Fifty Years of Quantum Chromodynamics (The Theory of The Strong Nuclear ...

Fifty Years of Quantum Chromodynamics (The Theory of The Strong Nuclear Force) - David Gross Institute for Advanced Study 144K subscribers Subscribe 144 Share Download Clip Save 5,127 views Jul 11, 2023 Prospects in Theoretical Physics 2023: Understanding Confinement Topic: Fifty Years of Quantum Chromodynamics (The Theory of The Strong Nuclear Force) Speaker: David Gross Affiliation: KITP, UCSB Date: July 10, 2023 Transcript Follow along using the transcript. Show transcript Transcript Search in video 0:14 okay so to cap off this uh wonderful first day of the school we have a 0:20 special treat a lecture by David Gross on 50 years of tcd and David is not only 0:28 one of the founders of qcd he's also one of the organizers of this school and uh 0:36 it's perfectly timed to be right around 50 years since the historic paper by uh 0:43 David and his graduate student at the time Frank wilcheck which was written 0:48 around a mile from here in chadwin Hall so yesterday was a 0:55 pretty nerve-wracking day because almost all flights to New York were canceled 1:02 and David's flight landed somewhere in Washington Dallas and then he said well 1:10 I think he set one of the world records in in the process of getting here maybe 1:16 for the number of uber calls and and the distance that this Uber 1:23 covered so so you truly won the extra mile or maybe 1:30 extra 200 miles on the way here so I'm very grateful to him for showing up and 1:37 we greatly look forward to this lecture thank you 1:47 um well this works foreign 1:53 it's really great to see you all we were 1:58 somewhat surprised the organizers in the workshop but what interest there is among the younger 2:04 generation and a 50 year old problem uh which we know has to do with the real 2:13 world and is enormously exciting and you 2:18 will learn a lot from the school I hope but you'll also it'll become clear that 2:25 there are a lot of open problems that you can help solve in the next 50 years 2:31 we don't want to you know qcd is going to be around for a few thousand years at least and uh it 2:38 would be nice to to finish it off or at least make significant progress this century 2:46 so I'll talk to you guys 50 years well 2:52 it's quite amazing uh to think back 50 years ago since on the one hand like 2:59 yesterday um especially coming back to Princeton on the other hand seems infinitely far 3:06 away um it's almost 100 years 3:11 anniversary to the birth of quantum mechanics in the form we know it still 3:19 um and so they're likely to be interesting celebrations next year the following 3:25 year um 3:30 but let me start the story at the beginning of nuclear physics with 3:37 110 years ago with Rutherford and boar 3:44 so the story really begins with Rutherford's discovery of the nucleus 3:50 over 100 years ago um which is my opinion 3:57 perhaps the greatest experiment of the 20th century there were three things that Rutherford 4:03 achieved in this experiment but first of course was to discover the nucleus the 4:08 second was to uh get a good classical picture of the atom and the 4:17 third was to immediately promote the quantum description of the atom and 4:22 the start of a 10 Years Journey to quantum mechanics As We Know 4:28 what he did of course was try to determine what the atom was made out of nobody had any really good idea 4:36 by scattering particles Alpha rays which alpha particles which he won the Nobel 4:43 Prize in chemistry for for discovering and using 4:49 you never won a Nobel Prize in physics I think if he had lived longer he probably would have for this much more important 4:55 discovery is students were the detector they sat in a dark room for hours and observed 5:02 the scintillations of a fluorescent screen where the scattered alpha particles scattered off 5:10 called nuclei gold atoms and uh he was very surprised that a lot 5:16 of the um particles came back at very large angles and uh 5:24 he calculated somebody told me that he didn't actually do the calculation that was disappointing 5:29 it's called the Rutherford cross-section but supposedly we had the help of a mathematician to 5:37 um use Maxwell's Theory to calculate the the cross-section and deduced 5:44 in fact that they're all of the massive or most of the mass and all the positive 5:49 charge in the atom was located in a very small region of space 5:56 you could only set a a 6:01 lower limit on what the radius of the nucleus was uh but that was a discovery of the 6:07 nucleus and in a sense LED immediately to a planetary model of the atom which 6:13 bore than turned into a 6:19 Bohr model of the atom and from there the quantum mechanics really required almost no new experimental evidence it 6:26 was all there you had to make sense out of out of this 6:32 uh the existence of quanta and the Bohr model of the atom which was very 6:39 successful most important perhaps 6:44 um Rutherford invented the method we still use today 6:50 to discover physics at Short distances you want to discover the properties of 6:56 matter at the summit Atomic lens scales smash protons on a fixed Target or 7:03 protons and protons or electrons and electrons scatter particles of particles lots of stuff comes out you analyze what 7:10 comes out probability distributions of the scattering events 7:16 uh we haven't improved conceptually on that tool in 110 years 7:22 even though it now we're reaching perhaps the limits of that tool from an economic and 7:30 technical point of view maybe we need to find some new method for exploring arbitrarily short 7:36 distances but we have it still Rutherford's of course we've greatly improved it all right see 7:43 greatly improved the detectors from these uh observations of pinpoints of light on a 7:52 scintillating screen but essentially it's the same thing so incredible experiment the atom the 8:00 nucleus and the experimental tool for for high 8:05 energy fundamental physics well 8:12 going forward 50 years or so as a graduate student so this talk by 8:17 the way is going to be um highly historical 8:23 the audience I see before me say most of the audience almost all the audience was 8:28 not alive 50 years ago but I also realized that for many of the 8:35 audience their parents weren't alive 50 years ago 8:43 um so 50 years or most of you seems like an awful long time 8:49 and I conjecture that very little of you very little of you actually know much 8:55 about the history of physics the history of the standard model is through qcd it's a mistake 9:02 it's not because it's interesting uh fascinating to understand your part 9:09 in a grand endeavor you know to join this incredible 9:16 march to understand the fundamental nature of 400 years old you're going to be part of it for the next 50 years 9:24 but also you'll learn a lot about how to think how to avoid going in Long directions 9:30 lots of things by studying history so might might be useful in addition to 9:37 enjoyable I admit this is my view of the history 9:44 of a personal point of view but that's always the case 9:52 the anyway so I entered graduates about 50 years ago at that point I was at 9:58 Berkeley which was the center of high energy physics that had the biggest accelerator in the world six TV sorry 10:05 six GB only down by a factor of a thousand but 10:11 it was enormously exciting because new particles were literally discovered every few weeks no hadrons 10:19 um and it was known by that time that there were two kinds of forces that acted 10:25 within the nuclei strong forces you know an electro what we now call electrolique or weak interactions 10:32 and I was always attracted by the strong interactions partly because well there were all these 10:37 continuing experimental discoveries very exciting but also it was especially 10:43 intractable new particles are being discovered all 10:49 the time they look just like the proton and the neutron which had been discovered 10:54 100 years before and they all seem equally fundamental equally Elementary 11:01 wasn't clear at all what were the basic constituents of nuclear matter 11:07 and then of course there was no principle to determine the Dynamics there was no 11:13 symmetry principle the principles that guided Einstein in the case of gravity 11:19 or there was no direct experimental clues that guided Maxwell 11:25 uh following in their footsteps people of course tried to construct theories of 11:31 Quantum field theories of pions and protons and but there was not based on any 11:38 under you know underlying principle and then if you wrote down a theory as 11:45 you power first tried to do introducing a carrier of the nuclear 11:51 force uh boson they would carry it was originally a spin one but then spin zero 11:59 a Pion which was shortly discovered one of the first particles in addition to 12:04 the neutron um it was clear that having introduced the 12:11 pine you could estimate the coupling of that Pi into nucleons to protons 12:16 and that coupling were strong so how do you calculate so we didn't know what the constituents 12:22 were we didn't know what the principle that underlies the Dynamics was and it was clear from the beginning that 12:29 you added the old storm coupling so you couldn't calculate 12:35 Tyson well known to this in this institute tried very hard after 12:42 the Triumph for QED which resurrected Quantum field Theory 12:47 on the doldrums of the 30s uh Dyson tried to construct a Quantum 12:54 field theory of nezons at protons and you know and to figure out how to deal 12:59 with strong coupling and uh I think somewhat characteristically of 13:06 Dyson after a few years of trying he was convinced that his ideas were 13:11 going nowhere there's a beautiful article I think it describes a discussion he had with Fermi 13:20 which deflated him so much that gave up and said uh 13:26 and said how it went to work on other things but again like many theorists 13:32 if they themselves give up they predict as Dyson did the correct Theory will not be found for a hundred years 13:42 the general feeling I must say was that something revolutionary would be 13:48 required and probably not in the framework of you 13:54 know not just following the lesson of QED the Revolution was needed 14:00 now in retrospect it's clear why the problem was shown was so big 14:07 and it's because we now know that the strong interactions are mediated by 14:13 gauge fields that couples to charges color to the colored quarks and gluons 14:20 and in the case of strong interactions the charges were completely hidden 14:26 the courts were hidden the ruins were hidden not not at all like QED where the 14:35 charges were quite visible and usable 14:41 there were no quarks and people started smashing protons 14:46 together uh right after the war when the government eager to build even 14:54 more powerful bonds or whatever we would come up with uh gave I created the field of high 15:00 energy and high dollar physics but no matter how hard you smashed had 15:07 grounds together all of it came out were other add-ons which looked similar to the original ones 15:14 and you never produced the visible charges of the basic 15:20 building blocks uh in addition Quantum field Theory 15:27 which was the framework that uh sort of was at the basis of 15:34 fundamental physics at the time and had a remarkable success in the case of 15:39 quantum electrodynamics in the late 40s early 50s 15:45 was under attack uh why was that well the big development 15:53 in the 40s 50s was renewalization hearing which was you know 16:00 retrospectively not such a big deal it was truly understanding 16:07 today we're in a profound way but then in a technical way how to deal with the 16:12 UV divergences that are that uh appeared in all 16:19 um quantum Beyond tree approximation in uh in any 16:25 Quantum field Theory but renewalization was thought by the 16:30 inventors and developers of the scheme to be a trick and sort of that's how it 16:36 was taught at the time this was a trick this was a way of sweeping Infinities under the rug 16:42 and it's so such a ugly way of dealing with what was 16:49 thought to be a profound problem the UV Infinities that uh it probably an 16:56 indication that a Revolution was needed they're also absolutely 17:02 in Quantum field Theory and among the high energy elementary particle physicists 17:09 really no useful non-perturbative methods um 17:15 in fact first course I took on Quantum field theory was from Stephen Weinberg 17:22 I'll show you many of you have read his books foreign 17:30 rules for higher spin particles and 17:36 he first lecture wrote on the Blackboard field Theory equals perturbation Theory 17:43 actually I don't think he said prohibition there I think he said field Theory equals final diagrams 17:54 there was a theory or we would call today an effective field Theory a phenomological theory of 18:00 the weak interactions Family Support family on Theory it was moderately successful 18:07 um there was no underlying principle behind it it was phenomenology but it worked 18:13 extremely well it's a very good effective field Theory but nobody had the Vegas idea how to 18:18 make it unitary how to renewableize it and so on it was not necessary at the 18:24 time experiments didn't demand it 18:30 Yang Mills Theory appeared very early 1954. but 18:37 it was plagued with massless posanswich nobody under stood how to remove seemed 18:44 to be there for an interesting mathematical extension of electrodynamics but not a realistic 18:52 Theory nonetheless of course here has tried to apply it 18:57 through strong interactions they coupled the gauge fields too the only symmetries 19:04 that they know about in the strong interactions which were flavors of the truth now today we 19:11 understand those are accidental symmetries there's no deep principle that says the 19:16 up and down Quark massages are are almost the same and so on it's not an exact symmetry it made no 19:23 sense and then there was a real Attack on 19:29 Quantum field Theory it came from my thesis advisor Jeffrey 2 19:36 and many other people but he was the leader of the Revolution 19:42 which said among the rest that all hydrants are equally fundamental they are looked the same 19:48 so let's assume they're no really Elementary constituents there might be an infinite number of hadrons somehow 19:54 they're all equally fundamental uh but then how do we construct a theory 20:02 so and there was a second idea called the bootstrap now the bootstrap is 20:08 something you've all heard of nowadays because uh it's come back in the Vogue and it's 20:14 very interesting it's so difficult to find a analytic unitary s Matrix as you know 20:24 very few exact examples some integral theories but 20:29 uh that wasn't understood at the time and um 20:38 and since there was no underlying principle and all 20:43 there's no Elementary particles maybe what you should do is try to find a 20:51 s Matrix that's what you observed uh based on that satisfies the general 20:57 principles especially analyticity which follows for locality 21:02 or causality and unit therapy that's the bootstrap program now the 21:09 boots that program back then was equally popular you could start doing stuff you 21:14 know writing some approximational left-hand side of the board and then 21:21 using by Crossing symmetry as a part for manual artisticity right something on 21:27 the right hand side and see if you could make them equal 21:32 General print those general principles the idea was would determine a unique 21:38 solution first we know nowadays that that's not the case there are many solutions for example qcd with any 21:46 number of flavor any number of colors 21:54 and we have now many examples of exact as major excuse 22:01 but that was the idea of the bootstrap and what it did was create a generation 22:08 you know a good part of a generation of young quizzes like yourselves who uh didn't know anything about 22:15 Quantum filter now on the other side of the Cold War 22:23 barrier of the Iron Curtain there was a perhaps more interesting attack on 22:28 Quantum field Theory the Landau poll or the problem of zero charge 22:34 which was uh 22:39 an investigation of an effective renewalization Group by a Russian 22:46 group that studied how the physical coupling 22:52 what over 137 in QED depends on the bear coupling 22:58 you include the quantum Corrections and take the ultraviolet cutoff to Infinity as you should to get stuck though 23:07 Continuum Theory and working the lowest order in if you 23:13 want in the beta functionalization group Lando in his group discovered what 23:19 that the physical correct coupling measures the value of the electric charge in a theory or the ultraviolet 23:27 cutoff Lambda vanishes no matter how big you make the bear coupling the coupling 23:33 if you want that defines the charge at the cutoff 23:39 vanishes which is a way of today we would argue 23:45 this such a theory the uh there is if true 23:51 and it probably is for Pure QED um although not proven uh means that 23:58 there is no Continuum limit QED does not exist as a a finite 24:06 well-behaved unitary Quantum filtering 24:11 and that's sort of what landed out concluded now the phenomenon of course 24:16 is screening you know if you put a charge into the vacuum 24:24 the virtual pairs that exist in the vacuum screen the charge 24:30 and that screening means that the electric force decreases you move away from the charge 24:38 increases the move towards the charge and at least in this lower starter 24:46 calculation uh that effect means that 24:52 keep the charge at Infinity which is how you do the measurement of the electric charge fixed the bear charge vanishes 25:01 and the theory is trivial so 25:06 Landau I mean this is probably true for all 25:13 nylons theories not an asymptotically free theory in four dimensions but 25:20 um land down certainly didn't have Good Grounds but you know he was 25:26 bald or jump to conclusions he said 1960 we 25:32 reached a conclusion that within the limits of formal electrodynamics by which he really means Quantum field 25:38 Theory a point interaction a local interaction is equivalent for any intensity 25:46 to no interaction at all fix the physical couplings 25:51 no interaction we are driven to the conclusions the 25:58 hamiltonian method for strong interaction is dead and must be buried although of course 26:05 with deserve an honor now 26:11 why do you say strong interaction same conclusion would be for QED but of 26:16 course he knew understood that the problem you know are so 26:21 the ultraviolet is so far away so much farther than the blank mass in 26:28 the case of QED the Lando poll if you want best 26:34 uh that it's irrelevant but for the strong interactions the 26:39 problem was immediate the coupling was strong and if you went higher and higher in energy it would diverge at some finite 26:46 energy so um now Orlando is a very powerful creature 26:53 and Soviet Union and uh 26:58 maybe it's not true for the whole Community but certainly his influence was enormous 27:04 and students young students like Sasha sitting back here 27:10 we're not allowed to work on Quantum field Theory they were smart enough to continue to 27:16 work on Quantum field Theory but under the guise that they were doing condensed matter here 27:22 right [Music] and he got into trouble she can tell you about some other time 27:32 so that was the situation theoretically 27:38 these are important lessons because you you know these were very powerful 27:44 people were very smart brilliant physicists land down two metal stem 27:52 uh so just remember that when all of us talk to you with great Authority 27:58 we might be equally wrong about not about most of the things 28:06 experimental situation was much brighter I must say I suppose just the opposite 28:11 of today theory was kind of um non-existent 28:17 but experimentally there were many new discoveries new particles new patterns 28:22 of particles new symmetries approximate symmetry is being discovered new 28:28 accelerators but even here there are interesting lessons 28:36 so many theorists and experimenters at the time believe that the secret of the strong interactions lay in the high 28:42 energy behavior of scattering amplitudes at low momentum transfer when you 28:47 scatter protons of protons 99.9 percent of the time what happens is they 28:55 just go forward spray a lot of particles in the forward Direction 29:00 and you can measure things like a total cross-section fraction scattered low momentum transfer 29:07 scattering now we know that that's not what we do 29:13 nowadays we look at that point one percent of the events which have larger momentum transfer that's how you look at 29:19 Short distances and discover new particles and new processes 29:25 so why were they doing this well 29:31 it was experimentally easy that's where most of the data works 29:39 they discovered some interesting patterns in diffraction scattering 29:44 Reggie Behavior constant total cross-section 29:50 the theorists develop theories or ideas about how to 29:56 describe diffraction scatter in total cost section and so on so there was a sort of mutual 30:03 confirmation of each other's biases for experimenters 30:09 it was good to study low momentum transfer because you had bigger statistics 30:15 always good theorists are told them that's what's really 30:21 interesting because they were studying Reggie polls and other things 30:28 and no one was really interested in the high momentum transfer experiments 30:35 which like Rutherford just turned out to be where the secret 30:40 of the strong interactions were to be found the theorists well 30:46 misguided the experimentalists were these were 30:53 harder experiments and nobody told them it might be interesting 31:02 so back to me my story after escaping from the bootstrap and 31:10 nuclear democracy which was dominant on the west coast of 31:16 the United States I came to Harvard where field theory was 31:23 acceptable yeah and I started 31:30 studying the properties what we would call today 31:35 the short distance behavior of composite operators and Quantum field Theory although again you know most of the 31:42 tools have not yet been developed operator products expansions kind of formal filter it was really nothing 31:50 but it was one could do this without 31:55 trying to invent our Quantum field Theory because there were operators that are experimentally observable 32:02 not in PP collisions but in EP collisions where you can measure Matrix 32:08 elements and correlation functions of current parenthesis couples electromagnetism or 32:15 to the weak horses so 32:23 at Harvard Callan and I would play the game of scene 32:30 what operator products in modern language would look like if you just made some model or more precisely used 32:37 free field Theory and uh 32:43 among the rest we wrote a sum rule for the structure function of deepened 32:49 elastic scattering so which will come to in a woman which measures 32:55 which is in modern language proportional to the energy medium tensor 33:01 and uh this could be measured in deep elastic 33:07 scattering deep elastic scattering was an experiment that was planned for the 33:14 slack the new slack facility which had an electron beam scattered electrons of 33:21 protons large momentum transfer but Rutherford experiment again but this 33:27 time using photons to explore the structure of the proton 33:34 and beer cane uh looked at the sub Rule and noticed that you know this this and 33:41 dimensional reasoning suggests yeah it could be written without any scale if you 33:48 notice this has dimensions of energy this has dimensions of energy 33:56 and suggested that perhaps these things scale 34:02 there are other some rules one could derive one of the most important actually was one with 34:09 Callan where again you know taking products like this how do you calculate well you assume some model which means 34:16 some free field Theory and well what turned out was that the 34:22 two cross-sections when you scatter Polo photons which have two velicities of 34:27 protons look at the total cross-section and it turns out that the ratio of 34:33 longitudinal to transverse polarizations in the photon are sharp leaders different whether the 34:40 constituents of the proton have spin one half or spin zero or spin one using free 34:48 filter and then 34:55 there are other some rules the experiments showed started in 1968 35:01 and 69 were reported deep elastic scattering at Slack 35:07 now this is an interesting experiment again it in effect discovered that the proton 35:14 looked at like it was made out of freely moving quarks 35:21 and not only that the scaling behavior that 35:26 is sort of inherent in free particles free point-like Behavior 35:33 seemed to work their Quark model some rules seem to 35:38 work and this of course 35:45 was the discovery of quarks scrum metal discovery of forks 35:50 now these guys were kind of bold at the time none of 35:56 the you know when you build a new Exciter a lot of proposals for experiments 36:01 this was so this is the Rutherford experiment in effect so 36:06 it was very strange historically that these are the only people who wanted to do it most of the proposals 36:14 were not even sure what they are because they go back and look but 36:20 it's really there wasn't a clamor to do this experiment everybody thought it would be boring 36:28 the lower energy lower momentum transfer experiments that measured the form 36:34 factor of the proton the distribution of charge within the proton showed that the 36:39 protons kind of diffuse so they thought the cross-section would be diffused like 36:45 Rutherford before Rutherford everyone believed that the structure of the atom was some kind of diffused mess 36:53 not interesting actually like most particles 36:59 experimentalists what they wanted to do was not understand the structure of the 37:05 proton but produce no particles 37:10 experimental hydrophysics always want to see bumps they want to see new particles they 37:16 don't necessarily are motivated by understanding new structure a new process 37:26 anyway it turned out much like Rutherford easiest explanation of these experiments 37:33 was the protons is made out of freely moving Point particles 37:40 pythons was feynman's work that looked as far as you could test them using 37:46 totally ill-motivated sub rules like quarks 37:51 so this was enormously confusing 37:57 now the fact the fact that some of my some rules worked and the experiments together 38:03 convinced me that hadrons are made out of Point like constituents 38:09 the constituents were quarks quirks were real 38:15 but then how could that possibly be you never produced quarks and experiments it 38:20 had to be confined we now say within has to be some strong coupling that 38:26 holds them there throwing interactions I think as we're strong so how come you 38:32 you could get free Behavior how could one explain scaling the 38:39 absence of course now again you might think okay everybody 38:45 got this message but that's not true it was only a small group of people who were 38:53 obsessed with this most people 38:59 uh for good reasons said well this is interesting but one 39:05 it'll probably go away now when something truly new is discovered 39:11 that isn't a sharp resonance and obvious discovery of a new particle 39:18 people are justifiably skeptical the errors in the initial experiments 39:24 were 20 30 percent hey girl boss 39:29 the energy was extremely low 20 GV 39:36 you're at every right to be skeptical on the other hand the 39:42 the conclusions were so striking that it uh 39:48 some people it was became an obsession certainly for me 39:53 so the obvious thing was scaling now scaling had not played any role in 40:01 in uh matching Quantum field Theory until that point 40:07 because scaling is not a exact symmetry of nature anywhere 40:14 in elementary particle Foods 40:19 somewhat later it became you know a big thing in commence metaphysics but 40:25 and the study of critical phenomena but that connection was still someone in the 40:30 future and at that time there was no interaction between people who were doing column filtering particle physics 40:38 and people doing what we uh 40:44 previous generation of my mentors called Squall of State visit 40:55 so I really was obsessed with trying to understand how this could be 41:03 it was clear that was trying to understand the scaling in the context of quantum field 41:10 Theory with Quantum effects destroyed scaling uh immediately 41:16 these some rules that we derived were simply invalid in an interacting Quantum 41:22 filter 41:27 by 1972 I had a plan a very definite plan 41:33 about how to make my advisor happy and kill quality filter 41:41 so idea was scaling was real the experiments were getting better it was holding up some rules were even better 41:47 satisfying um I really understood that the only way 41:53 scaling Could Happen would be that the opposite of QED the 42:00 at small distances large amount of transfer deeper elastic scattering your 42:05 probing the behavior of the theory in the ultraviolet short distances 42:11 if the coupling could vanish asymptotic Freedom which we call it 42:19 later um would solve the problem 42:24 and one thing we tried to prove was that that was actually required 42:30 and uh based on initial study of Parisi kurtalin and I showed that for 42:38 in a sense about uh the framework or Quantum field theory if you want to 42:44 explain asympatic Freedom you have to have sorry if you want to explain scaling if that's exact 42:51 phenomenal nature then you require us encountered freedom we did not discuss notable engage theories 42:59 because they are in fact well methods proof broke down 43:07 and then the idea was to show that asymptotically free theories not that there are no accidentally free theories 43:13 and that was you know conceivable to prove because if you have asymptotic freedom 43:20 at Short distances then you can trust motivation period short distance so you can do essentially one Loop calculations 43:27 and decide whether any given Quantum field theory is or is not asymptotically free 43:33 and they're accountable theories which are renormalizable 43:39 so that was actually proved by Coleman and I all of this 43:44 happened in 19 came out 1973 43:50 and uh there we showed that any Theory any number of scalar Fields been runoff 43:56 fields or barely engaged particles are a synthetically free 44:07 then the idea was to take the final Theory um 44:13 so at this point my first graduates appeared 72 44:20 or 70 yeah um thank wheelchair 44:25 when he still had hair and uh 44:32 and like you know excellent I have one more thing to do let's 44:39 calculate I want to be engaged theories and this indeed is just about a year 50 44:46 years ago so um 44:52 not only being engaged series of course had been around since 54. 44:58 1968 Weinberg wrote a lot a great paper called a model 45:05 of leptons which put forward what is now the issue to course you on Weinberg Salam Weinberg 45:12 rash house the theory of the electroweak interactions without the cork 45:18 why didn't he put in quarks Weinberg didn't believe in courts 45:23 at all you just thought that was not nonsense for obvious reasons 45:29 nobody's ever seen a pork and how could there be corkscreen so it was a model of leptons 45:37 but it wasn't renormalizable or he didn't know how to make it be normalizable he didn't know how to show 45:42 that it was a normalizable and so there was almost no work on the record week 45:48 again a lesson for you guys there might be things like that that nobody's 45:53 talking about that you won't hear about in these courses just lying around 45:59 but when it hooked development showed that you could renormalize a theory and 46:05 calculate suddenly there was a 46:10 and aside from the development of Electro week 46:16 Theory that had the impact of introducing people to functional 46:24 methods path integrals which were gave the understanding of how to write down 46:30 perturbation Theory or Edge theories ghosts and so on body of 46:37 public everyone had totally ignored the use of functional intervals in elementary 46:43 particle physics until then again there might be all sorts of tools 46:50 out there that nobody uses for 20 years more 46:58 and of course not to be engaged there so this was the last 47:04 and when we discovered that these theories possessed the remarkable theory of asymptotic abortion free filtered to 47:11 me by that time it was a complete surprise because I expected that there were no no 47:21 it was so Universal in for non lessons other freedom 47:26 that that's what I expected 47:31 and but the implications for me Rob is once you have that you that the only way 47:40 following the previous arguments to explain to you okay and scaling and one should look for a non-appeal 47:47 engage theory of the storm interactions and there was from then on there was no 47:55 choice these arguments the only possible theory 48:00 was the Angel's Theory as far as the gauge part of the theory 48:05 goes well 48:11 it had already been known phenomologically that quarks which have been identified 48:19 as which had been suggested as a way of organizing the 48:24 flavor symmetries of the strong interactions the approximate symmetries 48:29 of matter um there had to be an addition three labels 48:35 degenerate courts with three colors in order to explain 48:43 the magnitude of the E plus C minus photorecrusters Pi North BK 48:50 any other suggestions but courts are really thought to be 48:56 mathematical objects no problem attaching three labels to them although there were suggestions 49:02 that they might be associated with the with the quantum numbers of 49:08 of su3 gauge group so the 49:15 if you buy the colors in the three flavors that we knew about at the 49:22 time there was no choice but hey model based on three triplets of fermions within 49:28 issue three color gauge group to provide the strong interactions 49:35 and um that's qcv now why does qcds 49:43 why is it not synthetically free well it's obviously the opposite of screening it's anti-screening 49:49 and what is the physical motivation well it's best to think about it magnetically 49:55 you know euclidean Theory that's just 50:01 or in any Theory you could think about magnetically electrically magnetically 50:07 luas we put a an external magnet into the vacuum an 50:14 external colored magnet to the vacuum the virtual gluons are like permanent 50:20 magnets 50:27 so are by the white Forks but the Magnetic Moment of the Bruins much 50:32 bigger and they act like uh little magnets would do in the 50:39 presence of another magnet they align paramagnetism where the fourth at large distances 50:46 increases and at Short distances decreases 50:51 paramagnetism is the opposite of diamagnetism if you want to use an elect 50:58 a magnetic point of view and uh 51:04 that's that's a lot of freedom for you and you can work it out numerically 51:11 calculate the beta function that way that's of course not what we did originally 51:18 um oops anyway I'd always wonder why Landau 51:24 you know didn't say could have discovered all of this 51:30 um back in the late 50s by saying okay and anti-screening diode dielectric 51:37 vacuum is a dielectric medium that's really bad but what if it was 51:44 paramagnetic and he could have invented well the 51:49 English Theory already exists he could have come that way to this discovery but 51:57 hello yep 52:18 no it doesn't doesn't work actually it's harder 52:23 sorry why why doesn't it work well I actually we taught I tried that's 52:31 uh and didn't succe you know so why doesn't in QED 52:37 uh if you were if you're thinking physically about screening 52:43 right you conclude that the QED is not asymptotically free 52:49 you can also just think about the diagrams that contribute to charging normalization it's just a one little 52:54 diagram vacuum polarization diagram its sign is fixed by unitarity that's it 53:01 an immunity gauge theories the gluons are charged it's a more complicated effect in fact 53:07 the real origin of asymptotic freedom is because of the coupling of this external 53:13 charge to the virtual charges so it's really the triple glow on vertex doesn't 53:18 exist so couldn't give a general argument indeed 53:25 partly because they're more diagrams partly because it isn't in fact true 53:32 but to do that you had to do the calculation so that was so I didn't do that really 53:38 that was the point of doing the calculation with my new graduate 53:44 student who needed something to work on 53:50 anyway there are you know the one thing the great thing about history is what you 53:57 learn when you go back and read the original papers you learn a lot a lot more physics than 54:03 you do by reading the paper the many papers that come after second you learn about the mistakes that 54:10 people make you learn from that experience and what you 54:17 then there's the fantasy which I like to engage in but historians abhor which is what if 54:24 imagining other scenarios and there's if you think about it there's so many ways 54:30 that one could have arrived at what we know to be true nowadays 54:37 that's kind of fun to like Landau looking for paramagnetic 54:43 Quantum field theories prevent anyway let's we're making the mistake of 54:52 I always like so um 54:57 anyway that explained gave us a theory and a unique Theory there was really no 55:03 argument about what you could adding scalars to the theory 55:10 we tried can't destroyed asymptotic Freedom there was no 55:15 and everything we knew about the strong interactions were the few exceptions of outstanding problems like the U1 55:22 Ada Prime so on um we're consistent with qcd 55:31 but there was still the problem of why you always see quarks and uh in our 55:38 larger version of that letter uh we were 55:45 uh considered the possibility that that the Symmetry wasn't broken like the weak 55:51 interactions but exact ly and then well why don't we see these 55:56 Mass these blue ones and and 56:04 noted that there could be little connection between the free lagrangian to write down the spectrum of States 56:10 because of strong coupling and it could be that that suppresses all 56:17 but color schemer at stake now the idea was really at that point nothing more 56:22 than the fact that since the force gets stronger 56:30 extrapolation the perturbation Theory uh that could confine the courts 56:36 but that was a very uncomfortable idea because there was no 56:43 real understanding We Now understand 56:48 how that occurs well so 56:56 classically of course the force between quarks falls off like one of our gauss's 57:01 law and 57:07 atoms can be ionized and at the classical level of course 57:13 that's what would happen more or less it's the properties of virtual particles 57:21 in the vacuum or the vacuum which we now have pictures of from lattice cage 57:26 theory that modify the flex lines that 57:33 and squeeze the flux through a flux tube 57:39 now this picture uh was put forward 57:45 pretty rapidly by manual stand with others of an electronic a dual analog of 57:53 vortex flux tubes in a superconduct and very easily 58:01 electrical confinement is a kind of dual to Magnetic confinement of multiples in 58:08 the superconductor and as you heard in the previous talk point we talked 58:15 that or very vague analogy is one of the 58:20 directions where we might hope to prove or use 58:27 such ideas to understand mesons as flux tubes of electrical flux 58:33 and of course that kind of confinement produces a 58:38 linear confinement because the flux now 58:46 penetrating a finite area and we can see that on the lettuce 58:55 so that was a physical picture that came along and it's still after 50 almost 50 59:03 years we're trying to make that into a picture that has mathematical rigor 59:10 and more important analytic control 59:16 that's the goal of this collaboration won't confirmed 59:21 now historically qcd was not accepted by everybody of course 59:27 but it was immediate for a very small group of what I call spark people 59:36 it was really interesting to see how people reacted with an idea 59:42 like that but for the majority of my colleagues 59:52 the people on the West Coast didn't understand Quantum filter at all 59:59 Ken Wilson who should have discovered this was 1:00:04 motivated by the idea that all interesting 1:00:10 fundamental questions in physics are consequences of what emerges in the 1:00:15 infrared from some ultraviolet Theory which you didn't care about 1:00:21 and that the experiments which suggested simple feeling of short distances 1:00:26 would this would that was not just experimental too early to tell would disappear 1:00:35 um and then there were people liked my 1:00:41 colleague at Princeton Eugene vigner who could not accept that you could possibly describe a theory in terms of 1:00:48 particles you couldn't produce as asymptotic States because his definition of an elementary 1:00:55 particle was an irreducible unitary representation of the barnacore group 1:01:01 that you could produce the symbolic states to the observables which were estimated 1:01:09 but for this group and probably many others it was almost immediate 1:01:16 and in addition you could calculate 1:01:22 easily using perturbation Theory could make predictions 1:01:28 and that's what physicists like to do so there was a slow but steady growth 1:01:36 lots of calculations interesting applications immediately 1:01:42 but the real or yeah then there were very important theoretical milestones at 1:01:47 least for me one was latter stage Theory so Ken you 1:01:52 know who was misguided what he was looking for switched overnight 1:02:00 which I admire enormously and said okay let's calculate the mass 1:02:06 spectrum of headlines 1974 lattice Gage Theory what you heard 1:02:12 about today it has taken 40 years 50 almost 50 years 1:02:18 to get to the stage where you can now calculate the hydronic Mass Spectrum to better than a 1:02:25 percent and an increase in power 1:02:31 which Mike didn't mention by a factor of 10 to the 18. 1:02:37 10 to the 9 of that has come from Moore's Law computers 1974. 1:02:43 those of us who were alive 1:02:49 we remember how totally primitive it was 10 to the 9 came from Theory from 1:02:56 algorithm development understanding vermions better finite size effect 1:03:04 so that is quite you saw some of the evidence today it's quite amazing what they can 1:03:10 do but gotta remember one can have you know let's calculate 1:03:17 the Spectrum Wilson by the way gave up after a few 1:03:23 years says it can't can't work nonetheless people persisted luckily 1:03:28 but it did take an increase in power of by 18 orders and 1:03:38 um for me the cyborg is high squared Theory two-dimensional Theory 1:03:45 with no parameters except for coupling and developed design dynamical Mass Gap 1:03:51 and dimensional Transportation was very important to see and asymptotically free 1:03:56 Theory albeit in two Dimensions that for a large n you could solve exactly and develop the kind of non-preservative 1:04:04 mascot that we expect in qcd also at hoof's solution for large n 1:04:12 again of two-dimensional qcd which kind of confines trivially in a 1:04:18 way but still exhibited a model where you could see the fundamental quarks such or 1:04:25 distances but are never produced as a symbolic States was very reassuring of that possibility 1:04:31 in qcd but the real thing that changed the fear 1:04:39 acceptance was electron positron annihilation 1:04:45 are there the story is interesting the ratio of the total cross-section probably plus C minus to go to hadron 1:04:52 compared to electrons predicted by qcd 1:04:58 by just scaling to be a constant by qcd to 1:05:03 depend number of quarks number of colors charges of the quarks 1:05:08 and it worked very well one of the try early triumphs 1:05:14 of these ideas this is a plot of what the data looked 1:05:21 like in the summer of 1974. I was at a conference at the ictp and Trieste Burke 1:05:29 Richter showed this plot new experiments 1:05:35 at the E plus e minus storage ring at uh 1:05:42 Slack and he concentrated on this 1:05:47 these new data points which he said shows that the cross-section this ratio is not constant 1:05:54 it's rising linearly qcd is dead all those ideas that's all 1:06:02 scaling actually he knew about understood PCD 1:06:07 but scaling whatever it's all dead well as you know this is what happened 1:06:14 just a few months later now many of us in the audience 1:06:20 I think uh Jr and 1:06:25 um 1:06:31 pollitzer and George I had analyzed what I a charm 1:06:38 well the Japes eye would look like it would be very narrow because the coupling is going to zero it's a cool 1:06:44 object down state charm was accepted by all the smart 1:06:50 theorists had to be there had to be another Quark who crossed an electro-week anomalies and su2 1:06:57 like per week Symmetry and so on so on so 1:07:02 theorists again at that time were very timid compared to theorists nowadays they 1:07:09 hadn't had many successes so we didn't scream that well you've just seen charm 1:07:16 we said you're just seeing her 1:07:22 but then very dramatically this resonance uh this Porter Germany was discovered 1:07:30 and there were Within months 1:07:35 within days hours no archive then but rapid 1:07:41 pre-publication of lots of explanations but all the smart people knew well this 1:07:47 is charm and they could start calculating not just harmonium but other 1:07:54 uh charm excited States 1:08:01 and uh all the other explanations were garbage so this totally convinced lots of people 1:08:09 that qcd was on the right track and 1:08:15 well this is just the last 40 Years of using this probe 1:08:21 of Vector mesons as well as porcante pork 1:08:27 bottom not top 1:08:33 what about the experimental tests of asymptotic freedom of qcd 1:08:38 well as mitot said in his lecture logarithms run very slowly 1:08:44 so what is starting with a coupling which we now know is about 0.1 1:08:51 at the range of a few hundred GV and making predictions about what's 1:08:57 going to happen higher energies with logarithmic Behavior so this is uh 16 years after 1:09:05 qcd uh looks pretty good A bunch of experiments they all agree with the 1:09:11 running of the coupling but so there's a straight line to be honest 1:09:18 it's not easy to measure precisely logarithmically changing phenomena 1:09:25 and it really took more like 30 years to get precise tests this is 1:09:31 40 years pretty good uh nowadays these are deviations from 1:09:39 precise scaling that Hera these are just cross-sections 1:09:46 this is the best I could find today 1:09:52 there are updates but this is an indication of the Precision of the tests from many many different 1:10:00 experiments so that you can now measure but if you want the scale of the strong 1:10:07 interactions or the coupling at that scale to 1:10:13 less than a percent but 1:10:18 that's sort of the direct test of the investment letter Kratom but 1:10:24 using that let me calculate the cross-section the deviation from scaling 1:10:29 and the unbelievably precise tests of qcd at LHC 1:10:38 which is most of the events um 1:10:43 since they have any other any surprise discoveries uh 1:10:49 have greatly improved our faith and qcd 1:10:56 but the other you know we really wanted to calculate immediately as soon as possible was the Mass Spectrum of 1:11:03 hadrons and that's a hard problem that's not preservative that's not 1:11:08 the control of the vacuum all we really have analytically getting controllably 1:11:14 as lattice qcd which you started to hear about 1:11:19 today and uh as I said it does require it did require 1:11:29 noon technology and new ideas over many many orders and I do 1:11:37 but now is extraordinarily successful you'll hear a lot about that 1:11:43 there also been impressive calculations uh preservative calculations where one can 1:11:51 just use motivation Theory beta function now this is I just got a 1:11:56 preprint is now like five loops amplitudes which are used for the 1:12:03 backgrounds for new discoveries are required for the 1:12:09 back calculate the backgrounds for new discoveries and or tests of qcd 1:12:15 uh well you all heard much the last decade about credible developments and 1:12:24 understanding deeply new structures that allow one to improve the calculations 1:12:32 uh beyond the leading order which we did to Max deleted over next to next next to 1:12:37 the next to next very beautiful 1:12:43 um hinting at many deep structures enormously powerful and that will slowly 1:12:48 continue I just want to spend a moment on what I regard as the the truly still 1:12:57 for me the most appealing and remarkable features of this 1:13:03 perfect Theory and I I should really speak perfect field Theory 1:13:08 first you know they're really no infinities Noel for violent affinities no 1:13:15 Infinities at all no adjustable parameters and no new physics at Short distances 1:13:22 so what continues to Astound me is is 1:13:28 how we've ended up with an example uh so far of what a perfect quantum 1:13:34 mechanical theory of the world could look like of course we are 1:13:40 like a lot more but this is the minimum for the future 1:13:47 so they're clearly knowledge about Infinities I think that was clear from the lattice calculation the lattice as 1:13:56 Mike explained the lattice calculation you never 1:14:02 get anything that's infinite to find the coupling on the lattice you take it to zero not to Infinity 1:14:10 as you remove the cutoff and you know how to do that and uh 1:14:17 oil observables are calculable and you never have to 1:14:24 subtract Infinities or multiply by Infinity 1:14:29 so the conceptual problem with bugged people from the Iraq and 1:14:36 Eisenberg one of the beginnings of quantum field Theory through the 60s and seven of the existence of UV Infinities 1:14:43 kind of disappears 1:14:49 memory come to uh 1:14:55 no adjustable parameters the reason I read a beautiful article by 1:15:01 Feynman 1961 solve a conference gave a review of the present status of 1:15:09 quantum electronomics which was incredibly successful that a lot of 1:15:14 experimental measurements which he summarized in great depth was about 10 years after 1:15:20 you know the big to do and he he wanted he was dissatisfied he 1:15:27 wanted to see could Quantum electrodynamics really be a kind of perfect Theory 1:15:35 and so he said consider pure QED or QED has two parameters the mass of 1:15:42 the electron the points structure constant well let's put the mass to zero 1:15:47 could we calculate the mass could we calculate the fine structure constant people have been trying to calculate 1:15:54 Define structure constant wherever partly motivated by the fact 1:15:59 that its inverse is almost pure number 1:16:05 so consider pure QED with only zero Mass photons and electrons and protons 1:16:11 interacting with no other particles and having no cutoff 1:16:18 such a theory could not produce a finite electron mass now you might think that's because the theory is a chiral symmetry 1:16:26 but five minutes at that point made it clearer and remarked on it that he understood lambus recent work and chiral 1:16:34 symmetry could be broken you that's not an obstacle he said but the system is also invariant 1:16:42 to a change of scale conformal learner illness and there's no parameter to determine a 1:16:50 length and yet an electron with a mass involves such a letter 1:16:55 I'm not certain ly was always careful but it appears to be impossible to 1:17:01 generate a specific length from those scale whatsoever 1:17:08 but that's exact so he could have already concluded that QED somehow can't 1:17:15 be improved can't calculate the electron Mass can't calculate the fund structure constant 1:17:24 but as we know Quantum effects always break scaling Burns can not always but 1:17:32 can produce a physical Mass and then determine the coupling at this scale and that's exactly what UCD does 1:17:40 there are no essential adjustable parameters now of 1:17:47 course in the real world like in the case of QED there are other things the cork masses we understand have 1:17:55 nothing to do with PCD they have to do with the Higgs sector which 1:18:00 the number of colors who determines that 1:18:05 the number of Orcs 1:18:11 but yeah you do like Simon said throw away the quarks they're not essential 1:18:19 or keep the quarks Mass listed and uh then 1:18:25 because of dimensional transmutation which I summarize here the argument that 1:18:31 you're I hope know it uh all physical parameters 1:18:39 are in units of choose one the mass of the proton 1:18:47 calculable dimensionless numbers so qcd 1:18:53 is exactly the kind of thing 5 Min was dreaming of achieving a theory where 1:18:59 except for some scale which you need to measure masses Energies 1:19:07 everything is comfortable and that's true 1:19:12 true and your job is to figure out how to calculate at least in some rigorously 1:19:22 um systematic procedure 1:19:29 no adjustable parameter so for example that I always lay audiences explain that the mass of 1:19:38 the proton is really the confined NASA kinetic energy of the massless glons and 1:19:46 quarks that are rattling around in a confined region which sets the scale and 1:19:51 determines the mass and all mass ratios are calculable 1:19:59 and they're strong interaction of course defines that scale and is therefore 1:20:05 calculable as well now from a practical theoretical point 1:20:11 of view the most important feature the short term of qcd was that no new 1:20:19 physics at Short distances or high energies was necessary 1:20:24 and all theories we've had before knew that something was going to happen 1:20:29 at Short distances and that's what people expected but to the contrary asymptotic Freedom 1:20:36 means becomes simpler and simpler perturbation Theory becomes more and 1:20:41 more exact an immediate application of this 1:20:47 was that for a whole community of people so at that time was seriously sort of 10 years after CMB 1:20:56 tried to study early cosmology the early universe could be simple 1:21:04 also we had no problem extrapolating to to higher energies where the forces 1:21:10 could unify and finally a large number 1:21:18 so the implications of this 1:21:25 for the early Universe were really important because 1:21:30 if you imagine having to think about cosmology using nuclear physics oh my 1:21:36 God hadron Pals to Cork soup 1:21:41 pretty soon as understood that you've got a high enough densities in and temperatures 1:21:47 nuclear Quark protons had runs become settler they melt you have core grown 1:21:54 plasma we thought would be missed the cork glue on liquid that has 1:22:02 been discovered at Rickman but anyway that's all understandable perturbatively 1:22:09 more or less and so there was no obstacle 1:22:15 to extrapolating from CMB back to 1:22:21 as we do now through very very early times 1:22:27 their phase transitions along the way and this is another fascinating area of qcd which is 1:22:35 very different crowd than the people suited here but there are literally hundreds of theorists and experimenters 1:22:43 and experiments 1:22:48 and then of course the ability to to extrapolate to high Energies 1:22:54 uh LED very quickly after the standard 1:23:00 model was finalized to discovering that unification might very 1:23:06 well occur at extremely high energies these logarithms one very slowly 1:23:12 and that of course is the most important clue we have 1:23:18 to what happens near the plant grant them how the forces might unify together 1:23:24 with gravity and dominated speculative physics for the 1:23:30 last 40 years but also to some extent one of the very 1:23:35 large numbers that govern the structure of the universe that tell us why 1:23:40 why gravity is so weak at our scale of energies and why we are not black holes 1:23:46 is the Dirac large number problem one of his large numbers 1:23:52 which he said look nobody's ever going to calculate 10 to the minus 19. it's impossible 1:23:58 can't imagine a theory which could Calculate 10 to the minus 19. 1:24:04 and what he did was try to equative 1:24:10 invoked anthropic arguments because you know if this number would be 10 to the minus a thousand we would be 1:24:17 black holes right life couldn't exist right 1:24:23 gravity would be much stronger only a thousand nuclei would have 1:24:32 collapsed no black hole but the Iraq 1:24:37 like an asterox biography did he know about anthropic arguments 1:24:43 people were speculated using that those arguments already in the 19th century 1:24:49 instead he was better he was direct he uh he related this small number 1:24:57 two other small numbers the size of an atom compared to the size of the universe 1:25:03 some of them are time dependent so he said I can test this idea because then 1:25:10 some of the fundamental units that the Planck mass or so on would vary with time like the size 1:25:18 of the universe and that could be tested and has been tested and so far no 1:25:25 indication of because of a law cosmological variation of these fundamental constants 1:25:30 but qcd can calculate this number more or less 1:25:37 or at least you know from uh great qualitative point of view simply 1:25:44 it's the same picture always if at the unification scale which is 1:25:50 around the plank Mass the coupling is you know of order of our extrapolation of the 1:25:57 electromagnetic coupling and then you ask where does the proton 1:26:03 Mass arise from well it arises from the scale where the coupling becomes strong enough to confine these masses 1:26:10 corks and bronze that is given by the same kind of formula 1:26:15 and that from what we actually know is about 10 1:26:20 to the minus 19 19 plus or minus 2 which is good enough 1:26:27 so that we understand that large number 1:26:34 so it is kind of a perfect Theory it's a first example of a complete Theory 1:26:40 no adjustable parameters no indication of where it would break down 1:26:46 infinite bandwidth if you will but of course it's not the real world 1:26:54 luckily there are many questions we don't 1:26:59 understand and then of course we know there are other forces in nature the rest of the standard model and of 1:27:05 course gravity now I want to end with discussing 1:27:11 the other aspect of qcd which is remarkable and probably 1:27:16 a thousand years from now will be regarded as the most important thing 1:27:22 that we've learned which is the relation to string theory 1:27:27 which developed as a theory of restoring interactions at around the same time 1:27:34 motivated by experiment by the fact that Reggie trajectories spinning strings 1:27:40 were linear dualities between 1:27:45 one expressing physics in terms of particles in one channel and by crossing the other 1:27:53 channel etc etc so so much 1:27:59 and then of course nowadays by a theoretical experiments where we see 1:28:05 the fat strings that appear in lattice UCD and then of course by 1:28:14 um what we now have is in special cases 1:28:20 the beginnings of a precise Duality between our description of mesons as 1:28:28 either confining flux tubes or open strings in a fundamental string 1:28:35 theory and our understanding that uh gauge 1:28:42 Theory and string theory are not really different kinds of theories they're just different often different ways of 1:28:48 looking at the same phenomena or certainly part of the same conceptual framework 1:28:56 as was the understood early in pushing fundamental String Theory once 1:29:03 you have open strings mesons you have closed strings qcd you have blue balls 1:29:12 and that is the basis for a lot of the approaches you'll hear about in trying to 1:29:19 hopefully come up with a controllable 1:29:25 are analytic control of qcd 1:29:31 large nqcd is clearly an interesting limit in which 1:29:40 uh we understand that the theory must consist of 1:29:49 infinitely narrow stable blue balls I'm now ignoring the quarks 1:29:58 non-attractive theory of blue balls which is if you want the free field 1:30:04 limit of some Theory whose classical limit is 1:30:12 and goes to Infinity straight PCD 1 over n is kind of 1:30:19 the Quantum of action 1:30:25 might be a string theory might be something else but there is a classical limit which is the of something 1:30:33 looks a lot has to have an infinite number of glue balls and many other features which make sense 1:30:41 in string theory and it's probably a string theory although not the a string 1:30:47 theory we have yet constructed although much progress has been made towards guessing 1:30:54 what that theory might be in adscft of course gives us enormous guidance 1:31:02 the evidence of courses the structure of the perturbative 1:31:07 structure of qcd and Matrix theory in general one over n 1:31:13 one over one third phenomologically works very well much better than you might expect in 1:31:20 comparing prediction one of our own predictions of qcd with uh 1:31:26 with nature and of course the strongest argument currently is edsc FD 1:31:34 and you all know what that is so the Hope really is you know one hope 1:31:41 would be solve n equals four supersymmetric qcd that's a scale 1:31:47 invariant Theory it's almost solved 1:31:53 this perturbatively construct the dual string theory 1:32:01 well that's hard um 1:32:06 but not well that's hard 1:32:12 I'm running out of time okay that's not qcd it has super 1:32:18 symmetry which is not a future qcd um but you can easily break super Symmetry 1:32:25 by you know uh 1:32:30 pushing the masses of the extra particles for Infinity 1:32:36 but to form it then equals one or even real qcd in principle 1:32:41 you have to include quarks and interactions those can be treated 1:32:46 systematically in a 1 over NC expansion is a conceivable Direction 1:32:59 so let me just make a few remarks about the future 1:33:06 qcd has much more life than I ever expected the structures and of two-dimension of 1:33:16 perturbative diagnose Theory it's quite remarkable hints at something deep 1:33:23 but so far hasn't produced it but those calculational methods for 1:33:31 perturbative gauge theories are incredibly useful for experimenter and for theorists so that will continue 1:33:38 for sure numerical methods have reached a true level of maturity and uh they should be 1:33:47 available to people who are more interested in theoretical questions one of the purposes this collaboration is to 1:33:53 try to develop that connection because you now have a tool which has been improved by a factor of 10 to the 18 we 1:34:00 could take use of them make use of it better and of course there are new technologies 1:34:05 that will come along there are many many questions in qcd 1:34:11 that are great interest and Open Door experiments you can make predictions mostly having to do with time dependent 1:34:18 phenomena which so far a lot discussable by lattice cage Theory 1:34:24 oh and that experiment is still going and 1:34:30 although they're not discovering supersymmetry yet or new particles they are 1:34:36 measuring the properties of hadrons at LHC at 1:34:44 Rick at the electron ion collider which is a 1:34:50 colliding electron proton beam um in the next 10 you know over the next 1:34:57 decades all right see lhcb Alice the EIC the electronic equator there'll 1:35:05 be a lot of great experiments and you could be in the lucky position that my generation was in to actually in this 1:35:12 field make prediction and have them tested 1:35:19 it's a lot of pleasure coming from solving problems discovering new things in physics but believe me there's 1:35:25 nothing more satisfying than making predictions that experimenters put to 1:35:31 the test of nature and nature says so far it works 1:35:37 so far it worked never says yes it says maybe but with a smile 1:35:46 so I tried to summarize what have we learned fireman was once said you know 1:35:54 I love this in some imagine that today he would have said AI takes over 1:36:02 what statement would could we pass on to the Cockroaches or to the robots 1:36:09 that would contain the most information about physical world in the fewest words 1:36:15 and uh it's not an easy task one sentence all things are made of atoms little 1:36:22 particles that move around a perpetual motion attracting each other when they are a little distance apart but 1:36:28 repelling upon being squeezed into one another 1:36:34 I think I really I would like to see somebody do better 1:36:39 in one sentence no equation what have we learned from the standard 1:36:46 model well takes a whole slide matter is made out of 1:36:52 in one half fermions forces describe our Quantum gauge field three phases coulomb 1:37:00 screened or Higgs and a natural scalar fuel sector and 1:37:07 a confined strong phase with su3 color gay true 1:37:12 that's a one slide see if you can do better after 50 years what have we learned from 1:37:19 qcd well most important it is the theory of the 1:37:25 nuclear force Dynamics can determine all masses and 1:37:31 couples shouldn't be satisfied at this stage of any Theory it goes beyond 1:37:38 the well that even gener you know improves on the standard 1:37:45 models so all masses and couplings are calculable 1:37:50 but most importantly I think it's gate string duality 1:37:58 um the real hero is not a really engaged Theory page Theory 1:38:05 and it is the basic qcd and the rest of the standard model most of it 1:38:11 and of string theory or dual to it 1:38:16 and therefore of space-time and gravity that's the real hero of the story 1:38:23 but meanwhile I've come to the end 1:38:28 but not the end of the qcd Institute for Advanced Study 144K subscribers 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