 Okay Hi David. Good morning. Good morning Good morning. Okay. I Don't see you. You cannot see me. Okay in a second I really okay David. Can you see the theater at the hall and? Can you see me? It's not really Okay Okay great Okay, so we can get started now Okay, maybe let's wait a minute people are still coming in Okay, so I think we can start. It's a great pleasure and honor for me to Welcome professor David Gross after today's colloquium Good afternoon everybody And thank you very much David David was supposed to be here. In fact, he came all the way to London He had to turn back to Santa Barbara for some family emergency, but I really appreciate that you're up And awake at five o'clock in the morning at California time For this colloquium. So thank you very much It's really a wonderful opportunity for ICTP to have this colloquium on What I would say is a historic as 50 years of a historic discovery And the theory of strong interactions in fact Four years ago three years ago We had a very nice colloquium by Weinberg on the electro week interactions the theory of electromagnetic and weak interactions For which he shared the Nobel Prize together with ICTP founding director Salam and today's talk is about the strong interactions and that it's a quite amazing that between the two it pretty much covers the everything that we know about the Microscopic world and that this knowledge is only 50 years old in the history of human humankind so So David of course has a very distinguished career In 2004 he was awarded the Nobel Prize in physics together with Frank Wilczek and David Pulitzer For the discovery of asymptotic freedom in the theory of strong interactions Which really completely changed our perspective about strong interactions because it showed that the strong interactions are really asymptotically or at very high energies become weak and weak and you could actually understand them and And after this very major difficult problem David turned his attention to easier problems like unifications of all interactions including gravity so They wrote a famous paper in 1986 on the heterotic string which together with so gross Harvey Martinic Rome Which came to be known as the Princeton string quartet which unified all these three interactions that I talked about together with gravity and It's a strange twist of fate. I would say that the recent developments string theory really exploded in unimaginable directions and By a strange twist of fate The theory of strong interactions is in fact in some ways connected to theory of gravity Which perhaps David will talk about this is known as holography So this is Of course, he has many other contributions to physics, but I think these two should suffice I should add that David was also a very inspiring teacher And he led a great school in theoretical high-energy physics and in Princeton for many years He counts a Nobel laureate and fields medal among his former graduate students Had the good fortune of taking a Quantum field theory course from him really learn quantum field theory from the horse's mouth so to speak But I particularly remember a course that he taught on selected topics in theoretical high-energy physics I don't know if David remembers this, but this is how it went. It was taught in a very David grow style All the senior graduate students received a note in their mailbox saying that you have registered For this course on selected topics in high-energy physics and it was really one of the most wonderful courses. I've taken here. Really It a panorama of the theoretical high-energy physics every week He would lecture and the student would lecture and in 15 weeks we covered basically Many many all the advanced topics in physics, but what this showed was really that David really cared about Students he felt that all this Princeton graduate students didn't know enough physics and he wanted to correct this situation and This concern and engagement really he carries On a bigger stage. He has really been a senior statesman for theoretical high-energy physics or even science On the global stage. I know that he takes it very seriously. I've seen him in action in India and in recently we were together in the UAE and he was trying to persuade the president and the policymakers to invest in basic science so and this He I have I want to add that with ICTP. He has really been very keenly engaged He really gave a lot of time to ICTP. He has been Even from strategic matters to really in integrity of how to implement some strategic ideas He's on our advisory board on our scientific council And so I want to thank I want to take this opportunity David to thank you for this your engagement with ICTP Okay, so I'm sorry this introduction is a bit long but it's because The list of his accomplishments is a bit long But I will try to summarize it in one line if you look at my t-shirt basically his talk is about this term in the Lagrangian in the low energy Lagrangian if you include the quarks you have to include this term in the fundamental Lagrangian and As I was saying it somehow is related to this is the Dirac action This is the Yang-Mills action and it's somehow related to the Einstein-Hilbert term in the Lagrangian so I Think this is a good summary. So He told me that he might go over time for maybe an hour and 20 minutes But I gladly agreed. I really think this is a very important chapter in the history of theoretical physics and I would love to hear more from him if it goes over time. It's fine But if some of you have some other engagements, you know, please feed me to you can leave quietly around four o'clock and The session will be moderated by a professor Giovanni Velladoro my colleague So it's over to you David Okay, can you Hear me, can you hear me? Well, yes, nothing clear Out in clear. Okay. Well, thank you at the east. I really am very sorry that I wasn't able to be there and I Sttp in person, but this I guess is second best This year actually almost precisely is 50 years of What became known as QCD the theory of the strong nuclear force? our paper was arrived at physical review letters exactly one year and three weeks ago, so Still astonishes me This is Well-defined 50th anniversary of QCD And 50 years at least said that's a short time perhaps in human history, but in for a human seems like an awfully long time I 1973 Are was roughly the 50th anniversary of quantum mechanics, which was born around That time 50 years ago and I must say at that time in my life quantum mechanics seemed to be Have emerged Years and years ago way way in the past. I Would imagine that if I look at this audience Most of you probably at least half of the audience was not alive in 1973 And you might think that that is ancient history And you probably don't know that much about the actual history So part of my talk is going to be describing how QCD emerged 50 years ago and I'll also discuss how QCD looks today what we've learned and then Along the lines that a teach hinted at The view towards the future So let me start at historically at the beginning which is not 1973 nor 1923 but really 1911 What so that's now 60 years ago Sorry, that's a hundred and ten years ago when Ernst Rutherford discovered the nucleus of atoms Not only did he discover the proton the nucleus He invented Experimental high-energy physics he invented the techniques that we still use today Namely in order to probe the high-energy behavior of nuclei In order to probe atoms and then nuclei at ever shorting shorted Shorter distances we smash particles together and measure the cross sections of the Gathering you see what comes out. We try to deduce what happened inside the atom inside the nucleus This was really the first time that was done and that is the method we continue to pursue today What Rutherford did precisely was use alpha particles She had discovered and used emitted in radioactive decays created a collimated beam of alpha particles and He aimed them at a gold foil with the idea of Deducing what the gold atoms actually contained within them by measuring the deflection of the alpha particles from the gold? nuclei gold atoms His detector was rather primitive there was a fluorescent screen the particles that scattered Created points of light on the screen and his undergraduate student and postdoc sat in a darkened room and Measured the Position and the angle of deflection of these particles the alphabet particles And remarkably they observed that Although most of the particles were scattered just a little bit went off in the four directions Some of them came back right at you Rutherford was astonished by this Previously it was thought that the atom was some kind of diffused Object containing protons and who know what else? But it seemed that the only way he could explain these cross sections which he measured using electromagnetism classical electromagnetism was that if all of the Char positive charge in the atom And most of its mass were located in a very very small region at the center of the atom He discovered the nucleus of atoms he couldn't measure the radius, but he could bound it from above at Roughly one Part in a hundred thousandth of the radius of the atom So he discovered the nucleus of atoms which as you know gave rise to a kind of planetary model of the atom Made sensible by Bohr just two years later with the Bohr atom Which really was the crucial impetus Over the next ten years to developing the quantum theory of the atom and then quantum mechanics The atom The nucleus that the atom was very hard to study it required much better microscopes. It took 20 years for Chadwick to discover the neutron Neutrons and protons were present within the nucleus and The study of the nucleus and of the nuclear force then became one of the main focuses of high-energy physics a particle physics for For the next century Using Essentially Rutherford's Technique smash particles together. This is the modern version of LHC with a detector Which is slightly more complicated than his detector and the energies of course now the TV range And we scatter protons off protons not fixed targets But you know Rutherford invented this technique and we haven't done better in a hundred and 12 years We still don't have better ways or other ways of studying matter At very very short distances except by smashing particles into particle When I started graduate school They're already been after World War two Very high-energy GV Energies Accelerators being built in Berkeley where I was a student was the Learn of its day. It had the highest energy proton proton fixed target proton Accelerator and It was very exciting because lots of new particles and New patterns were being discovered weekly But the strong force by then it was known that there really must be two forces acting within the nucleus The weak force responsible for radioactivity But the strong force was the force responsible for holding the nuclei together and For whatever went on inside the nuclei and for creating perhaps all these new particles which are being Discovered but it was especially Intractable it wasn't at all clear which particles were elementary What were the basic constituents of nuclear matter the proton and the neutron of course But then dozens of new particles were being discovered starting with a pion of Yukawa But then heavier particles like the proton like the neutron but with different quantum numbers. They all seem rather similar Which are the elementary particles from which we might construct all the other ones Which are the constituent basic constituents of matter and then what are the dynamics? What principles determine the dynamics? Where how can you write down a theory of the forces among these particles? And then even if you could write down a theory and people tried from the time of Yukawa onward It was clear and measurable that the couplings the strength of the forces between these particles was very strong So how could you calculate because you couldn't just do perturbation theory like you could in electrodynamics? Dyson Who had contributed so much to the understanding of quantum electrodynamics tried very hard along with others? but then gave up after a few years and famously said in 1960 That the correct theory of the nuclear force will not be found in the next 100 years But he said in 1960 it only took about 13 years And the general feeling I must say at that time was that some new revolution Like quantum mechanics something new was required Because one had no idea how to explain these forces and In retrospect, it's kind of clear what the problem was The problem really was that the sources of the force which we now know are the colored Charges of quarks and gluons were completely hidden from view You couldn't see the basic of matter the case of atomic physics of course the electron was quite visible and you could Pull it out of atoms Measure its properties do all sorts of wonderful things with electrons Basis of the industrial revolution But quarks and gluons were invisible Mashing particles together like smashing atoms together you would think the basic constituents would be revealed But in all the experiments no matter how high the energy was is at that time and up to today quarks colored particles Never emerged. So the only thing you could see were these new particles like the proton and neutron Which we now know our color neutral So the charges were completely hidden For quite a while and then the conceptual tool the framework of relativistic quantum mechanics was really not very well understood Quantum field theory invented shortly after the birth of quantum mechanics was plagued with difficulties and Those difficulties were partially understood please technically in the 50s with the ability To renormalize the theory and get finite results for physical observables That was extremely successful We had a small parameter to find structure constant one could calculate But the basic dynamics of quantum field theory Renormalization was not understood and by and large it was thought to be a trick And the infinities the ultraviolet divergences that one encountered Were being swept under the rug, but something was unphysical about that and probably suggested That One needed something better than quantum field theory There were also quantum field theory was essentially only treatable in perturbation theory and this was fine for QED where the Coupling is very small but not for more stronger interactions that were hypothesized So a teach took my course in quantum mechanics I took Steve Weinberg's course in quantum mechanics as a graduate student and in the first lecture He wrote on the blackboard field theory under attack at that time in Berkeley is equal is essentially Bi-man diagrams perturbation theory no useful non-perturbative methods were known at that time and as we know now Many of the main properties of UCD are due to non-perturbative effects Which we still can't completely control but we have a lot more insight into many new methods There was a phenomenological theory of the weak interactions Fermi's or fermion theory current current interactions and it was moderately successful As long as you only used it in lowest order for an approximation, which was all that was required at low energy But how to make it unitary how to make it into a complete theory how to Normalize it it appeared unrenormalizable was unknown Yang-Mills theory non-abelian gauge theories the non-Abelian generalization of Maxwell's abelian gauge theory was immersed in the very early 1950s by Yang and Mills, but it was plagued with mathless bosons and Which did not exist in nature were not observed so what do you do and People tried immediately almost to construct non-Abelian gauge theories of the strong interaction Again the question was what are the charges that coupled to the gate that source the gauge field You need some kind of conserved symmetry some gauge symmetry of the of Nature and the only symmetries that were being discovered at that time in the case of the strong interaction addition to the abelian E&M symmetry Were flavor symmetries either spin etc. Strangeness, but those were not exact symmetries and it Inconsistent as people rapidly he came aware of to construct a gauge theory of broken Robles symmetries So theory was in trouble and in addition there were philosophical deep reasons why Many felt that field theory had to be replaced by something by another revolution like quantum mechanics are different My mentor my advisor at Berkeley was Jeffrey Chu who led this attack in the United States on quantum field theory and he introduced two ideas pushed two ideas that were very Influential one was nuclear democracy all the had runs that one knew Every week or two at Berkeley at that time a new particle was being discovered And they're all equally fundamental you can think of say the 33 resonance for the first Resonance date discovered by Fermi in the early 50s at Chicago Could be thought of as a bound state of nucleons and pion Or but you could also think of the pion as a bound state of protons and neutrons anti-neutron You could see no real difference if you ignore the weak interaction between Any had run why consider the proton and neutron and pion is fundamental Chu said all had runs are equally fundamental to the real difference between democracies and All right, there are no basic it's that you want like electrons and nuclei from which you build all composite atoms and then Since there was no real idea of how to write down a theory of the forces Chu said well, let's and it was so hard by the way to construct a relativistic quantum mechanical theory that was soluble or understandable because the forces are strong and Because a particle production any relativistic quantum mechanical theory Is that many body theory? To the idea was that you use general principles that we believe in and are even more fundamental than quantum field theory itself like causality causality and The uniterity which must be satisfied it would seem philosophically for any theory Perhaps if those general principles determine a unique Ethmetrics so hard to construct the scattering amplitude that one measures in these scattering Experiments that maybe there's only one answer and that's the dynamical principle now in Russia another thing happened that Attacked quantum field theory and that was What is called the problem of zero charge? No force free theory or the land of Paul and that was the Conclusion that ran out and the disciples reached by studying The short-distance behavior of quantum field theories mostly they studied quantum of electrodynamics was successful theory of E&M and In all theories they studied The physical coupling the one you actually measure we call the fine structure constant Vanished no matter how small they made the bare coupling the coupling that was needed to define the theory As you remove the ultraviolet cutoff that you needed to introduce in order to make sense out of the theory to begin with This conclusion was based largely on summing subclass of perturbation theory diagrams requirement diagrams with the conclusion they reached Was that? No, if you really Try to go to arbitrarily high energies remove the ultraviolet cutoff that you couldn't get end up with a finite coupling Like the fine structure constant. In fact, it would have to vanish Otherwise, you would get singularities poles and ample truth that violated causality and landow Concluded oh so so what? What is the origin of this problem from a physical point of view? Well, this is really a manifestation of screening of the fact that in a dielectric medium Electric chargers are screen now the vacuum is a dielectric medium in quantum electrodynamics Because the vacuum is not full of is empty of course but not as we know it can appear to be full of virtual elect Electron positron pairs which can pop in and out of the vacuum in very short time So we must think of the vacuum as a medium which contains dipoles Electron positron pairs and those orient themselves and when you put in an external charge To screen the charge that you insert into the vacuum There's a dielectric dielectric medium screens a charge and that leads to the fact that the electric force actually Decreases as you at larger and larger distances because there's more screening and conversely increases at very short distances Where there's less screening and this is actually true. We know now for all non-ethnologically free theories and Landout concluded we reached the conclusion again 1960 sort of a low point of theoretical physics At the time there was in the limits of formal electrodynamics a point interaction So if you really go to find the theory with no cutoff in the UV You introduce a coupling that measures the strength of that force at arbitrarily short distances No matter how strong you make that coupling Then at physical distances, there's no interaction at all And so Landau old as he was was driven to the conclusion that the Hamiltonian method on a field theory For strong for the strong interactions is dead and must be buried Although of course with deserved honor Now he he noted the strong interaction this problem Exists for electrodynamics, which is probably by itself Inconsistent at high energies or short distances But in the case of electrodynamics, we know that that only occurs at extraordinarily high energy way beyond Measurement at that time or today or far in the future But in the case of the story interactions But the coupling starts out stronger. This was an immediate problem and that was yet landouts conclusion and Landau was very powerful in the Soviet Union and For bad his group, which was by far the best in that time To work on quantum field theory and my young my colleagues Hoya Kosei and McDowell We're not allowed to work on quantum field theory and when they did they had to Disguise this by saying they were working on many body condensed physics Where you were allowed to use quantum field theory, but that was not a fundamental theory in any case That was the atmosphere When I was a graduate student What about experiment well unlike today theory was in shambles nobody knew what to do Nobody could calculate anything theorists were Were not very successful by and large, but experimenters were they just started constructing these wonderful accelerators And the pace of discovery was amazing There were great experimental excitement many new discoveries new particles new patterns that were emerging Reggie Paul Reggie theory Simit flavor symmetries strangeness new patterns of particles and Were being discovered all the time so just sort of the opposite of the situation today When theory is supreme and experimenters For many years have been largely confirming our Fantastic standard model and then there was a very interesting development which was Experimentally and theoretically important, but not didn't get to the part of the matter Because many theorists and experimenters believe that the real secret of this strong interaction Lay in the high energy behavior of scattering Well, that makes sense, but only at very low momentum transfer diffraction scattering Or as you probably know when you scatter particles most of them go in the forward direction And that's what contributes most of the Total cross-section and theorists were Very interested in that and discovered regularities in particular Reggie behavior, which seems to just give a framework for understanding high energy near-forward scattering So one had a situation where most of the data was forward scattering theorists were very interested in forward scattering so they told experimenters go study forward scattering Miners were happy because there were a lot of data in the forward direction to get better statistics and No one particularly was interested in the high momentum transfer scattering events Where the secret of the strong interactions were to be found Very they should have realized. Okay. How did Rutherford discover the nucleus? Well what? motivated him to hypothesize the nucleus was the few very few events where the alpha particles Gathered backwards or at very large angles Not many of them cross-section is much smaller, but those tell you Really about short distance structure of the atom Until 1968 really by that time I was Already dissuaded from the bootstrap philosophy and I was a postdoc at Harvard and Beginning to study Some rules people are very interested in Electromagnetic and other currents that coupled to Electromagnetism or to the currents that appeared in Fermi's weak interaction theory and Those were physical which you couldn't measure them an electron had run scattering or neutrino had done scattering so those were okay and They get common game among some of us were to use very simple illegitimate perturbed of free field techniques or models like the sugar our model Where you could try to analyze the product the short-distance product of These physically measurable currents and for example Kallen and I derived a sum rule that could be permanently tested in and the scattering amplitudes of electrons of protons For the cross-actions that f2 is one of the structure functions in the cross-section of electron protons scattering and Bjork pain notice this thumb rule well suggests that and dimensional reasoning suggests that these cross-actions should scale that there's no scale in them and We can we use other even less Meaningful techniques or a lot more techniques just free field theory to derive Some very specific relations some rules abstracted from the quark model By that time quarks have been discovered had been suggested by Gell-Mann and Sprague as As a way of mathematically summarizing the flavor symmetries of Hadron Quarks were a fractionally charged which was really weird because nobody'd ever seen practically charged particles But we showed that if you just assume that these currents Be had a short distance expansion. We would operate a product Short-distance behavior If you just take from the quark model to say that the currents are made out of quarks Quarks carry the charge inside Hadron's But otherwise there's no force Free field theory and there there's been one half quarks have been one half there man's Then the longitudinal cross-sections vanish at very high energies Whereas if there would be spin one or spin zero charged constituents, it would be the opposite the transverse cross-section would vanish So there were these vague ideas Mostly abstracted from perturbation theory or even free field theory and the remarkable phenomena happened that as soon as The slack deep in elastics gathering accelerators, so this was very high energy electrons gathering off photons fixed target The scaling of the cross-sections and some of these thumb rules appeared to be roughly correct That was an enormous surprise because people expected just as Rutherford expected that the Cross-sections would fall up very rapidly at large. I Nobody really wanted to do this experiment by the way. This was not where slack The community imagined that new discoveries would be made at this new machine but Kendall and Taylor boldly went ahead and They discovered that the proton looked Like it was made out of freely moving quarks They were the Nobel Prize for the discovery of So if you assume that inside the bottom there were freely moving quarks There in the electric charge inside the proton then you do And you cheated them as freely moving particles you You in fact that scaling if the cork masses were very small as we now know they are and the quark model some rules like Been some rule work Now the data was very crude. It was very low energy by today's standards and the errors were big But this behavior sort of stood out Easy to understand it if you just said you had three quarks moving around inside the proton as far as I Was concerned this was an amazing discovery But incredibly confusing confusing The experiments in the thumb rules convinced me that It looked like hadrons were indeed made out of points like constituent and to that charged constituents that Electrons and neutrinos coupled to work quark The quarks are real. They're not just mathematical devices direct to construct representations of labor symmetries but actually charges The representations that the basic constituents were made out of But on the other hand the quarks never came out were never produced at slack or anywhere else So they must interact very strongly to create composite color singlet baits Or to create composite states with integer charges electric charging So how could one explain scaling and it was clear that in quantum field theory But once scaling this idea of no scale physics at a very high energies emerged People started studying how you know scaling a feature of quantum field theory if you have a theory without a scale practically like Electrodynamics if you make the electron master zero Is that really a property of quantum field theory? No, it's not quantum effects destroy naive scale invariance They also completely Muck up the some rules that were being tested at slack so that confusion but in addition the discovery of Upstate you know upset me for the next five years. I of course had a detour into string theory Which developed at the same time string theory is even older in QCD and it was supposed to be a theory of historic interactions without a Hamiltonian without basic constituents Originally without strings just a bootstrap approach But then it became rapidly clear that strings were very soft the large momentum transfers scattering fell off exponentially Certainly not like what one thought slack So string theory could not explain scaling But by 1972 I had a plan Program I was going to kill quantum field theory forever The idea was I would take the experiments the slack experiments seriously not everybody did because nobody could explain them and It was quite reasonable to say I don't believe the experiments Experiments were a very low energy at that time we all the precocious skill and And the errors were big it always happens by the way in physics or often happens when you make an Experimental discovery the evidence is questionable for quite a while And so very smart people like Ken Wilson who was teaching us so much about non-perturbative quantum field theory renormalization, etc Really believed that in modern terms the dynamics of the strong interaction would be governed by a Some kind of fixed-point theory which we would discover But the idea that it will be governed by free theory at short distance to seem seem crazy and And he really believed the experiments were were going to change One went to high enough energies There would be corrections to the scaling, but I said let's take the experiment seriously and Kill quantum field theory and the idea was that It seemed that scaling required what we called a symptotic freedom Namely that as one went to arbitrarily short distances The couplings that control the force the interaction vanished Which we now call asymptotic freedom and based on an idea You know a result of per easy Helen and I showed also 1973 For all quantum field theory that made sense renormalizable quantum field theories at the time She took the experiment seriously you had really scaling Naive scaling That for all theories except not to be engaged theories, which were we didn't There is our theorem didn't hold Had to be asymptotically free The one had to have this theory where the short distance behavior It was the opposite of what it was for QED where the coupling became stronger at QED the coupling becomes stronger toward distance is Asymptotically free theories it becomes weaker And then the idea was to show that asymptotically free theories do not exist and That seemed reasonable just like Landau assumed that in effect surgery normalization is vacuum polarization is screening and Charges the coupling the force should should go to zero as Sorry, this is a typo Which I can't correct online, but Jesus go to infinity not zero at short distance. That was the Landau pole So That was the idea scaling requires that's not a freedom asymptotically free theories don't exist and Then okay quantum field theory cannot explain the experiment And indeed Coleman and I proved Same year in 1973 around the same time that no sensible field theory with any number of scalar particles or spin One-half particles or a billion gauge theories with arbitrary Interactions is asymptotically free. That seems like a very strong theorem But because if the theory is asymptotically free perturbation theory gets better and better at short distances and Therefore is controllable and therefore you can prove this theorem Not a billion gauge series again. It's actually of course wrong It is that's the other clear. Sorry. It is asymptotically free Okay, so that was the goal and The final portion of this plan was to rule out Yang-Mills theory or not a billion gauge theory which had been Where the problem of massless gauge bosons had been solved by eggs Angler and that hoofed in Beltman and Was now understood to be a physical renormalizable quantum filter And at this point Frank Wilczek became my first graduate student. I was still a young professor at Princeton Here is Frank and We discovered that within non-abillion gauge theories and no theories They possessed the remarkable feature perhaps unique among renormalizable theories and did unique And four dimensions of a symptomatically approaching free field theory and that will give your cane scaling We therefore suggest that one look to a non-abillion gauge theory of the strong interactions to provide the explanation for scaling For me this was a surprise Shouldn't have been but it was that There actually exists a asymptotically free theory. Oh, if you believe the experiment this is the theory you need to base the strong interaction on and The dynamics had to be non-abillion gauge theory Go on The gluons had to be electromagnetically neutral. You didn't see them in the experiments So there had to be charged matter Electrically charged matter and we already knew the patterns of flavor symmetries suggested that they were Fundamental representations of the flavor group and That they had three colors three quantum numbers that was necessary to explain Heidi by not the gay the total cross-seq annihilation cross-section of electrons positrons the Hadron's So there was really no choice as to what the dynamics was a Three triplet to Fermi as three flavors that all that one knew at the time now We know there's six maybe more but six flavors, of course SU three color gauge group because of color That's it nothing else you could do can't introduce scalars or other things because that would have Destroyed asymptotic Reno That's QCD In our larger paper We started oh, so why does QCD? not be Screen the color charge Well, because there's another effect in relativistic quantum mechanics of a gauge here And in particular in QCD in addition to having charged color charge Permions quark. They're also colored charge gluons, which are in the adjoint representation of QCD Quantum chromo dynamics So they carry the chuller of color of quark anti-quark there eight such SU three charge Blue on charges and they're kind of strong. They adjoint representation is bigger as a bigger cadmium than the Fundamental representation so forks anti-screen the charges Luons also screen the charges, but they in addition anti-screen the charges these permanent magnetic moments in the vacuum Exhibit not diamagnetism, but paramagnetism. These are permanent cycle on a spin one colored or glue on and Paramagnetism acts the opposite to diamagnetism It increases the force at large distances. There's more paramagnetism Decreases the force at short distances And what we discovered was that if there are enough fermions not if there aren't that many fermions like fermions less than 17 16 and a half Then the QCD force will be asymptotically free and our Bigger version of the letter we studied these non-abila asymptotically free gauge theories and Started to calculate the corrections to deep and elastic scattering because you know the force vanished At Arbitrally high energies, but before it got there it varied logarithmically and so though were measurable Logarithmic corrections to the scaling behavior, which one could calculate using in perturbation theory Well, you're all we're also tried to understand why the corks didn't get out. That was the deep mystery And we said well, maybe the symmetry is not broken like it was in the emerging theory of the weak interaction But an asymptotically free theory is that doesn't necessarily mean that you would seek works and go on There might be little connection between the free Lagrangian and the spectrum of states the infrared behavior of green functions This case is determined by the strong coupling limit of the theory And it might very well be that this is such so as to suppress all but color things that states neutral colors So that the color feels as well as the corks could be seen In the at large momentum, but never produced as real asymptotic slaves. This is what we call now confinement Now what's going on? Well, let's take electromagnetism. We have two charge char of electron positron has a Coulomb force Classically Which we calculate using Gauss's law, right? So we have the positive charge you draw a sphere around it Gauss's law tells you that the electric field times the area of the field of The sphere is equal to the charge contained within that sphere And that's why of course the force falls off like inverse distance Squared that coulomb's law if you calculate the work needed to pull the corks apart Well, it increases but saturates at the ionization energy so if you have more energy than I You can pull electrons out of atoms and make them Blowing wires and do work for you But in the case of the chromodynamic field the classical result would be the same But We know that the quantum vacuum is a medium a complicated medium pictured here Dude, you know with the help of lattice gauge theory full of fluctuating fields these virtual glue on magnetic dipoles that And that it moments that anthase screen and we believe that they squeeze the flux into tubes and The flux of the chromodynamic electric field This is sort of familiar in the dual magnetic case in superconductivity where which is where the Magnetic field lines they produced by monopole pairs would be squeezed into a vortex Or a flux to the magnetic lines and that of course is seen the mice are effected in in real superconductor That analogy was only understood after the fact as well But that's such a thing could could give a physical picture of confinement so now the calculation is that You calculate the electric field times the Area of the sphere around the charge But the area is now limited to this fixed area of the flux to So the force between the quark and that quark does not decrease Remains constant as you pull the quarks apart, so you try to pull quark away from an anti quark and a meson You have to overcome this constant force at large at any distance at large distances and the work Required then increases linearly and that's confinement It would take an infinite amount of energy to ionize a meson at least at zero temperature And this is a picture of what actually happens in QCD Thanks to that as gauge theory and you see the flux lines the of the gauge field Which one can deduce from these Monte Carlo simulations? Form unit flux to and By the way, it looks like a fat string no coincidence because It does look like a fat string now QCD You know everything sort of fit together. There was really no choice in Immediately hope of what the theory could be and it explained this these experiments and the acceptance was rather immediate for a small group of people which I defined as the smart people But most theorists I think ignored did not believe in this It seemed pretty crazy to base a theory on Particles that you could never see as asymptotic states. I had big arguments in Princeton with my senior colleague Eugene Vigner who had Defined particles as irreducible representations of the Baccarat group that you see as asymptotic states and the idea that you could base a theory on Particles that were confined and that you could never see as asymptotic states like corks go on to make no sense and then confinement now was a just a Very unsubstantiated conjecture still has not been proved but there was slow and steady growth and accepting and QCD over the two in years Finally because one could do things. I mean what really pushes physics rapidly forward often is the fact that a new idea allows you to calculate and as you all know people flock through the In the direction of some new development that allows them to Address old and even new physical problems easily and generate lots of interesting papers We started immediately deep elastic scattering because their corrections through the net e rules Could be tested also e pussy minus the hadrons their corrections to the naive rules and other Things that were old ideas, but now you could calculate But the most amazing and and then indeed in addition there were theoretical Milestones that made it some of these crazy ideas of confinement and so on less Objectionable In particular, but most importantly perhaps was the invention of lattice gauge theory by Wilson immediately Jumping on the problem and said okay. Let's put it on a computer and calculate the masses That was again very difficult took a long time before it became a precise tool but it gave one a strong coupling very far away from the continual limit Picture of what confinement consists of these string stringy picture Andrei Navarro and I studied a law a theory which one could solve exactly or large and in two dimensions, but it was gave us a precise and Analyzable example of dimensional transportation, which I'll get to but and as off solve large and QCD Now in two dimensions confinement is trivial because the force is perturbed of the confining but you could actually solve this theory and and very reassuringly Have a theory which made sense and was soluble of confined works Which didn't ever appear in there as asymptotic state But perhaps the most dramatic thing that happened in the next year or two was an experiment and In the measurement of the electron positron annihilation cross-section and here too A new accelerator came into play the E plus C minus storage Collider at slack So, you know if you just were naive refilled theory or QCD you could calculate the total cross-section for electron positive drones to annihilate the hadrons and it was a measurement of the Charge the square of the charge is a fundamental constituent of the proton of Hadron Sorry, I'll be stronger direction and that was one of the motivations for three color So the total cross-section pictured here F1 one gets beyond the the row Omega mesons Was predicted if there's three colors and three flavors and fractional charges and it seemed to work one of the motivations for scaling for QCD but at the end this new machine started seeing more events and Richter in 1974 at a meeting here there in Trieste, which I attended He announced With great horror that QCD is dead now. It wasn't QCD. He said the name hadn't been invented At that time, but all these silly ideas of Refilled theory behavior, etc. Were wrong the cross-section wasn't constant compared to lepton cross-sections Rising Now some of us at that meeting knew very well that Charm force had to be there to make sense of the electoral weak interaction and so on and Applequist and Bonsard actually Studied the properties using Q's asymptotic freedom of a charm anti charm non-state and Suggested it'd be rather narrow, although not as narrowly discovered and Felt that okay. This is not the death of QCD, but rather the indication that one is approaching the charm threshold But the experimenters were Didn't have much trust at that point in theory and theorists were rather timid They were you know hadn't had many successes in the past 60 years and so But then Richter went back to Stanford and in the fall He and Ting discovered Tom discovered this was a threshold for the emergence of charm particles and so there were more quarks or more what your Charm medicine has four-thirds a big charge it changes the cross-section and then of course that does agree with quantum chromatodynamics not only that this Measurement was extraordinarily informative not just about the strong interactions and these marvelous Tramonium or then bottom Adriatic states that could be analyzed perturbatively because of asymptotic freedom but also later the zebo zone of course and so that experimental Problem that was not a problem and resolved and led to calculable analysis of the Charmonium spectra gave an enormous push to the acceptance of QCD But what about the actual predictions in the test? Well This is 1989. This is 16 years 16 years later, and this is the measurement of the fine structure constant of QCD Which we know it depends on the scale. So this is measured at the energy of the Z meson Z boson and Theory and experiments seem to agree, but really I mean a straight line more or less passes through the data as well so the past the QCD took a lot of time, I mean the Deviations from only free field theory are only logarithmic and energy This is the measurement today. This is not the most Well, I'll ask them about a freedom This is what I actually I just heard that there's a better measurement now at The final atlas. Yeah, well, it is atlas. I've been able Using the fitting Production of the particles measuring jet distributions to measure the fine structure constant now to roughly a half of percent But this is many many different experiments all described by one coupling which is as we'll see in QCD Calculable so this is a an amazing development of experimental high-energy physics and But it took a long time This is the deviations the end of the Hera Deepen elastic machine. You see the deviations from totally straight lines is kind of small, but now quite precise But again, this is after 30 more years Today, of course, we have from the LHC incredibly impressive tests in 99% of the events at the LHC are QCD if you want to work in QCD is really a low on low on low on work work work scattering machine and Not only have the experiments gotten more precise Much higher energies, but the theory has gotten much more precise But what about actually described in the Hadron? The particles we actually see as asymptotic states to put on the neutron and all the others Understand them can't just use perturbation theory. We need to understand this complicated medium the QCD vacuum and that To be precise the only tool we have so far is lattice QCD and that Still remains the only tool if you're not going to go in the for the large distance structure of the theory For precise controllable first principle calculation of static Andronic properties But it's become an extraordinarily precise tool, which is necessary not just for testing QCD but also testing potential New physics beyond QCD be she'll want to understand the background From first principles precisely and that over the years enormous effort Has reached a new level of maturity due to well We are computers are billions of times more powerful than existed 40 years ago But also a lot of theoretical ingenuity and a lot of hard work And now we can calculate the spectrum of QCD again to about 1% precision including, you know Mass difference of neutron proton the neutron is slightly heavier than the proton and that's partly So partly because of the fact that the bear quarks Have slightly different masses sorry the math the quarks have slightly different masses Electromagnetism and the strong interactions so that is totally amazing Hadron masses can be calculated Another static hadronic properties form factor There are also on the theoretical side the ability to calculate so this is the beta function the crucial Thing that enters in the term and very high energies the behavior of the coupling has a synthetic freedom That we calculated, but This has now been pushed with only depicted the next three terms in the beta function, I think it's now up to six terms Unbelievable Calculational Developments have occurred Amplitudes have become a totally remarkable developments in in the ability to calculate Protervatively and in and not to be engaged theories to high order What appeared to be absolutely impossible now is standard and this is So in addition to the leading order say scaling results that we derived 50 years ago You can now calculate next the leading order and next the next the leading order and next the next next leading order and this is because of the discovery of a remarkable structure in simplicity of these scattering opportunities, which a priori seemed Beyond human control and that's going to continue those Seems to be no question that there's much more hidden in perturbation theory than just numbers There's structure of beauty and power okay, I want to just Emphasize some aspects of QCD, which I like to call a perfect theory and what do I mean by that? Well, it's certainly in The old-fashioned sense of what we might have expected Fifty years ago. It is a perfect theory. It has no infinities really none of these UV divergences are there really it has no adjustable parameters really I mean they're quark masses, but those set them equal. They're not part of QCD. They come from the Higgs mechanism if they were zero QCD would have no adjustable parameters and most importantly There's in just QCD by itself. There's no new physics that short distances or high integers It's a first theory which we know is more understandable simpler at arbitrarily short distances So let me discuss these points rapidly, but so there are no UV divergences in QCD and what do I mean by that? Well, look at the answer we can write down the answer using lattice gauge theory Which of course we have can only do if we introduce a cutoff to define the theory on our lattice But in QCD the coupling that we introduce at very short distances doesn't blow up like land down it vanishes So that's not the version infinities only appear if we try to express observable Which are actually measured of course at finite distances in terms of those measured at infinitely small distances Which is just asymptotically approachable And then sometimes if you try to express a finite thing in terms of zero, you're gonna get infinity one divided by zero So in lattice gauge theory for example for many static properties You can write down a multi-dimensional integral, which is a solution of QCD If you have a lattice version of QCD in a finite box It depends of course on the lattice spacing and the coupling you introduce is the lattice coupling at Distances of order of the lattice spacing a Which in the end you want to take the zero and you want to take the box to infinity And you know how to take the coupling to zero because the mass of data freedom and if you do so you'll find that physics proton mass Remains by night and it's countable and you never in this procedure doing lattice case theory ever encounter any infinities If the theory works, which it appears to no proof not rigorous But unquestionably true Infinite infinity never appears then No adjustable parameters So recently I read a wonderful Review of quantum electrodynamics that Feynman gave in 1961 the survey conference on the status of quantum electrodynamics, which in the previous Few years had become incredibly successful and Feynman Surveied that status, but then he asked Can we is this a complete theory all by itself? Ignore everything else, but she knew existed just QED So is it a theory that makes sense at all energies is that a theory where we could imagine Calculating the fundamental parameters that appear in the theory the mass of the electron the by instructor constant So He knew that the mass of you know if the electron mass was started out zero the bare mass was zero Mass that appears in Lagrange there would be a symmetry that protects that axial symmetry But he already knew at that time the ideas of spontaneous symmetry breaking were around So he knew you could produce a mass perhaps and so he asked what happens if we take pure QED with only zero mass electron and photon Interactive with no other particle having no cutoff energy He said well you can't produce a mass because the system is invariant to a change of scale So there's no parameter to define a length An electron with math with the mass defines a length the inverse mass So I'm not certain Feynman was always careful But it appears to be impossible to generate a specific length of no scale whatsoever So he concluded okay. I don't see how we could possibly calculate the electron mass. I don't see Also, how we could calculate the front structure comes but he was wrong quantum effect invariably rate scale invariant therefore and produce a physical mass and Incidentally Determine the coupling which varies with scale at that scale and that's what you see the It has no infinity and in fact no adjustable parameters But you have to ignore the cork masses which the light cork masses are quite totally ignorable and You could regard the number of colors the number of four as Three parameters, which you we don't know how to calculate, but now that was way beyond what Feynman were thinking Oh, and that works by what we call dimensional transportation Which we already kind of reviewed with lattice gauge theory. We must To define the theory to begin with we must introduce an arbitrary scale Where we cut the theory off or if you want to find what we mean by the coupling that it's measured at that scale and the famous beta function tells you how that if you can calculate it how that coupling Buries with the energy you've introduced to define that but a physical parameter P there can't depend on that scale at arbitrary and And therefore a normalization group tells you How this any physical parameter depends on this arbitrary scale mu and the coupling Which is defined at that scale It's total variation vanishes but that's because of explicit mu dependents listen cutoff dependents or Cutoff dependents due to the fact that the coupling depends on that color Now if you manage to have a physical Observable in the theory Just an obvious in a scale invariant theory that such a physical like a math would be produced Then it must have a completely specified dependent because it actually has physical dimensions of mu that's math scale and Therefore solving this equation. You know how it depends on the coupling And that's some technically free theories where the coupling vanishes that small at large energy small distance That means that roughly for weak coupling goes like mu the math Dale we've introduced either the minus one over G squared Or G squared of the coupling defined at mu Which is sort of a natural non-properative result in quantum mechanics and Non-authentically free theories like QED would have the totally bizarre behavior that the physical mass of some kind of Dynamically produced mass scale would blow up as G went to zero Crazy such theories probably don't make any sense as complete theory But if a mass scale is produced then any physical quantity must not depend on view Therefore be that physical math through the dimension the engineering dimension of the parameter and See the constant that multiplies that you know Can't depend on G can't depend on anything must be a calculable number So in a theory like QCD All physical parameters if there are no masses no cork masses to begin with are Calculable no adjustable parameters, so take the proton well the proton You know our orcs and gluons rattling around inside some region of space Defined by well where the coupling gets strong They try to pull them apart that at a roughly the radius of the proton You can't because the force is too strong that defines a length Where the force is strong enough to confine the corks and the gluons and if the corks were massless the energy in that volume was just the kinetic energy of the massless corks and gluons and and so the mass is Defines a length defines the strength of the coupling and All mass ratios dimensionless quantities you need some scale That's the mass say of the proton are calculable and the starting interaction. That's alpha s is Calculable So in QCD if you put the cork masses equal to zero Every physical object is calculable. There are no adjustable parameters except of course the number of colors and and and the number of corks In the real world the number of the cork masses and gravity and the other forces of nature finally the remarkable thing about QCD and at that time in physics, especially was that It said there's you know as far as target direction as goes. There's no new physics that shorted No, nothing bad or new happens at high energy to the contrary in high energies theory becomes simpler You can use perturbation theory better and better This had a lot of implication What's important one perhaps was that the early universe should be very simple Furthermore, one could imagine unifying the forces and then there's Another benefit of durox large number of problems. So at that time people, you know had discovered 10 years before less CMB Cosmology had been transformed into the big Big bang theory if you want, right? We knew the universe expanded from the very dense state hot state and We could see back to almost the beginning of using cosmic microwave background and Cosmologists astro particle physics started thinking about Go back further and we can see what was the universe like when it's very hot and very dense But as you just thought about that is nuclear physics and unknown quantum physics. It was a hadron hell But with QCD became a quark. So You could go back and You know start exploring what happens when addrons quarks and gluons are heated or made very dense and You could do that studying perturbation theory because theory becomes simple and Easily show that you expect there to be a phase transition very high temperatures protons will melt Rollins and quarks will escape and there will be some kind of arbitrary. I energy a quark along plasma Which we can actually explore in the lab but also in the early universe And that's a whole another subject enormous subject of the behavior of quarks and gluons hadrons at very high temperatures very high energies and You'd expect you know as a lot of freedoms today at very high energies. There's no force at all. So You can't have confinement Somehow very high energies should have free quarks and go on sending with density which introduces a Net barrier number can be effect neutron stars at their core perhaps and certainly observable at heavy-ion colliders like initially Rick and Alice now and become and finally you could extrapolate strong interactions to very high energies and see You know whether they become as already suggested by the big standard model Unified with the weak and electric that Interaction this extrapolation of a chap in 48 years ago remarkably discovered that That these forces become equal in strength added an extremely high energies Not that far from where gravity becomes an important quantum force and that of course has dominated speculative article physics Probably for me and my friends, but for most of the community for the last 40 years Try to understand what how this unification takes place and especially perhaps with gravity drink theory being the by far the most interesting idea that has emerged Okay, and finally it is probably The athletic freedom kind of solves a problem noticed by duroc back in 1937 that physics and a fundamental sense is plagued with very large numbers Like the ratio of the proton mass to the plank mass defined by Newton's constant with scale there So this fact of nature this 10 of the minus 19 is so crucial for us because it's You know it explains that have to be a large number roughly the Cube of the inverse of this number Protons inside a star before it collapses to a black hole So the fact that this number is so small in white gravity is still weak at low energies There's why big objects like planets or humans can exist without collapsing to black holes But where does this number come from and how could you possibly calculate it and dirac? Notice the few such large numbers all Tending to be multiples of products of 10 to the minus 20 or things like that for example the The size of the universe the observable universe compared to the size of the atom Now dirac would have tried to argue Anthropically right you could have said well, you know the proton mass was much bigger than Then to the minus 19 the plank mass Then a star couldn't have more than a thousand atoms before it became a black hole and we wouldn't exist So we're not in that portion of the multiverse But dirac was a sensible business. So he didn't go that way And he had no idea how to calculate such a number, but he tried to relate these different large numbers some of them were Time-dependent so he suggested okay, we can test this kind of the test an idea that the are all related somehow and Therefore the fundamental Parameters that define the plank mass or the strength of the coupling should vary with time And that could be tested because we have things like nuclear reactors, you know natural deposits of uranium that have been functioning as nuclear reactors for billions of years so you could try to measure time variation of these numbers and So far there's no evidence that there these are varying over cosmological times But we can now roughly calculate this ratio QCD not all these large numbers, but this one. Yes, and It's because the you know What imagines that at very high energies where the forces unified with gravity the coupling is of no order weaker than alpha at mz Continues to decrease, but only logarithmically so maybe it's the order of the fine structure constant Maybe so it'd be bigger and then you can calculate the mass of the proton because You go to lower and lower energies the force gets stronger at Increasing distances at some distance scale it becomes strong enough to confine the quarks that defines the radius of the proton that scale the mass of the proton and You can calculate just as we illustrated before And if you put in the numbers according to you know extrapolating what we know about the standard model a blank mass You get roughly 10 to the minus 19 maybe 10 to the minus 16 maybe 10 to the minus 22, but anyway roughly This large this is part of the solution of a large number poem of course there are many other Issues in the real world like the mass of the Higgs Don't understand why it's so small compared to the black man Okay, so QCD is the first example of a complete theory with no adjustable parameters No indication within the theory of a distance scale where it must break down infinite bandwidth But of course there's the rest of the world those We don't stand the number of colors the quark masses and mixings and of course There are other forces like electroweak and gravity My opinion the most important Development over the last 50 years in QCD Our understanding of non-abilling gauge theories, which are the basis of the standard model is their connection with string theory Now that began before QCD people anomalogically constructed mathematical models of the strong interactions The original model that led at the string theory because Mezzan do look like bat strings But it is clearly in my opinion a very One of the most profound thing we've learned is that gauge theories not a billion gauge theories and string theory They're really different aspects of the same underlying Fundamental concept of nature Early on it was realized and aren't there just these mezzan If you have open strings you can close them and it was discovered that don't look like gravity But not only can you Describe strong and electroweak and electromagnetic interactions using string theory but also gravity that Really given rise to a hope for getting analytical control of large of QCD in the infrared Because we know that at number of colors becomes big QCD is described by some kind of Non-interacting theory kind of classical theory with an infinite number of glue balls that are confined a theory an effective theory where When the number of you know one over the number of colors is the classical limit and This is probably a string theory a lot of evidence for this Painter diagram structure of large n matrix theory the fact that many aspects of QCD can be understood in a large n expansion and most importantly the ADSC DFT duality in which string theory open strings ending on brains our dual to closed strings and Certain setups a complete theory of age Interactions in gravity can be described either by quantum theory on the boundary of anti-dissidus space And this is true for super symmetric gauge theories in four dimensions close cousins of QCD or by Strings in this ABS bulk And the hope is that You know at least large n number of super symmetric charges So highly super symmetric one of chromodynamics could be solved exactly It's a scale invariant theory doesn't produce a mass gap or photon But it's a lot simpler When the number of colors becomes very big, it's integrable easy to solve sort of and And can't be deformed You know in principle to real QCD make the masses of Some of the extra part states you required to achieve super symmetry arbitrarily big Include quarks and their interactions and maybe At least get analytic control over the theory for a large number of colors and then do a one over number of color expansion So what do I expect in the future? Well, perturbatively there's going to be continuing discoveries of the incredibly rich and Suggestive in many directions behavior of perturbative non-abiliic gauge theory of QCD and its partners numerically lattice gauge theory is Incredible tool, but now massive efforts have been beginning to to explore lattice gauge theory For time-dependent phenomena using Hamiltonian And eventually quantum computers and Then one can begin to study numerically or solve the theory numerically in other larynx in regime and calculate some of the Ingredients that go into calculating LHC backgrounds and you know, how do quarks turn into head on how to They break up and so on and going back to Reggie behavior the fraction scattering But most interesting they're going to be a lot of new experiments and experimental probes There's of course LHC, which is now starting up with increased luminosity and there LHC be And at let's concede a mess as well have discovered lots of new quark bounce states Which are seem to be described not three quarks in a proton About four Penta quarks tetra quarks fascinating roughly 50 new particles have been discovered at the LHC Alice is continuing to Probe the quark low on plasma where a phase transition to deconfine quarks and go on thick slice fascinating and important perhaps for astroparticle physical contribution to astrophysics and cosmology Even elastic scattering which started the whole thing is now going to have a major development with the electron ion collider Which is going to be built at Brookhaven and collide electrons off ion And that will enable well even elastic scattering to be probed with incredibly increased precision and large momentum transfer or actually more diffraction scattering As well as nucleon the structure of nuclear physics which nuclear physics is really quark low on physics And it's turning now from a rather phenomenological theory of particle exchange based on you know to quark and go on physics And that's very exciting So let me just end by you know I finally was once asked or once said If in some cataclysm all of scientific knowledge were to be destroyed in only one sentence passed on to the next generation of creatures Perhaps silicon creatures who knows what statement would contain the most information in the fewest words So he tried to summarize this message in one sentence It's almost impossible But he said I believe it's the atomic hypothesis or the atomic fact or whatever you want to call that one sentence All things are made of atoms little particles that move around in perpetual motion Attracting each other when they're a little distance apart But repelling Upon being squeezed into one another Now you think about it. That's one sentence of which really summarizes You know our standard theory of Ordinary matter um So I tried to do the same for the standard model Impossible in one sentence But this was sort of one slide Standard model what have we learned Matter is made out of charge spin one half fermion With forces described by quantum non-abelian gauge theory Which exists in three phases? Electromagnetism or u1 in a long range forces below phase weak interactions generated by su2 gauge theory, which is screen or higgs By a somewhat unnatural Scalar field sector and finally qcd the strong interactions su3 gauge field Which exists in the confined phase This is one that not one thing again, but what we've learned on one slide from qcd You are the most important thing, of course, is that it is undeniably the theory of the strong nuclear force But It's the first theory where the dynamics can determine all masses and all couplings And finally the most important thing eventually I believe is the gate string duality So the hero indeed in the last 50 years of Particle physics has been on a big healing gauge theory Which provides a basis for qcd and it Solves its mysteries as well as the whole standard model electrolyte electromagnetism and qcd But the real secret of all of this Is connected. I believe in a deeper level which we have yet to understand fully with string theory And uh, which remarkably Will connect the standard model to quantum gravity as well as What space time really is and how it emerges And with that I come to the end the end of this talk But not the end of qcd. Thank you Thank you for taking us to this beautiful journey Of qcd and the strong interactions. So I will collect The question from the audience. Yes Hi, thank you for the very nice talk I was Wondering since even matthesh both mentioned it was studies of qcd and its closed cousins that Eventually led to the stunning development of the adscft correspondence I was wondering about your Perspectives on the other way of the symbiosis namely What Some close analog of the adscft or holographic correspondence Can teach us about Qcd in the real world as we know it with n equals 3 which is non super symmetric and non conformal Maybe you know in the context of some Noticable efforts such as those by wheat and sakai suki mattho and others I'm not sure. I totally got your question, but yeah Any adcft Is Has been a remarkable development which enables us to probe this Gauge string duality but You know, it's a it's a duality between string theory It doesn't really have except through the duality a definition non perturbative definition And gauge theories, which do Gauge theories we understand for wheat coupling You know and that works beautifully In asymptotically pre-theory In some domain We want to understand the strong coupling Gate quantum gravity or string theory we understand very well classically But we are want to understand and in flat or You know almost flat space But we want to understand it in highly curved spaces and for strong coupling So that's those are the difficult aspects of the So we're just it's just beginning to be some understanding of one of the most interesting corners Which is the wheat coupling limit of non-abundant gauge theory Asymptotically free regime where you start with free field theory and quantum gravity or string theory So that correspondence on the string theory side requires an extremely highly curved Place not a very almost flat space So it requires An 80th dual with a very small radius compared to the dreamland So that Very little understanding on the string side Very good understanding on the perturbative qcd side And there there have been recent developments of trying to Truly understand the string theory side and You know See if the duality how the duality holds and what is the best way of Understanding and being able to calculate on that string side Uh So from point of view qcd the main guidance there is what does that Effective string theory that describes qcd defense looks like right We could only really probe that using ad at tfp In the supersymmetric theory, but that should give us some hint One can also probe that effective string theory for qcd using lattice gauge theory using Just general ideas for and you know that Is one area where some recent progress has Has come about lots of interesting hints And I think it's a very very exciting area And uh one could imagine Um One understands that the effective string theory that describes mesons really making some rapid and important progress And there's the other side what we learn about quantum gravity in situation for The classical limit is not good and that Is proceeding as well, so You know this rule is extremely powerful But so far it's been mostly employed to test through Path and confirm and it's slightly extrapolate New old physics that one knew about before But if it really you know Is a complete duality One is going to in the future be able to use it and other ideas To fill out our understanding of say gauge theory for strong coupling that's We need to really understand nuclear physics As well as quantum gravity For highly curved faces And strong coupling, so it's a very exciting time Uh, at least for theorists It would be even bigger if one could Could get some important experimental clues Thank you In cold dense matter if we can avoid gravitational collapse Is the confinement inevitable at some critical density? Yeah, sure Yes, and it's roughly calculable and it's roughly of course the mass scale of qcd 100 few hundred mev but Well, I mean depend what you mean by reeling If you're asking for rigor Okay, that's That's a different standard than we usually employ Maybe there'll be a rigorous Don't you know, there's a clay millennium prize for a million bucks if you can Prove that qcd Makes sense And produces a mass scale I talked about the dimensional transportation That's the essence of an asymptotically free theory and If that you can rigorously prove That you get a million bucks that's proving confinement if you want that I haven't been proved but it's really true so And there's direct You know lattice gauge theory can be used to Prove the high temperature behavior. It's much harder the high density But what the high temperature behavior of qcd? There's no question. There's a decompining phase transition you can extract, you know use The non-rigorous techniques of our business to show that okay, you're arbitrarily weak coupling You can use perturbation theory to show there's confinement and And then there's experiment So at wreck we've been scattering for the ions on ions produce a fireball with an effective temperature for short times inside which is Above the decompining transition and the properties, you know, what comes out depends on the phase of that date and What there was a bit of surprise I must say But we sort of expected Able at the beginning that there'll be Uh, you get above the decompining phase transitions and free quarks who go on not totally free Plasma very complicated but a plasma like state Instead what was discovered that There was deconfinement, all right But more like a kind of strongly interacting liquid phase, which is Very interesting and uh and actually ADSE fd duality Which relates those fireballs To black holes has been very useful in studying the behavior of that fluid As we get the higher and higher energies, then it will be very interesting this electron ion collider that's turning on will We'll learn more about that phase, but eventually now there should be a deconfine Weekly interacting Quarks and fluids That's not a simple plasma because it's not a billion that vector But it's not like a electromagnetic plasma where the photons are neutral here the Charms much more complicated And interesting, but I can't prove any of this more questions Maybe I have one So, uh, where do you expect the next revolution in qcd? Maybe from the gravity side like you mentioned or maybe uh quantum computers for Solving the strongly capital theory You know there needs to be many many new developments so actually I am now part of a A new collaboration that's been set up. You know what one of these simon foundation collaborations at qcd Which is trying to bring together a lot of different kinds of people To attack some of these problems in a fresh way and And to attract some of The younger people perhaps in this audience Who think that physics began with a gscfd? Who these post of fantastic issues which can can be now and in the future even better tested experimentally This collaboration brings together People doing a larger thing if you want people doing Really mathematical physics in gauge theories and And super symmetric gauge theories people doing riders gauge theory People doing anything There are many different ways of approaching this problem and they all are connected and can give insight Uh, the physics, you know the the physical phenomena that remain to be understood are gonna Are going to be around forever We still are studying 150 years later after Maxwell Electromagnetism and discovering new phenomena and deeper understanding and by the standards of not to be linkage theories or strength theory Electromagnetism is a pretty trivial theory So, you know, this is going to go on for centuries But If you're looking for new ideas and don't want to necessarily work in a field where everyone else is working Not a bad A bet and you might have some new ideas of new insight Not a bad deal to move into at this point in time A lot a lot of new possibilities Thank you All right, so if there are no more questions, I would thank professor gloss again Thank you very much and for all of us. There will be a coffee break attending us in the terrace Thank you