantum gravity which keep bouncing since I was a kid else and people just haven't read the last 25 years of literature full general relativity I don't have a general solution of Einstein's equation but let me go to a physical problem like a spherical black hole then I know what the solution is or a certain cosmology then I what the solution is and so loop quantum gravity has done the same thing and concrete proposals for dynamics have been made have been verified I have been checked in many many ways in these models and I think today the look on to where it is in a good shape there is a theory we know the basic equations we know that in the classical limit it gives general relativity we have a lot of indications of that is it the right theory or it's not the right theory and for that we need to apply it in order to determine if a loop quantum gravity is right it has to make novel predictions about nature which are confirmed by experiments and perhaps the most stunning application of loo quantum gravity has been to the very origin of our expanding cosmos the Big Bang when it was first put put forward and you know also studied by people like la Mettrie in detail the Big Bang was really the beginning of the universe I mean then now I think the working cosmologists of early universe think about the Big Bang as really the time at which the universe became hot so it is a time at the end of inflation if say for example in the inflationary scenario during the fashion era people don't often emphasize this what is using Connor field theory at zero temperature Big Bang is really a prediction of general relativity in a regime where it is not applicable Martin Boyer Walt was a postdoc who had come from Germany in from Aachen and he had done his PhD thesis precisely on cosmological models on the classical side little bit of corn upside mostly classical side and so he really knew the mathematical background very very much so you took up this problem and he did some simplifying assumptions and found that in fact the Big Bang singularity was resolved so the current expanding face of our universe is in fact just the continuation of a previous pace in which there is a contraction and then an expansion the wall back in time one reaches a point of maximum density so this is what we call a bounce the universe collapses and bounces back one of the problems of Bowyer Walt's work was that the bounce could occur when the density was far too low to be realistic by 2006 a stroke are pavlovsky and Singh made a key breakthrough showing the bounce only occurs when the universe is close to the Planck density I thought it was too good to be true so to say I mean that it is something that is and I wanted to be very very careful so the question was whether this was an artifact of our specific choices for example the choice of state we made so we varied States still keeping them picked but we changed states we changed various things in the computational models I kept asking a thousand questions to the postdocs we're doing the simulations to make sure that something nothing was slipping us but the prediction remain robust in all this thing and after over eight months of this checks and rechecks we decided that we should write it up and publish it and that's what we did if there is a bouncer if there is a contracting phase how can we check this we expect this to live a signature in the CMB in the Cosmic Microwave Background at the Big Bounce you are in a very strange state the universe still has some curvature and therefore your initial conditions for perturbations are not the same as in the usual model and this might change as a spectrum that is the shape of the perturbations and this could be seen in the future experiments devoted to CMB the observations have certain anomalies the standard inflation predicts some what is called as power spectrum how much there is how much energy there is per unit frequency the power spectrum tells you how much power you have into fluctuations as a function of the l multipole which is the inverse of the considered length scale so intuitively speaking big L large L means small scales and small values of L in large scale so if you have a power suppression or excess at small L value it means that you have a poor suppression or excess for very huge distances in the universe this power spectrum agrees with the standard inflationary predictions for suitable potentials such as the one that was put forward by Stravinsky for example very well at small angular scales but atom largest angular scales it does not because this effector corresponds to the larger scale in in the in the sky these are also the ones on which we have a most uncertainty so there is a competing effect between the uncertainty coming from the observations and the effect that we are predicting so this makes the research particularly hard but predictions are there and maybe with future observations we will be able to look at the details the moment the happy situation is that look on cosmology has some predictions at low else these are compatible with the power suppression for example of observations but we also have predictions for other observables which is called the electric electric polarization correlation function electric polarization a temperature correlation functions if in fact observations for example falsify one of these predictions then we'll really examine what is happening what went into this prediction is two things one is of course the basics of Luke on cosmology and second is initial conditions and to me of course the most fragile things are initial conditions and so on or try to go back and see you know are there some better initial conditions communities are very well motivated in some fundamental aspects but of course they are not compulsory for a long time mathematical theories of quantum gravity remained rather far removed from experiments it's exciting that Luke quantum cosmology has provided a concrete bridge between fundamental theory and observations contributions from several young researchers especially Agnew Nelson and group have been vital in this process in addition younger researchers are also pursuing other directions and although these are not yet fully formed and therefore speculative they lead to fascinating new ideas in general relativity we can test the assumptions because we compare the theory with the observations in a regime where we know how to test it in quadrant gravity we cannot just compare with a real world in the regime where the theory is useful therefore you have to guess what are the correct assumptions and at this point nobody knows which as assumptions are illegitimate and which are not so depending on the assumptions you made you make you end up with a different loop quantum cosmology effective theory so we have the same model theory but we don't have the same effect if there to describe the universe one path has been followed by a by a car and its colleagues I follow another path and in this specific framework then we find a disappearance of time we cannot know if this is real but is it extremely exciting because it has been postulated into the past for example by Hottel and Hawking in one of our previous films we discussed the hartle-hawking no boundary proposal which is often associated with a beginning of the universe but as we saw in the film there is a real and imaginary time component the beginning takes place in imaginary time whereas in real time like in loop quantum cosmology there is a bounce so I know that by is not a big fan of this idea of disappearance of of time and appearance of a four-dimensional space but I think it's worth being considered because it is a possibility for sure this is not demonstrated and this is not a very hard consequence of rube column gravity but it is something that would be so fascinating with your experimental consequences so in my opinion it should be considered and calculation should be continued so doesn't matter how you do it no singularities can form in the early universe so there will be no big bang independently of how you construct your quantum cosmological Theory from loop quantum gravity so whether it's a symmetric a symmetric or some other kind of oscillations but still some kind of balance should be there you come from the past and you can predict the duration of inflation you know inflation is the expansion of the universe just after the bounds huge expansion so you have a clear prediction sank to the bounds on the duration of inflation and this could be experimentally problem to me this is highly reliable and very interesting because you don't have that in the standard cosmological Big Bang model from the observational viewpoint we know that the minimum number of defaults should be around 60 and we end up with a value which is of the order of 100 100 is above 60 so this is good otherwise model would be excluded but it is not much above so this is very good news for testability there are more things so that we can do to check the Big Bounce so there's the possibility to have access to effects that comes from the universe before the bounce so or any embargo with his collaborator so recently has push an idea about the possibility of observing gravitational waves that comes from explosions from objects that were present before the bounce if the universe did not begin at the Big Bang does that mean that the universe has always existed what about even before eons and eons before we just don't know there could be a past history of the universe which is infinitely long an argument against an eternal universe is that we would never get to now if the past stretched back for eternity these arguments to me seem a little bit like the arguments that were used by neo conscious in the early days of general relativity to say that general relativity is observed because in general relativity matter and energy curves space-time but how could it be possible because you want to talk about matter and energy one leave space-time so how could it curve space-time and very serious people argued this a very philosophical give this and I I I mean we know that this is completely vacuous argument I mean it's not that there is space-time first and then that curves but everything comes together so similarly here I don't see what the argument is there is actually detailed prediction of how we got here and and that's the last that that's it for me suppose you were to start a race and I'm going to start my starting guy wait for infinite amount of time before I shoot the gun you ever never start the race because you have to wait for an infinite amount of time so we never get to know right but there's no doubt that it started with the statement is that I mean that many philosophy is for example in Buddhist philosophy the universe is eternal and there are no problem in fact they emphasize that and that universe cannot have finite beginning so these people will just say that well I just dismiss it because I mean the statement there is no God and not for thinking the universe began with the Big Bang our singularity theorems that show that the evolution of the universe cannot be pushed indefinitely into the past there are the similarity terms but you can always interpret them as telling you that a quantum theory of gravity should not have one of the hypothesis that you are using to formulate this theorem there was an initial singularity theorems that were proved quite some time ago by Penrose Hawking and Goro and others which made some assumptions about the kind of matter that that sources gravity in the unit on that is used in inflation these conditions are violated and therefore there's some hope that perhaps in inflationary models there is no initial singularity no big banks in your allottee but a theorem proved by board a good and B Lincoln sure that this is not the case even if you don't make any restrictions on matter of the type that very made before but if you assume that the US was always expanding in the even in the past always expanding then they proved that the there is inevitable big bang type singularity now this conclusion is evaded in look on cosmology initially the universe is contracting and it expands only after the mouth and therefore the assumptions that they made is violated a new singularity theorem proposed by Aaron Waugh uses concepts from thermodynamics to argue for a beginning what is assuming that some version of second law of thermodynamics is really fundamental and should be a fundamental ingredient in the series we determine whether there will be singularity or not and I don't agree with that I think that it really is something that would emerge in approximate sense in the semi-classical regime whether our universe began to expand bang or bounce there still remains the mystery of why did it have such a low entropy stage I think we can gain much clarity if we realize the fact that entropy is not an absolute concept in fact it's a relative concept entropy is really counting the number of microscopic configurations which lead to the same macroscopic configuration when we look at a system depending on the way in which we interact with it we can associate with the system different entropies really add a very interesting view of entropy as something which is not purely objective but which is somehow related with the way systems interact one with the other so clearly stated it would mean that the entropy of the universe was not low at a Big Bounce it means that it appears to be low for us as living creatures for sure it's not a social construction I mean the entropy of the Sun does not depend on the fact that you look at it or not still it depends on the way microscopic systems interact one with the other so it's not completely subjective but it's not either completely subjective because the way you copper to another system depends on the kind of system you are in one of our previous films Sir Roger Penrose proposed that dark matter may be related to the remnants of black holes in the pre Big Bang universe Penrose dubbed these remnants era bonds after the primordial God of Darkness Erebos such black holes may have a large volume even though they have a small area and this can be used to explain the low entropy condition at the bounds if I look from the outside the universe seems to be so small and there are not a lot of ways in which things can be a lot can be arranged in such a small space so this is a way in which we say that the universe had a very small entropy but if now I can describe the universe as a wall including also the interior of this Ruben's of these black holes then there is a lot of space there is a lot of way in which a canary around feels and then the entropy of the universe is just whatever is not special is not particularly low is only low if I am a special observer that's it outside of these black holes I can only probe the physics associated with the exterior of the black holes when we look at the oldest light in the universe the Cosmic Microwave Background we find that it is not low entropy its high entropy so the low entropy condition of the universe must be in gravity rather than matter and this brings us to another possibility for explaining the low entropy and the balance what we need to consider is not just the entropy a pattern but also entropy associated with horizons so to say the gravitation entropy in the contracting branch because the dynamics is different from Einstein's theory a horizon develops but it grows extremely rapidly and in fact therefore the entropy really grows extremely rapidly and and as usual with black hole horizons this entropic dominates completely the entropy of matter therefore the entropy is really growing very rapidly but then at the bounce the geometry is such which is non-trivial statement is such that in fact the area of this horizon becomes infinite and then after that low horizon simply disappears so if you like what one has to do is to reset the entropy clock in a certain sense at the bounds and then then prepare again increases with the scheme that we have precise scheme we have the entropy is always increasing towards future there is no rebirth of a bad of time in one of our previous films Alex Vilenkin argued that a contracting universe and stable we have absolutely no idea of what exists in the universe before the bounds so it's very speculative and very difficult to decide whether the situation is stable or not and even in the case where the situation is unstable usually what it means is that the simplifications we use in standard cosmology cannot be used anymore but you know nature doesn't care about the fact that it is hard to calculate so when we say that something that won't work most of the time it means that the model we'll use to describe these things doesn't work anymore for example you cannot use a Freedman equation because a universe is so Hinnom a genius that the Freedman equation is not usable anymore so what do you need urgent care that we have to invent another equation I think the correct way to think is not to go from the past to the future is to start from what we know what we know is what we see around us today the universe is expanding we have started with galaxies we have gravity we have a theory and then we go backward in time and we see what happened and in our theory when we go backward in time we do not end up with a singularity we end up with a bounce string theory seems to predict the existence of a huge multiverse and it's often said this can solve the problem of why the constants of nature appear to be fine-tuned for life but many in the loop community are skeptical of the multiverse so how did they deal with the problem of fine-tuning one solution given by Lee Smolin is called cosmological natural selection will detail that in a future film but other alternatives are that fine-tuning itself is an illusion and that the constants are set by necessity there could be necessary I mean in the sense of I agree that all the detail arguments it's a hard subject but all the detailed arguments people do is always keep this end constant the same and varied this and then look something will go wrong but that is never there's no small perturbative solution around us but that there is no other solution is not some obvious to me at all that could be all constants are different and yet life forms that to me is not at all impossible we have to be very careful in the way in which we treat probabilities so when you say something is probable or something is improbable its with respect to which sets of events or which set of things and which set of possibilities you are imagining that there are many universes I imagine that we have only this universe and people say yes but if the constants were a little bit different there will be nothing and that's incredible intellectual arrogance and stupidity because nobody would be able today from the standard model to predict chemistry and life nobody at all so how do you know what would happen if you change the value of the Kosmo of a constant imagine you change the mass of a pork you know a coupling of the Higgs a little bit of course it would not be cardboard there would not be stars but there would be something else and nobody is able to say correctly in implausibly incredibly that the universe will not be complex and whatever because we're not able to do these calculations one example of fine-tuning that loop quantum cosmology directly addresses is the initial conditions for inflation in general relativity the question is actually imposed because the answer depends on the type that you choose to ask the question if the initial conditions are specified at a given time is it highly probable or very low probability is very low that the inflation would actually happen with enough number of in foldings in lieu quantum cosmology because wherever bounds are preferred instead of time we have an unambiguous answer that almost all initial data specified at the bounds will actually evolve into giving us enough number of a foldings in the future a key difficulty for any quantum gravity theory is how it deals with a fundamental feature of relativity known as Lorentz invariants this is a beautiful story of Lorentz invariants means being compatible special relativity wasting time can be translated into zero time interval doesn't mean anything lucemon tonight is not well defined in special TV that was a great discovery by i standing in 1905 and we're very sure about that in the quantum gravity there's a minimum length wait a minute you move fast and you see something smaller than the minimum length but that contradicts with the prediction so people say oh it's impossible to have Lorentz invariance in in quantum gravity and this solution is going to mechanics you can have discreteness and continuity at the same time and in the the typical example of that is angular momentum if you measure the angular if you have something spinning and you measure the angular moment in one direction quantum mechanic predict this is zero or 1/2 H bar or one H bar receiver so it's discrete okay but if I turn my head a little bit the component of the angular momentum One Direction shrinks become smaller and smaller continuously so it seems incompatible the discreteness of a component of angular momentum with the rotation environments okay but everybody knows that in quantum mechanics there is a rotation invariance and also discrete this how does it work well it works that when I turn my head what happens is not that this shrink is that you have a probability of seeing this full length and a probability of saying it's smaller and the probabilities move but you only see either this long of this long only the discrete values let answer so the answers probability and the loop quantum gravity is compatible with Lorentz invariants in spite of the minimum length for the same reason namely if you if you have something minimum length that you move fast you have a probability of seeing it's minimal length or probability of seeing it zero length you never see it half a plan clicked so it's a beautiful example of how quantum mechanics allows discreteness and continuous symmetry groups to say together so here we are completely the regime where you have to deal with effective theory so you will try to guess how the high-energy quantum gravity effects will manifest themselves on a very low energy phenomena because your gamma rays it's maybe 10 or 15 orders of magnitudes below the long scale so it is very low energy with respect to the Planck scale and once again there are different gazers so if you see nothing as it is required up to now you cannot conclude because it's perfectly possible that Lorentz invariance is indeed preserved by Lube cotton gravity but in the case you see something you see that the speed of light depends on the energy even in vacuum then this would be probably a smoking gun for quantum gravity maybe not for a specific theory of quantum gravity but was the idea that space is not continuous and that you have some atoms of space like webhead homes in in Christ or if loop quantum gravity predicts the existence of space-time atoms where do the new atoms come from when the universe expands well that's the same question as asking you classical general relativity they knew us expands and the volume increases so can just ask the question what creates is volume its dynamics which creates this morning so there is no conservation law for volume and there is no conservation law for the number of atoms of the geometry another route to exploring experimental testing of the theory comes from nature's most mysterious objects black holes a few years back we have detected for the first time gravitational waves directly thanks to the LIGO antennas in the US and revere go antenna in Europe and what were basically seen is require a sense merging of two black holes and then the relaxation of the big black hole coming from that and the well his big black hole is losing its energy from gravitational waves might depend on quantum gravity Corrections so it is still speculative at this stage but it is possible that some quantum Corrections outside the horizon of the black hole change this relaxed relaxation modes of the black hole and this could be probed by gravitational waves there are similar ideas in string theory with of course different consequences hopefully but it's very exciting that we now think about possible ways of testing quantum gravities thanks to gravitational waves because the way the black hole moves tells you something about the proper modes of space-time in itself we got convinced in recent years and we are try to convince the community that when you go to the center it doesn't end there the universe you continue you continue you go through a quantum region but very rapid did you go out is going to region and what you have on the other side of the quantum region is very simple it's just the time reversal of what went in and the physics is simple if you take a ball you let it fall it goes down and what happened when it hits the ground well the first approximation just comes up and how does it come up it's exactly time reversal or when it went down the way in which this effects are manifested as we know from quantum cosmology is in the form of an effective a repulsive force so this collapsing phase should evolve into an expanding phase and why bounce as well because you cannot compress things too much there's a limit in compression or if you want there is a Heisenberg uncertainty principle now the same reason an electron cannot be squeezed in an atom you know classical theory predicts that the electron is going to fall on the nucleus of an atom but it doesn't why doesn't because of quantum theory quantum theory forbids something to be to squeeze something to match what happened is that the matter bounces but seen from the outside this bow steak very very very long time so the physics if you fall in is you go to the minimal and you bounce up but seen from the outside use this you see these things in some sense slowly slow is long and strong is long down billions of years and then slowly long is slowing coming up I'm convinced that what a black hole does is go through the quantum phase becoming the time of reversal of a black hole which is called a white hole and bounce out so what do we need to do to test this idea that a black ball evolves into a white hole what we need are black holes that are older and small enough to be exploding today so this means that we need what were what is what are called primordial black holes prime black holes that format in the very early universe just got calling me and said well you know that this faster audibles sorry I know no idea what is faster any words so she said look at that Google fast why do Ballas and it turns out that there was an article nature's at the biggest mystery in astronomy so what a big mistake as well we have hardly seen these very violent signals with a radio telescope we see in Australia and recibo and they were so strong that they didn't believe them at the beginning we have too many explanations today we have more explanations than the number of actual detection we have this idea that perhaps faster ripples are exploding white holes for gamma-ray bursts we know now that depending on their duration they probably have different physical origins so it could very well be that fast radio bursts do not all have the same origin there are so many registers in our universe of many sorts of radiations so how can we distinguish radiation coming from exploding black hole from all the other sources with Covelli and Francesca video we try to make a full phenomenology of the phenomena that could take place due to Zeus bouncing black holes so it's correct that fast radio bursts could be explained I have also shown that the Fermi excess that is too many gamma rays we see from the galactic center could also be explained by this model but you know this is not very convincing because when you are a theorist and you have a model you can basically explain nearly everything we know how to discriminate between this model and all the other models that are currently available and the reason for that is that redshift dependence that is a way the signal coming from bouncing black hole depends at a distance at which the source is will not be the same for our models and for all the other Astrophysical or particle physics models if I have Barstow coming from this exploding black holes I can put them on a graph in which I have the observed wavelength and their distance and this would form a lie instead of forming a line as the only other Astrophysical objects that would form would form a curve that is flattened in for a higher values it would be fantastic because it would be a genuine quantum gravity phenomenon directly observed in the sky there's a lot of if in this story the main if is whether a black hole explodes when is large or when is small and we don't know it's it's it's recent stuff so ideas change but I'm pretty convinced that the the bounce from a black hole to a white hole which was our main idea that's solid if it happens at the very end of the vibration what what comes out is a black hole but the black hole sorry is a white hole but a white hole which varies teeny and this can stay there for very very long time before it dissipates all what is inside and can come out and these are the white holes that if they existed in a previous yield sort of before the bounds before the big bang my had gone through the through the bounds a couple of student of mine did calculations which seemed to indicate that no no no no no is very improbable that they explored what they are we we compute the probability the probability of explosion as a function of time and their first calculation is that probably is very too low too small so it becomes high only when the clock or shrinks down to small now it's a first oil calculation whether to go to next order we don't know how reliable it is but it goes the other way so we're in the middle of this whether it happens when the black hole is large and then this is it might be connected to the faster reverse or what it happens when it's it's very small and then they hope to detecting it is through these these remnants and connecting to the Dark Matter the reason of the discrepancy of the debate of the controversy is about a stability of the bounds and it's true that the bounds can be made more stable if it is not symmetric but it will not change the energy of the emitted quanta so as far as observations are concerned you don't really care about knowing if the bounce is symmetric or not the key point right now is how much time does a bounce take it might be less than the edge of the universe for some light black holes or it should be much more for the very same black holes depending on the assumptions instantly we've worked out a rather speculative idea which is the possibility that some something goes through the well this cloud of small black hole remnants my go through the mounds and constitute a part or maybe a large part of dark matter I'm just mentioned this idea is speculative new is not tested but if it works it would be pretty good evidence of a balance any kind of matter content will be destroyed by the bounds this is obvious in my view the only two things that could survive the bounds are gravitational waves because they are very weakly coupled to measure and black holes so black holes and gravitational waves might survive bounds and there might be footprints of what happened before there are interesting mathematical developments that show that it is possible they are exact solution of the equations of motion for example with the bounds and black holes so it is possible that they survive do they really survive in real life it's extremely difficult to say because we don't even know what was the matter content of universe at this time loop quantum gravity is one of the most exciting prospects for a quantum theory of gravity and it may well be on the verge of experimental testing these impressive advances have occurred thanks to sustained creative contributions of several hundred researchers including Barbara team Varadero John Lewandowski Barrett Friedel Dietrich Giza and many others so what comes next we have a foundation of loop quantum gravity that is very solid we have control about the fact that whatever the shape of the 2-d will be in ten years some things will not change the discreteness of the area the volume and so on will not change the fact that curvature is bounded will not change so we need to use this information to make predictions to study the phenomenology at the same time of course there are open questions also in the mathematical aspects of the theory and I think that so far and has been under appreciated the fact that there is a beautiful mathematics related to group theory with Charles Simon theory with the quantum groups and so on that is really very interesting to develop so I hope that both mathematicians and astrophysicists will get more and more interested in what we are doing and will try to help us to develop this theory in both directions loop quantum gravity has been around for more than three decades and while it has made impressive advances there is still of course the possibility that it is wrong so how does it feel to risk so much time and effort without knowing if it will really pay off there are very few times that theoretical physics as the opportunity to be part of starting at an area starting a research program and it flew quantum gravity if we don't yet know that it's the right theory of quantum gravity we know something absolutely marvelous in my opinion which is that it's a way that the world might be it's been an enormous privilege to be to have the experience of being part of the adventure of the quantum gravity and I'm grateful to everybody involved or by Carlo Ted Lewis and all the others and then I'm also so just amazed by all the young people who come in and so brilliantly develop the subject so it's a real it's the privilege of my life not a chance to be involved with you whether it's right or not sometimes I ask myself well why didn't I work on some more concrete problem which I could see the a yes or no in but you know life it was it is and if all the work I've done in my life will turn out to be significantly for understanding nature I'm super happy if only small part or nothing at all of it well I'll still be happy less happy but look at it being a fantastic life I was working with friends discovering trying to discover stuff investigating nature is great fun and we have to try there's no alternative than than trying

Channel: skydivephil

Views: 201,686

Rating: 4.7346869 out of 5

Keywords: cosmology, relativty, physics, science, astronomy, stephen hawking, roger penrsoe, multiverse, multiverse theory, cosmic microwave background, big bang, big bang theory, string theory, loop quantum gravity, parallel universes, universe, cosmos, before the big bang, what happened before the big bang, big bounce, black holes, nasa, fine tuning, quantum physics, quanutm gravity, carlo rovelli, abhay ashtekar, lee smolin, einstien, space, dark matter, dark energy

Id: x9jYH5VIF9E

Length: 75min 11sec (4511 seconds)

Published: Wed May 22 2019