David J. Gross (2015) - The Future of Particle Physics

Every year is a celebration of many anniversaries. There are three wonderful celebrations that we celebrate this year, in a way. Of course, one is 100 years of general relativity. of space and time and changed fundamental physics forever. And we're celebrating this centenary year here, although I must say this occasion has not been mentioned in Lindau until now. It's also 90 years of quantum mechanics which sort of came to its complete form in 1925. Perhaps the greatest conceptual revolution of the 20th century and one that informs all of microscopic, atomic, subatomic, fundamental physics. And finally in a sense, it is the 40th anniversary of the completion of the standard model of elementary particle physics. The comprehensive theory that was developed in mostly the 20th century that gave us a rather complete understanding of the basic constituents of matter and the forces that act on them. OK. Well, this time does not count against my meagre allotment here. Now let me see, let me unplug your monitor. Is that the problem? Which I don't like because then I can't use my thing. It's not suitable but that's, that's fine, we'll get that. This is irrelevant. So as I was saying, 2015 is a celebration of many things, perhaps the most amazing important in the centenary of general relativity, but also 90 years of quantum mechanics and 40 years of the standard model of elementary particles. I want to say a bit about Einstein because this is perhaps the only scientific meeting in the world this year that hasn't mentioned or celebrated Albert Einstein's remarkable contribution which occurred at the end of 1915. In one week, he published four papers. At that time, he could submit a paper to the Prussian Academy and get it published within two days. And in the final paper, he wrote down in final and complete form so-called Einstein's equations that describe the curvature of space-time sourced by matter, which is the source of energy and momentum of matter that curves space-time and gives rise to what we otherwise call gravity. Einstein left a legacy that is unmatched since perhaps Newton, one that will persist for generations. Beyond the specific form of the equations that describe gravity at its core, there are really three contributions that will live on way beyond his specific equations which will be superseded. First, he finally realized his dream of making space-time into a dynamical, physical object. Not an inert frame that is set down in some Kantian fashion, but rather a dynamical, physical object whose metric is subject to variation. It responds to the presence of energy and matter, curves, and that then affects matter. And we still are struggling in trying to understand what it means especially in quantum mechanics to have space and time itself be a dynamical, fluctuating entity. He also made possible physical cosmology. Before Einstein, cosmology was the domain of religion and theology and philosophy. After Einstein, it became physics, and immediately following his theory people and began to construct models of the universe and that has made possible 100 years of amazing developments in astronomy, astrophysics and cosmology. And finally, he dreamed and motivated generations of physicists after him to be as ambitious as he was and to try to unify the forces of nature and get to the core of physical reality. The first point, space-time be dynamical, was a major change in our notion of what space and time is and the dynamics of course is that space and time responds to energy and matter, curves, then gives rise to gravity. So it's the curvature of space-time that causes the earth to rotate around the sun and Einstein's equations describe how you understand this quantitatively. It also predicted strange new objects in the universe that might occur when there's so much matter that it curves space and time so dramatically, creating regions from which light cannot escape, otherwise known as black holes. Black holes were appeared months after Einstein's equations were put down by Karl Schwarzschild, a brilliant theoretical physicist who died a few years later on the western front. We're celebrating Einstein's equations which occurred, were written down during World War I. But Einstein never believed in the existence of such crazy objects and most of physicists were quite suspicious, although now we know that they are abundant throughout the galaxy. Indeed, throughout the universe. Indeed, at the centre of every galaxy. As far as we can tell, there is a big black hole, including our own, here it is, and we can measure the orbits of stars around this black hole and confirm that in the middle here, which you can't really see, there's a very small region which is entirely black and contains a mass of a million suns and is responsible for these orbits. Black holes also appear in other places in the galaxy. They form when stars collapse, supernova, leaving behind black holes which power through these accretion discs, these ultra-relativistic jets we see as quasars or gamma ray bursts. And they are the subject of continual theoretical experiments since their properties are so weird. So for example, we still struggle with the following paradox: One takes a well-defined quantum-mechanical system, say here consistently you have two particles that are correlated, as we would say, in quantum mechanics, they're entangled, and then we drop one of them into the black hole. It can never be in communication with us again according to the classical laws of general relativity. We don't know what state it's in, therefore we've lost information and can't predict a final state. Naively, information is lost and over 45 years ago, Hawking, around 40 years ago, Hawking posed a paradox in describing the quantum-mechanical properties of black holes, and suggested that this phenomena is information loss at the basic level and that indeed the quantum-mechanics in general relativity were somewhat inconsistent. He advocated giving up some aspects of quantum mechanics such as the preservation of information. People like me and others who came from particle physics community believed that we'll have to change and modify Einstein's theory as he always suspected, and we'll keep quantum mechanics. I think our side won, in fact Hawking admitted as much and paid off some debts a few years ago. Physical cosmology is the other great achievement of the Einsteinian framework. Before Einstein, we knew nothing and understood nothing, first approximation about the universe. There were all these bright things there, stars, we thought the Milky Way was the whole universe, we thought it was static, unchanging, we didn't know what stars were, anything. But after Einstein one could construct mathematical models of the history of the universe, that's about now a physical question and over that 100 years we have mapped out that history in extraordinary detail. We understand the 17.7 billion years of expansion. First rapid, then slowed down and now accelerated. The formation of structure from the hot Big Bang that is seen about 400 million years after the beginning. And finally, unification and one of the most astounding demonstrations of our unified, or partially-unified theory of elementary particles which is devoted to discovering, observing and understanding the basic building blocks of matter and the forces that act on them. We've made extraordinary progress in roughly 75 years. I'm almost 75 years old so in one lifetime, we've gone from no elementary particle and no understanding of the forces acting on them, except for electricity and magnetism, to a rather complete theory, quite remarkable. In a sense, experimental elementary particles began with Rutherford's discovery of the nucleon, the nucleus of atoms. He wanted to understand what goes on inside an atom so he invented a technique which we still use today, he bombarded gold foil, gold nuclei, gold atoms with alpha particles which were omitted nuclei of helium from radioactive substances and his students observed little dots of light on fluorescent screens and could measure the deviation of particles scattering off the gold nucleus. He deduced from this using theory. Rutherford was a good theorist too, he knew electromagnetism, he assuming the force between these alpha particles in the nucleus was electromagnetic, he could determine the size of the locus of the positive charge of atoms and their mass. He came to the conclusion that in the centre of atoms, 100,000 times smaller than the size of the atom was where all the mass, most of the mass, and all of the positive charge was located. That was the discovery of the nucleus of atoms. And of course in the last, since then for over 100 years, we have been exploring experimentally and constructing theories of what goes on inside the nucleus of atoms. But using exactly, conceptually the same idea: if you want to discover what something like this is made of, you take something else like this, you smash it together, see what comes out, and try to figure out what's going on. Make new particles, try to figure out the laws. We of course use much bigger accelerators than Rutherford had available. This is LHC. at CERN; this is CERN airport as you know. That's a massive 20 kilometre accelerator which accelerates protons, smashing protons at around a trillion, a million million electron volts. And then we detect the pictures of what comes out with these massive detectors and try to figure out, find one event out of a hundred-billion that might be some indication of new physics. Well, developing this theory, the standard model, it's called, has been recognized by Nobel prizes, many Nobel prizes. I did the exercise of counting how many and there are 52 Nobel Laureates who've contributed to the development of the standard model. And I'm leaving out, by the way, people like Einstein, Dirac, Heisenberg, Schrödinger, who created the theory of quantum mechanics, the framework in which of course the standard model is embedded. So even leaving them out, there are 52 laureates spread over 30 Nobel Prizes in the last 75 years. Also interesting, 20 of these prizes are for experiment, 10 for theory. So the lesson is if you want to get a Nobel Prize, experiment is the way. The other hand, in particle physics, if you want to do experimental high-energy physics, you have to join a group often consisting of thousands and the Nobel Prize limits to three, so it's a bit of a problem nowadays. Out of these 52 laureates, four are present here in Lindau. Why so few? Good question. This is an illustration of the standard model of elementary particle physics. It's the list and properties of basic constituents of matter, quarks, leptons, here you see the electron, the neutrino, the up and down quarks that make up the nuclei in our body, two other families, and three forces that act within the atom in the nucleus. Very similar at a very basic level. But different because of the strange quantum properties of the vacuum. Electromagnetism, that was already there in the 19th century, of course, and the weak and strong nuclear forces that act within the nucleus. And then of course the Higgs sector, or the Brout-Englert-Higgs sector which has been added on to account for properties of the weak and nuclear force. So these are the people who contributed. Starting with J.J. Thomson who at the end of the 19th century discovered the first elementary particle, the first basic constituent of matter, the electron. And then Rutherford, who discovered the nucleus, although actually he never got the physics prize, he got a chemistry prize for radioactivity but he deserves clearly to be on this list. Niels Bohr, the theorists are marked in red, Niels Bohr, who constructed the first model of the atom, of the structure of matter based on E and M, electricity magnetism and quantum mechanics, which was essential. Chadwick, who discovered the neutron. Took a long time from the nucleons to protons to the neutron. Carl David Anderson, who discovered the first antiparticle, predicted by Dirac, who I haven't put on this list, and discovered the positron, the anti-electron. Ernest Lawrence, who developed the cyclotrons, the modern particle accelerators we use. Blackett, who developed cloud chambers to use the cosmic rays accelerated throughout the universe as accelerators. Yukawa, who in 1949 made the first attempt to construct a theory of the nuclear force and predicted an existence of a new particle called the pion, which was then discovered by Powell. Then we have after World War II, the big development that followed the new scientific tools that were made available, like radar. Of course then Lamb who discovered anomalies in quantum electro-dynamics. Lee and Yang who proposed that parity might be violated in the weak force, in the weak interactions. Glaser, who developed the bubble chamber. Hofstadter, who probed the structure of the nucleon. Segrè and Chamberlain who discovered the antiparticle of the proton, the antiproton. And then Tomonaga, Schwinger and Feynman, who perfected, completed the understanding of quantum electro-dynamics. Luis Alvarez, who built a bubble chamber and the modern way of analysing high-energy physics experiments. Murray Gell-Mann, who discovered symmetry patterns among the nuclear particles that were being produced experimentally. Burt Richter and Sam Ting who discovered the J/Psi particle, or the charmed quarks. Glashow, Salam and Weinberg, who were the developers of the electroweak theory, the weak nuclear force. Jim Cronin who is here, the yellow stars are people who are here, the few, and Fitch who discovered C.P., or time reversal non-invariance in particle decays. Carlo Rubbia, and van der Meer who discovered the carriers of the weak force, the W and Z particles. Lederman, Schwartz, and Steinberger who discovered the two neutrinos. Friedman, Kendall, and Taylor, who did the experiments that for me were totally crucial, illustrating that inside protons, there really are quarks and this prize is in a sense for the discovery of quarks. Georges Charpak who developed many crucial experimental detectors for high-energy physics. Perl and Reines. Perl for discovering the tau lepton and Reines for the neutrino. Hooft and Veltman, again teaming here for understanding or normalize the properties of the gauge theories we use in the standard model. Finally the 21st century, Davis and Koshiba for discovering neutrino oscillations, in fact the neutrinos do have some mass. This is one of my favourites. The discovery of asymptotic freedom and the theory of the strong nuclear force. Oops, uh, where were we? And then uh... And then... And last but not least, and for the first time on the Nobel website where I took all these pictures in colour, Englert and Higgs for the discovery of the Brout-Englert-Higgs mechanism and again, Englert is not only here, but here. OK, and what came out of these 52 men whom I'm proud to be one of, it's quite a crowd, and unfortunately, not one woman, is the standard model, but it really is a theory, so it's a theory you can see because you can put it all on one t-shirt. And this Lagrangian, as far as we know, describes just about anything in a fundamental reductionist sense, all of science. I add in here Einstein's general relativity, which so far we truly only understand classically, and I'm adding here the cosmological constant responsible for the acceleration of the expansion of the universe. It's an unbelievably successful theory. The goal of millennia, of science of course, but the real development of this took only 75 years, from J.J. Thompson to 1975-ish. Unbelievably successful, as far as we know it works down perhaps to the smallest conceivable scales and to the edge of the universe. In a reductionist sense, and all physicists are reductionists, all of physics and therefore chemistry and biology et cetera, all are contained here if you all need to work hard enough to solve these equations. There are elements of this totally beautiful t-shirt that are still somewhat mysterious. For example, this term here is the one that accounts for the quark and lepton and neutrino masses. We don't understand its origin, it has a lot of parameters we have to measure and cannot calculate. Something is missing in our understanding of this term. Then there's this term which is... this term is another problem we don't truly understand. Then there's Einstein's term which we don't truly understand how to quantize, how to make its system in quantum mechanics. Then there's dark energy, whose form was predicted by Einstein and well tested, a great triumph in general relativity, but the magnitude of this term is an incredible, theoretical mystery. And then there are of course, many, many measurements, direct and indirect and puzzles that inform us, like Einstein, that even this fantastic standard theory must be provisional or must be physics beyond it, including dark matter which I'll come to, neutrino masses, baryon, why are there baryons left after the Big Bang, the acceleration of the universe. And theoretical mysteries, like how do the forces unify? Various, enormous disparancies of the scales of fundamental physics. The properties of the quarks and gluons, their masses and so on, and the theory of the universe. Some of those puzzles seem to be resolved by a very beautiful idea of super symmetry which I've discussed in Lindau before, in which we still wait for any evidence at the LHC. Dark matter however is there for sure, it has been observed indirectly throughout the universe by astronomers. They see something that affects matter and light and therefore they know there's matter there and indeed, most of the matter in the universe is not made of stuff that we are made out of and therefore it's called dark, it doesn't radiate. There are intensive searches to detect or produce such matter. I have no doubt that will happen in the next decade. Theorists want to unify following Einstein's dream and we're in the position to do so because we understand all of these forces now and when we extrapolate their properties, we find that they all sort of come together and look very similar and fit together at an extraordinarily high-energy short distance. Happens to be very close to where gravity becomes a strong force and we must take it into account. This is the strong intent that has motivated us for the last 40 years, to try to go beyond the standard theory of the strong, weak and electromagnetic forces through a unified theory, perhaps of all the forces, leading us to string theory, for example. Where we could imagine that all the different forces and quanta of the fields that describe these forces, all the particles and forces, are due to different vibrations of a single superstring. Now in that picture, that enormously high energy, enormously short distance, is known as the Planck scale. Discovered by Max Planck when he discovered the Planck's constant. He realized he now had three parameters which were clearly fundamental in physics. The velocity of light, the strength of gravity, and the constant he needed for his radiation law. And with three dimensionfull units, you can construct natural units for physics. And he did. And he advocated using these to communicate with E.T., extra-terrestrial civilizations, you know? They would say, "How big are you guys?" And a million years later we would tell them, "Well, we're two meters." Come on, what's a meter? No, we would tell them we are 10^35 basic units of length and anywhere, any physicist anywhere in the universe would know what that meant. That's the Planck length, it's awfully small. The Planck time is awfully fast. The Planck energy scale is awfully big, but that's a fact of nature. It's not a choice of theorists who like to probe domains which are inaccessible, it's a fact. And we have to live with it and it's what I call the curse of logarithms. So if you try to measure energy on a scale using a scale that is relevant to physics, meaningful, then you really should measure logarithm of energy. You increase the energy by factors of 10, 10, 10. That's always what we're doing by the way, particle physics. We always want to build a bigger accelerator by a factor of 10 so you should really use a logarithmic scale. On that scale is a scale where physics changes from energy to energy scale. So on that scale, Rutherford, in units of billion electron volts. Rutherford was down to 10^-3 when he was probing the structure of the atom. The strong interactions are characteristically probed at 10 to 100 Gev. Proton weighs 1 Gev and the weak interaction scale is maybe a TeV, a trillion electron volts being probed at the LHC, or 10 TeV, and we would really like to get to the unification scale, the Planck scale and that's 10^19 Gev. We're hoping to build an accelerator that'll go to 100 TeV, that goes a bit farther, but on a logarithmic scale, you see, going from the 75 years from Rutherford to the standard theory, this is, you know, a big step but it's only about as much as we've done before. And that's why we can, as theorists, speculate and work without being able directly to measure. And one of the reasons we can't directly measure up here is that there's another scale, called dollars. Oops, something again happened. I'll get back. God Almighty. Let's go to... God Almighty. Play, play. OK, back to the curse of the logarithms. So the real scale is, the scale of physics, it goes log of the energy, we've made a lot of progress, we have to get to here, but society uses dollars. Now it turns out that dollars increase and building accelerators like the square of the energy at best, and that's exponential then, in the scale of physics. And exponentials are really bad. So on the same scale, Fermilab, which cost about a billion dollars is down here, and then the LHC, which cost maybe six billion dollars is up here, that's a long way. And then the new machine that we would like to build at 100 TeV costs about 10 billion dollars, and the machine we really need to probe the Planck scale... I don't know what's in this direction, Munich? So on this scale, it's probably in Munich. So that's a fact of life and we have to deal with that and there are all sorts of strategies. And it certainly affects the way we look at the future, because if you look at the present and future of particle physics, you can either be extremely optimistic, as I tend to be, or extremely pessimistic, as even I tend to be sometimes. From the extremely pessimistic point of view, you could say well, "Standard theory works so well." In fact, recently, finally confirmed Higgs sector agrees with a prediction, the simplest predictions of the simplest models that were considered 50 years ago extraordinarily well. And that's disappointing in a way, it works but of course it doesn't tell us anything we didn't know, in a way. There also is no signal for these new particles and new symmetries we imagine, dark matter has not yet been directly observed and we're not guaranteed that it will be in the next decade or two. We have no direct experimentally, provable indication of where the next new threshold is and it might be as high as the LHC, as the Planck scale. What do you do if that turns out to be the case? And we'll know that in the next three, to five, 10 years for sure. Now the extremely optimistic scenario, which I subscribe to more fervently is that, well, there probably are a bit of deviations from the simplest Higgs model. Super symmetric particles will be observed in the next run of the LHC, which began a few weeks ago and dark matter will be detected on the sky or underground, produced at the LHC. And that'll give us enormously strong guidance for the next steps, and they're many steps, experimental steps of various colliders. In both cases, however, in both scenarios, the lesson I take away is that we must fully explore the next scale, 10 times greater. This really gets us into the scale where the electroweak sector of the standard model can be really understood. And we can do it, in fact, the United States did this 20 years ago. Then Newt Gingrich came along and shut it down. And we need the SSC. If it had not been killed, having gone through its first upgrade, would be running at about 100 TeV. Its design and energy was 40 TeV, would've been easily extendable with modern magnets to 100 TeV, so we already built it before Newt Gingrich got his hands on congress. And there are all sorts of plans by Geneva and most excitingly by China, which is now perhaps if not today, next week, the biggest economy in the world. They can afford to do this. It was a very exciting Chinese proposal to build a 100 TeV collider around here... and that decision will be taken probably by the end of this year. CERN has its own plans but a longer time schedule since they must finish with the LHC. Meanwhile, theorists can go on speculating as we have been so successful in the last 100 years, but now the questions we're asking are really profound in some sense but they build on previous knowledge. Space-time was altered in a totally fundamental way 100 years ago by Einstein, but once we add the quantum mechanics to the game, it remains the deepest mysteries. And many of the properties of space and time we take for granted, which when as infants we construct this model to navigate the world, appear to have no real fundamental meaning and many of us are convinced that space and time is truly an emergent concept. At the Planck scale, it's simply not a good way of describing, there's something more fundamental, space and time are emergent. We're now beginning to understand how that can happen and how to see what properties of a quantum-mechanical system underline our usual space-time descriptions. Space time emerges, of course gravity is just dynamical space-time, so it's also an emergent force. But this is difficult, very difficult conceptually. We have to, in a sense imagine, how do you start from a more fundamental basis for physics in which space and time are not there to begin with? How do you formulate the rules of physics without postulating space and time? And then both cosmologist and fundamental physicists of all types are now faced, given this understanding of the history of the universe, 100 years of story with understanding the Big Bang. Can't avoid it anymore. That again is now taken away from religion and philosophy and becomes a matter of physics. To find a solution to go beyond, you know say, "Well at some point it was hot and dense." But to really go back to the Big Bang requires confronting a question that physics has been able to avoid for millenia, which is, "How did the universe begin?" And, "Is this a question physics can address?" Can we determine the initial condition? I believe, that this question has to be confronted because what we do now and speculate, all speculations about unifying the forces of nature with gravity, the theory of space-time, in the end, the consistent answer to that has to describe how the universe began. Or modify the question as we transform our notion of what space-time is to one that can be answered. But it's obviously very difficult. And it's very difficult to see signals from very early times. We had a lot of hopes earlier this year with an experiment called BICEP 2 looking for relic gravitational waves, ripples in the metric of space-time that came very close to the very beginning. Unfortunately it turned out that they ignored the dust and we haven't yet seen such gravitational waves, but eventually we'll probably see them. So I am very optimistic and all the young people here should be optimistic for the following reasons. It's my reading of history and my own life's experience is that once a fundamental question becomes a well formulated scientific question, which means that it can be approached by experiment, by observation and by theory, mathematical modelling, it will be answered in your lifetime. So I've posed some of these really interesting questions, they will be answered in your lifetime, I didn't say my lifetime. Also, once an important scientific instrument is technically feasible and addresses a fundamental scientific question, like the 100 TeV collider, it will be built in your lifetime, maybe not mine but, you know... So I'm optimistic for particle fundamental physics because they're new discoveries I believe are around the corner. There are new tools that are being developed to try to deal with this incredible hierarchy of scales, there are wonderful new ideas and theoretical experiments. We have a wonderful theory of elementary particles but the most exciting questions remain to be answered and as always in science, well, there are incredible experimental opportunities. To be specific, I am very excited about the possibility of China coming into the game. The new experiments and new accelerators that are coming and new discoveries around the corner. So the best is yet to come, thank you.

David J. Gross (2015)

The Future of Particle Physics

David J. Gross (2015)

The Future of Particle Physics

Abstract

Elementary Particle Physics seeks to discover the basic constituents of matter and understand the fundamental forces that act on them. In this lecture I shall review the current state of particle physics, the grand success of the “standard model”, as well as the many questions that remain unanswered. I will discuss some of the theoretical ideas and speculations that have been advanced to answer these questions, and the possibility of testing these ideas at the Large Hadron Collider and it successors.

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