In conversation with Nima Arkani-Hamed

it's a great pleasure for me to be sitting next to the preeminent theoretical physicist of his generation Nima our Connie Hamed whose breath and death never cease to amaze me believe me you're in for quite a ride tonight what I'd like to do is to spend the first 15 minutes or so just giving a sense of context we're talking tonight about the future of fundamental physics so I just wanted to get a sense of where it is now and some of the big questions that Nima and his colleagues are looking at so Lehmann let's let me ask you to summarize what you think are the key results of 20th century physics in say the next two or three minutes well that's easy we learned in the 20th century that this incredible variety of phenomenon that we see in nature many of which on the face of it seem utterly unconnected to each other are in the end a consequence of a few very very simple basic general principles some of these general principles we didn't even know about 100 years ago but but we discovered them in the early part of the 20th century there are the these are the the basic principles of the laws of relativity is discovered by Einstein and the laws of quantum mechanics and those are the two big revolutions of the first part of the 20th century and much of the rest of the 20th century was devoted to figure out it was devoted to figuring out how these two great principles could cooperate with each other and could and simultaneously describe how everything around us actually works so we discovered that this tremendous variety of phenomenon actually boils down to very simple interactions between elementary particles and the kinds of elementary particles were allowed to have and the sorts of interactions they're allowed to have are very largely dictated by these general basic principles it's an amazing fact something that that we didn't even quite appreciate 50 years ago but but which we appreciate and understand very well now the structure of the laws of the universe at sufficiently long distances the sort that we probe where long distances here even means the tiny tiny distance scales that were probing at experiments like the LHC but the structure of the laws of nature at sufficiently large distances is almost completely dictated by these general principles of relativity and quantum mechanics and the story of the Higgs boson is essentially the completion of that structure so we we many of us knew the Higgs have to exist for a long time we believed on theoretical grounds that it should exist it's actually quite shocking that something as simple as the Higgs is associated with the phenomenon that it does explain but but many of us anticipated that it should be there and it's an incredible triumph or experiment of course it was discovered but it's also a triumph or this theoretical structure that that we've built up over the last century and so finally wherever we're at a point by the end of the 20th century which as a great was suggesting to me the other day should be defined by a physicist is July 4th 2012 and by the end of the of the 20th century when the Higgs was discovered we have a basic understanding of how things work the mechanics of how elementary particles interact with each other and at some 0th order how the world works and the questions that are then on the table for the 21st century's are deeper structural questions about why we can't got this sort of universe we got which turns out to be incredibly strange and bizarre and well I assume we'll spend some time talking about you make it sound a bit easy if I may say so yeah yeah you really say that there are two theories that the quantum theory that began its life 1900 then became through the hands of people like Dirac and Heisenberg a theory of nature on the small scale and you have relativity which in its general form is a theory of the drive force that shapes shapes the universe are you really saying all the stuff we've been hearing about quarks and strange and it's all all these are details compared with those giant theories there are very very important details but but it if I could describe it in the following way you could imagine handing a bunch of sufficiently competent theoretical physicists they have to be sufficiently competent example I wouldn't be able to pull it off but but you have to give sufficiently competent theoretical physicists these basic laws that the basic principles of relativity of special relativity even and of quantum mechanics and you could lock these theoretical visit up in a room and refused to let them look at the world outside and just ask them what kind of universe could you imagine that that's consistent with these general principles and they would discover something very interesting I would say look if we only had quantum mechanics there zillions impossible universes we could imagine even zillions of kinds of laws zillions of kinds of elementary particles or if we just had a relativity there's gazillions of possible universes we could imagine with their own details but you take these two principles together and all of a sudden it's shockingly difficult to make anything consistent with both of them and things boil down very quickly to a kindly menu of possible kinds of elementary particle and a very tiny menu of possible ways they can interact with each other so that that doesn't mean that that you can uniquely predict what the universe looks like um we have particles like quarks and electrons and photons and gravitons and so on we can't we can't predict ahead of time exactly how many of each kind of particle there are and the strengths with which they interact with each other but there's a very small menu that we get to choose from and all of our uncertainty about all the choices in what the universe can look like it just boils down to choosing out of that menu and so that that's a tremendous accomplishment it's it's it's an enormous reduction in our freedom for describing for describing nature and the fact that we've discovered that these basic principles continue to hold and they continued to describe the structure of all of matter all of the courses as we understand down to the tiniest distances that were exploring at the Large Hadron Collider those of you who don't know or describe we're probing distances that are around a thousand times smaller than the nucleus of the atom at the LHC the nucleus of the atom itself is around a million times smaller than the atom itself so so we're talking about unbelievably tiny distances which we need unbelievably high energies to probe and the same basic principles continue to work and in fact the Higgs had to be there the Higgs have to be there in order for these basic principles not to break down so that's that's why that's why it's a real triumph of this whole structure that was handed down to us by our intellectual ancestors in the early part of the 20th century that all of these things continue to work ok I don't think anyone is gonna disagree with you that if you go to the Large Hadron Collider you go to the other particle physics authorities you and your colleagues done a great job of predicting two-hump teen significant figures what the results of the experiments are you just said that but hold on Miss B let's just be boring realist for a minute if I ask you or your colleagues to predict the variety of organisms in this room the distances between the planets the size of the Sun could you do that nope does that not worry actually sorry sorry I was too quick we can predict the size of the Sun there there there there there are some there are some there's some very basic things like that that actually follow quite beautifully from general principles but but some but some many complicated and very important and fascinating phenomenon which 99.99% of science concerns itself with understanding is not what we're what we're talking about in this business we don't need all of the fancy-schmancy business about quantum mechanics and relativity even to get to a situation where you know almost all of the world around us is is governed into a very good approximation by classical physics and yet we don't understand if you give me a if someone's smoking I don't know if anyone does that anymore but but you watch that the smoke coming off the end of the cigarette and you see these like beautiful strange chaotic turbulent patterns it makes us as it rises we don't understand how that works and that's something that that we're all the essential equations that govern that phenomenon were understood already in the mid-1800s and yet they're complicated phenomenon that that arise from from from solving them which which we still don't understand well to this day so there's all kinds of things about the world around us that are complicated emergent phenomenon from the simple basic underlying laws it's that's what most of science concerns itself with quite properly but in our part of physics in fundamental physics we have a different goal in mind we're trying to understand in the simplest possible way the smallest set of basic principles from which everything else in principle follows and one of the reasons we do this is that it keeps working it's very surprising that as we continue to understand things more and more deeply fewer and fewer principles underlie more and more divergent phenomena and since we've been on that track really ever since Newton got us started down this trajectory 400 years ago or so we keep understanding that things that appear on the face of it to be completely different you know Newton told us that that the same force that drags the Apple to the earth is what keeps the moon going around the earth that's insane right it's totally not obvious it's something like that it's true and yet it's true and later we discovered that the things like electricity that the ancients knew about from lightning and magnetism that they had these funny rocks that sometimes repelled each other and sometimes attracted each other these were different aspects of the same thing as we keep learning more and more we discover that more and more of disparate phenomenon at the bottom at the most basic level are described by fewer and fewer principles and the 20th century took that trend of unification into much deeper territory much further removed from ordinary human experience but even more and more so I just claimed a moment ago that a very few to basic principles almost completely characterize and dictate what the world can possibly look like at long distances we find this fascinating and and this part of physics is not concerned itself with explaining all the rest of the incredible variety of phenomenon in nature which is amazingly interesting but but but trying to understand in the opposite direction the extremely simple ultimately mathematical principles that seem to underlie it all mm-hmm it doesn't worry you though but we can't explain the size of the Sun and the size of the earth and all sorts of you know any gross feature of the universe around us any decent theoretical physicist should be able to estimate and explain to to a factor of two or three okay but it doesn't trouble you that if you say if some some little set that Charles says NEMA can you explain the shape of this cauliflower you can't do it no absolutely can't do it I mean it does it I mean it doesn't trouble me I'm just as in it I'm just as interested as a next person about what what what about where those things are of course they're there they're wonderful complex phenomenon that that that somehow somehow arise from these basic underlying laws this is incidentally it's a really deep thing we don't have a very good understanding for why it is that the laws of nature organize themselves in this strange wonderful hierarchical way so that really simple underlying laws at short distances at long distances are replaced by a new sort of effective principles and at longer distances by other effective principals and so on I mean oh we understand in many ways technically how it works but at a deep level it's still rather rather mysterious mm-hmm okay now one of the things that we've got to get right is that you said about all the great things that science is done but the thing that keeps scientists busy is that they basically work on problems the whole time all right so you said that this is a great triumph that we've got we've completed the standard model as it's called based on special relativity and quantum mechanics with the Higgs Higgs boson just can you just give us a selection of what of the things that keep you awake at night the biggest problems that you see with this kind of physics you've been talking about don't think of any no no it's nothing it's nothing it's not that that I can't think of any you I don't know where to start so I could summarize what we said before about 20th century physics by saying that in some ways these big revolutions in the first part of the 20th century taught us that relativity and quantum mechanics largely make the structure of the universe inevitable which is amazing but if I have to summarize in a slogan what the difficulty is with physics today and in a sense we have to plow through understanding all these things to come to this point to be able to ask really I do want to emphasize this we're not at a garden variety location in the trajectory of our of the development of our field we're at a we're at a rather special time in that we're starting to ask essentially new kinds of questions which are rather different in character than the questions that were asking then we're asking you before and and as I said they're deeper they're more structural much of the 20th century was devoted to developing the principles that we learned about in the early part of the century and now from a variety of point of view we have some some some fundamental difficulties with these ideas that we have to overcome so and there are really two classes of questions the first is is in a sense the most profound and most deeply conceptual difficulties these two basic principles of relativity and quantum mechanics and in particular what relativity taught us is that we shouldn't think of space and time as two completely different things but they're somehow merged into this idea of space-time and furthermore that as things move around in space and time they interact with each other not in completely random ways but only when they eventually touch at points in space stuff so that idea the idea that there is an underlying space Sun we know from many points of view theoretical from many theoretical arguments we strongly believe in too strong a word but we very strongly believe that space time doesn't really exist and we and somehow has to emerge from more primitive building blocks so that's that's so the slow cold hold out of it you say that say that just just a bit more slowly you will convince the space time doesn't exist yes so the slogan we all like to say is that space is worried you ladies and gentlemen I think that says that the slogan is that space time is doomed and that something has to replace it right okay that's the first problem a little bit closer to home there's another class of questions about the universe it's such a simple question you would think we'd have an awesome answer to this question before moving on to all the fancier things that we spend all this time talking about but but it turns out we do not have an awesome answer to this question quite the opposite one of the most basic features of the universe is that it's big universe is a big place it's an enormous place and yet it's made out of microscopic things and and as we've discovered over the last hundred years not only is it ultimately the basic constituents and the basic interactions of all of these little point-like elementary particles that are whizzing around but even a little more deeply the essential character of the laws that describe them is is most clear most manifest at very very short distances or ultimately as by studying what the what the particles do at very high energies so that's a little bit odd that the basic structure of the laws is is is defined in incredibly short distances and yet the universe is an enormous and big place okay it's a little bit strange but it might not you might not think that it should keep you up at night but it turns out that it needs to keep us up at night because because of a funny feature of quantum mechanics again something you've probably all all heard about is is this famous uncertainty principle of behinds and bergs and one consequence of that is that if you try to probe very very short distances and times you need to do it with with collisions of particles at very very high energies so like a hollow like that so that's that's why the LHC so it's a wonderful irony right the LHC is the largest experiment human beings have ever built you know every superlatively you want you can attach to this machine largest greatest most people right - most humongous detectors like that why why is it so big ironically it's so big because it's designed to understand the laws of nature at the tiniest distances we've ever done before and and and ultimately the reason for that is this uncertainty principle of behind brings that in order the probe shorter and shorter distances we need higher and higher energies when we smack the particles into each other at enormous energies at the LHC in order to do just that we create new particles and those like the Higgs like lots of other elementary particles were more familiar with perhaps other ones that we're going to hopefully perhaps discover when the LHC turns on again and they come out they come especially when they typically decay back to ordinary particles those orbiting particles because the guys came in at enormous energies come out in enormous energies so you have to put these gigantic contraptions around them in order to stop them study them in detail and so on so so the it's it's it's wonderful that the enormous size of these experiments is a direct reflection of what we're trying to do which is to study the the tiniest distances that that we that we've ever ever probed so but but if I if I come back to to the - to the question of why there is a big universe we have these because of the uncertainty principle there is another phenomenon that if you even look at empty space what you would think of is the most boring thing possible right you know I try to examine what's going on here well I suck out all the air molecules in the room I'm really left with what what you might think of as the vacuum okay well even the vacuum can be exciting and that's because if you try to check whether this region of space is really a vacuum you need a microscope or you need a microscope like the LHC this will come to in a second but but just the act of probing very very small distances needs higher and higher energies and at some point you have so much energy that you can create a particle and it's antiparticle the existence of anti particles is one of the profound consequences discovered by Paul Dirac is one of the profound consequences of putting together relativity and quantum mechanics that every particle in nature has to have an antiparticle exactly the same mass but opposite charge and so that means that nothing stops you if you look at a really tiny region from producing an electron and a positron for example so you're active arif trying to check that this region is empty instead sees that it doesn't look empty it has an electron and and a and an electron and a positron comes out in fact if you probe more and more deeply higher and higher energies shorter and shorter distances you see more and more of a roiling cloud of virtual particles and antiparticles just in the vacuum so the vacuum itself is an exciting place and and accelerators like the LHC which are often described as the world's biggest microscopes you can ask what they're looking at what they're taking snapshots are the snapshots of the vacuum of this roiling mass of particles and antiparticles but there are these enormous quantum mechanical fluctuations at shorter and shorter distances and that's the origin of the problem of why it's so difficult to understand why there's a big universe we have enormous this roiling massive quantum fluctuations and very short distances and then it makes you wonder why if everything is fluctuating so violently which it is why does the universe over here look anything like it does over here look anything like it does over there and in fact as we understand it in more detail if you took the same group of theorists who we locked up in their room and asked them to predict what the universe would look like given the laws that we know they would come to the conclusion that this violent mass of fluctuations would curl the universe up to a miniscule size billions and billions and billions and billions okay I don't even know if I had known but as many buildings as you like smaller than it is and they would also conclude that every single one of us would be you know a million a thousand million million times heavier than we are we'd all be collapsed into black holes and none of that is true we live in an enormous one full hospitable universe none of us are black holes and and this is a this is this does not follow in a simple way from our laws in fact we have to make seemingly absurd choices for some of these parameters that describe what the laws of nature look like in order to just accommodate something as simple as why is there a big universe so anyway those are the two central questions two of the central questions I think what probably most people would say are the two central questions of fundamental physics in the 21st century what replaces space-time space-time is doomed ultimately question one and question two why do we why do we get this big universe when when there's violent roiling mass of quantum fluctuations at distance that is a stunning point though as I said we read so much in celebration we hear on TV about how great these theories are and then you hear as you say that the theoreticians given their head here predict that the universe is timing they give it their sighs completely wrong right well so so the reason why this isn't a blatant contradiction and also because and also that we're not lying to you is that there's that it that especially coming to this coming to the second point of why the universe is big there is something we can do to our equations to accommodate a large universe when I said you lock up these these theorists in their room they would make an estimate for the size of the universe and that estimate would be off by you know 60 decimal places so that's not so hot right but it but it's an estimate if you went back and told them you know what you know was actually 60 or they would say well jeez and then then go back and say well there is this thing I could do to accommodate that right but the thing that they could do to accommodate that is to pick some some parameter here like you know the mass of the electron and that picks some other parameter they're like the I don't know that the strength of the gravitational force okay and let's say if I if I adjust these two things relative to each other there are some sort of there's some magic value where I have to you know very finely adjust these things so that I can make the University as big as big as it is but in order to make this happen we have to finally balance things against each other out to the one hundred and twentieth decimal place okay so and that's really that's that's what we actually do yeah so so for obvious reasons this is called fine tuning it's like seeing a pencil standing on its so an analogy I use very often it's like you walk into a room and you see a pencil standing on its tip to within you know oh point zero zero zero zero zero 120 zeroes one degrees of vertical it's possible nothing that's that's that's a consistent configuration of pencil right but if you saw it you would think something is probably up right you would maybe look to see if it's if the pencil is hanging from a screen on the ceiling or maybe you'll look really carefully you see there's a little hand holding it up or you you look for something that would explain that funny state of affairs even though it's not inconsistent okay so that's the state of affairs with our understanding of why the universe is big the only understanding we can give for the largest of the universe involves making a very fine very finely adjusted choice of of the parameters of our theory and similarly with with the the closely related question and why we're not all black holes why all the masses of all the elementary particles that were made of why why there why they're small enough so the force of gravity is weak enough so that we're not all collapsed into into into black holes that also involves a fine adjustment in our in the parameters of our theory to a mere 30 decimal places but that's why we suspect we're missing something big and that's also why I said it's not something mechanical about how the universe works because once we take these basic facts as given we can accommodate it and we can accommodate it quite spectacularly we can predict the existence of a particle that that that we theorized about 50 years ago and then finally discover and experiments half-century later so at some level we know what we're doing but but but the questions that are left now the new questions are the more structural ones about why we got those that kind of universe to begin with and that seems deeply deeply mysterious and I was just wondering if you could expand on what you're saying about space and time and that non-existing did you hear that oh yes so I was just making any jokes about not having the time to do it so yeah so the all the difficulties with space-time I have to do with the existence of both quantum mechanics and gravity and they're there there are various aspects of this their various aspects of this problem I'll start with the width I'll start with the most vivid one although it's not the most important one from from some points of view the most vivid one is the following so let's say we just we just talked a moment ago about how if you want to probe smaller and smaller regions in space and time you need higher and higher energies like like like we do these experiments at the LHC and in a world without gravity there's no limits to the tiny distances you can probe in this way just have to find you know larger and larger multinational governments the larger accelerators higher and higher energies to probe shorter and shorter distances but because of gravity something new happens eventually which is that you end up putting so much energy into such a tiny region of space that you collapse the region of space you're trying to look at and study into a black hole this is this is a feature that that many of you probably know about if you put enough mass in a small enough region then then even light can't even light can't escape from it now and and that's time is your attempt to understand what's going on in that in that tiny region of space and time now if you take what we know about gravity you can make it you can make a prediction for when that would first happen and the distances you have to probe where that happens are around 10 to the minus 33 centimeters that's a famous length scale known as the pluck length and to calibrate that length scale is you know around 16 orders of magnitude smaller than what we're probing at the LHC so it's ridiculously ridiculously ridiculously tiny length scale which is and which is related to the fact that gravity is incredibly weak force compared to all the other forces we know of in nature if you take you know if you take two electrons they ever they gravitationally attract each other and as like charged particles they also repel each other but the force of electric repulsion is you know one followed with 42 zeroes times stronger than moved on the gravitational attraction so the incredible weakness of gravity compared to all the other forces is related to the fact that you've got to go to these really miniscule distances before the act of probing space and time collapse in the region into a black hole by the way what if you get frustrated by the fact that you made this little mini black hole and decided to build an even bigger accelerator to use even higher energies what happens you make an even bigger black hole and so there's just nothing you can do to probe distances and times that start getting to this plunk ian's territory now you could take the attitude that that space sometimes still exists but you just can't measure them right but that attitude is never paid off in physics every time there's some concept that you can't even in principle nevermind practical experiments but you can't even in principle imagine sharply probing it means that those ideas are approximate and they somehow have to emerge for more primitive principles but in this case it's startling because the thing which has to emerge from more primitive principles are space and time itself so that's the that's that's the that's the simplest origin of this difficulty this general subject is the subject of quantum gravity and it's sometimes described I just want to say this quickly sometimes the difficulties with putting quantum mechanics and gravity described in a way that the grand - sort of roughly alluded to a second ago which is that we have a great theory for the very small just quantum mechanics we have a wonderful theory for a very large which is gravity and we don't know we don't know how to combine the two and and that's not quite accurate in fact if we want to describe the effects of quantum mechanical effects of gravity at long distances we don't how to do it we know how to calculate them if they're minuscule effects nobody cares about but if you wanted to know what is the what's the first tiny quantum mechanical correction to Newton's laws we know what they are ahead of time I can't even show you a formula it's got strange PI's and factors you can be sure someone has done some work to get it okay it's possible the difficulties are not that we can't have the world of the large and the world of the small at the same time the real difficulties we don't know what's going on with space-time at very short distances and and and that and that the idea of space-time has got to break down as we approach these these tiny clunky and distances and end times one thing just a note of that yeah if I may say you're pretty well certain space and time I've got to go yeah I think almost all of us are certain that space and time I've gotta go okay another question we could take the microphone here please sir so what's the LHC going to be used for in the future well so of these two questions the LHC is going to weigh in in a really significant way on the second one why there's a big universe and the question of why there is in the universe is very close it's very intimately related to the Higgs particle so and there there are actually two aspects to the problem of why there's a big universe first why is the universe itself big and secondly why does it have these big things in it like us and the earth and things like that and why aren't we all crushed to really tiny minuscule black holes and more specifically the LHC is gonna have something to say that second question okay why why why the universe why this big universe has big things in it that's related as we as we said to the question of why the elementary particle masses are what they are and aren't much much much heavier than than what they are and that's ultimately through a slightly indirect set of arguments related to why the Higgs has the mass that it has and the difficulty is that these wild quantum mechanical fluctuations in the vacuum that we talked about while there's a very good reason why they don't give an enormous mass to particles like the photon the particle of light or particles like electrons and the quarks shouldn't be very good reason why these enormous fluctuations don't badly affect and those guys we have no good understanding for why they don't badly affect and make enormous ly more massive the Higgs particle and if they made the IG's particle enormous ly more massive the entire consistency of the theoretical structure would require us to move everything up with it okay so it's really the fact that we discovered the Higgs on the one hand it was I made it sound inevitable yeah but on the other hand it's utterly shocking but something like the Higgs is associated with this physics elsewhere in nature similar phenomenon I mean not in exactly the same setting but sort of analogous phenomenon to the ones that that that that that that we probe at the LHC have been seen but no where have they been associated with something like the Higgs particles okay no where they've always been associated the much more complicated physics and not something as astonishing simple as the single particle which on the other hand we have no understanding for why it's masses where it is then not vastly and not vastly heavier so so there's two possibilities if I go back to my analogy with the pencil standing on its tip the two possibilities were the ones we talked about either the pencil just is standing on its tip it should be very strange but it's possible or you find some mechanism that that makes a seemingly unstable situation secretly stable okay like it's hanging from a string or you like you see a little hand holding it up and actually the best analogy is like seeing a little hand holding it up because it would say if you look at sufficiently small distances you'll see something new which removes these big quantum mechanical fluctuations so most theoretical physicists for the last twenty thirty years have believed that it won't just look like it's finely tuned and that we'll discover some mechanism and this isn't just you know it's not just an aesthetic hope a number of times in the last hundred years two or three times in the last hundred years we've been confronted with a similar situation where there was an apparent fine-tuning and in every case it turned out there was an explanation there was the analog of a hand holding it give us something some one example well there's one one example it's act is very very analogous to the situation with the Higgs around 110 years ago there was a there was a crisis in theoretical physics involving the electron okay so the electron is a charged particle you probably all remember from high school that it has you imagine that it's surrounded by an electric field on the electric field dies off as you go to larger and larger distances but you also you also may know that there are some energy in that electric field and you can you can compute how much energy is in the electric field surrounding the electron and if you imagine the electrons the point-like particle you it's a point you discover to your dismay that there's an infinite amount of energy stored in the electron okay and that that should really bother you because then how the heck is it moving around if it's dragging this infinite amount of energy so something you could say so again something you could say is well there is that energy in the electric field and there's some other energy and they cancel against each other I don't know where the other one comes from but they just cancel against each other to give me the ultimate value of the of the the mass or the mass times C squared of the electron and that's possible that would be like seeing the pencil standing on its tip and the on the table but what many theoretical physicists in the early part of the 1900s imagined is that no it can't be like that and there's got to be that there's got to be something has got to change the the logic of this argument now you can actually make an estimate for where something new one's got to happen you say where do we get this infinity from we got this infinite energy by imagining the electron as a point so what's the most imaginative thing you could do is say it's a little shell right it's not a point so if it's a little shell then then the energy won't be infinite but you can ask how big would the shell have to be in order just as an estimate in order for this energy electric field not to be vastly too big and people did this estimate and it's a size it's around ten to the minus thirteen centimeters okay and they tried to make realistic theories like this and they failed badly and today we know that's not the answer something new does happen but is much more radical than they anticipated in fact it had to do with the discovery of quantum mechanics and particles and antiparticles in this whole picture we talked about about the roiling cloud of particles and antiparticles that surround the electron which already at a much larger distance scale of 10 to the minus 11 centimeters remove this sort of classical point like picture of the electron so but it's interesting that while their logic was wrong at sorry well their models were wrong their logic was perfectly correct something new did happen in fact it happened a hundred times earlier than it had to happen and and changed the the basic calculus of this discussion so so many theoretical physicists assume that the same thing is going to happen with the Higgs and that has two exciting consequences one is that there has to be something else the Higgs can't be alone the Higgs has to come along with some other particles okay and to those other particles can't be arbitrarily heavier than the Higgs they have to be sort of right around the corner right around where the Higgs is and and that that that basic logic as guided much of the thinking in our field for the last 20 years so and and three I should say whatever this new physics is can't be some random garden-variety junk it has a job to do it has to do something which sounds like a tall order it has to remove these violent quantum mechanical fluctuations in the vacuum that are ubiquitous part of quantum mechanics so if it's if it's gonna manage to accomplish that it has to have some it has some really new structures some idea in it it's not just some some it's not just something random so so that's what the LHC is looking for it's looking for are there are new particles beyond the Higgs could those particles be associated with being the little hand that holds up the pencil explaining why the mass of the Higgs is what it is and if there aren't if we don't see these new particles and and in a sense what we've seen at the LHC so far namely we discover the Higgs but none of these new particles yet while it hasn't ruled this idea out it's putting the idea that we will see particles like this under some more pressure than then then you would have naively expected although from indirect reasons many of us are not particularly surprised about what's been going on so far because these new particles could have already put an appearance earlier in a variety of other places that we haven't seen but but it doesn't mean that that they won't show up so either we'll see particles like this and we'll have a good explanation for why the Higgs is where it is or we won't and and if we don't it's an even bigger paradigm shift if we don't it's an even bigger sort of shock to the system because we would have really proved in a sense for the first time really striking very hard to ignore evidence that there was some fine tuning in in in the parameters that go into describing the laws of nature we haven't there are other situations where we suspect we may be seeing things like that but we're not completely sure if we see it for the Higgs it would be much concrete too much sharper evidence that something like that is going on so right a very interesting bifurcation we're a very interesting fork in the road whatever the LHC sees if it sees new particles spectacular if it doesn't see new particles in a sense even more spectacular because it's it's an even bigger shock to the theoretical system and we'll have to digest it and try to understand what it means well how about that how about that for an answer whatever that happens it's going to be fantastic alright ok another another question question at microphone over here please I just want to set how your job works so ah so feels like you described there's a lot of perspiration to what you describe what here's an experiment I have is it a source of inspiration he doesn't do experiments at all sorry I did I did but but no one paid up no no so so it's it's a very it's an it's an excellent question so first there's experimentalists and theorists and and again in most parts of science there isn't such a clear separation between experiment and theory it's it's it's the consequence of how mature I mean we've been at this business depending on how you count for 2000 years or okay that's giving the Greeks too much credit but we really started with ballet on Kepler and Newton for 400 some odd years and so it's by far the most mature of the of the sciences and so it's developed to a point where it makes sense to experiment people who specialize in experiment people specialize in theory and our lives are very different the experimentalist that I mean that they have inspiration to I mean it's not just just someone who's got to figure out and build these humongous things and make them work and and they're absolutely I could I could never do in a million years and and there are absolutely extraordinary people who managed to are doing but it's a very different set of skills it's a very different set of the people that built this very interesting so so and it's definitely not just perspiration there's some brilliant ideas needed to make every little component of that detector work so but but what theorists do it is is rather different and you might get the impression that well so we have these problems and we have to invent theories right we have to try to invent something that goes beyond what what we know in order to attack them and you might get the impression that since the experiments take 50 years to come along and confirm or deny a theory that the job of a theorist is really easy right we're just sit in your office you put your feet up on your desk and you and to theory here's one this is what's going on ah they're not gonna you know verify it for 30 or 40 or 50 years anyway so I'll just have fun while they and there's another one here's another one right you just dream up as I sometimes like to say you know fairies and leprechauns and nymphs and dryads around every corner and then experiment comes along every 50 years Judgment Day and kills 99.99% of ideas but until that time there's a proliferation of crap and you might have that that's that impression um a would that it were so okay but in fact again and this is closely related to why we separated an experiment in theory to begin with things don't work that way and the reason is exactly what I alluded to in the earlier part of the discussion what we already we don't know the answer to all the questions back we have very profound mysteries but what we already know about the way the world works is so constraining that it's almost impossible since we have to change something right in order to we have to do something new it's almost impossible to have a new idea which doesn't destroy everything that came before it so even without a single new experiment just agreement with all the old experiments is enough to kill almost every idea that you might have okay so for instance this this question about the kind of new particles that might come along that explain why the mass of the Higgs is what it is right you might think today I get up at once you go to grad school maybe you get up and you say oh this is a model here there something that can do it or you give them silly names and and you just keep making them up in fact in the 30 years that people worked on my problem purely theoretically as a theoretical problem just try to find it an idea that actually is the little hand that holds up the pencil they found two classes of ideas that work it's almost impossible to solve these problems you can't roll out of bed in the morning and just solve them precisely because we know so much already that anything you do is bound to screw everything up so if you manage to find one idea that's not obviously wrong it's a big accomplishment now that's not to say that it's right but not obviously being wrong is already a huge accomplishment in this in this field and so that's the job of the theoretical physicist is to try to come up with ideas that are not obviously wrong and and then hopefully you come up with and it serves two roles one of them is that one of them might end up being right the other one is that it directs experimentalist that where to look and the kinds of experiments to do and you know these are enormous things that enormous machines that are made enormous detectors enormous resources go into them we have to make sure that nothing is missed right and it will be one thing if it was just a barrage of crazy possibilities but because of what I told you it's not like that and so anyway that's that's that's that's why we have a job and isn't trivial okay just to go back to your previous comments about the kind of meaningless questions that we can ask in the English language in any other language about matter I'm sure that it's quite a big thing to say in a way because most people in a world probably disagree with you and say they are meaningful questions in some way is it not possible that perhaps they're meaningless in the current paradigm of our scientific models rather than utterly meaningless whether or not they could become meaningful if if and when and new theories come and can actually answer them good questions it's a car so um so I think yeah so if we unpack if we unpack what I said a little bit more so obviously they're meaningful in some sense they're meaningful in the English language when I say they're meaningless I mean they're meaningless statement they're meaningless statements and statements about the universe right so so let's say we take the statement I said that what is the position and velocity of that electron right I think perhaps it's perhaps it's possible in some far-off future theory that that might be a meaningful question but it will only be a meaningful question at what we mean by position and velocity and electron change okay if what we mean by those things are what we mean by them now it is a meaningless question and I think we can know that it is a meaningless question given as a meaningless meaning the statements mean it's a question about about the universe so this is something again that that that that that the that the often the concepts that end up being relevant that we end up needing to understand things more deeply our so foreign to to the ideas we have now that we can't even articulate the correct question before we happen to be in the neighborhood of the right answer okay and that's that's that's a it's a fascinating thing about the way this part of science works that's that's completely not like the sort of cookie cutter description that someone makes a hypothesis and then an experimentalist goes and it checks whether the hypothesis is true yes you're right no you're not right they go back to make another hypothesis oh it's this ridiculous picture of how science works is not remotely close to the truth and in fact especially in this business especially involving involving deep conceptual questions as I said we don't even know for asking the right question until we happen to be in the vicinity of the right answer and the car becomes carpet it comes before the horse a lot and very often we have the correct equations for a number of years before we know what they mean and so so nothing works in this straightforward nothing works in this straightforward way but but that means that we always have to be wary about about what precisely the words are supposed to me and now today we have a we we have a precise thing that we mean by electron even what we mean today by the electron is not quite what was meant 200 years ago by the electron because they didn't know about quantum mechanics then and we know something about quantum mechanics now so it won't be very useful to you but but what I really mean when I say the electron right now is an irreducible representation of the punker a group with spin 1/2 okay that's not something that JT I mean doesn't mean anything to you maybe maybe it does but that's not the way JJ Thompson who discovered the electron would would describe it so even the same word which for good reasons of course they're very good reasons to keep calling up the electron because it's the it's the quantum version of that thing that he discovered which was what the look the which is associated with the language I just used to describe it but we have to continually update what our words mean as we learn more about nature so I'll come back and say what I said therefore it's conceivable now I think it's incredibly unlikely and there's lots of reasons to believe that that we're never going to return to these deterministic pictures of the world and we'll never that that there is no more primitive theory underlying quantum mechanics in that at least in that sort of simple-minded way but if there are some miraculous loophole then then that sentence will only make sense if every word in it means something completely different than what it means now it's a deep question one one last question but if it could be a short question and anemic answer it briefly yes please there's it passed over here thank you all right thanks I wanted to ask you about that space-time theory being out the window why is it a problem and why is discovering that which has gone out the window sorry the space-time at least I'm yeah what's its space times over it's dead background that right right well it's not a theory I mean that what what yeah that's it's it's a problem so so well the idea of space-time is one of the two pillars of our current understanding of physics so there was relativity and quantum mechanics relativity really means that there's their space-time and as I as I waxed on about at length you take these two basic ideas of space diamond quantum mechanics and the rest of the universe largely follows so that's that's that's wonderful to have such a deep principle which has so many profound consequences which makes it more painful to realize that that idea can't somehow is itself an approximation to a something else but it's not a theory we have a specific you know it's a mystery it's a it's it's it's it's a difficulty we can do thought experiments where we try to measure distances and times to very small distances like we just did and we discovered that it doesn't make sense and we don't know what doesn't make sense yet so we're always in that situation in our business that there are things we understand very well and there are things that we don't understand and we have to obviously we have to go we have to go to the funny intersection between them and try to clean things up and so these kind of arguments that I made by the way one of the wonderful things about one of one about one of the wonderful things about fundamental physics today is that the way I describe the difficulties with the space-time to you is not far from the way I describe it to a graduate student and and also via difficulty with why there is a large universe it's not far from there I mean it's not they're very closely related to our actual official technical understanding of what the problems are the basic difficulties are big they're simple to state and they're big and that that's that's been the characteristic of the profound questions that have driven the development of physics for hundreds of years the basic issues are simple and big and easy to state the possible answers are not so easy to state often but but the questions are simple to to a motivate and so but anyway the reason we care is because space-time has served us shockingly well and it's going to be painful to lose it but it's also a big clue that we have to lose it somehow and and and it's also good in many other ways you see there are things we don't we don't understand what happens as we take our expanding universe and of all that back in time and eventually run into these singularities at the Big Bang these sort of deepest questions about origins about why we got the universe we got and so on it's somehow it's at least good that were confused about the idea of time itself because it gives an opportunity that perhaps understanding that the general question might shed some light on these other questions that we also care about at the same time

Channel: Science Museum

Views: 88,397

Rating: 4.8866081 out of 5

Keywords: Physics (Field Of Study), Nima Arkani-Hamed (Academic), Theoretical Physics (Field Of Study), Large Hadron Collider

Id: pup3s86oJXU

Length: 54min 39sec (3279 seconds)

Published: Mon Jun 30 2014