Brian Greene and Priyamvada Natarajan: World Science U Q+A Session

hey everyone welcome again to one of our live sessions live q a and discussion session here of world science you world science festival your daily equation whatever you want to call it a hybrid of all of the above and as advertised i think some of you saw that we're going to have a conversation about dark matter and you know second part of the live q a today where we're going to talk about some wonderful new research that has been spearheaded by a friend of mine and colleague from yale priya natarajan and we'll be bringing her in and having a conversation about some ideas that at least have the potential to upend our understanding about dark matter and that's that's really exciting i mean dark matter as we'll discuss in that conversation coming up has been around as an idea for nearly a hundred years well actually references to dark matter turns out go even further back than a hundred years as it turns out as we may discuss but still we don't know what the dark matter is and in fact we're not even completely sure that the dark matter exists so that'll be part of our upcoming conversation begin with a little bit of a little bit of news you may have heard this exciting and curious result that may hint at the existence of life on venus the discovery of phosphine in the atmosphere venus now look when we think about life in the solar system the dominant place that we've long turned as a potential habitat for life beyond earth is mars right i mean you see that in in so many science fiction episodes obviously we've sent rovers to mars in search of evidence of life venus has always been looked at as a much more forbidding place i mean there's a good reason for that right i mean the temperature is it's extremely hot so close to the sun it's got this atmosphere that holds in the heat over 800 degrees i think very high pressure but some scientists are at least suggesting and i don't think anybody's going out on a limb and saying that this is proof of the existence of life maybe people are saying that i haven't really heard that but the mechanism by which this particular molecule phosphate can be produced in the abundance that appears to be the case a current living system seems to be the best explanation that people can come up with so that's tentative it's not very indirect nobody scooped up some surface material on venus and located microbes or anything of that sort but think about how exciting that would be in fact i'm just curious for those of you who are willing to share your thoughts maybe in the chat or something do you think that there is life out there in the cosmos beyond earth that's big question number one is your life out there and big number question number two is do you think there's life in the solar system be a venus via mars be it enceladus europa whatever the many places that have been suggested as possible forms for life but the third and most important question i see most of you are saying yes life exists yes we think it does obviously for question one but it's not so obvious for question number one right i mean it could be it could be that life is this incredibly rare occurrence that even though there's so many planets around so many stars in so many galaxies if you call that number n and the odds the probability of the requisite conditions for life occurring if that is much much smaller than one over n then it's not so obvious but yes i certainly share your intuition i would have voted yes on question number one but here's question number three at least we have one no on question number two question number three life is one thing intelligent life a completely different category of life i mean they're seamlessly connected but the discoveries would have a radically different impact i mean there's just so much of a conversation you can have with a microbe it's an interesting and powerful moment in the history of scientific discovery to be certain that life exists on other worlds but to be able to have communication and to share knowledge and and heritage and insights about the nature of the world to no longer be having what might be called a species-wide monologue right we humans you only talk to ourselves i mean yeah sometimes to talk to other like i talk to my dog right so i don't mean it in in a very strict literal sense but the kinds of communication where there's a back and forth where the information is truly being shared as equals we tend to do that only from human to human and if we could do that human to other species on other worlds wow right what an amazing moment that would be and more than that more than that here's the thing all life on planet earth shares basic structures shares the basic way in which genetic information is stored in dna it's the same code the same codons across effectively all life the same energy extraction mechanisms the same energy distribution mechanisms for the most part i mean there are some outliers but for the most part it's all the same it would be spectacular to come upon extraterrestrial life in which it was just completely different some different method of storing information that has been built up over the course of species or maybe there isn't even an information storage mechanism maybe it's completely different i don't know but that would really be the most amazing thing so yes that's exciting news who knows if it'll stand up to further scrutiny but that would be a wonderful moment if the we could really locate life on another world like venus i should say you know it's a course i teach where i have the kids read ray bradbury short story ray bradbury always was ahead of the curve and thinking about the way the world is and might unfold and i don't know if you've read this short story i think it's called the longest day anybody read that short story ray bradbury fans out there read the longest day i think that's what it's called but it's a story where the explorers are on venus and they set up you know colony and they've been there for a long time so virtually none of them have a memory of planet earth but then an interloper from earth arrives and joins a a classroom a school girl joins a classroom and the sun comes out once every seven years on venus and they're all preparing for this miraculous moment where the clouds and the rain the incessant rain will finally clear away and this one student this girl who'd come from earth you know a while back she remembers what the sun is like and the other students are all jealous so they lock her in a closet for this once in a seven year appearance of the sun on venus during this like one hour respite from the rain and the clouds it's a it's kind of a wonderful kind of a wonderful story so you should you should check that out all summers in a day yeah yeah yeah thank you rayna pratt i had the the title wrong it didn't sound right to me the longest day all summer in a day is that story absolutely a a hat tip to reign at pratt i appreciate that all right so let's um let's get to some questions if you uh if you have any physics math universe anything at all i seem to have driven everybody into uh into a direction that's somewhat off topic um all right so um some weird questions tribe sale does death hurt like physically to the dying i presume it depends on the method by which death arrives so i don't really have a whole lot to say about that uh what else we have here uh susanth der garaju can you explain kalabian manifolds sure vaguely i can in fact i don't know maybe we can even bring up a picture of a clapia manifold while i'm giving this explanation that's always fun when we can do that in real time but collabion manifolds are a mathematical description of the extra dimensions that naturally come from string theory so many of you are familiar we've been trying for decades to realize albert einstein's dream of unified theory of physics and many people i'm included in that think the most promising proposal that has been developed in say the last 50 years is this notion that the fundamental constituents of matter are not little dots they're little vibrating filaments and that small change this is an amazing thing right that small change from a dot like particle to a vibrating string like particle if the string is small enough it looks like a dot that little change winds up allowing us to put gravity and quantum mechanics into a consistent mathematical framework which has been the stumbling point for a very long time in trying to come up with a unified theory of physics okay good but here's the thing when you study the mathematics of string theory you find kind of amazingly and remarkably into some you know this is uh the death toll of the theory for some it's the most amazing insight of the theory depends on your perspective but the math requires that the universe have more than three dimensions of space if string theory is to be right so we all know about left right back forth up down it's like the dimensions around me they're around you we move through them freely in day-to-day life those are the familiar dimensions of space the math of string theory demands more than those three dimensions of space so the natural question that we've struggled with for a long time is okay right if this theory has any chance of succeeding we better explain where those extra dimensions of space are where do they reside and one answer and look this is what i worked on as a graduate student and it's been around for a long time is this so you got the three dimensions of space which are like this grid here but if you go smaller and smaller and smaller according to these ideas you find extra curled up dimensions right so this animations those dimensions curled up into the shape of a circle that's just for ease of visualization and the idea is if a microscopic being is walking around in that environment they'll know about the big dimensions that you and i know about the grid but look the microscopic bin can also walk around the curled up dimensions so small that we don't see them but the math of string theory doesn't say that the extra dimensions are these little circles and now we come to the question that was asked this is the kind of shape that the math of string theory seems to require for the extra dimensions of space and these are known as kalabi yao manifolds there's math behind this these are complex killer manifolds a vanishing first churn class but you know if math is not your thing that's fine this picture is really worth whatever i've almost said a thousand words already this picture shows the shape a possible shape for the extra dimensions and string theory and those beautiful curled up components of space represent the extra dimensions of the columbi yao manifolds now when i was a graduate student when i was starting out in the 1980s when my hair was dark and the hairline was further down you know looked like a young man and thank you for all these who remind me that i'm aging as we all are frankly but as a young man we only thought the very first paper written on columbia manifolds only had five examples i showed you one picture but there were five known examples of columbia manifolds and the beauty is that the math of string theory linked very precisely the shape of the claudio manifold with the physics that we would observe like the mass of particles and the strengths of particular forces depending upon the size of the columbia manifolds and so forth so the dream was and in some sense still remains that if we knew the exact shape of the curled up component of the universe we could use that shape to make mathematical calculations of physics in the world that we can observe so you can't observe the shape directly it's too small but the shape casts a shadow into the realm of physics that we have access to that was the idea so as a graduate student i and a couple friends of mine kelly kirkland paul myron our advisor at the time graham ross set out to calculate for a given shape of the extra dimensions the physics that we might observe and we picked the most promising of the known shapes at that time and the physics that we extracted wasn't wasn't so distant from what we observed but it was not really possible to do the calculations with enough precision and to link things up precisely and it was awkward it was tricky you had to jump through hoops to make these shape this particular shape i should say compatible with the physics that we observe so the thought was well look you know just go through the rest of the shapes only four others at the time and maybe one of those does a better job and is bang on accurate in its predictions of how the world actually is the problem is over time in the 90s and 2000s the number of columbia shapes again i showed you that one beautiful purplish blue pinkish picture the number of shapes has exploded in terms of people mathematically discovering different shapes that meet the equations of string theory so that now the number of shapes is not like five they grew to a thousand into ten thousand a million it's exploded to numbers that are not astronomical would be a paltry description they're far bigger than astronomical 10 to the 500 is a number that people kick around and we have that many shapes obviously you can't examine them all so some have viewed this as the collapse of string theory because now there's so many different shapes and you don't know which is the right one therefore you never can make any precise predictions is the thought but others like me say hey it's early in the game we've been at this for decades sure but it's early in the game when you're talking about a theory that might address the deepest questions of the ages right why the universe is the way it is right that's the big question of them all so many of us just say hey we got to work harder got to keep doing our mathematical research you got to keep trying to find indirect ways to link up the unobservable qualities of string theory to the observable qualities of the world and that's that's that's really where we're pressing onward so that that perhaps susanth was a longer answer than you expected but that that's what a claudia manifilter is the curled up extra dimensions of string theory and that's the role that they play in our understanding of the world okay uh andrew hanny sir we are waiting for a general relativity course on world science you by you yeah i know i know i know i feel guilty i sit here with my head hung low it's hard to put these courses together because i like to do it right and look one way i could do it i could i could just do it completely and formally out of board i don't know i just feel like i want to do it with the right visuals i want to do it with the right interactive demonstrations so you can get a visceral feel for things like curved space and geodesics and the nature of curve time warp time so it's just a lot of preparation but i am thinking that this fall since i'm not going to be doing any traveling obviously at least i'm not going to be doing any traveling this fall this may be a time when i can carve out the necessary time to to get it done so andrew i'm i'm it's not it's not lost it's just uh taking a little bit longer but yes that's it's on my mind all right neil world science festival more physics panels please dr brian keating yeah i like brian kitty he's great um uh oh when is the next world science festival good question neil so we're gonna start rolling out world science festival programs done you know in our look we could have just knocked out the world science festival programs by having a collection of zoom conversations replacing the stage conversations but zoom conversations again are just hard to watch after a while they're kind of limited in their visual appeal the content can be good of course but you know us we like things to to really be visually compelling and and not for bells and whistles sake but to really try to communicate the ideas in the most interesting and most impactful way possible so we have spent we took a breather when we had to cancel the live 2020 festival and have been thinking about ways to bring programs out in to the digital space that will have the same kind of impact although it will be different obviously but the same kind of impact as our live programs in theaters do and i think we've come upon some pretty good ideas and we're going to be implementing those in the coming weeks so i don't know probably by mid-october we'll start to release world science festival programs and i think you're going to find them an exciting collection of topics done in a way that is visually interesting and impactful so so that's that's that's the uh the status of the world science festival very much in production and very much on the verge of being available so just hang tight and all that cool information will become in fact i think the first program that we're going to release if i'm not mistaken is a program that examines the question does math reveal reality right again you guys can chime in on this you know is math something that we humans come up with in order to basically encapsulate the patterns of the world that we encounter through observation and experiment or is math out there right is math actually out there and there are a variety of views on this my view is more toward the former that we come up with this stuff but the conversation will be with philosopher david albert philosopher shelly goldstein who have different views on these ideas i mean shelley in particular i understand believes that consciousness transcends mathematics so there's something in the world that is not accessible to the reach of mathematics and therefore it's a limitation on math from his perspective i do not agree with that but we'll argue it out we also have max tegmark many of you know max through his books and various uh signs for the public undertakings that max does and max has perhaps the most extreme position that all math of any stripe is real it's actually out there so any math that you find in any textbook anywhere or any paper however esoteric that paper might seem max would say you're actually describing something not that came from the human mind you're describing something that's out there in the world and i look at that i'm like seriously but yeah he's serious and he's got arguments for this and we'll discuss it and you know i will respond and you know philosophers like david albert if you know him from our other programs david albert does not suffer fools right and he does not take in you know it'd be kind of interesting to have a debate between donald trump and david albert and that would be that would be a major mismatch of the minds maybe it would be too much of a mismatch sort of like i don't know if einstein was to talk to an aunt about general relativity that's not a bad analogy i hope the secret service is not going to show up at my door now but um yeah so so how did i get there so my point is that david and shelly and you know we'll be really pressing max to see whether his argument that all math is real stands up to to scrutiny uh sylvia jonas will also be in that program philosopher of math who has spectacularly interesting ideas about where math comes from and the role that it plays in in human thinking and society so anyway that's the kind of stuff that uh will be coming your way in the very near future okay let's see what else we've got here on the uh the live chat window here all right there's just one that i like but it then i touched my screen there it is uh uh raj oven on perez green can the imagination of human mind be faster than the speed of light well so there's no process in the human brain where information is communicated from place to place faster than the speed of light but um when you talk about speed you better have a very clear sense of some kind of entity of some sort starting at one location ending at another location and you are determining the distance that it traveled divided by the duration that it took to complete that journey and if you're talking about the human imagination it's not obvious to me beyond the physical biological physiological processes in the brain i don't really know how to measure a speed because i'm not in the context beyond the actual physical of the processes i'm not in the context of something going from here to here a duration in in particular duration so i'm sort of not sure how to answer the question um um let's see uh arjun can we compare one over zero to singularities and yes i think that's a very good comparison so oftentimes when we're talking about space-time and black holes the general theory of relativity the kinds of ideas that emerge we talk about space-time singularities as a quality that can infect a solution to einstein's equations a black hole right at the center point the r equal to zero location the metric that is the distance relationships on that space blow up it's a singularity and when we talk about that singularity what do we mean and exactly as you suspect arjun the one over zero is a pretty good model of what we're talking about and when we talk about the curvature of say the black hole solution right the curvature goes like one over r to a particular power thing is r to the sixth i could don't quote me on that but i think that's the right number which means is r go to zero as r goes to zero you do get something that looks like one divided by zero so the curvature blows up in exactly the way that you suggest so yes that's right alexander seber brianski talks about minus 1 12 equaling infinity and yeah that is a that's a curious it's a curious fact that emerges in a lot of different areas of physics certainly in string theory it plays a very prominent role and what alexander is referring to is if i ask you the question and many of you are familiar with this from from youtube or i've seen it like as a meme on social media but if i ask you to add up 1 plus 2 plus 3 plus 4 plus 5 to infinity you know naturally we would say well that must add up to infinity right i mean it gets bigger and bigger and bigger but we in physics and mathematicians more generally often find a way of making sense of certain kinds of infinite expressions by fitting them within a larger rubric of ideas where you don't just look at one plus two plus three up to infinity but you look at something similar to that for instance imagine you look at one over n to the s as s goes from one to infinity and notice when s is minus 1 1 over n is just n so that function is called the riemann zeta function the riemann zeta function at -1 is the sum that we were looking at but the riemann zeta function you can evaluate it not for s equals minus one you can evaluate it for any value of s and then if you're nearby to minus one you can take a limit as the parameter goes to minus one and define if you will the zeta function at minus one to be the result that you get from that limit if you excuse me follow that procedure you find that the zeta function at minus one does add up to minus the twelfth and that's the way you can get one plus two plus three plus four plus five up to infinity to sum to minus a twelve so it's kind of a weird idea but that's the weirdness of infinity and it's the way you contain certain infinite quantities and yeah in string theory the quantum mechanical vibrations of a string winds up including that particular sum and that minus 1 over 12 it turns out if you multiply that by a half it gives you minus 1 over 24. and flip that upside down that minus 24 is actually turns into -26 from using the fact that you can have dimensions that are space-like and time-like and if you take that all in stride that's where the 26 dimensions of the bosonic string comes from it's effectively minus 1 over 12 times a half flip that guy upside down subtract 2 from it get minus 26 and then the 26 the magnitude of that is the number of dimensions and a similar calculation gets you the 10 dimensions of the super string the super symmetric version of string theory so yeah these calculations really do matter and maybe sometime i'll do an actual video explaining that i i can't imagine that what i described was clear unless you've sort of encountered it before but the bottom line is certain infinite quantities can be defined in a way that's completely rigorous and sensible and gives them a finite value okay so what do we got here on the uh let me look on the list of questions that we got on here um so udvash mainly asked a question that we will take up in just a little while which is what is dark matter you know is it an object is it a particle is it energy is it both so yeah that's a good question it's dark so we don't see it we believe it's there by virtue it's gravitational influence as we will discuss but nobody can answer that question yet that's the big question whoever answers the question what is the dark matter or whoever proves that the dark matter paradigm is wrong and there's a new explanation and proves that explanation they they will be getting a trip to stockholm so this is a big deal and that's what we will be talking about shrindi cabra please explain why string theory requires more than three dimensions gosh i wish i could i mean i know why it requires more three dimensions of course but to explain that in non-technical language has always stumped me i mean i can give the rough idea you know when a string vibrates okay if the universe say only has two dimensions of space so that strings vibrations are only say like on a flat table top the number of distinct vibrations of course is less than if that string can also vibrate in a third dimension which is less than if the string can also vibrate and a fourth dimension or a fifth dimension you can't picture that one but you can get the pattern that the more dimensions there are the more distinct vibrations a string has access to and it turns out in essence that there's an equation in string theory that requires the number of distinct vibrational patterns of a certain energy say to equal a particular value for the theory to be mathematically consistent so if you then start to examine a universe with two dementia like oh no it doesn't work math doesn't work there aren't enough distinct vibrations for a string i say okay well look let's go to three dimensions we look around there are three dimensions and you have more and you're like oh darn yeah there are more vibrational patterns but still not the right number not enough to satisfy the constraint of mathematical consistency within strength you say well okay look let me be maverick about this let me think outside the box let me imagine this is four dimensions of space yet more vibrational patterns of a string okay does that saturate the equations does that meet the constraint no it's still not enough and you keep on upping the number of dimensions increasing the number of vibrational patterns accessible to a string until it turns out wonderfully that when you get to 10 dimensions it all works the math comes together and that's not obvious that there'd be any number of dimensions that would work but it works that's impressive but then you're faced with the question that we started out with which is if you take this seriously that the world has to have more dimensions of space then you have to explain where those extra dimensions are and that takes us right back to the question about columbia manifolds that we started out with so i can't explain why any particular number of dimensions is singled out by string theory that that's truly from the mathematics but i hope that gives you some semblance of an intuition about why more dimensions are required okay that's a good question um what else do we have uh martin topinka from prague can you explain how a photon can reach infinity and beyond in the conformal cyclic universe yeah so again it's a tough question but distances in a universe are measured by something known as a metric i mentioned it before the general theory of relativity and different metrics yield different notions of distance and there can be certain metrics that are such that you can reach the infinity of space from the standpoint of say a euclidean observer they'd say that's infinitely far away but this other notion of distance that comes from this conformally scaled metric can yield a finite distance so it's it's it's basically the idea that infinity in some sense is in the eye of the metric and it's the eye of the person who commits to using a particular metric and that's why you can have different notions of which part of the space are accessible versus those that are not it's totally dependent on the way in which you measure distances totally dependent on the so-called metric of general relativity and certain metrics have counter-intuitive properties allowing you to say get to infinity in a finite length of time okay um let's see what else we have what time is it oh i should uh i know we have a little time let me just do a few more questions before we get to our dark matter conversation let me see what kind of uh cool things you guys are asking about um um uh let's see an ankit kumar says there are 56 dimensions according to the vedas uh yes yes on kit i sort of know that not that particular number by any means but i may have mentioned in previous conversations that my brother who's a writer and a filmmaker but he is also a hari krishna devotee has been since the 1960s before many of you were even born and over the course i mean if you read my latest book until the end of time i do mention this in the nicest of ways but basically over the decades anytime sort of we would find something interesting in strength here physics and my brother and i who we have sort of sporadic conversations we we don't speak every day by any means and there are long stretches when we don't have a whole lot of communication but it has been interesting it was more in the old days than more recently that when we'd come up with some discovery he would say aha we know that already it's like in this or that vedic text so i was curious about this and i and i did a little bit of research i can't say i became in any way shape or form a vedic scholar you know but i was just curious and and the connections that i found that he was referring to and maybe i misinterpreted so if you guys want to correct me please feel free but the connections that he was largely finding were poetic connections metaphorical connections like when we talk about the number of dimensions in string view it's coming from a rigorous mathematical equation that i tried to give you a sense for that equation in words but of course it's just so far words can take you but when you see the various numbers and the various ideas in the vedic text they don't emerge from that kind of a systematic inevitable chain of thinking as far as i can tell so so there is a poetic resonance between some of the ideas of modern physics and some of the ideas in sacred text which is which is kind of cool it's interesting but that cannot really be taken as a point of literal contact between the rigorous science and the more poetic metaphorical sacred text descriptions because look you know i'm fond of saying if you're interested in the magnetic moment of the electron you can calculate that with quantum field theory to 10 or 11 or 12 decimal places and there's nothing in any sacred text that i have ever seen that allows you to take insight about the external world and push it to that degree of precision and and and so that's what i mean by these connections are lofty high level poetic metaphorical whereas the connection that would get us fired up i mean really fired up if you go to an ancient sacred text and you're like there it is there's the calculation of that electron's magnetic moment and there are all the digits right there in the sacred text and here's how the sacred text came to that number wow then everything changes but i i don't suspect that will ever happen and that's why these connections are interesting and i find them beautiful i find it beautiful that certain ideas that modern physics has found interesting and powerful some of them have a poetic home in other ways of thinking about the world that's cool that's great that's inspiring but it's not as though it's the same and that distinction is important to keep in mind so summon name b says why do we need calculations for it to be true i'm not saying that you need calculations for something to be true i am saying that different modes of inquiry into the world come with different yardsticks for success and different acceptable modes of analysis and when you're talking about the scientific approach to the world the mode of analysis is mathematics and careful observation and experiment and calculation that attempts to explain the things that we see with mathematical precision that is one arena of human discovery and exploration if you're interested in the kinds of issues that the world's theological approaches tend to focus upon then your yardstick for success is simply different it's not better it's not worse it's just different and for most people a religious perspective on the world is judged if that's even the right word to use it's often not but is evaluated by the degree to which it feels right the degree to which it enriches life the degree to which it provides for some some kind of notion of value or meaning or a purpose and in that arena no you don't need to calculate anything that's not my point that i was making before my point making before is if you try to say these two modes of inquiry are in some sense the same they're not i don't think but if you try to say they're the same because they both use the word vibration or they both talk about extra dimensions if you try to say they're the same now they're no longer distinct modes of human inquiry and if they are meant to be the same you should investigate that you should interrogate that suggestion and to interrogate them being the same means take the modes of one and see whether they are relevant to the other and calculation is relevant to the scientific approach and i the point is making is not relevant to the theological approach and that to me is strong evidence that they're not the same there can be poetic resonances but they're not the same and that's the point that i was making okay um aguilat asks is our future and our present and our past do they all exist do they all exist simultaneously and look the bottom line is i'm not sure nobody really is but there are arguments that many of us find convincing that go really back to einstein that what one observer considers to be the present can be another observer's future and it can be a different observer's present if these folks are all moving relative to each other and so the subjective nature of the present and the past and the future suggests that the sharp demarcation that we usually make from the perspective of a single individual i'm here right now right that's now the clapping of my hands is now right sorry if i heard your ears right the past future and sometimes abound this moment but are not real or not as real as this moment that's how it feels to me as an individual but if the past is real to somebody else and if what i call the future i should have said what i call the past is real to somebody else and what i call the future is real to somebody else if they're moving the right way then who am i to say that my very subjective notion of what's not real holds any more weight than anybody else's subjective notion so one way to deal with that is to say that everybody's subjective notion is on an equal footing and if that's the case then things that i call the past are real to somebody else so they are real things that i call the future are real to somebody else and therefore are just as real as what i consider to be real which is the present and that's what einstein basically meant when he wrote to the widow of michaeli besso right that was his friend that he had innumerable deeply important profound conversations over the course of their 50 60 year friendship when michaeli besso died einstein wrote to besso's widow saying that you know he's departed this world a little ahead of me but to we convinced physicists the distinction between past present i'm going to say that again because i got a little getting choked up really really no it just had a little catch in my throat to weak convinced physicists the distinction between past present and future is only an illusion however persistent and and and that's a again a very poetic way of saying what i was describing from from special relativity so in that sense yes it's possible that the past present and future all do exist together okay so uh zachary asked how did we get dna that's a good question zachary a vital one and if you're interested in a more fleshed out version of the answer about to give again get my book from a library i'm not a salesperson for my book you know me i hate that kind of stuff but this is something that i go into some detail in until the end of time because i wanted the origin of the universe and the origin of life and the origin of the fine structures that life requires and the origin of mind and then of course the cessation of all of those processes i wanted that to kind of be put into a coherent framework so that is something that i discussed but briefly put we're all familiar with darwin's evolution by natural selection but most of us learn that i certainly did in a biology class in its relationship to the many species on planet earth so the question darwin addressed is how could there be such a broad range of species and of course the answer he gave is that over time there can be mutations in the genetic makeup of a given individual member of a species and if that genetic mutation makes that individual better adapted to survive in the environment then that individual is better or more likely to produce progeny who will inherit that mutation and spread that mutation widely and over time the accumulation of many of these mutations can yield gradual transformations that ultimately result in distinct species and if you allow this process to play out over you know billions of years time skills that are very hard for the human brain to fully process then starting from some uniform beginning you can imagine species branch off through these accumulated genetic mutations now that's evolution by natural selection at the level of life but as i emphasize in my book and it's it's not my idea but this is well known there can be a version of evolution by natural selection not at the level of living species there can be a version of that that comes in at the level of molecules so in some sense molecules can battle molecules in an effort if i anthropomorphize to gain access to the greatest amount of raw material in the environment and so molecules can then mutate once they learn to replicate this is the key thing that i should have stressed at the outset of this answer once a molecule learns the trick of replication it can dominate the environment if it learns to replicate better than other molecules faster than other molecules more stable than other molecules so over time mutations in that replication process can yield ever better adapted molecules and over time those molecules and therefore become ever more complex and when a molecule not only learns a trick of replication but the trick of storing information then that's an incredibly powerful molecule because it can hold on to the evolutionary lessons of the past not have to reinvent the wheel so that sophisticated molecule holds on to information that can then sculpt other molecules in the environment and now we're talking about a replicating molecule that holds on to information does that ring a bell dna is an example prime example of that kind of a molecular structure so that's the way in which dna could just have emerged through the natural blind processes of physics sculpted by a version of darwinian evolution right down at the level of molecules that's that's an idea for an answer to that question again if you want to see that in more spell that in more detail you can check out until the end of time and um but that that that should give you a feel for it all right a few more questions before so when will priya priya i'm going to bring prion in about 10 minutes so arnov uh yes you're chomping at the bit for that conversation so priya should be with us in about five or ten minutes um let's see uh cutie menadi why does light always travel at the speed of light in a vacuum it depends on the level of what you're asking that question in some sense the vacuum by definition is an arena that is homogeneous it doesn't change from place to place to place and therefore there's nothing in that environment to influence light's speed so if light travels at a particular speed at one section in one area one region of the vacuum you would expect it would travel at the same speed in another region of the vacuum because those regions are in effect physically the same there's no distinction between them because of the homogeneity more precisely when you look at the equations of electromagnetism maxwell's equations you find within those equations disturbances to electromagnetic field travel at a particular speed that comes out of the math and that speed is independent of where you are so long as the environment is emptied so long as the vacuum so our observations and our math all converge on exactly the same idea subam saaho asks what is the speed of dark i don't know if you're referring maybe you saw a youtube talk that i gave some time ago that when my son was quite young and i was telling him bedtime stories about aliens traveling near the speed of light he turned to me and asked the question what is the speed of dark so i don't know if you're echoing my five-year-old's cue is not 15. so it was a long time ago uh yeah the speed of dark what is it that's a beautiful idea i was kind of surprised when when my son asked that question but if dark is the absence of light then of course the speed of dark is the speed of light because if light is leaving a region it leaves that region at the speed of light so the speed of dark the increasing domain over which light is absent would be equal to the speed of light which is the speed at which that light can leave that region let's see what else we have uh before we turn to dark matter um prayage one of our long time stalwart attendees at these gatherings asks a question which is a good one what is time symmetry what is time symmetry well it's almost easier to answer the question what is time asymmetry and talk about symmetry in response to that but time symmetry is generally the idea that one direction of time and the other direction of time are in completely equal footing within our physical understanding of the world and that's a weird idea because if you think about your life thinking about the future and thinking about the past those are completely different temporal arenas the past usually you think of as it's gone it's like set in stone it's finished the future is open-ended has yet to occur you can remember the past you generally don't remember the future you have the opportunity to mold the future you don't have the opportunity to mold or affect the past so from an everyday life perspective there's a big difference between past and future but you know if you studied any physics at all even the most basic equations of physics show there's no difference between past and future past and future in physics basically amounts to replacing a variable called t the time variable replacing it with minus t that's it positive t going this way negative t going that way and the equations that we have are generally insensitive to that replacement the equations don't care whether you're using them to start from this point in time and use the equation to predict what things will be like or to postdict what things were like in the past the equations don't care they don't make a distinction between those two and yet we do so time symmetry is the mathematical fact that the basic laws of physics as we understand them don't seem to show a distinction between past and future and that's puzzling and we have to try to explain why it is that we as human beings absolutely do see that distinction okay sorry about that i think i'm gonna i did not bring enough water today so if there's anybody in earshot of this plea for more water that would be great if uh if water could show up at my door socially distanced now it's all only my family here so we don't have to worry about that okay um john olsen asks artificial intelligences are currently generating realistic images without having to calculate movement of every single atom is it possible that reality is already simulated by an ai entity of course yeah that's the simulation hypothesis that we've spoken about from time to time in this gathering and yeah i mean it's sufficiently powerful computer ai whatever words you want to use could certainly cause us to think that we are in a physical world populated by particles and atoms and molecules when in reality all that's happening is we're being stimulated by the simulation into thinking that the world as we typically describe it is real when it may not be so there's not a whole lot you can do with that simulation hypothesis beyond you know feeling a bit you know upended feeling your world is undercut you know glancing around and wondering if the simulator is ever going to make himself or her himself or themself visible to us but yeah is it a possibility it is i don't go around my daily life really worrying that i'm living in a simulation but i do allow for that as a as a real possibility and that's about the best that we can say about such things at the moment a robb b do you fear the singularity i no i don't fear singularities i hope that they're kind of far away that that would be a happy a state of affairs oh good my son is bringing me water yes um so but i don't fear it i i don't i don't stand in um tremble at the possibility that there might be some space-time singular rather my view is and i think this is a view of most scientists singularities are the big opportunity of say theoretical physics maybe observational astronomy too they're the big opportunity because if we can learn how to cure mathematically certain space-time singularities that may represent a deep advance in our understanding and so you know some of you know i was fortunate to be involved in some work a while ago that showed how string theory can cure certain kinds of space-time singularities these are singularities that would arise if spacer to rip in a certain very specific mathematical way and so that perhaps fearful possibility of space ripping we neutered it right we showed that it could be cured in a way that doesn't have any dangerous physical consequences again within a very specific theory specific kind of rip but that's the general way that we approach singularities as theorists okay great um let me do one more question and then we'll get to our our dark matter conversation um okay what do we got how do stars form good question gravity basically pulls matter together until the density and temperature is so high that nuclear processes ignite so that's a quick one there um um uh lion king could a black hole be stabilized after almost fully evaporating by a hawking radiation yes this is the uh the idea of remnants black hole remnants i don't think most people take that idea that seriously these days any longer i mean some do it's possible it's definitely out there but yes it's possible that a black hole could evaporate get smaller and smaller and smaller and then it stops and what's left is a little tiny nugget and a little nugget might be the end point of black hole evaporation absolutely a possibility um do gravitons exist um somebody asked i don't know if the traditional way that we think about quantum mechanics if that has relevance in a theory of gravity to make a quantum mechanical theory of gravity like string theory then gravitons would exist but if there's a different way in which gravity and quantum mechanics talk to each other and it's not that you quantize gravity but rather you gravitize quantum whatever that means if the sort of gravity is dictating story maybe gravitons don't exist but i say most people who work on quantum gravity would answer the question yes that that gravitons do exist um avdut asked does cause and effect uh is that invalid in quantum mechanics and no the cause and effect is alive and well in quantum mechanics you have to be very careful about the causes and the effects you have to be careful in analyzing the processes because they're different from the newtonian classical processes but if you think about a quantum mechanical wave function it evolves in time by a deterministic equation called the schrodinger equation and that deterministic evolution takes how things were in the past feeds them into the equation and that then in that causative manner determines what that wave function is like in the future so yeah there's a version of cause and effect there's a version of causality that holds perfectly well in the quantum mechanical description of matter rami als bog asks or says i miss your daily equation uh yeah i know it was fun to do that you know for a while but to come up with a new equation every day and then to sit down and do a little discussion of it you know it might sound like it's trivial and at some level it is trivial to do but when you have a lot of trivial things to do and i have a lot of other trivial things that i have to do you want to realize that your whole life is being eaten up by i don't mean trivial sense of you know trivial that it's not like useful and important what i mean is trivia in terms of a time sync so don't get me wrong that's what i meant but you have a lot of trivial things a lot of little time sinks they add up to a major time sync and you want to get nothing else done so that's why i had to sort of uh wind down and retire the uh the daily equation but maybe i'll be energized at some point soon and we'll come back to it all right i think with that let me just take another sip of water here and uh turn to the uh the main headline event if you will of our gathering here today which is this discussion of dark matter with uh priya natarajan so priya is someone i've known for a long time great physicist she's a professor at the department of astronomy and physics at yale great place he's a theoretical astrophysicist who's interested in cosmology gravitational lensing and black hole physics and just recently wrote a really interesting paper on dark matter that we're going to be discussing let me bring priya into the discussion now so if priya can join us i think priya yes priya do you see yes you see me as i can great to see you how are things going good um it's actually my sabbatical year but my wings are clipped so i couldn't go very far from new haven so covet clipped or something else yeah covet clip yeah yeah and and so uh you're where are you then i went far i'm uh in cambridge massachusetts we went like an hour or two away something like that yeah are you at a university there uh no i was supposed to spend some time at the black hole initiative oh at harvard yeah but you know um nothing is happening at the moment remote seminars but no in person um and i haven't ventured to um go there i have allergic asthma so i'm a little nervous yeah yeah like you know my son has an autoimmune disorder so like we're being incredibly careful because of that so obviously not anything to mess around with yeah so um i mean do you get out at all i mean do you uh yeah i get out for walks and runs um and the nice thing is that where i'm located i'm pretty close to the river and so i go out for walks and yeah it's charles river yeah it's not it's it's not actually been easy so i actually live on my own so i'm kind of used to my own company i'm very social but you know yeah um this is hard because this is sort of you know not self-imposed exile but you know hermit hood uh due to circumstances and i really miss the thing i love is i love to cook so i love to have people over for dinner and you know it's harder it's harder you know there was another of course many physicists have turned to you know the black death the plague of the late 1600s as the model right because newton newton goes away goes into isolation and re-emerges with the universal law of gravity so all of us are like this is our newton moment right this is our plague you know where we're gonna disappear and come but you've actually now uh i can't say that i've come forward with anything like that but you you wrote this great paper it was just published like a couple days ago right right so first le i rolled my eyes big time for those of you who didn't catch it with the newton the plague comment i mean the way we sort of valorize this stuff is just crazy right brian yeah whatever but yeah this we had a we had a very exciting paper uh hey we have to have our we have to have our myths right every every tradition has got to have its mythic heroes and figures that that you know inspire us to go forward but yeah i agree when you examine any of these or not any but many of these stories you examine a little more deeply and uh they're not quite the story that you were told as a kid right and also i would personally feel more comfortable if there were a few more like heroines around yeah there's always that that niggles me that greatness greatness doesn't seem to stick to our kind as easily yeah well uh hopefully that's changing yeah hopefully certainly uh the history of uh of most sciences is not uh an emblem is not to be held up as the model of the way that that the world should work right hopefully so coming back to our paper so it's a paper that's led by massimo meneghetti one of my collaborators and um we found something really sort of surprising but you know i've been on the trail of this for a while so the idea is that you know we see uh gravitational lensing sort of the bending of light by some of the most massive structures in the universe you see it very clearly for clusters of galaxies and the idea is that um you know in our theoretical framework we know how these clusters form and we can now form them in these simulations in the box so we can simulate a piece of the universe and we let it go and you can form pretty much all the structures that you see that's how successful the model is at the moment so what this allows you to do then is to directly confront the observational data what you infer from the data with your simulated universe so you can make a mock observation which mimics your real observation and then see how do they tally um and so um i've been trying to do this and you know sort of one of their so first thing you have to do is to you have to conceptualize a cluster and observed real object in the universe in such a way that this comparison is amenable that you can actually compare with simulation so that i sort of wrote a paper actually it's one of my first papers in grad school and i'm going to date myself i hate to do this but you can go look it up so i might as well declare it's 1997 i was young heaven for a friend you were a graduate student in 1997. but it was my first year or so okay so okay so um and so the idea was that you conceptual the conceptual model is that you think of a cluster as a huge lump amount of dark matter like a mountain range but then that there are individual peaks and the and the reason for that conception was that you know most galaxies have their own individual halos of dark matter they have their own glob of dark matter so explain that i mean there's some folks who will follow everything that you're saying but you mind if we even unpack some of the ingredients as we go along so you're talking about galaxies and and dark matter so so like where does that i mean just give us a little bit of history of why we even think that there is dark matter in in in galaxies right so i think that there's a pretty rich history for this right so i think it goes back to the 1930s with fritz zwicky uh this cantankerous swiss physicist at uh caltech what he did is he measured the speeds of galaxies in a cluster so a cluster of galaxies oh that's right that's the iconic picture of him um so galaxies are held together by the gravity of the cluster and they're moving around and so in the coma cluster which is a nearby cluster to us he went and measured the speeds i think of about 12 of them and he found that they were whizzing around really fast they were whizzing around so fast that it didn't make sense that they were still held by the gravity of all the matter that you can see so you see stars and you see stars in these galaxies and he said well there's something funny going on because there has to be some missing matter here because there's more gravity that is holding them in their grip galaxies they would fly off otherwise yeah and so he proposed this term dunkel materi so dark matter and that stuck so he just said well you know you know that that term actually goes even even further back we did a little bit yeah like i think we may even have it i think poincare not mistaken uh made reference to uh do we have that that that poincare reference obscure the translation of that so yeah it's actually an idea that goes even further back but you're right i mean i think i think most people would would credit zwicky with the first time and the only footnote that i just want to put into that story because his daughter yeah contacted me after one of my books because i think all of us have this view that has been again maybe it's one of those lore like newton and you know during the plague coming up with you know brilliant ideas she thinks we're being very unfair to her father when when when we show that image yeah yeah yeah do you know her do you know i don't know her but i have to say that i actually admire him you know i mean i i say cankerous because that image i knew you were going to pop that one up and um no i mean the thing you know as a first of all right i think zwiki was the first one who mentioned this dark matter in the context of a cluster yeah yeah okay was in the context of the milky way right so um you know he had he was brilliant in terms of the number of ideas he came up with right and of course somewhere crazy and out there but the hit rate was very good and we forget that i mean this whole idea of supernovae and what you're seeing when a supernova goes off and tying it to the end states of stars that's remarkable we wouldn't have found dark energy without that right i mean so he has made enormous contributions right i totally agree that you know we tend to get sort of fixated because that images are so powerful that's why right yeah yeah i mean the phrase that i remember that he is all whenever you see zwicky quoted they always quote the phrase spherical bastard oh my god i can't even never heard that yeah we're allowed to say those words yeah yeah that's actually even uh i think it's one of the seven allowed words even on any any network but um the idea here is that apparently he would say what is a spherical bastard it's a bastard when viewed from any angle so so so so so so these kind of and now i'm i'm propagating the exact same pull back and say this is what it's typically said but you're right i mean look i mean any human beings or rich collection of events histories quotes sayings insights and and his insights but i think his daughter has a point that you know as a very important point to make first is the damage to his actual reputation the second is as you just pointed out right by repeating this brian somehow yeah physicists in our heads we also start to associate you know what you can be a jerk as long as you're brilliant and we need to break that too because you don't have to be i mean it and then yeah there's no need for that allowance but um anyway coming back to the dark matter so he made um so he made those measurements and he said okay there's probably and he figured there had to be a lot of dark matter and you know he wrote this paper it's kind of sunk without a trace nobody took it seriously they thought it's a bit too out there and you i mean and in astronomy right we've also had a pretty checkered past and history with invisible entities like ether which went away yep so people are like well you know whatever this invisible stuff well we'll see but you know he was relentless that's the other thing that i do like about him and so he realized the further implications that if indeed there was so much matter so this was the 30 so einstein's theory of general relativity was proven and you know was the star theory he said aha then it would have to bend light because any any glob of matter in the universe actually bends the path of light because in fact that's how general relativity was first you know the first major confirmation was a bending of distant starlight by by the sun and you're saying that any source of gravity doesn't have to be the sun it could be this dark matter it acts like a lens light travels through it the gravitational pull of that dark stuff pulls on the light itself that's right yeah and i think you had a really beautiful demo that i remember i think it was on on youtube right so this whole thing with space time and the warping of space-time the potholes and space-time due to mother didn't you have this like trampoline or something yeah yeah we built a version of the uh the spandex you know gravity simulator and we actually set it up in our living room at home so it was with my two kids who were much younger i remember i was gonna say that you had sort of uh you had a little bit of child labor in you i did i did in fact my daughter was charged with putting the sun in the middle of the space-time simulator but she was too little to reach so i remember in the video i had to pick her up and put the sun sun down there in the middle but yeah that that and it's not look i mean people often ask me how good of a model is that and you know it gives you the mental toe hold but the reason why this simulator and the spandex warp when you put a shot put in the middle is because earth's gravity is pulling on the spandex so of course you're using earth's gravity to explain the warp space which is meant to be gravity so it's kind of chasing your tail a little bit but but yeah i mean the basic ideas you're saying is when matter or energy is in space it warps the environment that's einstein's idea not because something is pulling on it it's just a fact of the way gravity works you know and so i mean i think as you said one of the places where that analogy kind of gets a little fuzzy is the fact that it's hard for people to understand that you know that is the entire universe there's nothing above it below it because when we demonstrate we're showing it in air or whatever so we are all em embedded in that space time so light has no choice but to propagate through every bump and yeah in fact korea i'm just wondering guys can we bring up the three so here's a 2d version so you got a distant star that's what you were referring to and then the the star light is going on that curved trajectory near the sun i'm just wondering can we bring up the 3d version that we've made of this where you see matter moving through a 3d environment and maybe that can be brought up at some point as we're talking because because i think i have found that that image really propels people's understanding because of the question that you just raised it's like there's a why under there's like not exactly quite above right or above or the side so it's actually all around the object that space is being burst so if we can bring that up if we can grab a hold of it that'd be great but yeah going forward light bending so he said okay you know there is this other prediction for the nature of um distortion of space by matter and so he said okay let me calculate the light deflection if indeed the coma cluster had the kind of mass that i think it does with the dark matter he said let me calculate what the light deflection would be it is finite um and it is calculable of course but it's tiny and it was beyond the remit of telescopes at the time remember like a hundred inch was like state and that's right so um oh speaking of which mount wilson which is in peril right now with the bobcat fire do you know i did i fired it looking very going on really close yeah but um anyway the um oh no maybe it's not mount wilson maybe i'm getting it wrong maybe it's the lick observatory mount hamilton i think my mother was wrong it was hubble but but uh yeah yeah but yeah so um anyway the um so you know he then said that the deflection was too small but so that was not persuasive either because you know it was not detectable so kind of sat around the idea kind of sat around uh but then you know completely as is apt to happen in physics as you know right we keep rediscovering things that others have come up with and often don't see the connection so aware rubin and kent ford were looking at a completely different set of systems so they were looking here wait what you're right now roughly 70s okay so i think following during her thesis in the late 60s she was trying to understand rotation of galaxies like do galaxies actually rotate what powers their rotation and then later she started working um in washington at carnegie dtm and saw her with ken ford and ford had built the spectrograph which allowed a new kind of measurement which is a measurement of the speeds of stars inside a galaxy like a spiral galaxy from the center out and i think you might have a visual phrase i think we do have a visual of spinning galaxies uh if we can bring that up that would be great and the idea of the visual if we if we get to it here it is so you so so i guess it was kent ford's measurements that could give you the speeds of the stars in that rotating galaxy and uh i gather the punch line which we have in another visual is that they're moving too fast that's right yeah and and then so there you so they should be being flung outward like bicycle the water drop is on a bicycle rim as it spins the water flies off but they're not being flung outward so i gather they came to the conclusion there got to be some dark stuff there giving right extra gravity super and yeah i mean you know and um very rubin charted what what are called these rotation curves so that's the speed of stars as a function of from the distance of it and so where when ford looked at them and they looked really peculiar they looked like they were it looked like the stars were being held up on the outside somehow right the speed instead of falling there didn't seem to be kind of an edge to the galaxy as it were in terms of gravity i mean there was an edge in terms of light you run out of stars but you know and you know they sat on this paper i spoke to vera i knew her quite well she was amazing um and she would tell me that you know they sat on this she sat on this for years because the engineers really two years because they waited to gather more and more data so they wanted more and more convincing data so it was not a you know it was not a one-hit wonder for this one peculiar galaxy right yeah and then they uh they went ahead and published it and the implication was that there was some unseen matter again factor of ten but you know i went back and looked and i even asked her and she said you know they did not connect the dots with zwicky because that was a cluster of galaxies they didn't think it was the same dark matter at that time when they were working on it of course the connections were made soon after when humans jumped in and this whole cold dark matter model was uh proposed and that accounted for the dark matter both on the scales of galaxies and clusters and you know this whole idea that dark matter is basically smeared everywhere in the universe very lightly and then it lumps in locations and those are the locations where you preferentially form galaxies and galaxy clusters and so on so when we think about a galaxy i think many of us have a mental image that's similar to the little animation that we showed but if you include as you say dark matter if you make it visible for visualization purposes then the edge of the galaxy is completely different from what it is so do we have a little image i think we have an image of a galaxy together with the the dark matter component made visible to get a sense of yeah the kinds of scales that that we're talking about so if we can bring that up that would be uh that would be great um but the idea is you you if you think about all the stuff visible matter and dark matter the scale of the galaxy can be much much larger than simply the the the edge of the spiral arms that's right absolutely i mean it's almost roughly speaking it could extend almost ten times right and times ten times i think we do have that visual someplace so maybe in just a moment um okay so now we're at a stage by sometime in the 70s you've taken us through 30s zwicky puts forward this idea that's largely ignored that there may be some dark stuff out there important for the understanding the motion of clusters of galaxies by the 70s the idea is out there that to understand the motion of individual galaxies there's also some dark component dark stuff and people begin to think about those as the same stuff that there is this dark stuff that's threaded throughout everything yeah so then the natural next question is like what is the dark stuff made when when do people start taking that question seriously 1978 onwards sort of theoretically people started to see how could we uh explain observations on these various scales where this dark matter is showing up and there was a little more sort of a deeper understanding of you know star formation galaxy formation by then and then the idea was how do you integrate the formation and the structuring of the universe that we see with this unseen component so i would say that you know the heyday was kind of 80s 80s and people started to do little simulations so make up mock universes to see how this dark matter would cluster and so as people started doing you know so the one of the things the cool things and uh about this cold dark matter model is that you know this model was refined along the way as we got more and more observations right so um so you know geller and hakra looked uh mapped out galaxies in the very nearby universe and i don't know if there's a stick man they found that galaxies are clustered yeah and and so they're like okay so that's something that the theory then has to explain what what is clustering and what's going on right so they were able to then incorporate the data as it came along and refine this model and this model therefore has you know a storyline of how you start from some initial conditions and then you know but that calculation of starting from the initial conditions and getting down to sort of the relic radiation that jim peebles did already in the 60s so the cosmic microwave background and the fact that the microwave background that is now streaming towards us would inevitably encounter all the galaxies and everything that's forming in the universe on route these things form later in the universe right and so you know all the pieces were kind of there already and then there was this sort of grand integration i think sort of i mean it's a way to put it that every all the pieces came together and so then we have this theory but then this theory requires to explain the observations right the theory requires that this dark matter interact only via gravity that it has no other interaction it's inert so no no electric charge no strong nuclear charge no weak nuclear charge the only way that it talks to particles in its vicinity is through gravity right and then the other and otherwise otherwise we would have seen it it will like those are the constraints that that's the kind of consideration right and so and the other key thread as i said right there was this kind of collection of things that kind of makes it made sense together so from the particle physics end the other thing that came in is that you know astronomers were able to do an inventory of how much dark matter you have how much do you have to have today the question then is that you know on cosmic scales we cannot single out the behavior of a single particle of dark matter right we're looking at aggregates of dark matter and how they behave it's cosmic scales whereas particle physicists are looking at you know what is this particle when was it made and then it became clear that in a standard model of particle physics which was also being developed along roughly the same time it was clear that it was permitted you could have many particles that are produced very early in the universe that would give you the right density of dark matter if they were dark matter so they're like ah bingo right we we kind of narrowed it down to candidates and so we all became fixated uh including myself i mean you know on neutral enos um sort of you know as a favored wimps so these sort of weakly interacting so you know you permit very weak interactions if at all but um massive particles and so you know then of course there you know you have to do both things right so we have all been astrophysically pushing to look at the collective behavior of dark matter and particle physicists have been looking at direct detection trying to actually nail down the particle so you know those two threads have been going on and how much how much of this dark stuff should there be i mean what how does it compare to the amount of visible stuff so i mean if you kind of do like a pie chart of all the matter in the universe about 90 percentage 94 95 percent really um 90 percenters should be dark matter so we're dominated by dark matter dominated by documentary dominated now and often when people show pie charts and i think we do have a pie chart we also want them thinking about dark energy so i know this is not our focus here today but just so that this pie chart is clear can you just tell us what briefly what the dark energy part of this pie chart is right so this this my chart is a full inventory of everything in the universe that counts and so you know the dominant component in this blue part of this um diagram is dark energy and this was um detected dark energy was detected in 1998 and we believe it is what powers the accelerating expansion of the universe at the moment we believe that it could really be be a property of space itself we really don't know what it is the nature remains an open question we have some ideas and they're being actively pursued but what we know is that it is the dominant constituent of our universe and that it does something quite intriguing compared to matter so you know when think of matter and we're going on about gravity which tends to aggregate it's an attractive force whereas this dark energy is sort of this repulsive thing because it causes the accelerating expansion of the universe it's almost like space the stretching of space the there's an acceleration of that expansion i mean just like you know when you're driving you have to accelerate you have to press the gas pedal and you have to provide some energy so the moniker dark energy um you know and what it is um we have no clue but but the main part for our conversation if you can leave that pie chart back up for half a second the main the main relevance for us is that green wedge is like more than five times the arc yeah so so the so you know we pride ourselves on the depth of our understanding of how material objects behave in the universe we have the standard model particle physics you made reference to it we can do these very very detailed precise calculations but all of that is focused upon that little orange slice of the pie chart that you referenced and that big green wedge we don't we don't know what that matter is we don't know what equations govern it we don't understand precisely how it talks to the other wedges in in in the diagram so that's a great opportunity for us to go forward and the point that you're making is there are candidates that people have put forward you know my very first paper you spoke about your early bit my first paper was on a on a dark matter candidate you know i'll update myself now so that paper was not in the mid 90s but in the mid 80s you know so so my first paper is in mid-80s and it was about a particle that naturally emerged in certain approaches to string theory so so we did the standard analysis to figure out how much of that particle would remain today if it were the dark matter candidate and as you're saying it's not that hard to write down models where particles from the big bang era would persist until today with the right abundance to be that green wedge in the pie chart exactly and for many of us and many of these are so-called super symmetric particles and some in our audience will recognize that word super symmetry but for many of us as theorists way back in the 80s when this was sort of first recognized it was remarkable that particles that were coming from theories that had nothing to do with dark matter exactly and you stuck them into your calculations and the amount that would remain today was exactly on par with what you needed for dark matter this was so convincing totally you know there are many candidates and maybe we haven't been as open-minded about um exploring that space but um can i share a little secret yeah we almost did string theory and was going to work with wafa cameron waffle okay but i didn't because i was prepared for it like i was an undergrad at mit did pure math and physics as majors and you know i was like the prime candidate for that yeah and work at the time but you know why i didn't do it because i did not like the fact that you could not confront observations to test it and you had a good reason you know rapha was on the previous yesterday and i think for me actually you know um i was this like intense kid right so for me it was like an existential crisis i actually uh took a break and i started a phd in history and philosophy of science is that right yeah yeah yeah i know that the background so so so so was it so you were at harvard i should know all the best i don't know i might not i wasn't like mit okay okay sds got it okay but you know it was twofold first was that i was you know i was one of those kids who took like a million courses way ahead did all the graduate courses i was ready to do research by the time i graduated i incidentally i did my undergraduate thesis uh in particle physics with alan booth finite temperature field theory calculated a mickey mouse diagram but tiny little nothing major guys no one should get impressed i mean you know it was it was just a little piece of work but um i found that i you know as i was at mit right i was interested in a lot of things always like super curious and i was always interested in history of ideas and and i read kuhn i was quite influenced by cohn but then i thought you know this is not enough um you know this is how i was like a little kid i was like you know what this is not enough i think that he's missing something because he's not an active scientist okay i mean the person if you sit outside you know and i was like you know this thing of being an insider and an outsider because that was my life right my entire life i've been a bit of an outsider and an insider at the same time uh growing up uh etc in my sort of history so that was an idea that appealed to me and i was like you know what i think which one should roll with that then that's what i want to be so i also want to be a sort of a philosopher of science and an active scientist so and i was young and i said okay i'm going to do this phd and then i'll finish it and then i'll go back and do physics and because i had sort of reached this impasse i wasn't sure about what i wanted to do right because i didn't want to do string theory that was the automatic next step and so then i took this detour and of course i did not finish that phd my life went in a different direction and the person who got me there was another you know i was just i mean brian i don't know do you think about this this i've had such good fortune i'm so grateful to be able to do what i'm doing i mean i kind of reached it you know i started out being very focused then i know there's a little bit of detours when i got to where i am in terms of always wanted to be a professor and a researcher and a scientist and um i met you know chance meetings right i met martin schwarzschild he's another he's one of my greatest heroes and he is no more and he was um you know so what i was going to do in history and philosophy of science um is in fields like cosmology where you cannot perform controlled experiments and simulations are your experiment how does that impact inference how can you validate a theory and test it with a simulation i mean it's all related to the work that i'm covering now so these are questions obviously they're top of mind for folks who have gone into string theory because oftentimes it's not even a simulation for us it's just analytic mathematical equations and internal consistency as opposed to comparison with observations so these issues are are very close to us but let me just ask i mean do you feel like an outsider still in some ways yes right i mean i um obviously intellectually i'm very much you know uh in um in sort of the an insider but you know as a woman woman of color um and as a person there are many ways in which i feel that i uh stand out uh and in um so also in the scientific style um that i work on my own unlike most astrophysicists who are now you know theorists are sort of joined into big collaborations of thousands of people as a pet theorist i've not done that uh and the reason i haven't done that um is sort of a choice because i want to take creative risks and you can do that you know in those sort of collaborations it's a lot of consensus building about science and the scientific questions and how you tackle them and i like to have this freedom of just running with an idea myself so i work with smaller groups of people so um i think in that sense i feel a little bit like an outsider and you know and and brian you must do right those of us who have interests that go well beyond we don't fit the stereotype of what this kind of uh you know intense scientist is supposed to be like i mean i i i know that we are we have a lot of things in common uh you know we love to write we like to engage with the written word as well as equations um and we think that science is a creative pursuit like many others we're interested in art in history this that and so you know i think there's an image of who is supposed to be a serious intellectual scientist and i think there are people who probably would count uh me a little bit on the outside that you know i think people want to box me i think and when i was young i found that really difficult because i so wanted to be belong you know i want to be one of the big boys right and then i actually realized now that you know not being able to box you i in any way put you on the inside or the outside or whatever is actually a very powerful position yeah yeah no totally i i completely agree i mean you know there was a time when it was a lot of soul-searching for me and perhaps for you too should i spend my time solely doing research when i'll write the most papers and and have the greatest impact in science or should i split my time between doing the research and doing other things like writing books or doing tv shows or doing this kind of conversation that we're having here right now and you know the thought occurred to me that you know you write a really good paper in physics and it influences a couple a handful of hundred people typically you know whereas you do something you write a really good book or do a really good television show you can influence millions of people and and so the influence is different and the impact is different but there's value to each and i've basically gotten to a point in my life perhaps similar to yours where i just do whatever feels right at a given moment as opposed to try to fit myself within a particular rubric you know the usual rubric as you say is you do your calculations you do your research you do it from the beginning you become a graduate student until you leave the field possibly in a box that's right and and and my view there's just more to life than that and so if you can flesh it out in interesting ways that have impact you know although all the better all the more power powered teams i just find that my mind is stimulated like i i find that um of course you know you are incredibly powerful in your reach and how amazing clarity you have in explanation um i find that uh for myself it's just that you know when i'm grappling with sort of ideas and histories of ideas and so on it actually makes my scientific work much better yeah yeah it pushes me and you know it is hard i am trying to do like both with uh sort of equal intensity it's difficult and i think i can pull it off i have to confess i pull it off because a i am a workaholic okay you know what whatever i am a workaholic and i don't happen to have young kids or i have to care for or whatever right so i don't have those kinds of uh responsibilities that bind you and it's all time everything is time but um anyway so i think you know the this kind of obsession with simulation so i went met martin schwarzschild and he said what are you doing this for do you want to do simulations i said no no i want to work with ideas so then he said oh i know the person you need to go work with i don't know why you're doing all these humanities you know that you can always do that later priya you know just learn how to write i said oh yeah that was a hard thing to learn because i had to take all these graduate humanities courses and then i went to cambridge england and then my life changed and then it was theoretical astrophysics and it was you know dark matter black holes and you know and then idea was i would go back and finish i'm abd i thought i'd go back and finish the phd but you know i realized that as you said that it's a better outlet to actually write a book and you know and to and to express what i uh have been learning and digesting and thinking so you still i do have any thoughts of of completing the other degree or uh well i do have a master's i mean i got him you know i went i was there for three and a half years so you know um and i actually quit actually one of the frustrations i was also frustrated with that was because this was the time again dating us uh when the sociologists of science were really kind of you know it was the high point for them so i was tired of having discussions with some of my brilliant classmates you know is this stable made of atoms are are atoms just a good way to represent whatever this table might be made of and i'm like no guys i've seen a diffraction pattern and i think yes there is a conceptual model but you know there's reality and i was like you know what i um i am very open-minded about critiques of science uh and sociologists critiques of science and i think that uh scientists needed that um but i'm not for that extreme strong program but you know that science there's no content to science everything is context i'm not there yeah neither nobody i i think it's not a sensible position it's not supported um but anyway that was the reason i wanted to quit i was ready to leave as well and get back to science so i'd love to return i want to return a moment to your dark matter but just since we've gone down this interesting trajectory i mean you know there may be young women watching this or will watch it on youtube do you have any advice you know for for young women who are in a position that you may have been in 20 years ago who want to be scientists what what words of advice would you give them well you know i mean i um i don't have any great sagely words of advice luckily things have changed a lot and there's a lot more um lot many more opportunities to pursue science and different kinds of science now i think you know when i look back um i find that the thing it's a it's a it's perseverance persisting you know it's a it's a quality that we don't give as much credit to in terms of uh in science but i i mean you know you you will face a lot of obstacles everybody is going to because i mean there are going to be intellectual obstacles and other obstacles intellectual because that's part of science constantly trying to push your mind to learn more so you have to love that frustration of pushing yourself more right now and i think you have to persist and not give up too easily because you can give up intellectually or you can give up because of circumstances if you don't feel welcome somewhere or i think those other situations the other circumstances are changing um and i wish they were changing faster but um i would say you know it's just very glib to say follow your passion but it is true you have to kind of be stubborn and you kind of have to stick with it wouldn't you say right i mean it's it's advice to any young person well i i mean i give similar advice i i think the advice that you gave is particularly useful for you as a role model to to other girls and women who would want to follow in this kind of trajectory the more general advice that i typically give emerges from the fact that i have and you probably have this too i have a lot of young science students come to me and they want to immediately jump to the forefront like i want to work on string theory or i want to work on you know because i'm like whoa have you taken classical exactly have you taken the electromagnet have you taken citizens so i really emphasize to kids like watch programs like this and read books and see television programs we read new scientists or scientific american whatever to get excited about what's happening at the forefront but you've got to focus in on the basics no and get your foundations absolutely the same thing you have a lot of high school kids writing to me i was like oh you know i love black holes let me do research could you give me a project yeah i'm like you know what uh research is there for you it's very exciting as you say follow it but why don't you just master the foundations yeah because that's what you really need if you master the foundations then you know any research field you can get in and and you can make a contribution and you can make original contribution there is i have no doubt because i think i personally believe that you know every kid no matter what color what gender every kid has a very unique gift to bring to whatever they do be it science be it the arts be it any profession the question the big journey and adventure of life is to find what it is that is a good match to what you can creatively do your creative juices match the problem or the the place where you can contribute and all of us who are so fortunate to have made that connection early in your in our lives but it's never too late yeah and i agree that that is the big that is the big question and uh i agree i do feel incredibly fortunate in exactly the way that you described that i sort of found something that fired me up that i was good enough at to make a contribution in and uh yeah i mean i sit here and often like i mean i'm in the middle of writing a book like writing my last book i said i can't believe that i'm given the freedom to do this by the way i loved your last book all right i love it yeah i love the ambition of it because i really like these sort of um ideas grand synthesis of ideas yeah i think it's very stimulating and it's um and it takes time before you can um write right so yeah i'm called up also writing yeah but you know i am getting distracted by my work yeah you're wonderful last book that was actually out just a couple of years ago four years ago that time flies yeah so you're in the middle of writing uh again you know i mean you know i'm on this trail of understanding sort of radical scientific ideas so that trail i'm still on that same trail kind of a new direction um but anyway coming back so you know the obsession with simulations and the eye testing simulations was kind of always there for me it was kind of a long thread and so you know i came i finished my phd i was very very lucky you had a great advisor did work really it was you know it was a time of great personal turmoil and i i mentioned this only because you know we always say that you know great stuff happens when you're really troubled sadly for me that was true that it was a moment of enormous personal difficulties in my life and when i moved to england and i poured myself into my work and you know you could tell that i gave it everything and and it became me and it was kind of hard because i became a super workaholic at that point yeah yeah and but um then i was very fortunate i barely did a postdoc i became a fellow of trinity college and then i got the job at yale so once i came to yale i continued this thread of lensing and lensing was new at that time so the observational data was just coming in and the reason is you needed hubble space telescope kind of resolution to start seeing it and once you have it you see lensing everywhere i mean and in clusters of galaxies if you go and look an image of taken bubble space telescope beautiful right by eye you can pick out all the distorted shapes even by eye you can see it right just because the galaxies are normally you know elliptical or spiral and they just have this sort of weird bend to them when you look at the imagery because as we're saying before the light is bent by the gravitational pull of the matter especially so you have this cluster of galaxies so you know i've honed in on the system of cluster of galaxies because they are the largest repositories of dark matter so any lensing effect that you find would be most dramatic and so you know you have all these distant galaxies you have this cluster that is in between us and this whole population of galaxies in the universe and you end up seeing very mangled images of the background and the idea so you know as part of the process you know because i was early in on this game develop the methodology to look at mangled shapes and infer back out instead not just look at those and analyze them but use that to figure out how much dark matter is in the lens got it got it and the reason you can do that you know it sounds odd like how could you really do that well you know galaxies are born with a distribution of native shapes they have birth shapes however clusters are extremely rare in the universe so they're very i mean you know so when you look at the night sky most often you are not looking through a cluster they are very rare so the rest of the night sky is not as distorted which means you have the you have the baseline you know what the baseline is and you know modulo that baseline you're able to figure out so in this work we've just been following so you know what you can do now is that as the data is getting better and better you can compare with the theoretical conception of the cluster as this mountain ranges with peaks going to a simulation look at let's look so you showed it before but people probably uh don't have it in mind so you were able literally from the observations of these mangled distorted galaxies you were able to in essence come up with a map yes of the distribution of dark matter in in that region and and can we bring an and the peaks and valleys that you're referring to is very dense dark matter here less dense dark matter here and so forth throughout throughout the region so if we can bring up a picture is this an example of what you're right this is from our work in uh 2017 where so this is a cluster this is the mass distribution in a cluster and i just want to show that this purple stuff is actually a mound it's above the baseline in the universe okay already heaped and then on top of that heap you have all these little peaks and these peaks correspond to the location of cluster galaxy so you know galaxy cluster has about a thousand ish galaxies that are held together and each of these peaks is associated with a cluster galaxy you know so that was the original conceptual model i told you right and that has stood its stead and that is sort of verified as a useful way to conceptualize the distribution of dark matter in a cluster and remember we talked about this is where the connection back to where ruben and the individual galaxies come right so we saw that individual galaxies have their own extended dark matter and when they fall into a cluster some of the dark matter may be lost in the fall but they retain their own clumps so each one has its own blob and gloop of dark matter and those are those peaks are heaped so dark matter is heaped in those peaks as well as an overall kind of mound which is a larger scale distribution and now you're able to compare this if i understand to the simulations that you're talking about and see whether the physics as we understand it that's embodied in that simulation is agreeing with the actual observations exactly through this indirect means is that your most recent paper is that what you do in that paper that's exactly and and you know we already so what you can do you can go into a simulation you can look into a region where you have a cluster and count up these peaks you can count up the peaks you can look at their locations roughly and and you and you what you do is you pick clusters in the simulation box in your universe that would produce the right amount of lensing so they have the same amount of mass and they are apt to give you and you do this statistically right so you take as many clusters in a box you can make many of them so that was the original challenge over the years when i was trying to do it these objects are rare so your simulated box has to be quite large to have enough of them so that you can actually do a statistical analysis right so you need a lot of computer power to have a box at large and stuff and that became available like the best kind of state-of-the-art simulation where you could do this became available in 2016. and that was the illustrious simulation so this image this dark matter map it's it's the most detailed map still spatially of a cluster of the dark matter distribution so what we did is we we found that intriguingly the number of clumps on average matched very well but there was something amiss in terms of the detailed distribution of the clubs so there was a hint that there was a bit of mismatch a very small hint and you know and i really wanted to follow through and that is where this new paper comes in so one of the things that you need to pin down this model to get this you know the heights of these peaks the heights of these peaks are basically the amount of matter that is associated dark matter with an individual cluster galaxy so then you have to play the vera ruben trick to go and measure the stars so that these clusters are so far away that we have not measured like she did you know the speeds all the way from the center to the outward but we can make a central measurement a key measurement of just the inner part how fast of the star is moving and that's what in this paper we were able to do with this wonderful spectrograph on vlt the telescope operated by the europeans so we were able to get spectra so detailed accurate measurements first to determine the membership without ambiguity that this cluster galaxy is actually sitting in the cluster it's not in front of us it's not behind us right so that's a crucial source of uncertainty in all the modeling before you can make any grand claims you have to do like the basic sort of checking out and then we made measurements of the speeds of stars in the centers of galaxies and we made sure that our they were incorporated into our mass model which means we have the best observationally constrained at the moment distribution of dark matter so then you can start doing some precision tests and because the hubble data is so deep what you can do is instead of just looking at the large scale lensing you can now zoom in and see if around any of these individual clumps do any of these individual galaxies in turn double split the light themselves so which means they produce smaller scale events yeah and you can see them in the insets right in this uh in this figure but we found that because the data is so good now right so this is what is wonderful about the convergence like the data is good and the simulations are very sophisticated so it's you know it's a it's a good time to do a detailed comparison of the kind that we were doing so what we found is that these small scale clumps with these individual peaks shown in the previous map they are doing their own little lensing on top of the big lens so it's almost like you know because you know this lensing as you know right we have often makes analogy with just the usual glass optics lenses that we are all familiar with it's almost like you have this one big smooth lens and on top of that you have a lot of lenses yeah and so what we did is we said okay let us look at a quantity let's device a metric that can quantify the capacity of a cluster to do a lot of small scale lensing and so we defined this metric uh it's called galaxy galaxy strong lensing probability and you can calculate that for real clusters in the universe and it we didn't do it just for one it was a full sample of 11. and the data that we used i told you with the speeds um you know the extremely well calibrated were three clusters it's very expensive in terms of telescope time but you know in general it kind of ratified our modeling and it showed so we were able so we use 11 and then we calculated this quantity which tells you the capacity of small-scale clumps to do strong lensing split images not just mildly stretch them but actually split them like the cluster does and like individual galaxies sitting in the field also do so that is known that it does happen individual galaxies can have enough mass to do the splitting themselves so we were very surprised that we found such events in the clusters but then we went into and with this metric we calculated the probability and we did the same calculation in the simulations and we found a mismatch of an order of magnitude i see and that is huge because and then big mismatch yeah yeah and so you know simulations do not have cluster galaxies uh concentrated enough to have the capacity to produce as many of these strong lensing events on small scales so what does that tell us well we could be missing an ingredient in the um simulations of clusters clusters are really complicated they're very dynamical and you know there are it could be as simple as like you don't think it's like a bug in the code or anything as simple as that because the agreement with the other qualities is just too close right and we tested it with three different sets of simulations done by different groups with you know different uh um methods and you know different approaches even computationally if you give us a sense these simulations like can you describe in sort of lines of code i mean how big are the computer programs necessary they're just huge i mean yeah i mean and you know they're modular so i mean some of them could be close to you know several hundred thousand uh lines of code because because these contain recipes for the formation of galaxies you know for black hole activity black hole do it some of these things we kind of have to put in by hand it's well motivated but you know we put in by hand so if you add in all the pieces of physics that have to be put in it's a lot of code and like how much how much how much general relativity is in in this code is is it mocked up in a newtonian way or or is it like full general relativity well it is patched right so the scales on which general relativity is relevant and important it is part you know so the expanding universe background is very much they it's they're quite realistic and then of course in the regions of the universe that have decoupled from the uh rest of the universe like an actual galaxy there it's all newtonian pretty much right so i think they patch in uh they're self-consistent these are amazing bits of code and it's an incredible achievement to have kind of um codes that we have and we have a multiplicity of them now and and they you know there there's all these adaptive meshes where you can refine and zoom in without losing the larger scale you know you don't make the kind of trade-offs i mean of course the number of particles is finite and you could do better and so on but it's pretty spectacular with the billions of particles right yeah so what we found was that this mismatch means that maybe there's a physical process um that is missing in the simulations when you say missing in the simulation you mean that perhaps we've just been missing right i mean this could be pointing to something that we're just unaware of and that's why it's not in the simulation right a physical process that we have assumed is not important or doesn't occur or we haven't even understood as you say occurs in clusters right now what about for dark matter though is it is this a potential challenge to the whole dark matter paradigm right so the other possibility is that there's something fundamental about the nature of dark matter we have missed so we have with this cold dark matter model quickly tell us why cold i don't think we ever talked about the core oh it's because these particles move extremely sluggishly compared to the speed of light so yeah good so going from there sorry just one not at all um and you know so these we have attributed only gravity to them which means they condense and aggregate only via gravity we there's no other force there's no other interaction that can squeeze them further and so you know they have a peculiar equation of state right sort of in chemistry we know from gases like we know what the equation of state like boyle's law so the pressure the volume and the temperature are related so if you squeeze if you squeeze gas with a piston you increase the pressure and so on but cold dark matter is really peculiar because the equation of state is p equals zero it's pressure-less which means it doesn't collide with itself because you know pressure you have only because molecules collide with themselves or with a container or whatever but since these particles are collision-less they don't collide with each other you can't condense them you can't you know they can't you you don't lose energy and dissipate and become more compact you can't concentrate dark matter to the levels that you can concentrate ordinary atoms which is why the centers of galaxies are so replete with stars right they are very high density of stars because gas can collapse and it gets denser and denser because it has pressure and the density goes up and i think you mentioned uh in one of the questions i was listening in earlier somebody was asking you about star formation so because this pressure and you can squeeze the density and you eventually ignite nuclear uh fusion um you can't do that for dark matter because you can't squeeze it in enough there's no force that permits you to squeeze it in so it's just gravity and it's kind of this loose aggregation so this suggests that in the centers of cluster galaxies somehow the matter distribution is much more concentrated so you have ordinary matter of course there and you have dark matter right and so it obviously we could have been we could be missing maybe there is you know i'm not going to speculate too widely right but it could be that dark matter either interacts with itself and squeezes somehow uh itself to get more concentrated or it interacts with ordinary atoms or baryons in a way that allows it to dissipate and squeeze and get more concentrated basically and you know and i think what is intriguing though about both of these options um is that they're both really significant implications for the field and our thinking about dark matter if you're missing you know simulations have been the backbone of this field because we can't do controlled experiments remember so this is our that's our experimental lab and if you're missing something there i think it behooves us to pay attention to figure out what it is right so i mean have there been any uh theoretical papers perhaps written before or written since probably too soon although people work very quickly that offer a a theoretical answer that meets the mismatch between the two well i mean i haven't and there's been nothing so first of all we kept our results under wraps like we are supposed to and so we didn't really leak it to anyone so no one people are just our colleagues are just learning about it yeah you know the hint that i found in our 2017 paper that's out there that's been out there i think people have been looking for looking at possibilities there's some suggestions and in this paper in science right you have all the supplementary material there's 14 pages of validity tests in which we actually kind of look at all the possibilities many of the obvious possibilities you know maybe the simulation isn't good enough maybe you missed something um you know maybe you are the black hole activity is moving out too much matter from the centers because that's what we want the black holes to do to explain other things that we see so you know we turned off we ran simulations in which we turned off black hole activity completely yes there you know we can get rid of a little bit of um the discrepancy but you know the thing is it's annoying gap it's like you know an order of magnitude is quite large factors of two we can see and there aren't factors of five different factors of two that would all combine in the right way i mean we just couldn't find anything which is why the paper is open-ended well that's great yeah the other question about the nature of dark matter is you know problems on small scales with the cold dark matter and people have suggested solutions but you know those problems go in the opposite direction you need galaxies at the centers of galaxies to be not so concentrated you need them to be smoothed out but those were problems that were detected not in a cluster environment but for individual galaxies galaxies like the milky way and not sitting in the middle of this kind of you know happening cluster yeah well look you know um although it may be a myth the with newton and the plague uh we'd like to story maybe this really will upend the dark matter paradigm in some significant way and uh you will have used your time away from ordinary world in a very productive way if indeed that's the impact of this work so it's a beautiful paper thank you so much i think it's going to have a lot of impact i think i think a lot of theorists are going to try to rise to the challenge and come up with ideas that bridge the gap between the simulations and the observations and i think it's going to be a very exciting time so thank you so much for spending some time going over you know all sorts of ideas i want to quickly mention for the audience that priya has a world science u course that is going to be released i think in the next week or two or three something of that sort i don't have the exact date in front of me but you guys should keep your eyes out from that it's again a wonderful exploration of these kinds of ideas and uh we look forward to uh whatever forthcoming book uh emerged from uh your uh you know radical studies about how science is done and uh it's been a pleasure speaking with you so thank you so much you speak again yeah i miss our i miss coming to the world science festival um i come even if i'm not speaking it's a train ride away for me so i really miss all the activities hopefully we can all uh do stuff in person before yeah i appreciate that and again you know we we the festival's not gonna stop we put it on hiatus to figure out the best way to do it digitally as i mentioned earlier in the conversation to the uh the youtube audience and so no doubt we'll have you in some world science festival programs going forward whether it's live or digital but for the moment of course it will be digital but yes i certainly look forward to uh our paths crossing in a literal as opposed to a virtual sense right and yeah i want to say thank you to you for all that you do brian uh it's very important work thank you very much and thanks for joining inspirational yeah appreciate that thank you all right so thank you very much and we're going to just do a little bit more q a before we wrap up i'm not going to go quite as long as i sometimes do for reasons that i will not articulate explicitly just to say that i have to get back in shape for sitting still for three hours when one is drinking a lot of water okay a couple questions before we wrap up here um which is a great conversation i hope you guys enjoyed that discussion of dark matter and the uh various ways in which scientists try to analyze its presence indirectly dark you don't see it but you analyze its presence through its effect on the things that you can see and through the way that it lenses as pre was described you can map it out and then you can compare that to the way the dark matter should be distributed based on your understanding of what it might be and the ways in which it interacts and that mismatch is uh a powerful clue perhaps to either what dark matter is or to the fundamental laws of the processes that it engages in okay so what else do we have here i mean we'll do a question two before we wrap up here so jim blonde says dark matter will go the way of the ether and it may it may well be the case that just as in an earlier generation the notion of an ether was hypothesized to make sense of observations observations having to do with light we have hypothesized some more invisible stuff to make sense of our observations today of motions of galaxies and most of the stars and galaxies and you're right that may be a stop gap measure which goes away with a deeper understanding of the laws of physics so i'm not in any way saying that that isn't how things will turn out we just do not know at the moment um alex asks oh daegu asked brian what would be a bigger discovery life on venus or or dark matter you know i i'm not one for creating hierarchies that much in discoveries i don't really people often say who's the better scientist was it newton or was it einstein i'm like you know at that level it's so spectacularly high that it it doesn't matter the hierarchy doesn't matter but um you know in some sense the discovery of dark matter would confirm something that we've long suspected to be true so i think many people if dark matter turns out to be a weakly interacting massive particle wimp has pre-described it you know if that turns out to be the case i think everybody would be wow that's that's spectacular but in the end we'd nod our head and having confirmed something that we thought would be the case you know life on venus is not something that people have really spent a lot of time developing and thinking about so its immediate impact on the scientific community might be larger but again the point is any of those discoveries would just be spectacular off scale once in a generation kind of insight so it's just exciting across the board without any need to order the various discoveries um alfred asked could you do a narration from the big bang to quantum physics a narration like sort of a show and and yeah i should say you know uh i mentioned my book before until the end of time again library i'm not selling it but i mention it in response to alfred's question because we're also in the early stages of developing a television adaptation of that and that would go from the big bang to quantum physics and beyond so there may be something of that sort in the not too distant future all right final final question before i um have to um to uh leave um um rajev asks why is light massless and and light didn't have to be massless our understanding of its dynamics of its physics how it behaves points in the direction of it being massless and our observations of it which are quite precise by this point in the game suggest strongly that it is massless but you know if it turned out that um way down in the future with ever greater precision somehow we're able to measure light and its property it has a little little tiny mass so small that today's detectors would be unable to be sensitive to it we would find a way to accommodate it and it would change fundamentally our understanding of the basic equations and the basic properties of light as a transmitter of the electromagnetic force but we would be able to accommodate it so it's just to say that light doesn't have to be masses in the sense that it would be logically inconsistent to ever imagine light as a massive particle i mean the transmitter of the weak interactions the w and z bosons are these are massive particles and they are transmitting a force much the same way that the photon does so it doesn't have to be logically speaking massless but everything that we know points to it being massless all right with that i'm gonna i'm gonna wrap it up for today again thanks for joining us for our first q a hour and then this discussion in the second hour with with priya on dark matter and we will announce i'm not exactly sure if we're going to stick at this time going forward but we're always open to hearing suggestions for a more convenient time and we try to take that into account scheduling these events if you want to make suggestions for who you'd like perhaps to join in for part of these discussions happy to hear those suggestions as well and with that said keep a look out for priya's world science u course take a look at my own world science u course on special relativity that's sort of a 12 hour version and there's the two hour version that are out there in the world space time and einstein and so until we meet again uh take care i've enjoyed the time we spent today together bye

Channel: World Science Festival

Views: 67,240

Rating: 4.8166771 out of 5

Keywords: Brian Greene, Priyamvada Natarajan, dark matter, electromagnetic radiation, dark energy, cosmology research, Special Relativity, Albert Einstein, Free online course, Special Relativity course, What is Special Relativity?, physics, best physics course, space and time, Relativity of Simultaneity, Clocks in Motion, World Science U, WSU, World, Science, 2020

Id: 7obuj1OHt2c

Length: 130min 11sec (7811 seconds)

Published: Wed Sep 16 2020