David Gross: The Coming Revolutions in Theoretical Physics

I'm Francis Hellman I'm a professor and I'm chair of the physics department and I'd like to welcome you to the first of what will I believe be many public lectures far it's sponsored by the Berkeley Center for Theoretical Physics so this Center and I think many of you were just at the reception for this was formed to bring together some of the greatest minds in theoretical physics to search for answers to some of the most pressing questions about the universe and our speaker today is an illustrious example of one of those minds so events that like this lecture are part of the mission of this Center to not only bring together those minds but to bring some of that science out to the public as well so I'd like to give you a brief biographical sketch of David gross David gross received his bachelor's and master's degree from the Hebrew University of Jerusalem in 1962 he received his PhD in physics from UC Berkeley in 1966 then spent three years as a junior fellow at Harvard 1973 he became professor of physics at Princeton he then assumed the title of director and holder of the Gluck chair at the Cavalier Institute at UC Santa Barbara he has made many many profound contributions to physics but is known particularly for his work in what is called the asymptotic freedom of quarks quarks are the fundamental particles that make both neutrons and protons among other less well-known particles the interactions between these quarks are what causes the protons and neutrons to form and what causes the strong nuclear force that binds these together in the nuclei of atoms David's work which was done in 1973 together with Frank will check at Princeton discovered the nature of these interactions and that in particular that the interactions between the quarks depended very strongly and very not intuitively I have to say on how far apart they are from each other when they're far apart they're pulled together extremely strongly so much so that they cannot escape each other so the consequence of that is that we do not observe single quarks despite the fact that we are quite confident that they really exist but when they're very very close together the interactions are so weak that they behave as free particles so this theory actually underpins virtually all current models for how nuclei work and fundamental particles in etc he's won a really large number of as awards and honors and there's one particularly that he's known for and I'm sure many in the audience think they know what it is but I'm willing to bet that you actually don't so I'd like to point to the fact that in approximately I'm going to guess somewhere between like 1964 he was one of eight hundred students arrested during the sit-in at Sproul hall so we're very very proud of all of our Berkeley students and very proud that there are long and illustrious history incidentally he also won the Nobel Prize in Physics so so and that Nobel Prize was was was given for his work on the asymptotic freedom of quarks so his his talk today is actually not looking backwards I'm told at the work at that work but his looking ahead instead at the coming revolutions in fundamental physics and so with that I would like to ask him to come up and well thank you can you see this well thank you Francis it's a really a great pleasure to be back in Berkeley sorry anyway it's really a great pleasure to be back in Berkeley where my real life and physics began 45 years ago when I arrived as a young graduate student Berkeley at that time was incredibly exciting it was the center of particle physics highest energy laboratory was at that time still at Berkeley and theoretical physics theoretical particle physics was enormous ly exciting as Francis remarked Berkeley was excited in many ways at that time but for me most of all in physics it's a delight to be back here some of my my thesis adviser my teachers versus true and Mandelstam were in the audience some of my fellow graduate students John Schwarz Ling Li Wang and I'm not sure whether they're others of our generation but it's really delightful to be back I really owe a lot to Berkeley and I was happy to repay it partly a few years ago when I served on a review committee that reviewed the Berkeley physics department and came down very hard on the Berkeley administration for not supporting enough this jewel of physics in the United States as a consequence of that within two weeks the administration gave physics department 12 million dollars which they use to create this incredible Center that that we all saw this or some of us saw this afternoon which will help continue the great tradition that Berkeley has played in American science and especially in theoretical physics so I did my part in getting the 12 million they still need a lot more it seems and you know I appealed to those of you who have the means to contribute as well to the success of the Berkeley Physics Department in the Center for Theoretical Physics so I'm going to talk today about becoming revolutions in fundamental physics and I'm going to briefly discuss the state of particle physics and the discoveries that we expect are around the corner when the Large Hadron Collider a large particle accelerator is completed next year at CERN in Geneva but mostly I'm going to be talking about string theory which I'm sure you all have heard of this crazy idea that things are made out of strings you know I'm a collector of a popular opinion about string theory especially cartoons and years ago at about 1987 I really was happy to see that the New York Times regarded string theory as something very difficult but no interesting and clearly very important this couple she's saying it's all string theory to me I felt it I felt I was happy that string theory had replaced Greek but recently string theory is you know because of some rather silly books has gotten a different reputation and the same New York joerger magazine had this cartoon in 2007 Harvey's place there is a discussion on string theory this depressed me quite a bit until a friend of mine sent me this this advertisement that he found in the same in New York Times magazine for lingerie string theory when physics gets physical restored my confidence in the American media so particle physics we have an extraordinarily successful theory of particle physics the culmination culmination by the end of the 20th century of a quest that is going on since the Bhakra teased over two millennia to understand the nature of matter and that quest proceeded as you know in discovering that ordinary matter is made of atoms point like objects that form larger objects due to the forces between them but in the 20th century we understood that atoms themself had structure and consisted of a point like nucleus and surrounded by a cloud of electrons electrons being the first of a family we now identify as the basic constituents of matter leptons the nucleus apparently point-like at first was also revealed in the later in the 20th century especially after the war to have a structure itself made out of point-like particles protons and neutrons which later in the 20th century were revealed to have a structure as it turned out to be made out of point like porks and that's sort of where the story ends at the moment as far as we understand all the matter we've ever seen in the laboratory is made out of quarks and which form nuclei leptons like the electron that revolve around the nuclei they form atoms and ordinary matter we've discovered different types of quarks in fact and leptons three families of quarks and three families of leptons the electron and the elusive neutrino so by the end of the 20th century we seem to have identified all the kinds of matter that we need to explain all the stuff that we've ever created or observed in the laboratory and we explain it in terms of understanding the forces that act on these basic elementary atomic constituents the forces are of course the force of electricity and magnetism which holds the electrons in orbits around the nucleus and the two nuclear forces that act at very short distances within nuclei and within nucleons the strong force that holds the quarks together permanently within nucleons and provides the source of the nuclear force it's a strong much stronger force than the electromagnetic force which is why hydrogen bombs are much more powerful than TNT and of course there's another force of nature that all of us are acquainted with and was understood way back by Newton and improved on by Einstein the force of gravity which at the atomic level plays very little role these these three families of matter leptons and quarks and these four forces constitute what we might call our standard model or standard theory of elementary particle physics of fundamental physics of the fundamental constituents of matter and the basic forces that operate on these fundamental constituents and as far as we can directly tell by observation the standard model of the electromagnetic weak and strong nuclear forces plus general relativity Einstein's theory of gravity is the theory of all the observed forces of nature and it is an incredibly successful theory it is not as widely appreciated by the general culture as it should be what a incredible achievement of theory of physics this has been after two millennia this theory has been tested down to extraordinarily short distances of order 10 to the minus 18 centimeters but some of my colleagues work on nano physics a branch of physics defined by size the size being 10 to the minus 9 meters well particle physics works at distances of nano nano centimeters and tests our theories down to such distances and with extraordinary accuracy the tests of this theoretical framework which is based on quantum mechanics and on local field theory quantum field theory has been tested now to in some cases an accuracy better than a part in a billion which means you can both measure and calculate a number of measurable quantity and you have to worry about the tenth decimal place it is unbelievable feat of both experiment and theory to achieve such accuracy but all of the components of the standard model not just electromagnetism including the weak and strong nuclear forces have now been tested in dozens of cases to accuracies of much better than a percent we also see no limitation of this theoretical model you sometimes you construct theories they contain within them the seeds of their own destruction they lead to paradoxes through problems you know that they have to go wrong at some scale well the only place that we know that this theory goes wrong is the so-called Planck length of 10 to the minus 33 centimeters and the short distance this framework might explain all of physics down to that length we see no reason why it must fail before then and since the same theoretical structure works at the level of planets and stars and galaxies indeed the whole universe we now describe using the standard model particle physics and general relativity we can say that this theoretical framework works from the Planck length to the edge of the universe at sixty orders of magnitude on a logarithmic scale we are halfway between the Planck length and the size of the universe and we have one theory one theoretical framework that seems to be able to explain all of this this might lead one to conclude that after with such great success particle physics high-energy physics or what I call fundamental physics the search for the basic constituents of matter and our understanding of the basic forces that act on them is over but of course the very success of the standard model our very understanding that we've achieved raises more questions or as many interesting questions that are answered I like to say that the most important product of knowledge is ignorance not the kind of ignorance that leads to political strife bigotry racism that's not the kind of ignorance I'm talking about I'm talking about the ignorance that lead you to ask why questions the ignorance that is the basis of scientific exploration the ignorance that we cherish informed intelligent ignorance that is based on deep understanding of what important interesting addressable scientific questions are and that ignorance requires knowledge and the standard model has produced such wonderful ignorance wonderful questions questions by why are all these forces the forces that we observe in nature electromagnetism weak strong nuclear force all have their origin we believe in a notion of symmetry a beautiful symmetry principle a local symmetry Ecore gauge invariance now there are many other kinds of forces that we can imagine but somehow local symmetry seems to be the secret of nature why is that and then there are all these different forces they look kind of different why are they different and they're characterized by different strengths and why are the strengths of the forces what they are now these are questions that you can't begin to ask until you understand how what the forces are and how they work and that we do but now we're driven to ask these questions and others we have discovered measured observed these various families the basic constituents of matter quarks and leptons and they have a strange and bizarre pattern of masses and mixings and we've been measuring with great precision these masses and properties of quartz electrons for the last 40 50 years and so far we haven't been able to discern any obvious pattern and certainly no explanation of their strange pattern of masses and mixings the wider why are there three families and why are these they have the values that they do and now that we're beginning as we'll say to incorporate gravity the theory the dynamical theory of space and time into our considerations we begin to ask questions that when I was a student here were regarded as philosophy and not physics but now our physic such as why is three space three-dimensional or is it maybe they're nine or ten spatial dimensions yyy this is what drives physics forward and that questions that the why questions we ask now are much more interesting than the why questions that were asked 45 years ago when I was a student here many of those we answered but these are still around and of course we'd like to answer them because we're curious we'd like to know can we answer them some people will see argue that all of them are answerable or calculable but it's more than that we need to know the answers of these questions to really understand the beginning the origin in the early history of the universe particle physics has constructed a standard model that explains almost everything we observed but so has in recent years astronomy astrophysics and cosmology we have learned an enormous amount about the history of the universe depicted in this rather complicated history starting with a Big Bang inflating rapidly suddenly down to a almost constant expansion starting in a very hot stage and dense and then cooling off as the universe expands we have directly observed much of this history indirectly understand even more and have a picture that is quite satisfy and it fits together a standard model of astrophysics and cosmology from close to the big the beginning whatever it was till now of course now the physics we see around us is physics of atoms that make up stars and galaxies and people but in earlier times which we can observe directly by seeing light that comes to us from far away therefore in the past we can see errors were totally different physics applied we go back to about 300,000 years after the beginning back in time from 13 billion years where we are now we reach the era where we can no longer see directly beyond at that point the universe was so hot that atoms were stripped of their electrons and lived in a plasma of electrons and nuclei and radiation a fog that prevents light from escaping so before that we can't directly observe the physics but indirectly we can using our very good theory understand that in previous eras like for example a hundred thousandth of a second after the Big Bang we believed that before that time the universe was so hot that nuclei actually melted and quarks escaped from their confined nucleons and one had a plasma quarks and gluons and electrons and radiation a state of matter which is of great interest and one that we are trying to reproduce directly in the laboratory by smashing heavy nuclei and high energies to create a quark-gluon plasma that might live for a short time and be observable if we go back even further however we believe that our understanding of the physics and evidence in evitt ibly will break down and yet it was crucial to the formation of the structure the fluctuations in that matter that led eventually to structure and the formation of galaxies and stars if we go back even further we arrive at the beginning the Big Bang that produced it seems a very rapid inflation of the universe and all of our equations break down well one of the roads that people have been have tried ever since the standard model was completed in the 1970s to try to answer these why questions that don't seem to be answerable within the standard framework was to search for unification Einsteins dream have unified all the forces of nature together and it wasn't just because of the belief that a unified theory would be more predictive more powerful more able perhaps to answer some of those why questions it was also because the very pattern of force and matter that was discovered in the standard model the three forces that act within the atom like the magnetism weak and strong nuclear forces and the two kinds of matter are pieces of a jigsaw puzzle that fit very naturally together it was realized very early on that the three forces fit together as if they really are part of a single force each of these forces arises because of a symmetry of nature we learned and they seem to fit together as if all of them arise together as a consequence of an even bigger symmetry of nature and the same with the matter the matter fits together beautifully as a representation of that symmetry it's as if we have one force of Nate one's atomic force and one form of matter for all of the quarks and leptons fit together into a representation of the basic symmetry group that underlies all of the forces but this is a bit strange because after all if all the forces are equal they would look equal if they came from a bigger symmetry we would have noted the consequences of that bigger symmetry and the only reason we can imagine such a unification taking place is because of two great lessons that we learned in constructing the standard model one from the success of building the electroweak theory the our understanding of electromagnetism and of the weak nuclear force that symmetries can be broken not manifest in the real world in the state of the world although explore all those symmetries of the laws of nature and the other is that could explain the other phenomena is related to the asymptotic freedom that Francis mentioned she explains how the forces could be the same even if they have very different strengths force of electricity magnetism is much much weaker than the nuclear force how could they be the same force well that was related to our understanding that the strength of forces in quantum theory depends on distance it can it could be the fact it could be the case that all of these forces are equal not at the largest if you think of the size of a nucleus as large distances we now probe but at much much smaller distances why is it that the strength of the forces depends on distance well that has to do with the properties of the vacuum this is a picture of the way many of you might think of the vacuum this is the classical vacuum the classical vacuum is empty that's what it looks like remove everything from this room all the people all the chairs all the air everything cool it down to absolute zero you're left with the vacuum this looks like this that's the classical picture of the vacuum but in quantum mechanics that's not what happens according to Heisenberg's uncertainty principle you can't have anything inert anything dynamical and remember the all of space-time is full potentially full of electromagnetic strong and weak nuclear fields that transmit the forces between the particles those fields can't be quiescent and in their ground state motionless inert in the vacuum as classically one might imagine because if you try to observe the vacuum you would disturb it and set those fields those dynamical objects in motion a pendulum classically considered rest but quantum mechanically if you observe it you give it a kick so it must always be moving what we call zero point motion here is a picture of the quantum vacuum and we know it's an accurate picture because we have a theory that works at least that nuclear scale so here is a picture of a quantum vacuum as calculated using our theory of the strong nuclear force quantum chromodynamics at which is gives the dominant contributions to what's going on in the vacuum has lent scales of a nucleus which is ten to the minus thirteen centimeters this is what the vacuum looks like those fluctuating stuff are the quantum fields that give rise to the nuclear force between quarks so the vacuum looks like this not that empty white space it is a dynamical medium a complicated place and in such a medium the force between two objects carrying some charge and interacting through these very fields that are fluctuating and depend on the distance just as a dielectric medium like water in screen electrical charge so that's what happens and that cuts that has the effect in the case of the strong nuclear force of leading to the fact that as we go to higher and higher energy of the quarks that interact with this strong force the force gets weaker and weaker high energy corresponds to short distances so going to high energy is the same as looking at a physical phenomena at short distance this is why you need very high energy beams like to probe short distances this diminution of the strong force as a function of energy or distance is what was referred to as asymptotic freedom well if this is the case that the strong force gets weaker at high energies or short distances maybe it could become weak enough to be comparable to the electric force and they might have a common origin so if we look at the standard model we have these three nuclear forces three forces that act within the atom and the nucleus the strong weak nuclear forces and electromagnetism and over the range where we observe today they all vary with energy roughly in this manner and we can use our theory to say how they vary with energy at higher energies we can extrapolate the standard model and what was discovered early on was that these forces seemed to come together and if they were a single force but only when you look at very high energies or very short distances they split because of this phenomenon of symmetry breaking but if you were to do experiments at this extraordinarily high energy 15 orders of magnitude higher than present-day observation you would see evidence that they were uniformed this was a very important remains a very important clue to one the unification of all the forces of nature and second of all the extraordinarily high energy scale at which they might unify it's a bit depressing than it has been for years it still is depressing because we are not going to have a chance at our lifetime if ever to do direct measurements at this scale of unification but that doesn't mean that there's lots not a lot of wonderful physics that might appear in between at lower energies which are accessible and in particular we're most interested today in what might appear at the Large Hadron Collider which puts it pushes present-day observation by about one order of magnitude and here some of us expect the true revolution to occur which is the discovery of something called supersymmetry one of whose inventors Bruno's amino I think is in the audience in the hood so I want to say a word about supersymmetry that's the first and most upcoming experimental the revolution that will some of us think might be discovered in a few years and to discuss what supersymmetry is I need to tell you what super space is because the best way of describing a symmetry is to describe the space on which the symmetry operates so an example of a symmetry is rotational symmetry as you know the laws of physics are rotationally invariant that means if I do an experiment like I drop this measure how long it takes and then I rotate the laboratory and I do the experiment again the same answer laws of physics are invariant under spatial rotations I rotate the x axis into the y axis so to explain a symmetry I should easiest way is to explain the space on which the symmetry transformations act and in the case of supersymmetry it's super space so what is super space well you all know what space time is right we in ordinary physics we measure the position of a particle and the time of an event by a point in space-time we have X Y Z spatial coordinates and time so to describe super space super space is a space with more dimensions more coordinates we need more coordinates to say where a particle is we have these extra court well so the symmetries for example tensional symmetry was rotating x2 y super space has some extra coordinates which i call theta here we use Greek letters because we measure these coordinates with a new kind of number different kind of number than we measure these coordinates so if I measure distances on this direction we use ordinary numbers 1 2 3 in this direction we use funny kinds of numbers called grassman numbers variables and they have the property that if you multiply two of them theta1 times theta2 you get the opposite answer and if you multiple that multiply them in the other order later 2 times theta 1 is minus theta 1 times theta 2 they anti commute they are not a commutative under multiplication well mathematicians can invent all sorts of crazy numbers square root of -1 right that's pretty crazy but these are kind of nice numbers the main multiplication table is very short because theta one squared is zero because it's equal to minus theta one squared so these are I consistent kinds of numbers that you could invent and it turns out that there is a generalization of ordinary space-time where you add extra coordinates new dimensions but not new dimensions of the usual type but new dimensions where you measure the dimensions in these anti-community numbers there's a beautiful generalization of space-time and there's a beautiful generalization of the symmetries of space-time where in addition to rotating X the Y you can also rotate X 2 theta and later 2 X mathematicians actually didn't discover that it was discovered by physicists and as we'll see the first hints came from string theory but it's a beautiful extension of the traditional notion of space-time and of the symmetries of space-time that underlie our understanding of the physical world relativistic quantum field theory general relativistic theory which underlies gravity again is beautifully generalized to super space which has these anti commuting coordinates in physics we have two kinds of particles in nature bosons and fermions they are they differ by their spin integer spin angular momentum rotation or half angular momentum they differ by their statistics bosons are things that sort of commute like X's fermions are like Thetas that anti commute you exchange two fermions you get a minus sign in our description of the physics and so these kinds of rotations change bosons into fermions fermions in the bosons when you have a theory of nature formulated in super space the laws of physics are invariant under that then under rotations in zubur space then these transformations lead to the remarkable prediction that for every particle you've ever seen in the ordinary kind of theory there has to exist a superpartner you get by making one of these new transformations so the immediate consequence of imagining that our world is super symmetric in some sense would be that quarks which we observe would be accompanied by something we call sports which are actually bosons of spin zero and electrons would be accompanied by things we call slect ron's photons play for tina's and so on every thing you've ever observed you have a super partner which we've never seen and you might say therefore this idea mathematically is beautiful but physically is absurd but we've learned as I get coming back in the 20th century that many of the symmetries of the world certainly some of the symmetries that underlie the standard model and certainly if there are unification based on bigger symmetries those symmetries are not manifest in the world around us now that's a common phenomenon if you look at this room it is not rotational invariant if I look that way I see all of you I rotate I don't see you the laws of physics are invariant under rotations but this room isn't well this room is a complicated place we put all these people here chairs they're not there what about I remove everything from the room will it look rotationally invariant well if you thought of that that's getting back to the vacuum if you thought of the vacuum as being this empty white sheet yeah it looks translate rotationally invariant to rotate the sheet any old way but think of that vacuum is that complicated mishmash of fluctuating fields it need not respect the symmetries of the law of nature just as this room doesn't so we can invent new symmetries new spaces and symmetries on which they act which act on those spaces and if the symmetry is broken then these particles new particles that are you know predicted to be there with the same masses as the previous ones could be very heavy and if the symmetry is broken with a scale of about a trillion electron volts then these particles although heavy can be produced at this new accelerator well this is very pretty and it is very pretty but why should we expect this to have any chance of being seen well there are three important clues that we have that supersymmetry might be there and it might be just around the corner first it actually helps us with that unification of the forces if we look at those that unification it doesn't really work here this is a plot a logarithmic plot of inverse couplings which are straight lines with such a plot and if the unification really worked these straight lines would meet at a point by now our experiments have improved so much and the theory as well that the extrapolation is gotten quite good and you can within the standard model these forces there's nothing else don't meet no unification on the other hand if you just take the standard model and assume it's part of a broken not manifest supersymmetric theory in the minimal fashion and you assume that that braking is at a t.v scale then again you can do the extrapolation there's a change when supersymmetry kicks in and the forces be precisely at a point more or less within better than a percent accuracy after extrapolating over 14 orders of magnitude that is it my opinion an extremely strong clue that unification is correct and supersymmetry is correct and the scale of supersymmetry breaking is down here just where we might observe it might be a coincidence supersymmetry also helps explain the hierarchy this enormous ratio of scales between the unification scale and the electroweak scale the scale of the weak nuclear force that's a very small number that's why the unification scale is so far away from present-day observation we think we can explain this very small number but we need something and super some extra symmetry to help us supersymmetry does it these two features of supersymmetry by the way were not the reason people invented supersymmetry supersymmetry happened to cure these problems and most remarkable is again a realization after supersymmetry had been around that there is a lot of matter in the universe which we don't understand what it is it seems to be a new form of matter skok so-called dark matter which I'm sure you have heard constitutes most of the matter in the universe galaxies don't really look like this our picture normal picture of a galaxy based on the stars that we see the light matter the matter that interacts with light and we see astronomers have concluded by measuring the orbits of these stars in the galaxies that there's not enough mass in the stars to hold the stars in orbit there has to be a lot of other mass a lot of other matter and they can deduce how much there is and 90% of the matter in the universe appears to be in the form of some kind of very heavy particle that doesn't interact very strongly with our kind of matter and light now we don't know what that dark matter is but in supersymmetric theories there is naturally a candidate for it which if it the scale of those this heavy new particles is where we can produce them at the LHC we can understand why 90% of the matter the universe is dark so these are three indirect but my opinion very compelling clothes to take this possibility of this symmetry very seriously and and to look for the LHC this incredibly incredible dual accelerator that is being completed in Geneva runs between Switzerland and France and bangs protons together at trillion electron volt energies this machine with it's unbelievably massive and complex detectors has a chance of discovering the super world this is a revolution if it happens because and when you read in the paper few years from now that physicists at CERN have claimed to have discovered supersymmetry you should remember that this is a symmetry of super space and that discovering the symmetry non manifest or broken as we say doesn't matter it really means that the world has quantum dimensions that we live not just an ordinary space-time but we live in super space so discovering new quantum dimensions of space-time I regard as a major revolution and that might be around the corner but you know this is still a bit disappointing because if we discover supersymmetry we're still not going to know directly what goes on at the unification scale but here we have one more clue which we haven't mentioned yet and that's gravity gravity is a totally ignore Abbas in atomic physics and nuclear physics sometimes I find it hard to explain to people that gravity is unimportant because it's the only force they know about the force they feel when they wake up in the morning and get out of bed or when they fall flat on their face the other forces our shield did like the electromagnetic and weak and strong forces all objects are neutral because the forces are so strong most of the time but gravity everybody feels but it's incredibly weak and the best if you wanted to demonstrate that to a someone who doesn't realize how weak gravity is just hold something up and tell them that you are exerting a minut amount of electrical energy electrical chemical force to hold this up even though the whole earth is pulling down on it so I can easily oppose the whole earth the attraction of 10 to the 53 protons in the earth by exerting a little bit of a much stronger by 40 orders of magnitude at the atomic scale electrical force but gravity increases rapidly as you go to higher energies two particles scatter at moving past each other at very high energies have a increased gravitational force which after all gravity is the force between mass it's proportional to mass squared and mass of course e equals mc-squared is energy so gravity raised it Rises quadratically as you increase the energy whereas these forces vary logarithmic Li and if you go up by 20 orders of magnitude gravity becomes equally strong at the atomic level at the this very small so-called Planck scale to the other forces of nature at just about the place where they unify and that is a very important hint the hint is that if we're going to unify the strong weak and electromagnetic forces that seem to fit together so neatly we're going to have to unify them together with gravity which was Einstein's dream but this picture tells us that you know we're driven to that and that is a serious constraint because you can imagine many ways of trying to unify strong weak in electromagnetic interactions and people did for years but gravity is a different story that's much harder and that brings us to strengthen quantum gravity doesn't really fit in to our framework it seems if we try to incorporate Einstein's theory of gravity in the framework we use to describe the other forces we run into problems Einstein taught us that gravity is the dynamics of space-time so we have the quat the object the gravitational field is really the metric the distances of space-time itself the very structure of space and time and like anything else in quantum theories that fluctuates all the time like that picture of the vacuum and so space-time is fluctuating and it seems that in Einstein's theory if you just blindly quantize it try to make a quantum theory out of it the fluctuations are uncontrollable they're uncontrollable at these very short distances and nobody was ever able to control them which might mean that well we just weren't smart enough or that Einstein's theory is only an approximation we have to go beyond it to some other theory that is perhaps more fundamental and reduces to Einstein's theory at large this this is where we use it so far and string theory is such a theory it goes beyond Einstein's theory much in the same way that Einstein's theory goes beyond Newton's theory of gravity Newton's theory is pretty good when you want to throw things up at the air in this room but but they're small Corrections and they can become big in certain circumstances and the same thing with string theory and string theory is a break with this past of point light constituents making up matter that has worked so well from atoms down to quarks and electrons the sum in some sense string theory breaks for that by we're saying that if you were to look very carefully imagine a microscope that could resolve this very small Planck scale distance you would see that quartz and electrons are vibrating strings and in fact since a string vibrates in many different ways the quark and electron turn out to be different vibrational modes of the same string not only that but all of the particles not just the elementary particles of matter but the elementary quanta of force which are particle like as well the photon a particle of light the graviton the particle of gravity all particles in string theory in some sense are different harmonics of the same strength it's a beautiful unifying idea so what is string theory well we don't actually know the answer string theory is sort of accidentally discovered and developed and it's far from a complete theory but the way so far that it's developed it is what I like to call a conservatively radical modification of the principles of physics it is very danger very dangerous to try to modify the principles of physics it's not dangerous to change modify models of areas of science where we have understand the stabbed and the basic fundamental laws but it is very dangerous to modify the principles of physics your your overwhelming likelihood to be wrong logically inconsistent with experiment or logic or both if you're going to modify anything you should modify as little as possible and to some extent string theory so far as modified very little of our traditional theoretical principles of physics if any its replaces particles by strings it says well we're not going to make things out of particles are going to make things out of strings well why not but every other principle of physics so far has been left unchanged I'll argue later that's not going to be enough but that's so far what's happened and the way that we've constructed this theory over the years string theory is next year will be 40 years old is to essentially take everything we know about particles quantum field theory the basis of the standard model and do the same for Strings using same principle so for example we discuss the motion of a particle and we like to understand how a particle moves well we draw the trajectory of the particle moving in space as time goes on and we can determine that motion by various ways one way is to find that minimum length path that leads to geodesic motion of a free particle it's a nice way of describing the laws of motion classically of a particle and we can do the same in the string a string move sweeps out a tube tube doesn't have a natural length but it has the generalization area we minimize the area or extremize the area of the world tube of the string you get the motion of a classical string it's really totally straightforward nothing new once you have this classical picture you can easily do quantum mechanics because Fineman told us that in quantum mechanics you simply sum over all trajectories all paths and you want to calculate the probability for the particle to go from this event a to this one B you sum over all paths you wait the path with a phase which is proportional to length you square that amplitude to get the probability that's five minutes picture of quantum mechanics do the same for Strings you consider not just one two but all tubes you sum over all of them and you weight the sum by the exponential the area totally straightforward nothing new mathematics is a little more complicated but in a hundred years it'll be taught in high school the remarkable thing happens however when you consider string interactions here there's a surprise Fineman tells us how do we describe interactions well we have a particle a and particle B they move along these trajectories quantum mechanically you sum over all trajectories they happen to be the same point at the same time they have some probability of turning into a third particle see at the interaction point this is a nasty point it's really a singularity of the graph it's where all of the uncertainty and the unknowables of our theoretical framework lie all of those numbers we can't calculate are have to be put in at these points to specify the probability that a B turns into C this is where all of our Y questions lie I mean that's what you do as particle so it's strings you have to blow up this picture to go from lines the tubes but there are two ways of doing it you could have string a and B and they could interact by coming together just like the particles overlapping and forming a new string C which goes off which gives rise to this picture we blown-up those lines a and B come together form C goes out this would have been a disaster if people had constructed string theory this way you would have had to specify probabilities for this to occur for each curve there infinite number of curves an infinite number of parameters that you couldn't determine would have to specify furthermore this would have been sold would have been bought with other the Virgin seas and sicknesses but there's another way these two strings can move along come together and turn into a third-string very different picture and that leads to a diagram which is called the pants diagram so let me here we have the two strings coming together overlapping completely this is off luckily no I never considered it no I never considered it by the way because when they were constructing string theory they never knew they were doing string theory they were playing around mathematical formulas what they constructed was this way of string a and B Turing the string seat like the sections of your pants as you go up to trial two cups turn into a string there's if the pants are well made there's no singular point on the plans there's no place where the interaction occurs there's nothing to insert in fact once you know how to make her cuffs you know how to make the whole pants you can only tell by looking at the end whether you have one string or you have two separate strings it's a remarkable thing this kind of interaction from this point of view it's sort of unique topological there's no place you can insert additional information that offers the tantalizing prospect of being able to calculate everything there no free parameters it also means you can't change anything in string theory you get what you get once you discover know how to describe free strings you now to describe interacting strings now of course it's not so simple because this is sort of a perturbative you start with free strings and then they direct once they direct twice it's a perturbative way of constructing and that's roughly how we understand string theory a whole bunch of rules for constructing solutions for turbit of lis but all of them are essentially unique if you have a good starting point totally remarkable and the first time in physics string theory has it amazing properties all of which I think are connected to that basic fact it actually started not as an attempt to unify all the forces of nature describe quantum gravity but rather to describe the nuclear forces and today we understand that because we understand that how the nuclear forces work and we can calculate and this again is a picture of two quarks through very heavy quarks which I'm holding their close together where they barely interact and then we pull them apart and we can calculate what the fields the chromodynamic fields that hold them together look like or lattice gauge theorists can and this is the picture and you see that the forces and the energy these fields blue stuff are the fields and where all the energies are confined to us kind of two between the quarks which is why they can't escape it's like a rubber band that you can never pull it apart pull them apart completely and that tube is sort of like a fat string that's why we now believe that such mesons made out of cork anti quark pairs have properties that are string like and that's what led people based on observation to invent strengthen but then it turned out that there were all sorts of surprises that came out of the theory now remember this is not a theory you can fiddle with you get what you get you have no choice unlike it and the other time in physics I must say so when people encountered the fact that space-time had have 26 dimensions or there were inconsistencies they were disturbed because it didn't look like that especially in the nuclear domain then they discovered supersymmetry in effect in order to have fermions like quarks particles happen de jure spin that reduced the number of dimensions to ten then they discovered that the theory had local interactions and symmetries which now we understand is that the basis of the nuclear force but not a property of the mesons and then they discovered that the theory had to contain gravity which they weren't looking for they were just looking for a theory the nuclear force luckily they were when the theory of nuclear force came along string theory by and large was abandoned except by some brave souls pioneers like John Schwarz was in the audience who persisted and extended and recently we've discovered many more amazing properties which I don't have time to explain we've discovered that string theory isn't really necessarily a theory of strings there's an equally important role for other an object's brains brains of various dimensions and we've discovered an incredible array of totally different descriptions of this theory whatever it is all related to one another by an intricate web of so called duality and much more well string theory has achieved quite a bit although it has also failed in substantial way its achievements are that it is some kind of extension of the conceptual framework of physics as I said but so far sort of minor one not on the same level of relativity quantum mechanics we've replaced strings but particles by strings but not much more at a fundamental level but some of us still believe that what string theory will end up being will be an equal revolution relativity in quantum mechanics involving the third dimensional parameter of physics the Planck length which is associated with gravity it has provided us with a consistent finite theory of quantum gravity which is very important but most important most intriguing exciting is that string theory has this incredibly rich structure and it is in some sense extremely unique as a theory so it seems to have the possibility of having all the ingredients that are net that we think are necessary we don't know of anything in the real world standard model and anywhere else that isn't contained naturally in string theory and it's an incredibly unique theory so I want to discuss the achievements before I discuss the failures quantum mechanics and gravity were hard to reconcile for 70 years the fact that string theory provides a consistent theory that reduces to Einstein's theory for large distances and it's consistent with quantum mechanics is a quantum mechanical theory is a proof that there there's nothing inconsistent between general relativity of Einstein and quantum mechanics but more important it now provides us with tools to explore the truly paradoxical and weird issues that arise for strong gravity a very hard task but we now have the tools and the paths there have been some successes the most the biggest success is the fate of black holes a problem that was raised by Hawking who noted that black holes quantum mechanically aren't really black treated by crude quantum mechanical tools black holes act as thermal objects and a bit thermal radiation but he realized that that's a bit of a problem because if you throw ordered stuff if you throw stuff into a black hole and then it emits thermal radiation you've lost something you've lost information and Hawking was so disturbed by that this well being a relativist concluded that the laws of quantum mechanics are indeed violated by general relativity in general in particular for black hole because this process violates the preservation of information which is character and one of the main features of quantum mechanics string theory truly saves the day here by giving us examples which we have we believe our representative of all black holes but examples where we can precisely map this description of this process onto a theory where information is clearly preserved and seems to most of us to resolve this problem in fact even Stephen Hawking has agreed that he was wrong that that black hole formation evaporation will not violate the laws of quantum mechanics but there are many other paradoxes in quantum gravity where in the regime where gravity is strong and the quantum effects are big that we don't yet understand the most important one of course is the origin of the universe could string theory actually explain what goes on at the beginning how could you possibly explain what goes on the beginning well this is one of the most important questions that we're studying and clearly the answer so far is not yet not at all we don't yet understand but we have the tools to continue exploring this problem and it might take a long time on the other hand string theory also suggests at least to some of us that are very notions of space-time which have been used so far in traditional gravity relativity also in string theory are going to have to be radically modified what about the goal of producing a unified theory answering those why questions reproducing the standard model in calculating its parameters well there it's not easy we seem to have all the ingredients but to actually do the job you'd have to do a lot since we can't do measurements at these very high energies we'd have to understand lots of hard dynamical problems like these extra dimensions of space why don't we see them and this supersymmetry which is so important in string theory how is it broken because it must be and then there's the problem of the cosmological constant so briefly these problems the new dimensions might not be seen because they're very small that's the traditional understanding goes back almost a hundred years to the imagining new dimensions that are curled up in little circles so you don't see him well actually in string theory there's something like at least in some ways of constructing approximations six extra dimensions which we can learn about by trying to solve the analog of Einstein's equations for the structure of these dimensions get beautiful structures emerge and in a way that would have pleased the Greeks if you try to construct such solutions of string theory the nature of the forces what kinds of gauge forces emerge the form of matter the values of the masses are all determined by the geometry the shape of these hidden dimensions it's very beautiful and there are approximate solutions things you try to construct along these lines that look a lot like the physics we observe however turns out there are zillions of other possibilities that emerges approximate solutions as well don't look like what we observe and then finally there's the cosmological constant problem so remember this is the vacuum this is the empty state this is the vacuum the vacuum is full of all these fluctuating quantum degrees of freedom these fluctuating fields they move back and forth and they all carry energy all that motion has a lot of energy that energy energy mass acts back on a metric of space and time that's the theory that's our theory of gravity it's preserved of course in string theory generalized but this energy in the vacuum will act on geometry and will lead to a cosmological constant which is the name for the vacuum energy and such a vacuum energy has effects on the structure of space and time leads ooh accelerated expansion or Excel or accelerated collapse in general now if you naively estimate the cosmological constant of the fluctuating fields in the vacuum the natural scale is this very high energy scale and that leads to an enormous vacuum energy one so enormous that the universe would have collapsed instantaneously or expanded so much that each elementary particle each quart each electron would be alone with no neighbors that could communicate with certainly not observed this problem has been around in theoretical physics for almost a hundred years and in pre stringy era you try to calculate this number you didn't get ten to the hundred twenty you got infinity well physicists have a very normal reaction when they encounter infinities in their equations they simply put them equal to zero they don't understand why but they say well it can't be infinite so it's got to be zero which was the attitude for years and years and years even in string theory after we got this big we got a theory where there are no infinities still it was too big so had to be zero this is order of magnitude maybe there's some deep reason why it's zero one of the attractions of supersymmetry when it was discovered for at least a week or two was the fact that because of a deep symmetry principle the cosmological constant was zero but then of course people realized that well it's perhaps to be broken and that introduces a scale and it's not zero it's 10 to the 60 the attitude of most of us for years was there has to be some reason why it's zero because it's too big so there's a zero nobody had a reason why was zero that was the attitude this became more and more of a severe problem because you couldn't find a reason for the zero and finally as you know measurement of the cosmological constant was reported by astronomers in fact Perlmutter led one of the groups here at Berkeley to observe the acceleration of the universe which is presumably almost default explanation is cosmological constant but very small 10 to the minus 4 electron volt square so if that's true we've not sure but probably looks like it much harder to say that it's zero because it's been measured never understood why was zero anyway so that's a big problem and in string theory if you try to construct the solution by this perturbative method of building up a solution where I describe you you can calculate what lambda is and you're you know you're you're going to end up with a value which is much like probably around here no matter what you do well this fact and the fact that there are so many different kinds of solutions of string theory these perturbative constructions have led people to following scenario some of the people are in the audience my colleagues the landscape the landscape is a is a picture of the possible solutions that you seem to be able to get out of string theory more or less with some confidence and these different places in the landscape are different kinds of solutions they have different shapes of these internal manifolds the compact dimensions different forces different matter different masses different cosmological constants different now you add to that the idea of eternal inflation of the inflationary cosmologists like Andrei Linde and others that the this inflation that occurs makes the our universe expand from a very small portion of a state that existed before could have happened many times in a bigger multiverse and you might end up they say in any one of these different states universe a bad place big cosmological constant universe expanded so rapidly galaxies never form planets never formed people were never there to ask the question so we're not there we're in universe B where the cosmological constant happened to be very unlikely but happened to be small enough so that galaxies form planets form we formed we're here that's why we're here this is the anthropic principle unfortunately I can't prove it it's wrong but the main argument for it is driving all of my otherwise rational colleagues is this how could we possibly explain this incredibly small number or you know or sort of what we observe to what we would predict well I want to make a little side comment for their benefit they haven't heard it some of them are that we should be able to it's not such a big deal you really have to take the fourth root of this number because then you get a mass scale I mean you so this is the ratio of the scale of because laws are constants of the scale of where we would have calculated then the minus 16 is still a small number and hard to explain Dirac was the first to face this problem of large or small numbers that are hard to explain he noticed that the proton mass to the Planck mass was 10 to the minus 19 he also noted for other large numbers like the number of protons in the observable universe which happened to be the 4th power 10 to the 80 about four times this inverse of this number Dirac said why is this number so small how could I possibly they explained it well he the only explanation he could think by the way he could have invoked anthropic arguments if this number were 100th we wouldn't be here a hundred protons together form a black hole so we can't live in a universe where the proton mass to the black mass is 10 to the minus 19 it better be at you know something of ordered this magnitude in order to have enough protons to have stable stars before without forming black holes but the Ark was much too girag to invoke and tropic arguments instead he suggested that all of these big numbers were related one another by methods by reasons he had no theory for but they were related and that remarkably led to a prediction so the number of protons in the visible universe increases so therefore the in this the inverse of this number should increase as well and you could should increase substantially in the Hubble time and so it is both the Newton's constant the fine-structure constant had time dependence and that was a prediction that you could test indeed and rule out and soda rocks theory is wrong the topic argument I'm one of the reasons I don't like them they might even be true but I don't like them because you can never precisely rule out out except by calculating well in the case of the rocks problem we actually calculated this number or this problem was solved by QCD the argument is very simple you start with a well with a few assumptions like forces are all going to be unified well we there's evidence for that at it's a very high energy scale so the strong force is more or less like the electromagnetic force within a factor of two or three at this scale that's rough the Planck scale that's where unification shall occur and then you run the coupling the straw uncoupling to lower energies and it increases that's uh some thought of freedom and that's what sets the scale of the nucleon the distance of which the pork luan fields are confined is set by the strength of the strong force that sets this scale relative to this scale and it's because the forces run logarithmically it exponentiate so because of the physics of the logarithmic running of the scale to explain this number you really have to explain the logarithm of this very small number which is much easier and is naturally explained indeed by the running of the strong coupling country so we can calculate this number that Dirac was so 10 to the minus 19 within a order enacted there are two other very small numbers or large numbers that we think we can calculate we have ways of doing like the mass hierarchy the scale of the electroweak to the gut scale 10 the -16 slightly bigger antenna minus 19 but it's also you know hard to calculate right that's like this like the cosmological constant well that we think again is logarithms same old trick with the additional physics of supersymmetry for example there's another very small scale the neutrino mass scale we think we have a different mechanism seesaw mechanism calculate that very small number compared to electroweak scale so these are three examples of being able to calculate numbers of order this incredibly difficult number to calculate the problem is not the fact that it's small problem is simply we don't understand how to calculate it but either did Dirac Drakh had absolutely no idea of the physics that could explain 10 to the minus 19 ratio of the fundamental scale of nature the planks and the size of the proton well I don't know the answer and maybe there isn't but just because we don't know the mechanism doesn't mean that it doesn't exist to conclude that there is no such understanding and that we have to resort to anthropic arguments is a little premature in my opinion especially since all of the arguments are based on our present understanding of fundamental physics including string theory and we don't really know what string theory is we have all these ways of constructing proximate solutions but you ask a string theorist get them alone get her alone beat on earth what is string theory they'll admit they don't know they don't know how to write down the basic laws that give you various descriptions all five kinds of string theory something called m-theory we know little about or some of the more modern and powerful descriptions of string theory which happened to be old-fashioned field theories in fact field theories which are very close to the things we use in the standard model so could argue string theory is not any different than field theory this is one of the failures of string theory we don't really know what it is after 40 years we don't really have a formulation you know in physics do understand a theory sort of when you can put it on a computer we have can't imagine putting string theory on a computer we could put a calculation a part of a perturbative proximation to the beginning of a construction on a computer we couldn't put the theory on a computer you just have array of dual formulations most of which are defined by perturbing around some consistent background something seems to be missing could be that string theory isn't there's something truly missing it's not a theory it's a framework quantum field theory is fury it's a framework and then you add symmetry principles or whatever to make a specific theory what's attractive about string theory is that you don't have that possibility there's nothing you can modify but it also doesn't smell yet like a theory we can't write down the equations or the principle so it's strange maybe something's missing but we don't know what it is but maybe it has to do with well or come to that in a moment the fact that we sort of need new rules and we need new rules to paraphrase Bill Maher because we are faced with new questions that we really haven't had to or have the tools to begin to address before in particle physics the rule the rules were formulated to answer the traditional questions of physics what is the state of the vacuum now and the small perturbations excitations about the vacuum how do we calculate experiments in the future given our knowledge of the present but now when discussing quantum gravity discussing the quantum cosmology we are forced to discuss the whole universe as Einstein stress in a theory of gravity a theory of the dynamics of space-time the answer is not the state of the universe at a given time the whole thing the beginning the what happens at the boundaries if there are boundaries again and we've never had to face these questions before but you can't avoid them in a theory of quantum gravity none of our solutions that none of that landscape that people use to discuss the universe has a consistent account of either of the beginning and the end or the boundaries what are the rules to construct a spacetime history of the universe and I don't think we really know what the rules are we might not have the right framework for discussing those rules which might be just as well because in the framework we have it's hard to imagine what the rules would be we might have the right framework because many of us are suspect that our understanding of the nature of space-time as revealed by string theory whatever it it will have to change ed Witten said that space and time may be doomed which scare some people and sigh bird I'm almost certain that space and time are illusions other pretty good illusions and I've gone much over one of them but we have evidence within this theory that we discover this theory that we've hit upon and it is sort of unique and you get what you get but one of the things you get is the realization that space is an emergent phenomena that it's somehow not fundamental it's a crude description of physics at large distances and we have many examples in which some or one example which all of space emerges as a large distance classical approximation to a different description and a feeling therefore that in if we ever had a fundamental description of string theory whatever it is it couldn't be based on space as a fundamental concept but in relativistic theory if we believe that space is immersion then space-time must be emerging space and time have been unified since Einstein and we have absolutely no idea at least I have absolutely no idea what it might mean to formulate physics without time we have examples now where space emerges all of space all of the dynamics of space gravity but time how would you start formulating physics without a concept of time physics is about predicting the future moving in time time evolution it's about time it's hard to imagine formulating and that I think is one of the the barrier to a our inability to see what string theory is because I suspect that the same with string theory is it would have to be in a framework in which both space and time were emergent concepts and that's very difficult and will require a true conceptual revolution which hasn't it come and probably is not right around the corner so to summarize string theory as many hopes for the future unified theory of all the forces new concepts of space and time and maybe resolving these real paradoxes of quantum gravity and cosmology these are difficult problems and will require true conceptual revolutions I believe so meanwhile luckily string theory has other things it is doing and can be usefully employed it gives us incredible new insights into the kinds of theories we use in the standard model and elsewhere into ordinary quantum field theories especially gauge because of these formulations sometimes which can describe string theory as a field theory as a theory of particle remarkable and useful for example the famous EDS CFT correspondence between string theory and a curved space-time five dimensions and a ordinary gauge Theory much like the standard model gauge theories the boundary of that space in ordinary flat dimensions space-time four dimensions with no no gravity a large amount of supersymmetry but that's not that essential can be used to for example discuss properties of these gauge theories that are very hard directly but easier using the string picture and that's been used in some remarkable new applications now to discuss this quark gluon plasma and even in kinetics to discuss quantum phase transitions and strongly correlated systems mathematics is another area where string theories had incredible applications because it seems to again throw up new kinds of mathematics or fascinating new phenomena djegal scenarios whereas string theory is not in a position to predict make predictions that could falsify the theory if we knew how to formulate the theory it can give totally new ideas for where you might look for experimental phenomena new kinds of experimental form and finally coming back to the origin it we especially with these relations between string theory and field theory we might truly be able to construct the original motive for string theory a theory of these meson flux tubes so I'm going to go very quick through the possible new things that might happen extra dimensions larger pots of interesting from the logical scenarios here there has been this QCD string the string that proximate string that describes these mess ons does we believe for sure now have a description in terms of a kind of string theory which people are making enormous progress in constructing expressively and hopefully someday will be a useful analytic tool for describing these nuclear mesons so the message we have a wonderful theory of elementary particles but the most exciting questions remain to be answered new accelerators and experiments are coming soon they might discover quantum dimensions of space and time exciting new discoveries are around the corner string theory as many successes much promise that the costs of its failures so far the best is yet to come thank you

Channel: UC Berkeley Events

Views: 178,519

Rating: 4.6535435 out of 5

Keywords: uc, berkeley, ucberkeley, cal, webcast, education, physics, phy_20071019, yt:quality=high

Id: AM7SnUlw-DU

Length: 98min 10sec (5890 seconds)

Published: Fri Oct 26 2007

Reddit Comments

[u/[deleted]]

I'm going to spend the next 1:38:00 attempting to watch this. Hopefully in the interim one of you geniuses will comment a tl;dr simple wiki explanation for the rest of us.