David Gross: Frontiers of Fundamental Physics

thank you clearly been a pleasure to visit here today I've had met some new friends and some old friends and we've had some wonderful conversations on fundamental problems in physics which I'm not going to tell you about tonight I'm gonna tell you about some other problems having to do with what I'm calling fundamental physics now fundamental is a loaded word and everyone likes to be fundamental nobody likes to be unfunded mental what I mean by fundamental physics is that branch of physics that tries to understand the underlying principles in a very reductionist sense of how the universe works what it's made out of what the elementary particles atoms Forks electrons that make up all of the matter that we can observe and what are the nature of the fundamental laws of physics that act on these particles and produce the dynamical structures that explain our observations of the physical world I will I do explain to you the current state of this field which used to be called elementary particle physics sometimes is called string theory nowadays and I will explain what the what we do understand but I will largely explain what we don't understand and the problems we are facing which in fact as was hinted at by my banquet speech are the frontiers of our subject now the field of elementary particle physics is one in which enormous advances were made in the last century where we largely figured out what goes on inside the atoms that make up all the matter we see around us and we identified both the constituents of matter and the nature of the forces that act on those constituents though the consistence of matter we learned our electrons and that surround the nucleus the very small nucleus of atoms and give rise to the structure of atoms structure of molecules to chemistry to biology to all of macroscopic physics and towards the end of the 20th century we started to probe inside the incredibly small nucleus at the center of atoms and learned that the nucleons that make up the nucleus the proton and the neutron are made out of elementary particles that we finally identified and understood to be quarks so in a sense all the matter that we have ever observed we now understand as the Greeks supposed a long long time ago are made out of little point like objects they call them atoms we now understand that the atoms themself have structure they have electrons surrounding nucleus which in is made out of quarks and the other part of the story is to understand the laws of nature the which are expressible in mathematical form and describe how these quarks and electrons interact often in such complicated ways that they give rise to the marvelous structure and variety of physical phenomena in the universe and it turned out that three forces are needed the force of electricity and magnetism that acts on the electron and on the nuclei and holds the electrons in place around the nucleus and was known already and understood in its classical form in the 19th century and to forces that act only within the nucleus the strong and the weak nuclear force the understanding of all of these forces and of the nature of the elementary particles the quarks and leptons constitute what particle physicists call the standard model of elementary particles it really should be called the standard theory it's a extremely precise rigorous well-defined theory that whose equations can be summarized on one t-shirt and which is schematically described by this diagram in which all the elementary particles that we have discovered and mostly measured most of their properties and all that are needed to describe all the matter we've ever produced in laboratories or observed on earth or in other stars and three forces forces which are mediated in our modern understanding by particles themselves the quanta of the fields that mediate the force are for example in the case of electromagnetism the quanta of light the photon the carrier of the electromagnetic force and ripples in that field up as light rays similarly we have the weak and strong nuclear forces are mediated by particles we call gluons and W and Z's ordinary matter the matter that we are made out of that is not so radioactive radioactive as to disappear immediately is made up out of protons and neutrons which are made up of up and down quarks electrons revolve around the nuclei and neutrinos are their partners in some sense well we've also discovered that there two other similar families of quarks which except for their masses they're quite much heavier are similar in all characteristics to the first and best-known family and we give them strange names like charm and strange quarks top and bottom quarks and and their neutrinos and heavy electrons like the muon a towel and so on in any case this is all the matter that we need it seems more with one exception and all the forces that we need with a major exception force that makes this go up down and fall the force of gravity which plays very little role in the structure of atoms the nuclei so at this point we ignore it now I'm going to teach you a bit about the standard model and one of its mysteries namely nature of these three totally apparently different forces force of electromagnetism mediated by the electromagnetic field and it's quanta which is the quanta of light force the strong force that holds the quarks together inside the nucleus mediated by an similar field called the chromodynamic field and couples quarks together and the weak nuclear force mediated by a weak nuclear field whose quanta are W and Z particles now these forces share a common characteristic that is very deep and profound in some sense all of these forces are the same kind of force and yet they manifest themselves in ways that are totally different in the world that we observe in macroscopic physics in atomic physics and nuclear physics fact they're all the same suggests that they are profoundly related in prayer even unified at some scale and that's one of the frontiers the current frontiers and the fact that they seem so different an ordinary atomic nuclear microscopic life has to do with a profound understanding of the quantum vacuum I'm going to try to explain that a bit but before I do let me just remark that this theory we have based on these four understanding of these forces and the measured properties of these elementary constituents of matter is an extraordinarily successful theory of fundamental physics it is extraordinarily well tested in thousands of different experiments with extraordinary precision sometimes precision which tests the comparison of theory to experiment to one part in a billion or even better and it works as far as we can tell for almost all observations that we have made from the scale of nano nanometers a billionth of a billionth of a centimeter or less through the edge of the universe the same forces and physics described by the standard model accounts for the structure and evolution of stars and of galaxies and together with a classical understanding of gravity accounts we're just about everything we have so far with one or two exceptions measured about the physical universe that's the scale of applicability of this theory standard theory is amazing 60 orders of magnitude six factors of ten same theory can account from quarks to nuclei to atoms to stars the universe but I don't want to spend a lot of time talking about its successes and tests and how well it works because as I said in Stockholm the most important product of knowledge is ignorance and as the Dean told you you know you should remember that but not you shouldn't applaud stupid ignorance stupid ignorance leads to bigotry into Wars and to hatreds and and to mistakes but informed ignorant the questions that we can ask once we understand things well enough we can ask in fact we were driven to ask new questions questions that are scientific and I distinguish between questions that are scientific and questions that are not in questions that are scientific are questions that can be answered to have a way of thinking about them that you can approach their answers observation or by controlled experiments or by theoretical modeling theoretical reasoning those are interesting questions and by the time science gets to the stage where it can ask a question in a way that it can be answered by these methods the answer is usually not far away through the history of science we've gone through periods where questions that were asked by philosophers and religious leaders and were not scientific questions became scientific and then were rapidly answered and so that for a scientist that's it's the ignorance that's the most exciting part in science once you've learned something and understood it okay it's fun but it's much more interesting to be challenged by the open questions that's what drives science we are always I like to say moving out in a sea of ignorance enlarging the realm of our knowledge I'll come back to this picture at the end of the talk where I will ask how long can this go on we go on learning new things forever might at end the moment as I said and Stockholm there is no danger of ending there are so many fascinating questions to ask their questions within this fundamental theory that we have there are aspects of it which were not totally sure of and like everything in science we must continually test the predictions of our theory in order to make sure that it's correct or perhaps there's some small and perfection which will tell us that in it either needs modification or needs to be replaced there are phenomena that we have observed recently that don't easily fit into the standard model dark matter that seems to pervade the universe is the best example of this dark energy a sort of energy of the vacuum and pressure of the vacuum that causes the accelerated expansion of the universe is another one of these puzzles somewhat of a theoretical puzzle but a puzzle none the list at the moment the fact that these forces that we need to describe the physical standard model seem very similar but are quite different suggests that we look for a way of unifying these forces and we are also being challenged nowadays to question the actual once again the most primary of our concepts about physical namely space and time itself so these are the issues I'll briefly describe where we stand with respect to them and and I'm going to try to tie them all together by relating them to the properties of the vacuum packed particle physics fundamental physics largely is a an attempt to understand nothing once we understand the vacuum the rest is kind of trivial usually particles that make up atoms and people and so on are all little ripples and the vacuum first thing to understand is the vacuum and I I made a joke of that and said well we the goal is to understand nothing because that is most people's vision of what the vacuum is vacuum is like this slide nothing that is a reasonable description of the classical vacuum but not a good description of the quantum vacuum and it's the properties of the quantum vacuum that I are going to gave what give rise to many of the reasons that explain why these three forces that act within the atom appear to be so different although fundamentally they're really the same kind of force it's the properties of the vacuum that explained why quarks are confined asymptotic freedom that among the rest explains why you can't pull quarks out of the nucleus like electrons out of the atom why there aren't wires that conduct quarks as there are wires that conduct electrons why the properties of the quantum vacuum that explained that this so-called Higgs mechanism that we're waiting for its final verification the LHC and give rise to the masses of the elementary quarks and leptons and it's the property of the hypothetical property of the quantum vacuum that allows us to imagine new and profound symmetries of space and time and I'll discuss one of them supersymmetry so in quantum mechanics relativistic quantum mechanics that we use to describe the real world the vacuum is a very interesting place it's not it doesn't look like this at all it's not the boring empty space like what you might imagine this is a description of what the room this room would look like if we removed everybody from the room and all the atoms and all the molecules and turned off all the fields no electric fields no magnetic fields you're left with nothing that's classical vacuum but classical vacuum is empty and quite boring it's the ground state of the system you remove all the energy everything relaxes quantum mechanics that can't case this was understood in the earliest days of quantum mechanics sort of a sense expressed by Heisenberg's uncertainty principle or a consequence of that that every dynamical object including all the quantum fields fluctuates because when you observe it you disturb it and it moves innocence though the wires you can picture a classical pendulum in its lowest energy state is having no motion at all and being totally still clearly that's quantum mechanically inconsistent say with the uncertainty principle since this has a definite position and no momentum definitely momentum and when I observe the pendulum I must inevitably interact with it the light that I shine on the pendulum to see if it's there and motionless gives it a kick and there's some what we call zero point motion of this dynamical variable of this little pendulum the same is true with quantum fields with the electromagnetic field in a quantum theory of electromagnetism which can be thought of as lots of little pendulum and when you take this empty vacuum and try to see determine whether it has all the electric and magnetic fields actually turned off and zero you interfere with this vacuum cause little fluctuations in the electromagnetic field itself but I'm going to show you a picture of what the vacuum looks like at a scale of resolution of one nucleus of one proton now it's I can't do that again show you a photograph or a movie of some way of picturing or observing the structure of the vacuum at that scale but I can show you the result of a calculation in our theory of the strong nuclear force and the strong nuclear fields that give rise to that for and this is what it looks like the quantum vacuum is full of fluctuating fields and this is one way of picturing them these are measures if you want of the fluctuate fluctuating regions of different colors are different ways of measuring the magnitudes of these fluctuating chromodynamic fields that act between quarks and give rise to the nuclear force this is not a simple boring empty state with our conventional description it would it looks like a complicated dynamical medium and it's a much better way of thinking about the lowest energy state or the vacuum in a quantum mechanical system of many electrons or of quantum fields now it's the properties of this state this nth vacuum that cause the various forces of nature that act within atoms and molecules and nuclei electromagnetism weak and strong forces to be different because at the core they turn out to be the same thing the only difference between them is 1 2 3 electromagnetism is a theory with one kind of charge Ettrick charge there's a field that acts whose source is electric charge the weak and strong nuclear forces that act within the nucleus are at their core theories with two kinds of charges or three kinds of charge but aside from that the second kind of theory charges that source these special kinds of fields that Maxwell and Faraday discovered in the case of one George and this earth site appears kind of crazy because first of all electromagnetism is a force that we feel it we've all felt that when you've shuffled along and felt a spark fly from your finger or you touch a wire and you feel the flow of electrons it's not necessarily very pleasant you've never felt the flow of quarks or a something like cork magnetism that would repel to magnetic the charged pork systems but why are they so different these different kinds of forces the force of the weak force is very weak and acts only at very short distances inside the nucleus no one has ever felt a weak force at macroscopic distances the force mediated by the electromagnetic field or by that photon you saw going back and forth is the easiest to understand and indeed you don't need much more than high school physics to learn about the Coulomb force that exists between charged electrons and charge of positive and negative charge between electrons and there say anti particles of opposite charge so a long-range force it falls off like a inverse square law you wiggle the charges you this field whose field lines are described here undulate those ripples are in fact light rays travel with a velocity of light in quantum theory of electromagnetism we say that the force is mediated by the quanta the particles quantized packets of energy and momentum that the so called photon the quanta of light that's a force law gives rise to Coulomb's law need a pull one electron away from another electron and ionize an atom the energy as a few plotted versus the distance between the electron and say the positron the anti-electron saturates if you pull them far enough away you can ionize the electron you can ionize the atom as we do in order to create electricity and charges running through wires a weak and nuclear and strong nuclear forces are if it wasn't for the properties of the quantum vacuum would be exactly the same in the case of electromagnetism you can forget about the quantum properties of the vacuum they are very small and have very little effect on this picture under most circumstances but that's not the case for the weak nuclear force there there are two charges and we give them names we say there's an up charge and a down charge but there are two kinds of quarks and they differ in their weak charge but they're both like the electric charge charges through just two of them and the same is true the electron in the neutrino they also come with this these two charges and just like an electromagnet ISM there are fields that mediate the force between these charges except now since there are two charges there are more fields actually two times two minus one it's got a remember the minus one those are described there quanta are the so-called W and Z bosons and they can do more interesting things they can for example turn a up quark into a down quark by emitting a quanta of this field and which then turns in say to a neutrino and I positron now these up Qin down quarks are usually inside the proton and when that the up quark turns into a down quark proton turns into a neutron with the emission of an electron and a neutrino when the nucleon is within a a nucleus by gold it can turn into platinum and this is known as radioactivity or the transportation of elements but so the having two charges you have more fields in these fields can do more interesting things they can turn one charge into another thereby changing the atomic number changing the l1 element into another but there's another reason why the weak force is different and that it had to do with the property of the vacuum that's the reason the weak force is not a force that we feel and atomic distances or macroscopic distances and that has to do with the so-called Higgs mechanism so if the vacuum were as trivial as we would imagine just emptiness nothing else could be it was excited or fly 2:18 then our corks oops our quarks and leptons would be totally massless and they would move with the speed of light through this bicycle symmetric vacuum the reason they don't the reason they have a mass is because there is some other stuff that lives a mother field some other stuff that lives in the quantum vacuum and is pervades it breaks a symmetry which I've indicated here let me not get into that we call it a condensate it's as if the vacuum is filled with this inert condensate of this field invented by Higgs and others and moving in that condensate the electrons and quarks bounce off the condensate if you want as if they're moving in some viscous medium and acquire a mass not only did they acquire mass but the areas of the force the W's and Z's that mediate that force between the quarks and the leptons acquire a mass and instead of giving rise to a force that acts at very large long distances or great force only acts at very short distances that's the so-called Higgs mechanism and ripples in this Higgs condensate would be observable as particles and that is the famous Higgs particle that is now being looked for at the LHC ripples in this condensate that give rise to the masses of the quarks and of the carriers and produced this very short range force now that yet has been seen the ripples of this condensate that's what we are looking for and there some of us think we've seen it but it's still somewhat of an open question now what about the strong nuclear force my favorite force well they are they're three charges each quark comes in through what we call three colors simply three charges up quark red blue these are just labels three kinds of quarks three kinds of charges and once again the force is mediated by a field very much like the electromagnetic field we call it the chromodynamic field because it couples the color and if it was just we just had this simple classical vacuum there would exist this field field lines between a cork and an anti quark and the force would fall off like 1 over R squared and it would just be like electromagnetism you could pull the quarks apart and have them running wires except you'd have three kinds of currents red current right blue current white cotton but now the structure of the quantum vacuum in this theory is much more powerful but that has to do with the nature of these carriers which themselves are colored or can change one color into another so they have in fact two colors and that these quarks are living in this medium and it turns out that's what we discovered in effect that this medium does not like to have this field running through it and it try to squeeze these field lines as much as it can but it can't completely because of these sources that create the field lines so it does the best it can and it squeezes them to form a tube like this in this complicated medium and that's the phenomena that we call asymptotic freedom it implies two things one that the quarks when they're us together interact weakly weakly more and more weakly that's not obvious from this picture what's obvious from this picture is that when you try to pull them apart the force doesn't saturate in fact the energy of this flux tube grows linearly without bound you can never without an expenditure of an infinite amount of energy pull the quarks apart so the force gets stronger at large distances the ionization energy is infinite you can never take a quark out of the nucleus that's called confinement and the converse of that is the fact that the force gets weak at short distances that's a synthetic freedom which is what we observe in the laboratory and which led us to understand that quarks are there inside nucleons and behave like free particles and that in fact led to this theory QCD of quarks with three kinds of charges and a force very similar to electromagnetism except that there are three kinds of charges and that the vacuum squeezes the flux tubes together in that theory indeed when you do the calculation and again draw these these pictures which show the quantum fields the quantum chromodynamics feel between the cork and the antiquark as you separate them you see that you get instead of them spreading out as they would without the strange properties the quantum vacuum you get this linear flux term well that discovery did lead to this very nice banquet in Sweden where you see you see the this is the Queen this is the Queen to be an hour alright she's down Sweden you don't have to be doesn't go to the first nail she is the eldest daughter so she will be Queen Victoria someday and when they give you a Nobel Prize aside from the cheque they and the metal they give you a very nice document beautifully with a original picture by a by an artist in this case describing three quarks trying very hard to get out of the proton bound together by this crazy force and this again is a picture of what actually happens in the real world when you pull these three quarks about you again see that the flux this quantum chromodynamics field has been squashed to form this kind of trident that again makes it impossible to pull a cork out of the protein so all these forces that we use to describe all of atomic molecular nuclear physics are all the same force the only difference between them is the properties of the quantum vacuum which suggests that we should be unified didn't have to be that way they could have been totally different kind of forces but they are there all these different ingredients it together seamlessly as if they all come from a single kind of force and single kind of matter and in fact there are very simple extensions of the standard model where you embed the whole thing in one force law with five charges you know if you have three and two and one you can make something with just five charges or ten this is actually the picture with ten oops and put all of the quarks and leptons into a natural system where they have beats want some combination of these five charges and account for all the matter we've ever seen elementary matter and and one force law and then you could recognize that pieces of this force law our electromagnetism pieces are the weak and strong nuclear forces but of course there's one problem with that is the electromagnetic force is much much weaker than the strong nuclear force that's why h-bombs are so much more powerful than TNT so dynamite works on chemistry which is electromagnetic interactions forces and atomic bombs thermonuclear weapons are work on nuclear forces actually even just a small part of nuclear forces the weakest possible to clear forces so how can these be the same if the forces are so different well that too has to do with this phenomena of asymptotic freedom because as you go the force is only strong when the quarks are far apart they're very very close together the force gets so it could be that this force strong force at very high energies or short distances would be of the same magnitude and one of the first things that was done when the standard model was completed in the 70s was to extrapolate our knowledge from where we now measure to very odd high energies and remarkably the three forces that act within the atom seemed to come together at a very high energy many many orders of magnitude beyond where we presently observed but the theory works and nothing prevents theorists from doing this extrapolation this fact is one of the most important clues that we have as to where the next frontier is observational II what scale of energies will something new happen this suggests that the new thing that will happen will be unification horses fit together and their magnitudes go outside if you look at the phenomena at very short distances when you look at that picture of the vacuum and scale down to very short distances all of the structure goes away so what you say is okay if we look at what goes on in the world and very very short distances and this distance here corresponds to something like 10 to the minus 33 centimeters you would see one force but most of our observations are made at distances like meters or centimeters or nanometers and there this vacuum the quantum vacuum has a lot of structure we just showed you a picture of it and that makes these different parts of the unique force look very different that's how we understand that all horses can be unified and yet appear to give rise to large-scale behavior that is very different so this is a nice picture you would like to understand and unification of the forces but it's also rather depressing because it's removed from present-day observation by fifteen orders of magnitude and there's no hope of building an accelerator that will directly probe such incredibly short distances so if we're ever going to learn about this we need either very good theoretical understanding or lots more clues and one place we might get clues is just around the corner from present-day observation your regime which is now becoming present-day observation a trillion electron volt machine like the LHC in Geneva and one of the things we expect from it is a discovery of a profound new symmetry of nature all supersymmetry now the LHC is supposed to discover the Higgs particle and give a nobel prize to phosphor on glare and Higgs and that will happen if it hasn't happened already it will be very nice but the really exciting thing for most of us is not that but discovery of something new and for which we have no direct observational evidence yet the most exciting thing for me is supersymmetry symmetry principles as we have learned over the last century from Einstein onward are in some way at the foundation of just about everything we've learned in physics and so I let me I'd like to explain to you briefly what supersymmetry is because it is a profound new symmetry of physics which we haven't yet discovered symmetries of physics that we have discovered for example our rotational physics emma trina the laws of physics are invariant under rotate do an experiment throw this up measure how long it takes to drop rotate my laboratory 90 degrees through the experiment again I get the same answer that's a symmetry of the laws of physics rotational symmetry so supersymmetry is exactly the same it's the laws of statement that the laws of physics are invariant under rotations of super space but now you understand what supersymmetry is right except I have to explain to you what super space it though super space well is an extension of space base time which is the arena in which we describe physical phenomena particles move as time goes on in space yields are functions of space and time and symmetries are invariance of the laws of physics under rotations of say the x axis into the y axis of rotations in space or in space-time now what is super space super space has another coordinate more coordinates it's as if we have more dimensions but not exactly these aren't ordinary dimensions I'm adding so instead of just X Y is Z we have theta but theta is not an ordinary a dimension we can also talk about that X real dimensions but here I'm talking about extra super dimensions and super dimensions our new coordinates that you need to describe particles fields but are measured with numbers that anti commute numbers whose multiplication depends on the order so theta one times theta two are two such numbers and you don't get the same answer if you multiply theta - I'm stator one you get a - same answer now mathematicians as you know can invent all sorts of crazy things crazy numbers like the square root of -1 it's called an imaginary number I squared is minus 1 now what kind of number square is minus 1 these numbers are perfectly good numbers they have rules you multiply theta one times theta 2 you get something but multiply opposite order you get the negative of that anyway there's a whole definition theory of such grassmannian numbers so call and there's a beautiful mathematical extension of space-time that we use in physics to include super coordinates measured with these anti-community numbers and of the symmetries that rotate these coordinates into each other you can rotate a super coordinate into a ordinary coordinate and so on that's super space and it's very appealing to a physicist because in physics we have two kinds of particles bosons and fermions which have very different kinds of properties statistically and we can it turns out associate them with these different coordinates in zubur space so it turns out that one can take every theory in physics that we've ever constructed and extend it to live instead of living in ordinary space to live in super space we imagine that particles are moving not just in space but also have a theta coordinate and they're moving in super space when you translate that back to ordinary Theory ordinary space and you can always do that a very simple way mathematically oops hit that then always do that simply mathematically by banding your functions in the so-called Taylor series and replacing motion of one particle in super space by two particles in ordinary space these particles have what we call in physics different statistics that's fascinating I won't go into that but it does mean that in supersymmetric extensions of our theories supersymmetric theories for every particle that exists and we describe an ordinary space there is a superpartner that describes if you want the motion of the same entity in super space if we do this to the standard model we have our quarks electrons and photons say we predict that there must be ask work we call an ass electron and a 50 no these are the funny names we've given to the partners of the observed particles and none of them has ever been seen now when you arrive to such a situation in physics who say okay that just the theories wrong but we could always say no the theories okay it's the fault of the quantum vacuum these particles maybe haven't been seen because the quantum vacuum doesn't respect the symmetry so for example the laws of physics are invariant under rotations right I walk into this room I say that must be wrong you're there and if I rotate 90 degrees I don't see anyone so how can physics be invariant under rotations physics isn't parent into rotations this room is not invariant under rotations I just look at this room it doesn't look rotationally invariant but take the room and remove everybody from the room all the people and the chairs and the molecules in the air and you're left with the vacuum and surely that is rotationally invariant and normally we think it is but maybe not remember that picture of the vacuum look pretty complicated could you really tell that it was rotationally invariant and if it's not rotationally invariant then the simple consequences of the symmetry of the laws of physics under rotations won't be evident there will be consequences but not as simple as simply saying that these particles have to appear with the same mass that we should have seen them maybe in fact they could be much heavier and that's the case it and we understand easily how that can occur the quantum vacuum is not invariant under this new symmetry of rotations in super space then this doubling of particles won't be evident until we go down to small enough scales where the vacuum looks invariant under rotation okay so that's a possibility but why do we believe that we might have a chance of seeing that seeing these new particles at energies of a TV which we can now explore and where we have perhaps some hope that the vacuum might begin to show that it is invariant under rotations and super space well they're a bunch of hints from nature one is the unification of the forces so I showed you this picture of the three forces this is a more accurate picture of the inverse of the strength of the forces so this curve here is describing the strong force this is one over the strength so small value means very strong so very strong at low energies and on a logarithmic scale it increases like a linear function of the energy so it's getting weaker and weaker this means getting weaker this is electromagnetism getting stronger and after thirty years of doing lots of very high precision measurements in order to extrapolate better and better calculations it's clear that in the standard model these forces these three lines don't meet at a point which means that okay maybe the forces don't unify we were just wrong before or maybe something's left out and if you just put in supersymmetry if you make the standard model super symmetric then you're adding all these supersymmetric particles that some energy begin to come in and play a role and it turns out bring these three lines together at a point to within one percent after extrapolating over 14 orders of magnitude which is kinda nice now that could be a indication that unification occurs and supersymmetry is evident at a TV which is what you need to make those curves turn over at a TV or could be a coincidence you never know in science so there is this regularity which could be explained by saying there's this new symmetry which would show up at a TV but you that just couldn't be a coincident the other important hint has to do with dark matter oh as you know astrophysicists tell us that most of the matter in the universe most of the stuff ordinary particles don't consist of quarks and electrons there's some new kind of matter that attracts things gravitationally and that's how we see it indirectly but they think it's very some kind of very heavy weakly interacting massive particle but what is it and we don't know we haven't seen this new particle directly in the laboratory haven't made these particles and we haven't detected the dark matter when that's passing through this room but supersymmetric theories naturally predict that the lightest of these new particles would be a perfect candidate for dark matter would be a very heavy the trillion of electron volts hundreds of times heavier than the proton a candidate for Dark Matter at those energies those particles would those masses those particles would interact with them cells very little and with quarks and leptons very little would be neutral wouldn't radiate this would be perfect particles for candidates for dark matter and it turns out that if those particles appear at around a trillion electron volts then you can predict how much dark matter there would be in a universe and it comes out right well we can test that we can try to produce these supersymmetric particles and see whether they have the properties to be dark matter but again this is an important clue but it's only a clue it could be again a coincidence there are other reasons I don't have time to tell you all but there are enough that make many of us think there's a very good chance that the Large Hadron Collider which is now operating at an energy which could start producing such hubris of particles and detecting them in these incredible enormous detectors could do so in the near future it could also verify that the ripples in the higgs condensate thing so-called Higgs particle this is an example of what such a eggs event would look like and there has been as you many of you have heard a few months ago in December the first reports of hints that the in fact has been discovered this is a compilation in one of the two experiments of data and this bump at a mass of 125 GV is a what would be in most fields overwhelming evidence of the existence of a new particle particle physicists are kind of spoiled because of the high precision and standards that they have imposed on themselves who announce so even with what would look to be pretty good evidence the experimenters and they are the judges at this point are not willing to to announce a discovery but they're very close to doing some and most likely the Higgs will be discovered within this year as this signal becomes stronger supersymmetry is harder to discover it's a very predictive new theory predicts lots of new particles and couplings but the typical Susy event this again is a simulation of an event in these massive detectors where you see various trajectories of particles in the event and what many of these particles can be identified and this is the kind of typical event which an experimental will look at and say aha here is perhaps a Dark Matter candidate because I don't see a large amount of missing energy and momentum that should be going out in this direction to make up the conservation of energy and momentum so what you're looking for one of the strongest signals for discovering new particles is the absence of something that's much harder to make sure that you just haven't missed some neutrino which also is hard to detect that supersymmetry hasn't yet even gotten to the stage of the Higgs which in my opinion has already been discovered but I'm not the experimentalist in the case of supersymmetry so far there have been very less meaningful but the discovery will be much more meaningful because once when you read in the newspaper a few years or next year or this year maybe that super suit that physicists at CERN have discovered supersymmetry what they're in fact discovering quantum demint once we understand that this is a symmetry of nature then we conclude from that we must that the correct mathematical description of of the space in which particles live and which fields depend on it's not just ordinary space-time but super space-time space in time we'll have quantum dimensions we might also discover evidence for new ordinary dimensions but that actually is much less likely well I don't really have time to tell you about the speculations about what happens at these very high energies where the forces we have these hints that the force is unified but one thing is clear from is that we have to consider as well the fourth force of nature the one that we all feel in the morning movie get out of bed in force of gravity gravity is the weakest force in the universe in the atom the force of gravity is 40 orders of magnitude less important than the force of electromagnetism or the weak and strong force it is totally negligible between say an electron and a nucleus the only reason we feel it and that it plays an important role in the universe is that it there is that there is essentially no anti-gravity gravity cannot be screened gravity is always attractive gravity the source of gravity is energy and momentum the charge of gravity which in many ways is similar to these other forces except that the charges energy and momentum and everything has positive energy so everything is attracting and that's why you can put together a big planet which exerts a measurable force on me just a lot of atoms pull all pulling and I can actually resist that force with just the expenditure of a little bit of chemical energy so little old me is resisting the whole earth is pulling down on this thing and just with a little bit of chemical G which is so much stronger inherently at ordinary distances I can resist it the remedy only becomes important under two circumstances when you have lots of matter together like planets or stars or you go to very high energies though because the source of gravity is the mass or the energy really the mass is just the measure of energy of objects that are at rest or moving slowly but it really couples to energy and to electrons which are moving very fast with respect to each other will attract each other gravitationally as strong as they will repel by electromagnetism the force of gravity pieces very rapidly quadratically with energy and if you go to very high energies or very short distances it in effect becomes comparable with the other forces of nature and so the attempts to unify the forces must take that into account and unify them with gravity which is what led most of us working on this subject to string theory which turned out to be a natural way of incorporating combining these kinds of forces electromagnetic kinds of forces with the force of gravity in a very natural way well we don't really know whether this works it has many many wonderful properties that give us hope that it contains a lot of the ingredients we need to describe all of the properties of the real world the atomic and nuclear forces in addition to gravity it is a but many mysteries remain its connection to the ordinary forces is and to the QCD in particular are quite profound in fact string theory began as an attempt to understand those flux tubes I showed you long before we understood the correct way of understanding the nature of these flux tubes that exist between quarks and antiquarks properties of the strong nuclear force led people to invent strings and that's not too surprising because these flux tubes between quarks and antiquarks is you pull them apart look like fat strings and it was in the development of string theory in the beginning what was in an attempt to understand the strong nuclear force not a unified theory of what everything these soap were so called open strings because they had ends to which you could attach quarks but then it was realized that these open strings could be closed in fact have to be closed and once they were closed it was discovered that the dynamics of these closed string looked like gravity the string theory turned out to unify the different kinds of forces of nature those that are responsible for strong weak electromagnetism forces in atoms and nuclei and gravitational forces automatically and has this interplay between these two descriptions of the theory either as closed strings and open strings or bottom fields that transmit the forces or gravity have been recently useful for understanding both aspects since this inevitably brings us to a theory of space and time which is what group Einstein taught us that gravity is many of the most fascinating questions we've been led to in the search for unification have to do with space and time for example one of the first things that emerged in string theory was that the strings looked like they lived in more than three dimensions next dimension six extraordinary dimensions and then extra super dimensions as well but six extraordinary dimensions and to be consistent with a real-world one therefore must look for solutions of that string theory in which those dimensions were curled up and there are such solutions where so at each point in space now there really are real ordinary extra dimensions all curled up into these little beautiful manifolds whose structure can be deduced by solving the analog of Einstein's equations that determine the curvature and geometry of this internal space now there are lots of such solutions we were happy at the beginning we found one or two but now there's zillions and zillions the fascinating thing about this is that the questions that within the standard approach to are impossible to answer like who decides what the nature of the forces are or what kind of matter you have or what are the masses of the elementary particles all these questions that the standard model is hopeless it just has to take them as given our GM GM Metra sized here they are determined by the shape of the hidden dimensions now since we have lots and lots of solutions doesn't mean we and predict what the masses are or etc we can look for solutions that look like the real world and there are lots of those as well but it's still in a remarkable clue to as to the direction of it hope in the end of trying to answer this is that there's a geometrical answer to such questions our real failure in string theory and a predictive sense is we really don't understand what the theory is it's one of the most remarkable developments I've ever seen in or know about and totally unique in history of physics that people have developed a theory about knowing what it is a theory which is totally rigid and rigorous and you can't modify it but you can't write down the equations which you're solving either well we really have our prescriptions for writing down solutions to equations we don't know and we're not sure what is yet missing in this approach bring so called theory is not a theory it's more like a framework but what is missing is not something we understand and it might have to do with fact that we are in string theory or in this line of fundamental physics where we are doomed to have to deal with the dynamics of space and time starting to ask new questions about the nature of physics and space-time itself first base time goes on our intuitive childlike versions of vision understanding of base time has changed in the history of physics the last two centuries dramatically dude Einstein in particular but string theory suggests as well as quantum gravity that we're gonna have to change it even more this is two quotes by two eminent string theorists ed Witten who said space and time may be doomed or space and time are illusions now those are very strong statements and and what could they mean space and time especially are absolutely fundamental in our form no physics so what would it mean to say that they're illusions or doomed but this is what string theory strongly suggests and and in some sense the major question that major challenge at the moment my opinion in fundamental physics what is the true nature of space and time so what to say it's space and time is doom really means that these are inappropriate they're not sharp enough concepts to discuss some of the issues that we are being forced to discuss in particular the behavior of physics at arbitrarily short distances times and that space and time should instead be regarded as a an approximate concept that you use for describing big things where we're used where it's a good approximation or long things facing time or for some kind of an emergent concept but then the question is what is it made out of and how do we make it that is a very difficult issue especially with respect to time we have many examples of under an increasing number of understanding of how space can be an emergent concept and we can have situations where a better description does not involve a fundamental concept of space-time points but time is very hard to eliminate and we don't know how to do that and then there are issues about space-time which have to do with the fact that we are faced with in fundamental physics with imagining a theory and a set of rules that give us a solution but in the context of the universe and context of dynamical space time it's hard to see how we can avoid answering questions like the universe begin because that's part of what we're trying to describe the dynamics of space and time and how could we determine an initial condition is that something that physics is supposed to determine what happened before at the Big Bang we know a lot about the universe but very little about the very beginning if anything even worse than that because we're preachers if are the object is the universe then that's the whole space-time history from beginning to the end and we need it would appear to specify a boundary conditions or that determine not just the nature of the beginning but equally well the nature of the end or what happens on the boundary if there is a boundary and here we simply don't know what the rules are so we're in a situation where we are maybe not able to get away with avoiding answering questions like this which used to be religious but are becoming more scientific and well formulated and maybe we're forced to answer them to get a answer to the questions such as why is the belong heavier than the electron well these are very hard questions conceivable we could even answer them or not so how long is this game going to go on of looking deeper and deeper into the fundamental structure and workings of matter can we go on forever though you know when I gave this banquet speech in Stockholm I was telling the Nobel foundation that they're very lucky because unless their new problems there are no new prizes and and they could run out but there's no worry about that but who knows we may construct the final theory that possible or maybe we're just not smart enough to do this shaking your head could be and we may lose the will to proceed so I'm going to say worded to about first two issues first has to do with what I call the geometry of knowledge always very nice to construct a geometrical model like and the usual model people have of knowledge I fight is really very bad it's called the onion model our knowledge is an onion you peel it away layer by layer and you get to the core but that's really exactly the opposite of what we do so remember that picture I had were moving out in a into a sea of ignorance and we're extending the domain of knowledge so just the opposite of the onion model and this geometrical model is better than the unequal because it explains certain facts that might appear mysterious about knowledge and ignorance and wisdom so first of all in this picture you see knowledge is growing everything inside that ellipse is what we know maybe we don't know it really but it's in the library so we can look it up or resort to Wikipedia which is usually wrong or often wrong but anyway knowledge increases like the volume of that red region all right now what about the currents the ignorance is also increasing there's a lot of ignorance in this picture all that black region but most of that ignorance were not where of I'm so you know the Greeks knew a lot but they were not aware that they didn't know how to unify the weakened electromagnetic and strong forces cuz they didn't know what those were but now we know enough to ask those very well formulated questions so the ignorance that we're aware of is just on the boundary of knowledge we can look out and see this stuff we don't know so ignorant increases like the surface so that explains by the way why you know the more we study the more we know but the more we are aware of what we don't know when we feel ignorant it also explains why wisdom increases us you know wisdom is sort of like the ratio of knowledge to ignorance and this is increases like the volume to the surface for a while at least so now we can use this model to explore the question can we construct the final theory so that question really reduces to is there a finite amount of ignorance now we know cases where that wasn't the case and we constructed a final theory it was called exploration though when people started exploring the surface of the earth they also had a model like this you know this is what we knew or what they knew and we were pushing out into a sea of ignorant and there were two possibilities the earth was flat and you could go on exploring forever and there your map would get bigger like the area of what you know and you'd know that there was this boundary which increased only like the you know the circumference you and you'd never run out of new territory to explore but of course isn't what happened it only looked flat locally and it was actually round and eventually we ran out of new territories to explore an explorer societies shut down we ran out of ignorance and exploration was over we had a final map actually you can now go into details but the global map was finished at least this current era so the question is is the sea of ignorance compact or not though there are two possibilities it's unbounded and infinite no like this and then we're in business forever and then we're in business forever and there's an infinite amount of ignorance and science can go on and fundamental physics at least can go on forever and ever and I don't see any reason why that couldn't be the case or it could be compact and finite like the surface of the earth and eventually we'll run out of ignorance and construct the final theory now that could also be the case I'm ready totally agnostic but there is an experimental test so how would you begin to know whether your this is the case or that it or it's flat well I think the signs would there would be a few signs that you'd first of all you'd begin to run out of interesting questions like we've ran out of places to explore you'd begin to notice that the ignorant was decreasing there's no sign of that so that's what I told him in Sweden if by the way if this happened if we did run out of ignorance would that be the end of physics not really because just like we with the earth you can even if you have this global theory of the earth you can still explore many other directions and so this wouldn't be the end of physics even in this reductionist sense there's all the infinite variety of stuff that perhaps in some reductions our scent would emerge from a final theory whatever it is but still itself has an infinite extent of exploration at the more macro level however there's another danger that is that we may be too dumb to proceed now this is something we should worry about because it's not easy to imagine things that we are too dumb to proceed to understand but we have many examples that that might be the case for example you know my dog cannot understand quantum mechanics no matter how hard I try to teach him and you know there's no question that the dog can learn certain things but it's not gonna cannot understand quantum mechanics there are limitations to the dogs intelligence are there limitations to our intelligence to our collective intelligence well I think there are many ways so and it's kind of hard to imagine that it would be kind of arrogant to imagine that there wouldn't be right who are we to claim that you know at this stage of human of evolution we are capable well there are a lot of outs to this pasta this danger as well first language has a certain infinite capacity capability as Chomsky remark a newborn infant can and often does utter a sentence that no one has ever uttered before can do so there's a certain infinite capacity to language and mathematics is the highest embodiment of language it clearly has some come measure of some infinite measure of capacity maybe not infinite enough but once you at least has certain a certain infinite capacity which might be enough second of all we is not a time invariant thing we can if we discover such limitations we might evolve or we might speed up the evolution that might would be necessary for us to develop additional capacity but again there is an experimental way of determining this what would be the signs that we are getting to the stage where we're too dumb to proceed where these problems which appear so difficult emergent space-time unification of all the forces whatever initial condition for the invert these are issues that are beyond our capacity what would be the indication now you can't ask old men like a a whether the fact you know you there's often the case that people who've struggled with problems for many many years comes of the conclusion that there is no solution because they haven't found it I think the best way of deciding whether we are faced with questions that cannot be answered by ourselves best indication that we're getting to that situation would be if people trying to approach the frontiers of science look longer and longer to begin to be make significant contributions then you would feel that they were coming up against some wall but it's not the case might appear superficially that we are faced now with much more difficult problems than we were 400 years ago when it was trivial but people the brilliant young minds are able today as 400 years ago and their early 20s to get to the frontiers of the most difficult areas of science and to make substantial progress I regard that as a sign that as a species were not nearing the point we seem there's no evidence that we're nearing the point of impossible tasks I think more likely the biggest danger is that we may lose the will and the means that go forward in areas of fundamental science that I described we would love to be able to do experiments that seem totally impossible and beyond all and it could very well be that in part astrophysics cosmology we're getting to a point where the resources and the will power of the sum'n society would not allow us to construct the instruments that would allow that would enable us to answer give us the clues necessary and the tests of our theories to address many of the most fundamental questions so I don't know what the answer is here I just hope not it would be pretty sad if that is what stops this effort there's again I seen no sign of that happening now but there are some signs that approaching the limit of our resources and here we just have to quote David Hilbert on his grave wrote must now we will know

Channel: Chapman University

Views: 15,191

Rating: 4.7647057 out of 5

Keywords: Chapman University, David Gross, Nuclear Physics, Research QCD, God Particle, String Theory, Asymptotic Freedom, Frank Wilczek, CERN, Nobel Prize Winner, Higgs Boson

Id: wk5xHbTLVOE

Length: 95min 25sec (5725 seconds)

Published: Fri Mar 23 2012