Loose Ends: String Theory and the Quest for the Ultimate Theory

For more than a century physicists have tried to find a unified theory a single mathematical framework capable of describing all of nature's forces Albert Einstein who unified space and time and a special and general theories of relativity pursued this goal for over 30 years never succeeding today scientists are continuing the quest seeking to combine general relativity describing the universe on large scale stars and galaxies with quantum mechanics describing the universe unsub atomic scales each of these two theories works amazingly well in its own domain but put them together like in the dense center of a black hole and the math falls apart in the 1970s scientists came upon a potential solution but from an unexpected starting point studies of nuclear processes in that era we were trying to understand the strong nuclear force it's just the force that holds the nucleus of the atom together and that was a complete mystery how to do it in trying to solve that mystery physicists were led to a promising equation that seemed to describe vibrating strings that would hold together atomic constituents but within just a few years a different theory proved to be the right solution for understanding the strong nuclear force and so strings no longer seemed necessary almost all of these several hundred people stopped working on string theory at that time and only a handful of diehards continued to pursue them the few of us who continued working on string theory felt that the mathematics was so beautiful and compelling that this theory had to be good for something much like Einstein who spent long periods working on ideas outside the mainstream Schwarz and his colleague Joelle Sherk continued to explore string theory we realized what string theory was good for and that was as a quantum theory of gravity at that point I was convinced that this is what I've been working on to the rest of career Schwarz was proposing that string theory which had failed as a theory of the strong nuclear force was actually the solution to one of the biggest problems in all of physics uniting gravity and quantum mechanics when Schwarz excitedly announced this prospect few people paid any attention the field just wasn't ready so Schwartz and a new convert Michael Greene spent years developing the theory mostly on their own then in 1984 they achieved a breakthrough showing that the mathematics of string theory deftly avoided potentially lethal technical issues and this time the community of physicists heard them Princeton between the University and the Institute for Advanced Study you had almost half the experts in the world who were working on related topics and I am told that at that point almost all of them dropped what they were doing and became string theorists as string theory rapidly progressed there was a sense that research was but a few years away from uncovering the final and unified theory of physics more than 30 years later the goal of a fully unified and thoroughly tested theory has yet to be achieved but enthusiasm remains high string theory has come closer than any other attempt to go beyond known physics and tackled the grand challenge of unification but the question remains is string theory and elegant mathematical chimera or our string theorist rapidly approaching the realization of einstein's dream welcome to tonight's program which is focusing upon a really ancient project the project as we described in that little segment project of unification and you can really trace the human urge to try to unify our understanding going all the way back I mean Democritus long time ago imagined that reality amounted to atoms and the void that's all that there was Galileo spoke about the book of nature being written by God in the language of mathematics that would be the unifying framework describing things all in terms of the symbols that we use to articulate mathematical equations and as in that little piece and as well as many of you no doubt know Albert Einstein spent 30 years trying to unify our understanding of the laws of physics but ultimately came up empty-handed and what we're going to discuss here tonight is the next chapter of that story a story that is ongoing and we're going to break down the discussion into three conversations which very roughly speaking will cover the past the present and the future of unification there will be overlaps so that way of thinking about things is not completely accurate but that's the general progression of the evening so let's get right down to it and we have a number of guests who are leaders in the field and thinking about these kinds of ideas and the first guest this evening is a professor of natural philosophy physics and astronomy at Dartmouth College is a fellow of the American Physical Society and winner of the 2019 Templeton Prize please welcome Marcelo gleiser so Marcelo thank you for being here today and you know when we talk about unification we know that throughout history there have been many steps and route to the approach that some of us have been pursuing in fact you pursued at one point as well string theory and we have a little road map that we can bring up on the screen which kind of shows some of the key steps now obviously there's a lot on this and we're not going to be able to cover it all but what I'd like to do in our conversation is sort of start at the bottom left-hand side which many people view as the first step in the modern program of unification which is putting electricity and magnetism together so can you take us a little bit through that history maybe going back to Faraday and Maxwell and what happened sure so we're talking about early 1800s and people had some experience of electricity by having shocks whenever they you know had a very dry environment and they were touched like a doorknob or something and they knew about magnetism since well for a long time but at least in 1601 the court physician of Elizabeth the first called William Gilbert he wrote a book about magnetism and he realized that the earth was a giant magnet and that's how compasses work you know the basically aligning with the north magnetic pole and so these two things were considered to be very different and until Michael Faraday started to make some experiments and and actually even before him Hans Christian Oersted in Copenhagen he realized that if you had an electric current going through a wire a magnet would move and so there was this and it was sort of an accidental discovery had a bunch of stuff on the table or doing demonstrations and then he figure that out so clearly there was a connection between the two and it took Michael Faraday's brilliance you know to develop this and he realized that it wasn't just an electric current that could effect magnetic compasses but it was actually magnetism just like the picture is showing up there that he realized that if you move a magnet you can't create an electric current as well so there was clearly a connection between the two and he started to visualize this and this was also very useful for us with the notion of a field you know that basically the presence of these sources of magnetism of electricity they they spread around space and this spreading around space is how say a charge will fill the presence of another charge through this field right and then later on came Maxwell maybe a couple of decades later and he then elaborated these connections between electricity and magnetism showing that you actually have a set of four equations right the ones that you see on a t-shirt right you have and God said four equations and there was light because what Maxwell did was he wrote the equations of how changes in electricity magnetism because of different sources or even in empty space propagated and he realized the propagation was at the speed of light so clearly the way light should be interpreted is actually as a moving electromagnetic field in space and hence the unification of the two now those those four equations that are on t-shirts that perhaps it's worth noting his formulation was a little more complicated so you know as we mature in our understanding in the field of science and this will be a theme actually throughout tonight we get better and better and finding ways of streamlining combining unifying in a sense even in the mathematical notation so today we have these four equations but back then it was kind of a mess fact you if you really use special relativity which we'll talk about soon you can actually write all of them in one equation yeah it's beautiful right exactly so why don't we turn to that so this is our first unification electricity and magnetism this deep unexpected connection Maxwell codifies it mathematically in the electromagnetic equations that we all teach the undergraduates and physics around the world Einstein perhaps is rightly credited with the next step in the toward unity by recognizing an unexpected link between space and time say can you sort of take us you already mentioned light so the speed of light being constant is a vital part of that story as well so when you take us through that though so I still realize that there were some inconsistencies with the formulation of Maxwell's equations and and nobody could figure out exactly what was going on one of the big mysteries in in the 19th century was that you know as we know from intuition every wave propagates in a medium right so if you throw a rock on water you know the energy of the rock is gonna hit the water you're gonna see the waves propagating on water Here I am talking to you that means you have sound waves growing through the air if there is no air there's no sound no explosions in outer space so every way seems to need a medium to propagate so the question then became okay so at does light propagate right there was a important question because you can see Stars which means the medium should be transparent right and it can't have any viscosity because otherwise the planets would slow down and fall into the Sun so he had a transparent no friction no weight clearly and he had to be somewhat rigid actually quite rigid but to propagate waves at that kind of speed so it's kind of like a magical thing which people call the ether right and for many many years Maxwell try to create models of the ether to kind of make sense of all this and in 1887 there was a very famous experiment actually by American physicist Michelson Morley where they actually were going to show how the eater interferes with the propagation of electromagnetic waves and they couldn't find it so there was like a mystery for a while how is that even possible all right and only 1905 Einstein came up with his way new way of thinking so he creates the special theory of relativity he's 26 years old and he comes up with two essential postulates one of them very reasonable known way before from the time of Galileo that the laws of physics should be the same for everybody at least everybody moving with with with constant speed relative to an order makes sense otherwise how could you actually have laws of nature if they change when people moved around and the second one which is a really amazing one is that the speed of light should always be the same for any observer irrespective of how the source is moving right and with these two things he showed that you could actually that the the speed of light and the way we make measurements of distances in space and of moments in time a duration in time were affected and that's where you have the very strange idea of time dilation you know time moves slower if your clock is moving and space contraction where little you know a rod for example or you if you're moving close to speed of light you're kind of shrinking the direction of the motion so schematically it's as if space and time as we see here are kind of adjusting themselves in order to keep this other thing the speed of light from change and keeping it constant at you know whatever units you like this one is miles per hour but this constancy of the speed of light winds up establishing an unexpected unity between space and time they need to work together in order to keep the speed of light constant so that's sort of the second big moment in the modern route to a unified understanding of nature now after the special theory of relativity mentioned that's 1905 Einstein is 26 years old he could have stopped there right I mean that that's the kind of achievement that many of us would say you know there's not much left for me to do but frying Stein he was just getting started right so the next step is he goes further and brings the force of gravity into the story so it just tell us a little bit about the next ten years and this further unification of space time and gravity so it did take about ten years for a chance to really get to the end of the story but his first intuitions were not much later after 1905 he said he had the Hat his happiest thought right which was the idea that if somebody is kind of dramatic if somebody's falling from a roof dog that person won't feel his or her own weight right so you're sort of this idea of weightlessness they were falling through space and you feel weightless that is was his intuition and you know all this because living in New York you go up and down elevators all the time and you know when you're going down an elevator from a high-rise very fast you feel lighter right now if televator just Falls you feel weightless so I just and realized that to talk about motion they had acceleration in it you also had to talk about gravity so as he tried to expand his theory of special relativity special here's just was about motions at constant speed to motions with acceleration he realized that that new theory had to be a theory that included gravity as well and that's a beautiful thing because as he goes to formulate the theory which we would say it's perhaps the most beautiful theory in physics the biased of course you know because that's what we do right that theory was a theory that related gravity not to this kind of mysterious action at a distance which was what Newton did he in a sense did a unification of the laws of gravity on earth and in space as well but it was a theory that describe gravity as the curvature of space and also the its effects time and how time flows if you have a very strong mass you know that to do that so that was his next big step so he realized that and of course we should also mention the connection between mass and energy there is a byproduct of all this so in 1905 there was a second paper on speciality which is the famous equals MC square paper right in which there is a deep connection between mass and energy and that mass had the property of affecting the geometry of space and because of that there is a change in the motion say of planets around the Sun which can be explained by this by this beautiful theory so we now have this part of the the map fairly set now in 1919 70s or so there was another important moment that we don't really have time to go through but it's quite analogous to what Maxwell did putting electricity and magnetism together and that the electroweak theory which puts together another force of nature which is the weak nuclear force together with the electromagnetic force they also proved to be different aspects of a single ingredient called the electroweak force so we get to this point by say roughly 1980 now we could stop here right we have gravity in the story we've got electricity magnetism weak nuclear force and so forth but we want to continue to move toward unity as best as we can now the next steps however go beyond what we have experimentally tested so so the question is what do we do so we can give a sense of the equations behind this which is not a bad thing to show because it helps to motivate us to go further so so what are we looking at right from one equation to this that's not good right yeah so this is essentially what summarizes what we call the standard model of particle physics which basically is a description of all 12 particles that we have measured in the laboratory especially like at the Large Hadron Collider at CERN and before that a formula these huge machines that can actually find these particles of nature so there are 12 of those and then there's also the Higgs which is a particle responsible for giving mass to all the other particles and basically this is the state of the art in a sense that we can actually measure and feel comfortable somewhat about what we know and this describes essentially three fundamental forces gravity I'll forget it gravity is not even there right so this is not about gravity this is about electromagnetism and the weak and strong nuclear forces so the weak nuclear force very quickly is the one responsible for radioactive decay for their activity for having you know how the Sun powers itself in a sense and the strong nuclear force is one that keeps say the atomic nucleus together and I have a bunch of protons they're all positive they should be running away from one another but they're glued together by the strong force who also keeps the quarks which are the fundamental particles you see here inside of protons and neutrons okay so this is it one thing about the electroweak unification though is that it is not as perfect as we would like it to be because the way we think about these forces that every force has a what we call it a coupling constant but basically a measurement of how strong that force is innocent so gravity has the G for neutrons constant right electromagnetism has Eve for the electric charge and it turns out that the electroweak unification does not have a single coupling constants to connect the two it's still describing the two forces somewhat differently they seem to review similar behaviors at very high energies which is the the inspiration we can actually show the rest of the road map but the road map from here unlike this equation which is experimentally tested as we go further these steps are not experimentally confirmed in fact we can even go further and show the rest of the map and basically everything that's above Row 3 is hypothetical and we have been developing the mathematics behind these next steps in the road toward unity for three decades if not longer so the deep question that we face and I posed to Marcello without having the experiments without having the observations without having the data that will allow us to go above the third row where we do have experiments and data that confirms everything where does that leave us and you have very specific views on whether we're perhaps doing the right thing and pushing forward with a purely mathematical motivation to go further right so a little bit of history is that I in 84 I was doing my PhD at King's College and with when Schwarz talked about his and Greene you know talking about that discoveries in the string theory I'm like wow this is it no you got to work on this it's beautiful it's at the time the idea was that you'd really find a unification of the four known forces of nature which is something we should talk about at some point four known forces of nature is very important so I jumped into this and and tried hard like you did and a bunch of other people in our generation you know try to to make sense of all of this and and the program was very sound in the sense that behind our expectation of trying to find this unified formulation there were very concrete predictions right I mean so in order for these string theories to work you had to impose a new symmetry of nature called supersymmetry and supersymmetry had a very specific prediction which is basically to every particle of the standard model the twelve particles I talked about there should be a mirror world so to speak of supersymmetric particles so the electron has a supersymmetric is called this electron the photon the particle of light a photino and so on and hopefully with big machines you should be able to find them right that was the big expectation of the time so you big bigger and bigger machines and one of the goals of the Large Hadron Collider in Switzerland was not just to find the Higgs which beautifully was done in 2012 but to also find the lightest of all supersymmetric particles and and it hasn't it's not there at least we haven't found it yet right and and so that rules out some versions of supersymmetric theories and then the question is what do you do as a theoretical physicist at this point right that you basically spend decades of your life working and of course we're going to have another guess here there is a real expert on this you spend decades of your life working on this thing and you know you're placing a bet on this and it's not coming through so what are you supposed to do at this point right so do you say I'm sorry I was wrong and you move on to other things well it turns out that supersymmetric theories they're so flexible in their formulation that you can always kind of change the parameters in ways where the particles that you were supposed to find are so massive that you could not find in the head-on Collider which basically means is a theory that in principle cannot be killed right because it could I mean you say look I haven't found it no problem when we have a accelerator which is a hundred times bigger you'll find it and then you can always to a certain extent drill this up and create more and more complicated models right and I don't think there is anything wrong with that because there's what I would call it creative perplexity right now related to this because clearly we want this beautiful Platonic dream of simplification of nature to be true but at the end of the day nature is the ultimate guide you know we have to listen to nature because that's the whole goal of physics is to explain what the world is like not what the world is like you'd like it to be and and so this is a very interesting time for us right now because things that we thought were going to be definitely discovered are not quite being discovered and the moment now is a little complicated and and it's a very important moment because what we decide to do now is going to impact the field of particle physics for decades to come because these accelerators are very complicated machines and it takes decades of planning and a lot of money to make them work now you have a particular philosophical orientation in terms of human knowledge more generally and I think you even have a slide that you that you gave us that summarizes that perspective so just take us through your view on the program to sort of try to find the deep fundamental laws that will describe all physical phenomenon versus your view of how knowledge progresses right so this is actually the title of a book of mine called the island of knowledge and it's basically a metaphor for how we humans understand the world around us so the idea is simple if everything that we know about the world fits in an island right as we know more about the world and about ourselves and about a place in the universe this island grows and as every good Island you know it's surrounded by an ocean and I call mine the ocean of the unknown so science in a sense is an exploration of what we don't know right that's the whole point we are expanding our views we're expanding of tools of exploration I call them reality amplifiers in our telescopes or particle detectors in ways that we can see more that we could see in the past so that is how we advance right so as the island grows into this ocean of the unknown the paradox of knowledge though is the fact that the boundary between what is known and what is don't what is not known grows so that means that as you learn more about the nature of reality you're become equipped to ask questions that you couldn't have asked before because you had no idea simple example of that the telescope so before Galileo built a telescope you know in 60 well it wasn't his first wasn't the first telescope but he said it was he was smart and then he sold it to all the you know the nobility of Venice made money etc but so he he builds his telescope he looks at the sky and he sees things that no one had ever seen before right and that was a profound change in our worldview and with many implications you know religious implications social implications for softer implications because of this expansion of knowledge and because of this new machine he was able to ask questions about the world about the universe that no one could have asked before right and so to me this is very much how science advances it's through this expansion into the unknown that allow us to begin to ask new questions for example we couldn't have asked anything about supersymmetry before we had the idea of supersymmetry and now we're looking for it and we don't find it let's assume we could find it and then this whole new way of thinking about the world is is going to happen because of this new theory now having said that presumably you do agree that there are chapters inside the book of knowledge that can be fully written they open up other areas but it's conceivable that we will have the theory that describes all of the fundamental forces describing all the fundamental particles of matter how they combine and the kinds of behaviors that aggregates can play out maybe something that is beyond our ability to ever fully articulate like I don't think a fundamental theory will ever for instance be able to predict the the kinds of socks the color sucks that I have on I don't even know what I've got but I've got black right now right so it's just unlikely that we'll be able to undertake those kinds of calculations but what about the fundamental theory that's on our map toward unity is that something that you think fits into the unknowable or is that something that you imagine could be no right so so that's a great question so I think in terms of what philosophers like to call the map and the territory okay and the idea is that we are map makers that's what we do our theories are maps of what we see of the world right the territory if you were to there's a famous short story by jorge luis borges about the map makers that wanted to make a very perfect map and every time they improved the map the map became bigger and bigger and bigger until they had the map as big as the country and that was the best map they could ever have and clear was a useless map right because maps are descriptions of what we can see and so the point is that our theories are maps of reality guided by what we can eventually measure right and given that our machines our technologies can never give us a complete picture of reality there is always a higher energy scale until you get to the Planck scale and we are very far from that that is really very dangerous I think to assume that the four forces of nature that we know now are the only forces of nature exist so what I would like to point is that the best that we can do is to construct a unified theory of the known forces of nature but to make a statement that that is the ultimate theory of how the funnel is to me really unjustified by the way we do science right but typically of course I think most physicists have the attitude that all theories are provisional they're the best description that we have at a given moment of the physics that we have access to but you know we all know that even Einstein's equations will be modified as we try to understand the universe in more extreme realms than his equations were developed to explain so let me finish up with one one question because we only have a couple minutes before I need to move on to the next participant tonight it's kind of a big one but maybe I have a quick thought on if it's too big just telling me hey I don't want to talk about that we can just end it right there but when we talk about complete theories some I'm often asked what do you think about girdle's incompleteness theorem and what does that imply for this program girl of course famously wrote down a theorem a long time ago that basically said that if you have a system with axioms that are sufficiently rich to be able to describe things like arithmetic that either the system will be inconsistent or if it's not inconsistent there will be true statements within that system that you'll never be able to establish to be true within the system itself does that have any bearing in your mind on the program of searching for the deep fundamental laws of physics I think it is in a sense because it's kind of like the Russian dolls right I mean so to bypass the giggles incompleteness theorems you have to create a bigger system that encompasses that one right but then of course that bigger system is going to have the same problem and then a bigger system and on a bigger system and the problem is that we just the idea of completeness of knowledge is is is a very dangerous one and I think it'll prove that in mathematics and the notion that in physics because we do need to validate empirically what we're doing it becomes even more complicated because we depend on technology on machine and so I'm very happy with the notion that we can't know everything there is to know at the level of fundamental particles because that's what's going to keep us working harder to move on and on and to me is the process this quest for knowledge that really matters not so much the unpredict and gives us something to do going forward exactly yeah ok Mia in the second part of the conversation we're going to turn specifically to string theory proper and try to get a sense of how far we've gone in connecting it to the physics of the real world imagine I have a beautiful tree that's filled with oranges and I asked myself what is the orange made of how do I answer that question well I want to look deeply inside the orange so I magnify it and I magnify it again and if I keep on doing it deep inside sooner or later I begin to see molecules come into view but molecules are not the end of the story because the molecules I can enlarge them and if I make them big enough deep inside I begin to see atoms atoms are not the end of the story too because we have electrons zooming around the nucleus deep inside mostly empty space in the atom but deep inside we see the nucleus so if I grab that and magnify it I see that the nucleus is itself made of particles neutrons and protons and if I grab one of the neutrons and magnify it I find yet further particles little tiny quarks inside now that is where the conventional ideas stop string theory comes along and suggests that inside these particles there is something else so if I take a little quark and I magnify it conventional idea says there's nothing inside but string theory says I'll find a little tiny filament a little filament of energy a little string like filament and just like the string on a violin I pluck it and it vibrates creates a little musical note that I can hear the little strings in string theory when they vibrate they don't produce musical notes they produce the particles themselves so what quark is nothing but a string vibrating in one pattern an electron is nothing but a string vibrating in a different pattern a neutrino nothing but a string vibrating and a different pattern still so if I take all of this back together I have my ordinary orange and if these ideas are right they are speculative but if they are right deep inside the orange or any other piece of matter there's nothing but a dancing vibrating cosmic symphony of strings that's the basic idea of string theory how far we've gone toward establishing or refuting that this idea actually describes the world around us and for that discussion I'm pleased to bring out our next guest he was recently elected to the National Academy of Sciences and is currently writing a book on the big questions confronting particle physicists and cosmologists please welcome Michael dine so Michael thanks thanks so much for joining us and you know we had this map of unification that we can bring back up where we see at the top this proposed idea of superstring theory also called m-theory maybe we'll get to that in the course of our conversation but the the key question is is that step of the story something that we can justify through experiment and observation I mean the mathematics is beautiful John Schwarz spoke about it in the opening that it puts gravity and quantum mechanics together the left-hand side we understand well quantum mechanically the right-hand side is problematic quantum account your string theory puts that all together so it's a beautiful compelling structure and the question is aligning it with experiment now in the previous conversation with Marcela we spoke about supersymmetry which is the super super string and that for a long time was held out as the smoking gun that we were going to prove the super symmetric quality of the universe so first of all when we talk about supersymmetry what does it mean for the spectrum of particles the stuff that should be out there well I suppose back up in the in you wrote these equations or we presented these equations for the standard model and they look really ugly yeah okay they're not really ugly they're really rather beautiful so the so the theory is biskits ocularly successful until recently we were missing one piece which was this Higgs particle and there was a lot of speculation about what that might be and now that's been found and and it acts sort of just like it's supposed to right so so so we had this this kind of very pretty story and the thing that sort of excited a lot of the interest in string theory and certainly part of your career and certainly mine was the and East rominger who we'll hear from next is that string theory seems to be able to produce this standard model at least in rough outline right in a very remarkable way so so so that's partly why I didn't want to bias this that way but your beautiful Theory comes out right when all sake comes out vertically you mean like the vibrations of the stream the kinds of particles that we see on the left they bring in a very remarkable way yeah and and so that part is is really quite pretty now there had been before string theory came along there were reasons to think they might that that might be they're mostly connected with puzzles are related to the higgs particle and a lot of us thought who worked on this at the time thought maybe this idea was a little contrived and then a long came string theory and among the things string theory did was oh there was okay me not these particles next extra things yes these Morton these additional vibrations were were there and I certainly was I actually should say I was sort of dragged kicking in streams screaming into the subject of string theory because I've read all your papers I thought you were a gung ho and duty I was initially I was part of part of this generation that viewed string theory is kind of passe this thing as you saw in John Schwartz's argument and yeah and there was a period where I would talk particularly the ed Witten and I would say how are you gonna solve this problem and you would say you scratch his head and they say I don't know and then he would come back two days later and say by the way so so it's my do I have a solution yes yeah right yes so so I got quite nervous and in particularly this fact that a lot of these features of supersymmetry were there that we had speculated on was got me quite excited right and I thought I think in the spirit of things you said that there were weeks away and I better get to work so but sweetly but I should say that it's sort of understood in fact supersymmetry is a little too much of a crutch in string theory the only string theories who really understand well have exact supersymmetry so all these other particles would be there there were in addition to the electron which they in addition to the electron which is which which is so familiar to us there would be this thing just like that except just a little bit different on this electron and it's not there it's now so back in back in the eighties when I started to work on string theory all the somewhat more established scientists such as yourselves gave the impression that this was the next frontier of particle physics we're building this big machine in Geneva and it's gonna find these particles and that I mean tell us where we stand on that program so the truth is there were reasons to be skeptical about this program you tell me for a while I actually if you listen to me carefully as the low post as many of my friends some of whom you were having in other parts of being you would have heard me say that but but but but what we're now in the situation that we have the the Large Hadron Collider at CERN which has now been running well for almost a decade has has this that has both established the cigs party really established the standard model with exquisite precision has allowed us to do all kinds of things both experimentally and theoretically which I wouldn't imagine we could do it's a number of years ago okay but one things that has done is is said I think as Marcello said that if supersymmetry is there the particles are heavier than we guessed and Marcello described the theory as rather elastic in a way it inside its elastic but it gets uglier as the masses of these particles get to reach of the large right so so to be specific for example the the what we know about the Higgs suggests that the now suggests that the supersymmetric particles if they're there are are out of reach of the LHC and out of reach of a lot of the things that we currently contemplate so so so it's so it's it's a trouble it's an idea in some tension in some trouble yeah I mean I should I should say I don't know I I recently did a sort of a quick just curious on the fraction of the papers that I've written in my career that assumed that supersymmetry is correct and it's a significant fraction it could be as much as 90% of the work that I've done so there's a certain kind of discomfort associated with the lack of finding these part do you share that discomfort or I don't know if it's 90% of my papers it's in it's a it's a it's a it's a number and yes I share I share I share this discomfort I think will come but I think will come shortly to some of the or a little bit to some of the reasons why perhaps some of our D ideas about where supersymmetry should be you have a certain amount of hubris yep and so let's change your slightly there and talk about another key quality of string theory that is really quite iconoclastic that that it requires that the universe have more than three dimensions of space this is sort of one of the other strange features and because just sort of you know in a cityscape like here you can consider the dimensions to be up down back forth left-right and if you zoom in according to the ideas of string theory you go small enough you're gonna find extra dimensions of space usually imagine that they're curled up really small in order that they evade direct detection as we look around the world around us and strings are imagined to be so small that they vibrate within these tiny curled up dimensions of space so depending on the formulation of the theory there could be six or seven of these extra dimensions we understand that distinction quite well the question is is it a matter of simply hiding away these extra dimensions that are sort of a weird mathematical feature of the theory or is there a way that we might indirectly find evidence that these extra dimensions are actually out there could they be the smoking gun there there well there are various ways to think about this question I mean I often when I talk to students or lecture about this say the real problem so first you get kind of dizzy when somebody introduces you to this idea and the next problem is they say how hard it is to if they're curled up to actually see any evidence so and there had been speculations on the possibility there actually some of these extra dimensions or would be large yeah sort of millimeters even size so as you could imagine sort of probing them with tweezers and people subsequently have done experiments and ruled out these kind of very what people call large very large extra dimensions there is still a chance on my own embedding is against it that we will we could find evidence that we will not know so I would bet against but there is the possible but I want to say there is the possibility and certainly people consider it and people in doing experiments consider what might be the evidence in trying to exclude or place limits on these possibilities but but as you say it also seems I am I strongly suspect that well not we may not know the whole picture that that extra dimensions do play some role will play some role in some larger understanding of the laws of nature now one of the things that excited me about this subject we sort of talked a little historically back when these ideas were first really being developed back in the 1980s the precise shape of the extra dimensions sort of played the role of a kind of DNA of the string theory universe because the shape of the extra dimensions would affect how the strings vibrate and as you saw in the little opening video how the strings vibrate determines the properties of the corresponding particle so the thought back in the 80s was if you understood the precise shape of the extra dimensions you might actually be able to calculate those numbers in that crazy formula that we showed before that had within it you probably couldn't see it the mass of the electron the mass of the quarks the mass the neutrinos the mass of the Higgs that all of that might be embodied by the geometry of these extra dimensions a unity if you will of all those numbers inside the geometry the shape of these extra dimensions that program presumably was something that was appealing to you in that era as well one of the problems of course is there's not just one shape we're showing one shape over here I mean tell us what happened in the eighties and nineties for the you're actually more of an expert on this than I am but the but but there really and Andy as well there there has been a proliferation pot of sorry it should perhaps back up and yes a basically we don't really understand string we understand string theories sort of we don't really understand this it that well we can't write the equations as nicely as Maxwell wrote dis equations but we know we sort of know when we found a solution of the equations right and we thought we had just a few at first and some of them looked like you and yourself worked on examples which looked like they had lots of features of the standard model but there has been a proliferation so then now I don't know what the right way to characterize it but ordered many orders of magnitude of of known solutions as many as many different shapes may differ from different features so so we're not I personally am NOT an optimist that one could find the one right I I view this as a problem that one will have two more probe at least for the moment in a sort of statistical fashion asking what's characteristic what's generic as opposed to trying to find this specific standard model if you will find those generic features here by alright japes so my fantasy for example is explaining you know these we're coming back to supersymmetry yeah a kind of question like might be among these is it typical that the scale of supersymmetry breaking that the the splittings between these particles in their masses is that something you should see if the LHC is it something that is somewhat larger scale could you figure that out right and there's so many could you to sort of do some statistics of these and figure it out now now one other sort of astounding development in the late 1990s was the discovery that the expansion of the universe is actually speeding out or accelerating and and that was an unexpected I thinked about certainly for me I presumably for you as well we thought that the expansion will be slowing down over time gravity tends to pull things back together but yet it's going faster and faster to explain that we needed to introduce this idea of dark energy that yields this repulsive push the weird thing is the amount of dark energy is a bizarre number right it's a starts with a decimal point has a huge number of zeros after it and you know there's a sort of convenient feel for it right here so this number trying to make sense of this observed number indirectly some people of other approaches what has this done to the program of trying to connect string theory to the observable world so let me go back to supersymmetry first and you kind of explain what the tension was I did try before and why I why I claimed I was at least cautious okay and so you've drawn here something I didn't count the zeros but I'm guessing you about 120 years there are 120 zeroes so supersymmetry was proposed to explain a similar problem where they're about 32 zeros yeah okay so 120 is a lot more than 32 and it is in many ways a more fundamental problem and we don't have really good ideas we have an idea which we'll talk about to perhaps understand this none of us are totally in love with it and it's different than the idea of supersymmetry so this is something we already we knew even before this was discovered that that there was some tension here significant tension here and it's gotten just worse with this with this understanding and discovery any other idea presumably referring to is the multiverse idea yeah so once you sort of take us through the thinking so you've already sort of alluded to it here that the we we have this proliferation of solutions of the string equations actually I should perhaps back up and say that the multiverse was proposed before string theory in a sense it was a formula this was proposed by Stephen Weinberg who who basically predicted the dark energy so he basically said a way you might understand the dark energy is to imagine that there are actually if you like many universes with different values of this crazy number most of which are big I think you have a slide it shows most of which are don't have so many zeroes yeah but but there's just so many that a few of them have these zeros and if you're some kind of star trekky kind of figure I think you know exploring is these many universes somehow once in a while you'll find one with this very small number and what's special about these ones with small numbers he pointed out is only those will have stars planets things were people or intelligent beings are less intelligent being soon be yeah and this so this this idea gives a lot of gives pause but it also successfully really predicted the amount of dark energy that's observed so string theory looks like it might provide a setting for this idea this multiverse it has the potential we're not sure it's also actually related to our lack of understanding of how supersymmetry is broken yes Turing theory but there's the possibility that string theory works this way the idea is that you could have all these different universes with different shapes for the extra dimensions giving rise to different amounts of dark and different values of all those coupling constants of all those numbers in the sly and now some people look at a solution like that quizzically and they say look you guys have all these problems you don't have a unique shape for the extra dimensions you don't have a unique universe and in order to leverage those problems you imagine that maybe all these universes are out there and that we're just 1 in this grand collection and aren't you guys just giving up or even worse papering over the problems that your theories have we have you respond to this I remember a talk that David gross noticed string theorists cave where he kept repeating Churchill's lines yes never never never yes and and there's a question of whether it's giving up so my own personal attitude and now I'm not I'm not speaking of a great movement or something is that a way we might deal with this is to understand the statistics of of these many states so I think for those of us who now live in a world of big data and so on you could imagine you know you somehow sample all these and you have some idea what's typical you're right and there's a one proach one one view one aspect of this which is related to things that you worked on as well is related to the fact that most of these many universes aren't stable they're sort of like unstable elements they would decay after a fall apart quickly yeah fall apart and quickly and we want to stick around for you know at least 15 billion years or so and these would fall apart typically in fractions of seconds okay so you could ask what might account for what what among as you sample these which are the stable ones and it turns out interesting enough the stable ones are tend to be the super symmetric ones so this doesn't quite predict supersymmetry at the LHC but this is a in the class of things we might hope to address right but there are I should say there are many people who reject this whole way of thinking entirely so you know we're getting toward the end of our of our time here and and I wanted to just ask you the file I don't know if this is a personal question role but if we go back to your frame of mind in say the 1980s when there was this incredible excitement about supersymmetry string theory super gravity unification if you were to project forward from the mid-1980s to 2019 how have we done we the good questions first so so I would say that we actually a little bit as our focus has changed yeah in some ways I think in fairness to myself again III there's something called the tyneside burg problem which is which basically deals with some of the problems we faced and and and I think we've come further to addressing it than we actually have I might have guessed so but we also have I think this is something that Marcello said we've also sharpened our questions okay so in ways that perhaps aren't even on these slides we understand the dark energy has now given us a sharp set of questions the dark matter which I don't think has figured so much in our discussion yeah how's also and again in a qualitative way string theory gives us some handles on these questions as well so I am I'm keeping busy I am I think there are lots of interesting questions I think some things are hard I mean I think the fact that you know everyone just to pause and say that the fact that the Higgs particle is there and actually acts you know it's not something I would have really expected it's really a very remarkable comes out of attention mathematics and Naturals really so so so looking looking forward final question imagine that in the next 20 years we don't find supersymmetry either at the LSE maybe we have a new Collider it doesn't find any evidence for supersymmetry we don't find the dark matter so we don't have that as an input to our story we we sort of don't have any connection to observable experimental physics what do you think happens to the program of unification the program of supersymmetry and in that situation I think it goes on but I think I am my own thing now I'm being I am being personal here I think well first of all of course in 20 years I'm older than you in 20 years I will be you know it won't matter but but the but i but i think i think it will go on but i think it will it will be a narrower field and it will be I mean I think you know there will be quiet there is always a question of what are both what we do theoretically and also what's feasible and affordable to do experimentally right now and these will be these will be serious issues right right great talking to you Michael thank you for your insights on these issues Michael dying everybody all right so the final section of the program we're going to turn to an approach to gain insight into some of these questions it's coming from a somewhat different angle from the study of black holes sort of extreme form of gravity and for that conversation we are pleased to have and astrologer he has numerous awards including the Dirac Medal and the breakthrough prize in fundamental physics and it's work has shed light on the black hole information paradox discovered by Stephen Hawking please welcome Andy Fromm so great to see you Andy and you know in this third part of this conversation on the search for unity the path toward realizing Einsteins dream and a more modern version and Einstein even thought about it we're gonna turn to the study of black holes and before we get into the details of that subject I mean just quickly tell us I think most everybody here is familiar but you know what is a black hole and moreover what was Einstein's view about black holes was the into this possibility comes out of his equations yeah so if you're on the surface of the earth and you want to get into outer space you have to jump at 11 kilometers a second on the moon it's a little easier if you jump at 7 kilometers a second you can go on forever scape the gravitational pull of the moon but if you have enough mass in one place to get away you would have to be able to jump at the speed of light and Einstein told us nobody can ever go faster than the speed of light and so nothing can escape from a region of space-time where gravity is that strong not even light and therefore it's called a black hole because the black light doesn't get out and the first solution to Einstein's equations was in fact a black hole Schwarz child family solution and Einstein encounters this solution to his own equations yes and and what does he what does he say what does he think I it's it's an incredible thing I mean the solution was found just a few months after the field equations in complete form exact solution of the equations and we spent a hundred years trying to understand it and Einstein himself the great man 30 years later sorry 25 years later wrote a paper in which he said black holes don't exist and it's not because Einstein was stupid it's because it was just there really seeing objects and we've been trying to understand them for a hundred years we understand a lot about them we know they do exist in a spectacular development last month yes a picture of it let me bring that picture up yep there it is yeah warmed all the hearts of those of us who've been working on it for many decades and but there's still huge puzzles about a black hole which came up with Stephen Hawking's work in the 70s and those we are still very confused about and oddly string theory has had a lot to say about them and string theory in the hands of Andrew Strom mature has had a lot to say about that and we'll get to that you don't have to be modest I understand but we want to get to the the incredible insight that you and kurma baphu had about black holes to get there can you take us back to one particular puzzle that was first articulated by John Wheeler which had to do with these ideas of entropy and black holes yeah so a black hole is a very kind of perfect simple object it's really kind of a hole in space which is very perfectly described by Einsteins solution of Einstein equation and in that way it's very different from for example a star so if we take two stars of the same mass they will differ in innumerable details they will have different composition of molecules the molecules would be moving in different ways but black holes are just every black hole of the same mass and they also have a spin is according to Einstein according to their equations exactly the same and this puzzled people and the great John Wheeler who also invented the word black hole describe this by saying a black hole has no hair that is it's a featureless object so if you have to bold people in the room you can't distinguish them by their hairdos if you do people with their you could say what as a short haircut the other one has a comb-over whatever there's many different distinguishing features but so stars are like people with hundred black holes or like bold people they're all the same and well if you can't see very well so he envisioned if you say threw a cup of tea I think it's the the metaphor that he is you take a cup of tea and you sort of throw it into a black hole he was worried that the the information carried by the tea the entropy if you will the disorder carabao the tea would have no way of showing up inside a black hole because like you say it has no hair and they all look the same right so this system was a puzzle that you could sort of get rid of entropy by throwing it inside a black hole and it would be gone from the universe and his student bekenstein began to think about this and he gave insights that ultimately led to your work what was his view of this puzzle well they were very bothered so going back to Newton and before there is an idea which is very important to physicists namely that there are laws of physics and moreover we like to think that the laws of physics govern everything that happens if we knew all of them and if we had perfect knowledge and part of that is that information should never be lost things can't just disappear and so if you throw a cup of coffee or a computer or your diary or something into a black hole according to the way that Einstein would have described them there's no trace of it left it's just van and that is antithetical to everything that physicists would like to believe so what do we do with that puzzle so is it you know do we have to bite the bullet and say that information is gone and we just have to deal with it or what do we do well this story got substantially enriched due to the amazing work of Stephen Hawking in the 70s there's a missing piece in this discussion was the inclusion of quantum mechanics and so Stephen asked the question well what happens if you have a black hole in the real world and we know that a black hole everything is subject to the quantum mechanical uncertainty principle it it allows no exceptions and so a black hole itself is somewhat uncertain and the location of the horizon of the black hole the edge of it wobbles around a little bit has some uncertainty to it and that allows things to kind of slip out quantum things can slip out and the way that they slip out was described in a kind of breathtakingly simple and elegant paper by Hawking in the mid 70s and I had a very precise formula maybe we could see that formula and it it is one of the important formulas in I would consider one of the several most important formulas that came out of the 20th century physics Stephen has it on his gravestone inscribed is that true yes and it's on the floor it was all with all that with all the terms yes Wow under what that cost that yeah I think somebody else paid for probably yeah it's all floor it's on the floor Westminster yeah it's very important formula we just talk a little bit about that formula so yeah so the s and there's is entropy we can think of this as a formula for how many gigabytes you can put inside a black hole and it turns out if you want to have a smartphone with more gigabytes the size the the number of gigabytes you could put into your smartphone is proportional to the volume inside yours right put the little chips in there you fill it up goes like the volume this formula isn't like that the formula for the gigabytes that you can put in a black hole and it's proportional to the area so it's as if you're only allowed to put chips on the surface of the black hole right and that's surprising because the formula goes against our intuition right that that the amount of information the amount of stuff should be proportional to how much volume is there but it's saying it just goes like the area right now when we study thermodynamics and entropy as undergraduates yeah if I could interrupt for yeah it's a lot of gigabytes yes all of the all of the gigabytes in the Google data banks could fit inside a black hole a trillionth of a trillionth of an inch they're thought to be the best information storage devices you did that hard to get they get hold of that information but maybe her name yeah but so so it's a lot of information but it scales in a funny way doesn't go with the volume it goes like the area but the point I was making before is that when we study thermodynamics as undergraduates we're told that entropy and you know it's related to information is account if you will the different ways of rearranging the ingredients that make up whatever system you're studying yes so a lot of entropy means there's a lot of rearrangements that macroscopically all pretty much look the same if it's low entropy then they're fewer such rearrangements exactly now you mentioned before that black holes are like the simplest thing right there's a horizon but there's kind of empty space inside so the puzzle still is what are we counting right in that formula right and you you gave insight deep insight into that yeah so I just just want to rephrase that that you know according to what Einstein and Schwarz Hilda the black holes were and wheeler they were the simplest most featureless object right and then according to Hawking they were the most complex conceivable objects with the most structure that you you could possibly have so this was a huge puzzle and we didn't it sat there for decades many people lost a lot of sleep over it we didn't know what to do about it and then string theory came along and had something to say about it and that was a surprise but maybe shouldn't have been such a surprise you know as you discussed earlier string theory started out we're scientists we're exploring the unknown there's no road map we just explore everything and we might find something that we didn't expect you know Columbus was looking for China and he might have been disappointed when he found America but in retrospect it was a good thing and so we started out with strings and Brian was in the beginning talking about this picture of quarks and leptons and neutrinos being made of of strings that picture could be right the jury is still out but later on and and I would say actually this this began and in a wonderful collaboration that Brian and Dave more exciting collaboration that Brian and Dave Morrison and I had some years ago in which we found that strings way in between becoming quarks and leptons could turn into little black holes and then you put a lot of them together and which I hey this is kind of interesting they can turn into little black holes then we started studying how they put put them together and we could make eventually learn how to make big black holes out of them and we learned in that work this was also the beginning with bright and devorah said with other many people it takes a village there are many people involved in this but we learned how string theory can pull off this magic feat of describing an object which at once looks like an empty hole and a maximally complex object so in some way that can be described with very precise mathematics something is in both incredibly complex and incredibly simple when something gets very complex it becomes simple again so with that you gave the world really the first way of quantitatively analyzing the rearrangement of ingredients that have put together in the right way yield in the right circumstance a black hole right so many viewed this a kind of key moment in string theory because while it's not predicting an observable feature of the world that we go out and sort of directly measure yeah it is making contact with the quantitative feature of the world that was already on the books for decades and this is the first time that there was a way of understanding it through our into mental theory right right so string theory found a solution to a problem that had been posed in a completely different field now this solution this trick the string theory has for accounting for all the gigabytes in a black hole we don't know if the trick its to say it's the trick that is actually used by m87 black hole m87 that we just saw a picture of but it there aren't we didn't know there were any tricks now we know that there's one trick and we're trying hard to understand if m87 uses the same trick that's this string theory uses so we can extract lessons from string theory about the real world and that's something we're working yes so this is a this is now as of no doubt you follow the news this is a black hole real one that's what 55 million light years away in different galaxies and 87 now observed making use of these radio telescopes that are scattered around the globe is it conceivable that we could use even more precise versions of these observations to make contact with some quality of string theory that would be kind of iconic to it that might actually single it out as the unique or among the few possible theoretical frameworks for understanding the observations it's it's it's possible and we're trying you know to make a link between this picture and the ideas in string theory and when we did this calculation in string theory it was a very complicated calculation a lot of mathematics algebraic geometry long complicated calculations at the end we got perfect agreement so we knew that we had done it right perfect agreement with Hawking's formula talk perfect agreement with Hawking's formula so you could describe Hawking's formula as you know you again going back to you have a smartphone you start putting gigabyte photos into it you put 16 of them in and then you can't put anymore in and you deduce that your smartphone can hold 16 gigabytes of data you don't know how it stores the data that's what Hawking did he deduced how much their information is in a black hole what we did using string theory where we have a lot of control over the equations was pry the thing open look at the bits count'em and show that it showed that it showed that it agreed now we did it in a very complicated way and as often happens in science you start to realize that you can do the calculation at a shorter and shorter and shorter way and ultimately we realized that the key ingredient that enabled us to do this calculation was a special symmetry that these black holes have called conformal symmetry and then we realize that some black holes in nature once that spin very very fast have this same conformal symmetry and so probably we can understand the construction of their what the chips inside it looked like using the same ideas that we used in string theory they haven't been able to deduce the spin of this yet and we are trying to understand that they will get a better image we will understand the form of the image better but we're trying to see if this black hole has that conformal symmetry that we the same one that we used in string theory and so we've written some there's a chance that that will work out yep and there's a lot of black holes out there there's you know thousands or billions of them up there this this is an image with only 16 pixels I'm expecting that you know this is the beginning of seeing black holes well this is like not even a black and white TV I'm I'm hoping that we will get you know high resolution images and we will see many black holes some of them we know are spinning at nearly the speed of light and and have the conformal symmetry and so we've made some predictions for what exactly what will the black hole will look like the detailed pattern of of the polarization of the light and the shape of the image and so on and in principle this could be seen and it would be an interesting right so that is our that is our prediction of what the polarization the light that comes from the black hole what they did with the Alex Lobosco Delila gates of dead cop it's what the polarization of the light will look like if you were looking down the barrel action we're looking at 15 degrees so it's actually an even more complex pattern yep but there's a very definite prediction now this is not proof of string theory but it is it is how science works it works in unexpected ways and we're talking to the event horizon team about how to best set up the actually little telescopes to try to look at this and yeah so so we're coming toward the end of the discussion a couple of the things I wanted to to discuss with you before we conclude so in part one we spoke to Marcelo gleiser about the general idea of unification and starting with Maxwell and electromagnetism and going ultimately towards string theory and he mentioned this possibility that you know they're perhaps islands of knowledge and maybe there are limits to what we can ever figure out in part two we spoke to Mike dine about trying to make linkages between the unification program and warm you know bread and butter visit particle physics supersymmetry things that we might actually see it the Large Hadron Collider and that program has yet to be successful so one way of looking at these sort of first two parts of the conversation is great try but you're sort of not really getting there in my conversations with you across the deck is you were one Columbia what's it would you have said that to Columbus when it came back we try but sorry I wanted China exactly exactly No so that really speaks to your optimism which has been completely clear to me over many decades but but just to give a sense where do you stand on on on your your view of string theory is it not has it reached what you'd hoped it would by this era is it is it in some ways you have some ways no and like what's your prognostication going forward well so there's a view of the nature of physics that perhaps started in the beginning of the 20th century that progress in physics was about understanding things called duction ISM understanding you know molecules are made of atoms atoms are made of electrons and protons protons are made of quarks and then we go down to strengths that's the reductionist program but it's a really kind of one-dimensional view of what science is and the original idea which many or but not all people had of what string theory was good for was it was the final resolution of the reductionist program now we knew what everything was we found the strings that's the end of it physics departments around the world can shutter their doors you know that's not how it works you know we keep exploring we keep finding new things strings had to be explored we're still exploring we found other things that we didn't expect we found out about black holes we found out about Holograms we learned we spawned new fields of mathematics we you know been used to describe superconductors you know walk all kinds of of wonderful things that we learned and in fact I think that the idea that people were excited about back in 1985 was really a small thing you know to kind of complete that table that you put down in the beginning of the spectrum of particles and then we could say okay you know this is last year the World Science Festival it's all it's all done now we've got you know it you know string theory was really the beginning and what has happened so we didn't do that we didn't predict new things that were going to be measured at the Large Hadron Collider but what has happened is so much more exciting and than than our original vision right that is we've learned in the process of trying to understand what makes up a black hole it turns out the interesting things were not the small things but the big things m87 this thing you could drop our solar system in it and not notice it and we don't understand it right so we've learned we've we've learned a lot about about the big things and we've learned things which we sadly don't have time to go into it but about the nature of space and time which in quantum mechanics and we're we're getting little hints of a radical new view of the nature of space and time in which it really is just an approximate concept and emergent from something deeper I mean that is really really more exciting I mean it's exciting as quantum mechanics or general relativity probably even more so right and I think that before we solve the puzzles that you've heard about tonight we will have a revolution in the way that we think about the universe at least as profound as what was brought on by quantum mechanics and general relativity and I would trade that for a slightly lengthened list of elementary particles any day another great point and on that that is a spectacular and it's Rominger conversation thank you folks

Channel: World Science Festival

Views: 153,538

Rating: 4.8651214 out of 5

Keywords: String Theory and the Quest for the Ultimate Theory, Brian Greene, Michael Dine, Andrew Strominger, Marcelo Gleiser, String Theory, particle physics, black holes, superstring theory, supersymmetry, quantum gravity, Einstein, extra dimensions of space, Bekenstein, Calabi-Yau, holographic worlds, multiple universes, supersymmetric quantum field theories, mathematical physics, Superstring theory, best science talks, New York City, World, Science, Festival, 2019

Id: YSWd21z2qqE

Length: 87min 24sec (5244 seconds)

Published: Fri Aug 02 2019