Brian Greene and Cumrun Vafa: World Science U Q+A Session

all right so back to science and the and the topic for today so i'm going to begin with a conversation with cameron vapha and then i'll segue into your questions we'll take your questions during that session as well cameron vaffa is the hollis professor of mathematics and natural philosophy at harvard university he has won so many awards that it would take half of our time to list them all the breakthrough prize is certainly one of those he's well known for tremendous ideas that have set the course of many areas of research into string theory again we'll talk about some of them but perhaps those of you who've read my books are familiar with the work that he did with andrew strominger giving us the first microphysical calculation of the entropy of a certain class of black holes which is rightly regarded as one of the real crowning achievements of string theory so let me bring in now my friend professor kumar baffa so welcome cameron nice to see you ryan good to see you thanks for for joining us i know you're from home at the moment i think yeah and um and and just uh just to sort of get into things so i i your professor at harvard and harvard just recently announced as many places are that it's going fully online with its education have you done online perhaps in the spring did you do online teaching of your students sabbatical leave on spring prevented me from teaching so i'm having the experience that i have given lectures by zoom but not the class it was it would be an interesting experience beginning this fall yeah you know most people in fact this even came up at a meeting at harvard because you know that i'm on on one of the overseers committees there that there's a general sense that teaching via zoom or online is always a pale reflection a poor approximation to the experience in the classroom but i was sort of the lone voice in that meeting and i said i kind of feel like the teaching can be just as good it's different than engaging one-on-one but you know in in in small ways and large i'll give you one small thing i don't know about you but i never can remember all my students names i really wish i could but i can't but in zoom all their names are like right there so i can like call on jim or call on mary and it actually adds a personal touch that like i can't do in the live class because i can't i can't actually remember that's sort of a minor thing account yeah yeah exactly uh but uh but yeah i think it will be a i'd be interested to talk to you after you know some of your sessions to see you know how you're thinking about things i also want to um just announce to folks that you've got a new book out well that's the wrong side of it there we go over here beautiful new book puzzles to unravel the universe and i was fortunate to receive uh a nice advanced copy from you and uh enjoyed reading it tremendously so just to sort of get into things what what you know it's a question that many authors are asked every book interview that i've had often they ask you know what is it that drove you to write this book so let me just start in in that way at all what was your motivation well um part of the motivation was to try to communicate with the public about the things that excites me about science and i thought that maybe maybe there's a room for hands-on kind of relation between the public and the science and i thought puzzles is a way that that could uh it's a good vehicle to try to bring this connection a bit more hands-on experience instead of just reading they will get to do something and feel what is going on really and in fact it's based on a course that i've been teaching actually originally suggested by my wife offering to originate this course to to kind of communicate with uh with freshmen uh at harvard uh this quote called this course called physics puzzles and math and based on this special seminar which has been going on for about 10 years now i gradually develop more and more puzzles and tools to kind of communicate with with people who are not necessarily going to be physicists but they're interested in learning about how the physics goes about and how what are the basic ideas and i thought puzzle is a fun way to get them into into seeing what are these deep ideas and you know the puzzles don't have to be complicated to kind of illustrate some of the points and uh the nice thing is that even simple puzzles can relate to you know the latest things we are doing in physics modern physics you know what are the uh latest and intermost interesting aspects of the fundamental physics can be connected to simple looking math puzzles and this was both a fun way as well as educational way to bring this about and that was uh through this experience from this course i decided that it would be great to have this in the form of a book that others can also uh benefit even those who are not possibly the students in my class were you uh were you a a puzzle-solving kid i mean is that something that fired you up when you were young yes i loved puzzles i always loved puzzles and i always tried to you know it was always fun thinking about them but i also had this view about things around me which later got me interested in science which also was a way like viewing the whole thing as a big puzzle and uh you know to me to me from the childhood kind of questions like you know why is this thing moon up there and it's not falling and why is this and that in this way and that way so these kind of things that are around us even as childhood kind of got me started thinking and and then puzzles was kind of a more simpler and more uh well-structured way of asking a simple question and it was much simpler than the complexity of the reality around us but it was a way to kind of get me started in thinking about things that i found amazing about our universe and this and that later on but uh well it started from my interests as shown in each as child now sometimes when people hear the word puzzle they think in terms of you know it's fun it's it's it's a game sometimes when we think about the questions of the universe we use very different language that has a more sort of gravitas more profound quality to it do you find with your students that there's any tension between those different sort of levels of profundity you know the gamification versus sort of the profound investigations into the nature of the world or do they seamlessly come together they actually seamlessly come together i would say because even even i would even as physicists even when we do research and you well know this of course brian that when we see something is is gets gets understood it is always in simple terms if something is complicated we don't feel we haven't understood it so it should eventually be the profound ideas should be so simple that quote unquote the kids should also be able to understand otherwise we don't think we have understood it so this simplification or understanding in simple terms is a requirement requirement really to say we really understood something and it's not like if you have something which is profound it should look complex in fact it's the opposite if it is complex means you really haven't quite understood it you have to bring it down to spare bone to simple terms and only then can you really feel you have understood it so most puzzles that we get set by you know in school or or even in your book they're made up by a human being when we are investigating the sort of the puzzles of nature do you feel that that is ultimately a holy human quest that we kind of look out at the world and we puzzle about things and we frame our questions perhaps in a mathematical language but ultimately as you say it's just a puzzle or is there com some kind of deeper source for these puzzles it's a good question i have thought quite a bit about the relation of puzzles magic and the universe and how this works to me universe has this magical quality and like puzzles which are sometimes magical and we try using scientific methods to try to explain this magic and there's a difference between the the the regular magics and the universe as a magic scientific explanation of it does not make it less magical in fact it adds to its spell so it's kind of a little different from regular puzzles so it's not something like we made it up it's kind of uh it's kind of there and we we try to understand it we don't know with our own terms it has a different quality of a puzzle it's a different quality of magic to it than than regular things that are that we can make of course we get inspired by things around us and we make we try to parallel what we see around us in the universe but i would say that it's not like something we have imposed uh from us to the universe but rather we are just we're just mirroring what we see in some form in some obviously imperfect way in our way to try to model it in some form right we thought it would be fun since it's a book on puzzles uh you know in a moment we'll turn to questions of string theory and space and time but we thought it'd be kind of fun during that conversation if there's some folks out there that want to puzzle over a puzzle and see if they can come to the solution so if you compose one of your puzzles and our thought was for those of you who are out there that are interested to try to solve it the solution should take the form of a picture that shows the solution to the puzzle and in the chat world science fest will post i believe it's an email address or some other place you can see it where you can post your solution and among those who get the right answer we'll do like a little random draw and you'll get a a signed copy of kermit vapha's new book as a prize that the festival will provide to you so give us one puzzle that that you think fits into this category of interesting things for people to ponder sure um well one puzzle is the following take consider four cities i gave them name a b and c and d and the four corners of a square so they are equally this i mean the neighbor ones are equally distant a and b and a and c are the same distance and they form corners of a square and the task is to find the highway system which connects all these cities now the highway system that you find should have the shortest total length it should be able to connect every city to every other city it does not have to be a direct connection between every city and every other city as long as you can go from any city to any other city that is fine and we just want to know what is the shortest highway system that connects all these cities of course shortest highway system is useful because you want to have you want to spend least amount of money if you want to connect old cities and so that's the puzzle and what it has to do with physics and so forth perhaps we can explain afterwards so the form of the solution will be someone's got to write down draw a highway system connecting the four cities take a picture of it send it in and really specify the details of the directions that those various roads are taking and exactly exactly no no no need for proof of it just yeah just the explain just the drawing and then the specification on the drawing sufficient all right so if anyone's still paying attention to our conversation as opposed to trying to solve the puzzle uh we're gonna we're gonna move on from there we'll come back to the solution uh toward the end of discussion so so your line of research you know in mine as well for a long time has been in the area of unified theories string theory of course as many people who are listening to us well know einstein himself was in search of a unified theory toward his final days on planet earth and and never really found it for you what's the motivation what's what's the drive to find this this sort of final theory or this this unified theory well i uh i think that i enjoy it without understanding why i enjoy it and uh i don't think it's an aim to find the final theory because i don't think i would i will for sure not be able to succeed i'm sure about that i don't think uh it would be something achievable in our lifetimes so it's not it's not a it's not a precise goal as as a final quest or something for me i think it's a kind of a journey more than anything and uh i feel that the thing that amazes me is that discovering something beyond us this discovering and connecting something bigger than ourselves discovering that there's something out there which is resonates in us and we can kind of understand pieces of it and make sense out of it which is longer than our short lifetimes and what and much much bigger in space and validity than the tiny place like earth that we live in it's it's applicable to the whole universe understanding these laws allows us to feel a little more connected to the meaning of the whole existence in a broader term that is really what i want that's what gives me satisfaction yeah when i try to connect things and try to see the connections it's like seeing a piece of art or enjoying it and just even if even if i'm not doing it and if somebody else is doing it and just watching it it's already inspiring and and it makes me really uh enjoy it and so that is also another aspect so it's just beauty of it beauty of appreciation of what's going on is another aspect so of course being part of it and doing minor contributions here and there makes it even more fun sure so you know i've asked and answered the following question like 10 000 times over the course of uh the last decade or so so i'm happy to seed it over to you can you give us a just for those who perhaps don't know what's the basic idea of string theory i've always wanted to ask that question i've always answered it i've never asked it before you are the expert on it beautifully i cannot do any any better than you for sure brian but uh i don't know what is the easy way to say what string theory is uh i would say that uh well i was i would start perhaps by by describing a little bit uh about what what what is the basic questions before i get to what string theory is of course uh you know from old days we know uh we know gravity uh from newton's day he understood the quantitative understanding of how gravity works and later we learned about uh you know particles uh fundamental particles things like electrons and uh when when when we understood electrons and how the atomic structure works we found quantum mechanics is crucial this was last century beginning of that century and so when people tried to put together newton's theory which was beautifully extended by geometrical idea of einstein when he wanted to combine that piece of physics with quantum mechanics which described the tiny pieces of atoms it didn't work and it came accidentally it was actually discovered by physicists trying to do something completely unrelated that if you start talking about the vibrating tiny objects instead of point particles instead of points like tiny littles like spaghetti loops but very very tiny then you somehow manage to get rid of these contradictions that einstein theory had with quantum mechanics which describe how electrons work so this was the emergence of the idea of string theory basically as a way to resolve the paradoxes between einstein's theory and quantum mechanics so the the einstein theory which is great in describing how things work at large distances and quantum mechanics which was great with short distances got unified using this this extra ingredient that fundamental objects are not points not point particles but extended little loops and these vibrations of these loops give rise to different kinds of particles and different kinds of interactions emerge from from strings let me just jump in right there though i want to ask you a question on that freeman dyson in an article that i read in the new york review of books but i think he's had said it in different locations as well he questioned whether we should ever even worry about the contradictions between general relativity and quantum mechanics he said there's there's there's no reason to worry about that they're just describing distinct realms of reality and the contradictions are arising because you don't want those realms to be described in the same language does that hold any water for you well i'm i'm not sure exactly what what you might refer to because we already know that there are many pieces of science which emerge from contradictions in some sense we welcome contradictions because contradictions point towards something we don't understand and potential resolution of that leads to new ideas however there is one sense in which i can i can perhaps understand what he may have meant and that is in the context of what we call duality and string theory so occasionally we come up with two completely different pictures which look contradictory contradictionary contradicting each other in some way but this contradiction gets resolved by viewing it in a very novel way so it's kind of like a different viewpoint so you have two different things which look like there's some inconsistency but there is not of course trying to figure out why it's not is the key it is the key to understanding what is going on so the apparent contradictions are there but not a real contradiction and i think maybe that's what he's referring to so we shouldn't be bothered by apparent contradictions but we should perhaps welcome them as a opportunity to get to understanding deeper reality behind why they are the same or why there are no conditions at all yeah no certainly i mean you think about special relativity emerging the contradictions between electromagnetic theory and newtonian mechanics you can think about quantum mechanics emerging as a contradiction between observational data of certain particular systems and the classical description of those systems so i i totally agree that contradictions have been the most fruitful things that we physicists have encountered i think what freeman dyson was referring to he he didn't believe that gravitons exist he didn't think that gravity would ever need to be quantized um i i'm not exactly sure how he justified that thinking because i mean we'll talk about in a minute but when you think about things like black holes or the big bang it seems quite clear that to gain the full understanding you need to put general relativity together with quantum mechanics because it's extreme realms of gravity and tiny realms of space and it's very hard to imagine a description that wouldn't unite them but i guess he was thinking that we would one day just have distinct descriptions that would never require gravity and quantum mechanics to talk to each other again it's hard for me to understand where he justified that thinking and uh i gather it's it's hard for you to see it as well yes i think i i think that the quest we have in science is unification of ideas to us having different realms which have nothing to do with each other and describe reality is not is not it's not a happy ending for understanding what is the ultimate goal of what is the how do the things relate it's kind of giving up yeah i don't think i will i'll subscribe with that to that viewpoint but uh but of course we have understood much more i think especially in recent times but connections usually you mentioned black holes uh and for example fundamental things that we mentioned electrons you might think what does an electron fundamental anti this tiny thing have to do this massive singularity and space we should have a picture of like a little black hole how could they have anything to do with each other and now we have learned via string theory that they are related and the simplest relation to explain is that we have learned the following fact is from string theory that gravity is always no matter what kind of imagine universe you might imagine is always the weakest force so for example the electron has attract has a repo if you have two electrons they have repulsion electric repulsion force which is much stronger than their gravitational attraction by a factor of 10 to the power of 40 or something so it's a huge difference between these forces gravity is always the weakest force the electrons really strongly repel compared to the tiny attractive force they feel and the question is why and we learned that this has to do with the properties of black holes so understanding black hole that thing which has nothing to do a priori with this tiny little electron gets to explain the fact that they're always gravity is always the weakest force so these kind of connections come from quantum aspects of gravity and gravitons existing and all that which are crucial in hanging the whole thing together so these elementary excitations like electrons or gravitons and these big excitations like big black hole are actually intimately deeply related and we have seen exactly many examples of that by now in strength yeah in a number of uh of the chat comments i'm just quickly looking at them by a number of the listeners like uh raju radi and one listener whose name indicates the question at hand their name is loop quantum gravity so a natural question is so string theory is an approach to putting general activity in quantum mechanics together there other approaches that have been put forward what do you think about loop quantum gravity i think no approach is on par with with the amount we understand in string theory so i think it's fair to say that the the the other approaches are attempts and many of them uh there are not too many attempts but none of them are as has developed as in context of string theory and perhaps there's some connections and we may discover between some some pieces of these ideas within string theory but i would say that they are not on par with strength here right yeah i uh i hold a similar view but i think we're both perhaps biased the other the other community might might have said the same thing in reverse but uh let them have their own discussion if they want to uh exactly exactly so you know one of when i was um a graduate student back in the 80s which is more or less the time when you were when did you get your your your degree 85 19 i got my 86 yeah so around the same time you know there was a general sense that we both can recall that some people were saying okay the end of physics is near right we have this new theory it's just going to unify gravity and quantum mechanics together give us the final description of everything and at that time the older physicists i think knew well that this was over exuberance that i remember i remember talking to ed whitton at the time uh and you know he was like this is extremely exciting but he was like you know slow it down a little bit in in your expectation for where this is going to go now we are say 35 years later what's your view in terms of how far string theory has gotten i mean if you were to grade string theory what what grade would you give it well string theory is like opening a room which is totally dark and beginning to shine light on pieces of it and you don't know how deep or how complicated what shapes you're gonna find so i can easily see that with 35 years ago or so we we might have thought you know the room is tiny you know one flashlight the whole thing is done we get the all aspects ready and understood and ready to go but of course if you have a you don't know if it's a tiny room or it might be actually a big tunnel or it could be completely complicated so so i think it's hard to give it a grade because uh because it's uh we don't still know what is the totality of this this thing that we are calling string theory we have it's like really walking and you know shedding light on this piece and that piece and saying oh no no this is not just a you know a little room it's actually a big cave or this big this and that's so we're beginning to get pieces of it understood and so it's going to take a while for us to really delve into all aspects of string theory what i would say is that we have discovered so many little corners in that room that are so exciting each little thing each little thing is like a gem and we have you know we said wow look at this little thing here isn't it cool let's try to understand what it means about the rest of physics and we got we get absorbed this for a few years i said oh no no no there's this other gem over there and so forth so it's like a kid walking around in this little that is a candy store and discovering this and that and so forth and putting all together and you might ask okay so are you done with this candy store no no we're still continuing our journey in this in this uh in this way and what grade i can give i would say we have found so many exciting stuff in string theory that it's it's it's you could give a grade of a plus but if you ask have you been able to connect to any single experimental result that you can actually explain using your theory the grade is not not a for sure we have not been able to do that so therefore depends on what is the exact aim and we can we can only be modest in that we can do continue this journey understand pieces of it we have connected many different aspects of physics together we have understood how quantum theory and uh gravity can be consistent we have seen how gravity is could be related to a manifestation of forces like electromagnetic forces or generalizations of it we have seen how dualities work something which is looks one way could be completely related to completely different things so these kind of different aspects have shed huge light about what we understand we have learned so much more about black holes we have learned many many things and of course the longer this journey is perhaps the more we're going to learn more new exciting things so therefore maybe we shouldn't aim to have a short journey so longer journey may not be so bad but of course it would be nice to get a few experiments on the way as we walk on this journey well some some string theorists have said i wonder what your view is they point to the developments that began in the late 1990s that showed a very deep and unexpected link between string theory and the more traditional way that physics has been able to describe elementary particles namely quantum field theory and the deep connection between string theory and quantum field through some string theorists that i've spoken to take the position that the experimental program of testing string theory needs to be seen in a different light they would say hey once you find this deep connection between strength here and quantum field three well quantum field theory you already know that certain quantum field theories are spectacularly good at describing real world physics and once you have the link between string theory and quantum field theory then in some sense the experimental program isn't as crucial as you perhaps once thought it was what's your view of that i would not i would not agree with that first of all we would need to have to just to say that some kind of gravity theory is consistent with our understanding of string theory and therefore let's let's not worry about the details of our universe that's basically not giving up on interesting things that could be related to the universe we live in or universes that could be possibly uh consistent and perhaps even reachable from our universe so i would say that uh saying that something is consistent less than not dealt any further is basically again giving up so i would say that to say that gravity might be related to other forces is of course great strength unifies them but we would like to know more we would like to understand what aspects of our universe is is related to the consistency of gravity with these other forces can we make further predictions are there other kind of particles that they should be there can we learn anything about dark matter can we learn anything about dark energy from from the string theory perspective we know that there's this uh non-vanishing energy permitting the whole space what does it have to do with einstein's theory of gravity what does it have to do with the fundamental forces are there any relations between them can we make a prediction about the fate of the universe all these exciting questions that we need to understand and i think they're extremely exciting to understand for our universe and understanding what predictions we have about the universe that we live in rather than some some hypothetical possible universe that we don't even know whether it's relevant for our universe so understanding more specific things about our universe is is something that we still lack in in our in our understanding of strength theory so do you consider string theory to say to be falsifiable string theory is falsifiable in many ways but to make it very falsifiable you have to have a very specific universe in mind so for example i would say that if you just would be an aspect of falsifiability suppose you have you have an accelerator which can produce energies which are 10 to the power of 20 let's say higher than the energy of a proton suppose you go to that high end energy which is if you can arrange such a huge amount of energy then uh if you don't see any evidence for extended objects of one form or another i would say that that falsifies the theory so so there is a prediction that you have to be able to see something coming up there of course that's very far from the realm of experience we can do today by many many orders of magnitude and energy our or our even the large hadron collider is is a very tiny energy compared to the energy we need to reach to falsify string theory so falsifiability of the string theory is at this technologically we are not at the point where we can actually do it now but here it but i mean at least theoretically we could see it falsified of course otherwise it won't be a piece of science sure you know when i look back on the papers that i've written in my career i'd say that i don't know 90 of them rely on something called super symmetry in one way or another and many of us who have the same perspective as you that in principle you can test strength if you have a big enough accelerator you can actually in principle try to detect the extended nature of a string and as you say if you don't see it then that would be pretty good evidence against it if you do see it that'd be pretty good evidence for it but of course we don't have those machines we have to deal with what we have and many of us thought that supersymmetry this quality of nature that for every known particle there'd be a partner particle many of us almost took it as a foregone conclusion that the large hadron collider would discover these super partner particles and that way confirm supersymmetry it didn't happen or at least it hasn't happened was that did you find that disappointing what's your reaction but i wouldn't say that we were predicting it necessarily i mean so let me let me say that so just like you i actually was hoping that we would see some evidence for supersymmetry in the large hadron collider but evidence was against it in the sense that the amount of things that we we saw from other experiments did not point towards being able to see supersymmetry in string theory we believe supersymmetry this this amazing symmetry between different kinds of particles bosons versus fermions should be true at some energy scale in string theory that energy scale has to be something that is uh is measurable in some sense but may not be at the low energy that we can probe in the large hadron collider it could be all the way up to the energies which we call the planck energy this huge energy that is the ultimate energy that we can reach so it could be that you need to get so high in energy to see the evidence for supersymmetry we were hoping you know who knows maybe we are lucky and you know that you could see it at low energy and then we'll be happy we didn't see it but i wouldn't say that that dismayed me in terms of believing in string theory let's say or to say that it had to be seen no we were just hoping that if we if it is there then maybe we can get more insight into the inner workings of string theory and how it connects to our universe so it's disappointment in that sense i wouldn't say it's a theoretical disappointment it's just like circumstantial disappointment of the universe we live in which does not have supersymmetry realize that social low energy of course it's not still over they could you know it's the large hadron collider experiment is still going on and you know they could have some evidences for that but it's getting i would say as you say less likely and and i wouldn't bet on it as as i would say it's less likely even than before the experiment but i would say that that does not change my view about string theory so turning away from say the experimental side of things which you know as we've indicated is less successful but not necessarily a black mark against strength they're in turning towards some of the great successes so your own work on on black holes an enormous success more recently there's also been a tremendous amount of activity trying to understand the true nature of space and time in a quantum mechanical theory of gravity within string theory have you been impressed by those developments any any thoughts on where you think the promising avenues of research are in trying to understand a question like that well there are aspects of trying to understand deeper questions about black holes how does it for example if you throw something to the black hole uh after black hole evaporates and goes goes away by hawking radiation what happens to the information of the things that you sent in that was one of the puzzles that we expect the answer to be a way to resolve it and get the information out but uh it it didn't it wasn't clear and it still is not to be honest i'm not clear how does the information come out so there are some simpler models in which how the information may come out can be understood in those cases so lower dimensional examples there are some toy models which do that so there's some progress in that i wouldn't say that this is a case where i would say that we have make a big breakthrough and really understanding the big issue of black hole but there are there are incremental steps in that direction i would say there are many other directions in string theory which are i would say still ongoing and it's actually one of them which i personally am interested relates to the super symmetry again which we already mentioned so in fact uh i would turn it around the fact that the fact that we didn't discover supersymmetry and the fact that our universe does not enjoy this amazing symmetry between botons and fermions the supersymmetry is telling us something about our universe which is deep and that deep fact is the following in string theory we have two type of uh universes that can exist the universes that can last forever and the universes that are temporary that will decay away and of course we would like to know which one of these two universes we live in it turns out all the universes that we know of in string theory that last forever enjoys supersymmetry an unbroken version of the supersymmetry should be realized in our universe at least in the low energy it's not realized which implies we do not live in the universe which will last forever therefore our universe is is going to be going away it's going to decay away so that's a fundamental lesson that lack of supersymmetry is teaching us our universe is not going to last forever so therefore the next question would be what is the fate of our universe when is it going to go away how is it going to go away so all these questions are exciting and so this is just leading us to the deeper question about the nature so i think these kinds of questions i find among the most amazing questions in string theory that can actually teach us something concrete about our universe that's what those are the kind of questions i find most exciting and i i would i'm working on those kind of questions myself i remember the uh the first time you told me about those results i don't know if you recall we just happened to run into each other in a cafe in cambridge massachusetts yes and it's funny because i was in the midst of finishing my own recent book until the end of time which has to do with possible well part of the book is possible endings of the universe so i remember you told me about that way of thinking about things and it certainly made it into a book as a as an endnote i wasn't able to work it into the main text itself but yes i mean thinking about the ultimate fate of the universe and to have any theory that can give you insight into questions of that sort is is of course inspiring again for those of you who are just joining us i'm speaking with cameron baffett this is his new book over here puzzles to unravel the universe those of you who are here early on note that kerman posed a puzzle early on in about 10 or 15 minutes he'll describe the solution to that puzzle but those of you who want to try to still solve it feel free to write down your solution for the most efficient highway network between four cities that are at all the corners of a square so what highway system would you use if you were charged with doing it for the least amount of money and your highway system the cost is proportional to the linear length of the highway itself so back to to questions of string ethereum and uh cosmology and unified theories but one one thing before we get to that one relates to puzzle yes i'm going to give you another puzzle as well but this puzzle i'm going to give the solution to okay so it's not part of that one but i think it's interesting because it also illustrates a point about what does puzzle have to do with physics yeah simple way so this is the puzzle uh can something be falling and not hitting the ground that's the puzzle what is falling what could be falling and not hitting the ground ever yeah no this sounds great are you asking me that question or no no no no no i'm sure you can't say it immediately but this is a puzzle this is a puzzle that uh you would think okay something is falling and it's not hitting the ground what could it be what does it mean yeah and so this sounds like you know it can can be some kind of a word puzzle it's not it's actual puzzle and it's a puzzle that newton solved newton's solved in the following way so this is a question i that you could raise is that there's a moon and first of all the question is why isn't the moon falling and the second question is that okay if it's falling then why is it not hitting the earth so those are the questions those those are the questions and the answer that newton gave was so elegant which is the solution to this puzzle so so newton should get a price for that is that no actually moon is falling but it just avoids hitting the earth now how does that work and this was this is beautifully illustrated in this book in prison mathematica which is shows that if you start if there's no earth if you get rid of the earth and if you look at the moon and if the moon is moving in some direction it will continue going straight but if you have an earth instead of going straight if you have anything going straight if you have like for example a cannonball shooting going straight if you have an earth it will fall on it so if you have an earth and if you have a cannonball shooting from it will fall somewhere on the earth if you if you shoot at a cannonball in a with a higher speed it will go farther down and farther and farther you can go from the north pole let's say all the way up to south pole of the earth if you give it enough kick but if you really give the huge kick it will go all the way around and it will continue going around so what's happening is that that ball is falling down because if it wasn't before the earth it would go straight so having the earth causes that cannonball to try to to hit the ground it's falling namely instead of going straight it's falling just going not straight falling towards the earth but since the earth is round it keeps on missing hitting the earth so the cannonball or the moon itself is the answer to the puzzle that's that is falling if it wasn't for the earth to go straight so it is falling but it's not hitting the earth because the earth is round so it's like a game between the roundness of this earth and this falling of this moon so this is an example where the puzzle and the poetry and physics all melt together and of course they can be beautifully explained by mathematical formula that newton discovered but the whole thing is a simple looking puzzle that kind of illustrates the basic link between physics and puzzles now it's a beautiful it's a beautiful example and it's perhaps worth noting that um there was a period of time when you and i and a few other of our friends would find ourselves at the same christmas party every year and our you know tracy and my wife your wife would all leave us because we'd all start to talk about these puzzles in one little corner of the room and they're off you know drinking and partying and having a good time and the one that you told me won one christmas party that was the following i'll just ask it to the audience and again it's one that people can try to solve which is you have a a picture and you want to hang the picture but you're going to hang the picture from two nails not from one nail such that it's completely stable if both nails are secured to the wall but if you remove one nail or the other and you just leave the other in the wall the picture will fall and uh it's a it's a for those of you who are more mathematically inclined this puzzle gives you insight into a subject of mathematics called homotopi and it has to do with non-ability homotope groups and the non-ability nature of the homotopia group of the plane punctured at two points so for those who speak mathematical language that's what this is about so again it's a so the the puzzle that cumberland gave is a beautiful one that puts the poetry of physics and motion in the universe together and here's one again that cameron gave me that i i went home after that party we got home at like two o'clock in the morning at six o'clock in the morning i was up and my little daughter who was like five at the time we did it we took a picture we put it up and yeah and you know what we're getting that picture yes absolutely yeah exactly you gave me some good puzzles in fact some of them my book about the about the glasses and then the and the symmetry breaking which is part of that book so yes we have had good fun with puzzles and the relation between physics and absolutely so i want to turn to one other thing that's in your book which was quite unusual and and for me quite inspiring which is you have a section on science and religion and and you know it's interesting you know at the world science festival which is what this program is part of on various occasions we've had programs on science and religion and we do them because we find it a fascinating conversation between various distinct ways of of looking at the world we also have conversations between artists and scientists artists and theology theologians and philosophers so we'd like science to reach out to the world more broadly but i have to tell you there were a number of scientists who came back at us really hard really angry that we were having a program on science and religion at the world science festival they said what in the world does religion have any of any role there and basically our answer and you know tracy you know who's the co-founder with me at the festival our our view was what are you afraid of i mean any conversation that's rich that allows scientific ideas to be in dialogue with other qualities and perspectives on the world it's a good thing it's not something to to to sean but nevertheless there can be a recoiling and and so were you concerned about having science and religion like like what led you to include that that section in your book well i think that uh religion and science have many commonalities so i think that uh the the deeper truth is what one is after and so uh so of course uh as described in the book i don't think that i think there are ultimately different realms of experience religious experience and scientific experience and i kind of describe them in different ways and i kind of i was i wanted to share that you know scientists are also have views on this and i i gave examples you know newton uh how how religious he was on some more recent ones how they viewed for example einstein how did he use religion and so forth not so much as to try to come to a conclusion really about what is the meaning of you know with what should we do about religion what should we do about science and so on but to say this interplay between between the two and i i view in fact brought more broadly what you just mentioned about the role of religion and what how scientists view in fact even broad more broadly i feel that there's this anti-philosophical bend in which i would say religion is an example of of that same thing among scientists that somehow the scientists feel that these should not be this kind of thing is bad for science and i think that's as you just said and i completely agree a little bit of immaturity not to be able to describe and discuss it and learn from aspects of it i think this attitude that you know we know it all and it's kind of like everything else which is not can be not formulated in this scientific form is a waste of my time and so on i think that attitude is is a little bit i think short-sighted and i think the human experience is much wider and i think we should benefit from it we should incorporate it we should not be antagonistic we should be welcoming and try to understand it and if there are pieces we can say something science has sheds light on some aspect we should and vice versa if there's some some shortfalls about the scientific methodology we should bring in other aspects and so i think we should be open and i don't feel that we should be worried about the dialogue between different domains of human thought or experience so so in your book you do as you mentioned go through in some very nice sections uh einstein and religion hawking and religion feynman and religion what what i didn't see there was vapha and religion if you were to have included a section of that what would it have said it wouldn't have said much it'd be empty blank page i think i think that i think that i i want to leave that to the to the it's a personal i view that as a personal feeling about what it is i think i think the the part of the problem is that the word religion is not definable and you know you might say x and somebody might think what you mean by x is a y and somebody might think it's a z and so forth and you might say something about x and they might misinterpret you but as if you're saying something about y and they'll be complete misunderstanding so i think anything i would have said i felt is that to be so qualified it would not be possible to explain so i'd rather in fact generally not share my specific view because it's not possible to to to say it accurately enough but i would say that i would say that the the the view that that religion is in conflict with science or sciences and conflict religion i don't buy that in general so i think that there's there's more harmony in in religion and science not any specific religion or specific science or i think in general the the modality of humans thinking is is not inconsistent with different views of course you can take a narrow view of one thing and then you get contradictions but broadly interpreted they are not contradictory and an example is this lamar trying to getting motivated by religion by the experience about you know how the universe could have come from a tiny small little primordial thing which was ultimately led to the big bang theory that he was advocating even though einstein was against it and he thought this is kind of mythology of maybe religious mythology of some kind so these kind of views that you know other perspectives brings we should welcome we should not necessarily be antagonistic towards and my view is let's be open that's generally that's my my bottom line is to be open about this so there's a lot of resonance between what you're saying and and my own views and sometimes when i discuss this in a public forum some have come back at me and said well you know you have the luxury of saying things like that because you live in a country that by and large has certain kinds of not completely i mean you know this also said that you know can a presidential candidate succeed if they're willing for instance to say that they're not religious that they don't believe in god i mean that issue has uh never really been fully tested but putting that to the side you are in a unique position right because you come from iran when did you when did you leave iran what year well i came i came from iran just before my high school in 1977. so i think if you hadn't left iran do you think that your your views would still be the same as what you just described yes i still go back to iran incidentally i visit iran i give lectures there in physics i uh i love visiting iran there are they're very much interested in science uh beautiful you know universities and you know educational centers and so on so there's there's i mean one of the one of the things i find about science is that's borderless really it doesn't matter where you are you know what time you live and so it's a borderless and timeless adventure in a way and i view you know me as an example i don't feel like might be iranian or american now or both iran american has any impact about my view about science i could have been there i could have been here it's just i happen to be in the u.s working here but i i find this borderless adventure of science is what's universal and i think it'd be great to use science as a vehicle to go between different cultures and stuff clashing instead of confronting so i think that science in general for me is a glue going between different cultures different domains of human experience like religion like philosophy like art like poetry all these things can be glued together via science rather than science being as differentiating and saying this is right that's wrong this is this this is that and so forth so i think in a way i i find science as a as a refuge in in terms of the craziness that we see around us in terms of all these conflicts that we see around us and culturally or otherwise so i think i don't see any contradiction and i feel science can bring things together rather than differentiate them no i i totally agree in fact i got um a letter i've mentioned this in other settings but it brings to mind what you just mentioned i got a letter years ago from an american soldier who was in iraq in the craziness of everything that was happening there and he wrote me that he had a copy of one of my books and that it gave him the refuge that you're talking about he could sort of shut off the bombs and the and and all and all the death that was around him and all the fighting and the and the strife and just enter into quantum mechanics or enter into relativity and it felt like a universal glue to use the language that you're using that just allowed him to cope with the difficult situation that that he was thrust into by virtue of the problems that beset the world so one final error before we get to the solution to the puzzle uh you know i know that the question of time is is one that's very close to your heart close to the heart of many physicists what is it where does it come from is it fundamental where do you think we are on that journey to try to understand the true nature of time uh well it's a very difficult and important question of course and we don't have a deep understanding even of space which is in something simpler so i think that understanding how what space really is we have learned at least enough about the nature of space and string theory to know that there is no there's no universal answer that is different corners of stan string string theory leads us to different pictures of what is space and these are what we call duality corners one one corner and the other corner they're compatible different descriptions of what really space is they look in in contradiction they look not they cannot be possibly both be true but they're both true and we have learned not to say what is the right viewpoint that is we have learned that for the notion that what is the best viewpoint is not a good question that that that that question of what is the best viewpoint depends on the question being asked and so for example what is space we learned that that is not a good question because what is space ends up having good answer here or a good answer here which look contradictory so so we have learned that much about space which is i would say much easier question than the harder question and more interesting question that you're posing which is what is time so for time we don't even have those corners as well navigated as space but i would interpret i would expect you will have the same kind of things emerging for understanding time the time will not be a unique answer what is time it will be different like snapshots pictures and for one question that's a good answer for the other one the other snapshot is a better one we're going to have these pictures next to each other and with the totality of all these pictures we'll get a feeling about what is time not for a single one of them not for one or two of them but the totality that's what physics is being driven towards that's what string theory we have we have learned from the notion of duality and the fact that this fabric of different kinds of pictures come together and there's not one thing that is the right thing but the totality is what explains it so for space the recent results that link together the spatial fabric and quantum entanglement is that a direction that you think is going to be continue to be a promising one going forward that's certainly one direction that has been explored recently and has good results about it i would say that is among other things they're both they're they're that's one of the corners that i was alluding to there are different corners that are interesting so i would say each corner has to be explored uh not that any one corner is the answer i'm not expecting the answer that this is the answer i'm expecting more totality the of the experience that string theory is teaching us which is this duality picture that all of them are perfectly fine depends on what question you want to ask if you want to ask questions hey that's the best way of looking at if you ask question b you go the other way so yes the entanglement and how the space and degrees of freedom about individual bits of information how did that come to describe the geometry of space and so on and how they are entangled is certainly an example of one of these corners right so let's now um turn to the solution to the puzzle that that we gave at the beginning of the discussion so maybe just quickly because there are there are many more people with us now than were at the start so maybe uh just re-uh reacquaint us with the nature of the puzzle and now you're gonna give us the solution yeah so we have uh four towns uh four cities a b c and d on the four corners of a square and the puzzle is to find the shortest highway system which connects all cities together and the the highway system does not have to directly connect any city to any other city as long as you can get from one to the other to this highway system that would be sufficient so that's the puzzle all right and what's the solution the solution is the naive solution one might think is let's go to the next slide this one right the most naive solution is that well you just go like x now before we go to the next slide i would like to say why would we even guess the solution well because it's symmetric right you start with a square and so the x is the one of the most symmetric ones and it gets you from any place to any other place you want to get so that seems like the answer so so what is guiding us is the symmetry the symmetry of the square is guiding us towards the solution or we might think there might be another symmetric solution which is the next one which is this one this is also symmetric you know you have four towns four cities you can have the whole edges that would be another possible symmetric solution so you say either this or that and then you begin to think you say wait a second it cannot be this one because if it's this one you can delete one of the one of the one of the roads and you still can't get from any city to any other so that's shorter and you say in fact you can put that uh city a because that the road can go from any other place like the middle so you might say well that looks that looks the same length but actually you can pull that rod a little inwards and you can put it a little inwards to be like that with making an angle 120 degrees and this turns out to be the shortest and this is surprising this is confusing because you start with the problem which is symmetric the four cities on the corners of a square enjoy the symmetries of the square like 90 degree rotation but this highway system does not enjoy this symmetry in other words to get from city a to city c is a shorter path than getting from city a to city b even though city a and city b and city a and dc have the same distance relative to each other so that's a puzzle so why is it that the solution does not enjoy the original symmetry this fact we call in physics spontaneous symmetry breaking and this underlies a lot of amazing phenomena around us in the universe spontaneous symmetry breaking is a key to understanding many things and in fact the ideas go farther back much longer history than we think we originally thought we discovered this idea in the past century the physicist but actually it goes farther back to the greek philosophers so so early greek philosophers already had they are very smart they had already discovered that there is round and they thought you know it's at the center of the universe and it's not moving so these are the their observations was this and so they were a bit puzzled why it's not moving so they had to come up with an explanation of why it's not moving and they try to use symmetry so this is how they reason go to the next slide we see is the picture of the earth and they said well okay suppose it moves if it moves in some direction let's say some direction let's move it to the to the next slide it goes somewhere it breaks the symmetry so it cannot move because it doesn't want to break the symmetry so they explained that the earth is at the center of the universe and it's not moving because if it were to move in any direction picks out the direction and the very nature of picking a direction breaks this spherical symmetry around the earth so it's not allowed so that's one of them said this is a proof earth is not going to move and then aristotle didn't like the argument he said nope not a good argument why it's not a good argument they asked said he reasoned as follows he said well suppose you put a person at the center of a circle and he put loaves of bread around that person symmetrically located on a circle now the person is at the center of the circle and you ask them whether or not that person is going to move well you would say no it's not going to move because at the center of the circle it breaks the symmetry the circular symmetry were to move of course you know that's not the answer after a while the person feels so hungry person will move in some direction to get that loaf of bread so therefore the person breaks the circular symmetry because that's not an optimal place to be in being near the bread and eating it is much more optimal so they say who cares about the symmetry i'm going to get my bread i'll break the symmetry that's exactly what happened with that highway system this shorter highway system breaks the symmetry and that's what the the highway system chooses to do so it's broken symmetry now what does it have to do with physics you might ask well first of all i would say that spontaneous symmetry breaking is imprinted in on our bodies you see we live in a in a year in it on the earth which basically has this 100 360 degree rotation symmetry around us generally speaking so you would think that our body should also enjoy a 360 degree rotational symmetry like we should have we should have our eyes all over a circle around rather than just in the front but we only have it in front you might ask why why do we have our eyes only in the front and not all over all over the circle which would be more symmetrical so so somehow our body has broken the symmetry of circular 360 degree rotation that's only in one direction why is that well the answer is aristotle so for the same reason you want to see where the food is to go towards it in other words the motion to go to from one place to another is optimal in a given direction and not you don't need the 360 degrees so with all the availability of evolutionary system of how the body is designed it says it makes sense that much more better uses of resources to just put the eyes in the front because that's really where you want to go anyhow so the symmetry is broken because you need to get to the food and in more more details related to more aspects of modern physics this was the discovery of what's called the higgs particle or the god particle or what is the origin of the mass is exactly the same circle that aristotle drew it turns out surprisingly a few thousand years ago he already had the same picture that we now draw in our in our in our books or in papers talking about how particles get their mass namely if you think about they put the the potential that is around us they look it looks like a valley it looks like a hill and a valley and they bought the top of the the top of the this uh this hill area is is the most symmetric point that person that person is standing is the most symmetric point but it's not an optimal point and it turns out that the optimal point of course will be down on the on the valley area and if you go down in that valley here it turns out that the there will be a mass related to how far away you are from the symmetric point so the being away from symmetry translates to giving mass so breaking symmetry means picking up mass so this picture about how we have the eye how we get the mass all are related into this and they evolve into this simple idea of this puzzle about the square so the higgs gets hungry the higgs field gets hungry and goes for the bread on the uh on the vanilla he's reflects our hunger and that's why we become massive that's good in fact in fact said nicely that the the the the one of the fields in his model ate ate another one another few that became massive that's called higgs park who gets its mass that way this is literally this picture that's fantastic well that's great so thanks for that mini mini discourse on on on connecting a puzzle with real stuff in the world around us those of you who were able to solve that puzzle i don't know how many it is but maybe world science festival people will let me know i don't know if it was they have written already we have a winner from the audience yeah wow from this channel i see we have a winner uh prior dash is that right oh he's a he's a regular in in these in these gatherings so that's congratulations yes absolutely congratulations i'll be glad to send my book to you bryo that is that is well done well done so we will work out the details of that so so again the book that we're talking about and we have a slide for it but just so you can sort of see it straight out puzzles to unravel the universe by the guest today professor kumar vaffa from harvard it is available all over the place you should get a copy it is a it's a wonderful way to introduce yourself to deep ideas of physics through an interactive and a real active as opposed to sitting back and letting the author preach to you which is how most books are this is one where you can get in on the action and do things and then that will really gain a intuition for for the ideas that that professor vaffa is describing to you so kerman thanks so much for joining us for this uh conversation thank you very much brian for this session i really enjoyed it and it's great to connect with your audience and good luck with all the great work we are doing with connecting science to the wider public i appreciate that thank you and we'll we'll be in touch and uh no doubt our paths will cross uh sometime soon so thanks a lot maybe in the cafe somewhere very good all right guys so we're gonna now just uh move into the more ordinary q a that we do in these discussions here i hope i have oh i do have maybe one of you out there one day is going to send me a pair of glasses these that i have today are cracked and broke and they're not my wife's glasses at least she is using them but uh at least now i can see for half a second so again sorry for looking ridiculous in one way or another and yes prior dosh congratulations on getting the uh the solution really well done and let me just scroll through and um just take some uh general questions so ramsey asks is allah personified by the higgs boson i'm not sure maybe this is one that i should have kept the kermit vapor on to answer but you know when one uses the phrase god particle when describing the higgs particle perhaps it's worth mentioning that you know there's a story behind that the particle physicist and nobel laureate leon letterman who was instrumental in many of the ideas associated with the higgs particle higgs field and so forth people were trying to find the higgs particle for instance at the fermilab accelerator where leon letterman was associated for for many years and it was very difficult to find the particle nobody was finding it so there was a general joke going around that it was the god damn particles because it was so goddamn difficult to find this particle and i think the story goes that when leon letterman was going to write his book and call it the goddamn particle the publisher or the editor or audience surveys whatever suggested that the god particle would be a more poetic title than the goddamn particle so it's just to say that no one is really associating the higgs particle with any religious tradition with any real religious outlook it is in some people's mind a more fundamental particle than any other because it endows other particles with mass remember how this goes the higgs particle is the smallest piece of the higgs field and higgs field is meant to be a field a substance an invisible substance i like to think of it as an invisible cosmic molasses that fills all of space and as a particle tries to go through space it has to plow through the cosmic molasses and that creates a resistance and that resistance on the particles attempt say to accelerate is what we call the mass of the particle so it's a universal mass giving field and the universal quality can bring to mind the universal quality of god if you will so some people draw the connection in that way but again it is just a field of physics like the electromagnetic field like the other fields that we make use of the gravitational field and so forth and it's a very special field but mathematic is just one more field so i wouldn't look for any theological connections any deeper than the poetic ones that i just mentioned okay heading onward sanjay powwar what is dark energy and again i think this is a question that we've taken up a few times during these various sessions but i'm always happy to cover familiar ground it's important dark energy is another kind of field we believe it's a field like the higgs field that does permeate space it's very related to what i was just describing but the difference with the dark energy is that the amount of energy density in the field is such that the field has a significant gravitational influence on the universe the higgs field does not have a significant gravitational influence i mean it does endow particles with mass so you've got gravity in that story as well but in terms of the intrinsic source for gravity that the higgs field is it's modest it's minor whereas the gravity sourced by dark energy is not modest it's not minor it's significant so significant that we believe it gives rise to the repulsive gravitational push that is driving space to expand ever more rapidly so it's again in some sense analogous to the higgs field but in terms of its impact on the way reality unfolds it's quite different because of the vast store of energy that yields an important gravitational contribution that yields this accelerated expansion having said that let me not leave you with the wrong impression the amount of energy in the dark energy while enough to yield a gravitational influence that's observable is still a small amount of energy by everyday standards i did a calculation and it's in my latest book it's yeah it's a calculation that you can do on the back of an envelope i could probably do it on top of my head but i'll just try to remember the result if you look at the amount of dark energy in a given cubic meter of space the energy in there could light a 100 watt light bulb like the old variety right well 100 watt doesn't matter the technology but i'm having in mind not the 100 watt equivalent of an led but actually 100 watt light bulb it could keep it illuminated for about one about five trillionths of a second i believe is the answer so that's a tiny stretch of time to keep that light bulb illuminated which is just to say that even though the dark energy has enough energy to yield an observable gravitational influence the amount of that energy on everyday scales is tiny absolutely tiny but that's what that is so enos abu salham asks what is dt dx dt by dx and that's an interesting interesting question we all know what well i hope we all know what the x dt is right that's the calculus expression for the change in position divided by the change in time and the change in position over the change in time is speed that's what we mean by speed how far you go divided by the duration the interval that it takes you to travel that distance what if you flip it upside down and consider dt dx and dtx is a number well obviously it's one over speed but you might ask yourself does that come into physics and in an interesting in an interesting way and it kind of does because and i won't go through it in detail but there's an analogous version of that which is d tau d t where tau is the invariant interval the invariant space time interval so you can talk about detail dt or dtd tau and that basically is the rate of change of one clock relative to the clock that's on your wrist the clock that's on your wrist measures invariant time it is intrinsic time if you will and you can have other clocks that are not moving relative to you and you can ask yourself what's the rate of change of your space-time interval compared to their space-time interval and that is one way of thinking about einsteinian time dilation and why it holds true because uh in some sense we are all traveling through space time at the same in variance speed which is equal to the speed of light and if i get up and start to move i divert some of my motion through time into my motion through space because i'm now moving through space i'm not just sitting still and that notion of moving through space affecting your passage through time naturally emerges by considering things like d t d tau and d tau d t which is quite related to d t d x so if you want the details on that i can send you to a little endnote in my book the elegant universe i think it's i forgot which endnote it is but it's in chapter two so you can see how sometimes flipping the calculus expressions upside down can give you important and deep insights and that's one specific example of that all right let me i've been only looking at the uh the twitter questions let me see if there are any interesting uh not twitter the uh the youtube questions i get my social media sources mixed up but let's see john olson asks and this is from this is from i guess this must be from facebook or twitter a black hole question can two or more massive black holes change the geometry of a small black hole's gravitational field in a way that mass can escape so it's certainly the case if you've got multiple black holes around [Music] the geometry of space time at a given point is dependent upon all three of those black holes so oftentimes when we write down a picture of a black hole we show a schematic of the geometry but we show it on its own we don't show what it would look like if there were a bunch of black holes together with all the various warpings of space time overlapping in interesting ways and we can do that it's it's harder to do sometimes you don't even have exact solutions in these multi-body versions of the general theory of relativity but you can certainly approximate the solution quite well in many interesting circumstances and it's not that the new geometry allows the mass of a previous black hole to escape but it does change for instance the trajectories that hawking radiation which can escape from any black hole the trajectories that those particles that are emitted by a black hole will take so the detailed dynamics in any given circumstance does intimately depend upon the masses and the positions of whatever black holes might be in the environment and that can affect the ways in which those black holes evaporate but it does not affect the classical picture that mass can't escape from a black hole itself all right so what else do we have here uh chavoc mandal again from and i'll go back to the to youtube feed in a second but from our social media feeds how can we verify the predictions given by string theory we don't understand the quantum behavior fully how can we look inside a quantum world it's too bizarre um so so a lot of questions in there but the primary one how can we verify the predictions given by string theory right that is a question that cameron vaf and i discussed just a little while ago and as he mentioned in principle if you built a sufficiently large accelerator not one like the large hadron collider that can reach you know depending on whatever units you look at 10 to the four times the mass of a proton you know that kind of scale as cumberland mentioned you need a detector that's within an accelerator that can reach 10 to the 20 times the mass of a proton so that's 16 orders of magnitude more powerful than the large hadron collider now where did that number come from well within string theory a key number is the planck mass and the planck mass is about 10 to the 19 10 to the 20 times the mass of a proton and it's at the plank mass that in conventional string theory you would expect the extended nature of strings to be visible so that's why cameron mentioned that 10 to the 20 number and so right so if we could build an accelerator with that magnitude of energy then at least in principle we would be able to test string theory so when people say that string theory is untestable they really should qualify it in a way that they might roll their eyes and say well yeah of course i mean that but they should qualify by saying we can't test string theory right now but that does not mean that string theory is untestable and to some that is a distinction that doesn't matter much but for those of us within string theory it's a distinction that really does matter because if string theory were permanently untestable if there were a quality of string theory that said this is fundamentally not testable by any means then we really wouldn't be doing physics any longer you know some people say you know psychoanalysis is fundamentally untestable because regardless of what the patient says you can interpret it within the rubric within the framework of freudian analysis and in that way there's effectively nothing that the patient could say that would fall outside that template but that's not the case with string theory it's just that we have limited experimental capacities we advanced human beings here in the 21st century have limited experimental capacities and string theory is such a bold theory it is such an ambitious theory that some of its most iconic qualities just don't emerge at everyday energy scales the theory is so bold and ambitious that some of its key qualities emerge at fantastically larger scales such as the scales that kerman vapha was mentioning so so how do we test string theory we may get lucky excuse me half a second here we may get lucky and one day some clever einsteinian level string theorist or einstein squared level of capacity that person comes along and says look at this and they do some fancy mathematical manipulation and out pops some prediction of string theory that we could test with existing technology and we thought that we might have that with super symmetry that's a quality of string theory that might be measurable at energies like that of the heart large hadron collider but we've not succeeded and that might mean that that quality of strength here is wrong and kerman mentioned some of the implications of that if string theory is fundamentally not realized in a universe that is governed by string theory that's the big assumption that strength here is right but the flavor of is relevant say to our universe is a flavor that does not involve the super symmetric quality at least at accessible energies that might suggest that the universe is unstable and it might suggest that you know i don't think he couldn't mention the number but it could be that within you know a factor of 10 100 or a thousand times the current age of the universe our universe disintegrates and that is a possibility that i explore in my book and in the book until the end of time the specific version that kerman mentioned as i noted there is just really an endnote but the possibility that our universe might disintegrate in that time frame is one that we may have to at some level accept but the point for shivok mandal's question is that supersymmetry was one of those qualities that emerges from the mountaintop of string theory and rolls down and in principally thought might come to rest at energy scales accessible to the large hadron collider but that seems not to be the case but there may be other qualities but if there are no such other qualities or if we don't have the brain power to extract those other qualities then the real way to test string theory may be to wait until we have fantastically powerful energetic accelerators or whatever kind of machine takes the place of acceleration the far future so it's very much up in the air now you can ask yourself so does that mean that you shouldn't work on string theory and clearly someone like kumar papa says that's the wrong reaction there are the corners of reality that he made reference to which string theory can illuminate now you might say well how do we know it's reality you don't you don't but that's what being at the cutting edge of science is regardless of what subject you're in you're pushing into frontiers that you don't know at least if you're doing theoretical work whether they're relevant to the real world that we live in and you take a chance and some people find it very exciting to take that chance and they go on into string theory or other speculative fields or other scientists that want hey i'm working on a project and i want it to be done in two or three months and regardless of what the answer is i want it to be interesting enough to write a paper on it and there's a lot of science that can be done that way interesting science it's just a different way of engaging with reality so it's all a matter of personal choice okay deepak ponmar what is what is an axion an axion is a hypothetical particle that naturally emerges from certain attempts to solve famous long-standing problems in particle physics so there's a a problem called a strong cp problem it afflicts our understanding of the strong nuclear force c and p have to do with certain symmetries of the mathematical equations charge conjugation parity symmetry and when you look in the strong nuclear force the equations that we study there are certain terms that in principle could be there but they seem to be highly suppressed based on the impact that they would have on experimental predictions that are not observed and so the question is how do you argue that these terms should be suppressors or a natural way to suppress them and it's an interesting long story but one way of suppressing them is to look at a certain kind of symmetry a u1 symmetry for those of you that like mathematical analyses and associated with that solution is this particle called the axion and the reason why the axion is spoken of in more recent descriptions of the universe is that it may be a dark matter candidate it may be the dark matter is perhaps a better way of saying it it is a dark matter candidate and the more traditional candidates that people have thought about for decades weekly interactive massive particles or so-called wimps not the nicest of names for these particles but these particles naturally emerge from supersymmetry which again may or may not be relevant to low energy physics but we've searched for these wimps and we haven't found them or i don't know if that's the best description there have been moments when we thought we found them and then the experiments could not make a sufficiently convincing case that the community of physicists had come around to believe that the dark matter had been detected so just about everybody in physics is of the mindset that we don't know yet what the dark matter is and with all these experiments looking for one class of particles called wimps and not finding them to most people's satisfaction other candidates have bubbled up to the surface and this axion particle is is one such possibility so that's what an action is and that's why we're interested in them and we just have to wait and see whether additional information comes around to perhaps convince us that axions are indeed the dark matter so diego asks is symmetry a property of the universe or a property of our understanding of the universe and and that's a variant on on the deep question is is mathematics a quality of the universe or is mathematics simply the language that we humans use to describe the universe and both questions diego's question and my variation on that question are questions that people argue about all the time right i mean you look out and you see patterns in the world and in order to encapsulate those patterns in succinct descriptions one of the things that you do is look for symmetries right i mean a circle is the prototypical example and a circle is highly symmetric in the sense that if you rotate it through any angle around its center it looks the same and because it looks the same that means you don't have to describe every single point on the circle if you have described one point on the circle you've described all points on the circle because via the symmetry they all look the same that's a very simple example of the power of symmetry allowing you to reduce the complexity of your description you can just describe one point on the circle describe its distance from the center and you are done because every other point is the same distance from the center and therefore you don't need to talk about them in any explicit manner and they are all related by a rotational symmetry around the center now you know is the circle a real thing you know plato had in mind that there was this other meta universe or perhaps real universe where perfect circles lived and perfect triangles and squares and all of the the perfect versions of the shapes that we described they are out there they're real but they're in this other parallel reality others think about circle as a figment of the human mind it is something of our own making that we impose on the external world and when we find things in the world that look like circles we impose this pristine version of the circle upon them and in that way we model stuff that's out there in the world via these constructs of the human mind right so simple example right so right here i got this little bottle top you know it looks pretty circular and if i were to describe it i'd say the two-dimensional projection of this i would say it is a circle but in keeping with diego's question we all know that this bottle cap is not really a circle if you were to zoom in on it right i'll get closer and closer and closer you see that it has rough edges associated with it if you took an electron microscope in there and you'd even be able to see the ridges or maybe even see the atomic constituents that are rag tag arranged in something that approximates or without a real circle so that's the question are these ideas of symmetries talking about real stuff in the external world or is it our description of the external world so if i was to give you my answer diego i would say it's our description of the world i i've come to the viewpoint in recent years that mathematics is a powerful language of human invention for describing things in the world and within that mathematical language there are certain idealized qualities and among those idealized qualities are things that are known as symmetries circular symmetry is one but they're more abstract symmetries that we usually use the mathematical field of lead groups and li algebras to describe those symmetries and i think that these are powerful tools that we invented to describe the patterns of reality but to say that they're actually out there in the world [Music] that's a step that i used to take that i that i don't take at least at the moment so diego long answer to your short question i would say that the symmetry is a property of our understanding of the universe and that understanding is officially powerful that it can make predictions about things that are out there so it's not as though it's solid solipsistic it's not as though it's all happening in here we can use that symmetry and that map to make predictions and the predictions are borne out by experiment so it's not as though it's all limited to stuff that happens inside of our heads but i do think that the description is something of our own making okay [Music] local titans asks is there a possibility of more fascinating undiscovered behaviors of space-time that might even more complex than our imagination and local titans i like that name local titans i think the answer of course i have to say is yes is that possible yeah i mean the history of physics is a history where we're astounded by the concepts that the mathematics forces upon us i mean black holes are absolutely beautiful example einstein writes down his general theory of relativity in 1915 a mathematical description of space time within a handful of years we learned say from carl schwarzschild who solves the equations first one to solve the equations we learned about this possibility of black holes they weren't called black holes back in 1917 rather they were called dark stars or frozen stars but this idea that there might be this monstrous structure in the cosmos that sucks in matter that gets too close it doesn't let anybody escape this ultimate warp in the fabric of space-time it comes out of the mathematics it does not come out of imagination and in fact it was so shocking that einstein's imagination couldn't really cope with it einstein wrote a handful of papers i think the last one was 1948 where he was trying to argue that black holes would not be real that they were just a mathematical figment that emerges from his equations but not real but as you all know i don't have a picture here i don't know how if our team can grab a hold of the the famous picture of a black hole that was just taken a couple of years ago right that's not the only evidence we have for black holes we had the orbit of stars around the center of our milky way galaxy and we had other indoor we had gravitational waves the particular ripple in the fabric of space time that we detected back in 2015 the supercomputer simulation showed that it had to emerge from from the collision of two black holes but now we actually have in some sense photograph of a black hole shep doleman and the event horizon telescope they took this famous there it is we took this famous photograph right so look at the journey from einstein in 1915 to the imagination of carl schwarzschild 1916 1917 through the observations of many in culminating with the image that you just saw so yeah local titans i think that pattern will be played out again and again as we go forward and there will be increasingly strange and weird ideas that don't come from human imagination that come from the mathematical description of the world and some of those ideas will be wrong and some of them will be right and some of them will be experimentally confirmed just like the idea of black holes was so i do think that that is the pattern that we will see persisting going forward little doubt on that okay let me go back to our questions here nylesh asked sir can you please explain bell's experiment and hidden variables i can i can i'll give you an abbreviated version it's a beautiful story it's one that you know as i'm wondering have we actually done a full world science festival program on bell's inequality and entanglement it's certainly been mentioned in many of our programs it's certainly in a theatrical piece that we are now developing anew on quantum mechanics it's called spooky action the drama of quantum mechanics so those of you who liked light falls our theatrical exploration of the general theory of relativity and those of you have not seen it again i'm not supposed to do this but i don't care there's a if you go to this youtube channel they should not have it up it's called the documentary channel i don't know who it is we didn't put it up because we weren't allowed to they're not allowed to either but whatever uh so you can watch it on the documentary channel that's a theatrical exploration of the discovery of the general theory of relativity uh if you like it leave a comment there i always love to see if you don't like it don't leave a comment please but if you do like it leave a comment i'd love to see those those positive comments but we're developing a version of that kind of presentation for quantum mechanics but anyway back to the question the spookiness in spooky action the title of that piece is quantum entanglement and here's the story very very quickly here's the story so einstein himself in 1935 now in the 85th anniversary of that famous paper einstein and two colleagues showed that according to the math of quantum mechanics you could have two very distant particles that behave as though they are one the canonical example this is not the example that einstein used this is an example that comes from david bowm a wonderful character in the unfolding of the history of quantum mechanics but a given particle can be either spinning up or spinning down that's the nature of particles and in einstein and colleagues reworking of that idea with this bomium version you can have two distant particles that are both spinning up and spinning down at the same time they're both in a fuzzy mixture spinning up and spinning down and here's the thing you can go and measure one of those particles and cause it to snap out of the quantum haze according to the math that einstein and colleagues showed the other particle would also snap out of the quantum haze at exactly the same moment even though you didn't do anything to it let me see if i can get in front of the camera here correctly so up and down the same time you measure say the one in my right hand boom snap up the one on my left hand snaps down oh come on be in the frame it's so counter-intuitive how to move these things so it's weird it's spooky you do something in one location it influences a particle another location einstein said no no no no no there's no such thing as spookiness he said but this experiment is really showing you is that these particles that you thought were in a fuzzy mixture up and down at the same time they were not they were always up or always down and when you measured it and you found it spinning up it always was up and the other one when you measure it you find it spinning down it was always down and if it was always down then there's no spooky connection it's not like when you measure the particle in new york it affected the particle in california because the one in new york was always up the one in california was always down period end of story no spooky connection between them whatsoever so who's right that's the question is there spookiness as the math of quantum mechanics says in quantum entanglement or is there no spookiness as einstein's interpretation of the experiment would suggest and many people at the time thought this was not a question that could really be answered wolfgang paulie i think described this as a question that was no better than asking how many angels can dance on the head of a pen it's just not answerable why is it not angerable because paulie was saying look if you want to know if this particle was spinning up and down simultaneously and when you measured it it snapped out of the haze or if it was always up and when you measured it it was just revealing its pre-existing characteristics paulie says how can you ever test that because to test that you've got to measure it and the point is once you measure it it's always spinning up or down you can never catch it in the act of spinning up and down simultaneously [Music] john bell to answer your question and whose question was this this is uh nye luscious question john bell came along and found surprisingly found an observational consequence of a particle being in a fuzzy mixture save up and down at the same time even if you're not measuring that particle there's an indirect influence of a being in that fuzzy mixture of up and down that is measurable you have to be more clever you can't just brute force measure the particle and say well it's up and therefore i don't know if it was up and down beforehand or if it was just up beforehand you got to do something more involved something more clever and that's what john bell did he found a more clever experiment that could give you insight into whether particles have definite features before you measure them or whether they're in a mixture before you measure them and to most people's satisfaction that proposed experiment which is then carried out concludes that einstein was not right on this idea that particles really can be in this fuzzy mixture and it only becomes definite upon measurement or observation or interaction of some sort and to really understand the details in that explanation it's you have to really look at it more closely and in in chapter um i don't know chapter four of fabric of the cosmos i go through i think it's that chapter a long time ago now i go through a very detailed analogy that i i believe really captures the heart and soul of john bell's inequalities in the experimental state of checking those inequalities yielding the insight that i just described that at least when it comes to definite properties einstein's view seems not to be borne out by the observations that's one place where you can learn about non-mathematically but the math is not hard so for those of you who are familiar with linear algebra a little bit of quantum mechanics you do not need much to really understand the original papers so i would encourage you to do that but nilesh i hope that gives you the basic idea of john bell and quantum entanglement okay so um just trying to find a good question to take us in a in a different direction um so siamak rajyabhi asks can you explain eric valente's theory versus the general theory of relativity and um again that's a that's an involved story to to describe in in detail but i can give you the flavor for it so often we talk about general relativity as a fundamental theory of the world now earlier we discussed what does that really mean is it stitched into the fabric of the world the general theory of relativity or is it a human description of the stuff that happens out there in the world and again i don't know the answer to that question but a related question is is the general theory of relativity a fundamental theory of the world or might it actually emerge from yet more fundamental considerations and with eric verlinde and and others ted jacobson maybe was the first who started along a very similar trajectory asked themselves could it be the case that just understanding statistical mechanics thermodynamics in a gravitational setting might be enough to extract einstein's equations as opposed to posit einstein's equations and and these studies have been successful in in in in certain ways so without even knowing einstein's equations but knowing certain very basic features about thermodynamics and certain very basic features of quantum mechanics these these folks have been able to extract aspects of einstein's general theory of relativity and thereby come to the conclusion that einstein's theory is an emergent theory an effective theory a theory that comes into play in certain contexts but rests upon more fundamental physical principles and that's an exciting idea it's it's resonant with a lot of things that we think about and discuss and work on in string theory i mean for linda is a string theorist i think he would call him a string he would call himself a string theorist at heart i don't think that is a description that he would consider inaccurate although more much more broad and is thinking about things but in string theory there are many who've come to the conclusion as we began to talk about with kermit in the first hour that space and time are not fundamental that they themselves are emergent that's a spectacularly interesting idea because every formulation of physics you go back to to to anything that newton wrote down or anything that galileo was thinking about or anything that really happened after and all of these theories certainly assume a pre-existing spatial environment a pre-existing notion of time that allows for a dynamism that allows for things and systems to change over time you can't talk about change over time without time being part of your description from the get-go but these newer ideas are suggesting that space and time are not fundamental that there's a description of reality where when you look at the basic vocabulary you don't even need to use the word space and time to get the theory off the ground and then space and time come into play when you apply that theory in certain domains of reality such as the domains that you and i are inhabiting at the moment but the space and time and the description that comes from the general theory of relativity of space and time may all be emergent may all come into play in certain environments but are not fundamentally part of how the world is put together that's the basic idea [Music] uh andrew haney asked will you present a gender relativity course on world science you [Music] and andrew i i think the answer to that is yes and just putting all cards on the table you know that i have this special relativity course and you may also know although some of you may not be aware we just released the the video of the non-mathematic the non-mathematical version of the special relativity course it's called spacetime and einstein we released it just a few weeks ago we filmed it a while ago of course but we took it out of the world signs you universe and put it out just on our youtube channel we put on two youtube channels actually the world science festival youtube channel and the world science u youtube channel it's out on both and yeah yeah i was surprised it's gotten a lot of activity a lot of views i think between those two channels has gotten over 300 000 views in the last few weeks that's fun to see people are interested and that sort of motivates me to to go ahead and maybe create the general relativity course and also the quantum course i'm thinking the quantum course i might do first we're actually building a little studio up here where we are we are huddled waiting for this virus to pass and if we can get that studio built hopefully it's in the next month ideally one of the projects that we'll do in there is to film a quantum course for world science u analogous to the special relativity course that already exists and the general relativity course so so i will give you a tentative yes andrew i don't know the time scale for the general activity course but i'm i'm going to go out on a bit of a limb here and i'm going to try to get the quantum course done by like december january something like that and again though i'm going to try to do a non-math version that's embedded in the math version so that those of you who don't like the math can take the non-math version and if you feel motivated you can then watch some of the surrounding material and see the the mathematics behind some of these ideas so that's certainly a project that is moving from the back burner heading toward the front burner so we'll see how how that affects uh things going forward so um as can the casimir effect describe gravity at the quantum level and that's a very interesting idea amjad because the casimir effect i think i i hope well maybe i should say what that is i don't know if i don't have any show and tell with me so i just have to use my hands but if you take two metal plates and you bring them very very very close together there's an interesting force that comes into play if these plates i mean forget about gravity just for the moment if these plates are have no electric charge you wouldn't expect to be any force between them it doesn't matter how close together how far apart if they're neutral objects and you're forgetting about gravity then there's no other force that would act but quantum mechanically there is a force and it has to do with the undulations of the fundamental fields like the electromagnetic field because the quantum undulations of the field outside the plate is fundamentally different from the quantum undulations that quantum mechanics allows inside the plates and that difference creates an imbalance that in certain circumstances can force the plates to come together or in other geometries can force the plates to push apart and that force is called the casimir force of the casimir effect and the beauty of it is it's measurable casimir did these experiments in 1948 dutch physicist i'm pretty sure his dutch physicist did his experiments in 1948 so it was measuring the difference between the quantum undulations in the vacuum between the plates versus exterior to the plates and and so it's this beautiful macroscopic way of testing quantum effects can you use those ideas to test gravitational effects and the answer is definitely yes the question is is it is it detectable and there's a beautiful relevant paper to these ideas that i just was reading by nobel laureate frank wilcheck and an old friend of mine i guess i can call him an old postdoc of mine actually named uh malik parrik who is now at in arizona is it arizona state university or university of arizona i'm not i get those mixed up i think it's i think it's arizona state university and they've written an interesting paper trying to see whether you know quantum effects quantum effects analogous to the cashmere effect really that come into play when you measure gravitational waves could those subtle quantum effects be used to perhaps gain insight or detect gravitons themselves gravitons are the the quantum of the gravitational field photons or the quantum of the electromagnetic field right you're seeing me right now because photons are emanating from your screen traveling banging into your eye hitting your retina causing a cascade of other particle interactions inside your eye sending a signal down your optic nerve to your brain that's how you are seeing me it's all coming from the seed of photons interacting with your eye gravitons do for gravity what photons do for the electromagnetic field so there's a debate among some physicists as to whether gravitons are actually real this is where that remark i mentioned in the first hour when i was talking to kumpapha that freeman dyson seem to not think that gravitons are real but if they are real then what will check and and malik park have suggested is that these kind of gravitational wave like technologies properly used and interpreted might give us insight into the nature of gravitons or even the reality of gravitons so so yes in principle the answer to that question is yes sanjay says i'm like team physicist young sheldon thanks for sharing sanjay i uh i'm not sure how to respond to that but i guess that i guess that's a good thing so i will uh leave that i will leave that at that anas asks what's the natural log of a matrix and that's a slightly technical question but what we do in physics or mathematics more generally when you want to talk about operations on a matrix the operations that we know easily for matrices we know how to add matrices you know how to multiply matrices those are the easy things to do and so what we do is when we look at more complicated functions of a matrix where like with a natural log you don't really know what to do when you apply the two matrix we do know how to create a series expansion that approximates the function that you might be talking about for instance you know for an exponential which is the inverse of the natural log well how do you exponentiate a matrix well we know how to have a series expansion for an exponential e to the x you know it's 1 plus x plus x squared over 2 factorial plus x cubed over 3 factorial that's a series expansion for an exponential if you were to not put a matrix up there in the exponent for an exponential you'd say what is e to a matrix i don't know but if you plug that matrix into the series expansion you do know what it is one becomes the identity matrix x is just the matrix itself plus x squared over 2. you know how to square a matrix square matrix you multiply it by itself and so on so if you have a series expansion then you can use that as the definition of that function of a matrix and you do have a series expansion within limits for the natural logarithm and that's how you define the log of a matrix so thanks for that question hope that wasn't too technical but that is how typically people interpret the natural log of a matrix mr space asks hello mr green if space is expanding everywhere does this mean that at the nano scale or even smaller that the distance between atoms is also expanding and the answer to that question is is yes and no what do i mean by that well when you talk about space expanding on large scales you're talking about scales that are so large that the gravitational influence between entities within space is so small that the attraction between them is not influenced by gravity or in fact any of the other forces of nature and therefore you just focus upon the movement that comes from the dynamics of spatial expansion itself and indeed that can drive distant galaxies to move away from each other when you talk about the same ideas on small scales like nanoscales or you don't have to go that small talk about these scales right are the atoms in my broken glasses are they moving apart from each other because of the expansion of space and generally we say no why don't we say no we say oh god my glasses are so dirty it's kind of gross we say no i'm going to clean my glasses as i'm answering the question we say no because my glasses are held together by the electromagnetic force not strong enough to hold this arm on the electromagnetic force was overwhelmed there by like my dog's teeth or something sitting on my glasses caused it to crack but the atoms in my lenses they're held together by the electromagnetic force which is stronger than the outward swelling of space and that's why the atoms in here are not moving apart in the way that that you suggest mister space but if the expansion of space were to get stronger and that's possible if the expansion of space is not only speeding up but the rate of speed up is speeding up then it's possible that in the future the outward swelling of space is so powerful that it would cause the atoms in the lens to move apart shattering my glasses and it would indeed cause the electron orbiting the nucleus of an atom to expand away from the nucleus causing the atom to disintegrate and if that were the case then that kind of expansion would destroy everything so as of today the answer to your question mr space is no atoms are not being driven apart by the swelling of space but in the future if the expansion picks up in its ferocity then individual atoms could be moved apart from each other and even electrons within atoms could be driven apart from the nucleus causing everything to fall apart that is a potential future that that we potentially face okay so what else i'm going to go back to uh my uh other list of questions that comes in from social media things anything fun there um let's see uh so kaphil sharma asked a question just like mr space but i'm not going to answer it again but very close um so nomad asks uh in inflation or cosmology more general we imagine that the earth is 13.8 billion light years away from the big bang i'm not sure i would exactly say it that way so i'll come back to that in a second um and that means that the age of the universe for us is 13.8 billion years i'll buy that what about celestial bodies that are farther away and can't be seen by us so a number of questions in there nomad nomad which is your full name so i wouldn't say that we're 13.8 billion years 13 8 billion light years away from where the big bang took place i would not say that i understand what you mean by that what you mean by that is if we were to see the big bang like let's be concrete when we look at the cosmic microwave background radiation which was released just you know a few hundred years after the big bang those photons have been traveling toward us for on the order of 13.8 billion years so they emanated from a point in space that is 13.8 billion light years away at least when that photon left at that point in space so so so what does that what does that mean when we talk about a photon that's been traveling through space for so long and i just realized what i just said was a little bit wrong so i should really clarify that um so so when we see something in space we are seeing it of course as it was a long long time ago and when we say that the universe is 13.8 billion years old what we mean is that the big bang happened that long ago and therefore where were to actually see that event you'd be seeing an event that happened far away in the distant past but you see i wouldn't say that the big bang happened at a distant location what i would say is all locations that are now separated were on top of each other at the moment of the big bang so the big bang happened where you are where i am where everything is located because all those locations that are now distinct for the same location so clarification number one i would not say that the big bang happened 13.8 light years away from us there was a bang that happened there but we don't need to distinguish that bang from our bank because those locations were the same location but in terms of the second question in terms of celestial bodies that are even further away that we can't see yeah we make a very sharp distinction in physics between the universe and the observable universe and the celestial bodies that we can't see nomad nomad are in the universe but they're not in the observable universe and so we make assumptions about what those celestial bodies might be like we generally assume that they're pretty much like what we recognize by close examination of things in the observable universe but that could be wrong it could be that the observable universe is completely misleading it suggests that their stars galaxies and planets and things of that sort and we imagine that well beyond that there are stars galaxies planets and things of that sort but it could be that beyond the observable universe i don't know maybe there's nothing maybe there's all sorts of other stuff that we've never even thought of that's quite possible since we can't see it yet or perhaps ever depending upon how the universe expands it's difficult for us to say anything our theories are well commensurate with this assumption of homogeneity that what we see here is a good guide to what we see out there you know just like you know you look at one little chunk of a circle even if you weren't looking at the rest of it if you know it's a circle as we discussed before that one chunk gives you a lot of information about other chunks on the circle we're looking at one chunk of the universe right now does it give us insight into other chunks that maybe as nomad nomad asks too far away for us to see and we don't know the inflationary theory allows for on incredibly large distances for the universe to be quite different it allows for us to be part of a multiverse where those other universes might have radically different properties from the properties of our universe so the homogeneity might be a local homogeneity that sort of works in our environment but if you go into other environments that emerge from other universes that come from other bangs in the inflationary theory the physics and properties of those universes might be quite different and that's that's an exciting idea and one that has been harnessed in string theory to give answers to questions like why does the dark energy have the particular value in our universe that it does we've not been able to answer that question through a straightforward first principles calculation in our universe so what have some people suggested well they suggest hey what if there are other universes as nomad nomad notes and what if as nomad nomad's question suggests those other universes have quite different properties and in particular what if those other universes have different amounts of dark energy and moreover what if the spectrum of possible amounts of dark energy out there in this multiverse varies enormously such that in effect every possible value of the dark energy is realized in some universe that's out there now in that totality of universes that would mean that every possible value of the dark energy is realized which means that the value that we have doesn't need explanation it's just one value of the spectrum of all possible values that are out there and therefore it had to be that there was one universe with our particular value and here we are living in that universe the analogy that i like to use it's imperfect but you know if you go to uh if you go to a suit shop you're looking for a dinner suit and they pull one off the rack and it and it fits should you be surprised that they had a dinner jacket that fit you well if they had only one size then that would be incredibly lucky and you should be surprised but most stores don't have just one size they have all possible sizes you know within reason you know they don't have a continuum of sizes they have all possible discrete sizes between some small and large and within that spectrum of possibilities of course there's going to be one that pretty much fits you because they've got all possible suit sizes on the rack so any you know in a realm that had one size surprise in a store that has one size you should be surprised in a store that has all sizes you should not be surprised that one fits you now take that idea to the universe if there's a multiverse then all possible sizes are out there so you shouldn't be surprised at our size by which i mean our amount of dark energy is out there if the universe was not a multiverse if we were more like that suit shop that had one size suit then yeah we should be surprised by the particular value of the dark energy that we've measured it's some strange number that we don't have any insight into but here's where nomad nomad's question helps us out if the other universes exist and if they've had different values for the dark energy we're just one of the collection and that's a potential solution to the amount of dark energy in our world the solution is you don't need to worry about we're just one of the complete collection that is out there all right back to questions on our youtube feed here [Music] rob vandenberg asks where do you stand on the existence of karma or luck well what i would say is we like to use that language and we like to impose those concepts and those concepts as high-level human ideas for some are useful some like to think hey you know i do good to the world and that is good for my karma and some good will come to me and when that good comes people in this way of thinking about things say yeah they're like oh yeah that confirmed it i did that nice thing last year and now this nice thing happens to me and there's a certain kind of coherence and explanatory satisfaction that comes from that way of looking at things if you view karma and luck in that light as higher level human ideas that some people find useful and satisfying and gratifying then i think that's a a fine a fine kind of structure to give coherence to experience but fundamentally do i believe that there truly is something called karma do i fundamentally believe that collections of particles come together and do something good today because collections of products came together and did something that we humans consider good yesterday no i don't i think the particles that make up matter that make up us that make up our brains are doing what they do because of the laws of physics not because of some higher level conceptual framework of luck or karma those ideas are what we humans employ to try to make sense of experience and in that regard they can definitely play a role of helping people make sense of experience but fundamentally no there's no such thing as karma that you'll find in the fundamental laws of physics there's no such thing as luck that you'll find in the fundamental laws of physics particles do what they do because they are following the fundamental laws now whether we have those laws or not i don't know i've said this before it could be that we have those laws or it could be that we don't it could be that we have a mirror mere approximation to deeper laws that will be revealed through experiment and observation and research over the next 10 20 100 000 10 000 years i don't know how long it's going to take but i have confidence that in whatever laws we come up with whatever laws our future brethren come up with i do not believe that fundamentally within the laws there will be anything that we can identify as karma or luck or consciousness or good or evil or wonder or love or surprise or anxiety or any of the qualities that are vital to understanding ourselves as human beings navigating the reality that we are within rather these higher level concepts are manufactured by the human brain or other brains may manufacture them too or other ideas may come from other brains but the point is these concepts are manufactured certainly by human beings because they're useful when human beings come together in communities and want to have an effective and efficient way of navigating the environment and surviving and so these ideas are powerful they're useful but they are not fundamental in the same way that einstein's equations or schrodinger's equations are fundamental they're just different josh bakshi asks are you vegan yes i am vegan i'll leave that at that follow up if you'd like strumento asks can we upload our brain well certainly not today we cannot upload our brains obviously in many sci-fi scenarios people have imagined that you might be able to upload your brain and i don't see any fundamental reason why we couldn't now look there's a big assumption in there i'm assuming that our brains and our thoughts and our consciousness and our sense of self our personal ability to conjure inner worlds of self-reflective experience i believe that all of that is nothing but the motion of particles coursing through a particular structure that is the template within which those processes happen and the substrate within which those experiences take place not everybody agrees with that right i mean there are some who suggest that consciousness is out there there's kind of like a consciousness field that we tap into i don't see any evidence for that can't prove it wrong but i do base my views on reality on the things that we have confirmed through observation and experiment and so focusing upon the things that we've established with experiment and observation it leads me to the materialist physicalist view that i just articulated now if that's the case if the physical stance is correct then in order to upload my brain or yours we just need to replicate the physical structure and we need to replicate the physical processes taking place in that physical structure so if you're able to do that then i don't see any reason why the resulting artificial manufactured structure would not think and feel exactly say as i do if it was modeled upon my brain at this particular moment and similarly for your brain can we do that no it's hard will we be able to that one day i don't see any fundamental obstacle but whether we'll gain the expertise to do that with adequate fidelity to really reproduce the brain that's inside your head in my head right now i don't know but fundamentally i don't think the laws of physics as we understand them stand in the way of that [Music] um so gulab asks what are time and space and can strengthen your answer this at a deeper level so i don't know if gulab if you were with us in the first hour where we uh had a conversation with string theorist cameron vapha and that was one of the things that that we discussed and as kumam was saying you know there are a lot of insights into the nature of space and time that are coming from string theory one of the most striking is one that we're going to do a full world science festival program on in the in the coming weeks has to do with quantum entanglement which is the answer to an earlier question on the nature of bell's inequalities that that i described before but quantum entanglement suggests that the threads stitching the fabric of space may actually be the threads of quantum entanglement that in some sense connect distant particles in the manner that i described before so that would suggest that we are heading toward a deeper understanding of space and time because we are beginning to identify the threads that stitch them but as kumra and vaffa describe descriptions of space and time in one domain and one theoretical framework can be distinct from and even contradict the descriptions coming from other corners of the illuminated landscape of string theory so it's not to say that we have an answer the answer for what space and time are much as kerman said it may be that the best we'll ever be able to do is say in this in this circumstance according to this and this way of describing that circumstance mathematically here is how you should think about space and time and then in the same or subsequent breath we may say but in this circumstance according to this theoretical framework using this mathematical articulation this is the way you should talk about space and time and it may not be that one of those is right and the other is wrong i mean that both are right it may be that both are right and complementary and maybe the fullest story comes from this mosaic this amalgam of distinct descriptions and it's only by taking in the full mosaic that will have the deepest and fullest understanding of what space and time are okay i'm gonna head back here to uh your youtube questions so i know that a lot of you are watching in different different channels uh although i'm focused right now on the world science you youtube i probably should have said that those of you who are watching on the world no no the world science festival youtube it's very confusing isn't it we have a lot of properties out there so in the world science festival youtube that's where i am right now ask any questions that you want if you're on the world science you youtube i'm not looking at those i can't look at multiple feeds but maybe someone on our team will pick out some questions there and add it to my alternate feed coming from the social channels so it's up to you where you're looking at this but uh i am focusing on the world's high special youtube feed and given that what do we have here okay so daryl dawkins asks how is it possible to express the age of the universe using years since years refer to the earth going around the sun how are the years calculated before a star even existed yeah good question right so we talk about three 13.8 billion years what does that mean if you're talking about a measure that's tightly tied to the motion of the earth around the sun and the earth's only been going around the sun for the last five billion years so what we do daryl is we use the duration of year as defined by the duration it takes right now in the common era for the earth to go on a full orbit around the sun we use that duration as our fundamental scale of time and then we use our mathematical equations to work out the duration back to the big bang and we express that duration in terms of how many times an earth would have had to go around the sun at the speed that it currently does and in that way we express the duration back to the big bang in this particular unit called the year we're not literally imagining that there were 13.8 billion orbits of the earth around the sun since the big bang because as you rightly point out for instance at the moment to the big bang the earth didn't exist the sun didn't exist rather we're being a little bit more abstract than that we're imagining the earth going around the sun is just providing us a definition of a standard duration called the year and then relative to that standard duration we calculate the time back to the big bang and calculate how many times the earth would have had to go around the sun to exhaust the duration back to the big bang and that's where the 13.8 billion years comes from and you know there's a related question to that which is you know we learned from special relativity that time varies depending upon your motion and the general theory of relativity we learned that time varies depending upon the gravitational field that you are experiencing so when we talk about 13.8 billion years what do we really mean by that which clock are we using are we using a clock that is in motion and therefore we differ from other clocks and so on and the answer is going back to the question before we imagine that the universe is homogeneous and isotropic which means that it looks the same in more or less every direction if you average things out and every chunk of the universe that's sufficiently large on average looks like every chunk of the universe and because of that uniformity we imagine that we have clocks that are experiencing that average motion of expansion that average gravitational influence of the things in the environment and because the averages are the same everywhere every clock is experiencing the same relativistic effects and therefore every clock is taking off time at the same rate so using those universal clocks that are experiencing the average density in the average expansion motion of space we are calculating that the duration back to the big bang would be 13.8 billion years that's what we mean by that all right few more what time is it now we've been going about two and a half hours so i'm gonna probably wrap this up relatively soon as i'm i am starving and thanks for coming at this different time by the way so as i mentioned last time that we met i have to teach at our old time of 2 p.m eastern standard time so this will be our new time going forward i think uh who knows maybe we'll move it around and get people in different time zones i don't know if that'll be more confusing than it's worth but we'll see so um a few more questions though uh arnov asks where do you teach i teach at columbia university columbia university in manhattan and it was just announced that columbia university is going to be fully online for the undergraduate education so i will be teaching fully online so in principle yeah no matter where you are you can take my classes i don't think it works that way though if colombia gave every class away like that they might have i don't know trouble attracting students to pay tuition especially if students are not coming to campus wow difficult interesting questions that people need to deal with in this era but yes i do teach at columbia university and i'm actually teaching a course i'm not teaching a technical course so maybe you guys wouldn't even be interested i am teaching a course on my new book on the book until the end of time and i do take the students right from the beginning of time to the end of time and there's a bright spotlight that we shine on the emergence of life the emergence of mind and that spotlight continues to illuminate what conscious minds do they develop language they tell stories they develop myths they develop religions they engage in artistic and creative expression and they develop science to try to be able to tell the most accurate story of the objective physical reality that allows for all of these life like activities to take place so that's what the course is about sort of a blending of science in the humanities and um yeah maybe you guys if you're interested some of you can audit that course i don't know how that works so i can't promise that but uh sure that's what i'll be teaching starting uh in a week or two all right what do we have here time traveler asks who first came up with the idea of string theory and how uh the very initial ideas of string theory i think are usually credited to the work of a a very nice physicist friend of mine known him for decades gabrielle veneziano and as the story goes gabrielle benesiano was trying to understand experimental data in the 1970s or even 1960s i'm sorry yeah having to do with a strong nuclear force and noticed a pattern in the data of the interactions of strongly interacting particles that could be described by a particular interesting formula that he wrote down and others i guess may have written down too and when other physicists began to study this formula that was very good at describing the data of the strong nuclear reactions they studied this formula and this is people like leonard suskind and people like i think holger nielsen and and others at the time they began to see that in the mathematical description of the nuclear force you could see vibrating filaments in the mathematics or the qualities of vibrating filaments were embedded in the mathematical description and this began to lead people to think about a theory based on strings and it's interesting because the data that gabrielle vanessana was trying to describe was subsequent subsequently and i was wrong with my speech at the moment i guess two and a half hours in i began to get dry mouth so hang on half a second here the data that's better the data that gabrielle veneziano was looking at was subsequently described by a completely different theory the theory of quantum chromodynamics so the motivation for the formula that beneziana wrote down was undercut by these subsequent developments in quantum chromodynamics but nevertheless the formula had already been written down and the conclusion that that formula described vibrating strings had already been met or made is perhaps a better word and so string theory carried on even after the motivation to describe the nuclear reactions evaporated and that mathematical endeavor ultimately revealed a vibrating string that had the properties of the graviton the hypothetical graviton you could see that right inside the vibrating filaments which themselves were right inside the formula that veneziana wrote down and with that people started to think wait if you got a graviton this must be a theory not of just the nuclear force but of the gravitational force and that's where the development of string theory as a quantum theory of gravity really took off excuse me okay a few more questions and then we will call it quits for today so anas asks does the romanesian summation have real life implications and and i guess it depends on what you mean by real life but a lot of the insights of ramanujan have had a role say in string theory string theory of real life i don't know but certainly the idea of the number of dimensions in string theory the the ideas associated with certain mathematical formulations make use of modular forms whose properties go back to insights of romanesians so i would say that a good deal of the mathematical framework of string theory has an intellectual root that trellis is back it's probably the wrong verb there intellectual root that kind of digs its way back to the insights of of ramanujans that's a very interesting and exciting link between abstract mathematical ideas and those of cutting-edge physics okay um [Music] uh abhishek asks can more than two black holes merge to form a single black hole absolutely so if you have a collection of black holes that that come together ultimately as they when they come together they will shake the fabric of space but as those merging black holes ring down as the fabric of space carries away enough energy from that collision they will coalesce into a single big black hole so yes although we often describe the merger as a binary operation between two black holes in principle it doesn't need to be that it could be sequential right two black holes come together yield one and then a third black hole joins with those to create an even larger black hole or it's unlikely but it's conceivable for three black holes to just come together in principle there's nothing that would prevent that from happening lahanash asks can quantum entanglement be used to transfer information faster than the speed of light and i think most of us believe that the answer to that question is no and i understand exactly where that question is coming from because as i described before if you have these two distant entangled particles that are kind of in this state of entanglement where say one is up and one is down you measure one the other pops out and spins down that according to the math of quantum mechanics is an instantaneous process so you measure the particle in new york you find it up at the same moment the particle in california spins down it feels like there is an instantaneous transfer of information but actually there's no information to be extracted whatsoever and the reason is because look in new york let's say you do this experiment over and over and over again and you look at your data what would your data be well you go and you measure the electron in new york spinning up on the first run and then you measure it and spinning down on the second one and you measure in the thermal spinning down on the third one and spinning up on the fourth one so you have a random sequence of ups and downs and ups and downs in new york if you look at the data in california it will also be a random sequence of ups and downs but the only interesting fact being that when this one in new york is up this one in california is down when this one in new york is down this one in california is up now that's an interesting correlation between the data but you can only recognize that correlation when you bring the two data sets together you get on the phone you talk to the person at the distant location you say hey my data was up up down down what was your data they say hey mine was down down up up you're like whoa complete correlation when yours is up mines down and vice versa or you could take your data set and you got an airplane you fly to the other side of the country and you can pay your data set in person my point though is you have to do that comparison in order for you to extract anything informationally interesting about the data but that comparison is through an ordinary classical slower than light channel so the only time you extract anything informationally interesting about these two data sets is through that slower than light comparison and that's why there's no faster than light information that's being transferred or can be extracted from quantum entanglement okay i think guys i'm gonna wrap it up today i uh am still getting my [Music] my ability to go three full hours back into shape right i have to sort of exercise more to do that so thanks for joining us today and i i don't know when our next session is it's either next friday or the friday after sort of play it by year some of these times we'll have a guest as we did today and many of you thought that was interesting last time and let me know if you thought it was interesting and useful today i i had a good time speaking with karma bath carmen bath about his book and uh next time we'll venture forth into again whatever directions you'd like to go i i love to hear from you guys through social media again i'm b green at on on twitter i'm b green at facebook i think i'm b green too but i haven't used it in a while so i have to figure that out you should subscribe i don't know where the subscribe button is it's like somewhere at the bottom of the screens here i'm not going to try to use oh is there a subscribe button i don't know i hope there's a subscribe button down there but uh if there is a subscribe button do join him because then you'll be alerted when these sessions and other programs are happening and uh yeah keep keep abreast of what we're doing we're going to have some new courses coming out in world science u we're going to talk to many of the faculty on world sciences whose courses we will be releasing and i'm always here to answer any of the questions that come to your mind be they in quantum mechanics cosmology general relativity basic physics string theory as you see i'm happy to talk about certain aspects of philosophy religion consciousness mind brain if you find what i say interesting fantastic if you don't that's fantastic too but i hope that you do and uh until next time then uh keep your interest going keep the questions coming and i look forward to uh having our conversation either next week or the week after until then take care you

Channel: World Science Festival

Views: 52,516

Rating: 4.8841629 out of 5

Keywords: Cumrun Vafa, Lessons from String Theory, calculate black hole entropy, Puzzles to Unravel the Universe, black hole, string theory, Harvard University, Dirac Medal, Eisenbud Prize, Breakthrough Prize in Fundamental Physics, World Science U, University, science unplugged, New York City, NYC, Physics, Stephen Hawking, Albert Einstein, Quantum Mechanics, General Relativity, WSU, World, Science, Festival, Brian Greene

Id: GfRJZbsywPQ

Length: 153min 3sec (9183 seconds)

Published: Fri Aug 21 2020