The Future of Particle Physics - David Gross - 5/12/2018

good morning everyone so my name is Clifford Chung I'm an assistant professor vertical physics here at Caltech it's great it's my great pleasure to introduce the first spec first speaker of the morning session david gross David is the Chancellor's chair professor of theoretical physics at the University of California in Santa Barbara and also the former director of the esteemed Cavalier Institute for Theoretical Physics so David is renowned in the field for numerous seminal contributions to elementary particle physics and also a string theory his work on the strong interactions also known as quantum chromodynamics is now a cornerstone of the modern standard of standard model of physics which describes the world for this work he was awarded a Nobel Prize which she shared with Frank we'll check and our own David Pulitzer here at Caltech he's also was also an early pioneer and discoverer of string theory and remains to this day an active leader at the forefront of the most recent developments in theoretical physics so please join me in welcoming David who will tell us about the future of particle physics so it's really a great pleasure and honor to be here at celebrating 5mins 100th birthday yesterday I'm sure you were all there last night was absolutely spectacular gratulations to the organizers so wonderful evening I don't think there's any way we can surpass that today or I feel rather humble Fineman was an amazing physicist an amazing teacher he inspired many people from all over not necessarily physics but for physics of my generation he was he was a model in many ways and an inspiration he was of course a as many have said a magician a magical physicist of the highest caliber whose contributions of science are truly amazing and and beautiful and fun he was a great performer it was a celebrity physicist a great communicator an inspired teacher but for many of us he sort of he taught us many lessons about how to approach science and how to think about the real world these messages that were shown last night on his blackboard when I cannot create I do not understand and you really have to and he tried I guess to solve every problem that to learn how to solve every problem that has ever been solved are really inspirational I don't think any of us can really live up to that but it is an inspiration but there's another thing which he sort of taught my generation which is very valuable which is a very healthy disrespect for authority and a total aversion to and it really changed the way theoretical physicists behave I think in a profound way everyone to some extent tried to emulate fireman's joy and passion for physics the disrespect Authority which is especially pleasant when you're young and and an aversion which would be very healthy in today's room you also taught us how to visualize physics he taught us with his famous fireman diagrams how to think about how to see how to feel what elementary particles are doing as described by quantum field theory I remember when I first studied quantum field theory to the course from Steve Weinberg he went to the class wrote on the blackboard field theory equals final diagrams and for a long time and still today even though it's only really a perturbative approach a weak coupling approach to quantum field theory it dominates the way we think about particles and forces and how to calculate women love the calculate his most important contribution in my view was the reformulations of quantum mechanics his thesis or came out of his thesis project to picture to describe quantum mechanical amplitudes whose squares give probabilities of say well how you calculate the probability of a particle as at Point a at time 0 it'll be at point B at time T by summing over all paths from A to B weighted by a phase factor which the mathematical formula is given here but but thinking about quantum mechanics as a sum over histories gave us both a very important calculational tool a formulation that led to many many applications and new methods in quantum field theory and quantum mechanics but also totally different and geometrical and deep way of thinking about quantum mechanics so this year indeed is the hundredth birthday of Richard firemen we celebrate occasions like this which have zeros it's also you know remarkably the fiftieth by some Counting birth of the standard model of particle physics to which why men made important contributions a standard model you know could be dated could be fifty-one years could be 45 years old any case it's close to 50 and they're going to be many celebrations of the standard model over the next few years so I thought it would be appropriate to in this very brief talk discuss the future of particle physics which although you know finally was interested in everything and one of the important things he taught us was that everything in nature is fascinating especially if you really get into the details and try to understand how it works and calculate about his perhaps his primary interest was the physics of elementary particles and how they interact and the standard model which is now 50 years old half of hymens life is our current theory of elementary particles which have been identified the last hundred years as quarks and leptons electrons and and two kinds of up and down quarks inside ordinary nuclei and atoms and two other families three families of quarks and leptons and more importantly we understand in great detail the forces that act on these particles the force of electromagnetism whose carrier is the electromagnetic field or the particle of light proton the strong nuclear force that is carried by gluons and acts on quarks and binds them permanently within nucleons and nuclei and finally the weak nuclear force that acts on quarks and leptons turning one kind of quark into another for an electron into a neutrino together with a Higgs field which has been added to this framework to break the electroweak symmetry we have the essence of the standard model to which Fineman made very important contributions of course the theory the quantized theory of electromagnetism quantum electrodynamics which he and Schwinger tominaga Dyson was the first formulation of a quantum field theory to agree to the experiment in gave the whole file the foundation for our basic tool in fundamental physics quantum field theory he also made major contributions to the electrode to the weak study of the weak force in particular with very Galman understanding the chiral or left-handed nature of the weak force this is an amazingly successful theory it as far as we could tell as far as we can tell by extrapolating could work down to extraordinarily short distances the famous Planck scale and as far as we can tell it works out to the edge of the universe explaining the structure of nuclei atoms molecules stars galaxies you of course have to add to this these three fundamental forces Einstein's theory of classical theory of gravity this is much more than a model it's really a theory you can tell it's a theory because you can put the equations or the principle this case the action that determines how to weight different paths of quantum fields on one t-shirt and if you add in Stein's contribution you have a rather complete with a few parameters description as far as we can tell of just about everything that we've ever measured and continue to measure and in a absurdly reductionist sense you start with the equations that follow from this action with a lot of work and powerful computers we believe you could calculate the results just about just about everything not everything but just about everything that we have observed or continue to measure unbelievably successful I hope very much that at the 50th anniversaries we will call a star colonists the standard theory it's by far the best theory physics has ever had now I could go on for the rest of the talk describing how well it works I'll just give a few slides Fineman when the standard model emerged in the 1970s Fryman leptin he made important contributions to its development but when it but he was obsessed with calculating cross-sections like this using originally very simple ideas but then QCD and I think he would have been astonished and pleased at this incredible agreement with experiment over you know twenty four orders of magnitude of cross-section this requires exclude the kind of exquisite control that Filan always sought to be able from the theory to calculate with high precision and of course it's a an amazing experimental triumph to be able to to measure these cross-sections with such incredible accuracy one more is quite amazing these are cross-sections for jet production and the collision of protons at energies up to a TV you see here at the LHC Fineman once gave a beautiful summary of physics he wondered if in some Cataclysm all of scientific knowledge was destroyed in one sentence passed on to the next generation of creatures cockroaches who know what statement would contain the most information in the fewest words and he said well that's probably the atomic hypothesis or fact whatever you want to call it that all things are made of atoms little particles and move around in perpetual motion attracting each other when they're close but repelling each other when squeezed into one another and you know a lot of physics and a fire fireman could explain a hell of a lot starting from this sentence so I tried to I wonder what whether we could boil the standard theory down to one sentence I found that impossible but I do think we one slide we can summarize what we've learned from the standard model so manner consists of charged spin 1/2 particles those are the quarks and leptons the forces are described by quantum field theory based on local symmetries of nature gauge theories and there are three kinds of charges there are particles like some of the quarks and leptons that have electric charge they're described by electromagnetism a u 1 gauge theory and that force exhibits a what we call a Coulomb phase long-range interactions forces that fall off like the inverse square of the distance then there are some of the particles the quarks actually all of the particles quarks and leptons have weak charges we call them flavor - charges now and their forces acting on those charges are described by a non abelian and su to gauge theory and what we see in nature is that that force is screened extremely screened or Higgs and we achieve that somewhat unfortunately by a rather unnatural scalar field sector introduced to screen that charge resulting in a very short range interaction and finally the quarks have three other charges called colors and that those forces are described by a su 3 gauge theory but it shows another phase in which the forces are anti scream they get bigger and bigger as you pull the quarks apart and thereby the color charges are permanently confined inside nutri inside neutral nucleons that's a that's a standard model and I have a feeling took a long time to understand this sorry took a long time to understand this partly because some of these phases are so different than electromagnetism and the charges especially these strong charges are confined and therefore not apparent but being presented with such a slide I'm sure fine in 1950 could have worked out the standard theory in a few weeks among these elements the standard model there's my favorite QCD which aside from describing the nuclear physics with and behavior of quarks with great precision it gives us an example of what we should ultimately aim for I call a perfect theory because if we ignore the court which makes life easier or set their masses to zero which changes very little in nuclear physics 1 there are no infinities in any case but also there are no adjustable parameters k CD without quarks or massless quarks everything is calculon's it's numbers and there's no new physics no matter how high energy or short distance you go how is that well in quantum chromodynamics you go to very short distances the coupling vanishes that's has some thought of freedom and the only way to ever find infinities the kind of infinities that plagued the inventors of quantum field theory the kind of infinities that were dealt with by Fineman and his friends formulating QED through normalization theory don't appear unless you Express observables as finite distances in terms of those measured infinitely small distances but if you write down the answer fineman's path integral for QCD this is actually how one does numerical calculations in QCD no infinities ever appear you put the theory on the lattice in terms of a coupling which the theory tells you how to adjust as you let the lattice spacing go to zero to get a continuum theory and at no stage do any infinities ever occur second of all there are no adjustable parameters if you want to calculate your mass the mass of the protons and make you up it arises not from the court passes which might be zero but rather from the confined kinetic energy of the quarks and of the gluon field that holds them together the scale which determines the amount of energy is simply determined by the size at which the strong force becomes strong and therefore in this theory all masses which are confined energy are calculable all mass ratios are calculated by now fifty years later with powerful computers lattice gauge theorists can calculate the mass spectrum of hadrons with extraordinary accuracy we can now calculate the mass difference of the neutron and the proton the binding energy starting from PCD of the deuteron and the masses of all these hadrons if you go to arbitrarily high energies nothing happens in QCD the coupling vanishes asymptotically and this actually explains why gravity is so weak compared to this size of nuclei or explains why the proton mass is so light compared to the Planck mass which is the energy scale where gravity becomes strong because as you extrapolate you get to the point where we believe expect that all the forces are unified if you just assume at this point the for the nuclear force is of order you know some typical small parameter like the electromagnetic force then it turns out that the ratio of the proton mass to the Planck mass is exponentially small the inverse of the coupling and that naturally gives the right order of magnitude into a factor of a hundred or ten or so in other words the reason gravity we are so big compared to the fundamental scale and also why gravity is so weak at the scale of atoms and nuclei is that there they might have the same equally strong at the basic fundamental scale but as you go down to larger distances or lower energies the scale of the nuclear force increases algorithmically whereas the scale of the gravitational force decreases like the energy squared or one over the distance squared now the standard model isn't the answer if 5-minute was around he wouldn't be you'd be working probably on the questions that drive us to to physics beyond the standard model and there are a whole bunch of issues the most important of which are driven by experiment the existence of dark matter is observed by astronomers neutrino masses that have been measured but not totally understood they're mixing which might explain the baryon asymmetry of the universe why there's matter rather than antimatter and the cosmic acceleration which we attribute to the vacuum energy and from a theoretical point of view we have other motivations which are wide questions for example unification of the forces which is called out for in the standard model other hierarchies of scales not just the strong scale but the electroweak scale the nature of matter itself and the masses that are are generated or not understood and of course the a problem that fireman 2 was obsessed with as we'll hear in the next talk of trying to understand the quantum nature of gravity quantized gravity and apply our understanding of gravity to cosmology inflation and the cosmological constant what one of these is the if I had to choose one that I would like to understand it's probably unification how the three forces that we see in the standard Theory like traumatic ism and the weak and strong nuclear forces are unified and we strongly suspect that they must be partly because the ingredients that of the standard model which are these three forces three kinds of charges one two and three and matter quarks and leptons fit together and this was understood 45 years ago almost 40 years ago put together very neatly into a like pieces of a jigsaw puzzle and over the last forty years although in intensive searches for new kinds of particles or new forces that would destroy this simple Union I haven't been discovered this so it need not be the case but they do fit together as if there's a deeper you know they are really one force that looks different at low energy because of a mechanism which we well understand in many parts of physics breaks the symmetry breaks this force into three pieces and because the forces vary with distance it's perfectly possible that they have the same strength the same nature at very high energy and in fact already forty years ago people extrapolated the standard model to very high energy and found that the forces do come together and unify just around the famous Planck scale scale we believe is the fundamental scale of physics defined in terms of the fundamental dimension for parameters of physics Planck's constant the speed of light and Newton's constant and that's the point where gravity becomes strong as well so one of the big motivations after found this formulation in this standard model is to take this clue very seriously and regarded as an indication that all the forces unify with gravity as extraordinarily this is shouldn't be TV this should be TV luckily but a very short distance of 10 to the minus 33 centimeters that could be a coincidence you'd never know in this business and many of my colleagues or some my colleagues are willing to give up this clue if they have some idea mostly it's as far as I can see it's not worth worth giving up this clue yeah so what is the state of particle physics well my opinion the bottleneck particle physics is experimental not theoretical easy for me to say why it's that Planck scale that scale fundamental scale identified by Planck we theorists have no problem extrapolating as I just showed to the Planck scale and that's because physics scales as you vary the energy in some sense like the log of the energy so nothing much happens as you scale to these extraordinary high energies and we do it all the time and nothing bad happens maybe something happens you get to the Planck scale because gravity becomes a strong quantum force but so we do that and it's not a large extent we've been focused on ideas that come from string theory but experimentally to build an accelerator to probe those very high energies or short distances the difficulties end the cost scale at least like the square of the energy and experimentalists cannot easily extrapolate even by one order of magnitude so that's what I call well that's a fact of nature and fireman's dressed facts are important can't do anything about it we have to learn to live with it there really is bothersome that's what I call the curse of logarithms again physics scales physics here is represented by s the action scales like the log of energy and on this scale going from the atom to understanding the nucleus probing the electoral weak forces within the next Collider we might build soon is quite a distance in in energy but the scale of gravity since things are only very logarithmic is only another step like this not so bad and if this was possible experimentally to probe I have no doubt that we'd finish the game in less than a century but problem is that the cost goes like e to the S to s and on this scale you know Fermilab is here the LHC is here and the planck accelerator is about 500 miles down the road unimaginable well that's a real it's a fact of nature and a deep problem for particle physics for fundamental physics oh the future well they're always two options I tend to be optimistic you can be pessimistic and the optimistic sin they're a pessimistic scenario is roughly what's happening today unfortunately we don't see any anything rather new or unexpected in the behavior of these Higgs particles recently discovered we don't see the supersymmetric particles we hope to see we don't see Dark Matter anywhere except gravitationally no indication of the next threshold could be very high what to do bill the next accelerator obviously we can it's not that expensive and we must do that what else the optimistic scenario is that we'll start seeing at the LHC or elsewhere deviations from the standard model will produce supersymmetric particles say and will detect dark matter this is perhaps the most immediate challenge because it's definitely there and we'll have guidance for the next steps and there are many proposed next steps there's an extension of the LHC or the great Collider project in China which they can certainly afford to do and to have the abyssum ambition but we also need new technology i mean if you really extrapolate to these very high energies it looks hopeless on the other hand one thing i learned from history is that it's impossible to predict technology and by mental advances who would have thought a hundred years ago that we could see and manipulate atoms and build materials atom by atom there are ideas very little money is being devoted to this very little effort needs to be done what about theorists however they have an easier task and perhaps they should follow Einstein's encouragement and warning Einstein said the successful attempt to derive delicate laws of nature along a purely mental path by following a belief in the form of the unity of the structure reality encourages continuations in this speculative direction the dangers of which everyone who vividly finds it follows it must keep in sight well we all know Einstein tried for most of his life after his great successes and failed but we have no choice to pursue these clothes and to explore theoretically and that's what theorists have been doing mostly using string theory ideas string theory developed also 50 years ago this year's of the 50th anniversary of string theory and all theory arising from probably in the properties of horks found in flux toads which looked like strings and understanding that the gauge theories that describe confined quarks look a lot like open string strings with quarks at the end what was amazing in string theory was that when these strings were closed as they had to be they described the quanta of gravity marathon's and that string theory automatically and surprisingly unified the gauge forces that are the basis of the standard theory with gravity so it's still a possibility that in some sense the particles we see and the forces arise from super string theory Fineman by the way didn't like this although Caltech was a center of string theory fine and was suspicious because those string theorists weren't calculating anything that he cared about and to quote him I feel strongly at this is nonsense so I can entertain future historians or celebrators and his birthday by saying I think although superstring stuff is crazy and it's in the wrong direction I don't like it they're not calculating anything what he was interested likely or what are the masses of the elementary particles the quarks all these numbers in the standard model and these string theorists are so arrogant yeah this unified theory they don't calculate anything well but he knew as well as anyone and I think Hiroshi will explain that there were in addition to calculating all those features of the standard model which we still can't do in string theory there are fundamental issues in relativistic quantum mechanics of space-time that need what must be addressed the quantum theory of gravity is problematic when you explored very short distances it seemed that space-time itself fluctuating so dramatically that it made no sense what we call space-time foam and that you'd probably have to modify Einstein's theory in some way situations string theory is that we stolen exactly know what string theory is but in the last decades we've discovered my colleagues have discovered a fantastic duality a relationship between quantum field theory the basis of the standard model in string theory which is related to the duality or the connection in string theory between open strings with ends and closed strings and that insight which is called duality or more technically a DSC of T one of its manifestations relates quantum mechanical ordinary quantum mechanical systems that are for example the standard model to theory string theories in higher dimensional spaces and the famous example of the duality is between quantum field theory and four-dimensional flat space with no gravity and string theory in ten dimensions or five dimensional hyperbolic space and xxx sitter space so what really has happened dominated particle theory or the goal to unify all the forces and understand quantum gravity in the last twenty years is the exploration of the theory and the connection between quantum field theory and string theory and it's had an enormous by-product it's given us new insights into the gauge theories of the standard model and to QCD and its various properties but also into condensed matter physics strongly correlated electron systems and beginning to resolve some of the puzzles of quantum gravity black holes and it strongly suggests new concepts of space and time leading many of us to believe space-time is best thought of as an emergent concept gravity as well so where are we well we have this incredible framework in which the standard model traditionally was formulated standard model is a quantum field theory of matter and then we have this string theory which was started out as a theory of quantized extended objects and we've learned that they're the same thing different picture different versions or ways of looking at the same physics and that the framework of theoretical physics at the bottom is we have no idea it contains at least strings field and who knows what and for decades now we've been exploring theoretically exploring this framework it has a solid foundation of course in the end connection to experiment which we can use and apply the standard theory the standard model but we still have absolutely no idea what picks out the particular theory from the framework of all possible quantum field theories that's what Fineman wanted to know what fixes those parameters in the standard model and we still have no idea but at the stage we're at and the example of juicy D is a perfect theory we're asking a lot no fireman was asking a lot what fixes the particular dynamics what picks the stay there model out of this whole framework we're also asking what fixes the initial and final state of the universe in fact I have a suspicion that these are related so in particle physics we'd like to unify the forces and we'd like to understand why all those parameters are what they are the forces are there and so on uniqueness of a particular theory out of the enormous framework but I suspect that that might be that question its answer might be related to another question we're beginning to ask which is how did the universe begin maybe how it ended because now we are cosmologists are faced with that problem and as theoretical physicists we too are being forced to countably questions like how did the universe begin and this is natural because Einstein taught us that gravity is the dynamics of space-time so the universe is that history of space-time which includes the beginning and the end and here again this is a very outrageous goal but it is now being addressed scientifically and any solution to the unification of all the forces with gravity among the rest to give a consistent history of cosmology consistent history the universe including the beginning in the end now all of our multiple candidates for theories in the framework of strings last field theory but so far are breakdown we go to the beginning of time beginning of the universe so these problems might very well be related but they're extraordinarily difficult and they're made even more difficult because our most fundamental concept out of space-time is itself being threatened as we probe the quantum nature of dynamical space-time of quantum gravity so that's where we are we are asking very difficult questions we're not yet at the point where we can calculate and we might not be until we begin to understand the framework this enormous framework of strings and fields that might give us a consistent history of the universe and pick out a unique dynamics from this marvelous primer so we have a wonderful theory of elementary particles playmon would have happy to see and celebrate its successes but the most important questions remain to be answered we do have fantastic experiments and instruments and even more fantastic speculations so the best is yet to come thank you [Applause] so thank you David for that beautiful I think we have time for just maybe one or two questions I think there are some microphones set up if people would like to come to ask but we're running a little bit late but I think it could maybe handle warming - hmm takers I think there's microphones if you can line up I should say there's microphones on either side of this yes please so one of the hot topics in public press these days about physics like the question of time the number of books have come out about it including some from Sean Carroll and here at Cal Tech what are your thoughts about time and and you know what do you think about that subject oh boy I could I could talk for a long time about that about time it so we many of us are convinced that space-time unified is an emergent concept that of course is really difficult to imagine for time so how could you formulate physics without time time has many many mysteries some we understand roughly like the arrow of time but why it's correlates with the arrow of the cosmological expansion is a mystery that goes back to the initial conditions which we I think are beginning to have to address there are many features of time that are truly mysterious like the concept of now physicists know that there's no such thing as now and in the equations we used to describe physical reality and yet we all feel that there's a now it's just past and this Ellen the past thing I know now is coming very shortly from the future and we're moving and but this doesn't is not in accord with our equations or our theories so it's an illusion how that works out it's partly a problem of understanding the human mind but also the physical universe so time is the most interesting of all of our physical concept and as many there are many mysteries the nice thing about it although you know this is that in addition to a lot of the philosophizing in books you're talking about we're now and some of us are able with the help of these very specific understanding of this framework of strings and fields are able to begin to address some of those issues the way firemen like to do it with calculations and equations and not just words but it's the most mysterious of all of our concepts you you

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Length: 49min 51sec (2991 seconds)

Published: Tue May 22 2018