Public Lecture | The End of Spacetime

so good evening and welcome to this special installment of the slack public lectures actually this week we're hosting at slack a conference called amplitudes which is about the scattering of elementary particles part of the discussion is about impossible high precision calculations for the Large Hadron Collider part of it is about calculations of particle scattering and theories of gravity and if you know what that is supersymmetry and part of Adventures off into what you might call fundamental fundamental physics and people have actually flown in from all over the world to participate in these discussions so that's a benefit to you we have really a very special treat for you this afternoon we sorry it's still on we have really a very special treat for you this evening the speaker today is Professor Nima our Connie Hamed of the Institute for Advanced Study in Princeton Nima is known as one of the really most creative people now working in theoretical physics he did his PhD at Berkeley he was here at slack for a postdoc he was after that a professor in Berkeley then at Harvard and now has this position at Princeton the Princeton Institute for Advanced Study professorship has very simple obligations namely no obligations except to sit and think deep thoughts and some people can actually take advantage of that and Nima is certainly one so he will talk to you tonight about his ideas on the nature of space-time actually he will try and convince you that the notion of space-time is obsolete and he'll tell you what he's going to replace it with so please welcome Nima or Connie Hamed thanks so much Michael for the wonderful introduction it's always wonderful to be back at slack necessary as an intellectual home away from home for me and it's it's it's always terrific to a be back so thanks to all of you for coming out for an evening of light entertainment on this easy breezy subject and I really enjoy talking about this subject because it it highlights something where many of us believe certainly I believe that we're in a relatively special place in the history that of the development of fundamental physics that big questions are afoot big things are afoot and there are quite precisely drawn sharp challenges to our generation of people who has the privilege of working on these problems in the 21st century and I'd like to tell you about some of them and at least a few of the angles I first want to tell you what the sort of broad problems are that justifies such a crazy and grandiose title and also a few of the angles in to this grand problem that some of my friends and I have been pursuing so why are we in a special place well fundamental physics is the in in many ways it's the oldest and most mature part of science depending on when you start counting we've been at this business for two thousand years or four hundred years and so we're really good at it and we have a spectacularly successful theoretical structure that describes everything we know about the elementary particles and their interactions all the way from the tiniest distances that we're probing in experiments today that around a thousand times smaller even than the nucleus of the atom all the way out to distant scale is comparable to the size of the observable universe around 10 billion light years or so and this incredible theoretical understanding of the world around us over this enormous range of scales is based on two sets of extremely simple but deep and power four principles that revolutionize our understanding of fundamental physics in the early part of the 20th century and these are the two big principles that went beyond Newtonian thinking of quantum mechanics and Einstein's ideas of space-time now one could as a theoretical physicist imagine universes that were governed by quantum mechanical laws but did not have this mixture of space and time implied by Einstein's theory of relativity one could imagine the opposite theories that that had the mixture of space and time implied by the specialist Theory relativity but didn't have all the other strange in many ways more peculiar and revolutionary features of quantum mechanics and either one of those universes could be consistent and decent all by themselves but we seem to have do have a universe that has both of these two sets of principles and having the world that's compatible with both of those two sets of principles turns out to be almost impossible it's very very difficult to describe laws of nature that are compatible with both of these ideas at the same time I can just give you a very rapid quick idea for why it's true Einstein told us that space and time are not quite as different from each other as you might think so what you might think is just some amount of space someone else moving relative to you with some velocity might think is some combination of space and time so space and time can get sort of mixed up with each other all right quantum mechanics changed everything about the nature of the laws of physics instead of having a deterministic clockwork universe we had to give that up and imagine we had things like Heisenberg's uncertainty principle that you couldn't know where particle was and how quick it was moving at the same time and more fundamentally that we couldn't make deterministic predictions for what happened next given certain initial conditions so quantum mechanics changed everything but one thing it did not change is the primacy of the notion of time you're supposed to specify what things look like at this time evolved for some later time and see what it does there so time was very special still in the quantum mechanical picture of the world whereas relativity tells you it's not quite as special as we thought it was before and so they still don't speak exactly the same language it's difficult to talk about theories that are compatible with the principles of quantum mechanics and space-time simultaneously but there's a theoretical framework that was developed in the first third of the 20th century after these two revolutions it's known as quantum field theory and it gives us a sort of tight straitjacket within which we can describe potential laws of nature that are compatible with these two principles at the same time and much of the rest of the 20th century certainly the second third of the 20th century three through to the mid-1970s was spent on finding a particular quantum field theory this is a famous standard model of particle physics boringly named for such a grand thing but which describes all the elementary particles all the interactions everything we know exactly the thing that I told you about is a very particular quantum field theory particular spectrum of elementary particles and interactions between them and so on that actually describes the real world but more broadly we can imagine this larger theoretical frameworks still very rigid that allows us to talk about these things at the same time so that's the big success of the 20th century is that these principles are shockingly constraining on what laws of nature can look like and that's what makes it particularly disturbing that we have strong theoretical circumstantial evidence that at least one may be both but certainly I won't focus on I'll focus more on the doom of space armament in this in this talk makes it more startling that one of the things that goes into this incredibly rigid and powerful and successful theoretical framework is likely an idea that cannot survive in a more complete and deeper understanding of physics and that's this notion of space time and there are very simple thought experiments that lead you to expect that because of both gravity and quantum mechanics this notion of space time cannot be fundamental it doesn't mean that it's not useful we walk around in space we live in time it definitely doesn't mean that's not a useful approximate idea but it can't be a fundamental one there are notions that cannot occur in the more complete description of physics that's what I want to spend a few slides talking about is justifying this slogan that because of gravity and quantum mechanics space-time is doomed all right so so there's some very simple bot experiments I have to apologize throughout this talk for the truly crappy quality of my artistry so you'll have to suffer hopefully the content will make up for it we'll see so this is supposed to be a magnifying glass ok so that's that's unfortunate my unfortunate artist rendition of a magnifying glass so um let's say you want to take a magnifying glass and look at this region of space and time right here okay you want to see what's going on in there well something we learned because of quantum mechanics and Heisenberg's uncertainty principle is that in order to probe what's going on in really tiny regions you need enormous amounts of energy that's the irony for why we why the the experiments like the Large Hadron Collider that probe the tiniest distances we've ever probed are the hugest experiments we've ever built and it's because to probe those really tiny distances you need to smash things into each other in with absolutely enormous energies and so you need these gargantuan contraptions to make that happen all right fine but in a world in a world without gravity there is no difficulty accept money for asking for larger and larger accelerators to probe smaller and smaller distances you just keep going forever larger and larger energies gets the smaller and smaller distances but at some point something bad happens because the world does have gravity and it's at at some point you have to pump so much energy into such a tiny region of space that something cataclysmic happens for your effort to see what's going on in there um you know that energy and mass are related via equals mc-squared so putting a huge amount of energy in a little region it's like putting a huge amount of mass there but you also know that if you put enough mass in a tiny region of space what happens you collapse that region into a black hole and nothing can can get out so your effort to see what's going on in there absolutely obstructs itself you produce a black hole that makes it impossible to see what's going on in there and what if you get frustrated and say damn give me more money I'll build an even bigger accelerator higher energy what happens you make an even bigger black hole okay so it's even harder to see what was going on inside so this means that there's no operational meaning to distances that are small enough distance isn't that are small enough there's no experiment you can even imagine doing even in principle that would tell you to let you resolve what's going on in very short distances now we can extrapolate from our theoretical understanding of gravity the distances at which this happens and these numbers are astonishingly small okay the numbers are around 10 to the minus 33 centimeters just as calibration the size of an atom is 10 to the minus 8 centimeters the size of a nucleus is 10 to the minus 14 centimeters the sizes were probing to the Large Hadron Collider 10 to the minus 17 centimeters so this is 16 orders of magnitude smaller than anything we can even hope to probe in the lab but nonetheless we can do this thought experiment and every time it's happened to us in physics before that there's a quantity phenomenon something that we can't even in principle give operational meaning to it means that that notion is approximate we can't the it's it's happened to us over and oh this this thing has happened to us a number of times in the past few hundred years and so but this is really startling cuz I think that's approximate is space and time itself physics has changed a lot over the past 400 years but it's always been about describing how things vary in space as they move through time and that's exactly the thing which is now on the chopping block so when do these effects become important they become important every time some some notion of tiny distances are tiny times becomes relevant so we can imagine running the picture of the expanding universe back in time and as we run it back in time the universe gets denser and denser and hotter and hotter and there's a there's a moment beyond which we can't extrapolate our theoretical understanding at all where the sort of curvatures in space and time become comparable to this number around 10 to the minus 33 centimeters that's the moment that we colloquially referred to as the Big Bang singularity and people asked what happens before the Big Bang but the it's very likely it's a notion of before that starts breaking down near the near the Big Bang it's a notion of time itself that's starting to stop that's that no longer makes sense there and analogous things happen if you throw yourself into a black hole that in many ways on the inside of a black hole is like a universe that's not expanding but sort of getting crunched in so it has some similarity so this cosmological picture so our theories just break down when gravity is strong and these effects of quantum mechanics and gravity are important or ocean of space-time breaks down there and has to be replaced by something else now there's another aspect to this problem that in a longer colloquium I would spend more time describing but that way of talking about things so I'll go over it a little more quickly that way of talking about things makes it seem like the difficulties are localized to really tiny tiny distances these like ridiculously minuscule 10 to the minus 33 centimeter scale distances but in fact there is a more fundamental problem there's a deeper problem that again has to do with quantum mechanics and gravity in a quantum mechanical world if you want to speak with precision about anything because the walls aren't deterministic you have to invoke you have to take some funny limits in order to speak about anything with perfect confidence so for instance we can only predict probabilities in we can only even talk about probabilities in a quantum mechanical world so for the notion of probability to be meaningful you have to imagine do an experiment over and over and over again sort of it an infinite number of times in order to be able to speak with as much precision as you like about the notion of some probability after all if if you say that you have a fair coin if you flip it three times that might come up you know heads twice and tails once and that that doesn't mean that it's not fair so in order to figure out that it's fair you have to imagine flipping it a billion times or 100 billion times or more and more and more until the fraction that's 5050 becomes becomes becomes visible right okay now something else that that you have to do everything in in quantum mechanics is sort of fluctuating all the time and just for the results to be robust just so that you know whatever whatever apparatus is recordings the result of your measurement to give a precise number the apparatus also has to have be infinitely big because otherwise it has some inevitable amount of jitter that that gives you an error to anything that you're trying to measure so because of quantum mechanics you you have to if you're going to talk about anything if to make intially measure many measurements with an infinitely large measuring apparatus and once again without gravity that's not a problem but with gravity it is a problem because if I try to do that let's say this room I try to make any measurements with the total precision in this room then as I try to make the apparatus bigger and bigger it inevitably gets heavier and heavier and before it gets infinitely big it gets so heavy that it collapses the whole room into a black hole and then it's impossible to do the experiment over and over again because you get sucked into the singularity of the black hole and die so so there's an inevitable degree of imprecision associated with any measurement that you make in finite regions of space and time and that's that's a profound fact that has again to do with the existence of both quantum mechanics and gravity and the only kind of precise measurements we can talk about and these are sort of very highfalutin arguments but the only and why do we care about these precise measurements because as physicists we want to imagine there's a precise theory of the world and they're governed by some mathematical equations and those equations should be about something the things that they should be about we should in principle be able to measure to any degree of accuracy we like well what can we talk about if we can't talk about measurements that can make in a fixed size room the only things we can talk about is to take apparatuses move them infinitely far away from each other so we can make them bigger and bigger without collapsing thing into a black hole how'd them shoot particles or other excitations into the inside of the space-time have them bang into each other and come back out and go far away where they're measured again by infinitely large apparatuses so these are the kind of observations that we can talk about and now these are very highfalutin as I said very highfalutin arguments about quantum mechanics and gravity fortunately particle physicists love these kind of experiments anyway those are the sort of experiments essentially we do all the time in order to figure out what's going on with the laws of nature at very short distances we smash particles into each other now it's true we don't smash them in from infinitely far away but protons are 10 to the minus 4 centimeters big the LHC ring is 27 kilometers around and 27 kilometers is more or less infinity compared to the size of the protons that we're smashing so these are very very close to the experiments that we do all the time anyway in fundamental physics so these are two important things here one because of gravity and quantum mechanics just conceptually these are the only kind of liberals we can talk about that go out to infinity and two those are the kind of experiments that we care about anyway for the bread and brother practical purposes at least practical when it comes to a high-energy physics all right there's been many theoretical developments over the last 20 years especially in string theory that have run with this basic idea that the that the the precise things that we talk about should live on the sort of walls of space-time at infinity rather than in the interior of space and time and I don't have any time to talk about these sort of toy models that have been studied intensely over the last 20 years other than to say that there are there are very interesting toy models which kind of put universes in a box physicists love to put universes and boxes and and and and study them and so there are some extremely interesting theories where you can start seeing this idea that that that space might not be a fundamental notion it could emerge from the from from the dynamics and interactions of elementary particles in an interesting way and there's a huge amount still going on there and huge amount left to be understood about a very important thing which is not touched in that story is the important notion of time on the one hand and these universes in a box don't look that much like our universe so we would like to talk about our universe okay so let's talk about our universe then and let's go back to the scattering processes that I just talked about as I mentioned these are of absolutely central significance in fundamental physics these are the kind of experiments we do colliding protons going around two ways at around 0.9999 nine nine seven 9s the speed of light around this 27 kilometer ring about a hundred meters underground in Geneva around Geneva Switzerland and and these protons are collided and at these huge energies they probe distances of around 10 to the minus 17 centimeters as I've already alluded to um so so we're doing these kind of so these are very important experiments for us for these practical reasons as well all right so I've told you two things one of them is that so far one of them is that the notion of space-time is approximate it has to be replaced by something else the arguments that lead you to this idea that space-time are approximate tell you on the one hand that you should only talk about these kind of observations that you can do far away and on the other hand those are the kind of experiments that we do anyway so these two things go together and then these two observations go together in a nice way and you see you might think from from what the the first description and what's really spelling the doom of space-time has to do with gravity right gravity in quantum mechanics so you might think that in order to probe these questions you have to somehow get to these plunky and energies to see what's going on there and of course that's hopeless experimentally so you might think that well absent doing that there's there's no other way of going about things what I'm going to talk about is a it's a it's a strategy for thinking about these questions that's much more down-to-earth and and takes a lot of inspiration from the second practical reason why we care about these scattering processes anyway so as you'll see in a moment there is there are lots of indications at least some of us believe that there's a lot of indications and magical properties in in in the results of completely ordinary particle collisions in regimes that we in principle think we understood very well things that are going on right now in collisions at the Large Hadron Collider for example that bear in them clues to some deeper structure that might be underlying space-time and quantum mechanics in any case if there is some deeper structure that underlies space-time in quantum mechanics it's clear ahead of time that has to involve some new physical ideas and some new mathematical ideas as well and after all if we're going to learn to talk about physics where these notions of space-time in quantum mechanics don't make an explicit appearance then our usual way of doing things certainly makes them make an appearance and gives us all sorts of intricate and interesting phenomenon and you know non-trivial formulas that to make predictions for for experiments if these are going to come out of some other way of doing things clearly we have to have both new physical ideas and some new mathematical machinery that has to that has to produce the same formulas and perhaps eventually extensions of them in a in a different way and that's what I want to tell you about in the rest of the talk yeah so okay so but before getting there just I want to make a few general remarks which is why why you shouldn't necessarily be put off or discouraged by the fact that the relevant phenomenon seems to be taking place at energies that are totally inaccessible to us by by experiments and for this we can take some inspiration from very important developments earlier in the history of fundamental physics which had a similarly dramatic character to them and I just want to point out that sometimes the most crucial clues are hiding in plain sight not in the attempt to explain some new experiment at the frontiers of what's going on and where we've made our measurements but by trying to understand old experiments trying to understand funny features of the existing theoretical framework okay so let me just give a few examples of this many of them are of course famously associated with Einstein so Einstein's special theory of relativity largely had ancient origins Einstein just noticed that there's a notion of relativity that einside did not invent that goes back to Galileo and he just noticed that that notion of relativity was incompatible with the idea that what you do here shouldn't affect what's what's going on there arbitrarily quickly okay so there's a notion of locality that what happens here should not affect what's going on in Alpha Centauri instantaneously and if you just take very conservatively the Galilean picture of relativity and make it compatible of this notion of locality you're led to Einstein's picture about of relativity all right there's another example that has very much to do with with the quantum mechanics so in the in the usual Newtonian picture of the world you say that a particle moves around governed by his famous Newton's law that the F equals MA that gives you this the sort of clockwork universe picture this deterministic picture that where the particle goes next is totally determined by what's happened what's happening to it now right so you say where it is now how quickly it's moving and the force tells you where the acceleration is so it tells you where it goes next and you just keep doing it over over and over again okay so that's in that picture determinism is absolutely hardwired into the structure of the laws ultimately though we know that the world is not deterministic we've learned because of quantum mechanics of the world is not deterministic and I like to imagine that there is the ghost of theoretical physicist future that goes back to the classical physicist in you know sometime in the middle 1800s and says I have news for you from the 1930s determinism is gone and then they disappear into the night as ghosts of zeros future or want to do okay and you can ask what would this person make of this crazy news from from from the future how could it possibly be that the turbine ISM is gone if it's so crucially tied in to this to this Newtonian clockwork universe picture and the clue that they could take despite the fact they don't know in detail what's happening a hundred years later is that well if it's really true that determinism isn't really there the determinism I think I have now under my feet in the description of the Newtonian universe must somehow be not necessary there must be some other way of talking about exactly the same physics where that idea that is secretly not there doesn't actually make a fundamental appearance if determinism isn't really there or we can't imagine the universe is a little non deterministic it's like being a little pregnant it's not possible okay so it has to be that the only way it makes sense is is if there's a second way of talking about my physics right now where there's no term fundamentally and somehow it comes out it comes out of the consequence of some other of some other rules and if I find that way of thinking about things maybe it'll give a better jumping-off point to the to the next description of a reality where determinism is just not there this for some reason happens to us a number of times in theoretical physics to me it's the most mysterious feature of the structure of the laws of nature that at any given moment in time that's exactly the same laws can be described from psychologically philosophically totally different points of view that lead in the end to exactly the same equations it's it's totally bizarre that such a thing is possible but somehow it's necessary it's the only way we can make these big leaps we have to take the laws as we discover them rotate to some other way of thinking about them that then makes it possible to see how they could be the limits of something that looks radically different okay now how could this classical physicist from the 1800s be be inspired by this ghosts of theorists future they can look around and see if it's possible to think about classical physics in a way that the notion of determinism is not is not fundamental but comes out and indeed such a notion exists and people discovered it people scientists like Euler and LaGrange and demos but we discovered that there is this remarkable second way of thinking about classical physics not where you say the particle is acted on by force that tells it where to go next but the particle goes from A to B by sniffing out every possible way it could go from A to B and choosing the path that minimizes a certain quantity the quantity is the average value of the kinetic energy minus the potential energy as along this trajectory something that we call the action in fact people notice that there are similar things that apply to light okay light travels from A to B in a way that minimizes the time it takes to go from A to B that actually beautifully explains things like things you learn in high school like when light bounces off a mirror you maybe remember that the angle that it comes out with is the same as the angle that it goes in with and that's that's what it has to do in order to minimize the time it takes to get from here to there so if you remember Snell's law from high school that if you go through a medium where where light slows down and has to bend in this way with a particular law that's actually what it does to minimize the amount of time it takes from A to B this is equally applicable to light as it is to a lifeguard who's trying to save a drowning person who can run faster in the sand they can swim in the water okay they would have to run to this particular place for the angle of for the angle in and the and the angle through to be related in exactly the same way they are with Snell's law in order to make that happen okay so that's very interesting that there's a second way of talking about classical physics where determinism is not part at all of the essential description you see in that picture it kind of looks the opposite looks like you're thinking about where you're going next in some way and then by this sort of global picture of everything deciding what what you want to do and why does the second way of thinking about classical physics exist it's because the world is not deterministic the world is quantum mechanical okay and you couldn't possibly jump from the deterministic Newtonian picture of the world to the quantum mechanical one they're not continuously related to each other but this second picture is continuously connected to a quantum mechanics as Richard Feynman famously explained to us in the 1940s in a very precise sense the particle actually does take every possible path between a and B and in a particular limit where the quantum mechanical effects are expected to be small it's approximately the case that it takes the path that minimizes the action but there's no determinism in this picture at all so that's the that's the that's the inspiration that we're taking from history we have this grand question about about the doom of space-time we don't have access to experiments that can go to the relevant energy scales where these where these phenomena are expect to hit us in the face there are many different strategies you can take towards attacking this problem and I'm describing one of them but the one that I'm describing has the advantage that you're looking for clues in a completely solid place you're looking for clues in the structure of laws that we know for a fact describe the real world already okay there they're just true things about the world but we're forcing ourselves to look at these laws from this perspective that we try to get rid of and not use these principles the idea that there's a space-time sitting there we try to think about them in a way that doesn't use those principles and see if they can somehow come out in a different way all right all of this stuff has been very airy-fairy very philosophical so let's come crashing back down to earth and see a second set of against circumstantial clues that something like this is going on and I have to say that if it was just this first set of words this set of words could have certainly rattled around in my own head for decades without ever being motivated to do anything about it and what really motivated me and certainly at least me to to think about the subject in this in this way is this second set of observations that are much more concrete and down-to-earth that were motivated by making actually predictions to make contact with the experiments that are going on at the LHC and it's when these highfalutin things and very concrete practical things start pointing you in the same direction that it becomes irresistible to start following the direction all right so what do we need to do in order to maximize what we'll learn from these particle collisions at the LHC well to the extent probably most of you hear about the LHC you hear that it's supposed to be a machine that might produce new particles beyond the things that we know about so far maybe you've heard of supersymmetry or extra dimensions or black holes or God knows what else but you should realize that particle looking for new things that the LHC is like looking for a needle in the haystack here's a horrible cartoon for to give you an idea of the problem of the scale of the problem it vastly exaggerated how easy things are but roughly speaking you know any process you'll talk about if you look at the sort of number of events that what actually happens um there are some there are some rates that are really high when the energies involved are low and they go down as you go to higher and higher energies so there's some humongous thing that's sitting there already from completely known you know you would think boring physics not this exciting new stuff that you're looking for and then at really high energies if you're producing some heavy new particles for example you might expect to see a blip above this known stuff and the blip is what you're looking for but to give you an idea of the magnitude of the problem there are billions of collisions a second at the LHC particles that took the the most giant accelerators on the planet in the 90s to produce like top quartz these are sort of things that part of the business love there may be esoteric to most of the rest of you but you know back in 1994 in 1995 you know roughly 20 of these guys were made and that was a big celebration and now they're sort of 10 of them are produced every second at the LHC and now those fancy-schmancy things supersymmetry all that kind of stuff if we're lucky and they're there we haven't seen anything yet but even if they were there they're being made you know one a minute one an hour all right so that's the magnitude problem you have in order to dig out these things you're making a tiny rates you have to understand and subtract this ordinary stuff and for instance one of the main ways of seeing supersymmetry would lead you to need to be able to control the and understand in great detail the following process so the protons are big messy bags of particles if they're there they have inside them quarks they're held together and prevent it from escaping from the protons by other particles called gluons you see we have very imaginative names for these things and so roughly speaking when these two protons smashed into each other mostly they just break up into into just complicated Jets of strongly interacting particles that go more or less in the direction the beams are coming to begin with but rarely these little elementary particles point-like particles inside them have head-on collisions and they produce things that come out at at with high energies and big angles from from the direction the particles were going in at and so in order to for example see see some of the particles predicted in supersymmetry you have to be able to understand this process where two of these gluons inside the proton come in and produce four of the gluons going out now they don't come out looking like gluons they again turn into you know big jets of energy that comes spraying out in various directions but this is the underlying process that you need to understand very well in order to look for the new thing okay so what do you have to do well in principle this is old physics in principle we've understood how to do this for a long time and Richard Fineman famously taught us how to understand these things you open up Michael petkins book on quantum field theory and you copy out the rules that tell you how to do this calculation and Fineman tells you that to figure out what happens when two gluons go in and four gluons come out you have to draw these little pictures okay these pictures represent in fineman's way of thinking every way this process could take place in space and time so these two gluons come in they bang into each other make two gluons going out for a while but then each you know spontaneously turn into two other sets of gluons or maybe it happen this way or that way or that way and you have to add up every possible way that it could happen there's lots and lots of diagrams lots and lots of terms and there's even more complicated diagrams than than this but these are the sort of leading these are the elite effects that we could be interested in and what do you actually get if you do the calculation this isn't even this isn't even for the Tuda two gluons in four out I think it's just two gluons three out and and this is you get something that looks looks like this so you get about a hundred horrible looking pages of algebra now other people might call it twenty but I write big so for me you'd probably be more like 100 okay okay so um now so what it looks complicated no one promised you a rose garden right not everything in physics is simple and part of the chauvinism of fundamental physics is we declare as interesting only those questions that have simple answers and everything else is some while some engineering it's something else right you know they're very important but all the fancy stuff all the deep stuff is in questions with simple answers so this looks complicated of course it's complicated it is complicated two things come in four things come out what do you expect why should it be simple okay all right but physics has a really great way of rewarding morally good behavior and punishing immoral behavior intellectually immoral behavior I can't speak for the other kind of immoral behavior okay and what happened and almost a little over 30 years ago that what I'm describing happened around a little over 30 years ago people did the calculation and after lots and lots of tricks and lots of work they found that these hundred pages of algebra actually collapsed to a single term okay now I won't define what these variables mean you just have to trust me they're not just defined to be the sum of a hundred pages of algebra okay so there's a so these variables they roughly keep track of the amount of energy and momentum in the India particles all right now this is just absolutely incredible right there's clearly something wrong with the standard picture of the world that that that that Fineman and and others gave us it's not incorrect of course it's perfectly correct but it fools you into thinking that this one term thing is actually a hundred complicated pages right and furthermore why is it fooling you into doing this it's not disconnected from those highfalutin things that we talked about in the first part of the talk you see what is fine this picture doing it is making as Manifest as possible those two big principles of the 20th century there is a space-time the particles come in they hit each other in every possible way and you have to add things up every possible it could happen because of quantum mechanics that's why finally became famous he managed to make those two big principles of relativity and quantum mechanics manifest and that's what his diagrams do that's their purpose in life so you want to make space-time and the rules of quantum mechanics manifest you can do it perfectly correctly it's spectacular its general it works in every possible situation but it also makes you think that one term is represented by a hundred pages of algebra okay and so somehow this awakens in you the idea that maybe there is another way of thinking about this and in that second way surely you can't beat Fineman for making space-time in quantum mechanics obvious there should be a second way of understanding things where space-time and quantum mechanics are not obvious but something else is obvious okay some other principles rules and laws are obvious and maybe if we understand what those principles and rules and laws are we'll begin to get an understanding of what might underlie space-time in quantum mechanics we can start from those new pictures and read out how they manage to give us something that has space-time and and quantum mechanics in it a little bit like once you have the principle of least action it's perfectly equivalent to Newton's picture of the world totally 100% equivalent but that second starting point is closer to the lift you have to make eventually to a quantum mechanics where the notion of determinism is lost so that's the hope here it's a strategy the strategy is to it's to pursue where this is coming from try to understand where it's coming from in more and more in more and more general situations as close to the real world as we can possibly get and try to see whether whether these simple expressions and other things that I'll show you are actually giving us clues to some new principles that might underlie space-time in quantum mechanics there are many other things I won't have time to explain any of these things in the detail it's not just that a hundred pages collapse to a single term it's that when you study these things in sufficient detail you see that the answers have things that physicists love physicists love symmetries every time you see a symmetry you're supposed to get excited and try to understand where the symmetry comes from well these these processes these these these processes have some hidden symmetries in them and which could have been in principle observed any time in the last 50 years but they weren't they weren't observed till around ten years ago and they're sitting around in the structure of these simple formulas our our our cemeteries we didn't know about something that's quite remarkable is that they might seem like fancy esoteric symmetries having to do with gluons inside the proton that the LHC and all of that stuff but in fact again nothing in physics is disconnected and these hidden symmetries are connected to an ancient symmetry that people realized a little bit after Newton gave his solution to the orbits problem that said the particles go around in ellipses people were kind of amazed that there was such a simple solution to what seemed like a very complicated problem in physics whenever there's a very simple solution is a very complicated problem there's often a symmetry that's associated with it and people discovered in a somewhat different language that that problem the orbit problem had a hidden symmetry around 350 years ago that hidden symmetry and this hidden symmetry are essentially identical this new one is an update and a generalization of the old one so even though we're talking about what looks like esoteric things with gluon collisions of the LHC there's a continuous thread of history that takes it all the way back to the understanding of why planets go around in ellipses okay so so there's something there's something deep going on here now one of the wonderful things about this subject is something I just alluded to there's a lot that's been sitting under our noses for decades in and the decades another wonderful aspect of it is that it kind of seamlessly connects a humongous variety of different fields of intellectual endeavor so on the one hand I give you the the practical motivation for the second part of the talk by the desire of to tell experimentalist at the LHC what ordinary physics processes look like when you collide particles and you're not going to understand anything about the spot this is just meant to show you that that these are supposed to represent rates for different kinds of collisions that we care about and I just want you to notice if they span an enormous that that you know they span by a factor of a million or more of rates that you need to care about for it sort of practical for practical purposes so on the one hand the the theoretical idea involved in this subject one of the most important aspects of them is that they do make direct contact with with experiment and you can't mess around and get things wrong by a factor of two it has to just work otherwise it's otherwise it's it's useless okay so you have to get things right by a factor of two that that's man things over you know six four or five six or more orders of magnitude so on the one hand there's a there's there's the experimental heart of the subject that connects to real-world experiments but there are connections to deep parts of theoretical physics old and new and as time has gone on more and more revelations of connections to very abstract parts of mathematics deep parts of mathematics that involve areas that you would not think would have anything to do with physics okay so you know if you know anything about about physics and math you know maybe you know that calculus describes how particles move around well that's that's wonderful but it's also maybe kind of obvious I mean you sort of draw graphs and you know velocity and distance and so on and now maybe it's not so crazy that something like calculus might be a useful for it it's less obvious that abstruse questions about number theory would have anything to do with particle collisions at the LHC and yet they're connected I mean and not just in some vague way and some very in some very concrete way I don't know of any subject where you can honestly say on the one hand the plots like this are relevant for its existence on the other hand you can talk about the following gentleman it's name is Alexander Watson deke he's a widely thought to be you know one of the greatest mathematicians of the 20th century and he looks a lot like Gandalf when he's old which is pretty cool okay anyway what technique was involved with a very mysterious subject I don't know anything about in any detail every time I get mathematicians try to explain to me what this theory is they look at me as if you know I was I was asked to explain Einstein's theory of relativity to my dog so that it's equally as as equally as much point but anyway this is some very very deep mathematical theory that's connected to aspects of geometry and numbers theory its associated with this this amazing character and and it's connected it's connected in a direct way sort of seamlessly to these real world these real world experiments okay I should also say since we're here at slack that if you get if you're excited by the content of this talk one of the true heroes of the subject is right here at slack glance Dixon is sitting right there and so if you have any questions you should ask Lance okay you can thank me later later all right so in the time that I have left we're not going to spend equal time on all the rest of the slides here don't don't worry I want to give you so I think I hope I've set things up okay so we're gonna try to understand where all this miraculous simplicity the symmetries all these things are coming from and I just want to give you a flavor of one angle into the subject there are many many others this is just one angle into the subject it's the angle that that me and a number of my friends have been pursuing for around 10 years but and we'll just go through things rapidly I'm not expecting you to understand any of this in detail just think of it as an something relatively impressionistic but what I want you to take away from it is that to remember that we're talking about ordinary collisions of ordinary particles but that the kind of ideas are going to show up have a certain simplicity to them they're going to be relatively but a certain abstractness of them as well and they're definitely going to seem pretty alien okay so it's not remotely gonna be obvious that the things that I'm talking about will have anything to do with the scattering of particles and that's part of the point right the part of the whole point here is that we're looking for some new kind of principles ideas both physical and mathematical that are going to that have to somehow produce results that we normally associate with the existence of space time in a new way so so it's guaranteed that it has to look strange and alien ahead of time and that's what I just want to give you some impression about before we start all right but so what I'm going to do is tell you what the simple physical ideas are and what some of the mathematical ideas are again purely impressionistic aliso you so you get an idea for the sort of thing that we're up to some of us are up to before starting I want to mention something very important that the basic building blocks for all of these scattering processes the most basic scattering process of all are where three particles come together at a point in space and time okay it doesn't make sense to talk about when two particles come together at a point because if you're going to conserve energy and momentum that just means the particles moving in a straight line okay so nothing is happening for anything to happen three particles have to come together at a point in space in time and those amplitudes especially for the collision of those gluons inside the proton those amplitudes are incredibly simple again you don't need to pay any attention to these formulas other than see how nice and simple they look maybe one little thing that I emphasize here and I even drew it wrong this looks identical to that I apologize but these particles these gluons inside the proton in some approximation when we're at really tiny distances deep inside the proton we think of these particles is massless of course we don't really see them as massless particles moving around in the world but in these very high energy experiments effectively behave like massless particles and so they're moving they're zipping around of the speed of light and they have a spin so as they're moving they spin either in the direction that they're moving in or opposite to the direction that they're moving in that's also sometimes called the holistic and so we like to keep track of whether they're spinning in the directions moving in or opposite two directions are moving in so that's associated with a plus or a minus if it's alright so there are two kinds of three particle interactions it turns out depending whether on two of the gluons have - Felicity + 1 + or the other way around that I apologize I didn't write properly here okay so one of those associated it with a little black dot here representing that as two minuses in a plus and the other will the white dot which is two pluses and a minus but all I want you to see from that is that it's very simple so if those are very simple why is the answer so complicated why do we get those hundred pages the reason we get those hundred page is that when we talk about more complicated processes in fineman's way of thinking about things the particles that come in are real particles the particles that go out or real particles but everywhere inside we invoke something called virtual particles and if you know anything at al a level about this stuff you've heard of this notion of a virtual particle all the evil has to do with this notion of virtual particle ok and as the language suggests these are sort of figments of the theoretical physicists imagination ok there are things that were sort of adding to our description of the physics why they're not the real things that show up that we see in our detectors and so on there are things that we are invoking in order to be able to give the description of what happened inside here a consistent space-time interpretation right so inside here for example this particle doesn't go out to infinity doesn't come in from infinity it propagates over a finite range in space and time that's the the virtual particles and they're kind of a fiction that's mean that's needed to manifest the rules of space-time and quantum mechanics now on the one hand these things are directly responsible for the horrible complexity ok just no two ways about it they're one-to-one connected with the horrible complexity and isn't that interesting that those are exactly the notions exactly making space-time and quantum mechanics manifest or exactly the things that we want to relax somehow we want to have a second picture where we don't care so much about that and we want to see other things so the physical idea is to eliminate virtual particles never talk about them never use them and somehow get the answers in in another way that's the physical idea okay so now you can do that and I won't tell you how you do it there's a many remarkable developments that were needed in order to be able to do that but the idea is that we're gonna take those basic building blocks those simple basic building blocks and we're gonna glue them together to make more complicated things now these are not fineman's pictures because everywhere inside here there's no virtual particles all the stuff inside everything is are in a in a precise technical sense everything or real particles in the middle but we still have the picture of building up more complicated things by gluing together simple ones and you can learn after quite a while you can learn different rules for building these amplitudes by gluing by by gluing these things together and making these so-called physical only processes okay where everything that occurs inside involves real particles so for instance if you glue these basic building blocks together in this way to get one and two goes out to three four five then just a single diagram gives you the full answer right that's what you get instead of thirty pages of Fineman diagrams or if you glue these things together for one and two particles going out to four out then just this one diagram plus just two more that look like it replace the thing that we talked about the horrible hundred pages okay so this already shows that this physical idea of eliminating virtual particles has legs okay with this physical idea you can you can see some real simplicity in the structure of the of the final answer but there's a remarkable thing about this representation again as you would expect something has to give so what gives what gives is that each one of these processes by itself like this one piece by itself does not have the interpretation cannot be given an interpretation of a process that takes place at points in space and time okay okay so each one of these interactions is not something that's localized to a point in space in time it's actually spread out infinitely far out along the so-called light cones that emanate from points in space and time so each one of those things does not represent some local interaction here but something averaged over all of space and time so if you stopped and said why couldn't this be the final answer by itself it would be manifestly incompatible with the usual principles of space-time and quantum mechanics so the building blocks are incredibly simple but cannot be given a space time interpretation and only this funny sum of all of them can be okay so we see we start seeing this interesting interplay if you give up the slavish adherence to the principles of space-time and quantum mechanics you can get these much simpler expressions which however don't individually by themselves manifest those rules there's other funny things about this representation there turns out to be a huge array of different ways of expressing the answer in this form we can write it this way you can write it lots and lots of different ways with different looking pictures unlike fineman's diagrams or you just draw one set of pictures and you're done okay so there are some flexibility in how you express the answer when you express them in terms of these funny pictures with black and white vertices okay so if you stir up these things long enough you start wondering what new world do these objects come from okay there are some I mean we can we can derive them and people did derive them in various very smart and clever ways from the from the usual rules but they seem to have a life of their own and so this is this is how far you get when you think when you follow this physical principle of eliminating virtual particles but to this point you can start getting a vague goal that you might want to find a new picture for what these amplitudes really are where these rules of space-time and quantum mechanics aren't primary they somehow emerge for more primitive ideas and somehow this picture that you can write the answer in lots of different ways in terms of very simple building blocks kind of looks like vaguely and there are there are more precisely reasons for thinking something like this they should think about the amplitude as kind of the volume of something the volume of the region of some region in some space and the different ways of representing it would correspond to different ways of calculating this volume by breaking it up into little pieces okay so that's how far you get from the that's what you're motivated to do and that's what a number of my friends and I for for many years have been pursuing and in a special class of theories that in the limit is related to what we care about in the real world but it's not certainly not exactly the real world something like this is is indeed seen to happen so if you're given the energies and the moment of all the particles that you care about in the collision given that data from that data you indeed build a picture of some geometric space and in a precise sense the volume of this geometric space does give you the amplitude for the process right and now let me tell you a little bit about what that what that space is and and and how you think about it so so we pursued the physical idea of eliminating virtual particles and we're led to these by these motivations to what I told you about so what are the new mathematical ideas involved and there there are a number of them but I want to highlight some of them that I can quickly talk about in a public talk to also give you an idea of how strange and and fun and unusual they are so imagine you were trying to describe to a mathematician this complicated scattering process that you're talking about so you have gluons they're already not paying attention right you say they've momenta they don't know about momenta that elicits E they don't know about Felicity so you say look they just have some labels one in two in three four five a let's say okay that that I can understand so this is the absolute simplest version of a scattering process that you could might imagine talking about just changing labels okay so labels one two three four five go in and labels they get interchanged three five to one four come out so that's the that's the simplest version of a scattering you could possibly imagine how lame is that right that's it seems totally dumb but in fact this picture is indeed associated with scattering processes and have been understood to be understood with scattering processes for over 30 years okay it's a remarkable connection and this is how it goes that's that that's a permutation so you just put one two three four or five down here one two three four five up there and you just draw lines you know one goes to 4 2 goes to 3 3 goes to 1 4 goes to 5 5 goes to 2 okay so I'm just represented that that that permutation but you see all of a sudden that now looks like a picture of particle scattering that are moving in one dimension of space in one dimension of time so here are some particles in moving with various velocities first two hits three then three hits one and all these collisions happen so this is one dimension of space one dimension of time and a particular picture of this sort is actually associated with a standard mathematical operation of taking the final permutation and breaking it up into the product of what are known as adjacent trans positions which are just ways of interchanging to nearby things at a time so I can start with that and and just interchange to nearby things at a time to go up all the way down to one two three four five so that's already an old thing that people had seen 30 40 years ago that connected something primitive and basic in combinatorics just a permutation to a scattering process in physics that's now people always found this interesting but it always also seen that it couldn't possibly apply to the real world and it couldn't apply to the real world for two simple reasons first most importantly just what I told you the fundamental interactions that we have in the real world for gluons for example involved three particles coming together at a point in space and time whereas here the fundamental interaction has two things in and two things out and associated with that there's no particle there's no creation or annihilation of particles in this simple toy model whereas in the real world we of course do have the creation annihilation of particles so many people thought this is a cool thing but it's a very special thing in toy theories with only one dimension of space and so there's not much else to be said about it but in the last 10-15 years mathematicians found a different way of associating pictures with permutations and this is what they did now I remind you the fact that these look like black and white vertices at the moment has nothing whatsoever to do with the what I just told you about 10 minutes ago this is mathematician sitting in their offices they could give a crap about the LHC they're just trying to represent permutations because they're weird okay that's just what they like to do with their lives okay so they found a different way of representing permutations so let's say you want to have this permutation where 1 goes to 3 2 goes to 1 3 goes to 2 instead of having these things in and out you put the guys you're trying to premiere about out on the boundaries of a circle and you put a little black dot in the middle okay and if you want to do the other one 1 2 2 2 2 3 3 2 1 you do the same thing you put a little white dot in the middle and the rule is that to see where everybody goes you walk into the picture and if you hit a black vertex you make a right turn and if you hit a white vertex you make a left turn okay very simple it's a different rule now so for instance if you draw this picture then if you just follow this left right left right paths this implements the permutation 1 2 3 2 2 4 3 2 1 4 2 2 okay and they discovered that every permutation can be represented in this way okay so they're just trying to represent permutations they start drawing these pictures of gluing together black and white vertices in in in in every possible way with certain interesting rules that can actually implement every possible permutation okay so that's that's interesting so there's a very primitive combinatorial idea here that involves precisely the same pictures with black and white vertices so so that that picture of particle scattering is now has a combinatorial backbone to it ok all right well there's more to the story I don't have time to to really describe it in in a detail but this shows you that at a very primitive level about interchanging labels and then associated with this basic primitive mathematical structure are a few levels above it that that involve various that involve natural extensions of this idea and here I'm just going to fly through things just say say some words again just to give you some very impressionistic idea of what's going on for example one notion that involves these pictures with black and white vertices is generalizing something really simple is generalizing the notion of like being on the inside of a triangle ok so you know we all like triangles as kids maybe so mathematicians also like triangles they like triangles or like higher dimensional versions of triangles and around 10 years ago they started studying generalizations of triangles into slightly fancier spaces than the kind of ordinary spaces we would draw these pictures in in in elementary school but but they're not all that they're not all that fancy just to give you an idea of how of how con concrete things are if you want to imagine being a point on the inside of this triangle one way of talking about a point on the inside of this triangle is to just say that it's some average value of the vertices of the triangle so I have to give you a bunch of numbers that are positive and and and I just take the sort of weighted sum of the of the of the of the the coordinates on the of the vertices of the triangle so there's this notion that being on the inside of this triangle involves some notion of positivity that they're a bunch of weights that are positive and that notion of positivity can get generalized in interesting ways instead of having a bunch of numbers that are positive you can make a matrix and so this this matrix is now an array of numbers and instead of saying that the numbers are individually positive you say that the little determinants here little 2 by 2 determinants that you could associate if this matrix are all positive so there's various motivations for doing this it's not maybe as strange as it appears but I just want to say that's the that's the kind of thing that we're talking about now it's now this is something you can explain to a kid in grade 4 this is something you explain to a kid in grade 10 or something okay so so these are not complicated spaces or complicated definitions ok and then let's say your goal in life is to learn how to build matrices that have the properties that all the determinants are positive that sounds complicated right you have to go sit check every determinant is positive it sounds like a big mess well what these guys figured out again around ten ten years ago little over ten years ago is that in order to build these matrices where all the determinants are positive you can't do it all at once you have to build them up gradually by gluing together little matrices and the rules for building them up are to draw pictures with black and white vertices okay exactly the same sort precisely the same sort we're talking about before but now there's some more decoration there are a number of associated with all these edges that tell you in a specific way how to build these big positive matrices with all these determinants being positive by gluing together little ones so that's like as I promised you we start with this primitive combinatorial idea it goes up one level to generalize the notion of triangle and now it involves the same pictures showing up again so is it a coincidence that we're seeing the same pictures over and over again of course it is not a coincidence and in the end there is a very concrete connection between these primitive combinatorial and geometric ideas and exactly those physical only processes that we talked about where those same pictures of black and white vertices are associated with particle collisions and yeah I won't describe any of this in in any detail other than to say that the final ingredient you have to have add is something that we give us as physicist we have to give the energies in momenta of the particles that we're interested in the scattering process that's not something that the mathematicians know about in these pictures but given that extra data there's a completely canonical way standard way of associating one of these pictures and an Associated contribution to an amplitude with it with both the geometries that I talked about and the combinatorial that I talked about and these are the building blocks out of which we make the amplitudes the final thing that's left is to figure out why we're gluing them together in that particular way why are we taking these bits and pieces why are we taking this piece and that piece and the other piece and putting them together to build a whole big object and the answer to that question involves this further notion a further generalization of a triangle so we can generalize a triangle the first way that I told you but there's a second generalization of a triangle to go from a triangle to a polygon and if you generalize a polygon in exactly the same way that we generalize a triangle we get a cousin of this sort of abstract object that we talked about which is the thing that really directly calculates scattering amplitudes all right now I didn't tell you anything about it in in detail but I can I can I can give you a little vignette so here's a vignette that I could explain to a kid in grade 11 okay so let's say you want to calculate what Fineman would tell you is something very very complicated involving particles coming in all these complicated virtual things going on on the inside for process with say two particles in and two particles out but arbitrarily complicated stuff going inside there okay so this is a calculation complicated looking calculation in quantum field theory well you associate with that the following little geometry problem okay here's a geometry problem in two dimensions you look at the upper quadrant okay so just in here and you fill this upper quadrant with two kind of two kinds of points red points and green points and there's only one rule the rule is that if you take any pair of red points and you draw the arrow between them then the corresponding pair of green points the arrow must be pointing in the opposite direction that's it right so that's a little geometry problem here's what it is technically and it's fully posed I'm done I've specified a geometry problem I could explain to a kid in high school the solution to this geometry problem via this construction that I told you about calculates that quantum field theory answer okay now I hope you can see that this problem seems to have absolutely nothing to do on the face of it with particles colliding moving around in space and time quantum mechanics all of that stuff this little geometry problem has all of that in it it has all it comes out of the solution to this little geometry problem and you should not be surprised that while I can explain this geometry problem to a kid in high school I do not expect the kid in high school to solve this geometry problem because I myself and my friends have failed to solve this geometry problem for four or five years if you can you I will happily give you my job at the Institute for Advanced Study okay so you will have earned it and and yeah I will be happy to know the answer so all right so as I said I just hope you got from there just a rough idea both the physical ideas and the and the simple but somewhat abstract and little alien mathematical ideas that are involved in the subject so so in the end the amplitudes are the volume of this interesting this interesting mathematical structure there are different ways of breaking it up into pieces some of the ways of breaking it up into the pieces have the interpretation that the individual pieces are these physical only processes that we talked about the individual pieces don't have space time for quantum mechanics interpretation in them but the full volume does and at least in this toy example the rules of space-time in quantum mechanics newer merge as derivative notions from these more primitive combinatorial geometric rules all right and as I told you to the leading order of approximation what's going on here is relevant to the real world so if you're interested in calculating this particular amplitude at the LHC then if you use finding diagrams as we hundreds of pages of algebra in this picture it's represented by looking at the volume of a little picture like that and of course the the people who are by now doing these things that professionally using all the best techniques available knows these answers without drawing these the pictures and in fact these pictures are giving a geometric interpretation for a lot of these remarkable formulas that people have been using in the past the 10 or 15 years anyway but I'm saying this not to say that this is the this is the current method being used to calculate this at the LHC by drawing this picture but to say that this funny strange set of ideas directly you know is relevant to the real world in some approximation it's not it's not disconnected in some toy world that has absolutely nothing to do with ours all right so and in this example we see beyond what I advertised in the title of the talk which is that the space-time is doomed in in all the various examples of this sort that that many of us have been studying for the past ten years or so we're seeing something even a little bit beyond that not just that space-time is doomed or that space-time is not there in this case in the fundamental formulation of the physics but neither is quantum mechanics it's both the things together both the space-time of the quantum mechanics that somehow come out of these or the consequences that come out from these more primitive rules all right so that's all I want to say again this is one of it's a one very small part I'm up here giving the talk which is why I'm talking about this but it's one very small part of an a really an incredible scientific Odyssey I think it's amazing and that there's some mysterious and spectacular new sets of both physical and mathematical ideas that that are of central and direct relevance to the physics of space-time in quantum mechanics in the actual real world and Michael mentioned that many of us are gathered for the annual meeting of people working in this field amplitudes 2018 I've alluded to it already and it's you know it's it's in the air that those of us who work in this field breathe but it is great to step back from it every now and then and just see to me how remarkable it is that there's a subject that brings people who talk to experimentalists Collider physicists string theorists quantum field theory and pure mathematicians and you know this is not one of those things where people are just talking to each other to be polite they're talking to each other because they have active interests in common and it's clear that a number of us are seeing exactly the same structures from radically different points of view and it's strange and and exhilarating when it happens I can tell you just one tiny anecdote that when we are when we're running into these kind of structures there was a mathematician at MIT who we understood was kind of an expert in these things and so we want to a part to him at the MIT and we thought as physicists we knew about these pictures with black and white vertices and the mathematicians didn't know anything about them so we deliberately didn't tell him anything about any of that stuff we just talked about we just talked about things in a more you know with more formulas and it was already very exciting it was clear we're seeing the same kind of things it was really cool then he went to lunch and over lunch he said it's really remarkable it's happening this is really great but there's something really strange in my way of thinking about things it's crucial to draw little pictures with black and white vertices right and we said what are those pictures look like and he drew it but bang on one of the ones that we would draw instantly okay and that's completely insane right you know we are motivated for drawing these pictures by particle collisions they're motivated by permutations and and and representing matrices that have positive determinant it's clear we're seeing the same objects from different points of view but nature knows much more than both of us do about what's going on and one of the one of the joys of the life of in theoretical physicists is that by following what Nature has has to say you always have a friend that's smarter than anyone else that you could talk to okay and and and the structures that nature is telling us to study are really bringing all these people together in an in a very exciting adventure so um I think it's not remotely the time to summarize what it all means or where it's going or what might happen eventually but I do encourage you to uh stay tuned right thank you very much [Applause] so I think Nima can take a few questions if you raise your hand and be recognized each of you has a microphone in front of you when you're recognized press the button in front of you the microphone will turn red and then you can ask your question so does anyone have a question way in the back there yes you so as I said push the button in front of your microphone there and now you have the floor first of all Thank You professor there was a great great lecture and so my question is about its space time ends wouldn't that violate the second law of thermodynamics well first of all the the the sense in which they're there to oh there are two senses in which you can imagine space-time atoms one of them is like actually ending like you know the beginning of the universe or the end of the universe or when you jump inside the black hole and so if so there's that sense the sense of most of the talk was something more theoretical that that it ends just in the sense that we should not use it anymore we should find some other ideas to replace it with but your question about whether what happens with the second law I mean this is this is tied up with the it's it's it's tied up with questions about the arrow of time and and all you need to know is that everything we know about the universe today is compatible with the picture that it started off very simple so you imagine that be that the initial state of the universe is the initial state is is is is very simple the second law of thermodynamics is an inevitable consequence of the statement that initial is simple and that has very little to do with these more esoteric things the second law of thermodynamics is essentially nothing to do with cosmology you've talked many times about the fact that this is a special time in the history of science I mean that there's some things happening now that I think though yes yes and do you have a do you have any intuition about the upcoming evolution of these ideas what kind of timeline are you either hoping for or thinking we might it's very hard to know it's it's I think it's it's it's very very hard to know I think what what you need to know about about this subject and I think it's it's kind of in distinction to a lot of other things that are going on in the modern world where you have the idea that the only things that are cool or interesting or special or what happened last week or last month or god knows maybe at most last year this is a subject that that thinks of itself on the timescale of centuries okay that's the time scale on which progress has been made in the past sometimes it goes faster sometimes it go slower we had these terrible dark ages for example that weren't so much fun okay so if you're going to get into this business you have to get into it with the knowledge that you know people have made this analogy many times for the experimental aspect of this subject but it's also true for the theoretical one it's like building cathedrals you may or may not be around to see the fruits of your labors or didn't even see what you're doing was remotely relevant for the for the right answer but but but but it's it's it's the nature of the beast it's very hard to know it's very hard to know that being said one of the exciting things about this kind of theoretical exploration is that it has an internal notion of right and wrong okay and you know this is not this is not speculative science we're talking about things that for sure describe the real world and have the magical properties in them so you can't mess around you can't just say uh kind of works no that's part of the point I so just work exactly right there's a big check you come along you say I have some whiz-bang new idea to get rid of space-time blah blah blah you give the first half of my talk and you stop there then you're like okay put up or shut up right they either it works or it doesn't work yeah to be able to agree with everything that's coming before you have to be able to compute other things that haven't been computed before because you know how to check if it's right or wrong every time you have an internal notion of right or wrong it helps greatly in in accelerating progress so so I mean I can say I don't know the timescale for the really grand answers to fully emerge but just speaking for me personally I can't speak for other people in the field but had you told me you know 10 years ago 15 years ago the things that we know about the subject now I would think it's ridiculous okay so it's so so in some sense the progress has been much has been surprising and faster on the other hand you know it might it might slow to a crawl just for purely internal theoretical reasons we might need we might need tools that you know won't be invented for another century yes we just or discovered for another nothing really is invented in this subject it's all out there in the world of ideas already and we just discover them but so I don't know the answer is the answer is I don't know but you do the best you can I've used this analogy many times I really think when I say that we're in a special period I really mean special you can really think of most of the last 400 years the analogy I like to use it's like you know that the last 400 years is like you got on you got on a plane you flew to Katmandu you you got out you found some Sherpas you started going through the foothills and after a long time you're finally a base camp of Mount Everest ok the last 400 years have been about getting to the base camp of Mount Everest 2,000 years ago 400 years ago you could wonder about the deep nature of space and time you could wonder all these things and it would be totally stupid and pointless because you know hundred years ago we didn't know why grass is green and we didn't know why water is wet so it's really completely pointless to think about these things about space and time and all the rest of it we know the answers to those questions now and more esoteric ones besides so the next question that's what you always have to work on in science is the next question the next question are these questions there are they're not like seven questions between them and the other ones but you know you're a base camp so what do you do you can either sit there and wait for helicopters to take you up or wait for oxygen Thanks or he can start trying to go up and maybe you'll fall off and die you know who knows what will happen but but you can start doing it that's that's what I'm trying to that's what I hope you you you took from this talk is that it's possible to begin climbing and many people are climbing with different routes and some people might make much more rapid progress than others early on but crap out at the end because they didn't have the basically the right strategy you don't know you just have to have to try so I'm not even gonna speculate but what I do want to say is it's possible to work in this subject honestly okay that there's an internal notion of right and wrong and you can't just you know BS all day because you have to you have to actually produce results and the results have to have to have checks that they a pass and and if you do be asked too much you have your your colleagues around who are very intolerant of it so not looking at anyone particular than the audience that her hurry up please well that wasn't helpful remark at all very fascinating for presentation but in your presentation is based on the fact or assumption that the quantum reality or quantum foundations are concrete and you develop your logic from that basis but going back to the quantum foundation itself where the consciousness related questions related to the like objective reduction and the quantum measurement itself what role consciousness plays answer this question so I'm gonna give an answer that your thirty years ago was the answer 99.9% or theoretical digital gives but it somehow politically incorrect to say these days so I will say it there is nothing interesting going on in the foundations of quantum mechanics zero okay there's nothing in this subject that's not understood unless the words gravity or cosmology make an appearance and then not only are they relevant but that's the first twenty minutes of this talk was actually exactly about those those issues you see what forced us to talk about these observables of live at infinity it's because quantum mechanics need to do to these experiments with infinitely large apparatuses all right that's where this business about the foundations has teeth it has teeth it has beef it has consequences and you fall off those consequences and you actually do learn sort of radical things about the way you're supposed to talk about the world but you don't go anywhere with them unless unless unless you know that and you pursue them where they matter there is nothing interesting or deep or strange going on when a random graduate student does a random quantum measurement in a random basement laboratory somewhere and there's an everything there is fully completely understood within the ordinary logic of quantum mechanics something by the way what the founders of quantum mechanics understood perfectly I realized this isn't this is not answering your your this is not this is not engaging your question but I'm making such a strong statement because I I want to tell you the right answer and then if you come up here later we can talk in more detail about why that's the right answer which one okay so for good or ill I have been kind of imprinted with this vision of space-time as that you know the rubber sheet with the bowling ball right so by the way it's a little unpleasant because it's the gravity of the well what does this so how am I supposed to continue to sort of imagine not perfectly you see that none of this is saying that that space-time is not an astonishingly useful idea okay just like I mean this is not different so much in kind from something again that you you may have heard as we as we transition from the classical world to the quantum one that Heisenberg told us we can't talk about the position and the velocity of some particle at the same time well if you're a baseball player you damn well better be able to talk about the position and velocity of a baseball at the same time and if you if you you know if you tell your manager that Heisenberg's uncertainty principle causes you to bat you know 133 they're not going to be very impressed okay so it's an incredibly useful idea it's an incredible useful idea positioned the word position and velocity is an incredibly useful idea to keep in your head as you around the world but if you care about fundamental physics it's a discrete binary difference it's not position and velocities position or velocity right and now that reflects the magnitude of the problem we have that's what I was trying to explain in the first part of the talk that when we make these really giant transitions we change the language we change the grammar we change the words and what the words mean and those are one of the really big transition well the really big developments are and the and the problem is how can it be that the words changed so dramatically when they work so well right that's the basic challenge but we went through it already but when we went from classical to quantum and us and it presumably will be something similar when we managed to replace space-time with something else one of the necessary features of anybody coming along and telling you that they replace space-time and in some approximation they should tell you why that rubber sheet picture is damn good okay so we're never going to get rid of it in that sense but it doesn't mean that the the magnitude of the conceptual shifts that we have to make is not going to be huge just as the manager of the conceptual shift we had to make and we went from classical to quantum was actually huge does that answer your question yes remind me not to list you as a reference you know yes okay so along those lines how does Dark Matter fit into this are there black holes in Dark Matter that sort of thing or they're not yeah I mean there is a there's a large number of really interesting questions in fundamental physics and and we could spend hours in many different public talks on many of them so you know it is it is indeed amazing that the 90 that a huge fraction of the the energy in the universe is not made of of us there's dark matter there's dark energy we don't know for a fact what either one of those things are what I'll say about that the problem is that at least the conventional pictures for what dark matter might be it's incredibly important to know what it is it could tell us a a lot about about that the story of the the you know the detailed structure of the elementary particles that make up the world but in most of the conventional pictures for what dark matter is it doesn't sort of shake the foundations of the of the subject like the like the stuff I was talking about like the like the basic mysteries here you know shake the foundations of the subject dark energy is a different story altogether and in a in a talk that is three times as long as this one I would have probably spent two thirds of it on the dark energy problem so I didn't mention at once but but thinking about about the the really deep conceptual mysteries associated with dark energy is a huge underlying motivation that you know that is a superstructure on top of all of these questions at least as far as I'm concerned so so there's a at least if our conventional picture which might not be right of the of the ways these problems might be solved is correct the sort of hierarchy is that dark matter is something really cool be great to know what it is once we know what it is it's fantastic great humongous deal several Nobel Prizes but but it does not it doesn't shake the the picture of the world like dark energy does and unlike these theoretical questions do but that could be wrong that's part of our theoretical prejudice bait so let's end the formal part of this let's think Nima again very much for this inspiring [Applause] [Music] you

Channel: SLAC National Accelerator Laboratory

Views: 97,564

Keywords: space, time, science, theoretical physics, SLAC National Accelerator Laboratory, Stanford University, physics, public lecture, spacetime, quantum mechanics, fundamental physics, Large Hadron Collider, cosmology

Id: t-C5RubqtRA

Length: 91min 50sec (5510 seconds)

Published: Wed Jun 20 2018